Polyamide 6 based block copolymers synthesized in
solution and in the solid state
Cakir, S.
DOI:
10.6100/IR730916
Published: 01/01/2012
Document Version
Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the author’s version of the article upon submission and before peer-review. There can be important differences
between the submitted version and the official published version of record. People interested in the research are advised to contact the
author for the final version of the publication, or visit the DOI to the publisher’s website.
• The final author version and the galley proof are versions of the publication after peer review.
• The final published version features the final layout of the paper including the volume, issue and page numbers.
Link to publication
Citation for published version (APA):
Cakir, S. (2012). Polyamide 6 based block copolymers synthesized in solution and in the solid state Eindhoven:
Technische Universiteit Eindhoven DOI: 10.6100/IR730916
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners
and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
• You may not further distribute the material or use it for any profit-making activity or commercial gain
• You may freely distribute the URL identifying the publication in the public portal ?
Take down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Download date: 16. Jun. 2017
Polyamide 6 Based Block Copolymers Synthesized in Solution and in the Solid State PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op maandag 19 maart 2012 om 16.00 uur door Seda Çakır geboren te Giresun, Turkije Dit proefschrift is goedgekeurd door de promotor: prof.dr. C.E. Koning Polyamide 6 based block copolymers synthesized in solution and in the solid state by Seda Çakır Technische Universiteit Eindhoven, 2012 A catalogue record is available from the Eindhoven University of Technology Library. ISBN: 978‐90‐386‐3113‐4 Copyright © 2012, Seda Çakır Cover design: Taylan Çakır Printed by: Proefschriftmaken.nl || Printyourthesis.com Published by: Uitgeverij BOXPress, Oisterwijk “Ya ölü yıldızlara hayatı götüreceğiz ya da dünyamıza inecek ölüm.” “Either we bring life to the dead stars or the death will descend to earth.” Nazım Hikmet To my mom, dad, Taylan
and İso…
Table of contents
Chapter 1 General Introduction 1.1 Introduction to polyamides and polyamide 6 1.2 Crystal structure of polyamide 6 1.3 Modification of polyamides 1.4 Modification of polyamides by solid‐state polymerization 1.5 Objectives and outline of the thesis References 2 5 8 9 12 13 Chapter 2 Partially Degradable Polyamide 6‐Polycaprolactone Multiblock Copolymers 2.1 Introduction 18 2.2 Experimental 20 2.2.1 Materials 20 2.2.2 Synthesis of diamine end‐capped PA6 21 2.2.3 Synthesis of hydroxyl end‐capped oligoester 21 2.2.4 Synthesis of diisocyanate end‐capped polycaprolactone 22 2.2.5 Copolymer synthesis 22 2.2.6 Enzymatic and non‐enzymatic hydrolysis 22 2.2.7 Characterization 23 2.2.7.1 Size Exclusion Chromatography (SEC) 24 2.3.7.2 Nuclear Magnetic Resonance (NMR) Spectroscopy 24 2.3.7.3 Differential Scanning Calorimetry (DSC) 24 2.3.7.4 Fourier Transform Infrared (FTIR) Spectroscopy 24 2.3.7.5 Potentiometric titration 24 2.3.7.6 Scanning Electron Microscopy (SEM) 25 2.3 Results and Discussion 25 2.3.1 Diamine end‐capped PA6 26 2.3.2 Hydroxyl and diisocyanate end‐capped polypropylene adipate 30 2.3.3 Diisocyanate end‐capped PCL (TPCL) 32 2.3.4 Multiblock copolymers of PA6C and TPCL 34 2.3.5 Hydrolytic and enzymatic degradation of PEA‐ASM 39 2.4 Conclusions 42 References 43 Chapter 3 Multiblock Copolymers of Polyamide 6 and Diepoxy Propylene Adipate Obtained by Solution and Solid‐State Polymerization 3.1 Introduction 46 3.2 Experimental 48 3.2.1 Materials 3.2.2 Model reactions of glycidyl phenyl ether and propanoic acid 48 3.2.3 Synthesis of fully carboxyl end‐capped polyamide 6 48 3.2.4 Polyamide 6‐poly(propylene glycol) diglycidyl ether model reactions 49 3.2.5 Polyamide 6‐diepoxy propylene adipate reactions 49 3.2.6 Characterization 50 I
Table of contents
3.2.6.1 Size Exclusion Chromatography (SEC) 3.3.6.2 Nuclear Magnetic Resonance Spectroscopy (NMR) 3.3.6.3 Differential Scanning Calorimetry (DSC) 3.3.6.4 Thermogravimetric Analysis (TGA) 3.3.6.5 Potentiometric titration 3.3 Results and Discussion 3.3.1 Model reactions with glycidyl phenyl ether and propanoic acid 3.3.2 Model reactions with poly(propylene glycol) diglycidyl ether (PPGE) and PA6 3.3.3 Diepoxy propylene adipate (DEPA) and PA6 reactions 3.4 Conclusions References Chapter 4 Incorporation of a Semi‐Aromatic Nylon Salt into Polyamide 6 by Solid State or Melt Polymerization 4.1 Introduction 4.2 Experimental 4.2.1 Materials 4.2.2 Dytek A‐isophthalic acid salt preparation 4.2.3 Solution mixing of PA6/Dytek A‐IPA nylon salt in HFIP 4.2.4 Solid‐state polymerization (SSP) 4.2.5 Melt polymerizations 4.2.5.1 Caprolactam (CL)/Dytek A‐IPA salt 4.2.5.2 Dytek A‐IPA homopolymer 4.2.6 Characterization 4.2.6.1 Size Exclusion Chromatography (SEC) 4.2.6.2 Differential Scanning Calorimetry (DSC) 4.2.6.3 Nuclear Magnetic Resonance (NMR) Spectroscopy 4.2.6.4 Potentiometric titration 4.3 Results and Discussion 4.3.1 Low molecular weight PA6/Dytek A‐IPA copolyamides via SSP and MP 4.3.1.1 Molecular characterization of PA6/Dytek A‐IPA 1
copolyamides by H NMR, SEC and titration 4.3.1.2 Thermal properties of PA6/Dytek A‐IPA copolyamides 4.3.2 High molecular weight PA6/Dytek A‐IPA copolyamides via SSP and MP 4.3.2.1 Molecular characterization of PA6/Dytek A‐IPA copolyamides 4.3.2.2 Sequence distribution and degree of randomness 13
analysis by C NMR 4.3.3.3 Thermal properties of the copolyamides prepared with limited Dytek A loss 4.4 Conclusions References II
50 50 50 51 51 51 51 54 63 70 71 74 75 75 76 76 76 78 78 78 79 79 79 79 80 80 83 83 90 93 93 96 101 107 108 Table of contents
Chapter 5 Investigation of Local Chain Conformation and Morphology of Polyamide 6 Modified by a Semi‐Aromatic Nylon Salt 5.1 Introduction 5.2 Experimental 5.2.1 Wide Angle X‐Ray Diffraction (WAXD) 5.2.2 Fourier Transform Infrared (FTIR) Spectroscopy 5.2.3 Solid State NMR 5.3 Results and Discussion 5.3.1 WAXD studies 5.3.2 FTIR analysis 5.3.3 Solid State NMR analysis 5.4 Conclusions References Chapter 6 Epilogue and technology assessment Appendix Summary Acknowledgements List of publications Curriculum vitae III
112 114 114 114 115 115 116 117 121 126 126 129 133 137 141 145 146 Table of contents
IV
Glossary α Alpha form γ Gamma form ∆HC Enthalpy of crystallization ∆Hm Enthalpy of melting [NH2] Amine end group content [COOH] Carboxylic acid end group content 1
Hydrogen‐1 nuclear magnetic resonance spectroscopy H NMR 13
C NMR Carbon‐13 nuclear magnetic resonance spectroscopy AA Adipic acid ACA 6‐Aminocaproic acid ATR Attenuated total reflectance C Concentration or carbon CDCl3 Deuterated chloroform CL ‐Caprolactam CP/MAS NMR Cross‐polarization magic angle spinning NMR spectroscopy D2O Deuterium oxide DBD Dibutyltin dilaurate DEPA Diepoxy propylene adipate DMA Dimethyl adipate DMAP 4‐Dimethylaminopyridine DMSO Dimethyl sulfoxide DSC Dynamic scanning calorimetry DyI Dytek A‐isophthalic acid salt Dytek A 1,5‐diamino‐2‐methylpentane FTIR Fourier transform infrared spectroscopy GPE Glycidyl phenyl ether HCl Hydrochloric cid HFIP 1,1,1,3,3,3‐Hexafluoro‐2‐propanol V
Glossary
IPA Isopropanol or isophthalic acid L Number average block length Mn Number average molecular weight Mw Weight average molecular weight meq Milliequivalent MP Melt polymerization Mp Peak maximum MW Molecular weight p Conversion PA Polyamide PBS Phosphate buffered saline PCL Polycaprolactone diol PD 1,3‐Propane diol PDI Polydispersity PE Polyester PEA Polyesteramide PEA‐ASM Polyesteramide after solution mixing PPA Polypropylene adipate or propanoic acid PPGE Poly(propylene glycol) diglycidyl ether p‐XDA p‐Xylylenediamine r Ratio of the reactants R Degree of randomness RT Room temperature SEC Size exclusion chromatography SEM Scanning electron microscopy SH Salt homopolymer SSP Solid‐state polymerization T5% Temperature at 5% weight loss Tc Crystallization temperature Tg Glass transition temperature VI
Glossary
Tm Melting temperature TBO Titanium(IV)butoxide TDI Toluene 2,4‐diisocyanate TEA Triethylamine TFE 2,2,2‐Trifluoroethanol TGA Thermogravimetric analysis THF Tetrahydrofuran TPCL Toluene diisocyanate end capped polycaprolactone VT Variable‐temperature WAXD Wide angle X‐ray diffraction Xc Percent crystallinity Xn Degree of polymerization
VII
CHAPTER 1 GENERAL INTRODUCTION Summary In this chapter a general introduction to polyamides and specifically to polyamide 6 is given. Synthetic techniques for the production of PA6 as well as its crystal structure are described. Possible modification techniques of the polyamides are covered and modification by solid‐state polymerization is discussed in detail. Finally, the objectives and the outline of this thesis are explained. 1
Chapter 1
1.1 Introduction to polyamides and polyamide 6 Polyamide, in its fully aliphatic form also known as nylon, is the first commercial synthetic polymer entering modern life. The chemical structure is similar to that of proteins and polypeptides such as silk and wool, which are formed by the coupling of amino acids in nature. Polyamides have a repeating amide group (–CONH–) in their molecular structure and the type of the repeating unit determines the properties of the polyamides. The structure of the amide bond and the chain dimensions are represented in Figure 1.1 as postulated by Flory in 1953.1 The first commercial polyamide was invented by the research group of Wallace Carothers at Du Pont in 1935 and was presented as the world’s first synthetic fiber. The polymer was called Nylon 66 (PA66) because of the six carbons in the diamine and respectively the diacid residues.2 So, here the repeat unit consists of two monomeric residues. The reaction for the preparation of PA66 is shown in Figure 1.2. Commercialization of Nylon 66 replaced the usage of silk and in first instance it was used for military supplies such as parachutes, vests, tires and ropes. Figure 1.1 Structure and the dimensions of the amide group in aliphatic polyamides. Figure 1.2 Reaction scheme for the synthesis of Polyamide 66. A few years after the invention of PA66, in 1938, Paul Schlack and his co‐workers at IG Farben were able to make a polyamide out of one starting material which was named 2
Chapter 1
Polyamide 6 (PA6).3 In this case the ‘6’ stands for the total number of carbon atoms present in the single amino acid residue representing the repeat unit. In 1940 the first polyamide stockings were introduced to the American market. Up till 1950 almost the total polyamide market consisted of PA66. Thereafter PA6 slowly found its place.4 Later on, many other polyamides were introduced to the market such as PA69, PA610, PA11, PA12 and PA46 as well as aromatic polyamides. The type of polyamide based on amino acids is called an AB polymer, whereas a polyamide based on diamines and dicarboxylic acids is a polymer of the AABB type. The most common synthetic technique for the preparation of PA6 is the hydrolytic ring opening polymerization of ε‐caprolactam (CL) at 250‐270 °C. This technique consists of three equilibrium reactions as shown in Figure 1.3. The first step involves the hydrolysis of CL forming ε‐aminocaproic acid followed by the direct addition by ring opening polymerization (ROP) of CL to the amine end group of a growing chain (which can also be the ε‐aminocaproic acid). Finally, the polycondensation reaction between the amine and carboxylic acid end groups leads to high molecular weight product where water is released. In practice, the ROP and the polycondensation reaction occur simultaneously during a significant part of the process. Figure 1.3 Hydrolytic ring opening polymerization of ε‐caprolactam for the synthesis of Polyamide 6. Hydrolytic ring opening of ε‐caprolactam (1), addition reaction of ε‐
caprolactam to a growing chain, the CL ROP (2) and polycondensation reaction between the end groups (3). 3
Chapter 1
The PA6 polymerization consists of equilibrium reactions and at the polymerization temperature around 260 °C at the end of process there are always around 10 wt% unreacted CL and cyclic oligomers present. These cyclic compounds, mainly CL, are formed by back biting reactions. Therefore, these low molecular weight extractables are removed by extraction with water after the reaction. PA6 is predominantly produced by a continuous multi‐step process in industry as schematically shown in Figure 1.4.5, 6 CL and water enter the top of the VK (Vereinfacht Kontinuierlich) tube, which operates at about 250 °C and 1 atm. As the polymer forms it moves down the column with increasing viscosity and a mixture of polymer, unreacted monomer, water and water soluable oligomers exits the bottom of the VK tube. This mixture enters a pelletizer, and the pellets containing extractables enter the top of a hot‐
water leacher. Water and the product stream of the VK tube flow countercurrently to remove caprolactam and oligomers from the polymer pellets. Finally, the extracted pellets enter a solid‐state polymerization reactor. Dry nitrogen gas entering the bottom of the reactor increases the temperature and drives the reaction equilibrium forward, leading to the formation of higher molecular weight polyamide 6 (Mn=24‐32 kg/mol). Figure 1.4 VK tube process for PA6 production.5 4
Chapter 1
Today, PA6 and PA66 continue being the most widely produced commercial products among all polyamides accounting for 90% of the nylon manufactured globally (3.4 x 106 ton/year).7 Nylon has replaced metal for mechanical performance by serving as an engineering plastic with good stiffness, strength, toughness, resistance to chemicals and thermal stability. The chemistry and properties of polyamides and specifically PA6 were well described by several authors.6, 8‐10 PA6 is mostly used for automotive, electrical and packaging applications. Additives used during the production provide end‐products for various applications. Drawback of PA6 is the relatively high moisture absorption (9.5% at 100% relative humidity and 22 °C), which results in a plasticizing effect and enhances toughness due to the drop of the glass transition temperature to a value below room temperature.6 1.2 Crystal structure of polyamide 6 Polyamides are semi‐crystalline polymers having regular crystalline lamellae separated by amorphous regions at room temperature. Semi‐crystallinity of polymers is desired for many applications where the crystalline part provides strength, stiffness and chemical resistance and the amorphous region provides flexibility and toughness. One of the main characteristics of the polyamides is the ability of the –N–H group to form strong intra and intermolecular hydrogen bonds with the –C=O group in the amide linkages within the same or neighboring chains. The chains are oriented in a way to maximize the hydrogen bonding which also provides high regularity (Figure 1.5.c).11‐13 The character of the hydrogen bonds and the electrostatic attraction between the electric dipoles contribute to the strength of the amide‐amide interactions.14 During the glass transition around 47‐57 °C dipolar interactions are broken, whereas during the melting process at 220‐223 °C most of the hydrogen bonds are broken.15 Although during the early years of polymer science polymer crystals were believed to be formed according to the fringed micelle model, Keller in 195716 showed that polymer chains are folding back and forth on themselves where folds occur at the faces as shown in Figure 1.5.a according to his electron diffraction experiments. This model was called 5
Chapter 1
the “adjacent re‐entry model” and was shown to be more predominant for solution‐
grown crystals than for crystals grown from the melt. Low molecular weight polymers tend to fold into this structure as well.17 This model is also divided into two different forms: the smooth surface model or the rough surface model where there is a sharp boundary between the crystal and the amorphous phase in the former model while large variations in the fold length may exist in the latter one.18 Later Flory suggested that a “switchboard model” is more probable for melt‐grown crystals where chains are randomly folding back into the same lamellae as shown in Figure 1.5.b.19 In this model the amount of adjacent re‐entry is small since the conditions are far from equilibrium so that adjacent folding depends on molecular weight and molecular architecture.20, 21 The driving force for the chain to uncoil from a high entropy conformation is the lowering of the enthalpy due to the formation of favorable secondary H‐bonding interactions. The extent to which a polymer will crystallize is determined firstly by thermodynamic forces favoring maximum potential crystallinity at equilibrium, and secondly by the kinetic forces determining the rate and extent to which the polymer may actually approach such a theoretical maximum degree of crystallinity. Thermodynamic forces that can be mentioned are regularity, symmetry, even or odd number of atoms in the monomeric unit, polarity and branching, while the kinetic forces include molecular flexibility and processing conditions.21 Figure 1.5 Two main fold models of polymer crystals: adjacent re‐entry model (a), switchboard model (b) and intramolecular hydrogen bonding in PA6 (c). 6
Chapter 1
In the most ideal PA6 crystallization case, i.e. from solution, chain folding and the formation of hydrogen bonds occur in lamellar sheets, named β‐sheets, as shown in Figure 1.5. The lowest enthalpy level for a folded molecule results in intramolecular hydrogen bonding which is only formed within the sheets. The sheets are connected to each other by van der Waals interactions. The most stable crystal packing for PA6 is called the “α” form. This phase consists of molecules in an extended chain conformation with hydrogen bonds between anti‐parallel chains (see anti‐parallel orientation in Figure 1.6.a). In this case within each β‐sheet all possible H‐bonds can be formed without any problem, which is why this crystal form is the most stable one. In the second form, which is less stable and is called the “γ” form, the chains within one β‐sheet are oriented in the parallel form (Figure 1.6.b) and complete H‐bonding is only possible if the chains are somewhat distorted. The amide groups are twisted out of the plane of the methylene groups, shortening the chain repeat distance and permitting intermolecular hydrogen bonding between the parallel chains.11, 22‐27 Both forms are shown in Figure 1.6. Figure 1.6 Two crystalline forms of PA6: α form (a) and γ form (b). 7
Chapter 1
1.3 Modification of polyamides In most cases polyamides are modified for industrial applications to end up with better properties in line with the desired applications. In this way, properties of the bulk polyamide can be modified to yield more flexibility, longer pack life, increased glass transition temperature, lower melting temperature, higher thermal/solvent/abrasion resistance, enhanced flame retardancy, improved shrinkage and mechanical properties, etc. All of these improvements can usually be obtained without following expensive production routes. The most common technique used for this modification is to copolymerize the standard monomers of a specific polyamide with desired comonomers in the melt by which a random distribution of the property‐changing comonomers is obtained. Another technique is blending the specific PA with a polymer improving the desired properties where the components are mixed only to some level to make a physical mixture. If a physical mixture of two step‐growth polymers is held in the molten state, interchain reactions can take place yielding block‐like copolymers which will convert into a totally random microstructure as the reaction proceeds. For instance a melt reaction of AB type monomers with AA and BB type monomers will result in a copolyamide with both AB and AABB type structures. However, depressions in melting and crystallization temperatures to below the original values of both polymers are obtained in the end.28‐31 This behavior is well described by Flory32 and Jo et al.33 theoretically. This depression might be prevented by blending two types of homopolyamides for just a sufficient time, or by sequential addition of monomers and preventing transamidation reactions, by which block‐like copolymers can be obtained.34, 35 The advantage of such blocky structures is that the physical properties of both original polyamides are still present in the final material, whereas a completely random copolyamide might lose the crystallinity and favorable physical properties of both blend components. 8
Chapter 1
Copolymerization of polyamides with non‐amidic units is also possible and widely used to make copolymers like poly(ester amide)s, poly(ether amide)s, poly(urea amide)s and poly(urethane amide)s where the strength, crystallinity and thermal stability of the polyamide can be combined with the desired properties of the other polymer type by the addition of the other components. Polyesteramides have gained much interest, mainly due to enhanced biodegradability by the incorporation of ester linkages. Polyamides are well known to be highly resistant to biodegradation in nature; however, it has been shown that the combination with aliphatic ester groups makes it liable to hydrolytic and enzymatic degradation. Preparation of biomaterials for tissue engineering or drug delivery is also possible by this method.36‐40 Different synthetic approaches such as ring opening polymerization, ester‐amide interchange reactions, anionic polymerization, interfacial polymerization and polycondensation in the melt can be used.41‐56 It is also possible to enhance properties like lower moisture absorption and better dimensional stability by incorporating polyesters such as polyethylene terephthalate (PET).57‐63 Thermoplastic polyether‐block‐amides (PEBA) elastomers are also an interesting class of copolymers where hard segments consisting of crystallizable polyamide blocks provide the strength and the soft ether blocks provide the flexibility. In these PEBAs hard segments can interact with each other by hydrogen bonds.64‐67 Preparation of poly(urea amide)s and poly(urethane amide)s give the possibility to obtain polyureas or polyurethanes with improved thermal, mechanical and solvent resistance 68‐72 or dendritic self‐assembly structures.73, 74 1.4 Modification of polyamides by solid‐state polymerization Solid‐state polymerization (SSP) implies heating the starting material, being either dry monomers or the prepolymer, at a temperature above the glass transition temperature but below the melting temperature , so that the mobile reactive groups are able to react but the material does not become sticky or a fluid. By‐products are removed by passing inert gas through the reaction medium or by maintaining reduced pressure. If SSP is performed starting with dry monomers it is referred to as direct SSP, whereas the latter is 9
Chapter 1
called post‐SSP (or solid state postcondensation). Although SSP can be used for chain‐
growth polymers in industry it is mainly used for polyamides and polyesters. It is for example an important finishing technique to obtain high molecular weight polyamides (Mn > 25 kg/mol) suitable for spinning, extrusion and injection.75 The kinetics and the influence of various parameters involved in the SSP reactions of polyamides75‐83 and polyesters76, 81‐84 have been investigated by several research groups until now. There are four main steps governing the rate of SSP:75, 82 i)
The intrinsic kinetics of the chemical reaction where the reaction temperature and the presence of catalyst are the most important factors. ii)
The diffusion of the reactive end groups which is mainly dependent on the reaction temperature, initial prepolymer molecular weight and crystallinity. iii)
The diffusion of the condensate in the solid reacting mass which is affected by the reaction temperature, particle size, gas flow rate and the presence of the catalyst. iv)
The transfer of the condensate from the reacting mass surface to the inert gas. Similar parameters as in the previous item (iii) are important. The intermolecular exchange reactions involved in the SSP of polyamides are acidolysis, aminolysis and amidolysis reactions as shown in Figure 1.7.75, 85 Acidolysis is the reaction between an alkyl carboxyl group and an amide linkage, aminolysis is the reaction between an alkyl amine and an amide group, whereas the amidolysis is the reaction between two amide groups. All these reactions result in linear products such as polyamides, oligomers and by‐products. On the other hand, intramolecular reactions result in the formation of cyclic compounds. 10
Chapter 1
Figure 1.7 Exchange reactions of polyamides: acidolysis (1), aminolysis (2), amidolysis (3). Possible side reactions observed after long reaction times during SSP of polyamides involve the formation of a secondary amine group from the reaction of two amine end groups which, after the reaction with a carboxyl end group, forms branched structures in the case of an AB type polyamide (like PA6) and crosslinked structures in the case of an AABB type of PA. Crosslinking is especially observed in case of PA66.86 During the SSP of PA46 the formation of high molecular weight polymers is inhibited by pyrrolidine formation, which is a chain stopper (Figure 1.8).87 The reaction of pyrrolidine end groups with water results in carboxyl end‐capped polymer chains which act as terminating agents. Figure 1.8 Pyrrolidine end‐group formation and its reaction with water to form carboxyl‐
terminated chains. SSP is a very efficient and mild technique not only to reach high molecular weight step‐
growth polymers without having too many side reactions or without suffering from a very high melt viscosity, but also to incorporate other monomers/polymers into the main chain of the step‐growth polymer. As discussed in the previous section, most of the modification techniques for semi‐crystalline polymers lead to randomization by which the crystalline 11
Chapter 1
phase is deteriorated, and as a result, mechanical and physical properties are reduced. However, SSP gives the possibility to modify step‐growth polymers by transreactions (see Figure 1.7) without the entire deterioration of the crystalline behavior. Previously, a three phase model has been proposed for semi‐crystalline polymers which consists of a crystalline fraction, mobile amorphous fraction (MAF) and rigid amorphous fraction (RAF).88, 89 During the SSP reactions, it is expected that only the mobile amorphous phase takes part in the aminolysis, acidolysis and amidolysis reactions so that the crystalline phase remains intact. This modification is represented in the picture in the first pages of this chapter and in Chapter 4. This concept accordingly should result in a block copolymer structure with crystalline homopolymer blocks and chemically modified and usually amorphous copolymer blocks. By this way, comonomers/polymers can be incorporated into PA6 backbone in the solid state and the resulting copolymers can retain their high melting temperatures, crystallization rates and good mechanical/physical properties. Recently, Jansen et al.89‐93 and Sablong et al.94, 95 studied the incorporation of diol monomers into poly(butylene terephthalate) (PBT) above Tg but below the melting temperature of PBT. Jansen and coworkers showed for the first time that copolyesters with non‐random distributions and high molecular weights were obtained after solution mixing of both components in a common solvent followed by subsequent removal of the solvent and SSP. Comparison with melt‐polymerized samples proved the superior properties obtained after the modification by SSP. Molecular and morphological structures were studied in detail via SEC, DSC, 1H NMR and 13C NMR and blocky microstructures were indeed confirmed after SSP reactions. 1.5 Objectives and outline of the thesis The objective of the work described in this thesis is to chemically modify polyamide 6 (PA6) for realizing enhanced properties by solution and/or solid‐state polymerization in such a way that good material properties can be retained. One of the aims is to make partially degradable PA6 by incorporating hydrolyzable ester groups into the backbone of PA6. This can be done either in solution or in the solid state, depending on the functional 12
Chapter 1
end groups which connect the short polyamide and oligoester/polyester blocks together. In this way multiblock copolymers of polyamide‐polyester can be prepared so that degradability is obtained in addition to the good properties of PA6. Chapter 2 describes the incorporation of diisocyanate end‐capped polyester into amino end‐capped PA6 in solution, whereas in Chapter 3 the incorporation of an epoxide end‐capped oligoester into carboxylic acid end‐capped PA6 is reported. Another aspect is to show that high molecular weight PA6 can be modified below its melting temperature by selective incorporation of a nylon salt where the salt is only incorporated in the amorphous phase, excluding the large crystalline fractions from the transreactions. For this purpose, as described in Chapter 4, a semi‐aromatic nylon salt with an irregular structure was chosen so that it cannot co‐
crystallize with the crystallizable PA6 segments and can be easily forced into the amorphous phase. It was shown that incorporation of the nylon salt into the amorphous phase via intermolecular exchange reactions without the deterioration of the crystalline phase is indeed possible. The effects of salt composition, reaction temperature and reaction time were investigated. Detailed characterization in terms of molecular weights, thermal properties and blockiness were performed. Morphological changes obtained after the SSP reactions via heating up to the melting temperature of the blocky copolyamide are also presented in Chapter 5. The thesis ends with a technology assessment (Chapter 6), describing the possible industrial implementation of the promising SSP concept for PA6 modification. References 1.
2.
3.
4.
5.
6.
Flory, P. J. Statistical Mechanics of Chain Molecules. Wiley‐Interscience: New York, 1969. Carothers, W. H. US 2071250, 1937. Schlack, P. US 2241321, 1941. Koslowski, H. J., Dictionary Of Man‐Made Fibers:Terms, Figures, Trademarks. International Business Press: 1998. Seavey, K. C.; Liu, Y. A. Step‐Growth Polymerization Process Modeling and Product Design. John Wiley & Sons, Inc: 2008. Kohan, M. I. Nylon Plastics Handbook. Hanser Publishers: Munich, Vienna, New York, 1995. 13
Chapter 1
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
Nylon 6, retrieved on October 10, 2011, from http://www.chemsystems.com/about/cs/news/items/PERP%200708S6_Nylon%206.cfm. Galanty, P. G. Nylon 6. Oxford University Press: 1999. Marchildon, K. Macromol. React. Eng. 2011, 5, (1), 22‐54. Aharoni, S. M. n‐Nylons Wiley: Chichester, New York, Weinheim, Brisbane, Singapore, Toronto, 1997. Murthy, N. S. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, (13), 1763‐1782. Schroeder, L. R.; Cooper, S. L. J. Appl. Phys. 1976, 47, (10), 4310‐4317. Vinken, E.; Terry, A. E.; Hoffmann, S.; Vanhaecht, B.; Koning, C. E.; Rastogi, S. Macromolecules 2006, 39, (7), 2546‐2552. Garcia, D.; Starkweather, H. W. J. Polym. Sci., Part B: Polym. Phys. 1985, 23, (3), 537‐555. Botta, A.; Decandia, F.; Palumbo, R. J. Appl. Polym. Sci. 1985, 30, (4), 1669‐1677. Keller, A. Phil. Mag. 1957, 2, (21), 1171‐1175. Elias, H. G. Macromolecules: Volume 3: Physical Structures and Properties. Wiley‐VCH: Weinheim, 2008. Hoffman, J. D.; Lauritzen, J. I. J. Res. Nat. Bur. Stand. 1961, A 65, (4), 297‐&. Flory, P. J. J. Am. Chem. Soc. 1962, 84, (15), 2857‐&. Rastogi, S.; Lippits, D.; Terry, A.; Lemstra, P.; Reiter, G.; Strobl, G. Progress in Understanding of Polymer Crystallization. In Springer Berlin / Heidelberg: 2007; Vol. 714, pp 285‐327. Dhanvijay, P. U.; Shertukde, V. V. Polym. Plast. Technol. Eng. 2011, 50, (13), 1289‐1304. Li, Y.; Goddard, W. A. Macromolecules 2002, 35, (22), 8440‐8455. Parker, J. P.; Lindenmeyer, P. H. J. Appl. Polym. Sci. 1977, 21, (3), 821‐837. Hatfield, G. R.; Glans, J. H.; Hammond, W. B. Macromolecules 1990, 23, (6), 1654‐1658. Arimoto, H. J. Polym. Sci., Part A: Polym. Chem. 1964, 2, (5), 2283‐2295. Arimoto, H.; Ishibashi, M.; Hirai, M.; Chatani, Y. J. Polym. Sci., Part A: Polym. Chem. 1965, 3, (1), 317‐
326. Holmes, D. R.; Bunn, C. W.; Smith, D. J. J. Polym. Sci. 1955, 17, (84), 159‐177. Harvey, E. D.; Hybart, F. J. J. Appl. Polym. Sci. 1970, 14, (8), 2133‐2143. Suehiro, K.; Egashira, T.; Imamura, K.; Nagano, Y. Acta Polym. 1989, 40, (1), 4‐8. Johnson, C. G.; Cypcar, C. C.; Mathias, L. J. Macromolecules 1995, 28, (25), 8535‐8540. Stouffer, J. M.; Starkweather Jr, H. W.; Hsiao, B. S.; Avakian, P.; Jones, G. A. Polymer 1996, 37, (7), 1217‐1228. Flory, P. J. J. Chem. Phys. 1949, 17, (3), 223‐240. Jo, W. H.; Baik, D. H. J. Polym. Sci., Part B: Polym. Phys. 1989, 27, (3), 673‐687. Williamson, D. T.; Wilson, T.; Forrester, M. E. US 2007293629 (A1), 2007. Coffman, D. D. US 2193529, 1940. Hemmrich, K.; Meersch, M.; Wiesemann, U.; Salber, J.; Klee, D.; Gries, T.; Pallua, N. Tissue Eng. 2006, 12, (12), 3557‐3565. Mihov, G.; Draaisma, G.; Dias, A.; Turnell, B.; Gomurashvili, Z. J. Controlled Release 2010, 148, (1), 46‐
47. Okada, M. Prog. Polym. Sci. 2002, 27, (1), 87‐133. Katsarava, R.; Beridze, V.; Arabuli, N.; Kharadze, D.; Chu, C. C.; Won, C. Y. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, (4), 391‐407. Guo, K.; Chu, C. C. J. Biomed. Mater. Res. Part B: Appl. Biomat. 2009, 89B, (2), 491‐500. Deshayes, G.; Delcourt, C.; Verbruggen, I.; Trouillet‐Fonti, L.; Touraud, F.; Fleury, E.; Degee, P.; Destarac, M.; Willem, R.; Dubois, P. React. Funct. Polym. 2008, 68, (9), 1392‐1407. Ellis, T. S. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, (9), 1109‐1125. Alla, A.; Rodriguez‐Galan, A.; Martinez de llarduya, A.; Munoz‐Guerra, S. Polymer 1997, 38, (19), 4935‐
4944. 14
Chapter 1
44. Lips, P. A. M.; Broos, R.; van Heeringen, M. J. M.; Dijkstra, P. J.; Feijen, J. Polymer 2005, 46, (19), 7834‐
7842. 45. Ferre T.; Franco, L.; Rodriguez‐Galan, A.; Puiggali, J. Polymer 2003, 44, (20), 6139‐6152. 46. Villuendas, I.; Molina, I.; Regano, C.; Bueno, M.; Martinez de Ilarduya, A.; Galbis, J. A.; Munoz‐Guerra, S. Macromolecules 1999, 32, (24), 8033‐8040. 47. Chromcova, D.; Baslerova, L.; Roda, J.; Brozek, J. Eur. Polym. J. 2008, 44, (6), 1733‐1742. 48. Tokiwa, Y.; Suzuki, T.; Ando, T. J. Appl. Polym. Sci. 1979, 24, (7), 1701‐1711. 49. Goodman, I.; Kehayoglou, A. H. Eur. Polym. J. 1983, 19, (4), 321‐325. 50. Gonsalves, K. E.; Chen, X.; Cameron, J. A. Macromolecules 1992, 25, (12), 3309‐3312. 51. Chromcova, D.; Bernaskova, A.; Brozek, J.; Prokopova, I.; Roda, J.; Nahlik, J.; Sasek, V. Polym. Degrad. Stab. 2005, 90, (3), 546‐554. 52. Jakisch, L.; Komber, H.; Bohme, F. Macromol. Mat. Eng. 2007, 292, (5), 557‐570. 53. Ramaraj, B.; Poomalai, P. J. Appl. Polym. Sci. 2005, 98, (6), 2339‐2346. 54. Kim, I.; White, J. L. J.Appl. Polym. Sci. 2003, 90, (14), 3797‐3805. 55. Stapert, H. R.; Bouwens, A. M.; Dijkstra, P. J.; Feijen, J. Macromol. Chem. Phys. 1999, 200, (8), 1921‐
1929. 56. Luckachan, G. E.; Pillai, C. K. S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, (10), 3250‐3260. 57. Gaymans, R. J. J. Polym. Sci., Part A: Polym. Chem. 1985, 23, (5), 1599‐1605. 58. Gaymans, R. J.; Aalto, S.; Maurer, F. H. J. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, (2), 423‐430. 59. Persyn, O.; Miri, V.; Lefebvre, J. M.; Ferreiro, V.; Brink, T.; Stroeks, A. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, (12), 1690‐1701. 60. Retolaza, A.; Eguiazábal, J. I.; Nazábal, J. J. Appl. Polym. Sci. 2005, 97, (2), 564‐574. 61. Aharoni, S. M. Int. J. Polymer. Mater. 1997, 38, (3‐4), 173‐203. 62. Denchev, Z.; Kricheldorf, H. R.; Fakirov, S. Macromol. Chem. Phys. 2001, 202, (4), 574‐586. 63. Bailly, C. M. E.; Chisholm, B.; De Jongh, R.; De Wit, G. US 5731389 (A), 1998. 64. Gupta, A.; Singhal, R.; Nagpal, A. K. J. Appl. Polym. Sci. 2004, 92, (2), 687‐697. 65. Sheth, J. P.; Xu, J. N.; Wilkes, G. L. Polymer 2003, 44, (3), 743‐756. 66. Yu, Y. C.; Jo, W. H. J. Appl. Polym. Sci. 1995, 56, (8), 895‐904. 67. Gaymans, R. J.; Schwering, P.; Dehaan, J. L. Polymer 1989, 30, (6), 974‐977. 68. Dutta, S.; Karak, N. Prog. Org. Coat. 2005, 53, (2), 147‐152. 69. Takeichi, T.; Suefuji, K.; Inoue, K. Polym. J. 2002, 34, (6), 455‐460. 70. Tanzi, M. C.; Barzaghi, B.; Anouchinsky, R.; Bilenkis, S.; Penhasi, A.; Cohn, D. Biomaterials 1992, 13, (7), 425‐431. 71. Arun, A.; Dullaert, K.; Gaymans, R. J. Macromol. Chem. Phys. 2009, 210, (1), 48‐59. 72. Gonzalez‐de los Santos, E. A.; Lopez‐Rodriguez, A. S.; Lozano‐Gonzalez, M. J.; Soriano‐Corral, F. J. Appl. Polym. Sci. 2001, 80, (13), 2483‐2494. 73. Yang, M.; Wang, W.; Lieberwirth, I.; Wegner, G. J. Am. Chem. Soc. 2009, 131, (17), 6283‐6292. 74. Yang, M.; Zhang, Z.; Yuan, F.; Wang, W.; Hess, S.; Lienkamp, K.; Lieberwirth, I.; Wegner, G. Chem. Eur. J. 2008, 14, (11), 3330‐3337. 75. Vouyiouka, S. N.; Papaspyrides, C. D., Kinetic Aspects of Polyamide Solid State Polymerization. Wiley: New Jersey, 2009. 76. Fakirov, S. Solid State Reactions In Linear Polycondensates Prentice Hall: New Jersey, 1990. 77. Gaymans, R. J.; Amirtharaj, J.; Kamp, H. J. Appl. Polym. Sci. 1982, 27, (7), 2513‐2526. 78. Mizerovskii, L. N.; Bazarov, Y. M. Fibre Chem. 2006, 38, (4), 313‐324. 79. Vouyiouka, S. N.; Papaspyrides, C. D.; Weber, J. N.; Marks, D. N. Polymer 2007, 48, (17), 4982‐4989. 80. Xie, J. J. J. Appl. Polym. Sci. 2002, 84, (3), 616‐621. 81. Almonacil, C.; Desai, P.; Abhiraman, A. S. Macromolecules 2001, 34, (12), 4186‐4199. 82. Vouyiouka, S. N.; Karakatsani, E. K.; Papaspyrides, C. D. Prog. Polym. Sci. 2005, 30, (1), 10‐37. 15
Chapter 1
83. Seavey, K. C.; Liu, Y. A. Fundamental Process Modeling and Product Design for the Solid State Polymerization of Polyamide 6 and Poly(ethylene terephthalate). Wiley: New Jersey, 2009. 84. Ma, Y.; Agarwal, U. S.; Sikkema, D. J.; Lemstra, P. J. Polymer 2003, 44, (15), 4085‐4096. 85. Kotliar, A. M. Macromol.Rev. Part D‐J. Polym. Sci. 1981, 16, 367‐395. 86. Korshak, V.; Frunze, T. Synthetic Hetero‐Chain Polyamides IPST: Jerusalem, 1964. 87. Roerdink, E.; Warnier, J. M. M. Polymer 1985, 26, (10), 1582‐1588. 88. Wunderlich B. Prog. Polym. Sci. 2003, (28), 383‐450. 89. Jansen, M. A. G.; Goossens, J. G. P.; de Wit, G.; Bailly, C.; Koning, C. E. Macromolecules 2005, 38, (7), 2659‐2664. 90. Jansen, M. A. G.; Goossens, J. G. P.; de Wit, G.; Bailly, C.; Koning, C. E. Anal. Chim. Acta 2006, 557, (1‐
2), 19‐30. 91. Jansen, M. A. G.; Goossens, J. G. P.; de Wit, G.; Bailly, C.; Schick, C.; Koning, C. E. Macromolecules 2005, 38, (26), 10658‐10666. 92. Jansen, M. A. G.; Goossens, J. G. P.; Wu, L. H.; de Wit, G.; Bailly, C.; Koning, C. E. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, (5), 882‐899. 93. Jansen, M. A. G.; Goossens, J. G. P.; Wu, L. H.; De Wit, G.; Bailly, C.; Koning, C. E.; Portale, G. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, (4), 1203‐1217. 94. Sablong, R.; Duchateau, R.; Koning, C. E.; de Wit, G.; van Es, D.; Koelewijn, R.; van Haveren, J. Biomacromolecules 2008, 9, (11), 3090‐3097. 95. Sablong, R.; Duchateau, R.; Koning, C. E.; Pospiech, D.; Korwitz, A.; Komber, H.; Starke, S.; Haussler, L.; Jehnichen, D.; Landwehr, M. A. D. Polym. Degrad. Stab. 2011, 96, (3), 334‐341. 16
CHAPTER 2 PARTIALLY DEGRADABLE POLYAMIDE 6‐
POLYCAPROLACTONE MULTIBLOCK COPOLYMERS Summary Low molecular weight polycaprolactone was successfully incorporated into polyamide 6 by solution and solid‐state polymerization after synthesis of both components with desired co‐reactive end groups. The structure and thermal properties of polymers before and after incorporation were analyzed by SEC, FTIR, NMR, titration analysis as well as TGA and DSC. DSC data, together with an increase in molecular weight pointed to a multiblock structure with almost maintained melting temperatures with respect to pure components. Degradation of polymers was performed via enzymatic and hydrolytic routes at 25 °C and followed by weight loss analysis, SEM and SEC. 17
Chapter 2
2.1 Introduction Polyamide 6 (PA6) is a high‐performance engineering plastic used for a wide range of applications in everyday life. Strong hydrogen bonding between the chains and high regularity in the crystalline phase provide excellent thermal and mechanical properties but on the other hand result in a highly resistant material to biodegradation in nature. As there is an increasing demand for disposable packaging applications the biodegradability of PA6 could be enhanced by incorporating hydrolyzable groups into the main chain. These hydrolyzable groups can be selected from various aliphatic polyesters which are well known to be biodegradable due to cleavable ester links. Polycaprolactone (PCL) is one of these polyesters which can be used both for biomedical and ecological applications.1‐3 As a special class of biodegradable polymeric materials, the synthesis and characterization of poly(ε‐caprolactam‐co‐ε‐caprolactone) copolymers have been studied by different research groups. Synthetic approaches include ester‐amide exchange reactions, anionic polymerization, interfacial polymerization, ring opening and polycondensation reactions.4‐
14 Most of these works showed that the resulting copolymers have a random structure, whereas only a few papers described di‐ or tri‐ block structures. Degradation studies were also described in several articles5, 9, 11, 15, 16 by using different methods proving that these type of copolymers are susceptible to degradation, although mostly enzymatically. If the ester groups are randomly introduced into the PA6 main chain, the crystallization behavior of PA6 will be negatively affected, the melting temperature will be significantly reduced and the mechanical and physical properties, crucial for packaging applications (such as barrier properties), will become worse. This fact is also seen in the literature covered above where there is a big decrease in melting temperatures as the amount of ε‐
caprolactone increases when random copolymers are prepared. To the best of our knowledge well‐defined multiblock copolymers of this type of polyesteramides have not been synthesized yet and have certainly not been tested as biodegradable materials. 18
Chapter 2
A promising synthetic method can be incorporating these degradable groups or blocks into the amorphous part of a relatively low molecular weight PA6 below the melting temperature of the PA6 crystals.17, 18 For this purpose, well‐known synthetic techniques can be applied to prepare this new type of PA6‐PCL block polymers by making use of isocyanate‐amine reactions at low temperatures. Until now D’Hollander et al.19 obtained shape memory polyurethane networks based on a triblock copolymer made by the reaction of isocyanate end‐capped PCL and excess of amine end‐capped poly(propylene oxide). Lee et al.20 prepared shape memory polyamides by linear chain extension of PCL and diamine‐terminated polyamide in the presence of hexamethylene diisocyanate (HDI). Their aim was to have shape recovery by using high fractions of (PCL‐HDI)n units (70%) compared to (polyamide‐HDI) units. According to the thermal analysis of the copolymers the highest melting temperature of the polyamide segments was 183 °C. It should be realized that the synthetic route used by Lee et al. results in a rather ill‐defined structure, since HDI can couple either two PA blocks, two PCL blocks or one PA and one PCL block. The aim of this chapter was to make well‐defined PA6‐based multiblock copolymers where the good properties of PA6 such as high melting temperature and crystallinity can still be maintained, whereas biodegradation can be an additional property. We followed a stepwise technique where low molecular weight amine end‐capped PA6 and isocyanate end‐capped PCL polymers were synthesized separately followed by solution and solid state step‐growth copolymerization of these telechelic building blocks at reduced temperatures. The relatively low temperatures should prevent aminolysis of the PCL ester groups by the PA6 amine end groups. In this way, the targeted reasonably well‐defined multiblock copolymers of PA6 and PCL could be obtained with PA6‐like thermal properties and partial biodegradability. We realize that the PA6 blocks are not degradable, but by degrading the PCL blocks the material may disentangle and fall apart into small fragments. Molecular weights of the synthesized polymers were characterized by using SEC, NMR and titration methods. SEC was also used as a useful tool to follow the reactions with time. Molecular structures of the products were investigated by FTIR spectroscopy. Thermal 19
Chapter 2
analysis was performed by using TGA and DSC. Hydrolytic and enzymatic degradations were done in PBS buffer solution followed by surface analysis of the films by using SEM. HN
O
O
H
O
O
O
O
n
Caprolactam
(CL)
O
O
H
n
Polycaprolactone (PCL, Mn=1,600 g/mol)
H2N
NH2 250 °C, 3 bar, 6 hours
OCN
H2O
H
N
H2N
O
O
n
NCO
65 °C, in THF, 2 hours
DBD
H3C
N
H
NH2
OCN
m
H3C
O
H
N
O
O
O
O
n
O
O
H
N
O
NCO
n
O
CH3
Diisocyanate end capped PCL
(M n=1,850 g/mol, titration)
PA6 (M n=2,500 g/mol, titration)
[PA 6-b-PCL]n multiblock copolymers
H O H
N C N
Figure 2.1 Schematic drawing of stepwise synthesis of polyamide 6‐polycaprolactone multiblock copolymers obtained by solution and solid‐state polymerization. 2.2 Experimental 2.2.1 Materials ‐Caprolactam (CL) was kindly provided by DSM. p‐Xylylenediamine (p‐XDA, >98 %) and toluene 2,4‐diisocyanate (TDI, >98 %) were purchased from Fluka. 1,3‐propane diol (PD), dimethyl adipate (DMA) and titanium(IV)butoxide (TBO) were obtained from Acros for polyester synthesis. Dibutyltin dilaurate (DBD, 97 %), polycaprolactone diol (PCL, average Mn=530 g/mol and 1250 g/mol) and 2,2,2‐trifluoroethanol (TFE, 99 %) were purchased from Aldrich. 1,1,1,3,3,3‐Hexafluoro‐2‐propanol (HFIP, 99 %), tetrahydrofuran (THF) and diethyl ether were obtained from Biosolve. Deuterated chloroform (CDCl3, 99 %) was purchased from Cambridge Isotope Laboratory, Inc. (CIL). Lipase from Aspergillus niger 20
Chapter 2
(184 U/g) was obtained from Sigma. A commercial grade PA6 (Akulon, Mn=31 kg/mol, PDI=2.0) was provided by DSM and was used as a reference for biodegradation analysis. All chemicals were used as received, unless otherwise mentioned. 2.2.2 Synthesis of diamine end‐capped PA6 For the synthesis of diamine end‐capped PA6 a batch reactor with a capacity of 380 mL was used. Temperature and pressure were controlled via a computer. First, 100 g (0.88 mol) CL was charged to the reactor and heated until complete melting. Later, 3, 6 or 8 g p‐
XDA (0.022, 0.044, 0.059 mol, respectively) and 3 g (0.17 mol) water were added. The polymerizations were carried out at 250 °C at 3 bar for 6 hours under the flow of N2 gas and with continuous mechanical stirring. Samples for SEC analysis were withdrawn at various time intervals. Final products were extracted with water at 80 °C for 20 hours, filtered under vacuum and dried in an oven at 80 °C for at least 24 hours. Samples were investigated by using SEC, NMR, DSC and titration analysis. 2.2.3 Synthesis of hydroxyl end‐capped oligoester For the synthesis of hydroxyl end‐capped oligoesters 3.5 g (46.4 mmol) or 4.0 g (52.2 mmol) 1,3‐propane diol (PD) and 5 g (29 mmol) dimethyl adipate (DMA) were put in a 100 mL three neck flask. All the reactions were performed under argon with strong agitation at 180 °C in the melt using 20 mg TBO catalyst. Temperature control was provided by a heating mantle connected to a temperature controller. The reactor was equipped with a distillation set up to remove the methanol that was produced during the polymerization. The reaction time varied between 2.5‐3 hours. After cooling of the polymer to room temperature, it was put in methanol, precipitated by immersing in a liquid N2 and acetone mixture and then filtered. In every case, these steps were carried out three times for the complete removal of the excess diol and the catalyst and later followed by drying in a rotary evaporator and a vacuum oven. The polymers were investigated by using SEC and NMR. 21
Chapter 2
2.2.4 Synthesis of diisocyanate end‐capped polycaprolactone 5.6 g (32 mmol) TDI was placed in a Schlenk vessel which was connected to argon. 10 g (8 mmol) PCL was dissolved in 20 ml THF and placed in an addition funnel. After the addition of 1 drop of dibutyltin dilaurate (DBD), the Schlenk flask was heated to 65 °C and the slow addition of PCL solution to TDI was started with a rate of 1 drop/2 sec under strong agitation. Heating and stirring were stopped after 2 hours. The product was slowly added into diethyl ether which was cooled in an acetone‐liquid N2 mixture, which resulted in precipitation of the polymer. The solvent was removed from the polymer‐diethyl ether mixture to another flask by using a filtrating cannula and a filter by applying a pressure difference. In every case these steps were carried out three times to assure complete removal of excess diisocyanate and the catalyst. Later, residual solvent was removed by using reduced pressure. The product was characterized by NMR and titration. 2.2.5 Copolymer synthesis Totally dry 10 g (4 mmol) diamine end‐capped PA6 and 5 g (4 mmol) diisocyanate end‐
capped polyester were put in a 100 mL three neck round bottom flask under argon and dissolved in 50 ml HFIP for mixing on the molecular level. After complete dissolution, HFIP was slowly removed by vacuum distillation. This was done at room temperature to avoid the reaction between isocyanate end groups of the PCL and hydroxyl groups of HFIP. Then, the lump of material was taken out of the flask, ground in liquid N2, sieved and reduced pressure was applied again. As soon as the particles were almost totally dry, the product was stirred and heated gradually up to 160 °C, which is below the melting temperature of the polyamide. Reaction was continued overnight. Polymer fractions were investigated via SEC, FTIR and DSC. 2.2.6 Enzymatic and non‐enzymatic hydrolysis Biodegradation studies were performed with and without enzyme. For both methods, polymer films (25‐30 mg) with an average thickness of 0.4 mm prepared by solvent casting 22
Chapter 2
in HFIP were incubated in separate tubes filled with 10 mL phosphate buffer solution (pH 7.5) which were kept at 25±1 °C. The reference PA6 film was prepared by compression molding. For the enzymatic degradation, lipase from Aspergillus niger (1.6 U/mL) was used and the media was replaced periodically. Films were removed from the media at specific time intervals, washed with distilled water, dried and weighed to determine the weight loss. The morphology of the films was investigated by SEM. 2.2.7 Characterization 2.2.7.1 Size Exclusion Chromatography (SEC) Size exclusion chromatography (SEC) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of polymer samples. For the PA6 samples and for the blocky polyesteramides SEC in HFIP was performed on a system equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2414 refractive index detector (35 °C), a Waters 2707 autosampler, and a PSS PFG guard column followed by 2 PFG‐linear‐XL (7 µm, 8*300 mm) columns in series at 40 °C. HFIP with potassium trifluoroacetate (3 g/L) was used as eluent at a flow rate of 0.8 mL/min. The molecular weights were calculated against poly(methyl methacrylate) standards (Polymer Laboratories, Mp = 1020 g/mol up to Mp = 1.9*106 g/mol). For the polyester samples SEC in THF was performed on a Waters Alliance system equipped with a Waters 2695 separation module, a Waters 2414 refractive index detector (40 °C), a Waters 2487 dual absorbance detector, and a PSS SDV 5 μ guard column followed by 2 PSS SDV linear XL columns in series of 5 μ (8*300) at 40 °C. THF, stabilized with 2,6‐di‐tert‐butyl‐4‐methylphenol (BHT), was used as eluent at a flow rate of 1 mL/min. The molecular weights were calculated with respect to polystyrene standards (Polymer Laboratories, Mp = 580 Da up to Mp = 7.1*106 Da). Before SEC analysis was performed, the samples were filtered through a 0.2 µm PTFE filter (13 mm, PP housing, Alltech). 23
Chapter 2
2.2.7.2 Nuclear Magnetic Resonance Spectroscopy (NMR) 1H NMR spectra of the polymers were recorded on a Varian 400 MHz spectrometer at 25 °C. PA6 containing samples were dissolved in a 2:1 vol% CDCl3:TFE mixture, whereas the analyses of PCL and its derivatives were performed in CDCl3. For the PA6 polymers, the number average molecular weight Mn was calculated from the NMR spectra by estimating the ratio of the integrals of the proton signals of repeat units to the corresponding end groups. 2.2.7.3 Differential Scanning Calorimetry (DSC) Melting (Tm) and crystallization temperatures (Tc) as well as enthalpies of melting (∆Hm) and crystallization (∆Hc) of the polymers were measured using a TA Instruments Q100 calorimeter. For all the measurements 4‐6 mg samples and a heating rate of 10°C min–1 were used. DSC measurements of fully amine end‐capped PA6 polymers were carried out from 0°C to 260°C whereas the rest of the samples were analyzed from ‐50°C to 220°C. For each measurement the second heating curve was used to determine the Tm. For the determination of both Tm and Tc peak maximums were taken into account. 2.2.7.4 Fourier Transform Infrared Spectroscopy (FTIR) The presence of various chemical linkages of the products was derived from FTIR‐ATR spectra that were obtained on a Bio‐Rad Excalibur FTS3000MX spectrophotometer. The measurements were performed by making 50 scans using a golden gate set‐up, equipped with a diamond ATR crystal. The Varian Resolution Pro software version 4.0.5.009 was used for the analysis of the spectra. 2.2.7.5 Potentiometric titration For the determination of amine [NH2] and carboxylic acid [COOH] end group content, potentiometric end group titrations were done at room temperature in non‐aqueous environment using phenolic solvents. Molecular weight of the polyamides were calculated by using the formula 2*106/([NH2]+[COOH]). Isocyanate end‐group titration was done by 24
Chapter 2
using the back titration method. The sample was dissolved in THF and then, mixed with 10 mL 2.0 M diisobutylamine solution and finally titrated with 1.0 M HCl solution in IPA. Both blank and sample measurements were repeated at least three times. Molecular weight of the polyester was calculated by using the formula MWKOH*2*103/[OH]. 2.2.7.6 Scanning Electron Microscopy (SEM) Surface changes of the polymer films after degradation were observed by using Quanta 3D FEG (FEI) scanning electron microscopy (SEM) equipped with a field emission electron source. High vacuum conditions were applied and a secondary electron detector was used for image acquisition. No additional sample treatment, such as surface etching or coating with a conductive layer, has been applied before surface scanning. Standard acquisition conditions for charge contrast imaging were used. 2.3 Results and Discussion Synthesis of polyamide 6‐polycaprolactone (PA6‐PCL) or polyamide 6‐polypropylene adipate (PA6‐PPA) block copolymers consisted of three synthetic steps as presented in Figure 2.1. Firstly, low molecular weight fully diamine end‐capped PA6 was synthesized. Later, fully diisocyanate end‐capped PPA and PCL oligoester was synthesized and finally solution and solid‐state polymerization was performed with the co‐reactive oligoester and PA6 components. For the diisocyanate end‐capped oligoester synthesis initially hydroxyl end‐capped polypropylene adipate was synthesized and later end‐capping with toluene diisocyanate was done. Since this polyester had poor properties at room temperature, later fully hydroxyl end‐capped polycaprolactone was used which is commercially available. 25
Chapter 2
2.3.1 Diamine end‐capped PA6 Figure 2.2 Reaction scheme of the synthesis of amine end‐capped PA6. The synthesis of amine end‐capped PA6 as shown in Figure 2.2 involved the reaction of ‐
caprolactam (CL) with p‐xylylenediamine (p‐XDA) in the presence of water. p‐XDA was chosen as the diamine to be used because of its high boiling point (230 °C) with respect to other diamines which limits its evaporation during the CL polymerization. Statistically p‐
XDA should be incorporated inside the polymer chain during the polymerization and should not be present as an end‐group. 8000
1.0
7000
0.9
0.8
0.7
5000
0.6
4000
0.5
Mn vs. time
CL conversion vs. time
3000
0.4
0.3
2000
CL conversion
Mn (g/mol)
6000
0.2
1000
0.1
0
0.0
0
1
2
3
4
Reaction time (h)
5
6
Figure 2.3 Mn (SEC) (□) and CL conversion (■) vs. reaction time of the amine end‐capped PA6 containing 6 wt% p‐XDA (PA6C in Table 2.1). Changes in molecular weight and CL conversion vs. time in case of the polymer containing 6 wt% p‐XDA are presented in Figure 2.3. The molecular weight change as a function of the CL conversion is shown in Figure 2.4. In order to analyze the CL conversion of the reaction, a calibration curve for different concentrations of CL was prepared by using SEC. Later, known copolymer concentrations of SEC samples were prepared from the polymer 26
Chapter 2
fractions taken during the polymerization and by monitoring the change in CL peak area the conversion was calculated. In view of the perfectly stable baselines in the SEC chromatograms and the fully separated peaks of the PA6 polymer and the CL monomer we believe that the determined CL conversions are reliable. From the obtained results it is observed that after 2.5 hours the CL conversion already reached ca. 90%. Figure 2.4 demonstrates that Mn keeps increasing when the CL conversion almost remains constant after 90% conversion. This is because in the first stages of hydrolytic CL polymerization ring opening and polyaddition reactions are taking place simultaneously, whereas during the last 3 hours the polycondensation reaction is dominant, leading to a dramatic increase in Mn. Residual monomer and cyclic dimers and oligomers (<10 %) were subsequently removed by extraction with water. The theoretical Mn for this PA6 which is prepared by 6 wt% p‐XDA addition can be calculated by using the Carother’s equation (Xn=(1+r)/(1+r‐2rp) where Xn is the degree of polymerization, r is the ratio of the reactants and p is the conversion). From this equation Mn is calculated as 1,230 g/mol if p=0.96 as found from SEC and calculated as 2,420 g/mol if p=1.0 which is closer to the real case. 7000
Mn (g/mol)
6000
5000
4000
3000
2000
1000
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
CL conversion
Figure 2.4 Mn (SEC) vs. conversion of the amine end‐capped PA6 containing 6 wt% p‐XDA (PA6C in Table 2.1). 27
Chapter 2
Titration results proved that with the addition of 3‐8 wt% p‐XDA almost all the polyamide end groups were amine groups (Table 2.1). The percentage of the carboxylic acid end groups with respect to the total number of end groups decreased from 4 to 1% as the p‐
XDA composition was increased from 3 to 8 wt%. It was also not possible to see any peaks in the NMR spectrum related to carboxylic acid end groups as their concentration is negligible (Figure 2.5). An increase of the p‐XDA amount with a constant amount of water resulted in an increase of the amine end group concentration with respect to carboxylic acid end groups, and resulted in a decrease of the molecular weight of the polymer. When the p‐XDA concentration was kept the same but when no water was added during one of the polymerizations, very high concentrations of amine end groups were detected. This is due to the elimination of water initiation. In this case, only p‐XDA acts as an initiator for the ring opening polymerization of CL which results in a higher amount of amine end groups than in the case of the previous reaction where water was also used. Table 2.1 Chemical compositions of the reactants used and PA6 molecular weights according to titration, SEC and NMR. COOH NH2 Mn Mn Mn (Titration) (NMR) (SEC) PA6 Water p‐XDA code (wt%)* (wt%)* PA6A 3 0
94
95
10,500
‐
20,300 PA6B 3 3
18
467
4,100
3,800 9,800 PA6C 3 6
22
787
2,500
2,550 6,800 PA6D 3 8
17
977
2,000
2,250 5,300 PA6E 0 8
15
1,622
1,200
1,300 3,400 (meq/kg) (meq/kg) (g/mol) (g/mol) (g/mol) *Based on CL weight. Full characterization of the synthesized diamine end‐capped PA6 polymers is presented in Table 2.1 and Table 2.2. From the obtained results it is visible that the SEC Mn values of the PA6 samples are roughly a factor 2.5 higher than the corresponding values obtained from the titration data. This difference is related to the differences in hydrodynamic 28
Chapter 2
volumes of the synthesized PA6 and the PMMA standards used during the SEC measurements. Molecular weights were also calculated from the analysis of the end groups in NMR spectra. The 1H NMR spectrum of a PA6 sample which was prepared with 3 wt% water and 6 wt% p‐XDA addition (PA6C) is shown in Figure 2.5. All the protons of the molecular structure were assigned with the corresponding letters as shown. From the 1H NMR spectra one can observe only trace amounts of unreacted CL left after extraction. The methylene group which is connected to the amine end group results in a triplet around 2.65 ppm, whereas the methylene group in the repeating unit gives a quadruplet around 3.15 ppm. From the integral values of these two peaks, which are assigned as (E) and (E’) respectively, it was possible to calculate the molecular weights of the different PA6 samples. Mn values calculated from 1H NMR were close to Mn values obtained from the titration data (See Table 2.1), confirming the overestimation of the Mn values by SEC. E'
H2N
C'
D'
H
N
A'
B'
O
E
C
H
N
O
B
D
N
H n
A
A
O
B
O
E
C
D
N
H
m
D'
B'
A'
C'
NH2
E'
A
E
C
1.50
11.33
2.00
22.51
0.38
10.98
2.50
B
D
L
C
'
A
0.38
1.00
9.77
0.41
3.00
L
C
'
E
L
C
ppm (t1)
Figure 2.5 1H NMR spectrum of amine end‐capped PA6 containing 6 wt% p‐XDA (PA6C). The spectrum was recorded in a TFE/CDCl3 mixture. Thermal properties of the PA6 polymers which were determined by DSC measurements are collected in Table 2.2. It is obvious that there is a correlation between the melting temperature s and the Mn of the synthesized PA6 polymers. With a decrease in molecular 29
Chapter 2
weight, a significant decrease in melting temperature is observed, as expected. A similar behavior is observed for the crystallization temperatures. The enthalpy of melting values (∆Hm1) during the 1st cycle show an increasing trend. This might be because of crystal perfectioning due to decreasing molecular weight. Once the sample is cooled and reheated, ∆Hm2 values show a decreasing trend with decreasing Mn, similar to a decrease in melting temperatures. Glass transition temperatures (Tg) were not detectable from the measured DSC samples since the observation of the Tg of the polyamides is difficult by using conventional DSC techniques. Table 2.2 Thermal properties of PA6 polymers determined by DSC. PA6 PA6A Mn Titration (g/mol) 10,500 Tc (°C) Tm1 (°C) ∆Hm1 (J/g) 222.0 70.6 185.0 74.3 220.8 82.8 ∆Hc
(J/g) Tm2 ∆Hm2 (°C) (J/g) PA6B 4,100 215.8 75.5 180.7 71.7 214.2 64.0 PA6C 2,500 208.7 87.5 175.7 73.5 206.4 68.9 PA6D 2,000 204.3 86.3 173.7 74.3 204.0 67.6 PA6E 1,200 180.2 122.6 137.3 52.2 173.9 51.7 2.3.2 Hydroxyl and diisocyanate end‐capped polypropylene adipate For the synthesis of hydroxyl end‐capped oligoester, dimethyl adipate (DMA) was reacted with an excess of 1,3‐propane diol (PD) at 180 °C (Figure 2.6). Tin(IV)butoxide (TBO) is a well‐known catalyst for methyl ester and diol reactions.21, 22 Reaction details are shown in Table 2.3. During the synthesis of the first polypropylene adipate (PPA1) a molar ratio of 1.6 was used for PD/DMA and reaction was allowed to proceed for 3 hours. The number average molecular weight was calculated as 1,400 g/mol from NMR and 3,100 g/mol from SEC. SEC was performed using polystyrene standards and due to the differences in hydrodynamic volumes the Mn calculated by SEC was an overestimation. When the molar ratio of PD/DMA was increased to 1.8 and the reaction time was kept the same, a 30
Chapter 2
decrease in molecular weight was observed both by SEC and NMR (PPA2). For the 3rd run the same reaction conditions were used except that the reaction time was slightly shorter. This resulted in a further decrease in Mn as expected (PPA3). Figure 2.6 Reaction scheme of the synthesis of hydroxyl end‐capped polypropylene adipates. Table 2.3 Feed compositions, reaction temperature, reaction time and number average molecular weights of the polypropylene adipate polymers as determined by SEC and NMR. PPA PD/DMA T (°C) Time (h) Mn (SEC) (g/mol) Mn (NMR) (g/mol) PPA1
PPA2
PPA3
1.6
1.8
1.8
180
180
180
3.0
3.0
2.5
3,100
2,600
2,200
1,400 1,100 900 1
H NMR was performed on samples taken during the reaction of PPA3. From the spectrum in Figure 2.7 it is clear that after 1 hour, the reaction medium is a mixture of PD, DMA and PPA. However at the end of the reaction (2.5 hours) the methyl ester (‐CH3) peak of the DMA, which appears at 3.609 ppm, was totally disappeared, meaning that all the DMA was consumed and converted into PPA together with PD. Excess of PD is also seen in the spectra taken after 1 and 2.5 hours around 3.7 ppm which could be totally removed together with the catalyst upon purification and totally hydroxyl end‐capped PPA was obtained. 31
3.630
3.620
3.610
b'
HO
3.603
3.609
3.618
3.633
Chapter 2
a''
a'
O
O
d
d
O
c
c
a
a
O
b
OH
n
3.600
a''
3.603
a
3.633
a'
3.618
1 hr
d
c
b
PD
3.630
3.620
3.610
3.600
PD
b'
2.5 hr
ppm (t1)
4.00
3.50
3.00
2.50
2.00
1.50
1
Figure 2.7 H NMR spectra of samples during the PPA synthesis after reaction times of 1 and 2.5 hours. All the synthesized PPA polymers were very soft and sticky, which made their handling very difficult. The second step before the solution mixing with PA6 involved diisocyanate end‐capping of the oligoesters. For this purpose PPA3 was chosen, exhibiting the lowest molecular weight. However, even after end capping resulting polymer was still sticky. Therefore free‐flowing, non‐sticky PCL was used instead of PPA. In the following parts diisocyanate end‐capping of PCL followed by the synthesis of PCL‐b‐PA6 copolymers and their characterization will be explained. 2.3.3 Diisocyanate end‐capped PCL (TPCL) For the synthesis of α,γ-diisocyanate terminated PCL, PCL prepolymers with molecular weights of 530 g/mol and 1,250 g/mol were used as well as TDI as the end capper. TDI was chosen for end‐capping because of the highly unequal reactivity of its two isocyanate groups.23 This difference in reactivity of the isocyanate groups together with the highly excess amounts of TDI should limit the amount of chain extension. During the end‐capping due to the low Tg of the low molecular weight TDI end‐capped PCL (TPCL), the purification 32
Chapter 2
by precipitation of the polymer was rather difficult. However, when PCL with a molecular weight of 1,250 g/mol was used it was possible to obtain a totally solid and purified TPCL. Figure 2.8 shows the 1H NMR spectra of PCL before and after TDI end‐capping with structural assignments. All PCL and TPCL peaks were assigned as the methylene groups in the repeating unit. The TDI spectrum was also shown for clarity. For a completely TDI end‐
capped PCL, the signal for the protons of the methylene group (a’), that is connected to the hydroxyl end groups, should disappear and form a new signal. This indeed was observed in the 1H NMR spectrum of the purified polymer after end‐capping. This peak shifted from 3.63 ppm to 4.14 ppm, and was assigned as (a”). This is strong evidence for TDI end‐capping of all hydroxyl end groups of the PCL polymer. It is also observed that the signals for the phenyl protons of TDI entirely shift to lower field between 7.0‐7.1 ppm as the isocyanate end groups react with the hydroxyl end groups of the PCL. From this, it can also be concluded that all the excess TDI was successfully removed from the product after purification. n
l
k
OCN
NCO
I
D
T
CH3 l
m
k
n
m
b
a
d
b
f
O
H
a'
L
C
P
a
O
n
O
O
e
O
O
a
d
b
n
O
f
H
N
O
a''
NCO
n
O
4.217
4.139
O
c
O
L
C
P
T
H3C
c
H
N
'
a
f
e
OCN
3.679
O
3.694
3.630
O
e
O
n
4.229
O
c
d
c
b
O
H
CH3
'
a
ppm (t1) 7.0
6.0
5.0
4.0
1
3.0
2.0
1.0
Figure 2.8 H NMR spectra and structural assignments of TDI, PCL and TPCL as recorded in CDCl3. 33
Chapter 2
Isocyanate end group titration was performed to calculate the absolute molecular weight of the TPCL. If the PCL is only end‐capped with 2 TDI units (MW2TDI≈350 g/mol) and if no chain extension has occurred, the total Mn after end‐capping should be 1,250+350=1,600 g/mol. However, the molecular weight calculated from the titration data results in an Mn of 1,850 g/mol, which points to a minor amount of chain‐extension. Number average and weight average molecular weights of TPCL were also measured by SEC and were found to be Mn =3,500 g/mol, Mw=5,200 g/mol, respectively, where PS was used as the standard and THF was used as the eluent. For low molecular weight PCL, SEC determined values are overestimated with a factor of almost 2 if PS standards are used.24 2.3.4 Multiblock copolymers of PA6C and TPCL Preparation of multiblock copolymers was done by using PA6C (Mn =2,500 g/mol calculated from titration results) and TPCL (Mn =1,850 g/mol calculated from titration). As explained in the experimental part, the procedure started with solution mixing at room temperature (RT) and then continued with heating up the polymer gradually after complete removal of the solvent. Solution mixing in an argon atmosphere and removal of solvent under reduced pressure were all done at RT. The temperature was not increased before complete removal of HFIP since HFIP and isocyanate groups are highly reactive at elevated temperatures. It was observed that TDI and HFIP already react to a full extent only after 1.5 hours at 65 °C, however almost no reaction was observed after 24 hours at RT. Table 2.4 shows the changes in the molecular weight during solution mixing and subsequent heating in the solid state according to SEC analysis. The results demonstrate that average molecular weights had increased with respect to the initial Mn values of separate building blocks, being 2,500 and 1,850 g/mol, for PA6C and TPCL, respectively, even after 1 hr of mixing at room temperature in HFIP. At the end of the solution mixing the desired molecular weights were obtained (PEA‐ASM‐Polyesteramide after solution mixing). 34
Chapter 2
Table 2.4 Molecular weight and polydispersity data of the fractions obtained during the synthesis of multiblock copolymers of fully amine‐terminated PA6C and fully isocyanate‐
terminated TPCL as obtained by SEC. Sample Mn (SEC)
(g/mol) Mw (SEC)
(g/mol) PDI Solution mixing‐1 hr
9,600
15,300
1.6 Solution mixing‐2 hr
10,300
19,400
1.9 Solution mixing‐3 hr
11,200
21,300
1.9 Solution mixing‐4 hr
12,600
23,800
1.9 Solution mixing‐20 hr*
22,500
54,300
2.4 SSP‐ 40 °C
23,000
54,700
2.4 SSP‐ 60 °C
22,000
60,000
2.7 SSP‐ 80 °C
24,350
66,200
2.7 SSP‐ 100 °C
23,700
59,000
2.5 SSP‐ 120 °C
22,500
57,350
2.5 SSP‐ 160 °C
15,700
68,500
4.4 * = end of solution mixing (PEA‐ASM=Polyesteramide after solution mixing); RT=room temperature; SSP=solid‐
state polymerization Figure 2.9 shows the disappearance of the TPCL and PA6C peaks after solution mixing during which a higher molecular weight polymer was formed at a lower retention time. Realizing that the Mn data obtained from SEC are roughly overestimated by a factor 2.5 (see earlier), and given the fact that the average Mn from titration or NMR of the initial building blocks is ca. 2,200 g/mol (ca. (2,500 + 1,850)/2), we can conclude that multiblock copolymers were formed at the end of the solution mixing process. From the SEC curves it can be observed that after solution mixing all the TPCL was consumed and that the PA6C peak had shifted to lower retention times. However, PDI also increased significantly above 2 which may be a result of some branching during multiblock copolymer formation. It is also seen that Mn does not change a lot during the heating process up to 120 °C, whereas 35
Chapter 2
an increase in Mw is observed. This means that heating is not necessary for these reactions, since incorporation of TPCL into PA6C already takes place at RT resulting in a system with a sufficiently high Mn. When the heating is continued up to 160 °C Mn decreased while Mw increased, resulting in an increase in polydispersity. Most likely, as the heating was continued at high temperatures the formation of biuret and allophanate groups via reactions of free isocyanate with urea‐urethane linkages resulted in extensive branching, and cross‐linking. Moreover, thermal degradation resulting in low molecular weight molecules, thereby lowering Mn, can also not be excluded. The mentioned branching reactions are highly favorable above 100 °C after sufficient reaction times, and may even result in an insoluble product which is removed by filtration before SEC analysis.25, 26 Normalized RI SEC signal
SSP-160 C
SSP-140 C
SSP-100 C
SSP-60 C
End of solution mixing
TPCL
PA6C
10
12
14
16
18
20
22
Elution time (min)
24
26
28
30
Figure 2.9 SEC chromatograms recorded during the multiblock formation. Infrared spectra of PEA‐ASM (polyesteramide after solution mixing) and TPCL are shown in Figure 2.10. In the case of the PEA‐ASM spectrum, the N‐H stretching vibration band at 3292 cm‐1 and the amide I (C=O stretch) and amide II (C‐N stretch and C(O)‐N‐H bend) bands, which are observed in the range of 1650‐1540 cm‐1, can be assigned to PA6C blocks whereas the ester carbonyl band (OC=O stretch) at 1724 cm‐1 corresponds to TPCL blocks. The ‐CH2‐ stretching band is seen between 2934‐2853 cm‐1 for both PA6C and TPCL blocks. The disappearance of the peak around 2270 cm‐1, which represents the absorbance of 36
Chapter 2
isocyanate groups, proves the complete reaction between the two polymer building blocks. Absorbance
Amide
I&II
NCO
TPCL
OC=O
CH2
N-H
PEA-ASM
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Figure 2.10 FTIR spectra of diisocyanate terminated polycaprolactone (TPCL) and polyesteramide copolymer after solution mixing (PEA‐ASM) after 20 hours of solution mixing. c
O
a
O
c
d
b
CH 3
O
O
n
O
N
H
N
H
E
N
H
C
D
H
N
A
B
O
m
A
d
+
c
+
C
+
B
+
D
E
b
ppm (t1) 3.00
2.50
2.00
1.50
1
Figure 2.11 H NMR spectrum and structural assignments of the polyesteramide copolymer after solution mixing (PEA‐ASM) as recorded in TFE:CDCl3 mixture. 37
Chapter 2
The 1H NMR spectrum of the PEA‐ASM presents the characteristic peaks from the repeating units of both components. The methylene groups of the TPCL and PA6C chains are identified as shown in Figure 2.11. From the disappearance of the peak around 2.65 ppm, which corresponds to the –CH2 connected to the end groups of PA6C, (Figure 2.5), it can be concluded that all the end groups reacted with the isocyanate end groups of the TPCL. The thermal stability of the synthesized multiblock copolymer was analyzed by performing TGA measurements. The results were compared with the neat polymers that were used for the reaction. The first steps of thermal decomposition described as 5% weight loss for PCL, TPCL, PA6C and PEA‐ASM were found at 272 °C, 254 °C, 334 °C, 282 °C, respectively (Table 2.5). The thermal stability of the PEA‐ASM copolymer is lower than the stability of PA6C polymer, however still sufficient for melt processing. DSC analysis allows a comparison between the melting temperatures of the TPCL, PA6C and PEA‐ASM copolymer. Before the copolymerization, the Tm of TPCL is 43.6 °C, whereas the Tm of PA6C is 206.4 °C. After solution mixing, the PEA‐ASM sample clearly shows two separate melting temperatures, where 36.6 °C represents the TPCL melting temperature and 199.3 °C the Tm of PA6. Two separate crystallization temperatures are observed as well, viz. ‐7.6 °C and 156.3 °C for TPCL and PA6C, respectively. Table 2.5 Thermal properties of starting components (TPCL, PA6C) and the multiblock copolymer (PEA‐ASM) determined by DSC. T5%wt loss Tm1 ∆Hm1 Tc ∆Hc Tm2 ∆Hm2 (°C) (°C) (J/g) (°C) (J/g) (°C) (J/g) TPCL 254 – – – – 43.6 68.1 PA6C 334 208.7 87.5 175.7 73.5 206.4 68.9 PEA‐ASM 282 201.6 28.6 ‐7.6 8.4 36.6 11.5 156.3 21.5 199.3 35.4 38
Chapter 2
In combination with the earlier described molecular weight increase after solution mixing of isocyanate‐terminated PCL and amine‐terminated PA6 this observation points to a block copolymer formation without destroying the crystallinity of PA6 (Figure 2.12). The melting temperatures after copolymerization do not change significantly when compared to the data of pure TPCL and PA6C blocks. However, as shown in Table 2.5, a distinct decrease in melting and crystallization enthalpies is observed which might be a result of the reduction of the lamellar thickness due to the chemical linkage to another polymer block. Heat flow (W/g) Endo down
TPCL
43.6°C
PA6C
156.3°C
206.4°C
cooling
-7.6°C
PEA-ASM
heating
36.6°C
-40 -20
0
199.3°C
20 40 60 80 100 120 140 160 180 200 220
Temperature (C)
Figure 2.12 DSC heating scans of TPCL and PA6C, and heating and cooling scans of PEA‐
ASM polymer. 2.3.5 Hydrolytic and enzymatic degradation of PEA‐ASM Degradation studies were carried out with and without enzyme on PEA‐ASM samples. The degradability of a commercial PA6 (Mn=31 kg/mol) was also analyzed as a reference. However, it was not possible to analyze the initial PA6C and TPCL building blocks which were used for the preparation of PEA‐ASM copolymers, since stable films could not be prepared. Lipase from Aspergillus niger was used as the enzyme and all degradation 39
Chapter 2
studies were done at 25 °C which is relatively close to natural conditions, for a period of 8 weeks. The remaining weight of the films as a function of degradation time is presented in Figure 2.13. From the obtained results it is visible that PA6 is totally non‐degradable, as expected, while PEA‐ASM films are degradable both enzymatically and hydrolytically. Remaining weight (%)
100
95
90
85
Enzymatic degradation of PA 6
Hydrolytic degradation of PEA-ASM
Enzymatic degradation of PEA-ASM
80
75
0
10
20
30
40
Degradation time (days)
50
60
Figure 2.13 Remaining weight (%) vs. time of degradation during (♦) enzymatic degradation of PEA‐ASM, (●) hydrolytic degradation of PEA‐ASM, (▲) enzymatic degradation of PA6. Almost 12 wt% loss was observed after 8 weeks of incubation in case of the enzymatic degradation of PEA‐ASM films. Since degradation mainly occurs as a surface erosion process12, 27and it is difficult to reach the PCL chain parts buried inside the film, it is not possible to degrade the PCL content entirely within 8 weeks. Another constraint is the crystallinity of the PCL chains which makes it less sensitive for a rapid degradation.28, 29 It is also interesting to observe that the non‐enzymatic degradation occurs up to almost 7 wt% in 8 weeks. PCL degradation is known to be very slow under hydrolytic conditions.1, 30 However, abiotic degradation of PCL can be enhanced due to increased hydrophilicity by the presence of the amide groups in the copolymer.12, 31, 32 SEC analysis of the remaining films after degradation showed a minor decrease in molecular weights (Table 2.6). This is 40
Chapter 2
an expected result for surface erosion where a significant loss in molecular weight is not observed for short degradation times.27, 33 As stated by Hakkarainen33, these results agree with the general observation that enzymatic degradation of PCL proceeds by rapid weight loss with minor reduction in molecular weight. On the contrary, hydrolytic degradation proceeds by a reduction in molecular weight combined with minor weight loss. Table 2.6 SEC data of PEA‐ASM films before degradation and after 4 and 8 weeks of enzymatic and hydrolytic degradation. (E=enzymatic, H=hydrolytic) (HFIP was used as eluent.) Degradation time (weeks) Deg. Mn Mw (g/mol) (g/mol) PDI 0 ‐ 22,500 54,300 2.4 4 E 21,800 51,300 2.3 8 E 20,200 54,800 2.7 4 H 20,800 52,400 2.5 8 H 19,500 60,000 2.9 Degradation can be easily visualized by SEM pictures (Figure 2.14). PEA‐ASM film before degradation has a rather smooth surface exhibiting some holes because of the solvent evaporation. Erosion on the surface is clearly visible after 4 weeks of enzymatic degradation. After 8 weeks the depth of the holes and the irregularities at the surface have increased significantly. The difference between the PA6 film and the multiblock copolymer [PA6‐b‐PCL]n (PEA‐ASM) films is very obvious after the same degradation time, showing the higher stability of PA6 while PEA‐ASM is being degraded. 41
Chapter 2
Figure 2.14 Scanning electron micrographs of the polymer films: (A) PEA‐ASM before degradation, (B) PEA‐ASM after 4 weeks of enzymatic degradation, (C) PEA‐ASM after 8 weeks of enzymatic degradation, (D) Commercial PA 6 after 8 weeks of enzymatic degradation. 2.4 Conclusions Amine terminated low molecular weight PA6 polymers were successfully prepared by ring opening and polycondensation polymerization of ‐caprolactam with the addition of 3‐8 wt% p‐XDA and water as initiator. Molecular weights and melting temperatures decreased with the increase of p‐XDA content in the PA6 polymers. Diisocyanate terminated PCL oligomer was also synthesized and characterized with 1H NMR. Reaction of both components in solution demonstrated the multiblock copolymer formation as desired, which was proven by molecular characterization and DSC analysis. Biodegradation studies showed the enhanced degradability of PA6‐based films after block copolymer formation with the ester groups being cleaved by hydrolytic and enzymatic degradation. After 8 weeks of enzymatic degradation a weight loss of 12% loss was achieved. It was shown that 42
Chapter 2
by applying this stepwise synthetic route the crystalline structure of the PA6 blocks and their relatively high Tm are retained, which renders novel materials with properties close to those of PA6, while the biodegradability is enhanced by the PCL incorporation. The combination of PA6‐like thermal and crystalline properties with biodegradability is a clear advantage with respect to earlier described random polyesteramides prepared from caprolactam and caprolactone. References 1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
Gan, Z. H.; Liang, Q. Z.; Zhang, J.; Jing, X. B. Polym. Degrad. Stab. 1997, 56, (2), 209‐213. Ikada, Y.; Tsuji, H. Macromol. Rapid Commun. 2000, 21, (3), 117‐132. Tokiwa, Y.; Ando, T.; Suzuki, T. J. Ferment. Tech. 1976, 54, (8), 603‐608. Bernaskova, A.; Chromcova, D.; Brozek, J.; Roda, J. Polymer 2004, 45, (7), 2141‐2148. Chromcova, D.; Bernaskova, A.; Brozek, J.; Prokopova, I.; Roda, J.; Nahlik, J.; Sasek, V. Polym. Degrad. Stab. 2005, 90, (3), 546‐554. Deshayes, G.; Delcourt, C.; Verbruggen, I.; Trouillet‐Fonti, L.; Touraud, F.; Fleury, E.; Degee, P.; Destarac, M.; Willem, R.; Dubois, P. React. Funct. Polym. 2008, 68, (9), 1392‐1407. Deshayes, G.; Delcourt, C.; Verbruggen, I.; Trouillet‐Fonti, L.; Touraud, F.; Fleury, E.; Degee, P.; Destarac, M.; Willem, R.; Dubois, P. Macromol. Chem. and Phys. 2009, 210, (12), 1033‐1043. Ellis, T. S. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, (9), 1109‐1125. Gonsalves, K. E.; Chen, X.; Cameron, J. A. Macromolecules 1992, 25, (12), 3309‐3312. Goodman, I.; Vachon, R. N. Eur. Polym. J. 1984, 20, (6), 539‐547. Iwamoto, A.; Tokiwa, Y. Polym. Degrad. Stab. 1994, 45, (2), 205‐213. Stapert, H. R. Environmentally Degradable Polyesters, Poly(ester‐amide)s and Poly(ester‐urethane)s. PhD Thesis, Universiteit Twente, 1998. Tokiwa, Y.; Suzuki, T.; Ando, T. J. Appl. Polym. Sci. 1979, 24, (7), 1701‐1711. Toncheva, N. V.; Mateva, R. P. Adv. Polym. Tech. 2007, 26, (2), 121‐131. Michell, R. M.; Muller, A. J.; Castelletto, V.; Hamley, I.; Deshayes, G.; Dubois, P. Macromolecules 2009, 42, (17), 6671‐6681. Sasek, V.; Vitasek, J.; Chromcova, D.; Prokopova, I.; Brozek, J.; Nahlik, J. Folia Microbiologica 2006, 51, (5), 425‐430. Jansen, M. A. G.; Wu, L. H.; Goossens, J. G. P.; De Wit, G.; Bailly, C.; Koning, C. E.; Portale, G. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, (4), 1203‐1217. Vouyiouka, S. N.; Karakatsani, E. K.; Papaspyrides, C. D. Prog. Polym. Sci. 2005, 30, (1), 10‐37. D'hollander, S.; Van Assche, G.; Van Mele, B.; Du Prez, F. Polymer 2009, 50, (19), 4447‐4454. Lee, H. Y.; Jeong, H. M.; Lee, J. S.; Kim, B. K. Polym. J. 2000, 32, (1), 23‐28. Kricheldorf, H. R.; Behnken, G.; Schwarz, G. Polymer 2005, 46, (25), 11219‐11224. Montaudo, G.; Rizzarelli, P. Polym. Degrad. Stab. 2000, 70, (2), 305‐314. Odian, G., Principles of Polymerization. 4 ed.; McGraw Hill: New York, 1970. Kricheldorf, H. R.; Eggerstedt, S. Macromol. Chem. Phys. 1998, 199, (2), 283‐290. Dusek, K.; Spirkova, M.; Havlicek, I. Macromolecules 1990, 23, (6), 1774‐1781. 43
Chapter 2
26. Lapprand, A.; Boisson, F.; Delolme, F.; Mechin, F.; Pascault, J. P. Polym. Degrad. Stab. 2005, 90, (2), 363‐373. 27. Bikiaris, D. N.; Papageorgiou, G. Z.; Achilias, D. S. Polym. Degrad. Stab. 2006, 91, (1), 31‐43. 28. Albertsson, A. C.; Varma, I. K. 2002, 157, 1‐40. 29. Mochizuki, M.; Hirano, M.; Kanmuri, Y.; Kudo, K.; Tokiwa, Y. J. Appl. Polym. Sci. 1995, 55, (2), 289‐296. 30. Eldsater, C.; Erlandsson, B.; Renstad, R.; Albertsson, A. C.; Karlsson, S. Polymer 2000, 41, (4), 1297‐
1304. 31. He, Y.; Liu, X. B.; Luo, D. W.; Chen, W. J. J. Appl. Polym. Sci. 2008, 108, (3), 1689‐1695. 32. Papadimitriou, S.; Bikiaris, D. N.; Chrissafis, K.; Paraskevopoulos, K. M.; Mourtas, S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, (22), 5076‐5090. 33. Hakkarainen, M. Degradable Aliphatic Polyesters 2002, 157, 113‐138. 44
CHAPTER 3 MULTIBLOCK COPOLYMERS OF POLYAMIDE 6 AND DIEPOXY PROPYLENE ADIPATE OBTAINED BY SOLUTION AND SOLID STATE POLYMERIZATION Summary Polyamide 6‐diepoxy propylene adipate multiblock copolymers were synthesized by solution and solid‐state polymerization by using carboxyl‐terminated PA6 and epoxide‐terminated oligoester. The structure and thermal properties of the copolymers were analyzed by SEC, NMR and DSC. DSC data, together with an increase in molecular weight pointed to a multiblock structure with an almost maintained melting temperature of PA6. However, the presence of side reactions restricted the formation of high molecular weight products. HOOC
COOH
Polyamide 6
+
O
O
Oligoester
O
C
O
CH2
CH
OH
45
CH2 O
O
C
Chapter 3
3.1 Introduction
Polyamide 6 (PA6) is an important engineering plastic and as such mainly used for automotive, electrical and packaging applications. However it is not susceptible to degradation like some other industrial plastics. It would be desirable for PA6 to be environmentally biodegradable, especially for the packaging applications. It was well described in the previous chapter that the biodegradation of PA6 can be enhanced by the incorporation of hydrolyzable ester groups into the PA6 backbone. However, if this is done via melt polymerization random copolymers are obtained which result in the deterioration of the crystallization behavior of the PA6. This finally negatively affects the good mechanical and physical properties of PA6 which is not desired for the applications. It was shown in Chapter 2 that the incorporation of degradable ester groups into the amorphous part of a relatively low molecular weight PA6 below the melting temperature of the PA6 crystals is possible without a significant deterioration of the crystalline region of the PA6. For this purpose, highly reactive isocyanate and amine end groups present at the chain ends of polyesters and polyamides, respectively, were used so that PA6 and polyester blocks can be coupled already at room temperature in solution. In this way, multiblock copolymers were obtained for which the high melting temperature and crystallinity of the PA6 blocks can still be maintained. The same approach can be used by making use of epoxide‐carboxyl reactions. These reactions are widely used to produce crosslinked coatings from epoxy resins.1‐7 It is also possible to synthesize linear polymers in bulk without crosslinking if moderate temperatures and proper reaction times are used.8‐17 In this study we used a similar step‐
wise approach as used in Chapter 2 where the functional end groups were changed. Firstly, a low molecular weight carboxyl end‐capped PA6 was synthesized and a degradable oligoester with epoxide end groups was obtained from the Biocatalysis Group of M. Martinelle at the Royal Institute of Technology at Stockholm. Both components 46
Chapter 3
were mixed in a common solvent at room temperature with the addition of a tertiary amine as a catalyst. After the complete evaporation of the solvent solid state step‐growth copolymerization was performed well below the melting temperature of the PA6. Figure 3.1 shows the reaction scheme. In this way, multiblock polyesteramides could be prepared where the degradability of PA6 is enhanced by the incorporation of the hydrolyzable ester groups. Molecular weights of the synthesized polymers were determined by using SEC, NMR and titration methods. SEC was also used as a useful tool to monitor reactions with time. Thermal analysis was performed by using TGA and DSC. Figure 3.1 Schematic overview of the stepwise synthesis of polyamide 6‐diepoxy propylene adipate multiblock copolymers obtained by solution and solid‐state polymerization. 47
Chapter 3
3.2 Experimental 3.2.1 Materials Dry ε‐Caprolactam (CL) was kindly provided by DSM (Geleen, The Netherlands). Adipic acid (AA) was purchased from Sigma. Propanoic acid (PPA) was obtained from Merck. Irganox 1330 was purchased from Ciba Speciality Chemicals.
Catalysts 4‐
dimethylaminopyridine (DMAP) and triethylamine (TEA) were obtained from Aldrich. Glycidyl phenyl ether (GPE) was purchased from Acros. Poly(propylene glycol) diglycidyl ether (PPGE, average MW=380 g/mol) was obtained from Aldrich. Diepoxy propylene adipate (DEPA, average MW=450 g/mol) was synthesized as described before.18 1,1,3,3,3‐
Hexafluoro‐2‐propanol (HFIP, 99%, Biosolve) and deuterated chloroform (CDCl3, 99.8%, Cambridge Isotope Laboratory) were used for NMR measurements. All the chemicals were used as received unless stated otherwise. 3.2.2 Model reactions of glycidyl phenyl ether and propanoic acid 0.27 g (1.8 mmol) glycidyl phenyl ether (GPE) and 0.54 g (1.8 mmol) propanoic acid (PPA) were mixed in a Schlenk reactor under argon atmosphere with 1.4‐5.6 mol% TEA addition as catalyst with respect to the total weight. The reaction was conducted at 60‐80 °C and samples were analyzed by NMR to follow the conversion with time. 3.2.3 Synthesis of carboxyl end‐capped polyamide 6 Carboxylic acid end‐capped PA6 was synthesized in a batch reactor with a capacity of 380 mL. Temperature and pressure were controlled via a computer. First, 100 g (0.88 mol) ε‐
caprolactam was charged to the reactor and heated until complete melting. Later, 5 g (0.034 mol) adipic acid, 1 g (0.056 mol) water and 1 g Irganox 1330 were added. The polymerizations were carried out at 250 °C at 3 bar for 5 hours under the flow of N2 gas and with continuous mechanical stirring. The product was extracted with water at 80 °C for 20 hours, filtered under vacuum and dried in an oven at 80 °C for at least 24 hours. The product was characterized by using SEC, NMR, DSC and titration analysis. 48
Chapter 3
3.2.4 Polyamide 6‐poly(propylene glycol) diglycidyl ether model reactions 3.63 g (1.58 mmol) dry carboxyl end‐capped PA6 (dried under vacuum at 80 °C for 24 hours) and 0.60 g (1.58 mmol) poly(propylene glycol) diglycidyl ether (PPGE) were put in a 100 mL three neck round bottom flask under argon atmosphere and dissolved in 30ml HFIP for mixing. Also 0.45‐1.35 wt% DMAP was added as catalyst, the amount calculated with respect to the total weight of PA6 and PPGE (see Table 3.2). After complete dissolution, HFIP was slowly removed by vacuum distillation at RT. Then, the lump of material was taken out of the flask, ground in liquid N2, sieved and reduced pressure was applied again at RT for 2 days. As soon as the particles were completely dry, the product was placed in a Schlenk connected to Argon and a solid state reaction was performed in the temperature range 70‐120 °C, which is below the melting temperature of the polyamide. Reaction was continued overnight. Polymer samples were characterized via SEC, NMR and DSC. 3.2.5 Polyamide 6‐diepoxy propylene adipate reactions Diepoxy propylene adipate (DEPA) was prepared by Eriksson et al. by following a synthetic procedure as explained before.18 For the reaction of 0.30 g (0.67 mmol) DEPA with 1.61 g (0.70 mmol) PA6 the same synthetic method as explained above for PA6/PPGE was used except that the molar ratio of PA6/DEPA was mostly 1.05/1 whereas in two cases it was 1:1. A first set of reactions were performed with the addition of DMAP as catalyst with the same concentration as in the case of the PA6/PPGE reactions (0.45‐1.35 wt% with respect to the total weight of PA6 and DEPA) (see Table 3.4). The second set of reactions were either performed with the addition of a TEA catalyst (2 and 5 wt% with respect to the total weight of PA6 and DEPA) or without. Polymer fractions were characterized via SEC, FTIR and DSC. 49
Chapter 3
3.2.6 Characterization 3.2.6.1 Size Exclusion Chromatography (SEC) SEC was used to determine molecular weights and molecular weight distributions, Mw/Mn, of the polymer samples. The system was equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2707 autosampler, a Waters 2487 dual absorbance UV detector, a Waters 2414 refractive index detector (35 °C) and a PSS PFG guard column followed by 2 PFG‐
linear‐XL (7 µm, 8*300 mm) columns in series. The temperature was 40 °C. Hexafluoroisopropanol with potassium trifluoroacetate (3 g/L) was used as eluent at a flow rate of 0.8 mL/min. Toluene was used as the internal standard. The molecular weights were calculated with respect to poly(methyl methacrylate) standards (Polymer Laboratories, Mp = 1020 g/mol up to Mp = 1.9*106 g/mol). 3.2.6.2 Nuclear Magnetic Resonance (NMR) Spectroscopy 1
H NMR spectra of the polymers were recorded on a Varian 400 MHz spectrometer at 25 °C. Samples were dissolved in a 1:1 vol:vol CDCl3:TFE mixture. 3.2.6.3 Differential Scanning Calorimetry (DSC) Melting (Tm) and crystallization temperatures (Tc) as well as melting (∆Hm) and crystallization enthalpies (∆Hc) were measured using a TA Instruments Q100 calorimeter. For all the measurements, 4‐6 mg samples and a heating rate of 10°C/min were used under N2 atmosphere. DSC measurements were carried out from ‐80 °C to 150 °C for oligomers and from ‐80°C to 240°C for the copolymers. During each measurement samples were equilibrated at ‐80 °C and 150 or 240 °C for 5 minutes. For the determination of both Tm and Tc peak maximums were taken into account. 50
Chapter 3
3.2.6.4 Thermogravimetric Analysis (TGA) Thermogravimetric analyses (TGA) were performed on a TA Instruments Q500 TGA in a nitrogen atmosphere. Samples were heated from 30 °C to 600 °C with a heating rate of 10 °C/min. 3.2.6.5 Potentiometric titration For the determination of amine [NH2] and carboxylic acid [COOH] end group concentrations, potentiometric titrations were done at room temperature in non‐aqueous environment using phenolic solvents. Both blank and sample measurements were repeated at least 3 times. Molecular weights were calculated by using the formula 2*106/([NH2]+[COOH]), thus assuming the presence of two end groups per chain. 3.3 Results and Discussion Epoxide‐carboxyl reactions are widely used to synthesize crosslinked structures for applications like coatings. However, it is also possible to get linear polymers if the right reaction conditions are used.9‐12, 14 For this purpose, firstly model bulk reactions were performed by using glycidyl phenyl ether (GPE) and propanoic acid (PPA) as model compounds for epoxide‐ and carboxylic acid‐terminated polymers and triethylamine (TEA) as catalyst. After finding the conditions rendering the highest conversion for these reactions, model reactions with poly(propylene glycol) diglycidyl ether (PPGE) and PA6 were performed, since PPGE is a low molecular weight diepoxy compound similar to diepoxy propylene adipate (DEPA) of which very limited amounts were available. PA6 and PPGE were mixed in HFIP and later SSP was performed. Finally, DEPA and PA6 were used to make polyesteramide block copolymers. 3.3.1 Model reactions with glycidyl phenyl ether and propanoic acid To find the optimum conditions for carboxyl‐epoxide reactions bulk reactions, of glycidyl phenyl ether (GPE) and propanoic acid (PPA) were performed with the action of triethylamine (TEA) as the base catalyst. The chemical structures of the components are 51
Chapter 3
shown in Figure 3.2 and the proposed mechanism under base catalysis is shown in Figure 3.3. Figure 3.2 Reaction scheme of glycidyl phenyl ether and propanoic acid with TEA as catalyst. An anionic mechanism is the most probable one for epoxide‐carboxyl reactions. Firstly, amine catalyst takes one proton from the carboxyl group and then the resulting nucleophilic carboxylate group attacks the epoxide ring resulting in the product as presented in Figure 3.2. Attack to both carbon atoms of the ring is possible although only one possibility is shown here. The type and the concentration of the catalyst control the reaction rate. Reactions of GPE and PPA were conducted at 60, 70 and 80 °C with different concentrations of TEA. Reactions were followed by 1H NMR and the highest conversion was determined for each reaction before the occurrence of any side reactions. Figure 3.3 Proposed anionic reaction mechanism of epoxide‐carboxyl groups under base catalysis. A summary of the results of the model reactions is presented in Table 3.1. Bulk reactions were performed at different temperatures, with different catalyst amounts and mostly 52
Chapter 3
with a PPA/GPE molar ratio of 1. The conversion was calculated from 1H NMR by putting the integral areas of the corresponding peaks in the formula 1‐
1
((b1+b2+c)/(d+e+f+d’+e’+f’)). A typical H NMR spectrum before any side reaction has occurred is shown in Figure 3.4. The highest conversion before the side reactions became significant was calculated for each reaction. The most probable side reaction is the reaction of an unreacted epoxide with the hydroxyl group of the reaction 3 in Figure 3.3, which is formed after the epoxide ring opening. Table 3.1 GPE and PPA reactions @ 60, 70 and 80 °C with TEA as catalyst and highest conversions recorded before any side reactions occurred. Highest PPA/GPE TEA TEA T Time (mol/mol) mol%* wt%** (°C) (h)*** 1 1
1.4
1.25
60
16
54 2 1
1.4
1.25
70
9
70 3 1
1.4
1.25
80
5
85 4 1
2.8
2.5
60
14
77 5 1
5.6
5.0
60
10
80 Rxn # conversion (%)**** *With respect to the total amount of moles. **With respect to the total weight. ***Time before the side reactions were observed. ****Data calculated from NMR according to GPE conversion. From the results collected in Table 3.1 it can be seen that when the catalyst concentration is the same with increasing temperature, the time of reaction before the first side product formation can be observed is decreasing and the GPE conversion is increasing. When the catalyst concentration is increased at the same reaction temperature (60 °C), the GPE conversion is increasing as well and the time of reaction until the first side reactions are taking place is decreasing. The highest conversion of 85% was achieved when the reaction temperature was 80 °C with 1.4 mol% TEA. 53
Chapter 3
b1
d
e
O
d
d a1
e
b2
c O
n
PPA
G PE
OH
d`
O
d `a '
d`
e`
1
m'
O
c'
x
n'
a '2 b ' 1 b ' 2 O
N
'
n
+
n
e`
C OO H
m
a2
y
TEA
P ro d u c t
'
m
+
m
'
c
+
'
2
b
+
'
1
b
+
'
2
a
+
'
1
a
+
2
a
+
1
a
'
f
+
'
e
+
'
d
+
f
+
e
+
d
6.0
5.0
4.0
3.0
y
2
b
1
b
x
c
m
p
p
7.0
2.0
1.0
Figure 3.4 1H NMR spectrum of a GPE‐PPA reaction (Rxn #3 in Table 3.1) in CDCl3. 3.3.2 Model reactions with poly(propylene glycol) diglycidyl ether (PPGE) and PA6 After determining the optimum conditions for the epoxide‐carboxyl reactions in bulk using low molecular weight model compounds, model reactions with poly(propylene glycol) diglycidyl ether (PPGE) and PA6 were done. For this purpose initially a carboxyl end‐
capped PA6 was prepared. It was explained in the previous chapter that the addition of a diamine during the ε‐caprolactam ring opening polymerization results in totally amine end‐capped low molecular weight PA6 of which the molecular weight changes according to the added percentage of the diamine. For the preparation of carboxyl end‐capped PA6 5 wt% adipic acid was put together with 1 wt% water as initiator and 1 wt% Irganox 1330 as the antioxidant. All weight percentages were calculated as added amounts to ε‐
caprolactam. The reaction scheme is shown in Figure 3.5. For the calculation of the molecular weight SEC and NMR were used and titration experiments were also done. From the SEC measurement an Mn of 3,500 g/mol was obtained (PDI=1.6). Potentiometric titration, however resulted in an Mn of 2,400 g/mol ([NH2]=9.5 meq/kg, [COOH]=838 meq/kg). This difference in molecular weights is caused by the SEC technique in which the molecular weights are determined according to PMMA standards. Mn was also calculated 54
Chapter 3
from the corresponding 1H NMR spectrum by using the integral values of the peaks which are assigned as (A) and (A’) and was found as 2,300 g/mol (Figure 3.6). (A) is one methylene group in the repeating unit and (A’) is the methylene connected to the carboxylic acid end groups. For the molar calculations the Mn value obtained from the titration was used since it is supposed to be the most accurate molecular weight determination method compared to others. Figure 3.5 Reaction scheme of carboxylic acid end‐capped PA6. C'
A'
HO
O
B'
O
E'
D'
N
H
B
A
C
O
H
N
D
E
O
C
E
N
H
B
D
n
H
N
A
mO
E'
O
B'
D'
C'
OH
A'
A
C
B
D
E
'
A
1.50
m
p
p
2.00
9.01
2.50
18.94
3.00
8.68
1.00
9.11
3.50
Figure 3.6 1H NMR of PA6 with only carboxyl end groups recorded in TFE:CDCl3 1:1 solvent mixture. To make multiblock copolymers based on PA6 by carboxyl‐epoxide end group reactions PPGE (Mn=380 g/mol, see Figure 3.7) was used. Although it has ether groups instead of ester groups present in the DEPA, it was chosen for model reactions with PA6, since it is epoxy end‐capped and has almost the same molecular weight as DEPA. As catalyst 4‐
(dimethylamino)pyridine (DMAP) was selected, since it has a higher boiling point (162 °C) 55
Chapter 3
than TEA (90 °C) so that a possible loss during the reaction can be prevented. A molar ratio of 1/1 PA6/PPGE was used and DMAP was added in amounts of 0.45, 0.9 and 1.35 wt% with respect to the weight of the PPGE. These values correspond to 11, 22 and 33 mol%, respectively. As explained in the experimental section, all components were charged to a reactor and dissolved in HFIP. After complete dissolution, vacuum was applied at room temperature (RT) to remove HFIP as much as possible. This was done at RT because epoxide groups are reactive with the hydroxyl groups of the HFIP at higher temperatures. As the mixture was solid it was removed from the flask and ground in liquid nitrogen. After that it was left under reduced pressure at RT for 4 days. Finally the totally dry mixture was placed in a Schlenk reactor and a solid‐state step‐growth reaction was started at various temperatures under an argon atmosphere. Figure 3.7 Chemical structure of poly(propylene glycol) diglycidyl ether (PPGE). Molecular weights and PDI values calculated from SEC measurements performed throughout the reactions, which were performed at 70 °C and 80 °C with different catalyst amounts, are shown in Figure 3.8. It is seen that at the highest temperature with the highest catalyst content (80 °C, 1.35 wt% catalyst) the fastest built‐up in molecular weight is obtained within the first 72 hours. During the rest of the reaction time molecular weight almost levels off while for reactions at 70 °C the Mn values keep increasing. However, even after such long reaction times, the highest maximum achieved Mn values were only around 10 kg/mol. Furthermore, a decrease in the Mn of the reaction at 80 °C with 0.45 wt% DMAP is observed, which is most probably due to degradation. This degradation might be due to acidolysis of the ester groups which are formed after the reaction of epoxide‐carboxyl end groups. 56
Chapter 3
a.
Mn SEC (g/mol)
11000
10000
9000
8000
70 °C, 0.45 wt% DMAP
70 °C, 0.90 wt% DMAP
60 °C, 1.35 wt% DMAP
80 °C, 0.45 wt% DMAP
80 °C, 0.90 wt% DMAP
80 °C, 1.35 wt% DMAP
7000
6000
5000
4000
3000
0
20 40 60 80 100 120 140 160
Time (h)
PDI
b.
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
70 °C, 0.45 wt% DMAP
70 °C, 0.90 wt% DMAP
60 °C, 1.35 wt% DMAP
80 °C, 0.45 wt% DMAP
80 °C, 0.90 wt% DMAP
80 °C, 1.35 wt% DMAP
0
20 40 60 80 100 120 140 160
Time (h)
Figure 3.8 Molecular weight (a) and PDI developments (b) of PA6/PPGE reactions at 70 °C and 80 °C with different weight % of DMAP (weight% DMAP based on total weight). PPGE/PA6 reactions were also done at 100 °C and 120 °C with the same catalyst amounts as before. As the reaction temperature was increased higher molecular weights were obtained as shown in Figure 3.9.a. The highest molecular weight was obtained at a reaction temperature of 120 °C with 0.45 wt% DMAP catalsyt. In the case of this reaction Mn reached 16,600 g/mol after 18 hours and later decreased to 15,800 g/mol, which can again be a result of degradation. Reactions with higher amounts of DMAP at the same temperature yielded somewhat lower Mn values. It should also be mentioned that in every case the last sample recorded had a high PDI and it was very difficult to filter the samples when they were dissolved in HFIP for SEC analysis. That is why the reactions were not continued further. In the case of reactions performed at 100 °C molecular weight values 57
Chapter 3
obtained at the end of the reactions decreased with increasing catalyst amounts. It can also be observed in both figures that in general PDI values are increasing with increasing reaction time. Formation of insoluble parts points to extensive branching and crosslinking which will be discussed in more detail according to Figure 3.10. a.
18000
Mn SEC (g/mol)
16000
14000
12000
100 °C, 0.45 wt%
100 °C, 0.90 wt%
100 °C, 1.35 wt%
120 °C, 0.45 wt%
120 °C, 0.90 wt%
120 °C, 1.35 wt%
10000
8000
6000
4000
DMAP
DMAP
DMAP
DMAP
DMAP
DMAP
2000
0
10
20
30
40
50
60
70
Time (h)
b.
3.5
PDI
3.0
100 °C, 0.45 wt% DMAP
100 °C, 0.90 wt% DMAP
100 °C, 1.35 wt% DMAP
120 °C, 0.45 wt% DMAP
120 °C, 0.90 wt% DMAP
120 °C, 1.35 wt% DMAP
2.5
2.0
1.5
0
10
20
30
40
50
60
70
Time (h)
Figure 3.9 Molecular weight (a) and PDI developments (b) of PA6/PPGE reactions at 100 °C and 120 °C with different weight % of DMAP (weight% DMAP based on total weight). 58
Chapter 3
Table 3.2 Overview of PA6/PPGE reactions. PPGE PA6 PA6/PPGE‐1 PA6/PPGE‐2 PA6/PPGE‐3 PA6/PPGE‐4 PA6/PPGE‐5 PA6/PPGE‐6 PA6/PPGE‐7 PA6/PPGE‐8 PA6/PPGE‐9 PA6/PPGE‐10 PA6/PPGE‐11 PA6/PPGE‐12 DMAP
wt%* DMAP mol%** T (°C) Time (h) Mn (SEC) PDI ‐ ‐ ‐ ‐ 120 120 72 72 1,100 3,500 1.2 1.6 0.45 0.90 1.35 0.45 0.90 1.35 0.45 0.90 1.35 0.45 0.90 1.35 5.2 10.5 15.7 5.2 10.5 15.7 5.2 10.5 15.7 5.2 10.5 15.2 70 70 70 80 80 80 100 100 100 120 120 120 168 168 168 168 168 168 72 72 72 24 24 24 10,200 10,300 10,200 8,200 10,400 9,300 14,600 12,000 10,600 15,800*** 12,000 13,300 2.4 2.3 1.9 3.0 2.0 2.1 2.8 2.8 2.3 3.5 2.6 2.8 *wt% DMAP is with respect to the total weight of PA6 and PPGE. **mol% DMAP is with respect to the total molar amount of PA6 and PPGE. ***After 18 h Mn=16,600 g/mol. An overview of all the reactions with feed ratios, final molecular weights, PDI and reaction times are shown in Table 3.2. In the first two rows Mn and PDI values of the PA6 and PPGE are presented which were heated at 120 °C for 72 hours. No self‐condensation reactions were observed as the same Mn and PDI values were obtained as those measured for the neat PA6 and PPGE. One should take into account that the Mn, Mw and PDI values of all the copolymers were determined for the HFIP‐soluble part of the multiblock copolymer. It was reported in literature that epoxide‐carboxyl reactions are not very straightforward and side reactions can occur easily.9, 10, 13 A reaction scheme is presented in Figure 3.10. The first reaction shown is the primary reaction of epoxide‐carboxyl reactions. As the hydroxyl groups form they can react with acid end groups as in reaction (2). However, this reaction is suppressed in the case of base catalysis. Hydroxyl groups of the primary product can also react with the epoxide groups forming an ether compound (see reaction 3). This reaction is more favored if the epoxide groups are in excess. Finally, water that was formed in reaction (2) can react with epoxide groups (4). 59
Chapter 3
Figure 3.10 Possible reactions and side reactions of epoxide‐carboxyl end groups and intermediates. For GPE/PPA reactions in bulk the right conditions were found resulting in reasonably high yields. However it is observed here that much lower conversions are reached in case of PA6/PPGE reactions. First of all, the reactivity of the end groups is significantly lower in solid‐state reactions compared to the bulk reactions of GPE/PPA which means that longer reaction times, higher reaction temperatures and/or higher amounts of catalysts are required. It can be seen from the previous reaction graphs that at 70 °C and 80 °C molecular weights are low and do not increase upon elongation of the reaction time. This as a result increases the possibility for side reactions, most likely the reaction of hydroxyl groups with unreacted epoxide rings, making it difficult to reach high molecular weight, linear multiblock copolymers as desired. DSC second heating and cooling traces of PA6, PPGE and PA6/PPGE‐10 were recorded and presented in Figure 3.11 and Figure 3.12 as well as in Table 3.3. PPGE has a Tg of ‐76.2 °C, but there is no melting endotherm as it is a totally amorphous oligomer. The heating of PPGE was only performed until 150 °C, since the thermal stability is quite low. Additionally, we were not able to go to lower temperatures during the DSC measurements, since ‐80 °C is the lowest temperature limit for the cooling device. 60
Chapter 3
It is not possible to determine the Tg of PA6 from DSC but the melting temperature is clearly observed at 205.5 °C with a small shoulder at a somewhat lower temperature. When the PA6/PPGE‐10 after 18 hours of reaction (Mn=16,600 g/mol) is investigated a glass transition is observed at 17.2 °C, whereas the melting of the PA6 blocks is observed at 202.4 °C. To check if this Tg value is obtained from the miscibility of the PA6 and PPGE blocks, a DCS measurement of the physical mixture of both components was performed. From this measurement no Tg value was detected. This result points to the fact that this Tg value is only related to PPGE blocks. The remarkable shift in Tg is related to the formation of hydroxyl and ester groups during the reaction of epoxide and carboxyl groups by which the flexibility of the PPGE chains is reduced.19, 20 The very small shift in the Tm indicates that the PA6 blocks stay intact. The slight decrease in Tm points to a slightly hindered crystallization and a reduction in crystal thickness due to the chemical connection of the oligoether blocks. When the first heating endotherms of PA6 and the copolymer are compared an increase is observed after the reaction which might be due to the annealing step at 120 °C. On the other hand, a lower melting enthalpy during the second heating is observed as well as a decrease in the crystallization enthalpy, which means that there is a decrease in the degree of crystallinity of the PA6 although high Tm and Tc values are retained. Additionally, although the thermal stability of PPGE oligomer is very low, a highly thermally stable copolymer was obtained with 5 wt% degradation at 298.2 °C which is quite close to the thermal stability of PA6 itself. The observations of the enhanced Tg of the PPGE block and the somewhat lowered Tm of the PA6 and the high thermal stability together with an increase in molecular weight up to 17 kg/mol strongly point to multiblock copolymer formation. 61
Heat flow (W/g) Endo down
Chapter 3
-76.2 C
PPGE
PA6
17.2 C
205.5 C
PA6/PPGE
202.4 C
-80
-40
0
40
80
120
160
200
Temperature (C)
Figure 3.11 2nd heating DSC traces of PPGE, PA6 and PA6/PPGE‐10 after 18 hours of reaction time of COOH‐terminated PA6 and PPGE. Heat flow (W/g) Endo down
166.1 C
PPGE
PA6
170.5 C
PA6/PPGE
-40
0
40
80
120
160
Temperature (C)
200
Figure 3.12 Cooling DSC traces of PPGE, PA6 and PA6/PPGE‐10 after 18 hours of reaction time of COOH‐terminated PA6 and PPGE. 62
Chapter 3
Table 3.3 Thermal properties of starting components (PPGE, PA6) and the multiblock copolymer (PA6/PPGE‐10) determined by DSC. T5% (°C) Tg (°C) Tm1 (°C) ∆Hm1 (J/g) Tc (°C) ∆Hc (J/g) Tm2 (°C) ∆Hm2 (J/g) PPGE 140.5 ‐76.2 – – – – – – PA6 PA6/PPGE‐10* 340.2
298.2
–
17.2
201.2
206.6
83.0
99.0
166.1
170.5
77.3
53.0
205.5 202.4 71.4 50.8 *After 18 hours of reaction (Mn=16,600 g/mol). 3.3.3 Diepoxy propylene adipate (DEPA) and PA6 reactions After reaching a molecular weight (Mn) of almost 17 kg/mol with PA6/PPGE reactions PPGE was replaced with DEPA in order to obtain multiblock copolymers of PA6/DEPA of which the polyester block should be biodegradable. A first set of reactions was performed at 80, 100, 120 and 140 °C, either with 0.45 or 0.90 wt% DMAP addition. Except two runs, all the reactions were performed with a 1.05/1 mol/mol PA6/DEPA molar ratio to make sure that epoxide end groups are not in excess in order to limit the epoxide‐hydroxyl side reaction mentioned in Figure 3.10. The molecular weight and PDI data from this first set of reactions are collected in Figure 3.13. It can be clearly observed that with increasing reaction temperature the reaction rate significantly increases (compare the initial slopes of the curves). The final Mn values for all the reactions vary from 8 to 10 kg/mol. It was not possible to reach higher molecular weights, neither by raising the temperature nor by reacting for a longer time. The results in Figure 3.13.b demonstrate that the PDI values are increasing throughout the reactions and even approaching 5 in the case of 140 °C only after 2 hours of reaction time. 63
Chapter 3
a.
10000
Mn SEC (g/mol)
9000
80 °C, 0.45 wt% DMAP
80 °C, 0.90 wt% DMAP
100 °C, 0.45 wt% DMAP
100 °C, 0.90 wt% DMAP
120 °C, 0.45 wt% DMAP
120 °C, 0.90 wt% DMAP
140 °C, 0.45 wt% DMAP
8000
7000
6000
5000
4000
3000
2000
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Time (h)
b.
5.0
4.5
80 °C, 0.45 wt% DMAP
80 °C, 0.90 wt% DMAP
100 °C, 0.45 wt% DMAP
100 °C, 0.90 wt% DMAP
120 °C, 0.45 wt% DMAP
120 °C, 0.90 wt% DMAP
140 °C, 0.45 wt% DMAP
PDI
4.0
3.5
3.0
2.5
2.0
1.5
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Time (h)
Figure 3.13 Molecular weight (a) and PDI developments (b) of PA6/DEPA reactions at 80‐
140 °C with different weight % of DMAP catalyst (weight % DMAP based on total weight). To check whether higher molecular weights could be obtained, TEA was used to catalyze the reactions and reactions without any catalyst were performed as well. These reactions were performed in the temperature range 80‐120 °C (Figure 3.14). It is interesting to observe that all the reactions at 80 °C reach almost the same molecular weight regardless of the presence of a catalyst and its concentration. At 100 °C, the final molecular weight reached increases when the amount of catalyst (TEA) is increased from 0 to 2 wt%. However crosslinking occurs rapidly with 5 wt% TEA at the same temperature. The highest molecular weight (10 kg/mol) was obtained without any catalyst addition at 120 °C. This means that these reactions can already occur without the presence of catalyst.9 PDI values 64
Chapter 3
are increase with increasing reaction time like in the previous set of DEPA reactions where a more rapid increase is observed during the reactions at 100 and 120 °C. The reaction at 100 °C with 5 wt% TEA addition reaches a PDI of 8.6 (also shown in Table 3.4) after 6 hours of reaction time. 11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
Mn SEC (g/mol)
a.
80 °C, no cat.
80 °C, 2 wt% TEA
80 °C, 5 wt% TEA
100 °C, no cat.
100 °C, 2 wt% TEA
100 °C, 5 wt% TEA
120 °C, no cat.
0
5 10 15 20 25 30 35 40 45 50
Time (h)
b.
9
8
80 °C, no cat.
80 °C, 2 wt% TEA
80 °C, 5 wt% TEA
100 °C, no cat.
100 °C, 2 wt% TEA
100 °C, 5 wt% TEA
120 °C, no cat.
PDI
7
6
5
4
3
2
1
0
5
10 15 20 25 30 35 40 45 50
Time (h)
Figure 3.14 Molecular weight (a) and PDI developments (b) of PA6/DEPA reactions at 80‐
120 °C with different weight % DMAP catalyst (weight % DMAP based on total weight).
65
Chapter 3
Table 3.4 Overview of PA6/DEPA reactions. PA6/DEPA mol/mol Cat. DEPA ‐ PA6 PA6/DEPA‐1 Cat. Cat. wt%* mol%** T
(°C) Time (h) Mn (SEC) PDI ‐ 120 72 1,100 1.3 ‐ ‐ 120 72 3,500 1.6 1.05 DMAP 0.45 5.1 80 64 7,900 2.3 PA6/DEPA‐2 1.05 DMAP 0.90 10.2 80 64 7,800 3.5 PA6/DEPA‐3 1.05 DMAP 0.45 5.1 100 48 9,800 3.0 PA6/DEPA‐4 1.05 DMAP 0.90 10.2 100 10 8,300 3.4 PA6/DEPA‐5 1.05 DMAP 0.45 5.1 120 6 8,500 3.4 PA6/DEPA‐6 PA6/DEPA‐7 1.05 1.05 DMAP DMAP 0.90 0.45 10.2 5.1 120 140 2 2 8,000 9,200 3.1 4.8 PA6/DEPA‐8 1.05 – – – 80 48 7,400 3.3 PA6/DEPA‐9 1.05 TEA 2 17 80 48 7,800 3.1 PA6/DEPA‐10 1.05 TEA 5 42 80 48 7,800 3.5 PA6/DEPA‐11 1.05 – – – 100 24 7,500 6.4 PA6/DEPA‐12 1.05 TEA 2 17 100 18 8,800 3.2 PA6/DEPA‐13 PA6/DEPA‐14 1.05 1.05 TEA – 5 – 42 – 100 120 6 24 8,000 10,300 8.6 5.8 PA6/DEPA‐15 PA6/DEPA‐16 1.0 1.0 DMAP TBA 0.45 2 5.1 14.3 100 100 52 52 7,700 7,500 7.3 4.0 *wt% cat. is with respect to the total weight of PA6 and DEPA. **mol% cat. is with respect to the total molar amount of PA6 and DEPA. An overview of all the reactions before crosslinking was observed is presented in Table 3.4. In the first two rows Mn and PDI values of the PA6 and DEPA are presented which were heated at 120 °C for 72 hours. No self‐condensation reactions were observed as the same Mn and PDI values were obtained as for the neat PA6 and DEPA. After these reaction times, it was not possible to dissolve the polymers in HFIP anymore. PDI values mainly vary from 3 to 9, but like Mw might be under‐estimated, because insoluble parts of the copolymer are not taken into account. Two other reactions were also performed, namely one with tributylamine (TBA) as catalyst, which is more difficult to evaporate at the reaction temperatures used (b.p.=214 °C), and another one with DMAP where the molar ratio of PA6/DEPA was 1/1. These reactions were performed at 100 °C and it was possible to reach only an Mn of almost 8 kg/mol for both reactions. 66
Chapter 3
A
C
c
+
B
+
D
E
h
0
2
1
A
P
E
D
/
6
A
P
e
d
+
b
a
a
h
2
1
2
1
A
P
E
D
/
6
A
P
m
p
p
3.00
2.50
2.00
1.50
Figure 3.15 1H NMR spectrum and structural assignments of the PA6 and DEPA blocks as recorded in a TFE:CDCl3 1:1 mixture. Although the same reaction conditions as in the case of PA6/PPGE reactions were used it was not possible to reach such high molecular weights. The main difference between the two types of reactions is the presence of ester groups in DEPA. These ester groups after splitting by acidolysis of the COOH groups of the PA6 might have an accelerating effect on the formation of crosslinks. As a consequence higher PDI values in comparison with the previous PA6/PPGE reactions are observed. In these additional crosslinking reactions 67
Chapter 3
possible low molecular weight and mobile acidolysis products of the dicarboxylic acid type might play a role. The NMR spectra of PA6/DEPA‐12 reaction at the start and after 12 hours of reaction time (Mn=9,200 g/mol) are shown in Figure 3.15 with the chemical structures of PA6, DEPA and the copolymer together with the structural assignments. After the reaction the methylene group of the epoxide ring (a) has otally vanished. However the chemical shifts of (b), (d) and (e) of the DEPA as well as of the PA6 are still observed at the same chemical shifts as before. It can be concluded from these spectra that all the epoxide‐terminated DEPA has reacted with the COOH‐terminated PA6. DSC analyses of DEPA and PA6/DEPA‐10 copolymer were compared with the previously analyzed PA6 (Figure 3.16). DEPA is also a totally amorphous oligomer like PPGE and has a Tg at ‐68.9 °C. Heating of DEPA was performed up to maximum 150 °C, since its thermal stability is low. The Tm of PA6 is at 205.5 °C as mentioned in the previous section. After the copolymer formation the Tm value is totally retained, whereas Tg value shifts to 4.9 °C. This shift in Tg can again be explained by the restricted movement of DEPA chain segments due to the chemically attached PA6 blocks. In comparison to neat PA6 a lower melting enthalpy during the second heating is observed as well as a decrease in the crystallization enthalpy, which means that there is a decrease in the degree of crystallinity of the PA6 as mentioned before, but nevertheless high Tm and Tc values are retained. The thermal stability of the copolymer is very close to the thermal stability of the neat PA6. In conclusion, SEC, NMR and DSC data point to a segmented structure where a few blocks of PA6 and DEPA are present. Although molecular weights only up to 10 kg/mol can be obtained these copolymers can be used for some special applications in e.g. the coatings area and should be partially degradable due to the ester linkages. 68
Chapter 3
Heat flow (W/g) Endo down
-68.9 C
DEPA
PA6
4.9 C
205.5 C
PA6/DEPA
205.0 C
-80
-40
0
40
80
120
160
200
Temperature (C)
Figure 3.16 2nd heating DSC traces of DEPA, PA6 and PA6/DEPA‐12 after 12 hours of Heat flow (W/g) Endo down
reaction time of COOH‐terminated PA6 and DEPA. DEPA
166.1 C
PA6
169.4 C
PA6/DEPA
-40
0
40
80
120
160
Temperature (C)
200
Figure 3.17 Cooling DSC traces of DEPA, PA6 and PA6/DEPA‐12 after 12 hours of reaction time of COOH‐terminated PA6 and DEPA. 69
Chapter 3
Table 3.5 Thermal properties of starting components (DEPA, PA6) and the copolymer (PA6/DEPA‐12) determined by DSC. T5%
Tg
Tm1 ∆Hm1 Tc ∆Hc Tm2 ∆Hm2 (°C) (°C) (°C) (J/g) (°C) (J/g) (°C) (J/g) DEPA 206.5 ‐68.9 – – – – – – PA6 340.2
–
201.2
82.9
166.1
77.3
205.5 71.4 PA6/DEPA‐12*
325.3
4.9
206.0
82.5
169.4
58.4
205.0 60.8 *After 12 hours of reaction. 3.4 Conclusions Fully carboxylic acid end‐capped low molecular weight PA6 polymer was successfully prepared by ring opening and polycondensation polymerization of ‐caprolactam with the addition of 5 wt% adipic acid and water as initiator. The purpose was then to react these PA6 telechelics with an epoxide‐terminated, hydrolyzable polyester, to obtain a PA6‐like but at least partially degradable block copolymer. To understand the nature of the epoxide‐carboxyl reactions first model reactions were performed with gylcidyl phenyl ether and propanoic acid and for which maximum conversion of 85% was achieved. A second series of model reactions was performed with carboxyl‐terminated PA6 and poly(propylene glycol) diglycidyl ether with the addition of DMAP as catalyst in different amounts and in the temperature range of 70‐120 °C. These reactions were carried out in the solid state after solution mixing in a common solvent. A high reaction temperature together with low catalyst amount yielded the highest achievable number average molecular weight product. However, side reactions resulted in crosslinking making it difficult to reach very high molecular weights. DSC measurements revealed an enhanced Tg of the PPGE block and a slightly lowered Tm of the PA6. The high melting temperature together with an increase in molecular weight up to 17 kg/mol strongly point to a multiblock copolymer formation. Finally, PA6‐diepoxy propylene adipate reactions with COOH‐terminated PA6 telechelics were performed with or without the addition of the catalysts DMAP or TEA. Various amounts of catalyst were used in a temperature range of 70
Chapter 3
80‐140 °C. Lower molecular weights were obtained as compared to PA6/PPGE reactions, most probably due to the additional side reactions with the ester groups of the DEPA. On the other hand, high Tm and Tc values of PA6 were retained. The melting temperature of the copolymer is very close to the melting temperature of the neat PA6. In conclusion, SEC, NMR and DSC data point to a blocky structure upon reacting carboxylic acid‐
terminated PA6 with epoxide‐terminated propylene adipate, where a few blocks of PA6 and DEPA are present. These copolymers might be suitable for certain applications where the enhanced degradability is of additional value for PA6. References 1. Drake, R. S.; Egan, D. R.; Murphy, W. T. ACS Symp. Ser. 1983, 221, 1‐20. 2. Steinmann, B. Polym. Bull. 1989, 22, (5‐6), 637‐644. 3. Ambrogi, V.; Giamberini, M.; Cerruti, P.; Pucci, P.; Menna, N.; Mascolo, R.; Carfagna, C. Polymer 2005, 46, (7), 2105‐2121. 4. Soucek, M. D.; Abu‐Shanab, O. L.; Anderson, C. D.; Wu, S. Macromol. Chem. Phys. 1998, 199, (6), 1035‐1042. 5. Bucknall, C. B.; Partridge, I. K. Polym. Eng. Sci. 1986, 26, (1), 54‐62. 6. Ratna, D. Polymer 2001, 42, (9), 4209‐4218. 7. Pearson, R. A.; Yee, A. F. J. Mater. Sci. 1989, 24, (7), 2571‐2580. 8. Alvey, F. B. J. Polym. Sci., Part A: Polym. Chem. 1969, 7, 2117‐2124. 9. Blank, W. J.; He, Z. A.; Picci, M. J. Coat. Technol. 2002, 74, (926), 33‐41. 10. Chalykh, A. E.; Zhavoronok, E. S.; Kochnova, Z. A.; Kiselev, M. R. Russ. J. Phys. Chem. B 2009, 3, (3), 507‐511. 11. Davy, K. W. M.; Kalachandra, S.; Pandain, M. S.; Braden, M. Biomaterials 1998, 19, (22), 2007‐2014. 12. Matejka, L.; Dusek, K. Polym. Bull. 1986, 15, (3), 215‐221. 13. Matejka, L.; Pokorny, S.; Dusek, K. Polym. Bull. 1982, 7, (2‐3), 123‐128. 14. Patel, B. K.; Patel, H. S. Polym. Plast. Technol. Eng. 2009, 48, (9), 966‐969. 15. Patel, H. S.; Patel, B. K. Int. J. Polymer. Mater. 2009, 58, (12), 654‐664. 16. Sheela, M. S.; Selvy, K. T.; Krishnan, V. K.; Pal, S. N. J. Appl. Polym. Sci. 1991, 42, (3), 561‐573. 17. Yeniad, B.; Albayrak, A. Z.; Olcum, N. C.; Avci, D. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, (6), 2290‐2299. 18. Eriksson, M.; Fogelstrom, L.; Hult, K.; Malmstrom, E.; Johansson, M.; Trey, S.; Martinelle, M. Biomacromolecules 2009, 10, (11), 3108‐3113. 19. Nielsen, L. E. J. Macromol. Sci., Rev. Macromol. Chem. 1969, C 3, (1), 69. 20. Hirose, S.; Hatakeyama, T.; Hatakeyama, H. Thermochim. Acta 2005, 431, (1‐2), 76‐80. 71
Chapter 3
72
CHAPTER 4 INCORPORATION OF A SEMI‐AROMATIC NYLON SALT INTO POLYAMIDE 6 BY EITHER SOLID STATE OR MELT POLYMERIZATION Summary The nylon salt of 1,5‐diamino‐2‐methylpentane (Dytek A) and isophthalic acid (IPA) was incorporated into the PA6 backbone via solid‐state polymerization (SSP) and melt polymerization (MP). It was shown that the incorporation of the salt already occurred 20 °C below the melting temperature of the PA6. Molecular characterization was done by SEC, 1H NMR and titration analysis. 13C NMR sequence distribution analysis together with detailed DSC analysis strongly pointed to a blocky microstructure after SSP and to a fully random microstructure after MP. 73
Chapter 4
4.1 Introduction PA6 has outstanding properties due to its high chain regularity and strong H‐bonding.1, 2 It is possible to adapt the properties of PA6 by melt modification with different monomers or polymers. However, melt polymerization usually results in random copolymers and thus in a deterioration of the crystallization behavior and in a deformation of the crystalline phase. This means a decrease in melting temperature, crystallization rate and the degree of crystallinity, leading to undesired change of mechanical and physical properties.3 Therefore, it is important to find an alternative modification method to make copolymers based on PA6 which would not only have a high crystallinity and a non‐random distribution but improved material properties as well. Very recently, Novitsky et al.4 prepared polyamide 6‐polyamide 12,T copolymers by anionic polymerization exhibiting a blocky microstructure. Another method for making non‐random step‐growth polymers is the solid‐state polymerization (SSP) which is traditionally used as a postcondensation technique to increase the molecular weight of polycondensates.5, 6 During SSP the temperature is set just below the melting temperature (Tm) of the polymer. This makes the chains in the amorphous phase mobile enough to undergo further polycondensation and/or modification by transreactions, while the well‐ordered chain fragments present in the crystals remain intact and do not participate in these interchange reactions. James et al.7, 8 were the first to show that block copolymers can be obtained in the solid state by blending poly(ethylene terephthalate) and poly(ethylene naphthalene). Later, Jansen et al.9‐13 and Sablong et. al.14, 15 studied the incorporation of different types of diol monomers into poly(butylene terephthalate) by SSP. Jansen and coworkers showed that copolyesters with non‐random distributions and high molecular weights were obtained after short SSP reaction times. Moreover they made comparisons with melt‐polymerized samples. They investigated the kinetics, the sequence distribution (characterized by the degree of randomness R) and morphology of the copolymers in detail. In view of its semi‐crystalline character and the fact that it is a step‐growth polymer, PA6 can also be modified in the same manner in the solid state. In this study, a nylon salt of 74
Chapter 4
1,5‐diamino‐2‐methylpentane (Dytek A) and isophthalic acid (IPA) was prepared and incorporated into PA6 in different weight percentages. This was done via a two‐step process. First, the salt and PA6 were solution mixed in a common solvent which was hexafluoroisopropanol (HFIP). After the removal of the solvent, the temperature was raised to 20 °C below the Tm of the PA6 which is high enough to force transamidation reactions. The microstructure of polyamides has been studied by solution 13C NMR by several researchers.16‐18 In addition to this, it has been shown that 13C NMR is a powerful tool for sequence distribution analysis by investigating the possible dyad or triad structures formed after copolymerization.19‐26 If the right parameters are used, it is a promising technique for quantitative analysis just like 1H NMR spectroscopy. Block lengths and degrees of randomness can be calculated using this method. Different types of deuterated solvents and solvent mixtures have been used for polyamide analysis. Lately, Novitsky et al.4 showed the possibility of quantitative 13C NMR measurements by preparing a highly concentrated polyamide sample in a HFIP/CDCl3 3/1 vol/vol mixture. In this chapter, we used the same solvent mixture to compare the microstructures of the copolyamides prepared by SSP and melt polymerization (MP) by calculating the block lengths and degrees of randomness. In addition to titration measurements, changes in the molecular weight distributions were monitored in time, while the incorporation of Dytek and IPA into the copolyamide was monitored separately. Detailed thermal analysis also revealed the differences in the thermal properties, related to different monomer sequence distributions of the copolyamides prepared by SSP and MP. 4.2 Experimental 4.2.1 Materials Commercial grade polyamide 6 (PA6) pellets (Mn=21 kg/mol, measured using amine‐ and acid‐ specific potentiometric titration methods) and ε‐caprolactam (CL) were kindly provided by DSM (Geleen, The Netherlands) and used as received after drying. 1,5‐
75
Chapter 4
diamino‐2‐methylpentane (Dytek A, Aldrich) and isophthalic acid (IPA, Aldrich) were used as received for the salt preparation. 6‐Aminocaproic acid (ACA) was purchased from Aldrich and dried before use. Irganox 1330 was available from Ciba Speciality Chemicals and used as an antioxidant. 1,1,1,3,3,3‐Hexafluoro‐2‐propanol (HFIP, 99 %) and ethanol were obtained from Biosolve. The deuterated NMR solvents chloroform (CDCl3, 99.8%), dimethyl sulfoxide (DMSO, 99.9%) and deuterium oxide (D2O, 99.9%) were purchased from Cambridge Isotope Laboratory, Inc. (CIL). 4.2.2 Dytek A‐isophthalic acid salt preparation To a mixture of isophthalic acid (66.5 g, 0.4 mol) in ethanol (150 mL) at 80 °C a solution of Dytek A (46.5 g, 0.4 mol) in ethanol (120 mL) was added dropwise. Precipitation started during the addition of the Dytek A solution. After all the Dytek A was added, stirring was continued for 1.5 h. The white precipitate was filtered, recrystallized from an ethanol/water mixture (10:1, v/v) and dried at 80 °C in an oven under vacuum. 4.2.3 Solution mixing of PA6/Dytek A‐IPA nylon salt in HFIP PA6 pellets were ground into powder by a mill (IKA, A11B) after cooling with liquid nitrogen and dried in a vacuum oven at 80 °C. Dried PA6 and nylon salt (5‐30 wt%, 4‐25.6 mol% with respect to the total amount of feed, see Table 4.1 and Table 4.4) were mixed together in a three‐neck round bottom flask under argon with a minimum amount of HFIP at 55 °C. After the full dissolution of both components HFIP was removed by applying a reduced pressure below 10 mbar. The lump of material was removed from the flask and ground to a powder in the mill mentioned earlier. The fine powder obtained after sieving was dried in a vacuum oven at 80 °C. 4.2.4 Solid‐state polymerization (SSP) Low molecular weight PA6/Dytek A‐IPA (DyI) salt copolyamides were synthesized by placing the polymer/salt mixture, which was prepared by solution mixing as described above, in a glass tube reactor. Salt weight compositions of 5, 10, 15, 20, 25 and 30 wt% 76
Chapter 4
were used with respect to the total amount of feed. The PA6/DyI powder was deposited on the sintered glass plate at the bottom of the reactor (diameter=2.5 cm) and covered with glass beads. Inert gas was introduced below this glass plate through a glass coil surrounding the reactor. The gas was heated while passing through this coil before entering the reactor. To reach the desired reaction temperature of 200 °C a salt bath consisting of KNO3 (53 wt%), NaNO2 (40 wt%) and NaNO3 (7 wt%) was used.9‐15 Samples were taken from the reactor until it reached the reaction time of 24 hours. The synthesized low molecular weight SSP copolyamides will be abbreviated as (CL/DyIx)S1 where x denotes the feed weight percentage of the DyI salt and S1 denotes the SSP reaction. Figure 4.1 Schematic representation of the set up used for solid‐state polymerization. For the synthesis of high molecular weight PA6/DyI copolyamides the following procedure was used. At the first stage of the solid‐state polymerization the reaction was conducted in a closed glass vial (r=1 cm, h=7.5 cm) which was pressurized with inert gas to avoid the evaporation of the diamine. After the total incorporation of the diamine (followed by SEC), the polymer‐salt mixture was transferred to a glass tube reactor as described above. SSP was performed in the vial at 180 °C for 6, 12 or 14 hours depending on the DyI salt content in the mixture (10, 20, 30 wt%, respectively) and was later continued in the reactor at 200 °C until a total reaction time of 24 hours. Another CL/DyI mixture with a 4:1 77
Chapter 4
weight ratio was prepared with the addition of 0.8 wt% excess Dytek A with respect to the total amount of the CL/DyI mixture during the solution mixing. Abbreviation for these copolyamides will be (CL/DyIx)S2 where x denotes the feed weight percentage of the DyI salt and S2 denotes the SSP resulting in high molecular weight copolyamides. The product resulting from the excess Dytek A addition is denoted as (CL/DyIEx.)S2 and this reaction was performed for 48 hours to reach the desired molecular weight. 4.2.5 Melt Polymerizations (MP) 4.2.5.1 Caprolactam (CL)/Dytek A‐IPA (DyI) salt Preparations of CL/DyI copolyamides via melt polymerization (MP) were performed in 100 mL cylindrical flasks connected to a condenser and placed in a heating carousel. During the first set of copolyamides prepared by MP, flasks were filled with 5, 10, 15, 20 and 25 wt% DyI salt according to the total amount and with CL (see Table 4.1 and Table 4.4). For the second set of reactions 10, 20 and 30 wt% DyI salt were used with an addition of 0.6, 1 and 0.6 wt% Dytek A diamine, respectively. For all MP reactions 6‐Aminocaproic acid (ACA) was added as the initiator. An excess Dytek A in the second set of experiments was used to compensate for the diamine loss during the polymerization under a nitrogen flow. Before starting the reactions, firstly vacuum was applied for at least 2 hours, until it was below 10 mbar to ensure that all O2 was removed. Then, the temperature was raised to 265 °C under a constant nitrogen flow. The reaction was carried out overnight with magnetic stirring. The product was removed from the flask, ground in liquid N2, extracted with demineralized water at 100 °C for at least 4 hours and filtered. Finally, it was dried under reduced pressure at 80 °C for 1 day. For the 1st set of MP reactions the synthesized MP copolyamides will be abbreviated as (CL/DyIx)M1, where x denotes the feed weight fraction of the DyI salt and M1 denotes the melt polymerization. For the 2nd set of MP polymerizations copolyamides will be abbreviated as (CL/DyIx)M2. 78
Chapter 4
4.2.5.2 Dytek A‐IPA homopolymer 4 g Dytek A‐IPA salt, 0.2 g excess Dytek A and 0.12 g Irganox (1330 type stabilizer) were charged to a three neck round bottom flask equipped with a mechanical stirrer, a vigreux column and a condenser. A continuous argon flow was applied throughout the reaction flask. The temperature was gradually raised to 290 °C and was kept at this temperature for 3 hours. Later, vacuum (4‐6 mbar) was applied for 2 hours to remove the reaction product water and increase the molecular weight. The product was removed from the flask and ground into a fine powder. 4.2.6 Characterization 4.2.6.1 Size Exclusion Chromatography (SEC) SEC was used to determine molecular weights and molecular weight distributions, Mw/Mn, of the polymer samples. The system was equipped with a Waters 1515 Isocratic HPLC pump, a Waters 2707 autosampler, a Waters 2487 dual absorbance UV detector, a Waters 2414 refractive index detector (35 °C) and a PSS PFG guard column followed by 2 PFG‐
linear‐XL (7 µm, 8*300 mm) columns in series. The temperature was 40 °C. Hexafluoroisopropanol with potassium trifluoroacetate (3 g/L) was used as eluent at a flow rate of 0.8 mL/min. Toluene was used as the internal standard. The molecular weights were calculated with respect to poly(methyl methacrylate) standards (Polymer Laboratories, Mp = 1020 g/mol up to Mp = 1.9*106 g/mol). 4.2.6.2 Differential Scanning Calorimetry (DSC) Melting (Tm) and crystallization temperatures (Tc) as well as melting (∆Hm) and crystallization enthalpies (∆Hc) were measured using a TA Instruments Q100 calorimeter. For all the measurements 4‐6 mg samples and a heating rate of 10°C/min were used under N2 atmosphere. DSC measurements were carried out from 0 to 240 °C. During each measurement, samples were equilibrated at 0 °C and 240 °C for 5 minutes. For the determination of both Tm and Tc peak maximums were taken into account. 79
Chapter 4
4.2.6.3 Nuclear Magnetic Resonance (NMR) Spectroscopy NMR samples were prepared by following the sample preparation procedure described by Novitsky et al.4 10 wt% polymer was dissolved in a 3/1 vol/vol hexafluoroisopropanol: CDCl3 mixture. 1H NMR spectra of the polymers were recorded on a Varian 400 MHz spectrometer at 25 °C. Quantitative solution 13C NMR spectra were recorded on a Varian Unity Inova 400 MHz spectrometer at 25 °C. TMS was used as the internal standard. A 90° pulse width of 5.8 µs was used. 35,000 scans were acquired with a relaxation delay of 5 s and an acquisition time of 1.3 s. The data was zero‐filled to 128,000 datapoints and filtered using 1 Hz line broadening. Quantitative analysis of the peak integrals was done after baseline correction and deconvolution of the overlapping peaks using Varian VNMRJ 2.2d software. 13C NMR measurements of PA6 and DyI homopolymer were done with a delay time of 0 seconds and thus are not quantitative. 4.2.6.4 Potentiometric titration For the determination of amine [NH2] and carboxylic acid [COOH] end group concentration, potentiometric end group titrations were done at room temperature in non‐aqueous environment using phenolic solvents. Both blank and sample measurements were repeated at least 3 times. Molecular weights were calculated by using the formula 2*106/([NH2]+[COOH]). 4.3 Results and Discussion Preparation of caprolactam (CL)/Dytek A‐IPA (DyI) copolyamides was done both by solid‐
state polymerization (SSP) and melt polymerization (MP). The nylon salt was synthesized from 1,5‐diamino‐2‐methylpentane (Dytek A) and isophthalic acid (IPA) as described in the experimental section 4.2.2. Dytek A is an isomer of hexamethylene diamine and contains one asymmetric carbon atom (carbon (b) in Figure 4.2.). It was chosen together with IPA for the incorporation into PA6 since it was presumed that Dytek A‐IPA salt could easily be distributed in the amorphous phase but not readily in the crystalline phase due to its chemical structure which strongly deviates from that of PA6 (see Chapter 5). 80
Chapter 4
Figure 4.2 Chemical structure of the DyI nylon salt with the labels used for the 1H NMR interpretation (see Figure 4.3). 1
H NMR was performed to investigate the structure of the DyI salt. All the expected chemical shifts of the Dytek A and IPA after the salt formation are shown in Figure 4.3. The molar ratio of Dytek A to IPA was calculated to be 1 by using the integral peak areas. SSP was performed to force the incorporation of the salt into the PA6 backbone by aminolysis, acidolysis and amidolysis reactions.27‐30 On the other hand, MP was used for the copolymerization of ε‐caprolactam and the salt in the melt state. The overall chemical structure of the expected copolyamides is shown in Figure 4.4. c
O
2
D
O
S
M
D
h
f
+
a
i
g
b
e
+
d
ppm
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
Figure 4.3 1H NMR of Dytek A‐IPA nylon salt performed in D20/DMSO 1/1 vol/vol mixture (see Figure 4.1 for the labels). 81
Chapter 4
Figure 4.4 Chemical structure of PA6/Dytek A‐IPA copolyamides synthesized by SSP and MP. Two sets of SSP reactions were performed following two different synthetic routes. For both sets similar solution mixing, grinding and drying steps were used for the preparation of the desired PA6/Dytek A‐IPA (DyI) mixtures. In the first synthetic route, these mixtures prepared with different weight percentages of DyI salt (see Table 4.1), were directly put in a SSP reactor at a reaction temperature of 200 °C. Due to the volatility of Dytek A at high temperatures (b.p.=193 °C) under a continuous flow of argon, evaporation of diamine occurred leading to a stoichiometric imbalance. This resulted in low molecular weight copolyamides. For comparison, copolyamides by MP were also prepared. Characteristics of all the copolyamides synthesized by SSP and MP can be seen in Table 4.1 (shown as S1 and M1). In the second set of SSP reactions, PA6/DyI mixtures were first brought into an inert gas‐
pressurized closed glass vial at 180 °C to minimize the loss of diamine. After the expected partial incorporation of Dytek A‐IPA salt into PA6, samples were transferred to the SSP reactor to complete the incorporation of the salt at 200 °C. To compensate for the diamine loss during the MP reactions excess of Dytek A was used. Characteristics of these copolyamides can be seen in Table 4.4 (shown as S2 and M2). In the first set of experiments presented in this chapter low molecular weight PA6/DyI copolyamides, in the second set of experiments high molecular weight copolyamides, synthesized via SSP and MP reactions, will be discussed in detail. 82
Chapter 4
4.3.1 Low molecular weight PA6/Dytek A‐IPA copolyamides via SSP and MP 4.3.1.1 Molecular characterization of PA6/Dytek A‐IPA copolyamides by 1H NMR, SEC and titration All PA6/Dytek A‐IPA copolymers were characterized by 1H NMR spectroscopy. The 1H NMR spectrum of (CL/DyI20)S1 is shown in Figure 4.5 with the structural assignments. The composition of the copolymers was calculated by using the peak integrals of the corresponding residues. For the composition calculations peaks represented as (g), (h), (i) were used for IPA, while (a), (f), (c) were used for Dytek A and (A) was used for PA6 based on 1 proton integral value for each residue. Compositions of all the copolymers calculated according to 1H NMR are shown in Table 4.1. Although (A’) is not shown in the chemical structure of the copolyamide as given in Figure 4.5 it stands for the methylene group connected to the carboxyl end groups. This is just given as additional information and is not used for the calculations. It is also visible from this spectrum that there are some traces of unreacted IPA observed below a chemical shift of 8 ppm. a b d
N
H
e
N
H
j
O
g
h
E
N
H
k
h
A
C
D
B
O
A
CH3
c
O
f
i
E
C
+
B
+
D
+
b
+
e
+
d
k
P
I
F
H
P
I
F
H
6.0
5.0
4.0
3.0
c
m
p
p
7.0
'
A
f
+
a
j
i
h
g
8.0
2.0
1.0
Figure 4.5 1H NMR spectrum of (CL/DyI20)S1 copolyamide recorded in a 3/1 vol/vol HFIP/CDCl3 mixture. 83
Chapter 4
It can be seen from Table 4.1 that in general, with respect to the composition feed, slightly lower molar fractions of the DyI salt were obtained after SSP reactions. A higher difference is observed when 30 wt% salt was mixed with PA6. In case of MP reactions, an opposite trend is seen: copolyamides had a slightly higher molar fraction of salt incorporated compared to the corresponding feed fractions. In both cases the differences are within the experimental error of 1H NMR, with the exception of the (CL/DyI30)S1 sample. Table 4.1 Characteristics of neat PA6 and CL/DyI copolymers with different DyI feed compositions prepared by SSP and MP reactions. Composition feed (CL/DyI) Composition after SSP and MP (CL/DyI) by 1H NMR*** Mn (SEC) PDI wt%* mol%* mol% kg/mol PA6 PA6SSP** 100/0 100/0 100/0 100/0 ‐ ‐ 35 79 1.9 2.0 (CL/DyI5)S1 95/5 96/4 96/4 10 2.7 (CL/DyI10)S1 90/10 92/8 91/9 9.3 3.2 (CL/DyI15)S1 85/15 88/12 90/10 6.8 2.8 (CL/DyI20)S1 80/20 83/17 86/14 6.0 2.8 (CL/DyI25)S1 75/25 79/21 83/17 5.6 2.7 (CL/DyI30)S1 70/30 74/26 5.1 3.2 (CL/DyI5)M1 95/5 96/4 95/5 25 2.1 (CL/DyI10)M1 90/10 92/8 90/10 19 2.2 (CL/DyI15)M1 85/15 88/12 84/16 15 2.3 (CL/DyI20)M1 80/20 83/17 81/19 12 2.3 DyI HP 0/100
0/100
0/100
24 2.5 82/18 *wt% and mol% were calculated by using the molecular weights of CL and DyI salt units. **PA6SSP was obtained for comparison after dissolving in HFIP, removal of solvent followed by SSP at 200 °C for 24 hours. ***mol% DyI salt was calculated from the total amount of Dytek A and IPA present. 84
Chapter 4
Incorporation of the DyI salt into PA6 can be monitored by Size Exclusion Chromatogram (SEC). Incorporation of Dytek A and IPA can be monitored by RI and UV detectors, respectively. In Figure 4.6 changes in the RI and UV polymer and monomer peaks in the SEC for (CL/DyI20)S1 samples recorded during the reaction are shown. The PA6 peak and the Dytek A peak can be easily seen at elution times of 21.6 and 29.2 minutes, respectively, at the start of the SSP. Already after 15 minutes, the intensity of the Dytek A peak decreases while the polymer peak becomes a bit broader and shifts to higher elution times. At the end of the first full hour of SSP the Dytek A peak had almost disappeared and the polymer peak continued shifting to higher elution times. This is the result of PA6 chain scission by the attack of the DyI salt. As can also be seen in Figure 4.7 Mn is decreasing between 0‐8 hours of SSP and after that only a minor increase in Mn is observed. After chain scission obviously postcondensation does not occur to a significant extent due to the evaporation of Dytek A. This means that the disappearance of the Dytek A peak is not only caused by the incorporation of the diamine via aminolysis reactions, but is also because of the loss of diamine at the SSP temperature used (200 °C). It is also interesting to observe that after 10 hours of SSP PDI increases but later decreases again. This might point to some branching and crosslinking reactions which result in an increase of PDI. However, as the degree of branching increases further in time, these parts might be filtered out before the measurement leading to a decrease in PDI. Additionally, an increase in PDI was observed during SSP. This is because the crystalline part does not participate in the polycondensation reactions and the total copolyamide does not have a Flory distribution.31 In Figure 4.6.b SEC UV chromatograms of the samples taken at various reaction times are given. The wavelength of the UV detector was 275 nm. At the start of SSP the only UV active part is the IPA part of the salt, visible at an elution time of 32.3 minutes. After 15 minutes of reaction time a small peak appears at 29 minutes, most probably because IPA is at least partly incorporated in oligomers. Compared to the rate at which the Dytek A diamine peak disappears (see Figure 4.6.b) the disappearance of IPA occurs much slower and only after 8 hours a clear but broad polymer peak at an elution time of about 23 85
Chapter 4
minutes is visible. As IPA takes part slowly in the reaction, it is incorporated into the polymer chain, making it UV‐active. After 24 hours only 3 wt% IPA remains unreacted. The copolyamide peak containing DyI is clearly visible with a high intensity at an elution time of 25.5 minutes. a.
b.
IPA
24 h
24 h
16 h
16 h
Normalized UV SEC signal
Normalized RI SEC signal
Dytek
10 h
8h
6h
4h
2h
1h
0.5 h
10 h
8h
6h
4h
2h
1h
0.5 h
0.25 h
0.25 h
0h
0h
16
18
20
22
24
26
28
30
16
18
20
Elution Time (min)
22 24 26 28 30
Elution Time (min)
32
34
Figure 4.6 SEC chromatogramphs of (CL/DyI20)S1 samples at different reaction times recorded with RI detector (a) and UV detector (λ=275 nm) (b). SEC was also used to calculate the amount of IPA incorporated into the PA6 backbone by using the UV detection. For this purpose a calibration curve was made by measuring several known concentrations of the DyI salt in HFIP and by calculating the corresponding UV areas of the UV‐active IPA part of the salt (See Figure 4.8 and equation 4.1). After that, samples submitted to 0‐24 hour SSP reaction time were taken and prepared with known concentrations. From the SEC measurements of these samples with known concentrations it is possible to calculate the UV peak areas under the IPA curve and the polymer curve after the incorporation of IPA. Finally, by using the equation in the calibration curve (y=7.9708x) the unreacted and reacted amounts of IPA during the SSP reaction can be estimated qualitatively (Equation 4.2 and 4.3). 86
Chapter 4
35
4.0
Mn
30
PDI
3.5
3.0
20
PDI
Mn (SEC)
25
2.5
15
10
2.0
5
1.5
0
4
8
12
16
20
24
Time (h)
Figure 4.7 Number average molecular weight and PDI development of (CL/DyI20)S1 as a function of SSP reaction time. CIPA*MWDytek A = (CDYI‐CIPA)*MWIPA (4.1) CIPA,sample = AIPA,sample/7.9708 (4.2) (4.3) wt% IPA = CIPA,sample*100/Csample where CIPA and CDYI are the concentrations of IPA and DyI salt, respectively, and MWDytek A and MWIPA are the molecular weights of Dytek A and IPA, respectively. CIPA,sample is the concentration of IPA and AIPA,sample is the UV area of the IPA in the measured sample obtained from the SSP reaction. Csample is the total concentration of the measured sample. The estimated weight percentages of unreacted and reacted IPA during the SSP reaction of the (CL/DyI20)S1 copolyamide can be seen in Figure 4.9. It is very clearly observed that the wt% of unreacted IPA is rapidly decreasing during the first 10 hours of reaction time and almost totally disappears after 24 hours. The wt% reacted IPA is increasing during the reaction reaching a value of 12.4 wt% of DyI at the end of the reaction, which is slightly higher than the amount of IPA in feed (11.8 wt%, 8.3 mol%). 87
Chapter 4
16
y=7.9708x
12
-6
UV Area (*10 )
14
10
8
6
4
2
0
0.0
0.4
0.8
1.2
1.6
2.0
IPA concentration (mg/ml)
Figure 4.8 UV peak area of IPA versus IPA concentration in HFIP. 14
12
IPA (wt%)
10
IPA reacted
IPA unreacted
8
6
4
2
0
0
2
4
6
8 10 12 14 16 18 20 22 24
Time (h)
Figure 4.9 Weight fraction of reacted and unreacted IPA during the (CL/DyI20)S1 copolyamide formation by SSP according to IPA‐related UV signals recorded by SEC with UV detection. Amine [NH2] and carboxyl [COOH] end group titrations of some copolyamides were performed and the calculated Mn values were compared with the Mn values obtained from SEC (Table 4.2). A striking difference between the [NH2] and [COOH] values is observed for all the copolyamides synthesized by SSP and MP. The evaporation of the diamine is visible from the data presented. As the diamine was lost from the reaction 88
Chapter 4
medium, most of the chains were end capped with IPA via acidolysis and condensation reactions, leading to very high [COOH] values compared to [NH2]. Additionally, when the ratio [COOH]/[NH2] is considered, it can be seen that the difference increases when the salt changes from 20 to 30 wt%. This can also explain the decrease in molecular weight as the salt content is increased (Table 4.1). The loss of diamine is probably more pronounced with increasing salt content. The [COOH]/[NH2] value is smaller in case of melt polymerization with 20 wt% salt. During the MP the reactivity of Dytek A can be higher due to the much higher applied temperatures than during SSP (265 °C versus 200 °C, respectively.) and so the amount of lost diamine during MP could be limited. Although the molecular weights of the copolyamides prepared by MP are higher compared to copolyamides prepared by SSP, the same trend in Mn is observed as for SSP. Molecular weights of the copolyamides synthesized by MP also decrease with the increasing salt fraction, but to a much smaller extent than in the SSP case. This trend again points to an increasing imbalance between the diamine and the diacid with increasing salt content, obviously because of the evaporation of the diamine. Table 4.2 [NH2] and [COOH] values of copolyamides obtained by potentiometric titration and molecular weights calculated from titration and determined by SEC. PA6 (CL/DyI20)S1 (CL/DyI30)S1 (CL/DyI20)M1 [NH2] (meq/kg) [COOH] (meq/kg) [COOH]/[NH2] Mn titration
(kg/mol) Mn SEC (kg/mol) 48 50 1.0 20.0 35 535 8.6 3.4 5.9 619 14.7 3.0 5.1 182 5.6 9.3 11.3 62 42 32 In summary, the molecular characterization study by 1H NMR, SEC and titration shows us that without special precaution part of the volatile diamine is lost and that the amount of incorporated IPA is significantly higher than the amount of incorporated Dytek A diamine. Especially for SSP the imbalance is dramatic, resulting in an incomplete restoration of the molecular weight after the initial chain scission. 89
Chapter 4
4.3.1.2 Thermal properties of PA6/DytekA‐IPA copolyamides Differences in chemical microstructures, reflected in thermal properties of the copolyamides synthesized by SSP and MP, can be indirectly investigated by DSC analyses. The thermal characteristics are summarized in Table 4.3 and also represented in Figure 4.10. Table 4.3 Melting and crystallization temperatures, and enthalpy of transitions analyzed from the first and second heating and cooling runs after 24 h SSP. ∆Hm1 (J/g) 77.3 Tc
(°C) 188.1 ∆Hc
(J/g) 64.1 Tm2 (°C) 220.6 ∆Hm2 (J/g) 64.1 Xc1* Xc2* PA6 Tm1 (°C) 222.2 33.6 27.9 PA6HFIP** 220.8 85.1 186.3 65.7 219.4 60.9 37.0 26.5 PA6SSP** 220.0 90.1 186.3 69.9 220.2 65.8 39.2 28.6 (CL/DyI5)S1 214.3 92.4 185.5 79.8 216.3 92.2 40.2 40.1 (CL/DyI10)S1 220.3 96.9 188.4 78.7 216.2 72.5 42.1 31.5 27.7 (CL/DyI15)S1 217.4 100.7 176.1 71.9 208.0 63.8 43.8 (CL/DyI20)S1 215.6 87.7 169.2 63.2 203.1 56.0 38.1 24.4 (CL/DyI25)S1 215.2 59.7 165.5 54.0 201.9 45.6 26.0 19.8 (CL/DyI30)S1 218.8 57.5 168.5 56.8 202.7 49.0 25.0 21.3 (CL/DyI5)M1 213.3 63.6 164.6 53.3 211.0 54.1 27.7 23.5 (CL/DyI10)M1 203.0 61.8 144.4 44.1 194.4 43.5 26.9 18.9 (CL/DyI15)M1 187.8 53.4 121.1 29.0 183.7 36.5 23.2 15.9 (CL/DyI20)M1 174.7 40.1 123.0 3.45 172.0 26.8 17.4 11.7 *Xc1 and Xc2 represent the degree of crystallinity during the first and second heating, respectively. **PA6HFIP was obtained after dissolving in HFIP and removal of the solvent. PA6SSP was obtained after dissolving in HFIP, removal of solvent followed by SSP at 200 °C for 24 hours. The homopolyamide of the DyI salt was also synthesized to analyze its molecular and thermal properties for comparison. This polymer has a very broad melting peak during the 1st melting, but shows no crystallization and melting during the cooling and 2nd heating, respectively. Polyamides from Dytek A were reported in literature before and it was shown that because of the chemical structure of this diamine only a very limited crystal formation can be possible.32‐34 These crystals totally disappear during the 2nd heating run. After recording the first heating DSC traces of the (CL/DyI)S1 copolyamides, it is seen that 90
Chapter 4
the high melting temperature of PA6 is almost retained after the incorporation of the DyI salt. This supports the hypothesis that DyI is not incorporated in the crystalline phase of the PA6 and only reacts in the amorphous phase, as expected. This supports the idea that after SSP still long homo PA6 chain segments exist. However, during the second heating there is a decrease of the melting temperature up to 18 °C with increasing salt content, when compared to the second heating of neat PA6 polymer. This is believed to be due to partial randomization during the second heating. Since the temperature was raised up to 240 °C the crystalline part was molten and mixed with the amorphous phase, and most probably some randomization cannot be avoided. That is why packing into perfect crystals was hindered during cooling and why the second heating results in a lower melting temperature. a. b. 230
190
220
170
Tm1 MP
200
Tm2 MP
190
Tm1 SSP
180
Tm2 SSP
170
Tc (°C)
Tm (°C)
210
150
Tc MP
130
Tc SSP
110
90
160
0
5
10 15 20 25
DyI in feed (wt%)
30
0
35
5
10 15 20 25 30 35
DyI in feed (wt%)
Figure 4.10 Melting temperatures from the first and second heating (a) and crystallization temperatures from the cooling of the copolyamides synthesized by SSP and MP (b). On the other hand, the crystallization temperatures decrease with increasing salt content. This behavior can be seen in Figure 4.10.b. The percent crystallinities (Xc) of the copolymers were also determined from the ratio of the melting enthalpy of the first heating run (∆Hm1) and the melting enthalpy of a completely crystalline PA6 (230 J/g).35 Observing the crystallization behavior of the copolymers with different salt compositions according to 1st and 2nd heating runs, it is interesting to see that for the SSP samples the crystallinity is increasing with respect to that of pure PA6 with increasing salt content from 91
Chapter 4
0 to 20 wt% salt. In case of 25 and 30 wt% added salt the values fall below the crystallinity of the neat PA6. There might be several possible reasons for this behavior: solvent effect, annealing effect, differences in molecular weight, and residual salt effect. The positive effect of “solvent‐induced crystallization” during the solution mixing and annealing effect at rather high reaction temperatures on the degree of crystallinity is well know. In Table 4.3 the thermal properties of PA6HFIP and PA6SSP are presented. PA6HFIP was prepared by dissolving PA6 in HFIP and full removal of the solvent whereas PA6SSP was prepared by performing SSP for 24 hours after HFIP dissolution similar to the copolymer synthesis by SSP. It is seen from the data shown that the crystallinity is increasing during the first heating run both via solvent dissolution and annealing, however, almost the same crystallinity values are obtained during the second heating. Thus, solution‐induced crystallization and annealing have a remarkable influence on the thickening of the crystals during the first heating but these effects disappear during the second heating where crystallization occurred from the melt. It is interesting to observe that although there is no solvent and annealing effect during the second heating, still higher degree of crystallinities (Xc2) compared to that of neat PA6 are observed up to 20 wt% salt content. Molecular weights of the copolymers decrease significantly compared to the molecular weight of the neat PA6, which would result in a higher crystallinity. Another possibility can be the effect of traces of unreacted IPA left (0.4 wt% of the total PA6/salt amount according to UV‐SEC characterization). This residual IPA can act as a diluent lowering the viscosity and facilitating the crystallization. However, since the weight ratio of this unreacted IPA is quite low it is more likely that the calculated Xc2 values are more dependent on decreasing Mn values. These data demonstrate that significantly lower Mn values of the copolyamides compared to the Mn of PA6 enhance crystallization. However, it is also observed that with increasing salt content this effect becomes less dominant as the salt part starts acting as an impurity. Incorporated salt residues are not expected to co‐crystallize with the PA6 blocks, but they will act as an impurity leading to less perfect crystal formation. 92
Chapter 4
In case of the copolyamides synthesized in the melt, the melting temperatures decrease with respect to neat PA6 even for low weight percentages of DyI salt in the feed. This is already the case for the first heating run. The same trend is seen for Tc and degree of crystallization Xc values. Tm values even drop down to 172 °C and Tc to 128 °C pointing to a deterioration of the crystallization behavior and a disruption of the crystalline phase by formation of a random microstructure after the melt synthesis. Xc data are also significantly lower than the Xc of the copolyamides prepared by SSP. st
SSP, 1 heating
nd
SSP, 2 heating
st
MP , 1 heating
nd
MP , 2 heating
45
Crystallinity (%)
40
35
30
25
20
15
10
0
5
10
15
20
25
30
DyI salt (wt%)
Figure 4.11 Crystallinity (Xc) of the copolyamides as a function of DyI wt% in feed. 4.3.2 High molecular weight PA6/Dytek A‐IPA copolyamides via SSP and MP 4.3.2.1 Molecular characterization of PA6/Dytek A‐IPA copolyamides As explained before, higher molecular weight PA6/Dytek A‐IPA copolyamides were prepared by improving the synthetic procedure and by preventing excessive Dytek A diamine loss during the synthesis. Salt amounts of 10, 20 and 30 wt% were used both for SSP and MP reactions. The characteristics of these copolymers are shown in Table 4.4. 93
Chapter 4
Table 4.4 Characteristics of homopolymers of PA6 and Dytek A‐IPA salt, and of copolymers prepared by adapted solid state (SSP) and melt (MP) reactions. Composition feed (CL/DyI) Composition Composition after SSP after SSP and MP and MP (CL/DyI) by (CL/DyI) by 1
13
H NMR C NMR mol% PA6 100/0 100/0 – – 36 1.9 PA6SSP 100/0 100/0 – – 79 2.0 (CL/DyI10)S2 90/10 92/8 93/7 n.a. 24 2.4 (CL/DyI20)S2 80/20 83/17 84/16 86/14 17 2.4 (CL/DyIEx.)S2 80/20 83/17 85/15 n.d. 21 2.5 (CL/DyI30)S2 70/30 74/26 75/25 78/22 12 2.4 (CL/DyI10)M2 90/10 92/8 92/8 90/10 27 1.9 (CL/DyI20)M2 80/20 83/17 85/15 83.1/17 23 1.8 (CL/DyI30)M2 70/30 74/26 77/23 73/27 22 2.0 0/100 0/100 0/100 24 2.5 0/100 mol% mol% PDI wt% DyI HP Mn (SEC) kg/mol 13
n.a.= not available. It was not possible to determine the composition of (CL/DyI10)S2 copolyamide from C NMR due to very low intensities of the salt peaks. 13
n.d.= not determined. Quantitave C NMR measurement of (CL/DyIEx.)S2 was not performed. In Figure 4.12.a changes in the RI polymer and monomer peaks in the RI SEC for (CL/DyI20)S2 samples recorded during the reaction are shown. Similar to the low molecular weight copolymers prepared by SSP initially (see earlier), a decrease in molecular weight is seen caused by chain scission reactions. When the peak of the more reactive Dytek A part of the DyI salt had completely vanished after 12 hours (in a closed vessel), the reaction temperature was raised to 200 °C and an inert gas stream was flushed through the SSP reactor to complete the incorporation of IPA and to promote the removal of condensation water, respectively. Consequently, the polymer peak shifts back to lower elution times because polycondensation takes place. Since in the previous set of experiments most chain ends carried COOH groups, polycondensation and restoration of the molecular weight was impossible. As IPA starts reacting with the excess of amine end groups 94
Chapter 4
polymer chains recombine resulting in an increase of the molecular weight. Although higher molecular weights were obtained compared to previous set of polymers, they still were not as high as the initial molecular weight of the PA6. This is most probably because there is still a small loss of diamine causing an imbalance of the amine and carboxylic acid end groups. Although a closed system was used at the first stage of the SSP, complete incorporation of the diamine was not possible. Some of the Dytek A was gathered at the top of the vial as vapor and therefore did not take part in the reaction. Dytek
IPA
b.
24 h
24 h
16 h
16 h
Normalized UVUA
SEC signal
MV
Normalized RI
SEC signal
a.
12 h
8h
4h
2h
1h
0.5 h
12 h
8h
4h
2h
1h
0.5 h
0h
0h
14
16
18
20
22
24
26
28
14 16 18 20 22 24 26 28 30 32 34 36 38
30
Elution time (min)
Elution time (min)
Figure 4.12 SEC chromatograms of (PA6/DyI20)S2 samples at different reaction times recorded with an RI detector (a) and an UV detector (b). Figure 4.12.b shows SEC UV chromatograms of the samples can be seen taken at various times. As IPA takes part in the reaction, especially after 12 hours when the mixture is transferred to the SSP reactor at 200 °C with a continuous inert gas stream, it is incorporated into the polymer chains which then become UV‐active. After 24 hours all the IPA has reacted and is not visible anymore at an elution time of 34.5 minutes. Chain scission via transreactions followed by polycondensation reactions can also be followed in Figure 4.13. During the first 8 hours of reaction PDI has an increasing trend. For reaction times longer than 8 hours PDI drops down as the molecular weight increases. A possible explanation for this behavior was already made in the previous section. PDI can increase due to branching and crosslinking reactions throughout the reaction. However, as the extent of these reactions increase then, it will not be possible to filter the insoluble parts 95
Chapter 4
formed which will result in a decrease of PDI. This also means that SEC data are underestimated due to the removal of some branched/crosslinked parts via filtration before the measurement. Higher molecular weights were also achieved in the case of MP reactions by adding excess amounts of Dytek. This indeed compensated for the diamine loss (see Table 4.4). 40
PDI
Mn SEC (kg/mol)
30
25
2.8
2.6
2.4
20
PDI
Mn
35
2.2
15
2.0
10
5
1.8
0
0
4
8
12
Time (h)
16
20
24
Figure 4.13 Molecular weight and PDI of (CL/DyI20)S2 as a function of SSP reaction time. 4.3.2.2 Sequence distribution and degree of randomness analyses by 13C NMR 13
C NMR is an effective tool for the structural characterization of copolymers synthesized via solid state and melt polymerization. As discussed before, Dytek A‐IPA salt should only be incorporated in the amorphous phase during the SSP, rendering a blocky structure consisting of copolyamide blocks (amorphous part + salt) and homo PA6 blocks (present in crystals during SSP), whereas a totally random distribution is expected in the case of the melt reaction. The differences in chemical microstructures of these copolymers can be established by dyad sequence analysis. The 13C NMR spectrum of (CL/DyI20)S2 copolyamide with assignments of the peaks is shown in Figure 4.14. The corresponding chemical shifts of the peaks are shown in Table 4.5. 96
Chapter 4
Table 4.5 13C NMR chemical shifts of corresponding peaks. Carbon c B C e D d b A E Chemical shift 16.2 24.9 25.7 25.9 27.9 30.8 32.7 35.9 39.6 g i n m p COOH Carbon f a Chemical shift 40.3 46.3 h 125.1 129.5 130 134.1 169.9 176.8 179.4 a
N
H
b
d
CH 3
c
f
e
O
O
g
N mn
H
h
n m co
h
i
D
H
N
p
C
E
O
B
A
y
x
D
A
E
B
C
p
P
I
F
H
P
I
F
H
80
70
60
50
40
c
90
e
d
b
f
a
g
i
h
n
m
m
p
p
180 170 160 150 140 130 120 110 100
30
20
Figure 4.14 13C NMR spectrum of (CL/DyI20)S2 copolyamide recorded in a 3/1 vol/vol HFIP/CDCl3 mixture. For the sequence analysis, the chemical shifts of the methylene carbon atoms connected to the amide linkage but with different chemical environments were monitored. All the expected chemical shifts for this carbon are shown in Table 4.6. The region where all the corresponding shifts are present is shown in Figure 4.15. This methylene carbon gives a shift at 39.59 ppm for the neat PA6. On the other hand, the reaction of Dytek A diamine with IPA results in two different chemical shifts due to the presence of the methyl group. This methyl group can either be connected to C2 or C4 of the Dytek A diamine residue (see Figure 4.2) resulting in two signals at 46.23 and 40.58 ppm for the Dytek A‐IPA salt homopolymer. For the copolymers of PA6 and the salt three additional dyad sequences 97
Chapter 4
are expected. The CL residue of the PA6 can be next to an IPA residue from the salt (40.27 ppm) and a Dytek diamine residue can be next to a CL unit (45.55 and 39.91 ppm). The methyl group of the Dytek residue will again give two different shifts, as in the case of the salt homopolymer. The peak at 42.9 ppm (see Figure 4.15) appears as a result of the additives present in the PA6 and is thus not related to dyad sequences. Since all the 13C NMR measurements were done for quantitative analysis (check the experimental section for details) it is possible to calculate the integral values and to find the degree of randomness for each copolymer. This was done after deconvolution of the peaks and by normalizing the sum of the peak areas to 1. The degrees of randomness (R) of the copolymers were calculated using the following equations:21 Fcopolymer = C1 + C2 + C3 (4.4) FCL = C1 + PA6 (4.5) Fsalt = C2 + C3 + SH1 + SH2 (4.6) R = Fcopolymer, total /(2∙FCL∙Fsalt ) (4.7) mol% salt = Fsalt /(FCL+Fsalt) (4.8) The corresponding chemical shifts for the abbreviations used in the equations are shown in Table 4.6. Fcopolymer, FCL and Fsalt are the mol fractions of the total amount of copolymers, caprolactam and salt, respectively. C1, C2, C3, PA6, SH1 and SH2 represent the peak areas of the corresponding shifts. It is possible to calculate the molar percentage of the salt present in the copolymers by using equation (4.8). These compositions are shown in Table 4.4. For the (PA6/DyI)S2 copolymers the mol percentages of the salt at the end of the reaction were lower than the feed compositions, whereas for (PA6/DyI)M2 values closer to the feed compositions were obtained. This is due to the more pronounced loss of Dytek A diamine during the SSP. During MP this loss was prevented by the addition of excess Dytek A. 98
Chapter 4
Table 4.6 Assignment of the methylene carbon resonances (marked with an asterisk) in the PA6‐DyI salt copolymers for possible dyad sequences. Since a statistical distribution of added comonomer is expected for melt copolymerizations R should be close to 1. An R value less than 1 points to a blocky structure. As presented in the last column of Table 4.7, the total degree of randomness for (CL/DyI20)S2 and (CL/DyI30)S2 is calculated to be 0.51 and 0.45, respectively, which indeed reveals that a non‐random distribution of the DyI salt is achieved after SSP. In case of copolyamides synthesized in the melt a random distribution of the salt into the PA6 backbone is proven, as the R values calculated from the 13C NMR data are very close to unity. These results show that the crystalline part of the PA6 is not involved in the transamidation reactions occurring during the SSP. 99
Chapter 4
Figure 4.15 Expanded part of the 13C NMR spectrum of PA6, salt homopolymer (DyI HP) and copolyamides prepared by SSP and MP with 30 wt% DyI salt in feed. Spectra are recorded in a HFIP/CDCl3 3/1 vol/vol solvent mixture. It is also possible to calculate the number average block lengths of the dyads from the equations shown below:21 LCL‐CL=(PA6/C1)+1 (4.9) LDY‐IPA=((SH1+SH2)/(C2+C3))+1 (4.10) LCL‐IPA=(C1/PA6)+1 (4.11) (4.12) LDY‐CL=((C2+C3)/(SH1+SH2))+1 (CL/DyI)S2 samples have long homologous block lengths for PA6 (CL‐CL) while (CL/DyI)M2 copolymers have shorter homoblocks which significantly decrease in length with the increasing amount of incorporated salt. Salt homoblock lengths (DY‐IPA) are also longer for (CL/DyI)S2 samples. The CL‐IPA block lengths are almost the same in both type of polymers whereas DY‐CL block lengths are longer for (CL/DyI)M2 copolyamides, indicating a more random distribution of Dytek A in the copolyamide chain. The microstructure of (PA6/DyI20)S2 copolymer was checked after leaving in melt to analyze the effect of melt 100
Chapter 4
processing (sample code: (CL/DyI20)S2‐MP). It was put into a vial, pressurized with argon and immersed in a salt bath at 250 °C for 10 minutes. It is seen that although the block lengths slightly change after the melting, the same degree of randomness value as before the melting was retained. This shows that within reasonable processing times the non‐
random microstructure of the copolymers are not changed. Table 4.7 Number average block lengths and degrees of randomness of the copolyamides prepared by SSP and MP. CL‐CL DY‐IPA CL‐IPA DY‐CL R (CL/DyI20)S2 11.82 2.80 1.09 1.56 0.51 (CL/DyI30)S2 9.07 3.09 1.12 1.48 0.45 (CL/DyI10)M2 10.35 1.13 1.10 8.67 0.97 (CL/DyI20)M2 5.78 1.13 1.21 9.03 1.03 (CL/DyI30)M2 3.61 1.37 1.39 3.71 1.02 (CL/DyI20)S2‐MP 14.31 2.33 1.08 1.75 0.51 *** This sample was prepared by keeping the (PA6/DyI20)S2 copolymer at 250 °C for 10 minutes. 4.3.3.3 Thermal properties of the copolyamides prepared with limited Dytek A loss In Section 4.3.2.1 the thermal properties of low molecular weight PA6/Dytek A‐IPA copolyamides and differences in crystallization behavior of copolymers obtained by SSP and MP were discussed in detail. It was shown that copolyamides prepared by SSP exhibited superior thermal properties to polymers prepared by MP. Thermal characteristics of the high molecular weight copolyamides are shown in Table 4.8 and from the presented data the same conclusion can also be drawn for these polymers. First and second heating runs of (CL/DyI)S2 and (CL/DyI)M2 copolyamides are shown in Figure 4.15. These DSC curves show that during SSP the Tm values are only slightly reduced compared to the Tm of neat PA6, while there is a significant difference compared to MP. This is best seen for 30 wt% salt addition, where in case of (CL/DyI30)M2 no melting endotherm is observed during the second heating, indicating that the chain regularity is totally destroyed with the presence of high amounts of randomly distributed Dytek A and 101
Chapter 4
IPA residues, whereas the (CL/DyI30)S2 copolyamide still exhibits distinct first and second heating melting peaks. As it was well described previously, for the increasing crystallinity during the first heating run of the copolyamides prepared by SSP there can be solvent‐induced crystallization and/or an annealing effect as well as molecular weight effect and diluent effect of the salt or residual IPA during the SSP. During the first heating, percent crystallinity (Xc1) of the copolymers prepared by SSP (except the one prepared by 30 wt% salt) are higher than that of neat PA6. This is because of annealing and solvent induced crystallization. For 30 wt% salt content in the feed these effects might be overruled by the impurity effect of the salt leading to a decrease in crystallinity. However, during the second heating lower Xc2 values for the (CL/DyI)S2 copolymers were obtained compared to that of neat PA6. In case of low molecular weight (CL/DyI)S1 copolymers higher values of ∆Hm2 were observed up to 20 wt% salt in feed. Since the influence of residual salt or IPA is expected to be negligible, this obvious difference in Xc2 values between the 1st and 2nd set of copolyamides should be due to the differences in molecular weights. Distinct increase in molecular weights achieved by keeping the diamine loss less, result in a lower degree of crystallinity compared to the first set of low molecular weight copolyamides (Table 4.3). For (CL/DyI)S2 copolyamides a narrow and sharp melting peak is obtained during the first heating run, which is generally observed during SSP since the crystals have enough time for reaching a more perfect chain packing. During the second heating however a decrease of a few degrees in Tm values is observed due to some randomization as explained in the previous section. Double melting peaks occur, most probably as a result of the initial melting of imperfect crystals followed by recrystallization into more perfect crystals upon heating and remelting at a higher temperature.36, 37 102
Chapter 4
Table 4.8 Melting and crystallization temperatures, and enthalpy of transitions derived from the first and second heating and cooling runs. Tm1 (°C) ∆Hm1 (J/g) Tc
(°C) ∆Hc
(J/g) Tm2
(°C) ∆Hm2
(J/g) Xc1* Xc1* PA6 222.2 77.3 188.1 64.1 220.6 64.1 33.5 27.9 PA6HFIP** 220.8 85.1 186.3 65.7 219.4 60.9 37.0 26.5 PA6SSP** 220.0 90.1 186.3 69.9 220.2 65.8 39.2 28.6 (PA6/DyI10)S2 219.0 95.0 183.0 62.0 216.4 61.3 41.3 26.7 (PA6/DyI20)S2 218.2 81.2 175.4 50.7 211.9 49.4 35.3 21.5 (PA6/DyIEx.)S2 217.5 79.3 175.8 52.4 211.5 47.1 34.5 20.5 (PA6/DyI30)S2 217.8 70.1 166.4 40.9 207.1 38.6 30.5 16.8 (PA6/DyI10)M2 203.5 56.9 155.9 46.2 194.8 43.2 24.7 18.8 (PA6/DyI20)M2 182.7 53.9 134.5 30.3 179.2 29.5 23.5 12.8 (PA6/DyI30)M2 173.3 33.2 — — — — 14.4 — (PA6/DyI20)S2‐
MP*** 213.7 54.9 174.1 54.7 209.7 51.1 21.9 22.2 *Xc1 and Xc2 represent the degree of crystallinity during the first and second heating, respectively. **PA6HFIP was obtained after dissolving in HFIP and removal of the solvent. PA6SSP was obtained after dissolving in HFIP, removal of solvent followed by SSP at 200 °C for 24 hours. *** This sample was prepared by keeping the (PA6/DyI20)S2 copolymer at 250 °C for 10 minutes. When the cooling runs of (CL/DyI)S2 and (CL/DyI)M2 copolyamides are studied in both cases the crystallization peak is getting smaller and broader with increasing salt fraction, which means that crystal growth is retarded in both cases because of the presence of non‐
crystallizable salt units (Figure 4.17). In case of (CL/DyI30)M2 no recrystallization is observed during cooling, which points to a totally amorphous microstructure. In Picture 4.1 a PA6 homopolymer and copolyamides formed by melt polymerization with 10, 20 and 30 wt% salt are shown. It can be seen in this picture that the opacity of the polymers is decreasing with increasing salt fraction, indicating the reduction of the crystalline phase. 103
Chapter 4
a.
b.
(CL/DyI10)M2
H eat flow , E ndo dow n (W /g)
Heat flow, Endo down (W /g)
(CL/DyI10)S2
(CL/DyI20)S2
(CL/DyI30)S2
(CL/DyI20)M2
(CL/DyI30)M2
40 60 80 100 120 140 160 180 200 220
º
º
40 60 80 100 120 140 160 180 200 220
Temperature ( C)
Temperature ( C)
Figure 4.16 DSC traces of the first (‐‐‐) and second heating (―) cycle of (CL/DyI)S2 (a) and (CL/DyI)M2 (b) copolyamides. H ea t flow , E nd o d ow n (W /g )
Heat flow, Endo down (W/g)
b.
a.
(CL/DyI10)S2
(CL/DyI20)S2
(CL/DyI30)S2
40 60 80 100 120 140 160 180 200 220
Temperature ( C)
(CL/DyI10)M2
(CL/DyI20)M2
(CL/DyI30)M2
º
º
40 60 80 100 120 140 160 180 200 220
Temperature ( C)
Figure 4.17 DSC traces of the cooling cycle of (CL/DyI)S2 (a) and (CL/DyI)M2 (b) copolyamides. In Figure 4.18 the crystallinities of all the copolyamides are presented. A similar behavior as in the case of low molecular weight copolyamides is observed and a very clear difference between the crystallinities of the copolyamides prepared by SSP and MP is seen. 104
Chapter 4
Picture 4.1 PA6, (CL/DyI10)M2 , (CL/DyI20)M2 and (CL/DyI30)M2 copolyamides just after the melt polymerization (from left to right). st
SSP, 1 heating
nd
SSP, 2 heating
st
MP , 1 heating
nd
MP , 2 heating
45
Crystallinity (%)
40
35
30
25
20
15
10
0
5
10
15
20
DyI salt (wt%)
25
30
Figure 4.18 Crystallinity (Xc) of the copolyamides as a function of DyI wt% in feed. To make the differences in thermal properties of copolyamides synthesized by SSP and MP more clear heating and cooling curves of PA6, (CL/DyI20)S2, (CL/DyIEx.)S2, (CL/DyI20)M2 and DyI homopolymer were put together and compared (Figure 4.19). These (CL/DyI20)S2, (CL/DyIEx.)S2 and (CL/DyI20)M2 copolyamides were all synthesized with 20 wt% salt in the feed, however, the Mn of (CL/DyIEx.)S2 (21 kg/mol) is better comparable to (CL/DyI20)M2 (23 kg/mol), so that the molecular weight effect on the thermal properties can be excluded. Although (CL/DyIEx.)S2 has a little bit higher molecular weight than (CL/DyI20)S2, their thermal properties are almost the same. However, when these two copolyamides are compared with (CL/DyI20)M2, a very significant difference is seen both for the melting and the cooling runs. There is a shift to much lower temperatures both in Tm and Tc for the random (CL/DyI20)M2 while they are almost retained in case of other blocky copolyamides. 105
Chapter 4
Even higher heating and crystallization enthalpies are observed for SSP when compared to PA6. These facts were already discussed earlier in this chapter. It is also shown that DyI salt homopolymer is totally amorphous, showing only a glass transition temperature around 140 °C. From these results, after excluding the possible influence of molecular weight, it is very obvious that salt incorporation of relatively high amounts of salt by SSP does not severely influence the crystallization behavior. In contrast, for low fractions of salt the crystallinity is improved, possibly by the plasticizing effect of some residual salt. On the other hand, melt polymerization leads to a random distribution of the salt in the polymer chain resulting in the deterioration of the chain regularity and as a consequence in a reduction of the thermal properties. a.
b.
PA6
(CL/DyI20)S2
(CL/DyIEx.)S2
(CL/DyI20)M2
DyI HP
60
(CL/DyI20)S2
(CL/DyIEx.)S2
(CL/DyI20)M2
DyI HP
40 60 80 100 120 140 160 180 200 220
80 100 120 140 160 180 200 220
º
º
40
Heat flow, Endo down (W/g)
H eat flow , E ndo dow n (W /g)
PA6
Temperature ( C)
Temperature ( C)
Figure 4.19 Comparison of first (‐‐‐) and second heating (―) curves (a) and cooling curves (b) of PA6, (CL/DyI20)S2, (CL/DyIEx.)S2, (CL/DyI20)M2 and DyI HP. To check the thermal properties after melt processing the (PA6/DyI20)S2 copolymer was put into a vial, pressurized with argon and immersed in a salt bath at 250 °C for 10 minutes as explained in the previous section. The thermal properties of this polymer are presented in the bottom row of Table 4.8 ((PA6/DyI20)S2‐MP). It can be seen that Tc, Tm2 and χheating2 values of this polymer are very close to that of (CL/DyI20)S2 copolymer (174.1 °C, 209.7 °C and 22.2, respectively) and much higher than the corresponding values for (CL/DyI20)M2. This shows that even after melt processing of the copolymers prepared by SSP, relatively high degree of crystallization can still be retained. 106
Chapter 4
4.4 Conclusions The incorporation of the Dytek A‐IPA (DyI) nylon salt into PA6 below the melting temperature of the polymer was successfully achieved. By this method low molecular weight and respectively higher molecular weight PA6/DyI copolyamides were obtained, the latter by improving the reaction procedure and by limiting the diamine loss during the SSP procedure. Reaction of PA6 with both components was monitored by RI and UV SEC. Incorporation of Dytek A and IPA can occur via aminolysis and acidolysis reactions, respectively. On the other hand the apparently more reactive diamine may break the PA6 chains and form an excess of amine end groups, to which the IPA monomer may add by a condensation reaction. If the loss of diamine is reduced, firstly, in the SEC chromatograms a decrease in molecular weight is observed due to chain scission by transamidation reactions and, at a later stage, at a higher temperature and in the presence of a continuous inert gas stream through the SSP reactor, a built‐up in molecular weight is seen due to polycondensation. Melt polymerizations (MP) with the same CL/DyI compositions were also done. A comparison of the melting temperatures of the copolymers prepared by SSP and MP revealed that the high Tm and Tc values of the PA6 are almost retained after SSP, while a significant decrease is observed after MP. Additionally, for the first set of copolyamides higher crystallinities were obtained by SSP compared to neat PA6 up to a certain amount of salt addition, which is believed to be caused by low molecular weights of the copolyamides. 13
C NMR was used as an effective tool to characterize the sequence distribution of the copolymers obtained after SSP and MP. The degree of randomness (R) of the copolyamides synthesized by SSP point to a blocky microstructure (R=0.4‐0.5) whereas the R values of the copolyamides obtained via MP point to a totally random distribution of the PA6 and the DyI salt (R≈1.0). Both results obtained by DSC and 13C NMR are in agreement and confirm the non‐random, block‐like distribution obtained after SSP reactions. This is realized by the exclusion of the long PA6 chain parts, present in the crystals during SSP, 107
Chapter 4
from the transreactions. The blocky microstructure obtained after SSP is almost completely retained after a melt treatment for 10 minutes at 250 °C as shown by both thermal analysis and by 13C NMR R calculations. This indicates that these copolyamides are suitable for processing without any reduction of the good properties. References 1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
Aharoni, S. M., n‐Nylons: Their Synthesis, Structure, and Properties. John Wiley & Sons Inc: West Sussex, 1997. Kohan, M. I., Nylon Plastics Handbook. Hanser Gardner: 1995. Jo, W. H.; Baik, D. H. J. Polym. Sci., Part B: Polym. Phys. 1989, 27, (3), 673‐687. Novitsky, T. F.; Mathias, L. J. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, (10), 2271‐2280. Vouyiouka, S. N.; Karakatsani, E. K.; Papaspyrides, C. D. Prog. Polym. Sci. 2005, 30, (1), 10‐37. Gaymans, R. J.; Amirtharaj, J.; Kamp, H. J. Appl. Polym. Sci. 1982, 27, (7), 2513‐2526. James, N. R.; Sivaram, S.; Ramesh, C. Macromol. Chem. Phys. 2001, 202, (11), 2267‐2274. James, N. R.; Sivaram, S.; Ramesh, C. Macromol. Chem. Phys. 2001, 202, (7), 1200‐1206. Jansen, M. A. G.; Goossens, J. G. P.; de Wit, G.; Bailly, C.; Koning, C. E. Macromolecules 2005, 38, (7), 2659‐2664. Jansen, M. A. G.; Goossens, J. G. P.; de Wit, G.; Bailly, C.; Koning, C. E. Anal. Chim. Acta 2006, 557, (1‐
2), 19‐30. Jansen, M. A. G.; Goossens, J. G. P.; de Wit, G.; Bailly, C.; Schick, C.; Koning, C. E. Macromolecules 2005, 38, (26), 10658‐10666. Jansen, M. A. G.; Goossens, J. G. P.; Wu, L. H.; de Wit, G.; Bailly, C.; Koning, C. E. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, (5), 882‐899. Jansen, M. A. G.; Goossens, J. G. P.; Wu, L. H.; De Wit, G.; Bailly, C.; Koning, C. E.; Portale, G. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, (4), 1203‐1217. Sablong, R.; Duchateau, R.; Koning, C. E.; Pospiech, D.; Korwitz, A.; Komber, H.; Starke, S.; Haussler, L.; Jehnichen, D.; Landwehr, M. A. D. Polym. Degrad. Stab. 2011, 96, (3), 334‐341. Sablong, R.; Duchateau, R.; Koning, C. E.; de Wit, G.; van Es, D.; Koelewijn, R.; van Haveren, J. Biomacromolecules 2008, 9, (11), 3090‐3097. Goodman, I.; Maitland, D. J.; Kehayoglou, A. H. Eur. Polym. J. 2000, 36, (7), 1301‐1311 Mathias, L. J.; Davis, R. D.; Steadman, S. J.; Jarrett, W. L. Macromolecules 2000, 33, (19), 7088‐7092. Mathias, L. J.; Davis, R. D.; Jarrett, W. L. Polymer 2001, 42, (6), 2621‐2626. Yamadera, R.; Murano, M. J. Polym. Sci., Part A: Polym. Chem. 1967, 5, (9Pa1), 2259‐&. Kricheldorf, H. R.; Hull, W. E. J. Polym. Sci., Part A: Polym. Chem. 1978, 16, (9), 2253‐2264. Aerdts, A. M.; Eersels, K. L. L.; Groeninckx, G. Macromolecules 1996, 29, (3), 1041‐1045. Eersels, K. L. L.; Aerdts, A. M.; Groeninckx, G. Macromolecules 1996, 29, (3), 1046‐1050. Kint, D. P. R.; de Ilarduya, A. M.; Munoz‐Guerra, S. Macromolecules 2002, 35, (1), 314‐317. Asakura, T.; Matsuda, H.; Miki, T. Macromolecules 2002, 35, (12), 4664‐4668. Samperi, F.; Montaudo, M. S.; Puglisi, C.; Di Giorgi, S.; Montaudo, G. Macromolecules 2004, 37, (17), 6449‐6459. Denchev, Z.; Kricheldorf, H. R.; Fakirov, S. Macromol. Chem. Phys. 2001, 202, (4), 574‐586. C.D. Papaspyrides, S. N. V., Solid‐state polymerization. John Wiley & Sons, Inc.: Hoboken, NJ, 2009. 108
Chapter 4
28. K.L.L. Eersels, A. M. Aerdts, G. Groeninckx, Reactive Melt Processing of Aliphatic/Aromatic Polyamide Blends: Effect on Molecular Structure, Semicrystalline Morphology, and Thermal Properties, in Transreactions in Condensation Polymers Wiley‐VCH Verlag GmbH: Weinheim, 2007; p 268. 29. Feijen, J.; Stapert, H. R.; Bouwens, A. M.; Dijkstra, P. J. Macromol. Chem. Phys. 1999, 200, (8), 1921‐
1929. 30. Kotliar, A. M. J. Polym. Sci., Part D: Macromol. Rev. 1981, 16, 367‐395. 31. Flory, P. J., Principles of Polymer Chemistry. Cornell University Press: 1953. 32. Keating, M. Y.; Gardner, K. H.; Ng, H.; Marks, D. N.; Yung, W. S.; Avakian, P.; Starkweather, H. W. J. Therm. Anal. Calorim. 1999, 56, (3), 1133‐1140. 33. Starkweather, H. W.; Avakian, P.; Gardner, K. H.; Hsiao, B. S.; Keating, M. Y.; Ng, H. J. Therm. Anal. Calorim. 2000, 59, (1‐2), 519‐530. 34. Stouffer, J. M.; Starkweather, H. W.; Hsiao, B. S.; Avakian, P.; Jones, G. A. Polymer 1996, 37, (7), 1217‐
1228. 35. Wunderlich, B., Macromolecular Physics. Academic Press: New York, 1980; Vol. 3. 36. Roberts, R. C. J. Polym. Sci., Part B: Polym. Lett. 1970, 8, (5), 381‐384. 37. Todoki, M.; Kawaguchi, T. J. Polym. Sci., Part B: Polym. Phys. 1977, 15, (6), 1067‐1075. 109
Chapter 4
110
CHAPTER 5 INVESTIGATION OF LOCAL CHAIN CONFORMATION AND MORPHOLOGY OF POLYAMIDE 6 MODIFIED BY A SEMI‐
AROMATIC NYLON SALT Summary Structural and conformational differences between the PA6 homopolymer and two high molecular weight copolymers of PA6 named as (CL/DyI20)S2 and (CL/DyI30)S2 and containing 20 and 30 wt% Dytek A‐IPA (DyI) salt in the feed, respectively, were investigated. Room temperature WAXD analysis together with solid state CP/MAS 13C NMR showed that there is no cocrystallization of the DyI salt with the PA6 repeat units. A steady decrease of the degrees of crystallinity with increasing amount of incorporated DyI salt as well as a transformation of trans conformers into gauche conformers were observed both by performing temperature‐dependent FTIR and solid state NMR measurements. 111
Chapter 5
5.1 Introduction Polyamide 6 (PA6) is a semi‐crystalline polymer which exhibits excellent physical and mechanical properties as well as chemical stability, mostly due to its well‐known strong hydrogen bonding ability.1, 2 Modification of PA6 with monomers or polymers can be required for some desired applications. However, this modification is mostly performed via melt reactions where random copolymers are obtained and, as a result, the crystallization behavior is negatively affected. In order to overcome this negative effect of melt polymerizations, which leads to worse mechanical and physical properties for end applications, an alternative technique can be used which is solid‐state polymerization (SSP).3 During the SSP reactions incorporation of other monomers or polymers into PA6 can be achieved well above the glass transition temperature but below the melting temperature of PA6, where the parts of the chains in the amorphous phase are mobile enough to take part in the polycondensation and/or transreactions. By this way, the crystalline phase of PA6 is not affected and remains intact while the amorphous phase is modified, resulting in a blocky chemical microstructure.4 The crystalline structure of PA6 was described in Chapter 1 where it was mentioned that PA6 exists in two major crystal forms: the monoclinic α form where the chains are in the fully extended configuration in an anti‐parallel fashion and the pseudohexagonal γ form where the parallel chains are twisted approximately 60° out of plane of the molecular sheets.5, 6 In the α form the amide linkages and methylene units lie in the same plane and hydrogen bonds only occur between the chains forming β sheets of hydrogen‐bonded chains and for PA6 this form is thermodynamically is the most favored one. The X‐ray diffraction profiles of the α phase display typical reflections at 2θ=20.4° and 23.7° with crystal planes of (200) and (002/202), respectively. On the other hand the γ phase possesses two diffraction peaks at 2θ=21.8° and 11° with reflections of the (001) and (020) crystal planes, respectively.7 There are also other crystal forms of PA6 reported in literature which are mainly variations of these two crystal forms, however, most of them are not stable.6, 8‐11 The molecular packing of PA6 is highly dependent on processing 112
Chapter 5
conditions, thermal treatment, applied stress and presence of moisture and additives.7, 12‐
15
It is well known that the α form can be transformed into the γ form by treatment with an aqueous iodine/potassium iodide solution.16, 17 Incorporation of comonomers/polymers can also influence the packing of the chains.18 In general, the α form is dominant when the polymer is melt crystallized at temperatures higher than 150 °C or slowly cooled, while quenching and low‐temperature crystallization promote the formation of the γ form.12, 14, 18‐20
The γ form can be transformed into the α form via slow heating or annealing at an elevated temperature.13, 21 Conformational changes of polyamides can be followed by different characterization techniques. X‐ray diffraction gives direct information about the crystalline forms of the polymers. On the other hand, with the help of FTIR spectroscopy the crystalline and amorphous signals of the polymer and the structural changes upon heating can be investigated.21‐25 Solid state NMR spectroscopy is another very useful tool to study the molecular conformations and structures of the polymers. Reorganization of the chains, differences in packing and weakening of the hydrogen bonding upon heating can be proven by cross‐polarization magic angle spinning solid state NMR spectroscopy (CP/MAS NMR).26‐28 This technique can provide detailed information about the changes in crystalline and non‐crystalline regions as a function of temperature. The effects of the anisotropy of the chemical shifts and of the spin‐spin interactions occurring during the solid state measurements are eliminated by applying the MAS technique.29 Synthetic routes for the incorporation of a semi‐aromatic nylon salt into the PA6 backbone with different compositions and the formation of a non‐random microstructure was explained in detail in Chapter 4. Comparison with melt‐polymerized copolymers revealed the fact that high crystallization rates, degree of crystallinities and melting temperatures were almost retained after SSP, whereas sharp decreases in all these properties were seen after melt polymerizations. In this chapter a more detailed analysis was performed in order to fully understand the crystalline structure of the blocky copolymers prepared and to investigate the influence of the presence of the salt residues 113
Chapter 5
in the polymer backbone and the effect of reaction temperature. For this purpose WAXD, CP/MAS 13C NMR and FTIR spectroscopy were used extensively to understand the structural behavior of the copolymers. Local chain dynamics and conformational changes were also investigated via heating until the melting temperatures of the copolymers. 5.2 Experimental 5.2.1 Wide Angle X‐Ray Diffraction (WAXD) X‐ray diffraction patterns were obtained employing a Bruker AXS HI‐STAR area detector installed on a P4 diffractometer, using graphite‐monochromated Cu‐Kα radiation (λ=1.5418 Å) and a 0.5 mm collimator. The data were collected at room temperature on synthesized powder contained in glass capillaries. The 2D data were subsequently background‐corrected and transformed into 1D profiles via azimuthal integration. All the samples were annealed at 100 °C for 24 hours prior to measurement. 5.2.2 Fourier Transform Infrared (FTIR) Spectroscopy Fourier Transform Infrared spectra (FTIR) were obtained using a Varian 610‐IR spectrometer equipped with FTIR microscope. The spectra were recorded in a transmission mode with a resolution of 2 cm‐1. PA films obtained after casting from 1,1,1,3,3,3‐hexafluoroisopropanol were analyzed on a zinc selenium disk and heated from 30 oC to slightly above the melting temperatures of the polyamides using a Linkam TMS600 hotstage and TMS94 controller. The samples were cooled in steps of 10 oC steps and reheated with the same heating rate. For the study, the spectra from the second heating run were collected. The Varian Resolution Pro software version 4.0.5.009 was used for the analysis of the spectra. For the measurements at 30 °C, samples annealed at 100 °C for 24 hours were used. For the temperature‐dependent measurements the 2nd heating runs were considered. 114
Chapter 5
5.2.3 Solid‐State NMR Variable‐temperature (VT) 13C magic‐angle spinning/cross‐polarization (CP/MAS) NMR experiments were carried out on a Bruker ASX‐500 spectrometer employing a double‐
resonance probe for rotors with 4.0 mm outside diameter. These experiments used 10.0 kHz MAS and a 4 μs /2 pulse for 1H. All VT 13C CP/MAS NMR spectra were recorded using a CP contact time of 3.0 ms and TPPM decoupling during acquisition. The temperature was controlled using a Bruker temperature control unit in the range from 41 °C to 212 °C. It was not possible to go higher temperatures due to the limitations of the instrument. The VT 13C CP/MAS NMR spectra were recorded under isothermal conditions at intervals of 20 °
C, employing a heating rate of 2 °C/min between the different measuring temperatures. Reported temperatures are corrected for friction‐induced heating due to spinning using 207
Pb MAS NMR of Pb(NO3)2 as a NMR thermometer. 5.3 Results and discussion In Chapter 4 firstly low molecular weight and later respectively higher molecular weight CL/DyI copolymers were prepared by the incorporation of semi‐aromatic Dytek A‐IPA nylon salt into the high molecular weight PA6 backbone with different compositions. Differences in crystallization behavior of the copolymers were investigated by DSC analysis, while quantitative liquid‐state 13C NMR spectroscopy was used as the tool for the structural characterization of the copolymers synthesized by both solid state and melt polymerization. Sequence distribution and degree of randomness analysis as performed by 13C NMR and the blocky microstructures of the copolymers synthesized by SSP were confirmed using this technique. In this chapter structural changes of the neat PA6 and high molecular weight copolymers of PA6 named as (CL/DyI20)S2 and (CL/DyI30)S2 will be investigated at room temperature and as a function of temperature. 115
Chapter 5
5.3.1
WAXD Studies To understand the crystalline structures, X‐ray analysis of the PA6 homopolymer and (CL/DyI20)S2 and (CL/DyI30)S2 copolymers was performed at room temperature. As it was already mentioned in the introduction of this chapter, the X‐ray diffraction profiles of the α phase of the PA6 is composed of typical reflections at 2θ=20.4° and 23.7° indexed as (200) and (002/202), respectively, as shown in Figure 5.1. The X‐ray diffraction profiles of the two copolymers present strong analogies with the X‐ray diffraction patterns of the corresponding PA6 homopolymer displaying the same polyamide reflections in the same 2θ range. This means that the copolymers have crystal structures similar to that of the PA6 homopolymer. This structure consists of stacked hydrogen‐bonded sheets, in which the polymer chains are arranged side‐by‐side.5 It can be observed from Figure 5.1 that with increasing Dytek content the (200) and (002/202) reflections decrease in intensity, but they do not show any shift in position. This suggests that the interchain/intersheet distances are not affected by increasing the DyI salt content of the copolyamides. As expected the semi‐aromatic DyI nylon salt does not co‐crystallize with the PA6 repeat units and it only affects the crystallinity of the copolymers. In particular, it is possible to observe how the crystallinity decreases while increasing the DyI content. Considering the complex polymorphism of the PA6 and expecting an eventual influence of the DyI content on the crystallinity of these materials, all the samples were annealed for 24 hrs at 100 0C in order to promote the formation of the α‐form. In case of the (CL/DyI30)S2 copolymer with 30 wt% salt in the feed it is possible to notice the presence of a very low intense peak/shoulder in the 2θ range 20‐23°, the typical (100) reflection of the γ‐phase.17 The γ‐phase can be described as the aggregates of conformationally disordered chain segments with cylindrical symmetry. The small amount of γ‐phase co‐existing together with the α‐phase could be directly attributed to the higher content of the DyI salt considering the same annealing condition used for all the samples. This α‐form is not affected by the presence of the very small fraction of the γ‐phase and has a Brill transition upon heating as will be described in the next section. 116
Chapter 5
002/202
Intensity
200
PA6
(CL/DyI20)S2
100
(CL/DyI30)S2
10
15
20
2 (deg)
25
30
Figure 5.1 X‐ray powder diffraction patterns of the homopolymer and copolymers of PA6 with 20 wt% (16.6 mol%) and 30 wt% (25.6 mol%) DyI salt in the feed which are (CL/DyI20)S2 and (CL/DyI30)S2, respectively. (Real compositions of the copolymers were calculated as 15.7 and 25.3 mol% after SSP, respectively, as indicated in Table 4.4 in Chapter 4.) 5.3.2
FTIR Analysis Dytek 30%
Temperature‐dependent FTIR analysis is a powerful spectroscopic technique to analyze the influence of the incorporation of the DyI salt into the PA6 backbone on the structural changes of the (co)polymer chains. In Figure 5.2 FTIR spectra of the PA6 homopolymer and of the (CL/DyI20)S2 and (CL/DyI30)S2 copolymers can be seen recorded at 30 °C in the range of 3600‐2600 cm‐1 and 1800‐800 cm‐1. The bands at 3300‐3290 cm‐1, 3080‐3070 cm‐
1
and 2930‐2860 cm‐1 are associated with the hydrogen‐bonded NH stretching vibration, and NH stretching vibration with the overtone of amide II and CH2 asymmetric stretching vibrations, respectively17, 30 (Figure 5.2.a). The band at 3470‐3480 cm‐1 is assigned to non‐
hydrogen bonded NH groups.31, 32 For the copolymer with 30 wt% salt in the feed this broad band is quite visible. According to the DSC data the crystallinity observed during the first heating of this copolymer was lower than that of the neat PA6, as described in Chapter 4. 117
3080
2934
3300
PA6
2939
3073
3370
3400
3300
3200
3100
3000
2900
(CL/DyI30)S2
2800
1265
1235
1200
1170
1120
960
928
Absorbance [a.u.]
1545
Wavenumber [cm-1]
1477
1463
1417
1371
3500
1635
3600
2868
(CL/DyI20)S2
3300
Absorbance [a.u.]
PA6
893
842
1287
1262
1215
1180
1134
1101
1540
1640
1715
(CL/DyI20)S2
1480
1465
1417
1376
2858
Chapter 5
(CL/DyI30)S2
1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800
Wavenumber [cm-1]
Figure 5.2 FTIR spectra of the PA6 homopolymer and the copolymers of PA6 with 20 wt% (16.6 mol%) and 30 wt% (25.6 mol%) DyI salt in the feed which are (CL/DyI20)S2 and (CL/DyI30)S2, respectively, recorded at 30 °C. (Real compositions of the copolymers were calculated as 15.7 and 25.3 mol% after SSP, respectively, as indicated in Table 4.4 in Chapter 4.) The infrared spectra in the region 1800‐800 cm‐1 show the bands at 1640‐1635 cm‐1 of amide I CO stretching, 1545‐1540 cm‐1 of amide II CN stretching and CO‐NH bend which Dytek 30%
are characteristic for trans conformations as well as 1480‐1477 cm‐1 for CH2 scissoring 118
Chapter 5
next to NH groups, 1417 cm‐1 of CH2 scissoring next to CO groups and 1376‐1371 cm‐1 of CH2 twisting.30, 31, 33 The Amide III band is observed at 1265‐1262 cm‐1 and 1215‐1200 cm‐1, whereas the CH2 twisting related to amide III is at 1235 cm‐1. Bands at 1180‐1170 cm‐1 and 1134‐1120 cm‐1 are assigned to the skeletal motion involving CO‐NH and C‐C stretching, respectively.17 Amide stretching vibrations of the copolymers are visible at 960 and 928 cm‐1. Signals at 1200, 960 and 928 cm‐1 are associated with the α crystalline phase, while the 1170 cm‐1 band is associated with the amorphous phase.23 The bands at 1287 cm‐1 (amide III), 1101 cm‐1 (asymmetric CO‐C stretching34), 893 and 842 cm‐1 (amide stretching) are only visible for the copolymers and are therefore revealing the incorporation of the salt into PA6. The intensities of these bands increase with the increasing weight percentage of the incorporated salt. Additionally, similar to the non‐hydrogen bonded amide NH groups, an increase in the non‐hydrogen bonded carbonyl groups is seen by the presence of the bands at 1715 cm‐1 with increasing salt content. FTIR spectra of the PA6 homopolymer and copolyamides (CL/DyI20)S2, (CL/DyI30)S2 as a function of temperature are shown in Figure 5.3. A sharp decrease in the band at 3300 cm‐1 and the disappearance of the bands at 3202 and 3075‐3067 cm‐1 are observed with increasing temperature. These bands are associated with the NH stretch vibrations and become less pronounced close to the melting temperatures of the polymers. Weakening and broadening of the bands at 1477 cm‐1 and 1417 cm‐1 are seen via heating, pointing to a decrease in hydrogen bonding and moreover the transformation of the trans conformers into the gauche conformers. These bands come from the CH2 scissoring next to NH groups and CO groups, respectively and their total disappearance at around 170‐180 °C is observed. In the same manner the signals at 1294, 1237, 1201, 960 and 928 cm‐1 disappear totally upon heating at around 160‐170 °C. The bands at around 1294 and 1201 cm‐1 are related to amide III bands whereas the 1237 cm‐1 is the CH2 twisting. The absorptions at 960 and 928 cm‐1 were already associated with the crystalline phase and their unexpected disappearance below the melting temperature is due to their low intensity making them less pronounced at higher temperatures. 119
Chapter 5
Figure 5.3 FTIR spectra of PA6 homopolymer (a), (CL/DyI20)S2 copolymer (20 wt% (16.6 mol%) salt in the feed) (b) and (CL/DyI30)S2 copolymer (30 wt% (25.6 mol%) salt in the feed) (c) as a function of temperature. (Real compositions of the copolymers were calculated as 15.7 and 25.3 mol% after SSP, respectively, as indicated in Table 4.4 in Chapter 4.) 120
Chapter 5
The bands at 1477, 1417, 1294, 1237 and 1201 cm‐1 can be qualified as Brill bands and the disappearance of these peaks indicate the Brill transition of the copolyamides at around 160‐180 °C.20,23 During this transition the interchain and intersheet distances of the chains become equal and the triclinic structure transforms into a pseudohexagonal structure.35 Other bands in the spectra are associated with the crystalline phase and the amorphous phase. The crystalline peaks disappear close to the melting temperature of the polymer while the amorphous peaks are still visible even above the melting temperature. In conclusion, with the increasing amount of salt content, an increase in non‐hydrogen bonded chains are seen pointing to a decrase in crystallinity. This result supports the DSC data discussed in Chapter 5. Upon heating transformation of the trans conformers into gauche conformers and Brill transition are observed where the α form transforms into a pseudohexagonal phase. 5.3.3 Solid‐State NMR Analysis Temperature‐dependent (VT) 13C magic‐angle spinning/cross‐polarization (CP/MAS) NMR spectroscopy can be used as a powerful technique to analyze the structural behavior and the dynamics of the crystalline and non‐crystalline chain fragments upon heating of the copolymers prepared by the incorporation of Dytek A‐IPA salt into the PA6 main chain. CP/MAS spectra of semi‐crystalline polymers contain the chemical shifts of both the amorphous and crystalline regions, however, the crystalline fraction dominates the spectrum.28 Temperature‐dependent solid‐state NMR is a powerful technique to determine different chain conformations which are present in the polymers. Solid state NMR experiments on PA6 were carried out by other research groups before.28, 36‐38 In our work the main attention will be focused on PA6 modified with DyI salt by SSP reactions. Peak assignments and their chemical shifts of PA6 homopolymer, (CL/DyI20)S2 copolymer with 20 wt% DyI salt in the feed and (CL/DyI30)S2 copolymer with 30 wt% salt in the feed are shown in Table 5.1. 121
Chapter 5
Table 5.1. Chemical shifts of CP/MAS 13C NMR of neat PA6 and the copolymer with 20 wt% (16.6 mol%) and 30 wt% (25.6 mol%) DyI salt in the feed which are (CL/DyI20)S2 and (CL/DyI30)S2, respectively. (Real compositions of the copolymers were calculated as 15.7 and 25.3 mol% after SSP, respectively, as indicated in Table 4.4 in Chapter 4.) Dytek 30%
PA6 (CL/DyI20)S2 (CL/DyI30)S2 41 °C 212 °C 41 °C 212 °C 30 °C 193 °C C1trans 42.9 ― 43.1 ― 43.1 ― C1gauche 39.8 40.4 40.0 40.2 40.0 40.2 C2, C3gauche C2, C3trans C4trans C4gauche C5trans C5gauche COPA6 COIPA IPA residue Dytek residue 29.9 29.9 26.1 ― 36.5 ― 173.2 ― ― 30.1 30.1 25.9 27.5 36.9 ― ― ― ― 29.9 29.9 26.0 27.4 36.9 ― ― ― ― ― 30.1 30.1 26.2 ― 36.5 ― 173.2 167.4 122.0‐140.0 overlapping with PA6 30.1 30.1 25.9 27.2 36.7 ― ― ― ― ― 29.9 29.9 26.1 ― 36.3 ― 173.2 167.0 122.0‐140.0 overlapping with PA6 ― ― By the solid‐state NMR analysis of polyamides it is possible to investigate changes of the trans and gauche conformers population, where the trans form is the conformation of lowest energy with an extended form of the chains and the gauche form is the disordered conformation of the polymer chains.29 The changes in the structure and conformations of the PA6 chain fragments upon heating are shown in Figure 5.4, 5.5 and 5.6. For the neat PA6 at 41 °C, C1trans signal is observed at 42.9 ppm whereas C1gauche signal is observed at 122
Chapter 5
39.8 ppm. This gauche transformation is mostly present in the non‐crystalline region of the polymer and undergoes rapid transitions between the trans and gauche conformations upon heating.37 The C2 and C3 signals have the same chemical shift value both for the trans and gauche conformers, which was found at 29.9 ppm. C4trans and C5trans have chemicals shifts of 26.1 and 36.5 ppm, respectively, but no gauche conformers of these carbon atoms were visible at 41 °C. Upon heating, C4gauche and C5gauche start to appear at around 136 °C. Additionally, C5gauche is more pronounced via heating due to enhanced mobility of the polymer chains. These observations suggest that the increased local molecular dynamics of the methylene units between hydrogen‐bonded amide groups first promotes the formation of gauche conformers in the fragments next to the NH moieties, while the methylene units next to the CO groups are affected at higher temperatures. On further heating, the transfer of the rotational motion to the hydrogen‐bonded moieties becomes even more pronounced.39 As melting starts around 193 °C, the narrowing and sharpening of the peaks as well as the disappearance of the C1trans and the carbonyl peak at 173.2 ppm are observed. The twisting motion and the weakening of intermolecular hydrogen bonding are accelerated for these carbon atoms and more pronounced compared to other carbon atoms of the repeating unit.25 Close to the melting temperature of the PA6 (212 °C as the highest temperature applied during the measurements) only the trans and gauche transformers of the rigid amorphous phase are visible. For the copolymers (CL/DyI20)S2 and (CL/DyI30)S2 between 45‐20 ppm almost the same chemical shifts and conformational changes are observed as in the temperature‐
dependent CP/MAS 13C NMR spectra of PA6 (Figure 5.4, 5.5 and 5.6). The chemical shifts of the Dytek A residue were also expected to be seen in this region however, due to the relatively broad signals of PA6, it was impossible to distinguish any differences in the peaks compared to neat PA6 spectra. This results not only from the low concentrations of Dytek in the copolymers but also from the overlapping of the 13C resonances of the PA6 123
Chapter 5
units and Dytek residue. On the other hand, 13C resonances of the IPA in the copolymer are well visible. The carbonyl signal shows up around 167 ppm, while the aromatic carbons are in the region of 140‐120 ppm. These signals are relatively broad which indicates sample heterogeneity and local susceptibility. 212 C
193 C
t g
t
g
t,g
t
174 C
155 C
136 C
117 C
98 C
79 C
C1
CO
180
160
140
55
50
45
60 C
C4
C2, C3
C5
40
41 C
35
30
25
20
15
10
ppm
Figure 5.4 Temperature‐dependent solid state 13C CP/MAS NMR spectra of neat PA6. Letters above the spectra indicate the positions of trans (t) and gauche (g) conformers while the assignments below are explained in Table 5.1. 212 C
193 C
t g
t
g
t,g
t
174 C
155 C
136 C
117 C
98 C
79 C
60 C
C4
CO COIPA
180
160
C1
IPA
140
55
50
45
C5
40
ppm
35
41 C
C2, C3
30
25
20
15
10
Figure 5.5 Temperature‐dependent solid state 13C CP/MAS NMR spectra of (CL/DyI20)S2 with 20 wt% (16.6 mol%) DyI salt in the feed. (Real composition was calculated as 15.7 124
Chapter 5
mol% after SSP, as indicated in Table 4.4 in Chapter 4.) Letters above the spectra indicate the positions of trans (t) and gauche (g) conformers while the assignments below are explained in Table 5.1. 193 C
t g t
g
t,g
174 C
t
155 C
136 C
117 C
98 C
79 C
60 C
C4
C1
CO COIPA
41 C
C5 C2, C3
IPA
180 160 140
50
40
30
20
10
ppm
Figure 5.6 Temperature‐dependent solid state 13C CP/MAS NMR spectra of (CL/DyI30)S2 with 30 wt% (25.6 mol%) DyI salt in the feed. (Real composition was calculated as 25.3 mol% after SSP, as indicated in Table 4.4 in Chapter 4.) Letters above the spectra indicate the positions of trans (t) and gauche (g) conformers while the assignments below are explained in Table 5.1. Upon heating, the intensities of both peaks start to decrease and finally totally disappear at around 98 °C. This can be better seen for the (CL/DyI30)S2 copolymer, since the composition of the IPA in the copolymer is higher. The disappearance of the peaks at this relatively low temperature supports the conclusions from our WAXD analysis that there is no co‐crytallization of IPA and PA6. These resonances disappear at lower temperature than those of PA6 in the vicinity of 45‐20 ppm indicating a higher chain mobility which is facilitated in the amorphous phase of these copolymers. This shows that IPA is present in the more flexible and mobile chain fragments which already disappear well below the melting temperature of the PA6. 125
Chapter 5
5.4 Conclusions Analyses of the structure, morphology and conformational changes of the PA6 homopolymer and of PA6 copolymers with 20 and 30 wt% semi‐aromatic DyI nylon salt fractions in the feed were performed by using wide‐angle X‐ray diffraction at room temperature as well as by performing temperature‐dependent FTIR and solid‐state 13C CP/MAS NMR experiments. WAXD measurements revealed the presence of the typical reflections of PA6 at 2θ=20.4° and 23.7° both for the homopolymer and the copolyamides suggesting that the DyI salt does not co‐crystallize with the PA6 repeat units. However, the decrease in the intensity of the diffraction peaks showed that with increasing DyI content, the crystallinity is decreasing. Additionally, for the copolymer modified with 30 wt% added DyI salt, the appearance of a very low intensity peak at 2θ range 20‐23° pointed to the onset of the formation of a pseudohexagonal phase which is the typical (100) reflection for the γ phase. FTIR studies at room temperature also revealed the mentioned decrease of crystallinity with increasing DyI content but also a decrease of the amount of in hydrogen bonding in the polymer. The disappearance of some specific bands upon heating at around 160‐180 °C pointed both for the homopolyamide and the copolyamides to a Brill transition, where a fully pseudohexagonal structure forms. In a similar manner, the transformation of trans conformers into gauche conformers upon heating was also proven by the solid‐state 13C NMR measurements. The disappearance of the resonances of the IPA in the copolymers well‐below the melting temperature of the crystals is in agreement with the WAXD analysis indicating that there is no co‐
crystallization. References 1.
2.
3.
4.
5.
Aharoni, S. M., n‐Nylons Wiley: Chichester, New York, Weinheim, Brisbane, Singapore, Toronto, 1997. Marchildon, K. Macromol. React. Eng. 2011, 5, (1), 22‐54. Vouyiouka, S. N.; Karakatsani, E. K.; Papaspyrides, C. D. Prog. Polym. Sci. 2005, 30, (1), 10‐37. Jansen, M. A. G.; Goossens, J. G. P.; Wu, L. H.; De Wit, G.; Bailly, C.; Koning, C. E.; Portale, G. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, (4), 1203‐1217. Holmes, D. R.; Bunn, C. W.; Smith, D. J. J. Polym. Sci. 1955, 17, (84), 159‐177. 126
Chapter 5
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
Murthy, N. S. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, (13), 1763‐1782. Murthy, N. S.; Minor, H.; Latif, R. A. J. Macromol. Sci.‐Phys. 1987, B26, (4), 427‐446. Li, Y.; Goddard, W. A. Macromolecules 2002, 35, (22), 8440‐8455. Ramesh, C.; Gowd, E. B. Macromolecules 2001, 34, (10), 3308‐3313. Auriemma, F.; Petraccone, V.; Parravicini, L.; Corradini, P. Macromolecules 1997, 30, (24), 7554‐7559. Murthy, N. S. Polym. Comm. 1991, 32, (10), 301‐305. Gogolewski, S.; Gasiorek, M.; Czerniawska, K.; Pennings, A. J. Colloid. Polym. Sci. 1982, 260, (9), 859‐
863. Gurato, G.; Fichera, A.; Grandi, F. Z.; Zannetti, R.; Canal, P. Macromol. Chem. Phys. 1974, 175, (3), 953‐
975. Murthy, N. S.; Aharoni, S. M.; Szollosi, A. B. J. Polym. Sci., Part B: Polym. Phys. 1985, 23, (12), 2549‐
2565. Zhao, X. Y.; Zhang, B. Z. J. Appl. Polym. Sci. 2010, 115, (3), 1688‐1694. Akio, K. Polymer 1992, 33, (18), 3981‐3984. Arimoto, H. J. Polym. Sci., Part A: Polym. Chem. 1964, 2, (5), 2283‐2295. Johnson, C. G.; Cypcar, C. C.; Mathias, L. J. Macromolecules 1995, 28, (25), 8535‐8540. Cavallo, D.; Gardella, L.; Alfonso, G. C.; Portale, G.; Balzano, L.; Androsch, R. Colloid. Polym. Sci. 2011, 289, (9), 1073‐1079. Fornes, T. D.; Paul, D. R. Polymer 2003, 44, (14), 3945‐3961. Vasanthan, N.; Salem, D. R. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, (5), 536‐547. Vinken, E.; Terry, A. E.; Hoffmann, S.; Vanhaecht, B.; Koning, C. E.; Rastogi, S. Macromolecules 2006, 39, (7), 2546‐2552. Vasanthan, N.; Murthy, N. S.; Bray, R. G. Macromolecules 1998, 31, (23), 8433‐8435. Nair, S. S.; Ramesh, C.; Tashiro, K. Macromolecules 2006, 39, (8), 2841‐2848. Yoshioka, Y.; Tashiro, K. J. Phys. Phys. Chem. B 2003, 107, (43), 11835‐11842. Clauss, J.; Schmidt‐Rohr, K.; Adam, A.; Boeffel, C.; Spiess, H. W. Macromolecules 1992, 25, (20), 5208‐
5214. Murthy, N. S.; Curran, S. A.; Aharoni, S. M.; Minor, H. Macromolecules 1991, 24, (11), 3215‐3220. Weeding, T. L.; Veeman, W. S.; Gaur, H. A.; Huysmans, W. G. B. Macromolecules 1988, 21, (7), 2028‐
2032. Bower, D. I., An Introduction to Polymer Physics. Cambridge University Press: Cambridge, UK, 2002. Kohan, M. I., Nylon Plastics Handbook. Hanser Publishers: Munich, Vienna, New York, 1995. Coleman, M. M.; Lee, K. H.; Skrovanek, D. J.; Painter, P. C. Macromolecules 1986, 19, (8), 2149‐2157. Cooper, S. J.; Coogan, M.; Everall, N.; Priestnall, I. Polymer 2001, 42, (26), 10119‐10132. Jasinska, L.; Villani, M.; Wu, J.; van Es, D.; Klop, E.; Rastogi, S.; Koning, C. E. Macromolecules 2011, 44, (9), 3458‐3466. Silverstein, R. M.; Webster, F. X., Spectrometric Identification of Organic Compounds. John Wiley&Sons 1998. Hirschinger, J.; Miura, H.; Gardner, K. H.; English, A. D. Macromolecules 1990, 23, (8), 2153‐2169. Hatfield, G. R.; Glans, J. H.; Hammond, W. B. Macromolecules 1990, 23, (6), 1654‐1658. Kubo, K.; Yamanobe, T.; Komoto, T.; Ando, I.; Shiibashi, T. J. Polym. Sci., Part B: Polym. Phys. 1989, 27, (4), 929‐937. Okada, A.; Kawasumi, M.; Tajima, I.; Kurauchi, T.; Kamigaito, O. J. Appl. Polym. Sci. 1989, 37, (5), 1363‐
1371. Harings, J. A. W.; Yao, Y. F.; Graf, R.; van Asselen, O.; Broos, R.; Rastogi, S. Langmuir 2009, 25, (13), 7652‐7666. 127
Chapter 5
128
CHAPTER 6 EPILOGUE AND TECHNOLOGY ASSESSMENT This study provided interesting insights into the modification of a well‐known, widely used commercial polymer, namely Polyamide 6 (PA6), rendering new possibilities with respect to its chemistry and properties. PA6 is conventionally modified with other components via melt processing resulting in the deterioration of the good properties. The aim of this study was to modify PA6 with other conomomers/polymers above the Tg and below the Tm of the PA6 by keeping the crystalline phase during this modification intact, by which the known favorable physical and mechanical properties of PA6 should stay intact and other properties could be improved. This can be achieved by solution and/or solid state polymerization (SSP) so that the mobile amorphous part can take part in the modification reaction whereas the reduction of the crystalline phase is limited. This thesis can be divided into two main parts. The first part is devoted to the modification of PA6 for enhanced biodegradability and the second part describes enhanced physical, thermal or mechanical properties which can be tuned for a specific application. Fostering the degradation of commercial plastics without losing the good properties of the material has been a hot topic during the last decades for environmental reasons. PA6 is not susceptible to (bio)degradation like some other commodity/engineering plastics. Our synthetic technique, making short chains of PA6 and polyesters (PE) with reactive end groups which can already react with one another in solution and in the solid state, results in multiblock copolymers which are partially degradable and still exhibit the good thermal and physical properties of the neat PA6 blocks as explained in Chapter 2. In our study we only presented such a multiblock copolymer formation by incorporating polycaprolactone into PA6, however, different biodegradable polymers from a wide variety of PEs can be selected with different degradation rates, the targeted thermal and mechanical behavior 129
Chapter 6
of the resulting materials depending on the desired application. Due to the low thermal stability of the urea and urethane linkages between the PE and PA6 blocks, it is difficult to process these materials at high temperatures. Another limitation is the possible occurrence of ester/amide exchange reactions during processing. These effects can be overcome, provided that short reaction times and low processing temperatures are employed. In Chapter 3, carboxylic acid‐epoxide reactions were used to make multiblock polyesteramide copolymers. It was shown that these reactions have a complicated nature and result in side reactions and crosslinking, limiting the formation of very high molecular weight copolymers. On the other hand, these materials could be used as starting materials to prepare crosslinked network structures for coating applications. Additionally, PA6 with totally amine end groups can be used with epoxide terminated PEs to make linear chains of both blocks. In Chapter 4, the incorporation of a semi‐aromatic nylon salt into a commercial grade PA6 below the melting temperature of PA6 but above its Tg resulted in an almost complete retention of the high melting temperature while a high crystallization rate was retained as well. To the best of our knowledge this was shown for the first time in the literature. The irregular chemical structure of this nylon salt makes it easier to be incorporated only in the amorphous phase without any cocrystallization with the PA6 repeat units, as shown in Chapter 5. These reactions provide the direct opportunity to play with the properties of PA6 by modifying the amorphous phase, e.g. resulting in a lower water uptake, a higher Tg, enhanced flame retardancy, etc. This incorporation is achieved by a two‐step process: first solution mixing of both the PA6 and the modifying component in hexafluoroisopropanol (HFIP), followed by SSP after complete removal of the solvent. In the beginning, due to the loss of significant parts of the diamine, relatively low Mn copolymers were obtained and after optimizing the reaction conditions higher Mn copolymers could be prepared. Although high Mn values are desired for many applications, in industry low Mn copolymers can be used for further modification for some certain applications. It is possible to prevent the loss of the diamine by adding some excess of this nylon salt component to prevent a stoichiometric imbalance, which is 130
Chapter 6
already an applied technique in industry. A diamine with a higher boiling point can also limit its evaporation at the processing temperature. Handling of HFIP is not feasible for mass production since it is an expensive, corrosive and toxic solvent. Although it is not entirely industrially feasible to use the same manufacturing approach for an industrial application as used here, the HFIP mixing part can be replaced by a very short pre‐melt mixing in an extruder. Our studies already demonstrated that short reaction times used at high reaction temperatures do not lead to significant changes in the crystallization behavior, and if a very short melt‐mixing of PA6 with the nylon salt that should be incorporated is followed by the described SSP procedure, the end product is expected to maintain a block‐like structure and good thermal properties. 131
Chapter 6
132
APPENDIX Validation of the Flory equation for the melt polymerized copolymers of CL/DyI In Chapter 4, firstly relatively low molecular weight and in a next step higher molecular weight copolymers of PA6 with semi‐aromatic DytekA/IPA (DyI) nylon salt were prepared both by solid‐state polymerization (SSP) and melt polymerization (MP) for comparison of the molecular, thermal and microstructural properties. DSC studies together with quantitative 13
C NMR analysis pointed to a totally random microstructure for the copolymers prepared by MP. Destruction of the crystalline phase with the incorporation of the DyI salt can also be proven by a theory developed by Flory. Flory developed the following equation for the equilibrium melting temperature of a random copolymer: ∆
where the Tm is the melting temperature of the copolymer (K), Tmo is the melting temperature of the homopolymer (494 K), R is the universal gas constant (8.314 J.K‐1mol‐
1
), ∆HL is the heat of fusion of the repeat unit of the homopolymer (7253.6 Jmol‐1 according to the DSC measurement) and Na is the mol fraction of the homopolymer (calculated from the 1H NMR measurement after MP as presented in Table 4.1 and Table 4.4). Weight and mol compositions of the copolymers as well as the melting temperatures calculated from the above equation and measured by DSC are shown in Table A.1. It is clear from Figure A.2 that the copolymers synthesized by melt copolymerization obey the Flory’s equation very well, which is in agreement with the DSC melting temperature data. The depression in melting temperature with the increasing DyI content is proven both by the measurements and the theoretical calculation. This reveals the deformation of the crystalline phase and randomization if the melt polymerization is employed. 133
Appendix
Table A.1 Weight/mol fractions and calculated/DSC melting temperatures of the copolymers synthesized by melt polymerization. DyI salt
PA6 content
(wt%)*
(mol%)**
low Mn copolymers
0
1
5
0.95
10
0.90
15
0.84
20
0.81
high Mn copolymers
10
0.93
20
0.85
30
0.77
Calculated Tm
(°C)
DSC Tm
(°C)
221.0
207.9
192.9
177.1
168.9
221.0
211.0
194.4
183.7
172.0
200.1
179.6
156.5
194.8
179.2
no Tm
* Weighed percentage in the feed. 1
**As calculated from H NMR after the melt polymerizations. a.
DSC Tm
Tm (C)
210
Tm (C)
b.
Calculated Tm
220
200
190
220
Calculated Tm
210
DSC Tm
200
190
180
180
170
170
160
150
160
0
5
10
15
0
20
5
10
15
20
25
30
wt% DyI salt
wt% DyI salt
Figure A.1 Melting behavior of the low molecular weight (a) and high molecular weight (b) CL/DyI copolymers synthesized by melt polymerization. Flory’s equation can also be applied for the low and high molecular weight copolymers synthesized by solid‐state polymerization (SSP) to show that non‐random copolymers are formed. Weight and mol compositions of the copolymers as well as the melting temperatures calculated from the above equation and measured by DSC are shown in 134
Appendix
Table A.2. It can be seen from Figure A.2 that the copolymers synthesized by SSP do not obey the Flory’s equation at all, since there is a clear deviation of the calculated melting temperatures from the DSC melting temperature data. After SSP, the melting temperatures of the copolymers are still close to that of PA6 homopolymer indicating that a blocky microstructure is formed. This reveals that the deformation of the crystalline phase and randomization is limited during SSP. Table A.2 Weight/mol fractions and calculated/DSC melting temperatures of the copolymers synthesized by solid‐state polymerization. Salt
PA6 content (wt%)* (mol%)** low Mn copolymers
0
1
5
0.96
10
0.91
15
0.90
20
0.86
25
0.82
30
0.83
high Mn copolymers
10
0.93
20
0.85
20Ex.
0.85
30
0.75
Calculated Tm
(°C) DSC Tm
(°C) 221.0
210.7
196.5
192.3
182.1
175.5
170.0
221.0
216.3
216.2
208.0
203.1
201.9
202.7
202.1
177.4
179.9
150.9
216.4
212.0
211.5
207.1
* Weighed percentage in the feed. 1
**As calculated from H NMR after the melt polymerizations. 135
Appendix
a.
b.
Calculated Tm
220
DSC Tm
Calculated Tm
DSC Tm
220
210
210
Tm ( C)
Tm (C)
200
200
190
190
180
170
180
160
170
150
0
5
10
15
20
25
0
30
5
10
15
20
25
30
wt% DyI salt
wt% DyI salt
Figure A.2 Melting behavior of the low molecular weight (a) high molecular (b) weight CL/DyI copolymers synthesized by solid‐state polymerization. References 1.
Flory, P. J. J. Chem. Phys. 1949, 17(3), 223‐240. 136
Summary Polyamide 6 based block copolymers synthesized in solution and in the solid state Polyamide 6 (PA6) is a well‐known engineering plastic which has outstanding properties such as excellent physical and mechanical strength and chemical stability mainly due to its high chain regularity and strong hydrogen bonding between the chains resulting in a strong crystalline phase. PA6 production is mainly performed for automotive, electrical and packaging applications. Material properties of PA6 can be modified according to the desired applications. This modification is conventionally done by copolymerization of comonomers with ε‐caprolactam in the melt or by reactive blending, both methods resulting in a random distribution of the comonomers introduced into the PA6 main chain. This random distribution of the introduced comonomers causes a decrease in melting temperature, crystallization rate and crystallinity, resulting in undesired mechanical and physical properties. The aim of this research was to modify PA6, either by starting from low molar mass end‐
functionalized PA6 blocks and react these with other blocks carrying functional groups reactive with the PA6 end groups, or by submitting high molar mass commercial PA6 by solid state polymerization (SSP) or modification without destroying the crystalline phase. For both methods the synthetic approach consisted of first molecular mixing all components in a common solvent, followed by an SSP treatment. If this is performed above the glass transition temperature but below the melting temperature of PA6, modification of the backbone occurs in the amorphous phase which is the only mobile part of the polymer in this temperature range. Biodegradability of PA6 can be enhanced by e.g. making polyesteramide copolymers containing both PA6 and aliphatic polyester segments, since the ester‐containing blocks are susceptible to degradation in nature. If this is done in solution and/or solid state, then multiblock copolymers based on PA6 can be obtained. In this way, the good properties of 137
Summary PA6 can be largely retained while degradation can be achieved as an additional property, an important advantage for e.g. packaging applications. The synthesis and characterization of polyesteramide multiblock copolymers were described in Chapters 2 and 3. In the former one, totally amine end‐capped PA6 polymers (Mn=1,200‐4,100 g/mol) and totally hydroxyl end‐capped polypropylene adipate (PPA) polymers (Mn=900‐1,400 g/mol) were synthesized. PPA with the lowest obtained Mn and a commercially available polycaprolactone diol (PCL) were toluenediisocyanate (TDI) end‐capped. Since the handling of the oily diisocyanate end‐capped PPA was difficult, for the preparation of PA6‐
based multiblock copolymers amine end‐capped PA6 and TPCL (TDI‐modified PCL) were used. Reaction between the end groups and formation of high molecular weight multiblock copolymers were already observed at room temperature in hexafluoroisopropanol (HFIP), making an SSP treatment superfluous in this case. Biodegradation studies on the polymer films showed the enhanced degradability after PCL incorporation, where up to a 12% weight loss was observed after 8 weeks of enzymatic incubation at 25 °C. A pure PA6 polymer showed virtually no weight loss after a similar treatment. Another reaction route was applied in Chapter 3 for the same purpose of obtaining enhanced biodegradability. This approach consisted of a base‐catalyzed reaction between diepoxy propylene adipate (DEPA) and low molecular weight carboxylic acid end‐capped PA6 after solution mixing in HFIP, followed by complete removal of the solvent and finally an SSP treatment. To determine the optimum reaction conditions firstly model reactions were performed by using a diepoxy oligoether and carboxylic acid end‐capped PA6. As a catalyst for the coupling of both blocks 4‐(dimethylamino) pyridine (DMAP) was used in different amounts in a temperature range of 70‐120 °C. The highest Mn value obtained was 17 kg/mol and further increase in molecular weight was restricted by side reactions and crosslinking. Later, PA6‐DEPA reactions were performed with or without the addition of DMAP and triethylamine (TEA) as catalyst between 80‐140 °C. In this case lower molecular weighs were obtained, however multiblock copolymers were achieved consisting of a few blocks of PA6 and the oligoester. 138
Summary SSP can also be used to modify high molecular weight commercial grade PA6, as shown in Chapter 4. For this purpose a semi‐aromatic nylon salt of 1,5‐diamino‐2‐methylpentane (Dytek A) and isophthalic acid (IPA) was prepared which has a highly irregular chemical structure and accordingly is expected to be incorporated only in the amorphous phase of the PA6 without any cocrystallization with the PA6 backbone (see furtheron in this summary). This modification was also performed in the solid state, between Tg and Tm of the PA6, after solution mixing of the Dytek A‐IPA salt and PA6 in HFIP and after evaporation of this solvent. Different amounts of salt were used and melt polymerizations were also carried out for comparison with the performed SSP reactions. Incorporation of the diamine of the salt occurs via aminolysis whereas the dicarboxylic acid incorporation occurs via acidolysis reactions. Therefore, in the first stages of the SSP reaction a dramatic decrease in molecular weight was observed because of chain scission, which later started increasing due to polycondensation. Restricted molecular weight growth after the initial chain scission was observed due to the loss of parts of the diamine, causing an unbalance in the functional end‐groups, which could be prevented by using a closed environment and by lowering the initial SSP temperature until complete incorporation of the Dytek A. In the second stage, the reaction temperature was increased and continuous Argon flow was applied to favor the incorporation of IPA and to remove the condensation water by coupling of the broken chains. By this way, copolymers with Mn values of 12‐24 kg/mol were obtained with 30‐10 wt% salt added in the feed, respectively. The block length and degree of randomness calculations, which were performed by quantitative 13C NMR together with thermal analysis, strongly pointed to a blocky microstructure in case of the SSP reaction products. Comparison of the copolymers synthesized by SSP and MP with the same salt compositions not only revealed a significant decrease in the thermal properties but also a random distribution of the Dytek A‐IPA salt in the PA6 main chain after MP. This revealed that the deformation of the crystalline phase was prevented during SSP below the melting temperature of PA6 and that the incorporation of the salt only occurs in the amorphous phase. By this way, block‐like copolymers with thermal properties quite similar to those of pure PA6 were obtained. 139
Summary Morphological analysis of the copolymers prepared by the incorporation of the Dytek A‐
IPA salt was expanded in Chapter 5. Wide angle X‐ray diffraction (WAXD), solid state 13C NMR and FT‐Infrared spectroscopy (FTIR) analyses provided a better understanding of the structural behavior of the blocky copolymers prepared by SSP. WAXD measurements revealed that the nylon salt does not co‐crystallize with the PA6 repeat units. However, with increasing salt content a decrease in crystallinity and the formation of a pseudohexagonal phase was observed. The decrease in crystallinity was also observed from FTIR analysis at room temperature while FTIR analysis upon heating showed a Brill transition at around 160‐180 °C. From the temperature‐dependent solid state 13C NMR analysis the disappearance of the resonances of the IPA in the copolymers was observed well‐below the melting temperature of the crystals and this was attributed to the absence of cocrystallization of the Dytek A‐IPA salt with the PA6 chain segments, in agreement with the WAXD analysis. 140
Acknowledgements The end seemed too far at the beginning but the time flies, especially if you are having a good time. I would like to thank all the people who contributed to this good time both academically and socially during my PhD in Eindhoven. My dear promoter and supervisor Prof. Cor Koning, thank you very much for accepting me to this wonderful research group. Although you were very busy most of the time as the manager of the group, you have always been very motivating, positive and supportive during my research. I appreciate all your valuable contributions in my scientific progress. I would also like to thank DSM for the financial support and to the people who contributed to this thesis with their fruitful discussions and support: Dr. Pim Janssen, Dr. Rudy Rulkens, Dr. René Kierkels and Dr. Ronald Ligthart as well as technical staff: Marcel Aussems and Victoria de Bruijn. The reading committee of this thesis, Prof. Jan Meuldijk, Prof. Iskender Yilgor, Dr. Han Goossens and Prof. Christian Bailly are sincerely thanked for their comments and input for the last version of the thesis and for being part of the core committee. Dr. Rob Duchateau and Dr. Albert Schenning are appreciated for joining the defense committee. Many people within and outside the university contributed to this thesis. Many thanks to the ex and present technical staff of our Polymer Chemistry (SPC) group for their help with analysis: Wieb, Martin F. and Harry for SEC, Carin and Hanneke for MALDI and Rinske for SEM. Thanks to other people who took responsibility in analytical measurements: Monique, Inge, Yingyuan, Jey, Lidka and Erik for DSC measurements, Gozde for her help with DMTA, and Maria and Weizhen from SKT group for measuring DSC and TGA samples. Some nice collaborations came out during this study. Many thanks to Marko from SMO group for many weekend 13C NMR measurements and discussions and to Maurizio from SKT for his valuable help with X‐Ray analysis. Dr. Magnus Eriksson and Dr. Mats Martinelle from Royal Institute of Technology Stockholm are thanked for sharing their polyester with 141
us. Dr. Michael Ryan Hansen is thanked for solid state NMR measurements in Max Planck Institute Mainz. If it comes to acknowledging other people who helped me during my PhD, I should start with Thierry who was my guide in the lab during my first months and spent a lot of time with me for the synthesis of polyamides in a scary reactor. Merci beaucoup! And then, I should continue with more people: Donglin, for running for my help whenever I needed something in the lab or in the office and making me smile with your smart jokes. My sweet lady Lidka, I always enjoyed working close to your fumehood and being supported by you not only for work related stuff but also for any other thing. Dziękuję dużo miodu! Many thanks to my hard working students Onder and Mohammed. Special thanks to the rest of the inhabitants of office STO 1.41 who shared it with me. Rubin, Charlotte, Lyazzat; thank you for the friendly atmosphere in the office. Mi chiquita Sandra, you were much more than an office mate. I loved sharing the office and the house with you. Muchas gracias for all the great times we had. M. Peppels, I will miss your energetic ‘good morning’s and our long discussions about many different things. Thanks to Martin O., Martin F., Mark B., Gozde, Bahar, Camille, Elham, Fabian and Julien for making the coffee room full of food and drinks for us. Many many thanks to all the other members of the group with whom we had countless amounts of coffee, lunch breaks, borrels, and spent many time together: Rafael, Patricia, Wouter, Roxana, Hector, Simona, Judit, Maurice, Syed, Ingeborg, Joris, Dirk, Bart, Hans, Alex, Rob, Hemant, Shaneesh, Gemma, Gijs, Jerome, Judith, Timo, Ece, Dogan, Pooja, Yun, Jing, Miloud, Mohammad and Evgeniy. Some special thanks to special ladies; Bahar (dostlugun icin cok sagol!), Gözde, Camille and MC for all the lovely times we had inside and outside the department. Also, many thanks to our secretaries Pleunie and Caroline who took care of everything. Many Thursday evenings gathered people together in Fort from other groups/departments: Olivier, Benjamin, Florence, Alberto, Hans, Sabriye, Seda C., Başar, Barış, Ali Can, Natalia, Matthieu, Daniele, Dario, Domenico, Gosia, Jovan and many others…Thank you for sharing the beer and the good time! 142
Well, this PhD would be difficult without the amazing Turkish gang. My lovely space box neighbors Önder, Fırat, Derya and Melike, it was perfect to have you there. Melike, yavrucum, ne desem az, iyi ki varsin! Canım arkadaşım Bestem, I feel so lucky that we both came here! And the very special rest of the gang; Merih, Can, Hakkı, Nimet, Levent, Güneş, Tunç, Ulaş, Nilhan, Ezgi, Oğuz, Sinan, Memo, Ekin, hepiniz tek tek sağolun canlar! All the innumerable activities, trips and other times we had together were great and it was always like home with you! Finally, I would like to thank my family for always being by my side and to İso for being my family here in Netherlands. Thanks to everyone who made it possible to live and work in such a gezellig environment and tot de volgende keer! Seda 143
List of publications 




S. Cakir, L. Jasinska‐Walz, M. Villani, M. R. Hansen, C. Koning “Investigation of local chain conformation and morphology of polyamide 6 based copolymers”, in preparation. S. Cakir, C. Koning “Polyamide 6 Based Multiblock Copolymers Synthesized in Solid State”, in preparation. S. Cakir, M. Nieuwenhuizen, P. Janssen, R. Rulkens, C. Koning “Incorporation of a semi‐aromatic nylon salt into polyamide 6 by solid state or melt polymerization: Influence on degree of randomness”, submitted. S. Cakir, R. Kierkels, C. Koning “Polyamide 6‐polycaprolactone multiblock mopolymers: Synthesis, mharacterization, and degradation” J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 2823‐2833. C. Oguz, M. A. Gallivan, S. Cakir, E. Yilgor, I. Yilgor “Influence of polymerization procedure on polymer topology and other structural properties in highly branched polymers obtained by A2 + B3 approach” Polymer 2008, 49, 1414‐1424. Conference Proceedings: 




S. Cakir, C. Koning, M. Eriksson, M. Martinelle “Multiblock copolymers of polyamide 6 and diepoxy propylene adipate obtained by solid state polymerization” European Polymer Congress, Granada, Spain, June 2011, 52. S. Cakir, R. Kierkels, C. Koning “Biodegradable polyamide 6‐polycaprolactone multiblock copolymers obtained by solution and solid state step growth polymerization” Proceedings of Polycondensation Conference, Kerkrade, the Netherlands, September 2010, 105. S. Cakir, C. Koning “Polyamide 6 based multiblock copolymers obtained by solid state step growth polymerization”, Abstracts of Papers, 238th ACS National Meeting, Washington, DC, United States, August 16‐20, 2009. C. Oguz, S. Cakir, E. Yilgor, M. G. Gallivan, I. Yilgor “Influence of polymerization procedure on the topology and structural properties of highly branched polymers in A2+B3 systems: A modeling study” Abstracts of Papers, 235th ACS National Meeting, New Orleans, LA, United States, April 6‐10, 2008. S. Cakir, E. Yilgor, I. Yilgor, C. Oguz, M. G. Gallivan “Highly branched, segmented polyurea elastomers through oligomeric A2+B3 approach” Abstracts of Papers, 233rd ACS National Meeting, Chicago, IL, United States, March 25‐29, 2007. 145
Curriculum Vitae Seda Çakır was born on the 29th of August 1983 in Giresun, Turkey. After finishing her high school education at Bursa Gazi Anatolian High School in 2001, she started her bachelors at Middle East Technical University in Ankara. In 2005 she graduated with a B.Sc. in Chemical Engineering. Subsequently, she won a scholarship from the Materials Science and Engineering Master’s Program at Koç University in Istanbul. She worked within the Polymer Science group under the supervision of Prof. İskender Yılgör on the research entitled: “Highly branched, segmented polyurea and polyurethane elastomers through oligomeric A2+B3 approach” and received a M.Sc. degree in July 2007. In September 2007 she started her PhD study at the Eindhoven University of Technology in the Polymer Chemistry group supervised by Prof. Cor E. Koning. The most important results of this study are presented in this dissertation. 146