Thermal Properties of Silk/Poly(lactic acid) Biocomposite
Hoi-Yan Cheung and Kin-Tak Lau*
Department of Mechanical Engineering, The Hong Kong Polytechnic University,
Kowloon, Hong Kong SAR
*Corresponding Author: [email protected]
ABSTRACT
Conventional composites fabricated with synthetic polymer matrix like epoxy accompanied by those synthetic fiber
reinforcements like glass and carbon fibers are widely accepted in nowadays structural applications. However, these types of
composites are still facing serious problems of their biocompatibility, bioresorbability and biodegradability once they are being
used for the clinical applications. Recently, there is a new trend in developing novel composites which are made of
biodegradable polymer matrix like polycaprolactone (PCL), poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and their copolymers, reinforcing with plant-based or animal-based natural/bio-fibers like kenaf, jute, sisal, wool, spider and silkworm silks.
These novel types of composites are so called “biocomposites”, and they have advantages over the traditional ones including
low weight, well developed physical and chemical structures, and excellent thermal and mechanical properties. In this research
study, thermal properties of a silkworm silk/PLA biocomposite was investigated by making use of three main thermal analysis
technique, differential scanning calorimetry (DSC) for revealing different significant thermal regions, dynamic mechanical
analysis (DMA) for estimating storage modulus, loss modulus and even the mechanical loss factor, and thermogravimetrical
analysis (TGA) for observing the thermal stability and transition regions of the biocomposite, in order to compare with that of
pure PLA matrix.
Introduction
Due to the fast development on bio-technologies, it is now possible for doctors to use patients’ cells to repair orthopedic
defects. In order to support the three-dimensional tissue formation, implants made by biocompatible and bioresorbable
polymers and composite materials, for providing temporary support of damaged body and cell structures have been developed.
The major concern in developing implants for different surgical and orthopedic operations is the selection of suitable
biomaterials. Potential materials that have been proven by experimental data on their validity for biomedical applications are
metal, ceramics, polymers and the combinations of these materials (composites). For metallic materials and ceramics, they
have contributed to lists of medical applications, particularly in orthopedic tissue replacements. However, there are three major
limitations; they are (i) not biodegradable except biodegradable bio-ceramics, (ii) poor processability, and (iii) necessity of an
additionally surgical operation which induces extra pain for the patients. Therefore, biocompatible and biodegradable polymers
have shown a tremendous promise in providing more viable alternatives for the tissue engineering applications.
Basically, there are four types of polymeric materials used for clinical applications; they are (i) natural polymers, (ii) synthetic
polymers, (iii) hydrogels and (iv) composites. Many natural polymers found in living organisms of known biocompatibility can
be used to replace or regenerate native tissue structures and allow positive cell interactions with surrounding tissues.
Conversely, synthetic polymers are formed through controllable chemical processes to achieve desirable material and
chemical properties for a wide range of bio-medical applications. Hydrogels are primarily composed of fluid that swells the
polymer network to form a biphasic construction. Although the hydrogels can be synthesized, they are always formed naturally.
Composite materials are attractive as they combine material properties in ways not found in nature. Such materials often result
in lightweight structures having tailored properties for specific applications. Broadly defined, biocomposites are composite
materials made from natural/bio-fiber and non-biodegradable polymers like polyethylene (PE) and epoxies or biopolymers like
PLA and PGA [1].
One of the notable synthetic bio-polymers, poly(lactic acid) (PLA), which is an alpha polyester used widely in medical
applications and it has been approved by the Food and Drug Administration (FDA) for implantation in human body. PLA is a
highly versatile biopolymer derived from renewable resources like starch or sugar-based materials such as corns [2]. Most
work in literatures on fully resorbable biocomposites has been done with the use of PLA. The reason is that PLAs possess two
major characteristics that make them an attractive bioabsorbable material: (i) they can be degraded inside the human body in
a controllable rate, and (ii) if crystallization of PLA is prevented, their degradation products are nontoxic, biocompatible, and
can be easily metabolized [3]. Biocomposites from natural/bio-fibers and PLA have useful properties for environmental and
biomedical applications. For reinforcements of biocomposites, spider and silkworm silks, are animal-based high strength
natural/bio-fibers that can be resorbed by the human body. Combining these silks with biodegradable polymers can produce a
moderate strength and durable biocompatible and bio-resorbable polymer-based composite for potential bio-medical and
orthopedic applications [4].
This study was mainly focused on the biomaterials that can be used as implants in human body, therefore, thermal properties
and stability of the biomaterials would be an essential consideration. In the study, a natural/bio-fiber reinforced biodegradable
polymeric biocomposite, silkworm silk/PLA biocomposite was fabricated and the comparison in terms of their thermal
properties, was made between this type of biocomposite and pure PLA. For thermal property tests, Differential Scanning
Calorimetry (DSC), Thermogravimetrical Analysis (TGA) and Dynamic Mechanical Analysis (DMA) were used to investigate
glass transition, melting and decomposition temperatures, thermal stability, and dynamic loss modulus, dynamic loss modulus
and tan(δ) as a function of temperature and frequency of the biocomposite respectively.
Materials Processing
In order to study the thermal properties of biocomposites, natural tussah or raw silk fibers which are the products of larvae of
moths were used as reinforcements of a biocompatible and biodegradable polymer, PLA which is formed from corn starch to
fabricate high-performance biocomposites. Tussah or raw silk fibers were chopped gently into short fibers with the length of
5mm, in order to make sure that they were not stressed plastically during the fabrication processes. The matrix used in this
study was PLA which is kindly supplied by East Link Degradable Materials Ltd., Hong Kong. Before fabricating the
o
biocomposite, silk fibers and PLA pellets were first undergone a dry treatment with heat applied (80 C) for 24 hours in an oven
so as to remove excessive moisture which may strongly affect the properties of this kind of thermoplastic due to the formation
of avoids after curing. In this study, a comparison was made between a pure PLA sample and a silk/PLA biocomposite with
fiber contents of 5wt%. All sets of samples were fabricated by extrusion and injection molding method to maintain results’
consistency. The fibers and PLA matrix were mixed at different ratios, fed into a twin-screw extruder and maintained a uniform
o
temperature of 183 C at all zones inside the machine. The screw speed and the mixing duration were set to 100rpm and 10
minutes, respectively. The first run of the extrusion was discarded and the strands of the extrusion products were then directly
collected by a pre-heated injection cylinder for further injection molding. The molten mixture was then transferred to a minio
o
injection-molding machine; the injection cylinder and the mold were pre-heated to the desired temperatures of 200 C and 45 C
respectively. The resultant composite was in a dumbbell shape according to ASTM D638 for further testing.
Measurements
Differential Scanning Calorimetry (DSC)
DSC is extensively used in many different industries, its application and usage in the plastics industry has been widely
accepted. It is used to characterize materials for melting points, glass transition temperatures, and other material and material
reaction characteristics such as specific heat, crystallinity, and reaction kinetics. In this study, the melting and crystallization
behavior of PLA and the silk /PLA biocomposite were studied by using DSC at ambient condition. The temperature range of
o
o
o
the experiment was programmed as start from 25 C and end at 300 C at a constant scanning rate of 10 C/min. All testing
samples with sample weight of approximately 5mg was placed and sealed into an aluminium pan and an empty aluminium pan
was used as reference. The heat flow and energy changes in the aluminium pans were recorded. By observing the difference
in heat flow between the samples and a reference sample, DSC is able to measure the amount of energy absorbed or
released during samples’ phase transition. And therefore, crystallization and melting temperatures can be read from the peaks
as shown on the graph and even more subtle phase changes like glass transition temperature (Tg) can be observed.
Dynamic Mechanical Analysis (DMA)
DMA is a technique in which the elastic and viscous responses of a sample under oscillating load are monitored against
temperature, time or frequency. The frequency of oscillation is proportional to the modulus (stiffness) of the material.
Properties obtained from DMA as a function of temperature include in-phase component - storage modulus (E’ or G’) which is
the measurement of energy stored during deformation and related to solid-like or elastic portion of the elastomer; out-of-phase
component - loss modulus (E” or G”) is the measurement of energy lost, usually as heat, during deformation and related to
liquid-like or viscous portion of the elastomer. E’ and E” were determined in this study for stretching deformations. Tan delta
(tan δ) which is related to material’s ability to dissipate energy in the form of heat. Elastomer melting point (Tm) and glass
transition temperature (Tg) can also be determined from DMA curves. The setting of the DMA here was scanned thermally
o
o
o
from 25 C to 100 C with a scanning rate of 2 C/min. Frequency was selected to be sinusoidal oscillation with 1Hz. According
to the specifications provided, for a dual-cantilever bending test, sample size in this study was processed to 50mm in length,
5mm in width and 1.5mm in thickness with both ends clamped. A load was applied in the middle of the samples.
Thermogravimetrical Analysis (TGA)
TGA is commonly employed in research and testing to determine characteristics of materials such as polymers, to determine
degradation temperatures, absorbed moisture content of materials, the level of inorganic and organic components in materials,
decomposition points of explosives, and solvent residues. Thermal stability of the both pure PLA sample and the silk/PLA
biocomposite were revealed by making use of the TGA technique at ambient condition in this study. The setting of
o
o
o
temperatures was start from 25 C and end at 500 C with the temperature scanning rate of 10 C/min and air flow rate was set
to be around 400cc/min.
Results and Discussions
Crystallization and melting behavior of PLA and silk/PLA biocomposite
By making use of DSC, glass transition temperature (Tg), crystallization temperature (Tc), melting temperature (Tm),
crystallization enthalpy (ΔHc) and melting enthalpy (ΔHm) can be determined from DSC curves. According to literature
reference [5], assuming PLA is purely crystalline, the value of melting enthalpy of PLA should be 93.7 J/g and therefore the
o
degree of crystallinity (X%) in the biocomposite can be obtained by calculating from the equation of X% = ΔHm / ΔHm x 100%.
All the results from DSC testing are summarized in Table 1, the values of Tg, Tm and ΔHm for both the pure PLA and the
o
o
silk/PLA biocomposite are around 62 C, 170 C and 35J/g respectively and they are quite close to each other, whereas for the
o
Tc and ΔHc, the value decreases with the addition of silk fiber reinforcements and the decrements are dropped by 14 C and
15J/g respectively. Moreover, the degree of crystallinity increases slightly in the presence of silk fibers and the value is about
38%.
Referring to other literatures [6-9], there are two main factors that can control the crystallization of polymeric composite
systems: (i) the additives hinder the migration and diffusion of polymer molecular chains to the surface of the growing polymer
crystal in the composites, thus, provide a negative effect on polymer crystallization and result a decrease in the Tc; (ii) the
additives have a nucleating effect which give a positive effect on polymer crystallization and an increase in Tc. However, in this
o
case, the Tc of the silk/PLA biocomposite dropped by 14 C. It may be concluded that in the presence of silk fibers, viscosity of
the composite mixture increases significantly which hinders the migration and diffusion of PLA molecular chains to the surface
of the nucleus in the biocomposite.
o
o
o
Sample / Biocomposite
Tg ( C)
Tc ( C)
Tm ( C)
ΔHc (J/g)
ΔHm (J/g)
X%
Pure PLA
58.9
114.0
171.8
34.5
35.4
37.8
Silk/PLA (5/95)
64.4
100.2
168.4
19.9
35.7
38.1
Table 1. Thermal properties of pure PLA sample and 5wt% silk/PLA biocomposite by the use of DSC.
Dynamic Mechanical Properties
Thermo-mechanical properties were tested by DMA dual-cantilever bending test in this study and the following curves show
the storage modulus (E’), loss modulus (E”) and tan delta (tan δ) as a function of temperature for both the pure PLA sample
and the silk/PLA biocomposite. According to Figure 1, the storage modulus against temperature, the E’ of the silk fiber
biocomposite is higher than that of the pure PLA sample. Modulus increases in the presence of silk fibers can be concluded as
a combination effect of the fibers embedded in a viscoelastic matrix and to the mechanical limitation introduced by the fibers at
high concentration which reduce the mobility and deformation of the matrix, and in this case, stress can be transferred from the
PLA matrix to those silk fiber reinforcements [10]. Also, it can be observed that the E’ of both the pure PLA sample and the
o
o
silk/PLA biocomposite drops drastically from 53 C to 67 C which is their glass transition regions. However in this glassy zone,
contribution of fiber stiffness to the composite modulus is minimal. Normally, short fiber composite properties are governed by
the fiber orientation, fiber length distribution, fiber dispersion and fiber-matrix adhesion. Thus, when hydrophilic silk fibers
mixed with hydrophobic PLA matrix even using the extrusion and injection machine with heat and pressure applied, fiber
dispersion would be a serious problem to deal with but still, the stiffness of the biocomposite was increased as the E’
increased.
1.2E+12
1E+12
(b)
Storage Modulus E' (Pa)
8E+11
(a)
6E+11
4E+11
2E+11
0
20
30
40
50
60
70
80
90
100
Temperature
(Cel)
T
t
(C
l)
Figure 1. Storage modulus as a function of temperature of (a) pure PLA sample; (b) 5wt% silk/PLA biocomposite
2.5E+11
(b)
Loss Modulus E" (Pa)
2E+11
1.5E+11
(a)
1E+11
5E+10
0
20
30
40
50
60
70
80
90
100
Temperature (Cel)
Figure 2. Loss modulus as a function of temperature of (a) pure PLA sample; (b) 5wt% silk/PLA biocomposite
1.2
(a)
1
Tan Delta
0.8
0.6
(b)
0.4
0.2
0
20
30
40
50
60
70
80
90
100
Temperature (Cel)
Figure 3. Tan delta as a function of temperature of (a) pure PLA sample; (b) 5wt% silk/PLA biocomposite
Figure 2 compares the loss modulus as a function of temperature for both the pure PLA sample and the silk/PLA biocomposite.
When comparing the peak of the E” of the two different samples, the Tg of the silk/PLA biocomposite is slightly shifted to higher
temperature and with a broader range of the transition region as compared with that of the pure PLA sample. As the loss
factors are sensitive to molecular motions, this means that the mobility of the polymer molecular chains decreases as the
chains are hindered by the silk fiber reinforcements and lead to the shift of Tg. And this may be a trend that as silk fiber content
increases, the mobility of polymer chains decrease so as the Tg increases.
According to Figure 3, tan delta as a function of temperature for both the pure PLA sample and the silk/PLA biocomposite,
from the relationship between the storage modulus (E’), the loss modulus (E”) and the tan delta (tan δ) [11], can be calculated
by the equation
tan δ = E”/ E’
tan δ decreases as seen from the graph in the presence of silk fibers. This may be again due to the mobility of polymer
molecular chains decrease because of hindered by reinforcements, this leads to a reduction of height and sharpness of the
peak in the curves. In addition, damping in the transition region measures imperfection in the elasticity and some of the energy
used to deform the material is directly dissipated into heat, thus, the mechanical loss overcomes the friction of intermolecular
chain is reduced with silk fiber additives [12,13]. From other literature [14], it is also claimed that the reduction in tan δ denotes
the improvement in hysteresis of the system and a reduction in internal friction.
Thermogravimetry
120
100
Weight (%)
80
(b)
60
(a)
40
20
0
0
100
200
300
400
500
600
Temperature (Cel)
Figure 4. Thermogravimetric curves as a function of temperature of (a) pure PLA sample; (b) 5wt% silk/PLA biocomposite
5
0
0
100
200
300
400
500
600
-5
DTG (%/min)
-10
(b)
-15
(a)
-20
-25
-30
-35
Temperature (Cel)
Figure 5. Derivative thermogravimetry curves as a function of temperature of (a) pure PLA sample; (b) 5wt% silk/PLA
biocomposite
From TGA testing, thermogravimetric (TG) curves and derivative thermogravimetry (DTG) curves as a function of temperature
of both the pure PLA sample and the silk/PLA biocomposite can be obtained and are shown in Figures 4 and 5. The TG curves
indicate thermal stability of the materials whereas the DTG curves show the decomposition temperature of the materials.
o
Generally, silk fibers are degraded through three main stages: (i) starting from 52 C, absorbed moisture releases from the silk
o
o
o
fibers; (ii) a second transition, from 265 C to 350 C, due to the degradation of silk fiber; and (iii) from 350 C onwards, the silk
fibers start to decompose. According to the two figures, both of the pure PLA sample and the silk/PLA biocomposite show very
similar results and the data of the curves are quite close to each other. In the TG graph, it can be observed that for both the
o
pure PLA sample and the silk/PLA biocomposite, the weight percentage is dropped significantly starting from 300 C which is
o
mainly due to the degradation of the materials. In addition, there is another transition starts at around 360 C where the
decomposition of the materials starts and that is just consistent with the results as shown in the DTG curves. In DTG graph, a
o
peak appears at around 350 C for both pure PLA sample and 5wt% silk/PLA biocomposite which is the decomposition region
of the materials.
Conclusions
The thermo-mechanical properties of the 5wt% silkworm silk/PLA biocomposite were investigated by different thermal analysis
techniques in this study so as to compare with pure PLA. According to DSC, DMA and TGA measuring results, various thermal
property parameters like glass transition, crystallization and melting temperatures, and their enthalpies were estimated, and
also storage and loss moduli, tan delta and the thermal stability of the biocomposite were determined. Although the silk fibers
hindered the migration and diffusion of PLA molecular chains to the surface of the nucleus in the biocomposite which led to a
negative effect on polymer crystallization and a decrement in crystallization temperature, in the presence of silk fiber
reinforcements could enhance the stiffness of the PLA sample as the storage modulus increased desirably of the biocomposite.
Moreover, the thermal stability of the biocomposite was improved as the glass transition temperature and the loss modulus as
a function of temperature curves were increased, which were reflected by DSC and DMA results. Additionally, in the tan delta
as a function of temperature graph, with silk fiber reinforcements, the biocomposite was a large decrement as compared to
pure PLA sample, and it might be claimed that it was an improvement in hysteresis of the system and a reduction in internal
friction. Therefore, the biocomposite can be a potential candidate for further development on structural and clinical applications
as they have desirable thermo-mechanical properties. Future work will firstly concentrate on the optimization of this type of
biocomposite, in relation to the silk fiber length, orientation and dispersion, and also the manufacturing process; secondly will
be focused on the biocompatibility and biodegradability of this novel tissue engineering biocomposite.
Acknowledgement
This project was supported by The Hong Kong Polytechnic University Grant. The authors also express their appreciation to
East Link Degradable Materials Ltd., Hong Kong for supplying poly(lactic acid).
References
1.
Mohanty A.K., Misra M., Drzal L.T., Selke S.E., Harte B.R. and Hinrichsen G.. Natural fibers, biopolymers, and
biocomposites: an introduction. In Mohanty A.K., Misra M. and Drzal L.T. (ed), Natural fibers, biopolymers, and
biocomposites. CRC Press, FL, 2005.
2. Sun X.S.. Plastics derived from starch and poly(lactic acids). In Wool R.P. and Sun X.S. (ed), Bio-based polymers
and composites. Elsevier Academic Press, UK, 2005.
3. Berger S.A., Goldsmith W. and Lewis E.R. (ed), Introduction to bioengineering. Oxford University Press, Oxford,
1996.
4. Lee S.M., Cho D.H., Park W.H., Lee S.G., Han S.O. and Drzal L.T.. Novel silk/poly(butylenes succinate)
biocomposites: the effect of short fiber content on their mechanical and thermal properties. Composites Science
and Technology 2005; 65: 647-657
5. Raya S.S., Yamada K., Okamoto M. and Ueda K.. Crystallization behavior and morphology of biodegradable
polylactide/layered silicate nano-composite. Polymer 2003; 44: 857-866.
6. Albano C., Papa J., Ichazo M., Gonzalez J. and Ustariz C.. Application of different macrokinetic models to the
isothermal crystallization of PP/talc blends. Composite Structures 2003; 62: 291-302.
7. Huda M.S., Drzal L.T., Mohanty A.K. and Misra M.. Chopped glass and recycled newspaper as reinforcement
fibers in injection molded poly(lactic acid) (PLA) composites: A comparative study. Composites Science and
Technology 2006; 66: 1813-1824.
8. Kokta B.V., Dembele F. and Daneult C.B.. Polymer Science and Technology. Carraher J.R. and Sperling L.H. (ed),
Plenum, New York, 1985; 33.
9. Houshyar S., Shanks R.A. and Hodzic A.. The effect of fiber concentration on mechanical and thermal properties
of fiber-reinforced polypropylene composites. Journal of Applied Polymer Science 2005; 96: 2260-2272.
10. Rana A.K., Mitra B.C. and Banerjee A.N.. Short jute fiber-reinforced polypropylene composites: dynamic
mechanical study. Journal of Applied Polymer Science 1999; 71: 531-539.
11. Groenewoud W.. Characterisation of Polymers by Thermal Analysis. Elsevier Science B.V., The Netherlands,
2001.
12. Bleach N.C., Nazhat S.N., Tanner K.E., Kellomaki M. and Tormala P.. Effect of filler content on mechanical and
dynamic mechanical properties of particulate biphasic calcium phosphate-polylactide composites. Biomaterials
2002; 23: 1579-1585.
13. Pothan L.A., Oommen Z. and Thomas S.. Dynamic mechanical analysis of banana fiber reinforced polyester
composites. Composites Science and Technology 2003; 63: 283-293.
14. Fay J.J., Murphy C.J., Thomas D.A. and Sperling L.H.. Effect of morphology, cross-link density, and miscibility on
inter-penetrating polymer network damping effectiveness. Polymer Engineering Science 1991; 31: 1731-1741.