Chinese Journal of Polymer Science Vol. 26, No. 4, (2008), 425−432
Chinese Journal of
Polymer Science
©2008 World Scientific
SYNTHESIS AND PROPERTIES OF HIGH MOLECULAR WEIGHT
POLY(LACTIC ACID) AND ITS RESULTANT FIBERS*
Wang-xi Zhang** and Yan-zhi Wang
School of Materials & Chemical Engineering, Zhongyuan University of Technology, Zhengzhou 451191, China
Abstract Direct melt/solid polycondensation of lactic acid (LA) was carried out to obtain high molecular weight poly(lactic
acid) (PLA) by a process using various catalysts in the first-step melt polycondensation, and followed solid polycondensation
by using p-toulenesulfonic acid monohydrate (TSA) as the catalyst in the second step. Effects of various catalysts and
reaction temperature on the molecular weight and crystallinity of resulting PLA polymers were examined. It was shown that
SnCl2·2H2O/TSA, SnCl2·2H2O/succinic anhydride, and SnCl2·2H2O/maleic anhydride binary catalysts should be effective
binary catalysts to obtain high molecular weight PLA of more than 1.2 × 105. A conventional melt spinning method was used
to spin PLA fibers, which displayed tensile strength of (382.76 ± 1.41) MPa and tensile modulus of (4.36 ± 0.07) GPa.
Keywords: Polycondensation; Polyesters; Mechanical properties; Solid-state polymerization; Synthesis.
INTRODUCTION
Recently, many researchers have been interested in the manufacture and applications of biodegradable polymers
especially have showed growing academic and industrial interests in one of the aliphatic polyesters generated
from lactic acid. As one of the most promising environmental friendly material, poly(lactic acid) (PLA) is a
thermoplastic, high-strength and high-modulus polymer which has been mainly used in the biomedical fields as
the types of fibers, films, microparticles or microspheres, etc., on account of its biodegradable, biocompatible
and bio-absorbable features and its raw material renewable. Nowadays, in order for PLA to be processed in a
mass scale into packing, textile and general plastic materials, the polymers must possess good enough
mechanical properties and stabilities to prevent degradation and maintain high molecular weight.
Two different methods have been used to prepare high molecular weight PLA polymers. The first is a
commonly commercialized one to ring-open polymerize (ROP) lactide (lactide is a cyclic diester and is
produced by thermal decomposition of the lactic acid oligomer) to yield pure and high molecular weight PLA.
The second one is also a commercialized route, established by Mitsui Chemicals Co. (Japan), wherein lactic acid
and catalyst are azeotropically dehydrated in a refluxing, high boiling point, aprotic solvent such as diphenyl
ether, under reduced pressure to obtain PLA with weight-average molecular weight Mw greater than 3.0 × 105[1].
The direct condensation of lactic acid was initially regarded as a process only to obtain a low molecular weight
polymer which was not useful[2], but now the direct polycondensation of lactic acid can also obtain PLA with a
significantly high molecular weight[3−5]. For example, Moon et al. [6] have succeeded in preparing PLA with
molecular weight of about 1.0 × 105 by conducting melt/solid polycondensation. Fukushima and co-workers[7]
also obtained PLA with high molecular weight (Mw = 2.66 × 105) by using a process combining twin screw
extrusion and solid-phase polycondensation with the reflux of free lactide.
*
This work was financially supported by the HAIPURT (No. 2006KYCX009), the National Natural Science Foundation of
Henan (No. 200510465008), and Henan Innovation Project (No. 0523021300).
**
Corresponding author: Wang-xi Zhang (张旺玺), E-mail: [email protected]
Received June 1, 2007; Revised July 20, 2007; Accepted July 30, 2007
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W.X. Zhang et al.
However, all these synthesis methods of PLA have some shortcomings. Firstly, PLA obtained by ringopening polymerization is unavoidably at high cost because exhaustive monomer purification is needed to attain
high molecular weight of the resultant polymers. Furthermore, high cost will limit their applications. Secondly,
if a lot of azeotropical organic solvent is used[8], it will make purification process complicated and difficult, and
also increase the product cost. Thirdly, although high molecular weight PLA can be obtained by the direct
polycondensation[9, 10], details have not been revealed. Some results even are inconsistent according to the
previous works. For example, the molecular weight of PLA polymers sometimes was very low, and sometimes
was very high. In this paper, high molecular weight PLA polymers were prepared by the use of a novel direct
melt/solid polycondensation process in which the effects of much more catalysts on the reactions were
examined. For the aim to prepare PLA fibers with good enough tensile properties, the catalysts such as
SnCl2·2H2O/p-toulenesulfonic acid monohydrate (TSA) and SnCl2·2H2O/maleic anhydride were used during the
first step melt polycondensation; TSA was used during the second step solid-phase polycondensation, in which
their effects on the molecular weight and crystallinity of resultant PLA polymers were studied in detail.
EXPERIMENTAL
Materials
L-lactic acid (LA) as a 90 wt% aqueous solution was purchased from Guangshui National Chemical Co
Ltd.(Guangshui, China) and was purified using a molecular distillation method to further reduce the water
content before use. Tin(II) chloride (SnCl2·2H2O) was purchased from Shanghai Shanpu Chemical Company
(Shanghai, China), and p-toulenesulfonic acid monohydrate (TSA) was purchased from Shanghai Sanpu
Chemical Company (Shanghai, China). Other reagents were supplied from Huaisheng Chemical Reagent Co
Ltd. (Zhengzhou, China). All these materials except for L-lactic acid were used as received.
Polymerization
A batch type reactor (1000 mL) in laboratory-scale was used to synthesize PLA. The reactor was equipped with
a vacuum pump and cooling traps for the water removal. High molecular weight PLA polymers were prepared
by melt/solid polycondensation including two steps. The first step was to conduct the melt polycondensation, in
which a condenser was installed to help reflux of free lactide formed in equilibrium with PLA. When the solidphase polycondensation was carried out in the second step, the reflux condenser was dismantled from the
reactor.
When a kind of catalyst or the mixture of two different catalysts was selected for the melt polycondensation
in the first step, 400 g distillated L-lactic acid and predetermined amounts of catalysts were fed into the reactor.
The reaction was initially conducted at 150°C for 4 h, then at 160°C for another 4 h with pressure stepwise
reduced to 500 Pa.
Upon completion of the first-step melt polycondensation, 0.4 wt% (relative to the starting L-lactic acid)
TSA was fed into the reactor. The resulting PLA prepolymers were performed the second-step solid
polycondensation at 180°C (unless necessarily changing it) and the pressure of 300 Pa for 10 h using TSA as
catalyst for all runs.
Spinning
The obtained final PLA polymers were dried in a vacuum oven. A laboratory-scale single screw extruder with a
Φ 6.5 mm screw and a Φ1.0 mm die hole was used to spin PLA fibers by using a conventional melt spinning
process. The extrusion was carried out at 9 r/min and the die temperature was ranging from 200°C to 230°C.
The as-spun fibers were subsequently drawn in nitrogen to get the final hot-drawn fibers.
Measurements
PLA polymers were dissolved in chloroform. The viscosity studies were carried out at 25°C by using a
Ubbelohde viscosimeter. The following Mark-Houwink equation, [η] = 5.45 × 10−4Mη0.73[1], was used to
calculate the viscosity-average molecular weight, Mη. The weight-average molecular weight, Mw, and
polydispersity index of polymers were also obtained by the use of a Waters 150C ALC/GPC relative to
Synthesis and Properties of High Molecular Weight Poly(lactic acid) and Its Resultant Fibers
427
polystyrene standard with chloroform as the eluent.
Fourier transform infrared (FT-IR) spectra were obtained by using a NICOLET 5700 FT−IR Spectrometer
(USA).
Differential scanning calorimetry (DSC) scans were performed using a Perkin-Elmer DSC 7. Samples were
dried in a vacuum oven at 40°C for 8 h before used to perform DSC studies with approximately 4 mg samples at
a heating rate of 10 K min−1 under nitrogen atmosphere. The crystallinity of PLA, XcDSC, was calculated based
on the enthalpy of heat fusion measured by DSC according to the theoretical heat of fusion (203.4 J·g−1)[4] of
ideal PLA crystals reported previously. Wide-angle X-ray diffraction (WAXD) under Cu Kα irradiation was
also measured on a Shimazu XD-D1 apparatus operated at 40 kV and 30 mA. The degree of crystallization
(XcWAXD) was also evaluated according to the intensity of the crystalline peak area and the overall intensity for
both the amorphous and crystalline areas.
The tensile properties of the obtained PLA fibers were measured by using a XQ-1 tensile-testing machine
(made by Donghua University, Shanghai, China) at a crosshead speed of 0.5 mm min-1 with a testing length of
20 mm and load cell of 10 g. In each case, at least 30 sample filaments were tested, from which an arithmetic
mean and a standard deviation were obtained.
RESULTS AND DISCUSSION
Effect of Catalysts on the Melt/Solid Polycondensation
The polycondensation system of PLA involves two equilibrium reactions[11]: (i) dehydration equilibrium for
esterification and (ii) ring-chain equilibrium involving the depolymerization of PLA into L-lactide (Eqs. (1), (2)
and (3)). For the former equilibrium, the condensed water must be removed effectively in order to further
enhance the chain extension. Therefore, the reaction parameters should be optimized to promote the removal of
water byproduct.
Conducting reactions at high temperature and in reduced pressure can promote the dehydrative
polycondensation. Catalysts also play a significant role in attaining high molecular weight of product. However,
unlike the well-established approach for the catalytic ring-opening polymerization of lactide, the direct
polycondensation of lactic acid has received much less attention. Therefore, an ideal way to promote
polycondensation is to activate the dehydrative reaction and deactivate the formation of L-lactide by the possible
selection of a catalyst. It has been revealed that direct polycondensation of lactic acid using various Lewis acid
as catalysts through continuous azeotropic dehydration, wherein molecular sieves keep water content in the
solvent within the lowest limit[12]. Ki et al.[8] and Sung and co-workers[11, 13] made an intensive screening test to
find an adequate catalyst for the melt polycondensation of lactic acid. They examined with various catalysts:
metallic and nonmetallic, organic and inorganic, and homogeneous and heterogeneous. The screening test
revealed that tin oxide and chloride were particularly effective for increasing the molecular weight of PLA
among the metal and metal oxide catalysts frequently used for usual esterification reactions. In addition, the
addition of proton acids could also effectively prevent discoloration of product[6].
Therefore, various catalysts were used here especially tin chloride/co-catalyst systems were in more detail
W.X. Zhang et al.
428
examined. On the basis of catalyst screening tests, as was shown in Table 1, it was sure that SnCl2·2H2O/TSA,
SnCl2·2H2O/succinic anhydride, or SnCl2·2H2O/maleic anhydride binary catalysts are effective catalysts to
obtain PLA with high molecular weight. It was also noted that the polarity of the reaction system greatly altered
with the progress of the melt polycondensation, resulting in a great deterioration of catalyst activity. So TSA was
added to further catalyze the second-step solid polycondensation after completion of the first-step melt
polycondensation. The viscosity-average molecular weight, Mη, of PLA (Run P10, P11, and P12) obtained in the
first-step could reach 4.1 × 104 or more, and it could reach 12.66 × 104 or more after completion of the secondstep solid polycondensation in the end. These results were also in agreement with the reaction mechanism
suggested by Sung and co-workers[11]. The terminal groups of PLA are coordinated within the catalyst center.
The dehydration is driven among the carboxylate and hydroxyl ligands with the formation of Sn―OH. The
proton acid added to the catalyst can work as a ligand of the catalyst site. Furthermore, because the proton acid
is not involved in the esterification, it can fill the open coordination sites of the catalyst to hinder the side
reaction. The tin chloride/co-catalyst systems were surely effective catalysts to enhance the molecular weight
and to prevent the discoloration of PLA.
Table 1. Effect of catalysts on the melt/solid polycondensation
Tmc
Yield
Mη
Mw
Catalyst (wt%)
PDIa
4
4
(%)
(10 )
(10 )
(°C)
SnCl2·2H2O (0.5)
78.24
3.5
3.9
2.1
146.2
P1
TSA (0.4)
6.6
7.3
ndb
168.9
Stannous octoate (0.5)
76.53
2.8
3.1
1.7
147.1
P2
TSA (0.4)
5.2
6.7
nd
167.2
Benzoic acid anhydride (0.5)
73.45
2.5
3.0
1.6
nd
P3
TSA (0.4)
4.6
6.2
nd
nd
Succinic anhydride (0.5)
78.01
3.2
4.2
2.1
137.8
P4
TSA (0.4)
5.2
6.8
nd
159.1
Maleic anhydride (0.5)
80.09
4.0
4.8
1.9
141.2
P5
TSA (0.4)
8.7
9.4
nd
160.1
TSA (0.5)
76.99
2.9
3.5
1.9
146.0
P6
TSA (0.4)
4.2
6.1
nd
150.9
CSA (0.5)
75.90
1.8
2.3
nd
nd
P7
TSA (0.4)
3.3
4.5
nd
nd
BSA (0.5)
78.11
2.1
2.6
2.7
nd
P8
TSA (0.4)
3.1
4.3
nd
nd
MSA (0.5)
76.56
3.2
4.2
1.8
nd
P9
TSA (0.4)
3.6
4.6
nd
nd
SnCl2·2H2O (0.5), TSA (0.4)
82.18
4.1
5.7
2.0
151.6
P10
TSA (0.4)
12.66
14.65
1.9
167.0
152.8
2.1
5.9
SnCl2·2H2O (0.5), Succinic
4.6
anhydride (0.4)
83.41
P11
162.1
2.0
16.14
TSA (0.4)
14.45
Solid
SnCl2·2H2O (0.5), Maleic
152.1
1.9
5.2
4.4
Melt
P12
anhydride (0.4)
82.13
Solid
167.9
1.8
16.21
TSA (0.4)
14.68
a
PDI: polydispersity index of PLA; b nd: not detected; c Measured by DSC (heating rate:10 K min−1)
Run
no.
Reaction
process
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
Solid
Melt
XcDSC
(%)
21.7
69.0
16.8
65.8
nd
nd
19.8
56.7
23.4
60.2
25.6
51.2
nd
nd
nd
nd
nd
nd
35.2
65.9
31.1
XcWAXD
(%)
11.2
32.7
4.7
27.3
nd
nd
5.1
16.5
5.6
26.7
5.4
19.8
nd
nd
nd
nd
nd
nd
10.9
31.5
9.1
61.2
35.6
29.9
12.8
62.1
30.1
The melting point and crystallinity of obtained PLA polymers were also examined. It was shown that the
melting point and crystallinity of PLA polymers obtained from the second-step solid polycondensation were
both higher than those of PLA prepolymers obtained from the first-step melt polycondensation. It was also
shown that the crystallinity of PLA polymers examined by a WAXD method was quite lower than that obtained
by the use of DSC method. The big difference between the crystallinity values determined by DSC and WAXD
is most likely due to the reason that the crystallization is not quite perfect. The crystallinity determined from heat
of fusion by DSC reflected the morphology of the whole crystallization area in which the separation amorphous
Synthesis and Properties of High Molecular Weight Poly(lactic acid) and Its Resultant Fibers
429
region out from crystallization area is practically difficult. PLA crystals have a large tendency to reorganize into
more stable structures, through continuous partial melting/recrystallization/crystal perfection processes that
occur during the subsequent heating scan that leads to fusion[14]. The melting process of PLA was also different
if it was analyzed at different heating rates. On the other hand, the crystallinity obtained from a WAXD method
reflected the individual crystal parameters such as spherulites sizes. The crystallization of PLA resulted from the
polycondensation process possibly obtained defective crystals, so crystal perfection need to be further
developed. As a result, the crystallinity of PLA polymers examined using a WAXD method was significantly
lower than that obtained by the use of DSC method.
Figure 1 shows the changes in the viscosity average molecular weight, and Fig. 2 shows changes in the
crystallinity of PLA during the reaction processes including melt and solid polycondensation. During the firststep melt polycondensation, both molecular weight and the crystallinity of PLA polymers increased with the
reaction time. Furthermore, the increase in molecular weight of PLA obtained from the first-step
polycondensation tends to level off with increase of the reaction time. However, the molecular weight was
increased sharply after the addition of TSA at reaction time of 8 h. This was because the catalytic activity was
greatly decreased with the changes in the polarity of the reaction system and the quantities of functional groups
with the progress of polycondensation. As a result, the catalyst was no longer to enhance the increasing in
molecular weight of product. However, when a fresh catalyst, TSA, was fed into the reaction system again, the
polymer terminals could be concentrated in the amorphous region and connected with each other effectively by
Fig. 1 Viscosity-average molecular weight of PLA as a function of reaction time
First step: 150°C (4 h), then 160°C (4 h); P10, SnCl2·2H2O 0.5 wt%, TSA 0.4 wt%, P12, SnCl2·2H2O
0.5 wt%, maleic anhydride 0.4 wt%. Second step: 180°C (10 h), P10 and P12, TSA 0.4 wt%
Fig. 2 Changes in the crystallinity of PLA as a function of reaction time
First step: 150°C (4 h), then 160°C (4 h); P10, SnCl2·2H2O 0.5 wt%, TSA 0.4 wt% P12, SnCl2·2H2O 0.5 wt%,
maleic anhydride 0.4 wt%. Second step: 180°C (10 h); P10 and P12, TSA 0.4 wt%
430
W.X. Zhang et al.
esterfication according to the mechanism suggested by Sung and co-workers[6]. Thus, the elongated polymer
chains easily access to the crystallization around the crystal-amorphous borders. It was also concluded that both
the catalyst systems of SnCl2·2H2O/TSA (Run no. P10) and SnCl2·2H2O/maleic anhydride (Run no. P12) have
similar effects on the polycondensation. It is much more effective for TSA in the amorphous parts to enhance
polycondensation if the crystallinity of prepolymers generated from the first-step polycondensation is lower.
Therefore, SnCl2·2H2O/maleic anhydride is more effective to increase the molecular weight of the resultant PLA
in spite of having lower crystallinity as compared to that of using SnCl2·2H2O/TSA system. In addition, TSA is
also useful to prevent the discoloration of product, as was discovered by Sung et al.[6].
Effect of Temperature on the Solid Polycondensation
The changes in molecular weight of PLA as a function of reaction temperature during the second-step solid
polycondensation are shown in Fig. 3. High reaction temperature was useful to promote the removal of water, so
molecular weight of PLA obtained from both run P10 and P12 increased with increase of reaction temperature,
reached the highest at 180°C, followed decreased sharply with further increasing of temperature due to the
occurrence of depolymerization.
Fig. 3 Changes in the viscosity-average molecular weight of PLA as a function of reaction temperature
First step: 150°C (4 h), then 160°C (4 h); SnCl2·2H2O 0.5 wt%, TSA 0.4 wt% (P10), SnCl2·2H2O 0.5 wt%,
maleic anhydride 0.4 wt% (P12). Second step: 10 h, TSA 0.4 wt% (P10 and P12)
Therefore, the optimum reaction temperature was determined at 180°C for various catalysts tests, as was
shown in Table 1. At the reaction temperature of 180°C, lactide formation and other side reactions could
efficiently be avoided because the reaction equilibrium was inclined to the direction of polymer formation. As an
important factor, however, when the reaction temperature was elevated beyond 180°C it would lower the yield
of PLA because of vacuum sublimation and various side reactions, such as hydrolytic action, thermal
degradation, and intramolecular esterification of oligo (lactic acid). If reaction temperature was too high, it
would even cause partial or full carbonization of reactants, especially when the reaction time was longer.
Thermal Properties of PLA
Figure 4 shows a FT-IR spectrum of PLA polymers obtained by using SnCl2·2H2O/maleic anhydride as cocatalyst. The spectrum exhibited characteristic absorption peaks of ester at 1760 cm−1, attributed for C＝O and
the band at 1090 cm−1 due to COC, respectively, and ―CH2,―CH3 groups at the band in 2850−3050 cm−1. The
characteristic absorption peaks of hydroxyl groups of lactic acid disappeared in the FT-IR spectrum of the PLA
polymers, indicating that a high molecular weight PLA was formed.
Synthesis and Properties of High Molecular Weight Poly(lactic acid) and Its Resultant Fibers
431
Fig. 4 Typical FT-IR spectrum of PLA obtained by using SnCl2·2H2O/maleic anhydride as catalyst
After completion of the first-step melt polycondensation of LA, the DSC thermogram of obtained PLA
showed a clear crystallization exotherm around 105°C as well as a melting endotherm of the PLA crystals at
142−156°C, indicating a partly-crystalline structure formed, as was shown in Fig. 5(a). Almost all PLA
polymers obtained from the first-step melt polycondensation of LA using various catalysts displayed similar
DSC patterns. However, the crystallinity of PLA was increased significantly after completion of the second-step
solid polycondensation, as was shown in Table 1. As a result, some PLAs displaying broad crystallization region
rather than revealing a clear crystallization exotherm in their DSC curves. For example, P10 (Fig. 5c) and P12
(Fig. 5b) displayed a clear melting endotherm at 174°C and 178°C, respectively. From the viewpoint of melt
spinning, a polymer should be melt before its decomposition temperature. Therefore, the PLA can be melt-spun
to fibers as a kind of thermoplastic polymer with its melt point lowering than its decomposition temperature.
Many researchers[15−22] have studied the preparing processes and the properties of the resultant fibers generated
from high molecular weight polylactide obtained by ring-opening polymerization of lactide.
Fig. 5 Typical DSC curves of the PLA obtained under various polycondensation process
conditions: (a) the melt PLA resulted from the first-step melt polycondensation, (b) the
solid-state PLA obtained from the second-step solid polycondensation for Run P12, and (c)
the solid-state PLA obtained from the second-step solid polycondensation for Run P10
Resulting PLA Fibers
Here, the obtained PLA polymers after completion of the second-step solid polycondensation for Run P10 and
P12 were selected to melt-spin into fibers. The effects of molecular weight of the PLA polymers on the tensile
properties of resulting PLA fibers were examined, as were shown in Table 2. If the PLA fibers were prepared in
W.X. Zhang et al.
432
the same spinning and hot-drawn conditions, it was shown that the increase in molecular weight of PLA
polymers would increase the tensile strength and modulus of resulting PLA fibers. It was also noted that the
tensile strength of P10 and P12 as-spun fibers was 78.12 MPa and 86.91 MPa, respectively. When they were hot
drawn, the tensile strength of final P10 and P12 fibers was increased greatly to 360.23 MPa and 382.76 MPa,
respectively. This was the reason that drawing was an effective way to increase the tensile strength of fibers.
Samples
Mη (104 )
P10
12.66
P12
14.68
Table 2. Tensile properties of the PLA fibers
Draw ratio
Tensile strength (MPa)
Tensile modulus (GPa)
As-spun
78.12 ± 4.82
1.76 ± 0.19
3
210.08 ± 3.63
3.28 ± 0.11
6
360.23 ± 1.78
4.12 ± 0.08
As-spun
86.91 ± 4.17
1.87 ± 0.15
3
222.49 ± 3.40
3.45 ± 0.08
6
382.76 ± 1.41
4.36 ± 0.07
Elongation (%)
389.12 ± 8.84
49.42 ± 5.29
24.46 ± 2.73
368.90 ± 8.46
47.86 ± 4.92
23.78 ± 2.23
CONCLUSIONS
Poly(lactic acid) was prepared from lactic acid by a direct melt/solid polycondensation process using various
binary catalysts which were very effective to synthesize the PLA with high molecular weight of over 1.2 × 105.
The resultant PLA fibers with tensile strength of (382.76 ± 1.41) MPa and tensile modulus of (4.36 ± 0.07) GPa
were obtained. This novel method will provide a useful manufacturing process for preparing PLA fibers.
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