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Improved hydrolytic stability of poly(dl -lactide) with epoxidized soybean oil
Polymer Degradation and Stability 95 (2010) 485e490
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Improved hydrolytic stability of poly(DL-lactide) with epoxidized soybean oil Chunhua Fu a, Bei Zhang a, Changshun Ruan a, Chengbo Hu a, Ya Fu a, b, Yuanliang Wang a, * a b
Research Center of Bioinspired Material Science and Engineering, Department of Bioengineering, Chongqing University, Chongqing 400030, PR China Chongqing University of Science and Technology of China, Chongqing University, Chongqing 400030, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2009 Received in revised form 24 December 2009 Accepted 11 January 2010 Available online 28 January 2010
The multi-arm star polymer (ESOPLA) was obtained by ring-opening polymerization of DL-lactide using multifunctional epoxidized soybean oil (ESO) as an initiator in the presence of a stannous actuate (SnOct2) catalyst. Gel permeation chromatography with multi-angle laser light scattering (GPC-MALLS), FTIR, 1H NMR, thermal analysis and in vitro degradation were used to qualitatively characterize the synthesized polymers. The results revealed that ESO plays an important role in increasing the molecular weight, polymerization rate and monomer conversion rate. Degradation analysis demonstrated that the decrease in molecular weight and the weight loss ratio of the star-shaped ESOPLA were lower than that of linear poly(DL-lactide) (PDLLA). The surface topography of pre- and post-degradation materials was characterized by scanning electron microscopy (SEM). These SEM images showed that the linear PDLLA films underwent water erosion more readily than the star-shaped polymer films. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: Epoxidized soybean oil Poly(DL-lactide) Multi-arm star polymer Degradation
1. Introduction Polylactic acid or polylactide (PLA) is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources such as starch and sugar. It is a promising polymer to replace the usual plastics that are produced from non-renewable petroleum and natural gas resources [1,2]. However, the degradation of PLA is difficult to control and undesired degradation may occur during storage and usage. If crude PLA is not stored in refrigerated or desiccated conditions, it would degrade severely after 3 months but purified PLA, from which the residual monomers have been removed by an organic solvent, does not need strict storage conditions. As one of the most interesting environmentally friendly plastics, a large amount of research on PLA has focused on improving its mechanical property and processing technology [3e5]. Relatively little attention has been paid to the degradation problem during storage. Epoxidized soybean oil has been used to reduce water absorption and to improve the service life of concrete [6]. As a nontoxic and degradable plasticizer, ESO has been blended with PLA and other plastics to improve properties such as melt rheology and tensile strength etc. [7e10]. The oil may, however, leach to the surface of polymers during its service life. Dong reported that a multifunctional initiator can minimize the residual lactide ratio and increase the polymer molecular weight [11]. In this study, ESO
was used to increase the waterproof property of PLA and to reduce the residual monomer content. Epoxy and hydroxyl groups in ESO can initiate DL-lactide ring-opening polymerization. The main advantages of ESO and PLA are their abundance and environmental characteristics. 2. Experimental 2.1. Materials DL-lactide was synthesized by polycondensation and depolymerization reactions from DL-lactic acid and the crude product was recrystallized from dried ethyl acetate. The melting point of the product was 126.3 C. PDLLAs of various molecular weights were synthesized in our laboratory. SnOct2 was purchased from Sigma and used without further purification. ESO was purchased from Kaiqi chemical company and the average number of epoxy groups per molecule was 4. HPLC grade tetrahydrofuran (THF) was purchased from SK Chemicals. Analytical grade chloroform and methanol was purchased from Chongqing Chemical Factory.
2.2. Polymerization Specific amounts of ESO, DL-lactide and SnOct2 (1:5000 on a molar basis) were added to dry flasks and the flasks were filled with nitrogen gas. The polymerization flasks were put into an oil bath at 150 C for a specific time. After the desired reaction time, the reaction flasks were cooled and the crude products were DL-lactide
* Corresponding author. Tel./fax: þ86 23 65102509. E-mail address: [email protected] (Y. Wang).
0141-3910/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.01.007
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C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490
purified by coprecipitation in a chloroform-methanol system. The purified polymer was dried in vacuum until it had a constant weight and the residual DL-lactide was detected by gravimetric evaluation of the pre- and post-purified products. 2.3. Characterization Scheme 1.
The molecular weight and polydispersion index (PDI) of the polymers were determined with GPC-MALLS using polystyrene standards. Three Agilent 1100 HPLC columns (300 8.0 mm) were used in series with THF as the eluent at a flow rate of 1.0 ml/min at 25 C, FTIR was carried out using a Spectrum One spectrometer (Perkin Elmer). 1H NMR was performed on a Bruker ARX-400 spectrometer (Bruker) and the dried samples were dissolved in CDCl3. Differential scanning calorimetry (DSC) was carried out using a STA449C thermal analysis system (NETZSCH) at a heating rate of 10 C/min under nitrogen. The surface morphology of the polymer film was determined using a Hitachi S-520 scanning electron microscope (Hitachi).
degraded polymer samples. The weight loss ratio was calculated according to the equation:
weight lossð%Þ ¼
W0 Wt 100% W0
Where W0 and Wt are the sample weights before and after hydrolytic degradation, respectively. The reported molecular weights and weight loss ratios were sample averages.
3. Results and discussion 2.4. Sample preparation for degradation tests
3.1. Synthesis of star-shaped ESOPLAs
Films of PDLLA and ESOPLA were prepared by a solution casting method using a 3.0 wt% chloroform solution and they were placed in glass Petri dishes (47 mm) [12]. The solvent was allowed to evaporate over 48 h and films had formed. These films were dried in vacuum until a constant weight was obtained. All experiments were performed at room temperature. The obtained glabrous transparent films were cut into 10 10 mm pieces. The obtained films were UV sterilized for 30 min before the degradation study [13]. 2.5. Degradation study The sterile PBS solution (0.1 M, pH 7.4) was used as a medium and 15 ml PBS per film. The solution was changed every two weeks without considering the autocatalysis effect caused by monomer degradation. All the vials were sealed and kept in an incubator at 37 0.5 C. Every two weeks the polymer films were removed, rinsed with distilled water and dried to a constant weight in vacuum for molecular weight and weight loss ratio measurements. GPC-MALLS was used to determine the molecular weight of the original and
3.1.1. Effect of the multifunctional initiator on polymerization Multi-arm star-shaped ESOPLA was synthesized according to the procedure shown in Scheme 1. The hydroxyl and the epoxy groups of ESO can initiate the ring-opening polymerization of DL-lactide and the structure of ESOPLA is shown in Scheme 2. Table 1 shows that the molecular weight and the DL-lactide monomer residue ratio are a function of ESO concentration. At higher ESO content and with a constant concentration of SnOct2, reaction temperature of 150 C and reaction time of 12 h, the molecular weight initially increased significantly and then gradually decreased. The molecular weight reached 165,900 Da at 3.0 wt% ESO but the DL-lactide residue ratio decreased at higher ESO ratios. The molecular weight distribution was narrow (1.130e1.783). The molecular weight of the polymer normally decreases as the initiator ratio increases. However, one ESO molecule has several catalytic sites and every star-shaped molecule is composed of several chains. This is why the molecular weight increases when an appropriate amount of ESO initiator is added. Fig. 1 illustrates the effect of ESO on the polymerization rate and it is obvious that the molecular weight and the polymerization rate are significantly
Scheme 2.
C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490 Table 1 The result of
DL-lactide
487
copolymerization with ESO. Mw (104)a
Sample
PDIa
DL-lactide b
residue
ratio (%)
control poly(DL-lactide) Copolymerization with 0.5wt% ESO copolymerization with 1.0wt% ESO copolymerization with 3.0wt% ESO copolymerization with 5.0wt% ESO copolymerization with 10.0wt% ESO
5.697 6.792 7.619 16.59 15.14 8.219
1.130 1.621 1.756 1.403 1.330 1.783
2.72 1.85 1.61 1.45 0.60 0.58
*Polymerization temperature of 150 C and polymerization time of 12 h. a Measured by GPC-MALLS. b Detected gravimetrically
enhanced in the presence of ESO compared with the control group. The hydroxyl, epoxy and amino active groups are efficient initiating sites for the polymerization of lactides in the presence of the Sn(Oct)2 catalyst, therefore, ESO can accelerate the polymerization ratio. 3.1.2. Characterization of copolymer structures IR and 1H NMR were used to characterize and confirm the structure of the linear PDLLA and the star-shaped ESOPLA. In the IR spectrum (Fig. 2), the terminal hydroxyl absorption peak at 3510 cm1 in ESOPLA (b) is obviously larger than that of the linear PDLLA (a). This is contrary to the molecular weights of the two polymers (MwPDLLA ¼ 36,790 Da, MwESOPLA ¼ 165,900 Da) and indicates that a multi-arm structure exists in the ESOPLA polymer. Fig. 3a shows the 1H NMR spectrum of the linear PDLLA and the two main signals at 1.58 and 5.21 ppm are due to the CH3 and CH protons of PDLLA, respectively. In the 1H NMR spectrum of ESOPLA (Fig. 3b), the peaks at 1.58 ppm and 5.21 ppm are assigned to the CH3(6) and CH(7) protons of the PDLLA chains in the multi-arm polymer. The signals at 0.87 and 1.29 ppm are attributed to the CH3 (1) and CH2 (2) protons of ESO, respectively.
Fig. 2. IR spectra of (a) linear PDLLA, Mw ¼ 36,790 Da and (b) Star-shaped ESOPLA (3.0 wt% ESO), Mw ¼ 165,900 Da.
improve the tensile strength property of the polymers. From Fig. 4, the onset decomposition temperatures (Tonset) of linear PDLLA (a) and star-shaped ESOPLA (b) were 272.2 C and 268.7 C, respectively. The thermal stability of ESOPLA is slightly poorer than that of
3.2. Thermal properties of star-shaped ESOPLA and linear PDLLA The thermal property of star-shaped ESOPLA was investigated by DSC and compared with that of linear PDLLA. The glass transport temperature of ESOPLA (54.7 C) is lower than that of linear PDLLA (60.2 C) and this is due to the flexible chains and the plasticized character of ESO. The low glass transition temperature may
Fig. 1. The effect of ESO on the polymerization reaction rate.
Fig. 3. 1H NMR spectra of (a) linear PDLLA, Mw ¼ 56,790 Da and (b) Star-shaped ESOPLA (3.0 wt% ESO), Mw ¼ 165,900 Da.
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C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490
The changes in weight loss ratio as well as the decrease in molecular weight and surface structure of the polymers that were degraded in PBS solution were investigated. The molecular weights of the three samples were similar. Fig. 5 shows the obtained weight loss ratios for PDLLA and ESOPLA as a function of time during incubation in the PBS solution. The degradation process includes two stages. In the first two weeks, the degree of degradation was slight and there was no significant difference between the linear PDLLA and the star-shaped polymer
at this stage. The weight loss ratios of all the samples were less than 5%, and the films were still transparent. In the next stage, the weight loss ratios increased but the degree of increase was different among the three samples. The weigh loss ratio of linear PDLLA was 51.2% after a degradation time of 10 weeks while ESOPLA1.0wt% and ESOPLA10.0wt% were 28.6% and 20.3%, respectively. The transparency of PDLLA was inferior to the other two samples. This result indicates that degradation of the star-shaped ESOPLA is slower than that of linear PDLLA. The molecular weight of linear PDLLA and star-shaped ESOPLA as a function of time during degradation in the PBS solution is plotted in Fig. 6. The linear polymer's molar mass decreased from 91,350 to 28,500 Da after 10 weeks of degradation, however, the molecular weight of ESOPLA10.0wt% decreased slowly from 82,190 to 61,200 Da. It can be concluded that the degradation rate of the star-shaped ESOPLA is slower than that of the linear PDLLA. According to conventional theory, the degradation of ESOPLA should be easier because the star-shaped ESOPLA contains more hydrophilic groups and much shorter poly(lactic acid) chains. Two reasonable explanations may be given for this phenomenon. One is that a strong intermolecular force may be present in the star-shaped polymer because of its long arm-chain undergoing twining behavior and this microstructure can prevent water erosion of the polymer. The other reason may be that ESO can minimize the amount of trapped water and, therefore, slow the permeation of water into the polymer occurs, which is due to the hydrophobic nature of ESO. Cai [14] and Zhao [15] synthesized a star-shaped polylactide with hydrophilic multifunctional initiators, and they found that the star-shaped polymer degraded faster than the linear polymer control. These hypothesizes were verified by observing the film surface topography by SEM (pre-degradation and 8 weeks post-degradation). The surface topography of the polymer is shown in Fig. 7 and there is a significant difference between the linear PDLLA film and the star-shaped ESOPLA film. After 8 weeks of degradation, the linear PDLLA membrane surface (Fig. 7b) was uniformly covered by potholes of about 10 mm in diameter, which indicates that serious degradation occurred. The star-shaped ESOPLA film's surface deterioration was much less over the same period and various sizes of potholes are present (Fig. 7d and f). The copolymer with 0.5wt% ESO had obviously retarded the degradation of PLA.
Fig. 5. Degradation behavior of the different polymer films; Mw(PDLLA) ¼ 91350 Da, Mw(1.0wt% ESO) ¼ 76,190 Da, Mw(10.0wt% ESO) ¼ 82,190 Da. Medium: sterile, 0.1 M, pH 7.4 PBS; T ¼ 37 0.5 C. Medium was refreshed every two weeks.
Fig. 6. Dependence of Mw on the time of degradation, Mw(PDLLA) ¼ 91350 Da, Mw(10.0wt% ESO) ¼ 82190 Da.
Fig. 4. TGA thermograms of (a) linear PDLLA, Mw ¼ 91,350 Da, (b) ESOPLA(3.0wt%), Mw ¼ 165.900 Da.
PDLLA, which is due to the star-shaped ESOPLA containing a branched structure and short chains simultaneously, which results in more thermally unstable hydroxyl groups. At temperatures above 360 C, the linear PDLLA decomposes completely but the ESOPLA residue was 4.6wt%, which corresponds to the initial ESO content. The reason for this is that the decomposition of the hydrocarbon chain of the ESO unit is more difficult than the ester bond of PDLLA. 3.3. Weight loss ratio changes of star-shaped ESOPLA and PDLLA during degradation
C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490
489
Fig. 7. Surface topography changes during the degradation of polymer films; (a) Linear PDLLA film before degradation, (b) PDLLA film after 8 weeks of degradation, (c) ESOPLA0.5% film before degradation, (d) ESOPLA0.5% film after 8 weeks of degradation, (e) ESOPLA10.0% film before degradation, (f) ESOPLA10.0% film after 8 weeks of degradation.
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C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490
4. Conclusions A multi-arm star poly(DL-lactide) polymer was successfully synthesized with epoxidized soybean oil as an initiator. ESO accelerates the reaction rate of the ring-opening polymerization and a higher molecular weight PLA is obtained. ESO also improves the polymer's stability against hydrolytic degradation. Furthermore, PLA modified with ESO can reduce the storage costs and prolong the service life of the polymer by minimizing monomerinduced autocatalysis during storage and usage.
References [1] Auras R, Harte B, Selke S. An overview of polylactides as packaging materials. Macromolecular Bioscience 2004;4:835e64. [2] Garlotta D. A literature review of poly(lactic acid). Journal of Polymers and the Environment 2001;9:63e84. [3] Lima LT, Aurasb R, Rubinob M. Processing technologies for poly(lactic acid). Progress in Polymer Science 2008;33:820e52. [4] Mathew AP, Oksman K, Sain M. Mechanical properties of biodegradable composites from poly lactic acid (PLA) and microcrystalline cellulose (MCC). Journal of Applied Polymer Science 2005;97:2014e25. [5] Serizawa S, Inoue K, Ij M. Kenaf-fiber-reinforced poly(lactic acid) used for electronic products. Journal of Applied Polymer Science 2006;100: 618e24.
[6] Massingill JL, Fowler DW. Waterproofing Concrete Highways. United States: Highway IDEA Program; 2004. [7] Japon S, Boogh L, Leterrier Y, Manson JAE. Reactive processing of poly(ethylene terephthalate) modified with multifunctional epoxy-based additives. Polymer 2000;41:5809e18. [8] Ali F, Young W, Chang YW, Kang SC, Yoon JY. Thermal, mechanical and theological properties of poly(lactic acid)/epoxidized soybean oil blends. Polymer Bulletin 2009;62:91e8. [9] Zhan GZ, Zhao L, Hu S, Gan WJ, Yu YF, Tang XL. A novel biobased resinepoxidized soybean oil modified cyanate ester. Polymer Engineering and Science 2008;48:1322e8. [10] Xu YQ, You M, Qu JP, Qu JP. In: Melt rheology of poly (lactic acid) plasticized by epoxidized soybean oil, vol. 14. Wuhan University Journals Press; 2009. 349e354. [11] Dong CM, Qiu KY, Gu ZW, Feng XD. Synthesis of star-shaped poly(D, L-lactic acid-alt-glycolic acid) with multifunctional initiator and SnOct2 catalyst. Polymer 2001;42:6891e6. [12] Qu XH, Wu Q, Liang J, Qu X, Wang SG, Chen GQ. Enhanced vascular-related cellular affinity on surface modified copolyesters of 3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx). Biomaterials 2005;26:6991e7001. [13] Luo YF, Wang YL, Niu XF, Fu CH, Wang SJ. Synthesis, characterization and biodegradation of butanediamine-grafted poly(DL-lactic acid). European Polymer Journal 2007;43:3856e64. [14] Cai Q, Zhao YL, Bei JZ, Xi F, Wang SG. Synthesis and properties of star-shaped polylactide attached to poly(amidoamine) dendrimer. Biomacromolecules 2003;4:828e34. [15] Yuan WZ, Zhu L, Huang XB, Zheng SX, Tang XZ. Synthesis, characterization and degradation of hexa-armed star-shaped poly(L-lactide)s and poly(D, L-lactide)s initiated with hydroxyl-terminated cyclotriphosphazene. Polymer Degradation and Stability 2005;87:503e9.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Improved hydrolytic stability of poly(DL-lactide) with epoxidized soybean oil Chunhua Fu a, Bei Zhang a, Changshun Ruan a, Chengbo Hu a, Ya Fu a, b, Yuanliang Wang a, * a b
Research Center of Bioinspired Material Science and Engineering, Department of Bioengineering, Chongqing University, Chongqing 400030, PR China Chongqing University of Science and Technology of China, Chongqing University, Chongqing 400030, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2009 Received in revised form 24 December 2009 Accepted 11 January 2010 Available online 28 January 2010
The multi-arm star polymer (ESOPLA) was obtained by ring-opening polymerization of DL-lactide using multifunctional epoxidized soybean oil (ESO) as an initiator in the presence of a stannous actuate (SnOct2) catalyst. Gel permeation chromatography with multi-angle laser light scattering (GPC-MALLS), FTIR, 1H NMR, thermal analysis and in vitro degradation were used to qualitatively characterize the synthesized polymers. The results revealed that ESO plays an important role in increasing the molecular weight, polymerization rate and monomer conversion rate. Degradation analysis demonstrated that the decrease in molecular weight and the weight loss ratio of the star-shaped ESOPLA were lower than that of linear poly(DL-lactide) (PDLLA). The surface topography of pre- and post-degradation materials was characterized by scanning electron microscopy (SEM). These SEM images showed that the linear PDLLA films underwent water erosion more readily than the star-shaped polymer films. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: Epoxidized soybean oil Poly(DL-lactide) Multi-arm star polymer Degradation
1. Introduction Polylactic acid or polylactide (PLA) is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources such as starch and sugar. It is a promising polymer to replace the usual plastics that are produced from non-renewable petroleum and natural gas resources [1,2]. However, the degradation of PLA is difficult to control and undesired degradation may occur during storage and usage. If crude PLA is not stored in refrigerated or desiccated conditions, it would degrade severely after 3 months but purified PLA, from which the residual monomers have been removed by an organic solvent, does not need strict storage conditions. As one of the most interesting environmentally friendly plastics, a large amount of research on PLA has focused on improving its mechanical property and processing technology [3e5]. Relatively little attention has been paid to the degradation problem during storage. Epoxidized soybean oil has been used to reduce water absorption and to improve the service life of concrete [6]. As a nontoxic and degradable plasticizer, ESO has been blended with PLA and other plastics to improve properties such as melt rheology and tensile strength etc. [7e10]. The oil may, however, leach to the surface of polymers during its service life. Dong reported that a multifunctional initiator can minimize the residual lactide ratio and increase the polymer molecular weight [11]. In this study, ESO
was used to increase the waterproof property of PLA and to reduce the residual monomer content. Epoxy and hydroxyl groups in ESO can initiate DL-lactide ring-opening polymerization. The main advantages of ESO and PLA are their abundance and environmental characteristics. 2. Experimental 2.1. Materials DL-lactide was synthesized by polycondensation and depolymerization reactions from DL-lactic acid and the crude product was recrystallized from dried ethyl acetate. The melting point of the product was 126.3 C. PDLLAs of various molecular weights were synthesized in our laboratory. SnOct2 was purchased from Sigma and used without further purification. ESO was purchased from Kaiqi chemical company and the average number of epoxy groups per molecule was 4. HPLC grade tetrahydrofuran (THF) was purchased from SK Chemicals. Analytical grade chloroform and methanol was purchased from Chongqing Chemical Factory.
2.2. Polymerization Specific amounts of ESO, DL-lactide and SnOct2 (1:5000 on a molar basis) were added to dry flasks and the flasks were filled with nitrogen gas. The polymerization flasks were put into an oil bath at 150 C for a specific time. After the desired reaction time, the reaction flasks were cooled and the crude products were DL-lactide
* Corresponding author. Tel./fax: þ86 23 65102509. E-mail address: [email protected] (Y. Wang).
0141-3910/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.01.007
486
C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490
purified by coprecipitation in a chloroform-methanol system. The purified polymer was dried in vacuum until it had a constant weight and the residual DL-lactide was detected by gravimetric evaluation of the pre- and post-purified products. 2.3. Characterization Scheme 1.
The molecular weight and polydispersion index (PDI) of the polymers were determined with GPC-MALLS using polystyrene standards. Three Agilent 1100 HPLC columns (300 8.0 mm) were used in series with THF as the eluent at a flow rate of 1.0 ml/min at 25 C, FTIR was carried out using a Spectrum One spectrometer (Perkin Elmer). 1H NMR was performed on a Bruker ARX-400 spectrometer (Bruker) and the dried samples were dissolved in CDCl3. Differential scanning calorimetry (DSC) was carried out using a STA449C thermal analysis system (NETZSCH) at a heating rate of 10 C/min under nitrogen. The surface morphology of the polymer film was determined using a Hitachi S-520 scanning electron microscope (Hitachi).
degraded polymer samples. The weight loss ratio was calculated according to the equation:
weight lossð%Þ ¼
W0 Wt 100% W0
Where W0 and Wt are the sample weights before and after hydrolytic degradation, respectively. The reported molecular weights and weight loss ratios were sample averages.
3. Results and discussion 2.4. Sample preparation for degradation tests
3.1. Synthesis of star-shaped ESOPLAs
Films of PDLLA and ESOPLA were prepared by a solution casting method using a 3.0 wt% chloroform solution and they were placed in glass Petri dishes (47 mm) [12]. The solvent was allowed to evaporate over 48 h and films had formed. These films were dried in vacuum until a constant weight was obtained. All experiments were performed at room temperature. The obtained glabrous transparent films were cut into 10 10 mm pieces. The obtained films were UV sterilized for 30 min before the degradation study [13]. 2.5. Degradation study The sterile PBS solution (0.1 M, pH 7.4) was used as a medium and 15 ml PBS per film. The solution was changed every two weeks without considering the autocatalysis effect caused by monomer degradation. All the vials were sealed and kept in an incubator at 37 0.5 C. Every two weeks the polymer films were removed, rinsed with distilled water and dried to a constant weight in vacuum for molecular weight and weight loss ratio measurements. GPC-MALLS was used to determine the molecular weight of the original and
3.1.1. Effect of the multifunctional initiator on polymerization Multi-arm star-shaped ESOPLA was synthesized according to the procedure shown in Scheme 1. The hydroxyl and the epoxy groups of ESO can initiate the ring-opening polymerization of DL-lactide and the structure of ESOPLA is shown in Scheme 2. Table 1 shows that the molecular weight and the DL-lactide monomer residue ratio are a function of ESO concentration. At higher ESO content and with a constant concentration of SnOct2, reaction temperature of 150 C and reaction time of 12 h, the molecular weight initially increased significantly and then gradually decreased. The molecular weight reached 165,900 Da at 3.0 wt% ESO but the DL-lactide residue ratio decreased at higher ESO ratios. The molecular weight distribution was narrow (1.130e1.783). The molecular weight of the polymer normally decreases as the initiator ratio increases. However, one ESO molecule has several catalytic sites and every star-shaped molecule is composed of several chains. This is why the molecular weight increases when an appropriate amount of ESO initiator is added. Fig. 1 illustrates the effect of ESO on the polymerization rate and it is obvious that the molecular weight and the polymerization rate are significantly
Scheme 2.
C. Fu et al. / Polymer Degradation and Stability 95 (2010) 485e490 Table 1 The result of
DL-lactide
487
copolymerization with ESO. Mw (104)a
Sample
PDIa
DL-lactide b
residue
ratio (%)
control poly(DL-lactide) Copolymerization with 0.5wt% ESO copolymerization with 1.0wt% ESO copolymerization with 3.0wt% ESO copolymerization with 5.0wt% ESO copolymerization with 10.0wt% ESO
5.697 6.792 7.619 16.59 15.14 8.219
1.130 1.621 1.756 1.403 1.330 1.783
2.72 1.85 1.61 1.45 0.60 0.58
*Polymerization temperature of 150 C and polymerization time of 12 h. a Measured by GPC-MALLS. b Detected gravimetrically
enhanced in the presence of ESO compared with the control group. The hydroxyl, epoxy and amino active groups are efficient initiating sites for the polymerization of lactides in the presence of the Sn(Oct)2 catalyst, therefore, ESO can accelerate the polymerization ratio. 3.1.2. Characterization of copolymer structures IR and 1H NMR were used to characterize and confirm the structure of the linear PDLLA and the star-shaped ESOPLA. In the IR spectrum (Fig. 2), the terminal hydroxyl absorption peak at 3510 cm1 in ESOPLA (b) is obviously larger than that of the linear PDLLA (a). This is contrary to the molecular weights of the two polymers (MwPDLLA ¼ 36,790 Da, MwESOPLA ¼ 165,900 Da) and indicates that a multi-arm structure exists in the ESOPLA polymer. Fig. 3a shows the 1H NMR spectrum of the linear PDLLA and the two main signals at 1.58 and 5.21 ppm are due to the CH3 and CH protons of PDLLA, respectively. In the 1H NMR spectrum of ESOPLA (Fig. 3b), the peaks at 1.58 ppm and 5.21 ppm are assigned to the CH3(6) and CH(7) protons of the PDLLA chains in the multi-arm polymer. The signals at 0.87 and 1.29 ppm are attributed to the CH3 (1) and CH2 (2) protons of ESO, respectively.
Fig. 2. IR spectra of (a) linear PDLLA, Mw ¼ 36,790 Da and (b) Star-shaped ESOPLA (3.0 wt% ESO), Mw ¼ 165,900 Da.
improve the tensile strength property of the polymers. From Fig. 4, the onset decomposition temperatures (Tonset) of linear PDLLA (a) and star-shaped ESOPLA (b) were 272.2 C and 268.7 C, respectively. The thermal stability of ESOPLA is slightly poorer than that of
3.2. Thermal properties of star-shaped ESOPLA and linear PDLLA The thermal property of star-shaped ESOPLA was investigated by DSC and compared with that of linear PDLLA. The glass transport temperature of ESOPLA (54.7 C) is lower than that of linear PDLLA (60.2 C) and this is due to the flexible chains and the plasticized character of ESO. The low glass transition temperature may
Fig. 1. The effect of ESO on the polymerization reaction rate.
Fig. 3. 1H NMR spectra of (a) linear PDLLA, Mw ¼ 56,790 Da and (b) Star-shaped ESOPLA (3.0 wt% ESO), Mw ¼ 165,900 Da.
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The changes in weight loss ratio as well as the decrease in molecular weight and surface structure of the polymers that were degraded in PBS solution were investigated. The molecular weights of the three samples were similar. Fig. 5 shows the obtained weight loss ratios for PDLLA and ESOPLA as a function of time during incubation in the PBS solution. The degradation process includes two stages. In the first two weeks, the degree of degradation was slight and there was no significant difference between the linear PDLLA and the star-shaped polymer
at this stage. The weight loss ratios of all the samples were less than 5%, and the films were still transparent. In the next stage, the weight loss ratios increased but the degree of increase was different among the three samples. The weigh loss ratio of linear PDLLA was 51.2% after a degradation time of 10 weeks while ESOPLA1.0wt% and ESOPLA10.0wt% were 28.6% and 20.3%, respectively. The transparency of PDLLA was inferior to the other two samples. This result indicates that degradation of the star-shaped ESOPLA is slower than that of linear PDLLA. The molecular weight of linear PDLLA and star-shaped ESOPLA as a function of time during degradation in the PBS solution is plotted in Fig. 6. The linear polymer's molar mass decreased from 91,350 to 28,500 Da after 10 weeks of degradation, however, the molecular weight of ESOPLA10.0wt% decreased slowly from 82,190 to 61,200 Da. It can be concluded that the degradation rate of the star-shaped ESOPLA is slower than that of the linear PDLLA. According to conventional theory, the degradation of ESOPLA should be easier because the star-shaped ESOPLA contains more hydrophilic groups and much shorter poly(lactic acid) chains. Two reasonable explanations may be given for this phenomenon. One is that a strong intermolecular force may be present in the star-shaped polymer because of its long arm-chain undergoing twining behavior and this microstructure can prevent water erosion of the polymer. The other reason may be that ESO can minimize the amount of trapped water and, therefore, slow the permeation of water into the polymer occurs, which is due to the hydrophobic nature of ESO. Cai [14] and Zhao [15] synthesized a star-shaped polylactide with hydrophilic multifunctional initiators, and they found that the star-shaped polymer degraded faster than the linear polymer control. These hypothesizes were verified by observing the film surface topography by SEM (pre-degradation and 8 weeks post-degradation). The surface topography of the polymer is shown in Fig. 7 and there is a significant difference between the linear PDLLA film and the star-shaped ESOPLA film. After 8 weeks of degradation, the linear PDLLA membrane surface (Fig. 7b) was uniformly covered by potholes of about 10 mm in diameter, which indicates that serious degradation occurred. The star-shaped ESOPLA film's surface deterioration was much less over the same period and various sizes of potholes are present (Fig. 7d and f). The copolymer with 0.5wt% ESO had obviously retarded the degradation of PLA.
Fig. 5. Degradation behavior of the different polymer films; Mw(PDLLA) ¼ 91350 Da, Mw(1.0wt% ESO) ¼ 76,190 Da, Mw(10.0wt% ESO) ¼ 82,190 Da. Medium: sterile, 0.1 M, pH 7.4 PBS; T ¼ 37 0.5 C. Medium was refreshed every two weeks.
Fig. 6. Dependence of Mw on the time of degradation, Mw(PDLLA) ¼ 91350 Da, Mw(10.0wt% ESO) ¼ 82190 Da.
Fig. 4. TGA thermograms of (a) linear PDLLA, Mw ¼ 91,350 Da, (b) ESOPLA(3.0wt%), Mw ¼ 165.900 Da.
PDLLA, which is due to the star-shaped ESOPLA containing a branched structure and short chains simultaneously, which results in more thermally unstable hydroxyl groups. At temperatures above 360 C, the linear PDLLA decomposes completely but the ESOPLA residue was 4.6wt%, which corresponds to the initial ESO content. The reason for this is that the decomposition of the hydrocarbon chain of the ESO unit is more difficult than the ester bond of PDLLA. 3.3. Weight loss ratio changes of star-shaped ESOPLA and PDLLA during degradation
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Fig. 7. Surface topography changes during the degradation of polymer films; (a) Linear PDLLA film before degradation, (b) PDLLA film after 8 weeks of degradation, (c) ESOPLA0.5% film before degradation, (d) ESOPLA0.5% film after 8 weeks of degradation, (e) ESOPLA10.0% film before degradation, (f) ESOPLA10.0% film after 8 weeks of degradation.
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4. Conclusions A multi-arm star poly(DL-lactide) polymer was successfully synthesized with epoxidized soybean oil as an initiator. ESO accelerates the reaction rate of the ring-opening polymerization and a higher molecular weight PLA is obtained. ESO also improves the polymer's stability against hydrolytic degradation. Furthermore, PLA modified with ESO can reduce the storage costs and prolong the service life of the polymer by minimizing monomerinduced autocatalysis during storage and usage.
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