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Promoting crystallization of polylactide by the formation of crosslinking bundles
Materials Letters 117 (2014) 171–174
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Promoting crystallization of polylactide by the formation of crosslinking bundles Yu Zhang a, Chengling Wang a, Hainan Du a, Xinpeng Li a, Dashan Mi a, Xiongwei Zhang a, Tao Wang a, Jie Zhang a,b,n a b
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China State Key Laboratory of Molecular Engineering of Polymers (Fudan University), Shanghai, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 August 2013 Accepted 1 December 2013 Available online 7 December 2013
In this study, the influence of crosslinking structures on the crystallization of polylactide (PLA) samples was investigated. Chemical-induced crosslinking method was adopted and crosslinking structures were successfully achieved in the presence of dicumyl peroxide (DCP) and triallyl isocyanurate (TAIC). Originally it was commonly suggested that crosslinking would block the motion of chains, thus hindering the crystallization process. However, our results indicate that low or moderate crosslinking bundles (not complete polymer network) can promote the crystallization of PLA samples to some extent. With further increasing crosslinking degree (from 12.1% to 41.2%), although the onset crystallization temperature can be increased as well, the perfection of crystal lamellas is inferior. Based on the experimental results, a schematic model is proposed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Crystal growth Crosslink
1. Introduction Polylactide (PLA) is a typical biodegradable polyester obtained by synthesis of lactic acid, which can be produced from renewable resources such as corn or sugarcane [1]. Unfortunately, owing to its intrinsic slow crystallization kinetics, PLA products are usually amorphous, especially under actual processing conditions such as conventional injection molding and extrusion, where a large cooling rate exists, leading to some undesirable properties such as poor barrier property and thermal resistance [2]. The methods to increase the crystallinity of PLA samples are the research hot spots, such as annealing [3], adding nucleation agent [4], or imposing shear flow [5]. Crosslinking has been proved to be an effective way to improve the heat stability and mechanical properties of PLA samples. The crosslinking structures of PLA can be formed by electron beam irradiation [6] or γ-irradiation [7] in the presence of a small amount of crosslinking agent triallyl isocyanurate (TAIC). An alternative approach to achieve crosslinking structure is chemical crosslinking [8]. Chemical reactions between crosslinking agent and polymer chains can be initiated by chemical treatments in place of irradiation. In the past, the research about the influence of crosslinking structures of PLA mainly lay in mechanical, thermal, rheology properties rather than their influence on the crystallization [6–8].
n
Corresponding author. Tel.: þ 8613086681699; fax: þ 8628 85405402. E-mail address: [email protected] (J. Zhang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.005
To the best of our knowledge, no research about the influence of the crosslinking structures on the crystallization of the PLA has been reported. In this article, we paid special attention to the influence of crosslinking structures induced by chemical method on the heating and cooling process of PLA samples. Interestingly, we found that the crosslinking structures could promote the crystallization of PLA samples to some extent. With further increasing crosslinking degree (from 12.1% to 41.2%), the onset crystallization temperature can be increased as well, but the perfection of crystal lamellas is inferior.
2. Material and methods A commercially available polylactide (PLA, 4032D) with a weight-averaged molecular weight of 9.96 104 g/mol and a polydispersity of 1.67, was purchased from NatureWorks. Triallyl isocyanurate (TAIC, crosslinking agent) and dicumyl peroxide (DCP, initiator) were supplied by West Reagent. PLA samples containing different concentrations of TAIC (0%, 0.1%, 0.2%, 0.5%, 1%) and constant content of DCP (0.2%) were mixed in a Haake mixer at 50 rpm, 180 1C for 10 min. Then, the samples were hot-pressed at 190 1C for 5 min into sheets with a thickness of 1 mm. For convenience, code “PLA/x” is used to describe each sample, where x represents the TAIC content. For comparison purpose, pure PLA was treated with the same procedure (labeled as PLA).
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Y. Zhang et al. / Materials Letters 117 (2014) 171–174
Differential scanning calorimetry (DSC) was carried out on a TA Q200 calorimeter in a nitrogen atmosphere. The sample was first heated up to 200 1C at a heating rate of 10 1C/min and held on for 3 min to release thermal history, then cooled down to 40 1C at 3 1C/min to record the crystallization behavior of the sample (cooling section). After maintaining at 40 1C for 1 min, it was heated up again to 200 1C at 3 1C/min to study the melting behavior of the sample (heating section). Temperature-dependent dynamic rheology properties were investigated using a rotational rheometer (Malvern Instruments, Bohlin Gemini 200). The measurement was performed at a constant shear rate of 2 rad/ s and the temperature varied from 200 1C to specific temperature (100–120 1C) at a cooling rate of 3 1C/min. Dynamic mechanical property (DMA) was measured with a TA-800A apparatus from TA Instrument. Experiments proceeded at a frequency of 1 Hz at the temperature range of 20–200 1C at a heating rate of 3 1C/min. The synchrotron 2D-WAXD was carried out on the BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF), China. The wavelength used was 0.124 nm. Hot Stage Polarized Light Microscopy (HS-PLM) was conducted using DX-1 (Jiang Xi Phoenix Optical Co., China) microscope. Each sample was heated to 200 1C first and kept for 3 min to release thermal history, then crystallized isothermally at the temperature of 134 1C for 30 min. The images reflect the final morphology.
3. Results
Fig. 1. Gel fraction and degree of swelling as a function of TAIC content.
It can be seen from Fig. 1(a) that with increasing TAIC content, gel fraction increases sharply at low TAIC contents (0.1% and 0.2%) while it nearly levels up at high TAIC concentrations (0.5% and 1%), indicating the formation of crosslinking structures. Variation of degree of swelling as a function of TAIC is shown in Fig. 1(b), indicating that the higher gel fraction (namely crosslinking degree) is, the inferior swelling ability it has. In order to investigate the effect of crosslinking degree on the crystallization, PLA/0.1
Fig. 2. Cooling (a) and heating (b) performance (3 1C/min) of various samples: (a-1) cooling section of DSC; (a-2) dynamic rheology; (b-1) heating section of DSC; (b-2) DMA.
Y. Zhang et al. / Materials Letters 117 (2014) 171–174
(12.1%) and PLA/1 (47.2%) are chosen as the representatives of low crosslinking and moderate crosslinking samples, respectively. Fig. 2(a) compares the performance of various samples during cooling process. The onset crystallization temperature and crystallization enthalpy (derived from cooling section of DSC) are summarized in Table 1. It indicates that pure PLA has the lowest crystallization ability [2–5]. With the addition of DCP, both the onset crystallization temperature and crystallization enthalpy increase, which can be attributed to the breakage of chains induced by DCP, and these shortened chains can stack in lamellas easily. However, when TAIC is incorporated, these “broken chains” can be reorganized by the crosslinking agent. With the crosslinking structures being formed, the crystallization temperature rises again. It can be seen from Fig. 2(a-2) Table 1 Comparison of crystallization ability of various samples during cooling process (derived from cooling section of DSC). Samples
Onset crystallization temperature (1C)
Crystallization enthalpy (J/g)
PLA PLA/0 PLA/0.1 PLA/1
119.3 125.7 127.3 131.0
17.7 25.3 50.3 49.0
Fig. 3. 1D-WAXD curves of various samples which are derived from 2D-WAXD patterns.
173
that the complex viscosity is much higher in crosslinking samples, proving that the crosslinking structure can survive from high temperature of 200 1C. During the heating process, the samples experience a glass transition at about 60 1C, as shown in Fig. 2(b). Then, with the temperature increasing, cold crystallization happens, as a result, the storage modulus increases obviously. The cold crystallization peak of crosslinking samples is the lowest, which is ascribed to higher crystallinity during the cooling process and higher crosslinking degree that are not propitious to the motion of chains. It should be noted from Fig. 2(b-1) and Fig. 2(b-2) that PLA/1 has a more obvious cold crystallization than PLA/0.1, for the reason that highly crosslinking chains may experience a much longer time to stack within the lattice in perfection, thus during the heating process, they suffer from a self-perfecting process. In contrast, the chains of PLA/0.1 have much higher motion ability. Accordingly, they can be packed in crystal lamella with nearly no defects during cooling process. Fig. 3 presents the 1D-WAXD curves of compression molded samples, proving that the PLA/0.1 sample presents better crystal perfection than PLA/1.
4. Discussion The crystallization ability of PLA is the lowest. After isothermal crystallization for 30 min, some relatively large spherulite comes into being, nevertheless, still a few chains cannot crystallize (the black part in Fig. 4(a′)), indicating that the crystallization is rather slow. However, the chains shortened by DCP would increase its crystallization ability to some extent. As exhibited in Fig. 4(b′), the spherulite becomes small and no “black part” exists, hinting better crystallization ability than pure PLA. With the further incorporation of TAIC, the crosslinking bundles are formed (not complete crosslinking network). The compact structure of these bundles can act as precursor to promote the crystallization. During crystallization, the chains that surround the crosslinking bundles can be absorbed to stack in lamellas to form spherulite. The formed spherulite is so small that they cannot be distinguished as a result of higher nucleation density than that of pure PLA (Fig. 4(c′) and Fig. 4(d′)). For PLA/0.1 sample, the surrounding chains can stack in lamellas in perfection due to their relatively good motion ability (Fig. 4(c)). However, with the crosslinking degree further increasing (from 12.1% to 41.2%), the motion of chains becomes relatively
Fig. 4. Mechanism of crosslinking bundles-promoted crystallization.
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Y. Zhang et al. / Materials Letters 117 (2014) 171–174
terrible due to moderate crosslinking (Fig. 4(d)), although the onset crystallization temperature can be increased as well, the perfection of crystal lamellas is inferior.
support. We are indebted to the Shanghai Synchrotron Radiation Facility (SSRF) in Shanghai, China for WAXD experiments. References
5. Conclusion Crosslinking structures were successfully achieved in the presence of initiator and crosslinking agent. Our results suggest that the low or moderate crosslinking bundles (not complete polymer crosslinking network) can promote the crystallization of PLA samples. With further increasing crosslinking degree (from 12.1% to 41.2%), although the onset crystallization temperature can be increased as well, the perfection of crystal lamellas is inferior. A schematic model is proposed to interpret the mechanism.
Acknowledgment We would like to express our great thanks to the National Natural Science Foundation of China (51010004) for financial
[1] Tsuji H, Daimon H, Fujie K. A new strategy for recycling and preparation of poly (L-lactic acid): hydrolysis in the melt. Biomacromolecules 2003;4:835–40. [2] Miyata T, Masuko T. Crystallization behavior of poly (L-lactide). Polymer 1998;39:5515–21. [3] Wang Y, Li M, Shen C. Effect of constrained annealing on the microstructures of extrusion cast polylactic acid films. Mater Lett 2011;65:3525–8. [4] Barrau S, Vanmansart C, Moreau M, Addad A, Stoclet G, Lefebvre J-M, et al. Crystallization behavior of carbon nanotube-polylactide nanocomposites. Macromolecules 2011;44:6496–502. [5] Tang H, Chen J-B, Wang Y, Xu J-Z, Hsiao BS, Zhong G-J, et al. Shear flow and carbon nanotubes synergistically induced nonisothermal crystallization of poly (lactic acid) and its application in injection molding. Biomacromolecules 2012;13:3858–67. [6] Nagasawa N, Kaneda A, Kanazawa S, Yagi T, Mitomo H, Yoshii F, et al. Application of poly (lactic acid) modified by radiation crosslinking. Nucl Instrum Methods Phys Res, Sect B 2005;236:611–6. [7] Jin F, Hyon SH, Iwata H, Tsutsumi S. Crosslinking of poly (L‐lactide) by γ‐irradiation. Macromol Rapid Commun 2002;23:909–12. [8] Yang S-L, Wu Z-H, Yang W, Yang M-B. Thermal and mechanical properties of chemical crosslinked polylactide (PLA). Polym Test 2008;27:957–63.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Promoting crystallization of polylactide by the formation of crosslinking bundles Yu Zhang a, Chengling Wang a, Hainan Du a, Xinpeng Li a, Dashan Mi a, Xiongwei Zhang a, Tao Wang a, Jie Zhang a,b,n a b
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China State Key Laboratory of Molecular Engineering of Polymers (Fudan University), Shanghai, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 August 2013 Accepted 1 December 2013 Available online 7 December 2013
In this study, the influence of crosslinking structures on the crystallization of polylactide (PLA) samples was investigated. Chemical-induced crosslinking method was adopted and crosslinking structures were successfully achieved in the presence of dicumyl peroxide (DCP) and triallyl isocyanurate (TAIC). Originally it was commonly suggested that crosslinking would block the motion of chains, thus hindering the crystallization process. However, our results indicate that low or moderate crosslinking bundles (not complete polymer network) can promote the crystallization of PLA samples to some extent. With further increasing crosslinking degree (from 12.1% to 41.2%), although the onset crystallization temperature can be increased as well, the perfection of crystal lamellas is inferior. Based on the experimental results, a schematic model is proposed. & 2013 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Crystal growth Crosslink
1. Introduction Polylactide (PLA) is a typical biodegradable polyester obtained by synthesis of lactic acid, which can be produced from renewable resources such as corn or sugarcane [1]. Unfortunately, owing to its intrinsic slow crystallization kinetics, PLA products are usually amorphous, especially under actual processing conditions such as conventional injection molding and extrusion, where a large cooling rate exists, leading to some undesirable properties such as poor barrier property and thermal resistance [2]. The methods to increase the crystallinity of PLA samples are the research hot spots, such as annealing [3], adding nucleation agent [4], or imposing shear flow [5]. Crosslinking has been proved to be an effective way to improve the heat stability and mechanical properties of PLA samples. The crosslinking structures of PLA can be formed by electron beam irradiation [6] or γ-irradiation [7] in the presence of a small amount of crosslinking agent triallyl isocyanurate (TAIC). An alternative approach to achieve crosslinking structure is chemical crosslinking [8]. Chemical reactions between crosslinking agent and polymer chains can be initiated by chemical treatments in place of irradiation. In the past, the research about the influence of crosslinking structures of PLA mainly lay in mechanical, thermal, rheology properties rather than their influence on the crystallization [6–8].
n
Corresponding author. Tel.: þ 8613086681699; fax: þ 8628 85405402. E-mail address: [email protected] (J. Zhang).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.005
To the best of our knowledge, no research about the influence of the crosslinking structures on the crystallization of the PLA has been reported. In this article, we paid special attention to the influence of crosslinking structures induced by chemical method on the heating and cooling process of PLA samples. Interestingly, we found that the crosslinking structures could promote the crystallization of PLA samples to some extent. With further increasing crosslinking degree (from 12.1% to 41.2%), the onset crystallization temperature can be increased as well, but the perfection of crystal lamellas is inferior.
2. Material and methods A commercially available polylactide (PLA, 4032D) with a weight-averaged molecular weight of 9.96 104 g/mol and a polydispersity of 1.67, was purchased from NatureWorks. Triallyl isocyanurate (TAIC, crosslinking agent) and dicumyl peroxide (DCP, initiator) were supplied by West Reagent. PLA samples containing different concentrations of TAIC (0%, 0.1%, 0.2%, 0.5%, 1%) and constant content of DCP (0.2%) were mixed in a Haake mixer at 50 rpm, 180 1C for 10 min. Then, the samples were hot-pressed at 190 1C for 5 min into sheets with a thickness of 1 mm. For convenience, code “PLA/x” is used to describe each sample, where x represents the TAIC content. For comparison purpose, pure PLA was treated with the same procedure (labeled as PLA).
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Y. Zhang et al. / Materials Letters 117 (2014) 171–174
Differential scanning calorimetry (DSC) was carried out on a TA Q200 calorimeter in a nitrogen atmosphere. The sample was first heated up to 200 1C at a heating rate of 10 1C/min and held on for 3 min to release thermal history, then cooled down to 40 1C at 3 1C/min to record the crystallization behavior of the sample (cooling section). After maintaining at 40 1C for 1 min, it was heated up again to 200 1C at 3 1C/min to study the melting behavior of the sample (heating section). Temperature-dependent dynamic rheology properties were investigated using a rotational rheometer (Malvern Instruments, Bohlin Gemini 200). The measurement was performed at a constant shear rate of 2 rad/ s and the temperature varied from 200 1C to specific temperature (100–120 1C) at a cooling rate of 3 1C/min. Dynamic mechanical property (DMA) was measured with a TA-800A apparatus from TA Instrument. Experiments proceeded at a frequency of 1 Hz at the temperature range of 20–200 1C at a heating rate of 3 1C/min. The synchrotron 2D-WAXD was carried out on the BL16B1 beamline in the Shanghai Synchrotron Radiation Facility (SSRF), China. The wavelength used was 0.124 nm. Hot Stage Polarized Light Microscopy (HS-PLM) was conducted using DX-1 (Jiang Xi Phoenix Optical Co., China) microscope. Each sample was heated to 200 1C first and kept for 3 min to release thermal history, then crystallized isothermally at the temperature of 134 1C for 30 min. The images reflect the final morphology.
3. Results
Fig. 1. Gel fraction and degree of swelling as a function of TAIC content.
It can be seen from Fig. 1(a) that with increasing TAIC content, gel fraction increases sharply at low TAIC contents (0.1% and 0.2%) while it nearly levels up at high TAIC concentrations (0.5% and 1%), indicating the formation of crosslinking structures. Variation of degree of swelling as a function of TAIC is shown in Fig. 1(b), indicating that the higher gel fraction (namely crosslinking degree) is, the inferior swelling ability it has. In order to investigate the effect of crosslinking degree on the crystallization, PLA/0.1
Fig. 2. Cooling (a) and heating (b) performance (3 1C/min) of various samples: (a-1) cooling section of DSC; (a-2) dynamic rheology; (b-1) heating section of DSC; (b-2) DMA.
Y. Zhang et al. / Materials Letters 117 (2014) 171–174
(12.1%) and PLA/1 (47.2%) are chosen as the representatives of low crosslinking and moderate crosslinking samples, respectively. Fig. 2(a) compares the performance of various samples during cooling process. The onset crystallization temperature and crystallization enthalpy (derived from cooling section of DSC) are summarized in Table 1. It indicates that pure PLA has the lowest crystallization ability [2–5]. With the addition of DCP, both the onset crystallization temperature and crystallization enthalpy increase, which can be attributed to the breakage of chains induced by DCP, and these shortened chains can stack in lamellas easily. However, when TAIC is incorporated, these “broken chains” can be reorganized by the crosslinking agent. With the crosslinking structures being formed, the crystallization temperature rises again. It can be seen from Fig. 2(a-2) Table 1 Comparison of crystallization ability of various samples during cooling process (derived from cooling section of DSC). Samples
Onset crystallization temperature (1C)
Crystallization enthalpy (J/g)
PLA PLA/0 PLA/0.1 PLA/1
119.3 125.7 127.3 131.0
17.7 25.3 50.3 49.0
Fig. 3. 1D-WAXD curves of various samples which are derived from 2D-WAXD patterns.
173
that the complex viscosity is much higher in crosslinking samples, proving that the crosslinking structure can survive from high temperature of 200 1C. During the heating process, the samples experience a glass transition at about 60 1C, as shown in Fig. 2(b). Then, with the temperature increasing, cold crystallization happens, as a result, the storage modulus increases obviously. The cold crystallization peak of crosslinking samples is the lowest, which is ascribed to higher crystallinity during the cooling process and higher crosslinking degree that are not propitious to the motion of chains. It should be noted from Fig. 2(b-1) and Fig. 2(b-2) that PLA/1 has a more obvious cold crystallization than PLA/0.1, for the reason that highly crosslinking chains may experience a much longer time to stack within the lattice in perfection, thus during the heating process, they suffer from a self-perfecting process. In contrast, the chains of PLA/0.1 have much higher motion ability. Accordingly, they can be packed in crystal lamella with nearly no defects during cooling process. Fig. 3 presents the 1D-WAXD curves of compression molded samples, proving that the PLA/0.1 sample presents better crystal perfection than PLA/1.
4. Discussion The crystallization ability of PLA is the lowest. After isothermal crystallization for 30 min, some relatively large spherulite comes into being, nevertheless, still a few chains cannot crystallize (the black part in Fig. 4(a′)), indicating that the crystallization is rather slow. However, the chains shortened by DCP would increase its crystallization ability to some extent. As exhibited in Fig. 4(b′), the spherulite becomes small and no “black part” exists, hinting better crystallization ability than pure PLA. With the further incorporation of TAIC, the crosslinking bundles are formed (not complete crosslinking network). The compact structure of these bundles can act as precursor to promote the crystallization. During crystallization, the chains that surround the crosslinking bundles can be absorbed to stack in lamellas to form spherulite. The formed spherulite is so small that they cannot be distinguished as a result of higher nucleation density than that of pure PLA (Fig. 4(c′) and Fig. 4(d′)). For PLA/0.1 sample, the surrounding chains can stack in lamellas in perfection due to their relatively good motion ability (Fig. 4(c)). However, with the crosslinking degree further increasing (from 12.1% to 41.2%), the motion of chains becomes relatively
Fig. 4. Mechanism of crosslinking bundles-promoted crystallization.
174
Y. Zhang et al. / Materials Letters 117 (2014) 171–174
terrible due to moderate crosslinking (Fig. 4(d)), although the onset crystallization temperature can be increased as well, the perfection of crystal lamellas is inferior.
support. We are indebted to the Shanghai Synchrotron Radiation Facility (SSRF) in Shanghai, China for WAXD experiments. References
5. Conclusion Crosslinking structures were successfully achieved in the presence of initiator and crosslinking agent. Our results suggest that the low or moderate crosslinking bundles (not complete polymer crosslinking network) can promote the crystallization of PLA samples. With further increasing crosslinking degree (from 12.1% to 41.2%), although the onset crystallization temperature can be increased as well, the perfection of crystal lamellas is inferior. A schematic model is proposed to interpret the mechanism.
Acknowledgment We would like to express our great thanks to the National Natural Science Foundation of China (51010004) for financial
[1] Tsuji H, Daimon H, Fujie K. A new strategy for recycling and preparation of poly (L-lactic acid): hydrolysis in the melt. Biomacromolecules 2003;4:835–40. [2] Miyata T, Masuko T. Crystallization behavior of poly (L-lactide). Polymer 1998;39:5515–21. [3] Wang Y, Li M, Shen C. Effect of constrained annealing on the microstructures of extrusion cast polylactic acid films. Mater Lett 2011;65:3525–8. [4] Barrau S, Vanmansart C, Moreau M, Addad A, Stoclet G, Lefebvre J-M, et al. Crystallization behavior of carbon nanotube-polylactide nanocomposites. Macromolecules 2011;44:6496–502. [5] Tang H, Chen J-B, Wang Y, Xu J-Z, Hsiao BS, Zhong G-J, et al. Shear flow and carbon nanotubes synergistically induced nonisothermal crystallization of poly (lactic acid) and its application in injection molding. Biomacromolecules 2012;13:3858–67. [6] Nagasawa N, Kaneda A, Kanazawa S, Yagi T, Mitomo H, Yoshii F, et al. Application of poly (lactic acid) modified by radiation crosslinking. Nucl Instrum Methods Phys Res, Sect B 2005;236:611–6. [7] Jin F, Hyon SH, Iwata H, Tsutsumi S. Crosslinking of poly (L‐lactide) by γ‐irradiation. Macromol Rapid Commun 2002;23:909–12. [8] Yang S-L, Wu Z-H, Yang W, Yang M-B. Thermal and mechanical properties of chemical crosslinked polylactide (PLA). Polym Test 2008;27:957–63.