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Effects of molten poly(d ,l -lactide) on nonisothermal crystallization in stereocomplex of poly(l -lactide) with poly(d -lactide)
Thermochimica Acta 573 (2013) 193–199
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Effects of molten poly(d,l-lactide) on nonisothermal crystallization in stereocomplex of poly(l-lactide) with poly(d-lactide) Yi Li a,b , Changyu Han a,∗ , Xin Zhang a , Qinglin Dong a , Lisong Dong a a b
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China College of Material Science and Engineering, Jilin Jianzhu University, Changchun 130118, China
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 27 September 2013 Accepted 27 September 2013 Available online 6 October 2013 Keywords: Poly(l-lactide) Poly(d-lactide) Stereocomplex Blend Nonisothermal crystallization
a b s t r a c t Poly(d,l-lactide) (PDLLA) and stereocomplex-poly (l- and d-lactide) (sc-PLA) blends were prepared by solution blending. The influence of the addition of PDLLA on nonisothermal crystallization behavior of scPLA was investigated. It was observed that the crystallization peak temperature of PDLLA/sc-PLA blends was marginally lower than that of neat sc-PLA at various cooling rates. Furthermore, the half-time for crystallization increased with an increase in PDLLA content, indicating the dilution effect of PDLLA on the sc-PLA component to restraint the overall crystallization process. The nonisothermal crystallization data was analyzed using Avrami, Ozawa and combined Ozawa–Avrami models methods. The validity of kinetic models on the nonisothermal crystallization process of neat sc-PLA and PDLLA/sc-PLA blends was discussed. The approach developed by combined Ozawa–Avrami models successfully described the nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends. The activation energy for nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends based on Friedman equation was evaluated. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Poly(lactic acid) (PLA) has been paid considerable attention from both fundamental and practical perspectives because it is derived completely from renewable resources, and hence production is sustainable [1–3], but also it can significantly contribute to the control of green-house gas (CO2 ) emission as a result of carbon capture during plant growth and the eventual complete biodegradability of the PLA matrix. The attributes of biodegradability, biocompatibility, good mechanical properties, and versatile fabrication processes make it one of the most promising materials for biomedical applications such as implant materials and controlled drug delivery systems, as well as for the conventional applications where common thermoplastics are employed [4–6]. PLA has two stereoregular enantiomers, poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) due to the presence of a chiral carbon in the skeletal chain. The polymerization of meso-lactide or racemic lactide leads to the formation of atactic poly(d,l-lactide) (PDLLA). A special crystalline structure termed stereocomplex based on CH3 . . .C O interactions of stereoselective van der Waals forces can be formed by blending PLLA and PDLA, which is easily formed by the melt blending, solution casting, or supercritical fluid methods
∗ Corresponding author. Fax: +86 431 85262244. E-mail address: [email protected] (C. Han). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.09.035
when mixing two polymers with identical chemical composition but different steric structures [7–9]. The pure PLLA or PDLA usually crystallizes in ␣-form with a 103 helical chain conformation, while the stereocomplex-PLA (sc-PLA) possesses a triclinic unit cell with a 31 helical chain conformation. This sc-PLA showed its Tm at about 230 ◦ C, which is about 50 ◦ C higher than that of pure PLLA or PDLA, so that sc-PLA should accordingly have better thermal and mechanical properties, and higher hydrolytic stability than neat PLLA or PDLA. Since the first report by Ikada et al., the influences of the homopolymer molecular weight, blending ratio (equimolar and non-equimolar), blending condition, and optical purity on the formation and properties of stereocomplexes have been well investigated [10–21]. However, for crystallization in bulk from the melt, the exclusive stereocomplex formation is limited to the PLLA and PDLA pair at least either of which has the molecular weight about 10,000, whereas solely homo-crystallites are formed in the equimolar PLLA/PDLA blends where both polymers have the molecular weights higher than 100,000 [9]. The crystallization kinetics of the melt-miscible blends of amorphous and crystalline polymers has been extensively investigated. When crystallization occurs below the melting point of the crystalline component, the process involves two types of polymer transport, namely, diffusion of the crystallizable component toward the crystal growth front and a simultaneous rejection of the amorphous component [22]. This crystallization process produces a liquid–solid phase separation, resulting into various
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morphological patterns closely controlled by the kinetics of the two types of polymer transport. In this case, the morphological formation may be kinetically governed by the thermal history and composition to achieve tailor-made properties for the blends. Therefore, investigation of the crystallization kinetics of polymer blends containing crystallizable components also has the practical importance [22,23]. Although the crystallization kinetics of neat sc-PLA has been extensively studied, however, studies have yet to be directed to investigation of influence of miscible dilutions on the crystallization kinetics of the sc-PLA. Therefore, PDLLA was selected in this work to study the effects of miscible blending on the nonisothermal crystallization behaviors of sc-PLA. The kinetics is analyzed using theoretical approach of Avrami, Ozawa, and combined Ozawa–Avrami models for nonisothermal crystallization. The activation energy describing the nonisothermal crystallization process is calculated based on Friedman method. On the other hand, the sc-PLA has higher strength, modulus and relatively slower degradation rate, making it an ideal material for load bearing devices [21]. In contrast, due to the amorphous nature, PDLLA shows weaker mechanical properties and faster degradation rate, making it a preferred candidate for developing low strength material. Consequently, blending sc-PLA and PDLLA can balance the mechanical properties and degradation rate and obtain the biomaterials with the controllable and improved performances.
2. Experimental PDLLA was purchased from Aldrich. It exhibited a weight–average molecular weight (Mw ) of 85,000, polydispersity of 1.64 (GPC analysis). PLLA was synthesized by the ring-opening polymerization of l-lactide using tin octanoate as a catalyst. It exhibited a weight–average molecular weight (Mw ) of 19,000, polydispersity of 1.83 (GPC analysis). PDLA was synthesized by the ring-opening polymerization of d-lactide using tin octanoate as a catalyst. It exhibited a weight–average molecular weight (Mw ) of 110,000, polydispersity of 1.92 (GPC analysis). Binary blends with PLLA and PDLA and ternary blends comprising PLLA, PDLA, and PDLLA were first separately prepared, then stereocomplexing sc-PLA and PDLLA mixtures were prepared by mixing the separately prepared solvent, followed with film-casting. PDLLA was first dissolved in chloroform (CHCl3 ) solution, and then further blended with solutions of PLLA and PDLA with proper ratios. Ratios of PLLA to PDLA in blends were maintained at 1/1 by weight in either binary or ternary blends. In ternary blends, PLLA/PDLA was fixed at 1/1, with the PDLLA contents ranging from 70/30, 50/50 and 30/70 in weight ratios, the first number referring to the weight percentage of PDLLA. The prepared solutions were mixed together with vigorous stirring, the solutions were cast onto petri dishes placed horizontally, and then the solvent was allowed to evaporate at room temperature for 12 h. The sample was further dried at 50 ◦ C under vacuum for 7 days to remove the solvent completely. The molecular weight parameters of neat PDLLA, PLLA and PDLA were measured using GPC with a Waters 410 GPC instrument equipped with two Waters Styragel columns (HT6E and HT3) and a differential refractometer detector. Measurements were performed at 25 ◦ C and at a flow rate of 1.0 mL min−1 using chloroform as eluent. The molecular weight was calibrated according to polystyrene standards (Polyscience Co.). Thermal analysis was carried out using a TA instruments differential scanning calorimetry (DSC) Q20 with a Universal Analysis 2000. All operations were performed under nitrogen purge, and the weight of the samples varied between 5 and 8 mg. For the glass transition temperature (Tg ) measurements, the PDLLA/sc-PLA
Fig. 1. DSC curves of heating scan at 10 ◦ C/min for neat PDLLA, sc-PLA, and PDLLA/scPLA blends.
blends were heated from 20 to 240 ◦ C at 10 ◦ C/min. For nonisothermal crystallization, the samples were initially melted at 240 ◦ C for 1 min in order to erase the previous thermal history and were performed at a cooling rate of 10, 20, 30, and 40 ◦ C/min, respectively. All measurements were carried out in nitrogen atmosphere. Samples were not used for second time in thermal analysis to avoid thermal degradation. 3. Results and discussion 3.1. Miscibility of the blends It is well known that Tg of a polymer blend is one of the most important criteria for the miscibility of components. So, it is necessary to establish the Tg value of PDLLA/sc-PLA blends. Fig. 1(a) and (b) shows the DSC heating curves of as-casing films of PDLLA/sc-PLA blends with various PDLLA contents. No melting peak is detected for neat PDLLA in the DSC measurements, confirming its amorphous nature. Moreover, a melting peak at high temperature ranging from 223 to 228 ◦ C can be observed for the neat sc-PLA and PDLLA/scPLA blends, indicating that the stereocomplex of PLLA with PDLA can be formed in the presence of PDLLA. The enthalpies of melt of the PDLLA/sc-PLA blends have been calculated from the enthalpy of melt normalized to the sc-PLA content, which are 70.8, 75.4, 70.0, and 73.2 J/g for neat sc-PLA, PDLLA/sc-PLA30/70, PDLLA/scPLA50/50, PDLLA/sc-PLA70/30 samples, respectively. It is clear that the enthalpy of the PDLLA/sc-PLA blends is very similar to that of neat sc-PLA, suggesting that the presence of PDLLA did not affect the degree of crystallinity of sc-PLA. As the stereocomplex forms in the PLLA/PDLA blends, the stereocomplex crystals will significantly hinder chains mobility, and increase the Tg . Therefore, the PDLLA and sc-PLA samples show the different Tg values and the miscibility of PDLLA and sc-PLA blends can be assessed by the change of Tg . The Tg values of neat PDLLA and sc-PLA are around 48 and 60 ◦ C, respectively, which are in well agreement with the values reported previously [21,24–26]. As seen in Fig. 1(b), all the blends with different compositions exhibit a single Tg that shifts to a higher temperature with an increase in the sc-PLA content. These results suggest the miscibility of the PDLLA and sc-PLA blends in the molten state. 3.2. Nonisothermal crystallization behavior The study of the nonisothermal crystallization behavior of polymers during nonisothermal processing is of great technical
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Table 1 Values of the heat evolved during crystallization (Hc ), the crystallization onset temperature (Tc ), the crystallization peak temperature (Tp ) and the half time of the crystallization (t0.5 ) for neat sc-PLA and PDLLA/sc-PLA blends. Cooling rate (◦ C/min)
Hc a
Tc (◦ C)
Tp (◦ C)
t0.5 (min)
Neat sc-PLA
10 20 30 40
65.4 61.8 59.0 66.0
217.1 213.0 211.0 208.6
204.6 198.7 194.4 191.0
1.21 0.71 0.55 0.46
PDLLA/sc-PLA30/70
10 20 30 40
65.2 69.9 69.1 72.8
213.2 207.8 207.1 202.7
200.5 193.0 189.7 184.3
1.24 0.75 0.60 0.48
PDLLA/sc-PLA50/50
10 20 30 40
72.4 63.0 66.1 62.9
207.8 202.9 201.7 197.9
195.2 185.7 183.9 179.0
1.26 0.88 0.61 0.50
PDLLA/sc-PLA70/30
10 20 30 40
63.5 68.3 59.5 69.6
200.9 193.0 192.9 189.0
185.7 174.6 173.2 167.9
1.52 1.00 0.68 0.56
Sample
a
Hc is corrected for the content of sc-PLA in the blends.
cooling rate, there is more time to activate nuclei, therefore, the crystallization can start at higher temperatures [28]. Secondly, the presence of PDLLA in sc-PLA leads to a significant decrease in Tp , and at a given cooling rate, Tp decreases with increasing PDLLA content. For example, at the cooling rate of 10 ◦ C/min, Tp for neat sc-PLA is 204.6 ◦ C, while for PDLLA/sc-PLA70/30 blend, it is 185.7 ◦ C. This is ascribed to the dilution effects of PDLLA on the sc-PLA component, because an extra energy is required for demixing the polymer components in the crystallization of the miscible blends and thus a higher energy is needed for the sc-PLA component to transport from the miscible melt to the growth front of crystal lamella [26]. 3.3. Nonisothermal crystallization kinetics The relative degree of crystallinity (Xt ), as a function of crystallization temperature (Tc ), is estimated according to Eq. (1):
T
Xt =
T
(dHc /dT ) dT
0 T∞
T0
(1)
(dHc /dT ) dT
Fig. 2. DSC thermograms of nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends at selected cooling rates of (a) neat sc-PLA and (b) PDLLA/scPLA 50/50.
importance, because most of the practical processing techniques are under nonisothermal conditions [27]. The effects of PDLLA on the crystallization behavior of sc-PLA were quantitatively analyzed through nonisothermal DSC experiments. Fig. 2 shows the crystallization thermograms of neat sc-PLA and PDLLA/sc-PLA blends at selected cooling rates between 10 and 40 ◦ C/min. From these plots, the crystallization temperature (Tc ) and crystallization peak temperature (Tp ) defined respectively, as the crystallization onset temperature and the crystalline peak, of sc-PLA and PDLLA/sc-PLA blends can be determined. Also, the heat evolved during crystallization (Hc ), which is normalized to the sc-PLA content, can be obtained; the results are summarized in Table 1. Table 1 and Fig. 3 shows the relationship between Tp and cooling rate for neat sc-PLA and PDLLA/sc-PLA blends. From Table 1 and Fig. 3, it is clear that both Tc and Tp decrease with increasing cooling rate. For example, Tp of neat sc-PLA decreases about 13 ◦ C, when cooling rate increases from 10 to 40 ◦ C/min. A similar behavior was observed for the PDLLA/sc-PLA blends. The slower the cooling rate, the higher the temperature range at which the crystallization occurs. At lower
Fig. 3. Crystallization peak temperature Tp vs. cooling rate for neat sc-PLA and PDLLA/sc-PLA blends.
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Fig. 4. Variation of the relative crystallinity (Xt ) with time for nonisothermal crystallization of (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
where T0 is the initial crystallization temperature, T and T∞ are the crystallization temperature at time t and the ultimate crystallization temperatures, respectively. The dHc /dT is the heat flow rate. By using Eq. (2), the temperature can be related to crystallization time scale in nonisothermal crystallization. t=
T0 − Tt
(2)
where is the cooling rate. The conversion from temperature to time is performed using a constant cooling rate. Fig. 4 shows the plots of the relative degree of crystallinity (Xt ) as a function of time. The results show that increasing the cooling rate reduces the time for completing crystallization. From these curves, the half crystallization time (t0.5 ), which is defined as the half period from the onset of crystallization to the time at which Xt is 50%, can be derived. The t0.5 for neat sc-PLA and its blends is listed in Table 1. As expected, the crystallization half time decreases with increasing cooling rate, since t0.5 is a measure of the crystallization rate. Furthermore, it is apparent that the value of half-time of crystallization for PDLLA/sc-PLA blends at various cooling rates is higher than that of neat sc-PLA and increases with increasing the content of PDLLA and can be explained as follows: addition of PDLLA
Fig. 5. Plots of ln [−ln (1 − Xt )] vs. ln t for crystallization of (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
could act as dilution agents to restraint the overall crystallization process. Consequently, t0.5 for crystallization tends to increase with increasing PDLLA content. In addition to t0.5 , other parameters, such as the kinetic rate coefficient, are commonly used to characterize the nonisothermal crystallization kinetics of polymers. The Avrami equation is frequently employed to analyze the nonisothermal crystallization kinetics of polymers, according to which the relative degree of crystallinity Xt dependent crystallization time t can be expressed as [29,30] 1 − Xt = exp (−kt n )
(3)
where Xt , k, t, n are the relative crystallinity, the rate constant, crystallization time and Avrami exponent, respectively. The linear form of Eq. (3) can be expressed as follows: ln [− ln (1 − Xt )] = ln k + n ln (t)
(4)
Fig. 5 shows the plots of ln [−ln (1 − Xt )] vs. ln t for neat scPLA and PDLLA/sc-PLA blends at various cooling rates. From the figure, it can be seen that straight lines portion are obtained in
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the early stage of crystallization and is followed by a gentle deviation at longer time for each plot. It should be noted that when Eq. (4) is used to nonisothermal crystallization, the parameters k and n have different physical meaning because the temperature changes instantaneously during the nonisothermal crystallization. It affects both the rates of nucleation and growth process. Thus, in the present work the Avrami analysis is considered to be inapplicable and will not be discussed below. Considering the nonisothermal process, k is the factor that should be considered. Based on the mathematical derivation of Evans, Ozawa modified the Avrami equation by incorporating the cooling rate factor and is given by [31]: 1 − X(t) = exp
−K(T ) m
(5)
where K(T) is Ozawa crystallization rate constant, and m is the Ozawa exponent. The double logarithmic form of Eq. (5) is: ln [− ln (1 − X(T ))] = ln K(T ) − m ln ()
(6)
Plots based on Eq. (6) for the nonisothermal crystallization data of neat sc-PLA and PDLLA/sc-PLA blends at given temperatures are presented in Fig. 6. If the Ozawa method is valid, the plot should result in a straight line. In our study, the Ozawa plots of neat scPLA and PDLLA/sc-PLA blends show deviation from linearity when cooling rate varies from 10 to 40 ◦ C/min, indicating that the Ozawa equation is not appropriate to describe the nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends. The reason for this is that Ozawa in his approach ignored secondary crystallization and dependence of the fold length on temperature [32]. From the above analysis, it is clearly that the Avrami and Ozawa methods do not describe the nonisothermal crystallization behavior of the neat sc-PLA and PDLLA/sc-PLA blends very satisfactorily. A modified method was proposed by Liu et al. [33] to study the nonisothermal crystallization behavior by combining the Avrami equation and Ozawa equation. The relation between the cooling rate and the crystallization time t (or temperature T) was defined for a given degree of crystallinity. Thus: ln = ln F(T ) − ˛ ln t
(7) F(T) = [K(T)/k]1/m
where the parameter refers to the cooling rate that needs to reach a defined degree of crystallinity at unit crystallization time and ˛ is the ratio between the Avrami exponent n and the Ozawa exponent m (n/m). Thus, at a given degree of crystallinity, from the plot of ln vs. ln t, the values of ˛ and F(T) can be obtained by fitting linear slopes and determining intercepts of the lines, respectively. The plots of ln vs. ln t for neat sc-PLA and PDLLA/sc-PLA blends are given in Fig. 7 and the values of F(T) and ˛ are listed in Table 2. It can be seen from Table 2 that the value of ˛ for neat sc-PLA varies from 1.40 to 1.55 and for PDLLA/sc-PLA blends varies from 1.31 to 1.62. The variation in the values of ˛ for both neat sc-PLA and PDLLA/sc-PLA blends is small, indicating that the combined Avrami–Ozawa equation is successful in describing the nonisothermal process of neat sc-PLA and PDLLA/sc-PLA blends. The similar results were also reported in polypropylene/clay nanocomposithes [34], polypropylene/layered double hydroxide nanocomposithes [35], poly(ethylene 2,6-naphthalate)/silica nanocomposites [36], and poly(ethylene 2,6-naphthalate)/multiwalled carbon nanotube nanocomposites [37]. The value of F(T) systematically increases with an increase in the relative crystallinity for neat sc-PLA and PDLLA/sc-PLA blends samples, and the F(T) values of neat sc-PLA are generally smaller than those obtained for PDLLA/sc-PLA blends. Here, F(T) mainly reflects the crystallization facilitation effect of PDLLA on sc-PLA matrix. The value of F(T) increases with increasing the content of PDLLA, indicating that the PDLLA/sc-PLA blends can achieve the same degree of
Fig. 6. Ozawa plots of ln [−ln (1 − Xt )] vs. ln for crystallization of (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
Table 2 Nonisothermal crystallization kinetic parameters for neat sc-PLA and PDLLA/scPLA blends at different relative degrees of crystallinity by combination of modified Avrami–Ozawa equation. Sample
Xt (%)
˛
F(T)
Neat sc-PLA
20 40 60 80
1.40 1.43 1.47 1.55
9.76 11.84 13.79 16.70
PDLLA/sc-PLA30/70
20 40 60 80
1.41 1.47 1.50 1.59
9.90 12.41 14.95 18.73
PDLLA/sc-PLA50/50
20 40 60 80
1.39 1.47 1.51 1.62
10.21 13.52 16.60 22.01
PDLLA/sc-PLA70/30
20 40 60 80
1.31 1.37 1.40 1.42
11.61 16.14 20.96 28.01
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Fig. 8. Dependence of the effective energy barrier on the extent of relative crystallinity.
plotting the left hand side of Eq. (8) with respect to 1/Tx a straight line must be obtained. Plots of ln (dX/dt) vs. 1/Tx at different relative crystallinity are obtained as a straight lines, permitting the calculation of the effective energy barrier at different degrees of crystallinity. The dependence of the effective activation energy on conversion based on Friedman equation is shown in Fig. 8. The activation energy increases with the increasing relative crystallinity. Higher activation energies are also obtained for blends ascribed to the dilution effects of PDLLA on the sc-PLA component, because an extra energy is required for demixing the polymer components in the crystallization of the miscible blends and thus a higher energy is needed for the sc-PLA component to transport from the miscible melt to the growth front of crystal lamella as discussed above. 4. Conclusion
Fig. 7. Plots of ln vs. ln t for (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
crystallinity slower than neat sc-PLA. This implies slower kinetics of crystallization [38]. For nonisothermal crystallization processes, it is also interesting to evaluate effective activation energy. Several mathematical procedures have been proposed in Literature [39,40]. Among them Kissinger method was often used for evaluating effective activation energy of nonisothermal crystallization. However, Vyazovkin et al. demonstrated that this method provided invalid results when applied to the processes that occurred on melt crystallization [41]. To this concern, differential isoconversional method of Friedman and Vyazovkin et al. [42] found most appropriate. The Friedman equation can be expressed as follows: ln
dX dt
x
= constant −
En RTx
(8)
where dX/dt is the instantaneous crystallization rate as a function of time at a given conversion X. According to this method, the Xt function obtained from the integration of the experimentally measured crystallization rates is initially differentiated with respect to time to obtain the instantaneous crystallization rate, dX/dt. Furthermore, by selecting appropriate degrees of crystallinity (i.e. from 10 to 90%) the values of dX/dt at a specific X are correlated to the corresponding crystallization temperature at this X, i.e. Tx . Then, by
The PDLLA/sc-PLA blend system shows a single composition dependent Tg in all composition, indicating miscibility between PDLLA and sc-PLA, which leads to that crystallization growth of sc-PLA in blends is hindered by the presence of PDLLA. The nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends was studied by DSC. The kinetic studies suggest that the addition of PDLLA to sc-PLA leads to an increase in the half-time for crystallization for the cooling rates investigated. This behavior is ascribed to the dilution effect of PDLLA to restraint the overall crystallization process. Kinetic models based on Avrami, Ozawa, and modified Avrami–Ozawa were used to analyze the nonisothermal crystallization behavior, the former two models were inapplicable to satisfactorily describe the nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends. However, the method proposed by modified Avrami–Ozawa successfully described the nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends. The activation energy for nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends determined using Kissinger method was estimated. The higher value of activation energy for crystallization obtained for PDLLA/sc-PLA blends confirmed the dilution effects of PDLLA on the sc-PLA component, because an extra energy is required for demixing the polymer components in the crystallization of the miscible blends and thus a higher energy is needed for the sc-PLA component to transport from the miscible melt to the growth front of crystal lamella.
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Effects of molten poly(d,l-lactide) on nonisothermal crystallization in stereocomplex of poly(l-lactide) with poly(d-lactide) Yi Li a,b , Changyu Han a,∗ , Xin Zhang a , Qinglin Dong a , Lisong Dong a a b
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China College of Material Science and Engineering, Jilin Jianzhu University, Changchun 130118, China
a r t i c l e
i n f o
Article history: Received 23 August 2013 Received in revised form 27 September 2013 Accepted 27 September 2013 Available online 6 October 2013 Keywords: Poly(l-lactide) Poly(d-lactide) Stereocomplex Blend Nonisothermal crystallization
a b s t r a c t Poly(d,l-lactide) (PDLLA) and stereocomplex-poly (l- and d-lactide) (sc-PLA) blends were prepared by solution blending. The influence of the addition of PDLLA on nonisothermal crystallization behavior of scPLA was investigated. It was observed that the crystallization peak temperature of PDLLA/sc-PLA blends was marginally lower than that of neat sc-PLA at various cooling rates. Furthermore, the half-time for crystallization increased with an increase in PDLLA content, indicating the dilution effect of PDLLA on the sc-PLA component to restraint the overall crystallization process. The nonisothermal crystallization data was analyzed using Avrami, Ozawa and combined Ozawa–Avrami models methods. The validity of kinetic models on the nonisothermal crystallization process of neat sc-PLA and PDLLA/sc-PLA blends was discussed. The approach developed by combined Ozawa–Avrami models successfully described the nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends. The activation energy for nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends based on Friedman equation was evaluated. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Poly(lactic acid) (PLA) has been paid considerable attention from both fundamental and practical perspectives because it is derived completely from renewable resources, and hence production is sustainable [1–3], but also it can significantly contribute to the control of green-house gas (CO2 ) emission as a result of carbon capture during plant growth and the eventual complete biodegradability of the PLA matrix. The attributes of biodegradability, biocompatibility, good mechanical properties, and versatile fabrication processes make it one of the most promising materials for biomedical applications such as implant materials and controlled drug delivery systems, as well as for the conventional applications where common thermoplastics are employed [4–6]. PLA has two stereoregular enantiomers, poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) due to the presence of a chiral carbon in the skeletal chain. The polymerization of meso-lactide or racemic lactide leads to the formation of atactic poly(d,l-lactide) (PDLLA). A special crystalline structure termed stereocomplex based on CH3 . . .C O interactions of stereoselective van der Waals forces can be formed by blending PLLA and PDLA, which is easily formed by the melt blending, solution casting, or supercritical fluid methods
∗ Corresponding author. Fax: +86 431 85262244. E-mail address: [email protected] (C. Han). 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.09.035
when mixing two polymers with identical chemical composition but different steric structures [7–9]. The pure PLLA or PDLA usually crystallizes in ␣-form with a 103 helical chain conformation, while the stereocomplex-PLA (sc-PLA) possesses a triclinic unit cell with a 31 helical chain conformation. This sc-PLA showed its Tm at about 230 ◦ C, which is about 50 ◦ C higher than that of pure PLLA or PDLA, so that sc-PLA should accordingly have better thermal and mechanical properties, and higher hydrolytic stability than neat PLLA or PDLA. Since the first report by Ikada et al., the influences of the homopolymer molecular weight, blending ratio (equimolar and non-equimolar), blending condition, and optical purity on the formation and properties of stereocomplexes have been well investigated [10–21]. However, for crystallization in bulk from the melt, the exclusive stereocomplex formation is limited to the PLLA and PDLA pair at least either of which has the molecular weight about 10,000, whereas solely homo-crystallites are formed in the equimolar PLLA/PDLA blends where both polymers have the molecular weights higher than 100,000 [9]. The crystallization kinetics of the melt-miscible blends of amorphous and crystalline polymers has been extensively investigated. When crystallization occurs below the melting point of the crystalline component, the process involves two types of polymer transport, namely, diffusion of the crystallizable component toward the crystal growth front and a simultaneous rejection of the amorphous component [22]. This crystallization process produces a liquid–solid phase separation, resulting into various
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morphological patterns closely controlled by the kinetics of the two types of polymer transport. In this case, the morphological formation may be kinetically governed by the thermal history and composition to achieve tailor-made properties for the blends. Therefore, investigation of the crystallization kinetics of polymer blends containing crystallizable components also has the practical importance [22,23]. Although the crystallization kinetics of neat sc-PLA has been extensively studied, however, studies have yet to be directed to investigation of influence of miscible dilutions on the crystallization kinetics of the sc-PLA. Therefore, PDLLA was selected in this work to study the effects of miscible blending on the nonisothermal crystallization behaviors of sc-PLA. The kinetics is analyzed using theoretical approach of Avrami, Ozawa, and combined Ozawa–Avrami models for nonisothermal crystallization. The activation energy describing the nonisothermal crystallization process is calculated based on Friedman method. On the other hand, the sc-PLA has higher strength, modulus and relatively slower degradation rate, making it an ideal material for load bearing devices [21]. In contrast, due to the amorphous nature, PDLLA shows weaker mechanical properties and faster degradation rate, making it a preferred candidate for developing low strength material. Consequently, blending sc-PLA and PDLLA can balance the mechanical properties and degradation rate and obtain the biomaterials with the controllable and improved performances.
2. Experimental PDLLA was purchased from Aldrich. It exhibited a weight–average molecular weight (Mw ) of 85,000, polydispersity of 1.64 (GPC analysis). PLLA was synthesized by the ring-opening polymerization of l-lactide using tin octanoate as a catalyst. It exhibited a weight–average molecular weight (Mw ) of 19,000, polydispersity of 1.83 (GPC analysis). PDLA was synthesized by the ring-opening polymerization of d-lactide using tin octanoate as a catalyst. It exhibited a weight–average molecular weight (Mw ) of 110,000, polydispersity of 1.92 (GPC analysis). Binary blends with PLLA and PDLA and ternary blends comprising PLLA, PDLA, and PDLLA were first separately prepared, then stereocomplexing sc-PLA and PDLLA mixtures were prepared by mixing the separately prepared solvent, followed with film-casting. PDLLA was first dissolved in chloroform (CHCl3 ) solution, and then further blended with solutions of PLLA and PDLA with proper ratios. Ratios of PLLA to PDLA in blends were maintained at 1/1 by weight in either binary or ternary blends. In ternary blends, PLLA/PDLA was fixed at 1/1, with the PDLLA contents ranging from 70/30, 50/50 and 30/70 in weight ratios, the first number referring to the weight percentage of PDLLA. The prepared solutions were mixed together with vigorous stirring, the solutions were cast onto petri dishes placed horizontally, and then the solvent was allowed to evaporate at room temperature for 12 h. The sample was further dried at 50 ◦ C under vacuum for 7 days to remove the solvent completely. The molecular weight parameters of neat PDLLA, PLLA and PDLA were measured using GPC with a Waters 410 GPC instrument equipped with two Waters Styragel columns (HT6E and HT3) and a differential refractometer detector. Measurements were performed at 25 ◦ C and at a flow rate of 1.0 mL min−1 using chloroform as eluent. The molecular weight was calibrated according to polystyrene standards (Polyscience Co.). Thermal analysis was carried out using a TA instruments differential scanning calorimetry (DSC) Q20 with a Universal Analysis 2000. All operations were performed under nitrogen purge, and the weight of the samples varied between 5 and 8 mg. For the glass transition temperature (Tg ) measurements, the PDLLA/sc-PLA
Fig. 1. DSC curves of heating scan at 10 ◦ C/min for neat PDLLA, sc-PLA, and PDLLA/scPLA blends.
blends were heated from 20 to 240 ◦ C at 10 ◦ C/min. For nonisothermal crystallization, the samples were initially melted at 240 ◦ C for 1 min in order to erase the previous thermal history and were performed at a cooling rate of 10, 20, 30, and 40 ◦ C/min, respectively. All measurements were carried out in nitrogen atmosphere. Samples were not used for second time in thermal analysis to avoid thermal degradation. 3. Results and discussion 3.1. Miscibility of the blends It is well known that Tg of a polymer blend is one of the most important criteria for the miscibility of components. So, it is necessary to establish the Tg value of PDLLA/sc-PLA blends. Fig. 1(a) and (b) shows the DSC heating curves of as-casing films of PDLLA/sc-PLA blends with various PDLLA contents. No melting peak is detected for neat PDLLA in the DSC measurements, confirming its amorphous nature. Moreover, a melting peak at high temperature ranging from 223 to 228 ◦ C can be observed for the neat sc-PLA and PDLLA/scPLA blends, indicating that the stereocomplex of PLLA with PDLA can be formed in the presence of PDLLA. The enthalpies of melt of the PDLLA/sc-PLA blends have been calculated from the enthalpy of melt normalized to the sc-PLA content, which are 70.8, 75.4, 70.0, and 73.2 J/g for neat sc-PLA, PDLLA/sc-PLA30/70, PDLLA/scPLA50/50, PDLLA/sc-PLA70/30 samples, respectively. It is clear that the enthalpy of the PDLLA/sc-PLA blends is very similar to that of neat sc-PLA, suggesting that the presence of PDLLA did not affect the degree of crystallinity of sc-PLA. As the stereocomplex forms in the PLLA/PDLA blends, the stereocomplex crystals will significantly hinder chains mobility, and increase the Tg . Therefore, the PDLLA and sc-PLA samples show the different Tg values and the miscibility of PDLLA and sc-PLA blends can be assessed by the change of Tg . The Tg values of neat PDLLA and sc-PLA are around 48 and 60 ◦ C, respectively, which are in well agreement with the values reported previously [21,24–26]. As seen in Fig. 1(b), all the blends with different compositions exhibit a single Tg that shifts to a higher temperature with an increase in the sc-PLA content. These results suggest the miscibility of the PDLLA and sc-PLA blends in the molten state. 3.2. Nonisothermal crystallization behavior The study of the nonisothermal crystallization behavior of polymers during nonisothermal processing is of great technical
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Table 1 Values of the heat evolved during crystallization (Hc ), the crystallization onset temperature (Tc ), the crystallization peak temperature (Tp ) and the half time of the crystallization (t0.5 ) for neat sc-PLA and PDLLA/sc-PLA blends. Cooling rate (◦ C/min)
Hc a
Tc (◦ C)
Tp (◦ C)
t0.5 (min)
Neat sc-PLA
10 20 30 40
65.4 61.8 59.0 66.0
217.1 213.0 211.0 208.6
204.6 198.7 194.4 191.0
1.21 0.71 0.55 0.46
PDLLA/sc-PLA30/70
10 20 30 40
65.2 69.9 69.1 72.8
213.2 207.8 207.1 202.7
200.5 193.0 189.7 184.3
1.24 0.75 0.60 0.48
PDLLA/sc-PLA50/50
10 20 30 40
72.4 63.0 66.1 62.9
207.8 202.9 201.7 197.9
195.2 185.7 183.9 179.0
1.26 0.88 0.61 0.50
PDLLA/sc-PLA70/30
10 20 30 40
63.5 68.3 59.5 69.6
200.9 193.0 192.9 189.0
185.7 174.6 173.2 167.9
1.52 1.00 0.68 0.56
Sample
a
Hc is corrected for the content of sc-PLA in the blends.
cooling rate, there is more time to activate nuclei, therefore, the crystallization can start at higher temperatures [28]. Secondly, the presence of PDLLA in sc-PLA leads to a significant decrease in Tp , and at a given cooling rate, Tp decreases with increasing PDLLA content. For example, at the cooling rate of 10 ◦ C/min, Tp for neat sc-PLA is 204.6 ◦ C, while for PDLLA/sc-PLA70/30 blend, it is 185.7 ◦ C. This is ascribed to the dilution effects of PDLLA on the sc-PLA component, because an extra energy is required for demixing the polymer components in the crystallization of the miscible blends and thus a higher energy is needed for the sc-PLA component to transport from the miscible melt to the growth front of crystal lamella [26]. 3.3. Nonisothermal crystallization kinetics The relative degree of crystallinity (Xt ), as a function of crystallization temperature (Tc ), is estimated according to Eq. (1):
T
Xt =
T
(dHc /dT ) dT
0 T∞
T0
(1)
(dHc /dT ) dT
Fig. 2. DSC thermograms of nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends at selected cooling rates of (a) neat sc-PLA and (b) PDLLA/scPLA 50/50.
importance, because most of the practical processing techniques are under nonisothermal conditions [27]. The effects of PDLLA on the crystallization behavior of sc-PLA were quantitatively analyzed through nonisothermal DSC experiments. Fig. 2 shows the crystallization thermograms of neat sc-PLA and PDLLA/sc-PLA blends at selected cooling rates between 10 and 40 ◦ C/min. From these plots, the crystallization temperature (Tc ) and crystallization peak temperature (Tp ) defined respectively, as the crystallization onset temperature and the crystalline peak, of sc-PLA and PDLLA/sc-PLA blends can be determined. Also, the heat evolved during crystallization (Hc ), which is normalized to the sc-PLA content, can be obtained; the results are summarized in Table 1. Table 1 and Fig. 3 shows the relationship between Tp and cooling rate for neat sc-PLA and PDLLA/sc-PLA blends. From Table 1 and Fig. 3, it is clear that both Tc and Tp decrease with increasing cooling rate. For example, Tp of neat sc-PLA decreases about 13 ◦ C, when cooling rate increases from 10 to 40 ◦ C/min. A similar behavior was observed for the PDLLA/sc-PLA blends. The slower the cooling rate, the higher the temperature range at which the crystallization occurs. At lower
Fig. 3. Crystallization peak temperature Tp vs. cooling rate for neat sc-PLA and PDLLA/sc-PLA blends.
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Fig. 4. Variation of the relative crystallinity (Xt ) with time for nonisothermal crystallization of (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
where T0 is the initial crystallization temperature, T and T∞ are the crystallization temperature at time t and the ultimate crystallization temperatures, respectively. The dHc /dT is the heat flow rate. By using Eq. (2), the temperature can be related to crystallization time scale in nonisothermal crystallization. t=
T0 − Tt
(2)
where is the cooling rate. The conversion from temperature to time is performed using a constant cooling rate. Fig. 4 shows the plots of the relative degree of crystallinity (Xt ) as a function of time. The results show that increasing the cooling rate reduces the time for completing crystallization. From these curves, the half crystallization time (t0.5 ), which is defined as the half period from the onset of crystallization to the time at which Xt is 50%, can be derived. The t0.5 for neat sc-PLA and its blends is listed in Table 1. As expected, the crystallization half time decreases with increasing cooling rate, since t0.5 is a measure of the crystallization rate. Furthermore, it is apparent that the value of half-time of crystallization for PDLLA/sc-PLA blends at various cooling rates is higher than that of neat sc-PLA and increases with increasing the content of PDLLA and can be explained as follows: addition of PDLLA
Fig. 5. Plots of ln [−ln (1 − Xt )] vs. ln t for crystallization of (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
could act as dilution agents to restraint the overall crystallization process. Consequently, t0.5 for crystallization tends to increase with increasing PDLLA content. In addition to t0.5 , other parameters, such as the kinetic rate coefficient, are commonly used to characterize the nonisothermal crystallization kinetics of polymers. The Avrami equation is frequently employed to analyze the nonisothermal crystallization kinetics of polymers, according to which the relative degree of crystallinity Xt dependent crystallization time t can be expressed as [29,30] 1 − Xt = exp (−kt n )
(3)
where Xt , k, t, n are the relative crystallinity, the rate constant, crystallization time and Avrami exponent, respectively. The linear form of Eq. (3) can be expressed as follows: ln [− ln (1 − Xt )] = ln k + n ln (t)
(4)
Fig. 5 shows the plots of ln [−ln (1 − Xt )] vs. ln t for neat scPLA and PDLLA/sc-PLA blends at various cooling rates. From the figure, it can be seen that straight lines portion are obtained in
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the early stage of crystallization and is followed by a gentle deviation at longer time for each plot. It should be noted that when Eq. (4) is used to nonisothermal crystallization, the parameters k and n have different physical meaning because the temperature changes instantaneously during the nonisothermal crystallization. It affects both the rates of nucleation and growth process. Thus, in the present work the Avrami analysis is considered to be inapplicable and will not be discussed below. Considering the nonisothermal process, k is the factor that should be considered. Based on the mathematical derivation of Evans, Ozawa modified the Avrami equation by incorporating the cooling rate factor and is given by [31]: 1 − X(t) = exp
−K(T ) m
(5)
where K(T) is Ozawa crystallization rate constant, and m is the Ozawa exponent. The double logarithmic form of Eq. (5) is: ln [− ln (1 − X(T ))] = ln K(T ) − m ln ()
(6)
Plots based on Eq. (6) for the nonisothermal crystallization data of neat sc-PLA and PDLLA/sc-PLA blends at given temperatures are presented in Fig. 6. If the Ozawa method is valid, the plot should result in a straight line. In our study, the Ozawa plots of neat scPLA and PDLLA/sc-PLA blends show deviation from linearity when cooling rate varies from 10 to 40 ◦ C/min, indicating that the Ozawa equation is not appropriate to describe the nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends. The reason for this is that Ozawa in his approach ignored secondary crystallization and dependence of the fold length on temperature [32]. From the above analysis, it is clearly that the Avrami and Ozawa methods do not describe the nonisothermal crystallization behavior of the neat sc-PLA and PDLLA/sc-PLA blends very satisfactorily. A modified method was proposed by Liu et al. [33] to study the nonisothermal crystallization behavior by combining the Avrami equation and Ozawa equation. The relation between the cooling rate and the crystallization time t (or temperature T) was defined for a given degree of crystallinity. Thus: ln = ln F(T ) − ˛ ln t
(7) F(T) = [K(T)/k]1/m
where the parameter refers to the cooling rate that needs to reach a defined degree of crystallinity at unit crystallization time and ˛ is the ratio between the Avrami exponent n and the Ozawa exponent m (n/m). Thus, at a given degree of crystallinity, from the plot of ln vs. ln t, the values of ˛ and F(T) can be obtained by fitting linear slopes and determining intercepts of the lines, respectively. The plots of ln vs. ln t for neat sc-PLA and PDLLA/sc-PLA blends are given in Fig. 7 and the values of F(T) and ˛ are listed in Table 2. It can be seen from Table 2 that the value of ˛ for neat sc-PLA varies from 1.40 to 1.55 and for PDLLA/sc-PLA blends varies from 1.31 to 1.62. The variation in the values of ˛ for both neat sc-PLA and PDLLA/sc-PLA blends is small, indicating that the combined Avrami–Ozawa equation is successful in describing the nonisothermal process of neat sc-PLA and PDLLA/sc-PLA blends. The similar results were also reported in polypropylene/clay nanocomposithes [34], polypropylene/layered double hydroxide nanocomposithes [35], poly(ethylene 2,6-naphthalate)/silica nanocomposites [36], and poly(ethylene 2,6-naphthalate)/multiwalled carbon nanotube nanocomposites [37]. The value of F(T) systematically increases with an increase in the relative crystallinity for neat sc-PLA and PDLLA/sc-PLA blends samples, and the F(T) values of neat sc-PLA are generally smaller than those obtained for PDLLA/sc-PLA blends. Here, F(T) mainly reflects the crystallization facilitation effect of PDLLA on sc-PLA matrix. The value of F(T) increases with increasing the content of PDLLA, indicating that the PDLLA/sc-PLA blends can achieve the same degree of
Fig. 6. Ozawa plots of ln [−ln (1 − Xt )] vs. ln for crystallization of (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
Table 2 Nonisothermal crystallization kinetic parameters for neat sc-PLA and PDLLA/scPLA blends at different relative degrees of crystallinity by combination of modified Avrami–Ozawa equation. Sample
Xt (%)
˛
F(T)
Neat sc-PLA
20 40 60 80
1.40 1.43 1.47 1.55
9.76 11.84 13.79 16.70
PDLLA/sc-PLA30/70
20 40 60 80
1.41 1.47 1.50 1.59
9.90 12.41 14.95 18.73
PDLLA/sc-PLA50/50
20 40 60 80
1.39 1.47 1.51 1.62
10.21 13.52 16.60 22.01
PDLLA/sc-PLA70/30
20 40 60 80
1.31 1.37 1.40 1.42
11.61 16.14 20.96 28.01
198
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Fig. 8. Dependence of the effective energy barrier on the extent of relative crystallinity.
plotting the left hand side of Eq. (8) with respect to 1/Tx a straight line must be obtained. Plots of ln (dX/dt) vs. 1/Tx at different relative crystallinity are obtained as a straight lines, permitting the calculation of the effective energy barrier at different degrees of crystallinity. The dependence of the effective activation energy on conversion based on Friedman equation is shown in Fig. 8. The activation energy increases with the increasing relative crystallinity. Higher activation energies are also obtained for blends ascribed to the dilution effects of PDLLA on the sc-PLA component, because an extra energy is required for demixing the polymer components in the crystallization of the miscible blends and thus a higher energy is needed for the sc-PLA component to transport from the miscible melt to the growth front of crystal lamella as discussed above. 4. Conclusion
Fig. 7. Plots of ln vs. ln t for (a) neat sc-PLA and (b) PDLLA/sc-PLA50/50.
crystallinity slower than neat sc-PLA. This implies slower kinetics of crystallization [38]. For nonisothermal crystallization processes, it is also interesting to evaluate effective activation energy. Several mathematical procedures have been proposed in Literature [39,40]. Among them Kissinger method was often used for evaluating effective activation energy of nonisothermal crystallization. However, Vyazovkin et al. demonstrated that this method provided invalid results when applied to the processes that occurred on melt crystallization [41]. To this concern, differential isoconversional method of Friedman and Vyazovkin et al. [42] found most appropriate. The Friedman equation can be expressed as follows: ln
dX dt
x
= constant −
En RTx
(8)
where dX/dt is the instantaneous crystallization rate as a function of time at a given conversion X. According to this method, the Xt function obtained from the integration of the experimentally measured crystallization rates is initially differentiated with respect to time to obtain the instantaneous crystallization rate, dX/dt. Furthermore, by selecting appropriate degrees of crystallinity (i.e. from 10 to 90%) the values of dX/dt at a specific X are correlated to the corresponding crystallization temperature at this X, i.e. Tx . Then, by
The PDLLA/sc-PLA blend system shows a single composition dependent Tg in all composition, indicating miscibility between PDLLA and sc-PLA, which leads to that crystallization growth of sc-PLA in blends is hindered by the presence of PDLLA. The nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends was studied by DSC. The kinetic studies suggest that the addition of PDLLA to sc-PLA leads to an increase in the half-time for crystallization for the cooling rates investigated. This behavior is ascribed to the dilution effect of PDLLA to restraint the overall crystallization process. Kinetic models based on Avrami, Ozawa, and modified Avrami–Ozawa were used to analyze the nonisothermal crystallization behavior, the former two models were inapplicable to satisfactorily describe the nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends. However, the method proposed by modified Avrami–Ozawa successfully described the nonisothermal crystallization behavior of neat sc-PLA and PDLLA/sc-PLA blends. The activation energy for nonisothermal crystallization of neat sc-PLA and PDLLA/sc-PLA blends determined using Kissinger method was estimated. The higher value of activation energy for crystallization obtained for PDLLA/sc-PLA blends confirmed the dilution effects of PDLLA on the sc-PLA component, because an extra energy is required for demixing the polymer components in the crystallization of the miscible blends and thus a higher energy is needed for the sc-PLA component to transport from the miscible melt to the growth front of crystal lamella.
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