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Nov 13, 2012 - (27) Fana, Y.; Nishidaa, H.; Shiraib, Y.; Tokiwac, Y.; Endo, T. Thermal degradation behaviour of poly(lactic acid) stereocomplex. Polym...
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Isothermal and Nonisothermal Cold Crystallization Behaviors of Asymmetric Poly(L‑lactide)/Poly(D‑lactide) Blends Yi Li† and Changyu Han‡,* †

College of Material Science and Engineering, Jilin Architectural and Civil Engineering Institute, ChangChun 130118, China Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

‡

S Supporting Information *

ABSTRACT: Isothermal and nonisothermal cold crystallization behaviors of asymmetric poly(L-lactide) (PLLA)/poly(Dlactide) (PDLA) blends at PDLA loadings of 0−20 wt % were investigated in this work. Formation of the stereocomplex in the blends was confirmed by differential scanning calorimetry and wide-angle X-ray diffraction. For both neat PLLA and its blends, the overall isothermal cold crystallization rates increase with increasing crystallization temperature; moreover, the overall isothermal cold crystallization rates of PLLA are faster in the blends than in neat PLLA, indicative of the nucleating agent effect of the stereocomplex formed in the blends. Crystallization mechanism and crystal structure of PLLA remain unchanged despite the PDLA loading. For the nonisothermal cold crystallization, the crystallization process of PLLA is accelerated by increasing both heating rate and the PDLA loading up to 10 wt %. The Ozawa equation failed to fit the crystallization process, while the Tobin equation could describe it well in the relative degree of crystallization range of 0−75%.

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INTRODUCTION Poly(L-lactide) (PLLA) has attracted much attention because it is biomass-derived, biodegradable, biocompatible, and nontoxic to the environment and human body. Recent innovation on the production process has lowered significantly the production cost, which further stimulates the studies on its property and potential applications.1−4 However, PLLA exhibits a rather slow crystallization rate and low crystallinity, which greatly limit its practical applications.5,6 Therefore, great efforts have been made to improve crystallization properties of PLLA.7−11 Among these efforts, stereocomplexation between PLLA and poly (D-lactide) (PDLA) is one of the most effective and promising methods for increasing the crystallization properties of poly(lactic acid) (PLA)-based materials.12−27 Ikada et al. first reported that the 1/1 blend of PLLA and PDLA produced a stereocomplex whose crystal structure was different from that of PLLA.13 This stereocomplex type poly(lactic acid) (sc-PLA) showed its Tm at 230 °C, which is 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 PLLA.12 Since this first report, the influences of the homopolymer molecular weight, blending ratio, blending condition, and optical purity on the formation and properties of stereocomplexes have been well investigated.13−28 It is well-established that thermal history plays an important role in affecting the crystalline structure and morphology of semicrystalline polymers. A semicrystalline polymer can crystallize not only when cooled from the melt but also when heated from the amorphous state.29,30 The former is so-called “melt crystallization”, and the latter one, is “cold crystallization”. The physical and mechanical properties of semicrystalline polymers, such as PLLA, are largely dependent on their solidstate morphology and level of crystallinity. Owing to a slow © 2012 American Chemical Society

crystallization rate, PLLA is generally amorphous after conventional extrusion or injection molding. Therefore, to further improve the physical and mechanical properties of PLLA such as high-temperature dimensional stability, modulus, tensile strength, and barriers properties, the annealing process above the glass transition temperature (Tg) (cold crystallization process) is very necessary to develop a certain level of crystallinity, which plays an important role in determining the physical and mechanical properties of PLLA in applications. A fundamental understanding of the cold crystallization kinetics is important because control of the crystallization factors allows for the design of materials with desirable properties. Most of the previous works on the crystallization behaviors of asymmetric PLLA/PDLA blends were on the melt crystallization.15,27 However, to the best of our knowledge, the cold crystallization behaviors of asymmetric PLLA/PDLA blends have not been explored in details. Therefore, the aim of the present work is to report the isothermal and nonisothermal cold crystallization kinetics of asymmetric PLLA/PDLA blends investigated by differential scanning calorimetry (DSC) and analyzed using the Avrami, Ozawa, and Tobin models. It is expected that the research reported herein is of great help for a better understanding of the crystallization behaviors of PLLA and for future industrial applications.

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EXPERIMENTAL SECTION PLLA (4032D) was a commercial product of Natureworks Co. Ltd., USA. It exhibited a weight-average molecular weight (Mw) of 207 000, polydispersity of 1.73 (GPC analysis).The D-isomer Received: Revised: Accepted: Published: 15927

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content of PLLA is approximately 2.0% (as per the manufacture). 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). PLLA and PDLA were dissolved in chloroform separately and were solution-blended at different PLLA/PDLA ratios, coded as PLLAx, where x represents the weight percentage of PDLA. 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 60 °C under vacuum for 7 days to remove the solvent completely. Then the samples were melted at 190 °C for 3 min, followed by quenching in liquid nitrogen. Stereocomplex crystallites that formed in the blends stayed unmelted at 190 °C and embedded in the PLLA matrix. 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 isothermal cold crystallization, the sample was heated to the chosen crystallization temperature at 100 °C/min and held for a period of time until the isothermal crystallization was complete. The crystallization temperatures chosen in this work were from 85 to 100 °C. The evolution of heat flow with crystallization time was recorded during the isothermal crystallization process for the later data analysis. For nonisothermal cold crystallization, the sample was heated to 190 °C at various heating rates, such as 2.5, 5, 7.5, and 10 °C/min. Wide angle X-ray diffraction (WAXD) experiments were performed on a D8 advance X-ray diffractometer (Bruker, Germany) at room temperature in the range of 5−40° with a scanning rate of 4°/min. The Cu Kα radiation (λ = 0.15418 nm) source was operated at 40 kV and 200 mA. The samples were first hot-pressed into films with a thickness of around 0.4 mm at 190 °C, quenched into liquid nitrogen to retain the stereocomplex crystallites, and then transferred into a vacuum oven at 90 °C for 24 h.

Figure 1. (a) DSC traces obtained on the quenched samples containing various content of PDLA at a heating rate of 10 °C/min; (b) WAXD profiles of the quenched samples containing various content of PDLA.

RESULTS AND DISCUSSION Stereocomplex Formation. Formation of the stereocomplex in the blends was confirmed by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). DSC traces obtained on the quenched samples containing various content of PDLA at a heating rate of 10 °C/min are given in Figure 1a. In all the samples, the cold crystallization and melting peak from the PLLA homopolymer is present around 100 and 168 °C, respectively, while the stereocomplex melting peak is present around 218 °C as reported in the literature.25 The area of the melting endotherm for the stereocomplex increased as the amount of PDLA in the blend increased showing that the initial composition of the blend can be used to control the amount of stereocomplex in the final material. Formation of the stereocomplex in the blends was further confirmed by WAXD. Figure 1b shows the WAXD profiles of the quenched samples containing various content of PDLA. The most intense peaks of the blended samples are observed at 2θ values of 12, 21, and 24°. These peaks are for the stereocomplex crystallized in a triclinic unit cell of dimensions: a = 0.916 nm, b = 0.916 nm, c = 0.870 nm, α = 109.2°, β = 109.2°, and γ = 109.8°, in which L-lactide and D-

lactide segments are packed in parallel taking a 31 helical conformation.12 Isothermal Cold Crystallization Kinetics of Neat PLLA and Its Blends. To investigate the effect of the incorporation of PDLA and their contents on the crystallization of PLLA in the blends from the amorphous state of PLLA, the overall isothermal cold crystallization kinetics of neat PLLA and its blends was studied with DSC first in a temperature range from 85 to 100 °C. Figure 2a,b shows the plots of relative degree of crystallinity versus crystallization time for neat PLLA and PLLA10. The plots of relative degree of crystallinity versus crystallization time for other samples are shown in Supporting Information, Figure 1S. It can be seen that all these curves have the similar sigmoid shape, and the corresponding crystallization time for all the samples becomes shorter with an increase in the crystallization temperature (Tc). The corresponding crystallization time for the PLLA/PDLA blends becomes shorter with increasing the PDLA content at the same Tc. For instance, it took neat PLLA about 16 min to finish crystallization at 85 °C, but for the PLLA5, PLLA10, and PLLA20 samples, the time required to finish crystallization became around 9.5, 8.5, and 9 min, respectively. No significant change of the corresponding

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Figure 2. Variation of relative degree of crystallinity with crystallization time at different Tcs for (a) neat PLLA, (b) PLLA10, and the related Avrami plots for (c) neat PLLA, (d) PLLA10.

Wc value. A very similar phenomenon was also observed in the melt crystallization process of asymmetric PLLA/PDLA blends.21 However, this tendency was opposite to the results obtained by Yamane et al.17 They reported that Wc values of the blended samples increased with PDLA content when the PDLA content was very low and tended to level off at higher PDLA content. The discrepancy between our results and those reported by Yamane et al.17 may be attributable to much lower PDLA content (1−5 wt %) and lower Mw of PDLA utilized (Mw of PDLA = 14 000) in their studies. The Avrami equation is frequently employed to analyze the isothermal crystallization kinetics of polymers, according to which the relative degree of crystallinity Xt dependent crystallization time t can be expressed as33,34

crystallization time for the PLLA/PDLA blends with PDLA content was observed. In addition, the values of the normalized crystallization enthalpy (ΔHc) and the degree of crystallinity (Wc) are listed in Supporting Information, Table 1S. The values of Wc were calculated using the following equation according to previous work.26 Wc = 100

ΔHc1 f ΔHm°

(1)

where ΔHc1 is the measured heat of fusion, f is the weight fraction of the PLLA in question, f = 1 − (2 × PDLA concentration (wt%)/100), and ΔHmo is the enthalpy of fusion for a crystal having infinite crystal thickness (93 J/g).9 It is found that the Wc values of the blended samples are decreased with the increase of PDLA. The Wc value of neat PLLA was about 36%, but for the PLLA20 sample, the Wc value became only around 12%. Similar results were also found by Schmidt et al.21 and Brochu et al.28 As the stereocomplex forms in the asymmetric PLLA/PDLA blends investigated, PLLA homopolymer chains can possibly become tethered to or even come between stereocomplex crystallites, thus reducing chain mobility. When large amounts of PDLA are present (20 wt %), more tethering sites are formed, and PLLA homopolymer chains may be tethered at multiple junctures.21 This will significantly hinder chain mobility,31,32 resulting in a reduced

1 − X t = exp( −kt n)

(2)

where Xt is the relative degree of crystallinity, n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is the overall rate constant associated with both nucleation and growth contributions. The linear form of eq 2 can be expressed as follows: Log[− ln(1 − X t)] = log k + n log t

(3)

The Avrami parameters n and k can be obtained from the slopes and the intercepts, respectively. 15929

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Figure 3. DSC heating traces from the amorphous state at various heating rates for (a) neat PLLA, (b) PLLA10, and plots of relative degree of crystallinity versus crystallization temperature from the amorphous state at various heating rates for (c) neat PLLA, (d) PLLA10.

In the case of the DSC experiment, Xt at t is defined as the ratio of the area under the exothermic curve between the onset crystallization time and t to the whole area under the exothermic curve from the onset crystallization time to the end crystallization time. Figure 2c,d shows the Avrami plots of neat PLLA and PLLA1, from which the Avrami parameters n and k can be obtained from the slopes and the interceptions, respectively. The Avrami parameters are summarized in Supporting Information, Table 1S for neat PLLA and its blends crystallized at different Tc values. The values for n depend on the crystallization mechanism, and n usually is in the range of an integer number between 1 and 4. It was found for various polymers that the n value adopts fractional numbers due to secondary crystallization. The values of n were found between 1.15 and 1.85, depending on the temperature and PDLA content. This result also shows that PDLA content has no significant effect on n values. The lower n value indicates that crystal growth may proceed in one dimension. The smaller n value may be due to a faster crystallization mechanism that does not provide enough time to grow in three dimensions.35 The value of n observed in this study is similar with the values reported by Vasanthan et al. for cold crystallization of poly(Llactic acid) and poly(L-lactic acid)/clay nanocomposites.35 In previous work, the isothermal melt crystallization kinetics of

neat PLLA and the asymmetric PLLA/PDLA blends has been investigated. The values of n are found to be around 3, suggesting that the growing of crystallites is three-dimensional and athermal.14 The crystallization mechanism for cold crystallization and melt crystallization are different due to the different crystallization processes. The values of k are also listed in Supporting Information, Table 1S. The k values are increased with increasing Tc from 85 to 100 °C during isothermal cold crystallization, indicative of a diffusion controlled crystallization process. However, it should be noted that it is difficult to compare the overall crystallization rate directly from the k values because the unit of k is min−n and n is not constant. Thus, the crystallization half-time (t0.5), the time required to achieve 50% of the final crystallinity of the samples, is introduced for comparing the overall crystallization rates. The value of t0.5 is calculated by the following equation:

t0.5 =

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠

(4)

The crystallization rate can thus be easily described by the reciprocal of t0.5. The variations of 1/t0.5 with Tc for neat PLLA and its blends are listed in Supporting Information, Table 1S, from which the effects of Tc and the PDLA content on the variation of overall crystallization rate can be obtained clearly. As shown in Table 1S, the 1/t0.5 values increase with increasing 15930

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heating rate. The Tp values of neat PLLA and its blends at various heating rates are all listed in Supporting Information, Table 2S. It is clear from Table 2S that Tp shifts toward higher temperatures with an increase in heating rate for all the samples. Moreover, in the blends, Tp values decrease apparently with an increase in the PDLA contents relative to neat PLLA at all heating rates. Such results indicate that the incorporation of stereocomplex crystals enhances the nonisothermal cold crystallization of PLLA matrix significantly, and the degree of enhancement in Tp is strongly dependent on the PDLA contents. It is obvious from the aforementioned results that the nonisothermal cold crystallization of asymmetric PLLA/PDLA blends is influenced by both the presence of PDLA and heating rate. Figure 3 panels c and d show the plots of relative degree of crystallinity versus crystallization temperature at various heating rates for neat PLLA and PLLA10. It can be seen from Figure 3c,d that the plots shift to a higher temperature range with increasing heating rate. Figure 4 shows plots of relative

Tc for both neat PLLA and its blends, indicating that the overall isothermal crystallization rate increases with increasing Tc. Such results are expected since it is much easier for the chain movement of PLLA at higher Tc, thereby leading to the increase of the overall crystallization rate. The 1/t0.5 values are larger in the blends than in neat PLLA at a given Tc, indicating that the stereocomplex crystals formed may play a significant role as nucleating agent during the isothermal cold crystallization process of PLLA in the PLLA/PDLA blends. It should be noted that the 1/t0.5 value first increases and then decreases with increasing the PDLA loading at a given Tc and exhibits the maximum value in the PLLA10 sample. For example, at a given Tc of 85 °C, 1/t0.5 is found to be around 0.19 min−1 for neat PLLA. In the case of the PLLA5 and PLLA10 samples, 1/t0.5 values shift to around 0.35 min−1 and 0.45 min−1, respectively; however, 1/t0.5 is around 0.34 min−1 for the PLLA20 sample. For all the other Tc used, 1/t0.5 of PLLA also increases with an increase in the PDLA content from 5 and 10 wt %; however, 1/ t0.5 then decreases with a further increase the PDLA content up to 20 wt %. The reason why 1/t0.5 shows such PDLA loading dependence will be discussed in the following section. In brief, the aforementioned results indicate that the isothermal melt crystallization of PLLA is enhanced by the presence of the stereocomplex in the blends, and the degree of enhancement in 1/t0.5 is affected apparently by the stereocomplex contents. It is also essential to study the effect of PDLA on the crystal structure of PLLA in the blends. As introduced in the Experimental Section, all the samples for the WAXD experiments were crystallized at 90 °C. For neat PLLA, two sharp characteristic diffraction peaks are shown at 16o and 19°, corresponding to (200)/(110) and (203) planes, respectively.11 Moreover, the peak at 2θ = 24°, a characteristic diffraction peak of α′-form, also appears in the WAXD patterns, indicating that the α′-form forms during the isothermal cold crystallization at 90 °C.36 For the PLLA/PDLA blends, the similar diffraction patterns are also observed in Supporting Information, Figure 3S, which indicates PDLA does not alter the crystal structure of PLLA in the PLLA/PDLA blends. In short, the crystal structure of PLLA remains unchanged despite the addition of PDLA in the blends. Nonisothermal Cold Crystallization Kinetics of Neat PLLA and Its Blends. The nonisothermal crystallization of neat PLLA and its blends from the amorphous state was further studied. As described in the Experimental Section, the samples were first quenched from the melt to liquid nitrogen to reach the amorphous state of PLLA and then heated to 190 °C at various heating rates. Both heating rate and the PDLA loading are the two main factors that affect the nonisothermal cold crystallization behavior of PLLA in the blends. The effect of heating rate and the PDLA loading on the nonisothermal cold crystallization behavior were studied. Figure 3a,b shows the crystallization exotherms of the samples at various heating rates. The crystallization exotherms of other samples are shown in Supporting Information, Figure 4S. With increasing heating rate, the crystallization exotherms become broader, and the cold crystallization peak temperature (Tp) shifts to higher temperature range. Moreover, at a given heating rate (for example, 5 °C/min), Tp of neat PLLA is around 105.5 °C, whereas Tp of PLLA5, PLLA10, and PLLA20 are around 93.5, 92.9, and 89.8 °C, respectively, shifting to lower temperature range with increasing the PDLA content in the blends. In addition, with an increase in the PDLA content up to 10 wt %, the crystallization exotherms become narrower at a given

Figure 4. Plots of relative crystallinity versus crystallization temperature for various blends at a heating rate of 2.5 °C/min.

crystallinity versus crystallization temperature for various blends at a heating rate of 2.5 °C/min, it can be easily found that at a given heating rate, the plots shift to lower temperature range with increasing the content of PDLA, which confirms that the presence of PDLA enhances the cold crystallization of PLLA. During the nonisothermal cold crystallization process, the relationship between crystallization time t and the corresponding temperature T can be represented as follows: t=

T − T0 φ

(5)

where T is the temperature at crystallization time t, T0 is the onset temperature of crystallization, and φ is the heating rate. The half-time of crystallization (t0.5) is the time required to achieve 50% of the final crystallinity of the samples. All the values of t0.5 for neat PLLA and its blends at different heating rates are summarized in Supporting Information, Table 2S. The variations of 1/t0.5 with φ for neat PLLA and its blends are also listed in Table 2S, from which the effects of φ and the PDLA content on the variation of overall crystallization rate can be obtained clearly. It should be noted that the 1/t0.5 value first increases and then decreases with increasing the PDLA loading 15931

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and exhibits the maximum value in the PLLA10 sample. It is clear that the variation of 1/t0.5 with the PDLA loading during the nonisothermal cold crystallization is similar to that of the isothermal cold crystallization shown in Supporting Information, Table 1S. Therefore, it is necessary to discuss the effect of the presence of PDLA and their loading on the crystallization behavior of PLLA in the blends. The overall crystallization process of polymers generally involves both nucleation and growth. The stereocomplex crystals formed in the blends play two different or competing roles in affecting the crystallization process of PLLA. On the one hand, stereocomplex crystals serving as a nucleating agent accelerate the isothermal crystallization of PLLA as discussed earlier. On the other hand, polymer chains are highly entangled in the melt state, and during the crystallization process, the polymer chains must overcome certain energy barriers to diffuse and attach onto the growing front of a crystal. The presence of stereocomplex crystals may act as a physical cross-linking point to restrict the movement of chain segments and hinder the crystal growth process by imposing the constrains upon the surrounding polymer chains especially when they have good interactions with polymer chains. At PDLA contents less than 10 wt %, the crystallization of PLLA is enhanced with increasing the PDLA loading because the nucleation effect induced by the stereocomplex crystals is predominated. With further increasing the PDLA content up to 20 wt %, although stereocomplex crystals may provide more nucleation sites, the presence of more stereocomplex crystals must impose a much more significant confinement effect on the crystal growth of PLLA. It is believed that this confinement effect overweighs the nucleation effect, thereby slowing down the overall crystallization rate. A similar confinement effect on melt crystallization in asymmetric PLLA/ PDLA blends was also observed in previous work by Hillmyer et al.21 It is of interest to evaluate the effect of PDLA on the crystallization rate of PLLA in the blends quantitatively. A crystallization rate parameter (CRP), corresponding to the crystallization rate of polymers, was proposed by Zhang et al.37,38 The CRP can be determined by the slope of a linear plot of 1/t0.5 versus heating rate, and a higher slope means a faster crystallization rate. Figure 5a shows the plots of 1/t0.5 versus heating rate for all the samples. The values of CRP are determined to be 0.026, 0.066, 0.067, and 0.065 for neat PLLA, PLLA5, PLLA10, and PLLA20, respectively. The higher values of CRP in the blends indicate that PDLA is efficient in enhancing the cold crystallization of the PLLA matrix; moreover, the values of CRP are also affected by the contents of PDLA, which indicates the cold crystallization rate is also increased by increasing the PDLA loading up to 10 wt %. Khanna developed a method to compare the crystallization rate of different polymer systems by means of a crystallization rate coefficient (CRC), representing a change in cooling rate required to bring about 1 °C change in the supercooling of the polymer melt.39 According to the method developed by Khanna, the CRC parameter can be applied to rank the polymer on a single scale of crystallization rates. The value of CRC could be determined from the slope of the linear plot of cooling rate versus Tm − Tp, where Tm and Tp are the melting point temperature and nonisothermal melt crystallization peak temperature, respectively. In the current work, crystallization processes of neat PLLA and its blends were investigated from the amorphous state of PLLA; therefore, an approach modified by Qiu et al. was used to determine CRC by using Tp − Tg

Figure 5. Effect of the PDLA loading on the crystallization rate of PLLA: (a) crystallization rate parameter and (b) crystallization rate coefficient.

instead of Tm − Tp, where Tp and Tg are nonisothermal cold crystallization peak temperature and glass transition temperature, representing a change in heating rate required to bring about 1 °C change in the superheating of the polymer amorphous phase.11 Figure 5b shows the plots of heating rate against Tp − Tg for all the samples. The values of CRC are around 0.536 for neat PLLA, 0.719 for PLLA5, 0.698 for PLLA10, and 0.511 for PLLA20, respectively, also indicating that the nonisothermal cold crystallization of PLLA has been enhanced apparently by the presence of PDLA in the blends. The Ozawa theory has been widely used to analyze the nonisothermal crystallization kinetics of polymers.40 According to Ozawa theory, the relative degree of crystallization (XT) at a temperature T, can be calculated from the following equation: 1 − X T = exp[−k(T )/α m]

(6)

where α is the cooling rate, k(T) is cooling (or heating) crystallization function and m is the Ozawa exponent that depends on the dimension of crystal growth. The double logarithmic form of eq 6 can be written as ln[− ln(1 − X T)] = ln k(T ) − m ln α 15932

(7)

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Figure 6. The Ozawa plots of (a) neat PLLA, (b) PLLA10, and the Tobin plots of (c) neat PLLA, (d) PLLA10.

governed directly by different types of nucleation and growth mechanism. Equation 8 could be rewritten as follows to calculate the Tobin crystallization kinetics parameters

If the above equation validly describes the nonisothermal crystallization kinetics, the plots of ln[−ln(1 − XT)] against ln α should result in a straight line and kinetic parameters k(T) and m can be obtained from the intercept and the slope of the lines, respectively. The Ozawa plots of ln[−ln(1 − XT)] against ln α for neat PLLA and PLLA10 at a given temperature are shown in Figure 6a,b. As is evident from the figures, accurate analysis of the nonisothermal crystallization data cannot be performed since the curves in the plots deviate from linearity and an increase in curvature is observed. The nonisothermal crystallization processes of neat PLLA and its blends do not follow the Ozawa equation. The reason may be due to the disregarded assumptions of slow secondary crystallization and dependence of the fold length of polymer chain on temperature in the Ozawa equation.41 Tobin proposed an approach that involved phase-transformation kinetics with growth site impingement to describe nonisothermal crystallization kinetics of polymers. 42 In accordance with this method, the equation of phase transition is

Xt =

k t t nt 1 + k t t nt

log(X t /(1 − X t )) = log K t + nt log t

(9)

The Tobin parameters nt and kt could be obtained from the plots of log(Xt/(1 − Xt)) versus log t. However, in the higher Xt range (≥75%), the Tobin method generally has the lower value than the experimental data.11,43−45 The reasons might be due to the fact that the model as shown in eq 8 was the simplified form of a rather more complicated model described in the original publications, or perhaps due to the overemphasis of the impingement effect.11 Figure 6 panels c and d show the plots of log(Xt/(1 − Xt)) versus log t for neat PLLA and PLLA10. It is obvious that the Tobin approach could describe the nonisothermal cold crystallization for PLLA more properly than did the Ozawa model in the Xt range of 0−75%. Hence, the nt and kt values of all the samples are obtained in the Xt range of 0− 75%. The nt and kt values of all the samples at various heating rates are also listed in Supporting Information, Table 2S. The nt values are all around 3, which indicate that both the heating rates and the incorporation of PDLA may not change the nonisothermal cold crystallization mechanism of PLLA. Similar results were also found for poly(L-lactide) (PLLA)/octavinylpolyhedral oligomeric silsesquioxanes (ovi-POSS) nanocomposites and poly(p-dioxanone) (PPDO).11,44

(8)

where Xt, kt, and nt are the relative degree of crystallinity as a function of time, the Tobin crystallization rate constant, and the Tobin exponent, respectively. On the basis of this proposition, we do not need to integrate the Tobin exponent nt, since it is 15933

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CONCLUSIONS In this work, isothermal and nonisothermal crystallization kinetics of neat PLLA and its asymmetric blends with PDLA at loadings of 0−20 wt % from the amorphous state were investigated in detail. Isothermal cold crystallization kinetics of neat PLLA and its blends was studied with DSC at various crystallization temperatures and analyzed by the Avrami equation. The overall crystallization rates of neat PLLA and its blends increase with increasing crystallization temperature. At a given crystallization temperature, the overall crystallization rates are faster in the asymmetric PLLA/PDLA blends than in neat PLLA, and are strongly dependent on the content of PDLA; however, the crystallization mechanism and crystal structure of PLLA remain unchanged despite the addition of PDLA. Nonisothermal cold crystallization behaviors of neat PLLA and the asymmetric PLLA/PDLA blends were also investigated at different heating rates. Both heating rate and the PDLA loading are the two main factors that influence the nonisothermal cold crystallization behavior of PLLA in the asymmetric PLLA/PDLA blends. On the one hand, with increasing heating rate, the crystallization exotherm shifts to higher temperature range, and the crystallization process is enhanced for both neat PLLA and its blends. On the other hand, at a given heating rate, the addition of PDLA enhances the nonisothermal cold crystallization of PLLA significantly; furthermore, the 1/t0.5 value first increases and then decreases with an increase in the PDLA loading and exhibits the maximum value in the PLLA10 sample. In addition, the Ozawa equation did not fit the nonisothermal cold crystallization very well due to the secondary crystallization of PLLA; however, the Tobin method could be used to describe the crystallization process properly. From the Tobin equation, it can be concluded that the incorporation of PDLA may not change the nonisothermal cold crystallization mechanism of PLLA.

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ASSOCIATED CONTENT

* Supporting Information S

Figures showing DSC and WAXD patterns for all samples. Tables showing isothermal and nonisothermal crystallization kinetics parameters for all samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-431-85262244. Fax: +86431-85685653. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (50703042). Part of this work is supported by Jilin Province Science and Technology Agency (20116025).

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