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Isothermal and Nonisothermal Cold Crystallization Behaviors of Biodegradable Poly(L-lactide)/Octavinyl-Polyhedral Oligomeric Silsesquioxanes Nanocomposites Jing Yu and Zhaobin Qiu* State Key Laboratory of Chemical Resource Engineering, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Isothermal and nonisothermal cold crystallization behaviors of biodegradable poly(L-lactide) (PLLA)/octavinylpolyhedral oligomeric silsesquioxanes (ovi-POSS) nanocomposites at low ovi-POSS loadings were investigated in this work. For both neat PLLA and its nanocomposites, the overall isothermal cold crystallization rates increase with increasing crystallization temperature; moreover, the overall isothermal cold crystallization rates of PLLA increase with increasing the ovi-POSS loading in the nanocomposites relative to neat PLLA, indicative of the nucleating agent effect of ovi-POSS. Crystallization mechanism and crystal structure of PLLA remain unchanged despite the ovi-POSS loading. For the nonisothermal cold crystallization, the crystallization process of PLLA is accelerated by increasing both heating rate and the ovi-POSS loading. The Ozawa equation failed to fit the crystallization process, while the Tobin equation could describe it well. The activation energies of nonisothermal cold crystallization were calculated by the Kissinger method, which increase with increasing the ovi-POSS loading in the nanocomposites relative to neat PLLA.
’ INTRODUCTION Poly(L-lactic acid) (PLLA) has attracted considerable attention in the fields of biomedical application and environmental protection due to its biodegradability and biocompatibility.13 Viewed as one of the most ideal candidates to substitute petroleum-based polymers, PLLA has also been widely used in the general-purpose plastics fields because of its high strength and high modulus.4 PLLA is a semicrystalline polymer, which can crystallize in α, α0 , β, and γ forms depending on different crystallization conditions.59 However, PLLA has some disadvantages such as poor mechanical properties, slow crystallization rate, and slow degradation rate.10 These disadvantages seriously restrict the practical applications of PLLA. One of the most effective ways to modify and improve its properties is by adding nanofillers, such as carbon nanotubes (CNTs), layered metal phosphonate, hydroxyapatite, and graphite oxide into PLLA matrix to form nanocomposites.1114 The nanofillers can significantly enhance the properties of PLLA even at a very low content. Polyhedral oligomeric silsesquioxanes (POSS) are a kind of three-dimensional nanoparticles including a cube-like core surrounded by eight active or inactive organic corner groups.15 POSS have been widely incorporated into polymer matrix to make organicinorganic nanocomposites due to the higher performance compared with other inorganic nanofillers and the recent commercial availability of many useful precursors.16,17 Considering the advantages of low cost and easy process, the simple blending method becomes an ideal way to incorporate POSS into a majority of polymers, such as polypropylene (PP), polyethylene (PE), and polystyrene (PS).1821 It has been discovered that POSS can achieve a fine dispersion and make great improvement on the properties of the polymer matrix. The crystallization kinetics of both polymer and polymer composites are particularly important for the analysis and design r 2011 American Chemical Society
of processing operations. A semicrystalline polymer is able to crystallize not only when cooled from the melt but also when heated from the amorphous state.2224 The former is so-called “melt crystallization”, and the latter one is “cold crystallization”. In our previous works, PLLA/octavinyl-polyhedral oligomeric silsesquioxanes (ovi-POSS) nanocomposites were prepared via solution and casting method at low loadings of ovi-POSS.25 It is found that POSS have played a role as an efficient nucleating agent and enhanced the nonisothermal and isothermal melt crystallization of PLLA significantly, especially with the increase of POSS content to 1 wt %. In this work, effect of ovi-POSS on the isothermal and nonisothermal cold crystallization kinetics of PLLA in the nanocomposites was further investigated. It is expected that the research reported herein is of great help for a better understanding of the structure and properties relationship of biodegradable polymer nanocomposites and for the future industrial applications.
’ EXPERIMENTAL SECTION PLLA (Mw = 1.69 105 g/mol) was kindly provided by Biomer Company, Germany. Ovi-POSS were purchased from Shenyang Amwest Techology Company, China. The PLLA/ovi-POSS nanocomposites were prepared through a solution and casting method with chloroform as the mutual solvent. On the one hand, the appropriate amount of ovi-POSS was added into chloroform at a concentration of 1 mg/mL. Then, the mixture was sonicated with a KQ-700DE ultrasonic generator for 2 h to make a uniformly dispersed solution. On the other hand, Received: August 1, 2011 Accepted: October 13, 2011 Revised: September 16, 2011 Published: October 13, 2011 12579
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Figure 1. Variation of relative degree of crystallinity with crystallization time at different Tcs for (a) neat PLLA, (b) POSS-0.5, and (c) POSS-1.
PLLA was placed into chloroform and stirred for 2 h to dissolve PLLA completely. Next, the ovi-POSS solution was added to the PLLA solution and further stirred for 4 h. The PLLA/ovi-POSS solution was poured into a dish to evaporate the solvent at room temperature for 12 h. The sample was further dried at 80 °C under vacuum for 3 days to remove the solvent completely. Through the aforementioned procedure, PLLA was mixed with the addition of 0.5 and 1 wt % ovi-POSS, respectively. For brevity, they were abbreviated as POSS-0.5 and POSS-1 from now on. For comparison, neat PLLA was stirred for the same time as the nanocomposites. Thermal analysis was carried out using a TA Instruments differential scanning calorimetry (DSC) Q100 with a Universal Analysis 2000. All operations were performed under nitrogen purge, and the weight of the samples varied between 4 and 6 mg. For isothermal cold crystallization, the sample was heated at 40 °C/min to 190 °C, held for 3 min to erase any thermal history, and then cooled to 0 °C at 40 °C/min to reach the amorphous state. Subsequently, 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 105 °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 at 40 °C/min, held for 3 min to erase any previous thermal history, and cooled to 0 °C at 40 °C/min to reach the amorphous state. Subsequently, the sample was heated to 190 °C again at various heating rates, such as 2.5, 5, 7.5, 10, and 12.5 °C/min. Wide angle X-ray diffraction (WAXD) experiments were performed on a Rigaku D/Max 2500 VB2t/PC X-ray diffractometer at room temperature in the range of 545° with a scanning rate of 4 °/min. The CuKα radiation (λ = 0.15418 nm) source was operated at 40 kV and 200 mA. The samples were first pressed into films with a thickness of around 0.6 mm on a hot stage at 190 °C, quenched into ice water to reach the amorphous state, and then transferred into a vacuum oven at 90 °C for 3 days.
’ RESULTS AND DISCUSSION Isothermal Cold Crystallization Kinetics of Neat PLLA and Its Nanocomposites. To investigate the effect of the presence of
ovi-POSS and their contents on the crystallization of PLLA in the nanocomposites from the amorphous state, the overall isothermal cold crystallization kinetics of neat PLLA and its nanocomposites was studied with DSC first in a temperature range from 85 to 105 °C. Figure 1 shows the plots of relative degree of crystallinity against crystallization time for all the samples. It is obvious from Figure 1 that all these curves have the similar sigmoid shape, and the corresponding crystallization time for all the samples becomes shorter with increasing the crystallization temperature (Tc). The corresponding crystallization time for the PLLA/ovi-POSS nanocomposites becomes shorter with increasing the ovi-POSS loading at the same Tc. For instance, it took neat PLLA about 29 min to finish crystallization at 90 °C, but for the POSS-0.5 and POSS-1 samples, the time required to finish crystallization became only around 20 and 12 min, respectively. In addition, the values of the crystallization enthalpy (ΔHc) and the degree of crystallinity (Wc) are listed in Table 1. On the basis of the heat of fusion of 100% crystalline (ΔHmo) PLLA (93 J/g),26 the Wc values of neat PLLA and its nanocomposites are determined and normalized with respect to the composition of each component in the composites. It is found that the ΔHc and Wc values of all the samples are increased with the increase of Tc. With the incorporation of ovi-POSS, the ΔHc and Wc values are also increased slightly at the same Tc. The well-known Avrami equation is often used to analyze the isothermal crystallization kinetics of polymers;27,28 it assumes that the relative degree of crystallinity develops with crystallization time as 1 Xt ¼ expð kt n Þ
ð1Þ
where Xt is the relative degree of crystallinity at crystallization time (t), n is the Avrami exponent depending on the nature of nucleation and growth geometry of the crystals, and k is the crystallization rate constant involving both nucleation and 12580
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growth rate parameters. 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 2 shows the Avrami plots of neat PLLA and its nanocomposites, from which the Avrami parameters n and k can be obtained from the slopes and the interceptions, respectively. The Avrami parameters are summarized in Table 1 for neat PLLA and its nanocomposites crystallized at different Tcs. It can be found that the average values of n are around 2.1 and almost unchanged with the addition of ovi-POSS, suggesting that the Table 1. Summary of the isothermal Crystallization Kinetics Parameters of Neat PLLA and Its Nanocomposites at Different Tcs Based on the Avrami Equation samples
Tc (°C)
ΔHc (J/g)
Wc (%)
n
k (minn)
neat PLLA
85
15.5
18.4
2.33
3.90 104
90
23.0
21.6
2.29
2.04 103
95 100
28.2 29.4
28.1 29.7
2.19 2.36
6.87 103 1.21 102
105
28.7
31.3
2.19
2.45 102
85
15.6
24.5
2.03
3.97 103
90
24.7
25.9
2.16
6.95 103
95
27.5
26.3
2.07
2.93 102
100
30.2
30.8
2.12
3.62 102
105
33.2
32.3
2.05
6.72 102
85
20.7
24.7
2.09
1.34 102
90
29.4
23.6
2.11
3.76 102
95
35.2
26.2
1.91
1.40 101
100
35.4
29.0
1.95
2.80 101
105
39.1
30.2
2.13
9.32 101
POSS-0.5
POSS-1
incorporation of ovi-POSS may not change the crystallization mechanism of PLLA in the PLLA/ovi-POSS nanocomposites.29 The values of k are also listed in Table 1. 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 minn 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 crystallization rate can thus be easily represented by the reciprocal of t0.5. The value of t0.5 is calculated by the following equation: 1=n ln 2 ð2Þ t0:5 ¼ k Figure 3 illustrates the variations of 1/t0.5 with Tc for neat PLLA and its nanocomposites, from which the effects of Tc and the ovi-POSS content on the variation of overall crystallization rate can be obtained clearly. As shown in Figure 3, the 1/t0.5 values increase with increasing Tc for both neat PLLA and its nanocomposites, indicating that the overall isothermal crystallization rate increases with increasing Tc. Such results are reasonable since it is much easier for the chain movement of PLLA at higher Tc, thereby resulting in the increase of the overall crystallization rate. The 1/t0.5 values are larger in the nanocomposites than in neat PLLA at a given Tc, suggesting that ovi-POSS may play a significant role as nucleating agent during the isothermal cold crystallization of PLLA in the PLLA/ovi-POSS nanocomposites. In addition, the 1/t0.5 values increase with increasing the ovi-POSS loading in the PLLA/ovi-POSS nanocomposites at a given Tc, suggesting that the ovi-POSS loading has a significant effect on the crystallization of PLLA. In brief, the overall isothermal cold crystallization of PLLA is accelerated by the presence of ovi-POSS in the PLLA/ovi-POSS nanocomposites relative to neat PLLA; moreover, the enhancement of the overall crystallization rate of PLLA is influenced by the ovi-POSS loading. In our previous work, the isothermal melt crystallization kinetics and morphology of neat PLLA and the PLLA/ovi-POSS
Figure 2. The related Avrami plots for (a) neat PLLA, (b) POSS-0.5, and (c) POSS-1. 12581
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Industrial & Engineering Chemistry Research nanocomposites has been investigated. The values of n are found to be around 2.3, suggesting that the crystallization may correspond to a truncated sphere growth with athermal nucleation.25 In the present work, although the similar n values have also been found, the crystallization mechanism might be different due to the different crystallization processes; thus, further morphology investigations are necessary for the crystallization mechanism study in the near future. In addition, the k values are decreased with increasing Tc from 120 to 135 °C during isothermal melt crystallization, indicative of a nucleation controlled crystallization process; however, the k values are increased with increasing Tc from 85 to 105 °C during isothermal cold crystallization, indicative of a diffusion controlled crystallization process. It is also essential to study the effect of ovi-POSS on the crystal structure of PLLA in the PLLA/ovi-POSS nanocomposites.
Figure 3. Temperature dependences of 1/t0.5 for neat PLLA and its nanocomposites at various Tcs.
Figure 4. WAXD patterns of neat PLLA and its nanocomposites.
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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 16.25° and 18.57°, corresponding to (200)/(110) and (203) planes, respectively.30 Moreover, the peak at 2θ = 24.4°, a characteristic diffraction peak of α0 -form,9 also appears in the WAXD patterns, indicating α0 -form forms during the isothermal cold crystallization at 90 °C. For the PLLA/ovi-POSS nanocomposites, the similar diffraction patterns are also observed in Figure 4, which indicates ovi-POSS does not modify the crystal structure of PLLA in the PLLA/ovi-POSS nanocomposites. In short, the crystal structure of PLLA remains unchanged despite the addition of ovi-POSS in the PLLA/ovi-POSS nanocomposites. Nonisothermal Cold Crystallization Kinetics of Neat PLLA and Its Nanocomposites. The nonisothermal crystallization of neat PLLA and its nanocomposites from the amorphous state was further studied. As described in the Experimental Section, the samples were first quenched from the melt to 0 °C to reach the amorphous state and then heated to 190 °C at various heating rates. Both heating rate and the ovi-POSS loading are the two main factors that affect the nonisothermal cold crystallization behavior of PLLA in the PLLA/ovi-POSS nanocomposites. The effect of heating rate on the nonisothermal cold crystallization behavior was studied, using POSS-1 as an example, and the influence of the ovi-POSS loading was also studied, using 5 °C/min as a given heating rate. Figure 5a shows the crystallization exotherms of POSS-1 at various heating rates. With increasing heating rate, the crystallization exotherms become broader, and the cold crystallization peak temperature (Tp) shifts to higher temperature range. Similar results were also found in neat PLLA and POSS-0.5. For brevity, the results are not shown here. Figure 5b shows the crystallization exotherms of neat PLLA and its nanocomposites from the amorphous state at 5 °C/min. In Figure 5b, Tp of neat PLLA is around 119.8 °C, whereas Tps of POSS-0.5 and POSS-1 are around 111.6 and 100.6 °C, respectively, shifting to lower temperature range with increasing the ovi-POSS content in the PLLA/ovi-POSS nanocomposites. In addition, with increasing the ovi-POSS content, the crystallization exotherms become narrower. The Tp values of neat PLLA and its nanocomposites at various heating rates are all listed in Table 2. It is clear from Table 2 that Tp shifts toward higher temperatures with increasing heating rate for all the samples. In addition, in the nanocomposites, Tp decreases apparently with increasing the ovi-POSS contents relative to neat PLLA at all heating rates. Such results indicate the incorporation of ovi-POSS enhances the nonisothermal cold
Figure 5. DSC heating traces of (a) POSS-1 from the amorphous state at various heating rates, and (b) neat PLLA and its nanocomposites from the amorphous state at 5 °C/min. 12582
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crystallization of PLLA matrix significantly, and the degree of enhancement in Tp is strongly dependent on the ovi-POSS contents. It is clear from the aforementioned results that the nonisothermal cold crystallization of PLLA/ovi-POSS nanocomposite is affected by both the presence of ovi-POSS and heating rate. Figure 6a shows the plots of relative degree of crystallinity versus crystallization temperature at various heating rates for POSS-1. It can be seen from Figure 6a that the plots shift to higher temperature range with increasing heating rate. The same trends are observed for the other samples, which are not shown here for brevity. Figure 6b compares the plots of neat PLLA and its nanocomposites at 5 °C/min. It can be easily found that the plots shift to lower temperature range with increasing the content of ovi-POSS, which confirms that the presence of ovi-POSS enhances 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 ¼
T T0 Φ
ð3Þ
Table 2. Summary of Cold Crystallization Peak Temperature, Crystallization Half-Time, The Values of nt and kt Based on the Tobin Equation of Neat PLLA and Its Nanocomposites from the Amorphous State at Various Heating Rates samples
Φ (°C/min)
Tp (°C)
t1/2 (min)
nt
kt (min nt)
neat PLLA
2.5
105.2
5.65
4.66
4.32 104
5
119.8
5.44
4.42
9.24 104
7.5
129.2
4.52
4.66
1.55 103
10 12.5
134.7 137.8
3.45 2.82
4.63 4.32
5.90 103 2.19 103
2.5
102.0
5.42
4.57
5.43 104
5
111.6
3.70
4.68
2.89 103
7.5
118.8
3.17
4.51
8.37 103
10
121.1
2.55
4.68
1.85 102
12.5
125.8
1.97
4.18
9.24 102
2.5
94.9
4.18
4.43
1.51 103
5
100.6
2.35
4.41
2.19 103
7.5
105.5
2.08
4.91
2.71 103
10
108.0
1.65
4.80
9.23 102
12.5
111.3
1.57
4.90
1.48 101
POSS-0.5
POSS-1
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 (t1/2), the time required to achieve 50% of the final crystallinity of the samples. All the values of t1/2 for neat PLLA and its nanocomposites at different heating rates are summarized in Table 2. It can be found that the values of t1/2 decrease with increasing heating rate for both neat PLLA and its nanocomposites, indicating that the overall nonisothermal cold crystallization rate becomes faster with increasing heating rate. The t1/2 values in the nanocomposites are smaller than that of neat PLLA at a given heating rate, indicating again that the addition of ovi-POSS accelerates the crystallization process of PLLA; moreover, the t1/2 values are reduced with the increase of ovi-POSS content in the PLLA/ovi-POSS nanocomposites, It is of great interest to evaluate the effect of ovi-POSS on the crystallization rate of PLLA in the nanocomposite quantitatively. A crystallization rate parameter (CRP), corresponding to the crystallization rate of polymers, was proposed by Zhang et al.3134 The CRP can be determined by the slope of a linear plot of 1/t1/2 versus heating rate, and a higher slope means a faster the crystallization rate. Figure 7a shows the plots of 1/t1/2 versus heating rate for all the samples. The values of CRP are determined to be 0.0184, 0.0307, and 0.0390 for neat PLLA, POSS-0.5, and POSS-1, respectively. The higher values of CRP in the PLLA/ovi-POSS nanocomposites indicate that ovi-POSS are efficient in enhancing the cold crystallization of PLLA matrix; furthermore, the values of CRP are also influenced by the contents of ovi-POSS, which means the cold crystallization rate is also enhanced by the increase of ovi-POSS loading. Khanna suggested comparing 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.35 According to Khanna, the CRC parameter can be used as a guide for ranking the polymer on a single scale of crystallization rates. The CRC values should be higher for faster crystallization systems. 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 and nonisothermal melt crystallization peak temperature. In the present work, crystallization behaviors of neat PLLA and its nanocomposites were studied from the amorphous state; therefore, we modified the determination of CRC using Tp Tg 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.
Figure 6. Plots of relative degree of crystallinity versus crystallization temperature for (a) POSS-1 from the amorphous state at various heating rates and (b) neat PLLA and its nanocomposites from the amorphous state at 5 °C/min. 12583
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Figure 7. Effect of the ovi-POSS loading on the crystallization rate of PLLA; (a) crystallization rate parameter and (b) crystallization rate coefficient.
that m is not constant with temperature, and K(T) can not be determined due to the curvature present in the curves. It is apparent that the Ozawa model failed to describe the nonisothermal cold crystallization of PLLA, which may be attributed to the fact that the Ozawa theory neglects the strong secondary crystallization.39,40 Similar results were also found for the other composite systems, such as PLLA/multiwalled carbon nanotubes (MWCNTs) nanocomposites, PLLA/carbon black composites, and PLLA/silica nanocomposites.4143 Since the Ozawa model failed to describe the nonisothermal cold crystallization for PLLA, another theory of the phase transformation kinetics with growth site impingement, proposed by Tobin,37 was used in this work. According to this approach, the equation of phase transition is
Figure 8. The Ozawa plots of POSS-1.
Figure 7b shows the plots of heating rate against Tp Tg for all the three samples. The values of CRC are around 0.302 for neat PLLA, 0.433 for POSS-0.5, and 0.653 for POSS-1, respectively, also indicating that the nonisothermal cold crystallization of PLLA has been enhanced apparently by the presence of oviPOSS in the nanocomposites. To study the kinetic parameters of nonisothermal crystallization, several methods have been developed, and the majority of the proposed methods are based on the Avrami equation.23,36,37 The most popular theory is the one proposed by Ozawa,38 which can be used when crystallization occurs at a constant cooling (or heating) rate. According to the Ozawa model, the time variable in the Avrami equation was replaced by a cooling (or heating) rate and the relative degree of crystallinity was derived as a function of constant cooling (or heating) rate Φ as KðTÞ ð4Þ Xt ¼ 1 exp Φm where K(T) is the cooling (or heating) function at crystallization temperature T and m is the Ozawa exponent, which depends on the type of nucleation and growth mechanism. Double logarithms of eq 4 and rearrangement results in the following form: logð lnð1 Xt ÞÞ ¼ log KðTÞ mlog Φ
ð5Þ
If the Ozawa equation fits the crystallization very well, the plot of the log(ln(1 Xt)) versus log Φ would result in a series of parallel lines, and the kinetic parameters m and K(T) can be derived from the slopes and the intercepts, respectively. The Ozawa plots for POSS-1 are illustrated in Figure 8. As shown in Figure 8, although the plots show some linearity, some curvature is also apparent. The continuous change in the slope indicates
Xt ¼
k t t nt 1 þ kt t nt
ð6Þ
Where Xt is the relative degree of crystallinity as a function of time, kt is the Tobin crystallization rate constant, and nt is the Tobin exponent. Based on this proposition, the Tobin exponent nt does not need to be integral, since it is controlled directly by different types of nucleation and growth mechanism. Equation 6 could be rewritten as follows to calculate the Tobin crystallization kinetics parameters logðXt =ð1 Xt ÞÞ ¼ log kt þ nt log t
ð7Þ
The Tobin parameters nt and kt could be obtained from the plots of log(Xt/(1 Xt)) versus log t shown in Figure 9 for POSS-0.5. It is obvious that the Tobin method could describe the nonisothermal cold crystallization for PLLA more properly than does the Ozawa model. The similar plots are observed for neat PLLA and POSS-1, which are not shown here for brevity. The nt and kt values of all the samples at various heating rates are also listed in Table 2. The nt values are all around 4.58, indicating that both the heating rates and the incorporation of ovi-POSS may not change the nonisothermal cold crystallization mechanism of PLLA. However, in the higher Xt range (g75%), the Tobin method always gave the lower value than the experimental data. The reasons might be that the model as shown in eq 6 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. Similar results were also found for poly(ethylene succinate) (PES) and syndiotactic polypropylenes (s-PP).24,44 The crystallization activation energy is also an important parameter in studying the nonisothermal cold crystallization of polymers. 12584
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PLLA/clay nanocomposites, and poly(ethylene terephthalate) (PET)/MWCNTs nanocomposites.41,46,47
Figure 9. The Tobin plots of POSS-0.5.
Figure 10. The Kissinger plots of neat PLLA and its nanocomposites for the estimation of crystallization activation energy in nonisothermal cold crystallization.
The activation energies for nonisothermal cold crystallization of neat PLLA and its nanocomposites were derived from the Kissinger method,45 which is given as follows: !! Φ d ln Tp 2 ΔE ! ð8Þ ¼ R 1 d Tp where ΔE is the activation energy, Tp is the crystallization peak temperature, and R is the universal gas constant. Figure 10 shows the Kissinger plots of neat PLLA and its nanocomposites, and the slopes of the plots of ln(Φ/Tp2) versus 1/Tp gave the value of ΔE/R. Thus, the ΔE values of nonisothermal cold crystallization are estimated to be 55.6 kJ/mol for neat PLLA and increased to 78.3 and 109.4 kJ/mol for POSS-0.5 and POSS-1, respectively. It is clear that the cold crystallization activation energies of the nanocomposites are higher than that of neat PLLA, and the ΔE values increase with increasing the ovi-POSS content in the PLLA/ovi-POSS nanocomposites. The existence of ovi-POSS has lowered the nonisothermal cold crystallization peak temperatures for the nanocomposites, thereby impeding the movement of PLLA chain segments to the growing surface, and causing the enhancement of the crystallization activation energy of PLLA in the nanocomposite. Similar results have also been recently found for PLLA/MWCNTs nanocomposites,
’ CONCLUSIONS In this work, isothermal and nonisothermal crystallization kinetics of neat PLLA and its nanocomposites at low loadings of ovi-POSS from the amorphous state were studied in detail. Isothermal cold crystallization kinetics of neat PLLA and its nanocomposites were studied with DSC at various crystallization temperatures and analyzed by the Avrami equation. The overall crystallization rates of neat PLLA and its nanocomposites increase with increasing crystallization temperature. At a given crystallization temperature, the overall crystallization rates are faster in the PLLA/ovi-POSS nanocomposites than in neat PLLA, and are strongly dependent on the content of ovi-POSS; however, the crystallization mechanism and crystal structure of PLLA remain unchanged despite the addition of ovi-POSS. Nonisothermal cold crystallization behaviors of neat PLLA and the PLLA/ovi-POSS nanocomposites were also investigated at different heating rates. Both heating rate and the ovi-POSS loading are the two main factors influencing the nonisothermal cold crystallization behavior of PLLA in the PLLA/ovi-POSS nanocomposites. 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 nanocomposites. On the other hand, at a given heating rate, the addition of ovi-POSS enhances the nonisothermal cold crystallization of PLLA significantly; furthermore, the higher the ovi-POSS content, the faster the crystallization rate. 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 ovi-POSS may not change the nonisothermal cold crystallization mechanism of PLLA. The cold crystallization activation energy of PLLA increases with increasing the ovi-POSS content. ’ AUTHOR INFORMATION Corresponding Author
*Fax: +86-10-64413161. E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Biomer, Germany for kindly supplying PLLA sample. Part of this work is financially supported by the Fundamental Research Funds for the Central Universities (ZZ1005). ’ REFERENCES (1) Garlotta, D. A literature review of poly(lactic acid). Polym. Environ. 2001, 9, 63–84. (2) Drumright, R.; Gruber, P.; Henton, D. Polylactic acid technology. Adv. Mater. 2000, 12, 1841–1846. (3) Ikada, Y.; Tsuji, H. Biodegradable polyesters for medical and ecological applications. Macromol. Rapid Commun. 2000, 21, 117–132. (4) Bogaert, J.; Coszac, P. Poly(lactic acids): A potential solution to plastic waste dilemma. Macromol. Symp. 2000, 153, 287–303. (5) Pan, P.; Inoue, Y. Polymorphism and isomorphism in biodegradable polyesters. Prog. Polym. Sci. 2009, 34, 605–640. (6) De Santis, P.; Kovacs, A. Molecular conformation of poly(S-lactic acid) biopolymers. Biopolymers 1968, 6, 607–612. 12585
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