ARTICLE pubs.acs.org/IECR
Crystallization Kinetics of Linear and Long-Chain-Branched Polylactide Mohammadreza Nofar,† Wenli Zhu,† Chul B. Park,*,† and Jed Randall‡ †
Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 ‡ NatureWorks LLC, 15305 Minnetonka Blvd, Minnetonka, Minnesota 55345, United States ABSTRACT: In this study, the non-isothermal cold crystallization and isothermal melt crystallization of both linear and long-chainbranched (LCB) polylactide (PLA) were investigated using a differential scanning calorimeter (DSC). Talc was used as a nucleating agent to promote crystallization. The effects of chain branching on PLA’s cold crystallization kinetics at different heating rates and on PLA’s melt crystallization kinetics under different temperatures were studied by using Avrami analysis. The results showed that LCB-PLAs have faster cold and melt crystallization rates than those of linear PLA, since branched chains can play a role of nucleating site. Talc is a powerful nucleating agent, especially for linear PLA, either in cold crystallization or melt crystallization process. It was seen that addition of talc to PLA improves the crystallinity of PLA samples with more linear structure, more effectively because of its role of crystal nucleation. In PLA samples with more branched structure, talc has the least effect on crystallinity suggesting that the branched structure dominated crystallization already regardless of the presence of talc. Isothermal melt crystallization experimental results also showed that branched PLAs crystallized much faster than linear PLA and talc could increase the melt crystallization rate of linear PLA, but not that of PLA with a more branched structure.
’ INTRODUCTION Polylactide (PLA), or poly (lactide acid), is derived from a seasonally renewable plant source and it is biodegradable and compostable. It has attracted extensive industrial interests, not only in biomedical but also in commodity applications, because it releases no toxic or noxious components during the manufacture. PLA foams, in particular PLA microcellular foams blown with supercritical carbon dioxide (ScCO2), are considered one of the most promising substitutes to replace polystyrene (PS) in packaging industries because of its biodegradability (compostability to be accurate), competitive material and processing costs, and comparable mechanical properties. Currently, the mass production of PLA microcellular foams is still quite challenging because of this material’s low melt strength associated with its linear molecular architecture. Recent research results showed that the introduction of chain branching, (i.e., connecting the terminal groups through a chain extender) is an efficient way to improve PLA’s extensional viscosity and foamability.15 Our latest research results also showed that long-chain-branched (LCB) PLAs have much higher melt strength than the corresponding linear materials, resulting in a microcellular closed-cell structure and a larger expansion ratio spanning a wider processing window when using supercritical carbon dioxide as a blowing agent.68 Unlike its microcell morphology, the dependency of the thermal and mechanical properties of PLA foams on the degree of final crystallinity is well-known. However, PLA crystallizes very slowly during the melt crystallization processes, compared with other semicrystalline polymers, such as polypropylene (PP) and polyethylene (PE). Hence, it is hard to achieve high crystallinity without further treatments in the PLA products manufactured via highthroughput thermoplastic processes, in which the supercooling rate is very high. To develop the final crystallinity, orientation processing methods, including uniaxially drawing spun fibers9 and biaxially stretching extruded films or sheets,10,11 are widely used r 2011 American Chemical Society
in the PLA industry. Annealing PLA products above the glass transition temperature (Tg) is another way to improve the final crystallinity because PLA can crystallize between Tg and Tm (melting temperature). That is due to cold crystallization which takes place during heating and isothermal processes. During the isothermal annealing, the crystallization kinetics can be explored through various models such as Avrami analysis.1218 The thermal behavior of PLA is different from other semicrystalline polymers (PP, PE, etc.) because it involves multiphase transitions during heating, such as glass transition, chain relaxation, cold crystallization, and melting of crystals. Most researchers have investigated the thermal behaviors and crystallization kinetics of linear PLA. However, relatively few published literatures elucidated the effect of chain branching on PLA’s thermal behaviors and crystallization kinetics. Dorgan et al. reported that branched PLA crystallized faster than the linear analogue.19 However, Ouchi et al. showed that the final crystallinity of branched PLA is lower than its linear counterpart, while the crystallinity of branched PLA can be controlled by tailoring the chain length of branching structure.20 On the other hand, Mihai et. al showed that addition chain extender improves the crystallization behavior of PLA samples.21 Although these results showed that chain branched PLA has different crystallization behavior from linear one, there is not much systematic study on the effect that the degree of chain branching has on PLA’s crystallization kinetics. The type of the chain extender and its functional groups affecting the chain’s order when the branched structure forms can be important factors in crystallization kinetics.4,5 Received: June 3, 2011 Accepted: October 31, 2011 Revised: October 14, 2011 Published: October 31, 2011 13789
dx.doi.org/10.1021/ie2011966 | Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Formulation and Molecular Characteristics of Materials sample
D-lactide
chain extender
Mn
no.
name
(mol %)
(wt %)
(g/mol)
Mw/Mn
1 2
linear PLA B1-PLA
4.6% 4.6%
0% 0.35 wt %
90,111 98,592
1.8 2.2
3
B2-PLA
4.5%
0.7 wt %
112,944
2.5
Figure 1. DSC second heating curves of neat PLA samples at heating rate of 10 °C/min.
One of the shortcomings of the PLA products is the low heat deflection temperature because of the low Tg and low crystallinity. But if the crystallinity is increased, the heat deflection temperature will be increased, and the utility of the PLA products will be enhanced significantly.22 As mentioned previously, PLA’s crystallization rate is relatively slow, and it is hard to get high crystallinity in the products without any further treatments, such as stretching or annealing. To increase the crystallinity and heatresistance temperature of final products, crystal nucleating agents are utilized to accelerate the crystallization rate. Several potential nucleating agents have been reported in the literature.2331 Among them, talc was recognized as a highly effective nucleating agent. Isothermal crystallization study showed that the minimum crystallization half time in the experimental temperature range can be dramatically decreased from 13.6 min to less than 1 min28 or from 40 min to 1.5 min,29 when just 1% talc is added. Harris and Lee reported that the addition of 2% talc speeds the crystallization rate nearly 65-fold,30 and Kolstad found that 6% talc gives a 500-fold increase in the nucleation density.31 During microcellular foaming process, talc is also a widely used cell nucleation agent.3236 Therefore, talc has dual roles in the microcellular foaming, that is, promoting both crystallization and cell nucleation. In our previous microcellular foaming experiments,68 LCB-PLA resins with 0.4 wt % talc were used as polymer matrix. For these reasons, it is important to study the effect of chain branching on PLA’s crystallization kinetics with the presence of talc. In this study, the isothermal and non-isothermal cold and melt crystallization kinetics of linear and branched PLAs with different degree of chain branching were systematically examined using a differential scanning calorimeter (DSC). The effect of talc as a nucleation agent on PLA’s crystallization behavior was also investigated. The effects of chain branching on PLA-talc materials’ crystallization behavior will help to better control the processing conditions and obtain desired crystallinity in the final products.
Figure 2. DSC heat graphs of PLA-talc samples at different heating rates after cooling 20 °C/min from melt condition (a) linear PLA-talc; (b) B1-PLA-talc; (c) B2-PLA-talc.
’ EXPERIMENTAL METHODOLOGY Materials and Sample Preparation. One linear and two kinds of long-chain-branched (LCB) PLA materials in the pellet form which are commercially available were supplied by NatureWorks LLC. According to the manufacturer, all these materials are semicrystalline polymers with relatively slow nucleation and crystallization rates, and have similar D-lactide molar contents. The linear PLA was commercially available Ingeo 8051D. 13790
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
Figure 3. Variation of Tcc (a) and Tg (b) vs heating rate for PLA samples with different branching degree.
LCB-PLA materials were formulated by reactive extrusion based on 8051D with an epoxy-based multifunctional oligomeric chain extender (Joncryl ADR-4368C, BASF Inc.). A description of the molecular structure and chain extension mechanism of this chain extender can be found elsewhere.37 The LCB-PLA containing 0.35 wt % chain extender is referred to as B1-PLA and the one with 0.7 wt % chain extender as B2-PLA, respectively. D-lactide and chain extender contents, and molecular characteristics of materials are shown in Table 1. As in the case of the talc-containing PLA samples, the neat PLA materials with zero talc content were also processed in a Minilab twin-screw extruder using the same processing condtions to maintain the same thermal history38 and to complete the reaction of the chain extender for the branched samples. To decrease the moisture level of PLA pellets, they were oven-dried at 65 °C for at least 6 h before processing. The solid densities of dried PLAs were 1.24 g/cm3 measured by a pycnometer. Mistron Vapor-R grade talc (Luzenac America) was used as a nucleating agent in this study. Talc was melt-blended at 0.4 wt % loading level with each PLA material at 180 °C for 4 min with screw rpm of 40, using a minilab twin-screw extruder. Then, both neat and composite materials were hot-pressed using a hydraulic compression machine (Carver Inc.) at 200 °C for 5 min into a 400 μm-thick film and then cooled in the air. The disk-shape PLA samples (typically 15 mg for each) were punched from prefabricated film and loaded in the aluminum pans with lids for the DSC experiments. Crystallization Kinetics Study. By using a DSC, Q2000 (TA Instruments, America), the following experiments were performed: (1) Non-isothermal cold crystallization study of neat PLA materials. Samples were heated from room temperature to 200 °C at a heating rate of 10 °C/min and then equilibrated at 200 °C for 10 min to completely eliminate the previous thermal and stress histories. Then they were cooled to 25 °C at a rate of 20 °C/min to expose all samples to the same thermal treatment. Finally they were reheated to 200 °C at a rate of 10 °C/min. (2) Non-isothermal cold crystallization study of PLA-talc materials. After eliminating the previous thermal and stress histories of Experiment (1), samples were cooled to 25 °C at a rate of 20 °C/min and reheated to 200 °C at a rate of 2, 5, 10, 20, and 30 °C/min, respectively. (3) Non-isothermal melt crystallization study of both neat PLA and PLA-talc materials. After eliminating the previous thermal and stress histories of Experiment (1), samples were cooled to 25 °C at a rate of 2 °C/min.
(4) Isothermal melt crystallization study of PLA-talc materials. After eliminating thermal and stress histories of Experiment (1), samples were cooled at a rate of 20 °C/min to one of several isothermal temperatures (90 °C, 100 °C, 110 °C, and 120 °C) and equilibrated at a predetermined temperature until crystallization completely finished. During all the heating and cooling processes, the DSC curves were recorded and analyzed using TA Universal software. The initial degree of crystallinity,χ, that is, the crystallinity of samples before reheating, was calculated by using the following Equation: χ¼
ΔHm ΔHcc 100% 93:6
ð1Þ
where ΔHm is the melting enthalpy, ΔHcc the cold crystallization enthalpy, and 93.6 the melting enthalpy in J/g of 100% crystalline PLA.39 The kinetics of isothermal melt crystallization at different isothermal temperatures was analyzed using the Avrami equation: 40 XðtÞ ¼ 1 expð kt n Þ
or
ln½ lnð1 XðtÞÞ ¼ n ln t þ ln k
ð2Þ
where X(t) is the relative crystallinity at crystallization time t, k is the crystallization kinetic constant for nucleation and growth rate, and n is the Avrami exponent reflecting the mechanisms of crystal nucleation and growth. By plotting ln[ln(1 X(t))] versus ln(t), the Avrami exponent, n, and the logarithm of the kinetic constant, ln k, were determined. Jeziorny’s theory41 modified by the Avrami equation was used to analyze the kinetics of non-isothermal cold crystallization by considering the effect of heating rates on the crystallization process; the parameter characterizing kinetics during nonisothermal crystallization was written as ln kc ¼ ln k=j
ð3Þ
where kc is the modified crystallization rate constant considering heating rate, j.
’ RESULTS AND DISCUSSION Non-isothermal Cold Crystallization Kinetics. Figure 1 shows the DSC second heating curves of neat PLAs at a heating rate of 10 °C/min. The glass transition (Tg) and melting (Tm) 13791
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
Figure 4. Cold crystallization and melting enthalpies of PLA-talc samples at different heating rates: (a) cold crystallization enthalpy; (b) melting enthalpy.
Figure 5. Time dependence of relative crystallinity (left) and Avrami double-log plots (right) for cold crystallization of PLA-talc samples at different heating rates. 13792
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
Table 2. Crystallization Half Time and Parameters from Avrami Analysis for Non-Isotherm Cold Crystallization of PLA-talc Samples samples linear PLA with talc
B1-PLA with talc
B2-PLA with talc
j (°C/min) t1/2 (min)
n
ln k
k
2
7.00
4.871 9.806 5.51 105
5
3.96
4.718 6.872 1.04 103
10
2.45
3.384 3.532 2.92 102
20
1.04
2.248 0.776 4.60 101
30 2
0.73 6.60
2.070 0.224 1.25 4.772 9.377 8.46 105
5
4.08
4.327 6.436 1.60 103
10
2.27
3.163 2.967 5.15 102
20
1.02
2.281 0.406 6.66 101
30
0.70
2.113
2
6.75
3.513 7.102 8.23 104
0.287 1.33
5
4.06
3.333 5.055 6.38 103
10 20
2.11 1.07
2.476 2.295 1.01 101 2.134 0.768 4.64 101
30
0.45
2.010
1.056 2.87
temperatures of all linear and LCB-PLAs are very similar and are almost 58 and 150 °C, respectively. When the samples were heated above the Tg, exothermic peaks related to cold crystallization behaviors for linear and LCB-PLA samples are shown in Figure 1. In this study, linear PLA shows a cold crystallization peak temperature (Tcc) higher than those of the two branched PLAs, because of its high chain mobility which disables the chains pack earlier. As seen, the Tcc of B1-PLA and B2-PLA are around 124.5 °C, whereas for linear PLA the cold crystallization takes place at a higher temperature of 131 °C. Also, the larger cold crystallization and melting enthalpies of two branched PLAs shown in Figure 1 indicate that higher crystallinity forms than in linear PLA because of the higher potential that branched structure can have for nucleating new crystals. This has been reported in the work of Mihai et. al.21 For all neat linear and branched PLA samples, initial crystallinity was almost zero, indicating that they were all essentially amorphous with the previous thermal treatments at a cooling rate of 20 °C/min. Figure 2(ac) shows the DSC heating curves of linear PLA, B1-PLA, and B2-PLA with talc at different heating rates. The cold crystallization peaks of the PLA-talc samples at a heating rate of 10 °C/min are much more remarkable than those of the neat PLA samples shown in Figure 1, indicating that talc is an efficient nucleating agent for PLA’s cold crystallization. Tcc and Tg data derived from these heating curves are summarized in Figure 3 (a-b). Compared with the data at the same heating rate shown Figure 1, it is evident that the increase of crystallization enthalpy for linear PLA is greater than those of B1-PLA and B2-PLA. This is because linear PLA has better chain mobility to crystallize with the presence of talc. In PLA samples with more branched structure, talc has the least effect on crystallinity suggesting that the branched structure dominated crystallization already regardless of the presence of talc. Figure 3 shows that, the Tg and Tcc for each material increased with an increase of the heating rate, because of a thermal delay. As shown, the Tg of all samples with a fixed heating rate is the same, whereas the Tcc is lower for samples with more branched structure and this, as mentioned, can be due to the higher crystal nucleating potential of the branched structure.4,21 Also, at a low heating rate, for example, 2 °C/min, there
are two melting peaks on the curves of both linear and branched PLAs. The low-temperature melting peak can be attributed to the melting of metastable or imperfect crystalline structures formed at the early stage of cold crystallization, and the high-temperature peak can be the result of the melting of more perfect crystalline structure that occurred at the later stage of cold crystallization, because of the increased lamellae thickness. When the heating rate increased to 5 °C/min, double-melting peaks disappeared from two branched PLAs, but were still shown on the linear PLA’s heating curve. The possible reason is that branched PLAs are less able than linear PLA to increase lamellae thickness because of their restricted chain mobility. When the heating rate increased to 10 °C/min, all the PLA-talc materials showed just a single melting peak, indicating that less crystal perfection occurred during faster heating processes. When the heating rate is 20 °C/min, the cold crystallization peaks of two LCB-PLAs are very small. They are hard to detect when the heating rate increased to 30 °C/min. However, cold crystallization peaks are shown on all the linear PLA’s heating curves, accompanied by remarkable melting peaks. The slower disappearance of cold crystallization peaks indicates that linear PLA has better segmental mobility than those two LCB-PLA materials. Considering the initial crystallinity of linear and branched PLA-talc samples, the original crystallinity for all samples is almost zero, very similarly to PLA samples with no talc, indicating that they were essentially amorphous, because of fast cooling before reheating. Figure 4 plots the cold crystallization and melting enthalpy versus heating rates. Crystallizability depends highly on chain architectures. At the same heating rate, the enthalpy of cold crystallization (ΔHcc) decreased gradually by having a higher degree of linear structure. Also the cold crystallization takes place sooner for branched structure compared to linear PLA samples because of the higher crystal nucleating potential of the branched structure. Figure 4a also shows that the ΔHcc decreased as the heating rate increased, because polymer chains do not have enough time to form crystal structures during faster heating processes, resulting in a decreased ΔHm, which is shown in Figure 4b. The melting enthalpy varies at a same pace as cold crystallization enthalpy, suggesting that the melting behaviors are mainly caused from melting crystals generated during the cold crystallization process. To depict the non-isothermal cold crystallization kinetics of PLA-talc materials, the relative cold crystallinity of three PLAs versus time at different heating rates is plotted in Figure 5 (a, c, and e) and their corresponding Avrami double-log plots are shown in Figure 5 (b, d, and f). The crystallization half time obtained from relative cold crystallinity curves and parameters (n, ln k, and k) derived from Avrami plots and Jeziorny methods are listed in Table 2. It is clear that the crystallization half time decreased as the heating rate increased. The Avrami exponent n gradually decreases with increasing heating rates. The n values are about 4 or 3 under the slower heating rates, suggesting a three-dimensional (3-D) growth. The n values decrease to around 2 under the heating rates of 20 and 30 °C/min, indicating that cold crystallization corresponds to a two-dimensional (2-D) growth of nucleation. An explanation for this would be that at higher heating rates, there is less time for polymer chains to organize in a 3-D order. Table 2 also shows that under the same heating rate, the n values decrease with an increasing degree of chain branching. It is speculated that the higher degree of chain branching suppressed the formation of crystallites with 3-D structure through talc during cold crystallization since the branched structure already dominated the crystallization in a 13793
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research homogeneous 2-D spherulitic form, regardless of the presence of talc. However, at a high heating rate, the crystallinity has less
Figure 6. DSC non-isothermal crystallization traces of PLA samples.
Figure 7. Degree of crystallinity of PLA samples with various branching degree, with and without talc.
ARTICLE
dependency to chain structure because of the limited time for cold crystallization. Non-Isothermal and Isothermal Melt Crystallization Kinetics. Figure 6 shows the non-isothermal melt crystallization DSC curves of neat PLA and PLA-talc samples at a cooling rate of 2 °C/min. There is no detectable exothermic peak on the cooling curves of neat linear PLA, indicating that the melt crystallization rate of neat linear PLA is very slow, whereas with inducing branched structures, the crystallization starts to increase significantly (Figure 7), suggesting that the branched structure through the chain’s end groups behaves as a crystal nucleating site. On the other hand, by adding 0.4% talc, crystallization peaks emerge on the cooling curves, and they are more pronounced for linear PLA and then B1-PLAs, whereas the crystallization of B2-PLA gets no virtual effect from talc. In fact, with a higher branched structure, talc has less effect on improving the crystallinity suggesting that the branched structure dominates the crystallization. The results of the degree of crystallinity in these samples are shown in Figure 7. The degree of crystallinity is calculated based on the crystal formation heat flows (J/g) derived from the cooling graphs shown in Figure 6, divided to 93.6 J/g as the melting enthalpy of 100% crystalline PLA. This experiment result is consistent with earlier results in the literature,2831 that is, talc is an effective nucleating agent for improving PLA’s melt crystallization, even at a very low concentration although as shown in this work it is more effective for linear PLA than B2-PLA. To examine the effects of the degree of chain branching on melt crystallization, with or without the inclusion of talc, isothermal crystallization experiments were performed in a temperature range from 90 to 120 °C at an interval of 10 °C. The DSC isothermal crystallization traces are shown in Figure 8. The Avrami double-log plots based in Figure 8 are shown in Figure 9. From Figure 9, the Avrami exponent n for linear and branched
Figure 8. DSC isothermal melt crystallization traces under different isothermal temperatures. 13794
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
Figure 9. Avrami double-log plots for linear and chain-branched neat PLA and PLA-talc samples under different isothermal temperatures.
PLAs under different isothermal temperature can be obtained and are shown in Table 3. It can be seen that the n values of all PLAs are around 2 and 3 in most cases; however, for B2-PLA the number gets closer to 2 in most cases, indicating that homogeneous nucleation and 2-D spherulitic growth are more likely to occur. Also around 100 and 110 °C the crystallization takes place faster and easier and in fact, the crystal growth rate is high and the n value increases more compared to that at 90 and 120 °C, where the crystallization takes longer. This is due to the temperature conditions that are better for crystallization expediting the crystal growth with a tendency for more heterogeneous crystallization. Figure 10 plots crystallization half time, the time corresponding to 50% of the final crystallinity, which is derived from the integration curves of their DSC isothermal curves. From Figure 10a, it can be seen that the linear PLA’s crystallization half time is very long, although in the range of 100 and 110 °C, the crystallization half time is reduced, compared to the other
temperatures. This range is in fact the efficient range of faster crystallization. With a branched chain structure, the time for complete crystallization dramatically decreases because indolent branching structures act as nucleation sites for linear segments with a higher local order. By comparing the crystallization half time of two branched PLAs shown in Figure 10a, it is clear that the crystallization rate of B2-PLA is slightly faster than B1-PLA, suggesting that the existence of denser and longer branching can further promote the self-nucleation in the amorphous phase although the overall crystal growth rate is very slow. Crystallization enthalpy, which represents the crystallinity developed during the isothermal process, is shown in Figure 11. Figure 11a demonstrates that the final crystallinities of linear PLA and branched PLAs are very similar, and they can be increased by increasing the isothermal temperature because of the higher chain mobility at a higher temperature. The effect of talc on the PLA’s isothermal crystallization behavior was also investigated at the same thermal conditions. 13795
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
Table 3. Avrami Exponent n as a Function of Isothermal Temperature Avrami exponent n isothermal crystallization temperature (°C)
linear PLA
linear PLA with talc
B1-PLA
B1-PLA with talc
B2-PLA
B2-PLA with talc
90
2.71
2.40
2.48
2.27
2.09
2.33
100
2.78
2.94
3.22
2.63
2.05
2.52
110
2.83
2.88
2.8
2.90
2.3
2.65
120
2.48
2.45
2.53
2.29
1.80
2.53
Figure 10. Crystallization half time of PLAs under different isothermal temperatures.
Figure 11. Crystallization enthalpy of PLA samples isothermally crystallized under different isothermal temperatures.
As shown in Figure 10b, linear PLA’s crystallization rate increased significantly at all temperatures by adding just 0.4% talc. This again confirms that talc is a highly effective nucleating agent for linear PLA. Talc also accelerated the crystallization of B1-PLAs, but not as significantly as that of linear PLAs. In PLA samples with more branched structure, talc has the least effect on crystallinity suggesting that the branched structure dominated crystallization already regardless of the presence of talc, as we have seen in the non-isothermal cold crystallization study. Figure 10b shows that, at temperatures of 100 and 110 °C the crystallization half time reduces, compared to the other temperatures for both B1 and B2-PLAs. However, Figure 11b shows that crystallization enthalpies are significantly influenced by adding talc, especially at higher temperatures and especially for linear PLA. At higher
temperatures, crystallization enthalpies of all PLA-talc samples increased, because of the higher chain mobility that increases the crystallization potential. With the presence of talc, the dependency of crystallization enthalpy on the degree of chain branching is significantly different. In linear PLA-talc, in the presence of talc the amount of crystallinity increases, because of the higher crystallization potential of linear chains, whereas in branched PLAs, the higher the degree of branching, the lower the potential degree of crystallization. Although the branching helps the crystal nucleation, chain mobility will decrease, and this in turn will decrease the chance for a higher crystallinity eventually. The isothermal melt crystallization study results can be tentatively interpreted as showing that branched chains can function as nucleating agents in accelerating crystallization. 13796
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
’ CONCLUSION Cold crystallization and melt crystallization of both linear PLA and LCB-PLAs were investigated using a DSC instrument. The effects of talc and chain branching on the cold crystallization and the melt crystallization of PLA were studied by using the Avrami analysis. Non-isothermal cold crystallization experimental results showed that PLA samples with a higher branching degree have higher crystallizability than the linear PLA samples. With the presence of talc, the cold crystallizability of PLA increased, but this was more pronounced for linear PLA than the branched structure. The non-isothermal crystallization during cooling showed that the branching increased the crystallinity of the PLA samples because of its crystal nucleation effect, the addition of talc increased the crystallization of PLA samples, especially for linear PLA, and the effect of talc diminished with a more branched structure. Isothermal melt crystallization experimental results also showed that branched PLAs crystallized much faster than linear PLA, and talc could increase the melt crystallization rate of linear PLA, but not that of PLA with a more branched structure. In PLA samples with more branched structure, talc has the least effect on crystallinity suggesting that the branched structure dominated crystallization already regardless of the presence of talc. Also, according to the Avrami analysis, it was shown that homogeneous nucleation and 2-D spherulitic growth are more likely to occur for PLAs with higher branching degree. For isothermal temperatures for which the crystallization growth rate is faster, the crystal growth has higher tendency to have 3-D and heterogeneous crystallization because of higher chain regularity. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +1-416-978-3053. Fax: +1-416-978-0947. E-mail: park@ mie.utoronto.ca.
’ REFERENCES (1) Di, Y.; Iannace, S.; Maio, E. D.; Nicolais, L. Reactively Modified Poly(lactic acid): Properties and Foam Processing. Macromol. Mater. Eng. 2005, 290, 1083–1090. (2) Pilla, S.; Kim, S. G.; Auer, G. K.; Gong, S.; Park, C. B. Microcellular Extrusion Foaming of Poly(Lactide)/Poly(Butylene AdipateCo-Terephthalate) Blends. Mater. Sci. Eng., C 2010, 30 (2), 255–262. (3) Sungsanit, K.; Kao, N.; Bhattacharya, S. N.; Pivsaart, S. Physical and Rheological Properties of Plasticized Linear and Branched PLA. Korea-Aust. Rheol. J. 2010, 22 (3), 187–195. (4) Pilla, S.; Kim, S. G.; Auer, G. K.; Gong, S.; Park, C. B. Microcellular Extrusion-Foaming of Polylactide with Chain-Extender. Polym. Eng. Sci. 2009, 49, 1653–1660. (5) Carlson, D.; Dubois, P. Free Radical Branching of Polylactide by Reactive Extrusion. Polym. Eng. Sci. 1998, 38, 311–321. (6) Zhu, W. L., Wang, J., Park, C. B., Pop-Iliev R., Randall, J. Effects of Chain Branching on the Foamability of Polylactide. SPE, ANTEC, Technical Papers, Paper #1045, Chicago, IL, June 2224, 2009. (7) Zhu, W. L., Wang, M. Y., Leung, S. N., Park, C. B., Randall, J. Microcellular Extrusion Foaming for Linear and Long-Chain-Branched Polylactide. SPE, ANTEC, Technical Papers, Paper #146, Orlando, FL, May 1620, 2010. (8) Wang, J., Zhu, W. L.; Park, C. B.; Randall, J. Impact of Molecular Branching on Microcellular Foaming of Polylactic Acid in Extrusion. SPE, ANTEC, Technical Papers, Paper #0530, Chicago, IL, June 2224, 2009. (9) Solarski, S. M.; Ferreira, E. Characterization of the Thermal Properties of PLA Fibers by Modulated Differential Scanning Calorimetry. Polymer 2005, 46, 11187–11192.
ARTICLE
(10) Yu, L. H.; Liu, F.; Xie, L.; Chen, X. Effect of Annealing and Orientation on Microstructures and Mechanical Properties of Polylactic Acid. Polym. Eng. Sci. 2008, 634–641. (11) Chinsirikul, W., Kerddonfag, N., Winotapun, C., Khumsang P., Hararak B. Processing and Elongational Deformation Behavior of Biaxially Stretched Polylactic Acid (PLA) Films. PPS-27, 27th World Congress of the Polymer Processing Society, Marrakech, Morocco, May 1014, 2011 (12) Tsuji, H.; Ikada, Y. Properties and Morphologies of Poly(L-Lactide): 1. Annealing Condition Effects on Properties and Morphologies of Poly(L-Lactide). Polymer 1995, 36, 2709–2716. (13) Yu, L.; Liu, H.; Dean, K.; Chen., L. Cold Crystallization and Postmelting Crystallization of PLA Plasticized by Compressed Carbon Dioxide. J. Polym. Sci., Part B: Polym. Phys. 2008, 46, 2630–2636. (14) Day, M.; Victoria Nawaby, A.; Liao, X. A DSC Study of the Crystallization Behaviour of Polylactic Acid and its Nanocomposites J. Therm. Anal. Calorim. 2006, 86 (3), 623–629. (15) Su, Z.; Liu, Y.; Guo, W.; Li, Q.; Wu, C. Crystallization Behavior of Poly (Lactic Acid) Filled with Modified Carbon Black. J. Macromol. Sci., Part B 2009, 48 (4), 670–683. (16) Liao, R.; Yang, B.; Yu, W.; Zhou, C. Isothermal Cold Crystallization Kinetics of Polylactide/Nucleating Agents. J. Appl. Polym. Sci. 2007, 104, 310–317. (17) Chu, M., Wu. T. Exp. Anal. Nano Eng. Mater. Struct. 2007, C, 27, 827-828 (18) Huang, J.; Hung, Y.; Wen, Y.; Kang, C.; Yeh, M. Polylactide/ Nano- and Micro-Scale Silica Composite Films. II. Melting Behavior and Cold Crystallization. J. Appl. Polym. Sci. 2009, 112, 3149–3156. (19) Dorgan, J.; Lehermeier, R. H.; Mang, M. Thermal and Rheological Properties of Commercial-Grade Poly(Lactic Acid)s. J. Polym. Env. 2000, 8, 1–9. (20) Ouchi, T.; Ichimura, S.; Ohya, Y. Synthesis of Branched Poly(Lactide) Using Polyglycidol and Thermal, Mechanical Properties of its Solution-Cast Film. Polymer 2006, 47, 429–434. (21) Mihai, M.; Huneault, M. A.; Favis, B. D. Rheology and Extrusion Foaming of Chain-Branched Poly(lactic acid). Polym. Eng. Sci. 2010, 50, 629–642. (22) Huda, M. S.; Drzal, L. T.; Mohanty, A. K.; Mirsa, M. Effect of Fiber Surface-Treatments on the Properties of Laminated Biocomposites From Poly(Lactic Acid) (PLA) and Kenaf Fibers. Compos. Sci. Technol. 2008, 68 (2), 424–432. (23) Nam, J. Y.; Ray, S. S.; Okamoto, M. Crystallization Behavior and Morphology of Biodegradable Polylactide/Layered Silicate Nanocomposite. Macromolecules 2003, 36, 7126–7131. (24) Tsuji, H.; Takai, H.; Saha, S. K. Isothermal and Non-Isothermal Crystallization Behavior of Poly (L-Lactic Acid): Effects of Stereocomplex as Nucleating Agent. Polymer 2006, 47, 3826–3837. (25) Xiao, H.; Yang, L.; Ren, X.; Jiang, T.; Yeh, J. T. Kinetics and Crystal Structure of Poly (Lactic Acid) Crystallized Non-isothermally: Effect of Plasticizer and Nucleating Agent. Polym. Compos. 2010, 2057–2068. (26) Pluta, M. Morphology and Properties of Polylactide Modified by Thermal Treatment, Filling with Layered Silicates and Plasticization. Polymer 2004, 45, 8239–8251. (27) Wu, D.; Wu, L.; Wu, L.; Xu, B.; Zhang, Y.; Zhang., M. Nonisothermal Cold Crystallization Behavior and Kinetics of Polylactide/ Clay Nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 1100–1113. (28) Ke, T.; Sun., X. Melting Behavior and Crystallization Kinetics of Starch and Poly (Lactic Acid) Composites. J. Appl. Polym. Sci. 2003, 89, 1203–1210. (29) Li, H.; Huneault., M. A. Effect of Nucleation and Plasticization on the Crystallization of Poly(Lactic Acid). Polymer 2007, 48, 6855–6866. (30) Harris, A. M.; Lee., E. C. Improving Mechanical Performance of Injection Molded PLA by Controlling Crystallinity. J. Appl. Polym. Sci. 2008, 107 (4), 2246–2255. (31) Kolstad, J. J. Crystallization Kinetics of Poly (L-Lactide-CoMeso-Lactide). J. Appl. Polym. Sci. 1996, 62, 1079–1091. 13797
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
Industrial & Engineering Chemistry Research
ARTICLE
(32) Kaewmesri, W. P.; Lee, C.; Park, C. B.; Pumchusak, J. Effects of CO2 and Talc Contents on Foaming Behavior of Recyclable High-MeltStrength PP. J. Cell. Plast. 2006, 42 (5), 405–428. (33) Naguib, H. E.; Park, C. B.; Lee., P. C. Effect of Talc Content on the Volume Expansion Ratio of Extruded PP Foams. J. Cell. Plast. 2003, 39 (6), 499–511. (34) Naguib, H. E.; Park, C. B.; Lee, P. C.; Xu., D. A Study on the Foaming Behaviors of PP Resins with Talc as Nucleating Agent. J. Polym. Eng. 2006, 26 (6), 565–587. (35) Chen, L.; Blizard, K.; Straff, R.; Wang., X. Effect of Filler Size on Cell Nucleation during Foaming Processes. J. Cell. Plast. 2002, 38 (2), 139–148. (36) Moon, Y.; Cha., S. W. Study on Viscosity Changes with Talc in Microcellular Foaming Process. Fibers Polym. 2007, 8, 393–398. (37) Villalobos, M.; Awojulu, A.; Greeley, T.; Turco, G.; Deeter., G. Oligomeric Chain Extenders for Economic Reprocessing and Recycling of Condensation Plastics. Energy 2006, 31, 3227–3234. (38) Pantani, R.; DeSantis, F.; Sorrentino, A.; DeMaio, F.; Titomanlio, G. Crystallization Kinetics of Virgin and Processed Poly (Lactic Acid). Polym. Degrad. Stab. 2010, 95, 1148–1159. (39) Fischer, E.; Sterzel, W.; Wegner, H.J. G.; Collo., K. Investigation of the Structure Of Solution Grown Crystals of Lactide Copolymers by Means of Chemical Reaction. Polym. Sci. 1973, 251, 980–990. (40) Avrami, M. Kinetics of Phase Change. I General Theory. J. Chem. Phys. 1940, 8, 212–224. (41) Jeziorny, A. Parameters Characterizing the Kinetics of the NonIsothermal Crystallization of Poly(Ethylene Terephthalate) determined by D.S.C. Polymer 1978, 19 (10), 1142–1144.
13798
dx.doi.org/10.1021/ie2011966 |Ind. Eng. Chem. Res. 2011, 50, 13789–13798
