Accelerated Crystallization of Poly(lactic acid): Synergistic Effect of

Article pubs.acs.org/IECR

Accelerated Crystallization of Poly(lactic acid): Synergistic Effect of Poly(ethylene glycol), Dibenzylidene Sorbitol, and Long-Chain Branching Jinxiu You, Wei Yu,* and Chixing Zhou Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China ABSTRACT: The crystallization of poly(lactic acid) (PLA) is usually slow and related to the content of D-lactide. A new approach is suggested in this work to accelerate the crystallization of PLA via the synergistic effect of nanofibril nucleating agent (dibenzylidene sorbitol, DBS), plasticizer (poly(ethylene glycol), PEG) and long-chain branching in PLA. It is found that premade DBS/PEG gel can act as an active nucleating agent of PLA, which makes the crystallization peak appear during cooling. The preparation of DBS/PEG gel before mixing with PLA is important because self-assembly of DBS directly in PLA melt is difficult even in the presence of PEG. The mixing temperature is also found to be critical, which determines the amount of residual nanofibrils after melt mixing. Tuning the chain structure into long-chain branching via multifunctional monomer pentaerythritol triacrylate (PETA) will further speed up the crystallization of PLA because of the additional interaction between DBS nanofibrils and the grafted monomers. It is proven that the acceleration of crystallization is not ascribed to the change of crystal form but is due to the dominating increase in the nucleation density as well as the faster growth rate of spherulites in the presence of the plasticizer. Therefore, the problems of low melt strength and slow crystallization of PLA can be solved simultaneously via the present approach. cooling (10 °C/min) and the half-crystallization time at 120 °C is less than 1 min.19 Recently, some biobased nucleators such as myo-inositol and TBC8-t were also reported. Kunioka and coworkers20 found that isothermal crystallization of PLLA with 5 wt % myo-inositol at 100 °C finished within 2 min after melting, while that of PLLA finished crystallization over 14 min under the same condition. Liang et al.21 synthesized a novel nucleating agent TBC8-t and found that TBC8-t could greatly enhance the crystallization rate of PLLA by increasing the nucleation rate rather than the crystal growth rate. Furthermore, the crystallization peak temperature (Tc) and crystallization rate of PLLA nucleated with TBC8-t were much higher than those with conventional nucleating agent such as talc. 1,3:2,4-Dibenzylidene D-sorbitol (DBS), a frequently used organic nucleating agent in some polymers such as polyethylene (PE), polypropylene (PP),22−24 poly(ethylene terephthalate) (PET), and poly (butylene terephthalate) (PBT),25 has also been tried to improve the crystallization behavior of PLA. The idea is to make DBS self-assemble into nanofibrils, which can be effective nucleating agents. For example, Lai26,27 prepared DBS/PLLA samples by solution casting. In DBS/ PLLA systems, DBS nanofibrils were found by scanning electron microscopy (SEM) and polarized optical microscopy (POM). The DBS nanofibrils largely formed outside of PLLA spherulites, but some nanofibrils were dispersed in the PLLA spherulites, which affected the orientation of PLLA lamellae and caused a change in birefringence to influence slightly the

1. INTRODUCTION Polylactic acid (PLA) is an environmentally friendly polymer, and it could be used as the traditional commodity plastics with high mechanical properties, thermoplasticity, and biocompatibility.1−3 However, because of the weak melt strength and slow crystallization speed of PLA, the practical application and development are limited.1−3 The slow crystallization rate of PLA makes it very difficult to form crystals during cooling from melt, especially under high cooling rate for PLA with high D-lactide content. There are many efforts to alter the crystallization behavior of PLA.4 The first approach is to utilize various inorganic particles in microor nanoscale with different shapes as nucleating agents, such as talc,5 metal oxide,6 carbonate,7 sulfate,7 clay,8 and carbon nanotubes.9−11 In most cases, the cold crystallization was greatly enhanced with decreased cold crystallization temperature, increased cold crystallization speed, and increased crystallinity. Crystallization during cooling from melt is hardly observed. The second approach makes use of organic materials to promote crystallization. Some organic molecules like polyethylene glycol (PEG)12−14 and carbon dioxide (CO2) under high pressure15 can decrease the glass transition temperature of PLA substantially, which actually increases the mobility of PLA chains at the crystallization temperature. The plasticizing effect of these materials can enhance the cold crystallization in most cases, but the crystallization during cooling is often not affected. Some researchers also tried to modify the chain topology of PLA, which has also been found to affect only the cold crystallization.16−18 It has been reported recently that a special zinc citrate complex can greatly promote the crystallization of PLA, where the melt crystallization happens at 128 °C during © 2013 American Chemical Society

Received: Revised: Accepted: Published: 1097

July 23, 2013 December 5, 2013 December 28, 2013 December 29, 2013 dx.doi.org/10.1021/ie402358h | Ind. Eng. Chem. Res. 2014, 53, 1097−1107

Industrial & Engineering Chemistry Research

Article

600 mixer (Haake Rheocord 90, Germany) using neat PLA with the monomer PETA (3%), radicals initiator DCP (0.3%), and antioxidant Irganox 1330 (0.2%). Melt mixing was operated at 180 °C. The rotor speed was initially set at 20 rpm during the addition of materials and was later increased to 60 rpm for 10 min. Rheology was used to quantify the chain structures in both linear and LCB samples. The LCB sample is believed to be a mixture of linear molecules and branched molecules. Quantitative information on molecular weight, molecular weight distribution, and topology of branched molecules are obtained by fitting the dynamic moduli using the branch-on-branch model. The LCB PLA is a mixture of linear chain and comblike chain. The weight fraction of the linear component is 0.66, and the weight average molecular weight is 80 kg/mol with polydispersity 1.4. It contains two kinds of comblike molecules: one has a backbone with molecular weight (Mb) 160 kg/mol and three arms, and the other has a backbone with Mb = 320 kg/mol and six arms. The arm length in both comblike chains are 80 kg/mol. The weight fractions of the two comblike molecules are 0.12 and 0.22, respectively. The polydispersity of backbone molecular weight and arm molecular weight is 1.5. The molecular weight of the linear component in LCB PLA is smaller than that of the original PLA (102 kg/mol), which is ascribed to the degradation of PLA during reactive processing. The details of structural analysis of long-chain branching and its effect on the crystallization of PLA can be found in a recent publication.16 2.4. Preparation of PLA Blend. PLA and LCB PLA were dried at 45 °C for at least 24h in vacuum before mixing. PLA blend samples with antioxidant Irganox 1330 (0.2%) were prepared by direct melting in a batch (Haake Rheocord 90, Germany). The rotor speed was initially set as 20 rpm during the addition of materials and was later increased to 60 rpm for 10 min. The detailed compositions are listed in Table 2. As a plasticizer, the content of PEG in PLA is usually less than 20%. Because the main content of the gel is PEG, the total DBS gel content in PLA is set at 10%. As a nucleating agent, the typical DBS content in polymer is around 1%. Then, the amount of DBS in the gel is chosen as 5%, 10%, or 15%, which corresponds to the DBS content 0.5%, 1%, or 1.5% in PLA with the addition of 10% gel. The content of talc is chosen to be the same as that of DBS (P-D/P1) in the system. 2.5. Transmission Electron Microscopy (TEM) Analysis. The sample was cut into ultrathin sections of 100 nm in thickness by 10 MPa Ultracut UC6 type of Ultramicrome (Leica Company, Germany). These ultrathin sections were stained using the vapor of RuO4(aq). The TEM analysis for D/ P physical gel was observed by a JEM-2010 HT transmission electron microscope (Electronics Corporation, Japan) with maximum accelerating voltage of 200 kV. 2.6. Differential Scanning Calorimetry (DSC) Analysis. The thermal characteristics and crystallization behavior of different PLA samples were studied with a differential scanning calorimeter (DSC, A Perkin-Elmer PYRIS) in a nitrogen atmosphere. About 6 mg samples were weighed for DSC tests. For the first scan, it was heated from 25 to 180 °C at the heating rate of 10 °C/min, and held for 3 min to eliminate thermal history. Then it was cooled to 25 °C at the same speed and held for 3 min. Finally, the sample was heated again to 180 °C at 10 °C/min. The crystallization temperature (Tc), the glass transition temperature (Tg), and the melting temperature (Tm) of neat PLA and PLA blend were measured in the cooling scan and the second heating scan. For DBS/PEG gel samples,

growth rate of PLLA. However, there is no relevant investigation of the crystallization of DBS/PLA blends produced by directly melt blending because the melting temperature of DBS (220−225 °C) is too high for melt processing of PLA. Melt mixing at that high temperature will result in serious degradation of PLA.28 The third approach utilizes both organic and inorganic materials as nucleating agents. The most frequently used combination was the plasticizer plus the nanoparticles, which are often used in PLA with a high D-lactide content (>4%). However, simple mixing of PEG with inorganic materials like clay29 and carbon black30 does not show great cooperative effect on crystallization. Another approach is to mix PLLA with PDLA to form stereocomplexes, which can easily crystallize and have a much higher melting point.31 In most of these efforts, only cold crystallization is affected and crystallization during cooling from melt is still difficult. In this work, we will suggest a new method to promote the crystallization of PLA. A combination of plasticizer (PEG) and organic nucleating agent (DBS) will be adopted. It will be demonstrated that the protocol to mix these additives into PLA is crucial for the crystallization of PLA. Moreover, we will illustrate that proper modification of the PLA chain using a mutilfunctional monomer helps to further speed up the crystallization of PLA. The mechanism of crystallization and the synergistic effect of plasticizer, nucleating agent, and chain topology will be studied in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. PLA was a Nature Works product (2002D). The content of L-lactide was about 96 wt % and the monomer was less than 0.3 wt %. The antioxidant Irganox 1330 (1,3,5tris(3,5-ditert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene) was provided by Ciba, Switzerland. DBS was obtained as a powder from Milliken Chemicals (Spartanburg, SC). PEG homopolymer with the number average molecular weight as 10000 g/mol and talc (3000 mesh) were received from Shanghai Chemicals Factory, P. R. China. Pentaerythritol triacrylate (PETA) was obtained from Tianjin Kermel Chemical Reagent Co., Ltd., P. R. China. All were used as received without any further treatment. Dicumyl peroxide (DCP) was received from Shanghai Chemicals Factory, P. R. which must be purified and recrystallized at least three times at room temperature. 2.2. Preparation of Physical Gel. A series of DBS/PEG (D/P) physical gel samples was prepared by melt mixing in a hot three-neck flask using DBS powder and PEG in nitrogen atmosphere at 230 °C. The rotor speed was set as 200 rpm for 30 min. The physical gels occurred during cooling to room temperature. The samples were dried at 110 °C for one week in vacuum before use. The detailed compositions are listed in Table 1. 2.3. Preparation of LCB PLA. PLA was dried at 45 °C for at least 24 h in vacuum before mixing. Grafted PLA samples were prepared by direct melt radicals reaction in a RheomixTable 1. Compositions of Different DBS/PEG Gels sample

DBS (%)

PEG (%)

temperature (°C)

D/P1 D/P2 D/P3

15 10 5

85 90 95

230 230 230 1098

dx.doi.org/10.1021/ie402358h | Ind. Eng. Chem. Res. 2014, 53, 1097−1107


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Table 2. Composition of Samples sample

PLA (%)

DBS (%)

PEG (%)

D/P1 (%)

D/P2 (%)

D/P3 (%)

talc (%)

temperature (°C)

P-talc P-PEG P-DBS P-D-P1 P-D/P1 P-D/P1L P-D/P1h P-D/P2 P-D/P3 LCBP-D/P1

98.5 91.5 98.5 90 90 90 90 90 90 90

− − 1.5 1.5 − − − − − −

− 8.5 − 8.5 − − − − − −

− − − − 10 10 10 − − 10

− − − − − − − 10 − −

− − − − − − − − 10 −

1.5 − − − − − − − − −

180 180 230 180 180 150 230 180 180 180

Figure 1. TEM images for PEG/DBS gels D/P1 (a), D/P2 (b), and D/P3 (c).

the melting temperature (Tm) was observed in the first heating scan. For isothermal crystallization, the sample was heated from 25 to 180 °C at 10 °C/min and held for 3 min. Then it was cooled at 100 °C/min to a certain temperature (110−130 °C in this work) for crystallization. 2.7. Polarized Optical Microscopy (POM) Analysis. A polarizing optical microscope (Leica DMLP, LECIA Corp., Germany) equipped with a programming temperature controller (TMS 94, Linkam Scientific Instruments Ltd., U.K.) and a Sony digital camera was used to investigate crystal morphologies of PLA samples. PLA samples were placed between two microscope cover glasses. Then it was heated at a rate of 50 °C/min to 180 °C from room temperature, and held at 180 °C for 5 min. Then it was rapidly cooled down to a preset crystallization temperature between 110 and 130 °C. The growth of spherulites were observed and recorded. 2.8. Wide-Angle X-ray Diffraction (WAXD) Analysis. WAXD analysis for neat PLA and PLA blend were obtained using an X-ray diffraction meter (Rigaku, Japan) by Cu Kα radiation (λ = 0.154 nm) at a voltage of 80 kV and a current of 20 mA. The neat PLA was annealed for 1 h at 110 °C before analysis. The PLA samples were scanned at room temperature at the scanning speed of 4°/min with the diffraction angle (2θ) in the range of 5°−40°.

observed during the same heat treatment for neat DBS, minor effect of acceleration in PLLA crystallization could be found. In this work, we would prove DBS could also be an effective nucleating agent for PLA when the samples were properly prepared. Actually, it has been reported that DBS could selfassemble in different solvents including PEG to form fibrils.32 The formed structure depends greatly on the polarity of the solvent. Figure 1 shows the nanofibril morphologies of the DBS/PEG (D/P) gel systems formed by DBS self-organization observed by transmission electron microscopy. It was obvious that there were more nanofibrils in D/P1 than in the other two because of the difference in DBS concentration. The diameter of nanofibrils is between 30 and 100 nm, which was much larger than the diameter of a single fibril (typical less than 20 nm). It implied that those observed by TEM in the three gels were aggregations of fibrils. This was consistent with our recent observations that the DBS concentrations in this work were much larger than the gel−cluster transition at about 3.5% in PEG.32 Figure 2 shows the melting behavior of D/P1, D/P2, and D/ P3 gel samples at the scanning rate of 10 °C/min. For D/P1, D/P2, and D/P3, the peaks of the melting temperature (Tm) were 163, 160, and 155 °C, respectively. Moreover, the end of melting temperatures were 171, 166, and 159 °C for the three gels, respectively. It was clear that the melting point (Tm) was influenced by the content of DBS in the D/P system, where more DBS resulted in a higher melting point because of thicker nanofibrils by DBS self-organization. 3.2. Nonisothermal Crystallization of Linear PLA with PEG/DBS Gel As Nucleating Agent. The premade DBS/ PEG gel was melt mixed with PLA in a batch mixer. Figure 3 showes the thermal behavior of P-D/P1 blend at the scanning rate of 10 °C/min. After melt mixing of PLA with P/D1 gel at 180 °C, an obvious exothermic peak on the cooling curve

3. RESULTS AND DISCUSSION 3.1. Morphology and Thermal Properties of DBS/PEG Gel. DBS has been found to be a good nucleating agent in polymers like polypropylene (PP)22−24 and polybutylene terephthalate (PBT).25 The mechanism of nucleation was the formation of DBS nanofibrils in these polymer melts, which acted as heterogeneous nucleating sites for crystallization. Lai et al. also tried to use DBS in PLLA by solution casting to assist crystallization.26,27 However, because no DBS fibrils were 1099



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Figure 2. Thermogram of D/P1, D/P2, and D/P3 samples at 10 °C/ min scanning rate.

Figure 4. Cooling (a) and second heating (b) curves of PLA blends samples at10 °C/min scanning rate.

The crystallization temperatures (Tcc and Tc for cold crystallization temperature during heating and melt crystallization temperature during cooling, respectively), and the melting temperatures (Tm1 and Tm2) for different samples are listed in Table 3. For the blends of PLA and PEG/DBS gel, the melt crystallization temperature (Tc) increases as the DBS content in the gel increases. Moreover, both the hightemperature melting point (148−151 °C) and the lowtemperature melting point (139−143 °C) increased slightly with DBS content, and the low-temperature melting peak became weaker as DBS content decreased. Such differences were related to the increasing content of PEG from D/P1 to D/P3. Our observations were consistent with the effect of PEG on the crystallization of PLA,33 where increment of plasticizer caused a weak decrease in the high-temperature melting point (Tm2), an evident decrease in low-temperature melting point (Tm1), and a smaller low-temperature melting peak. The degrees of crystallinity (Xc) of PLA samples are also shown in Table 3. Compared to the neat PLA, the total degree of crystallinity all increased after the addition of nucleating agent, especially in the PEG/DBS system. Similarly, the crystallinity decreases as the DBS content in the gel decreases. The half-crystallization times for PLA with different DBS/ PEG gels during cooling are also shown in Table 3. In contrast to the higher crystallinity in blend with more DBS (D/P1), the half-crystallization time was shorter in blend with more PEG (D/P3). The different trend manifests the different role of DBS and PEG in the crystallization. Because both components in gel could influence the crystallization of PLA, questions arise on their respective roles when the gel was added as a nucleating agent. Therefore, it was necessary to separate the effect of PEG

Figure 3. Thermogram of P-D/P1 samples at 10 °C/min scanning rate.

appeared. It was known that the crystallization behavior of PLA was greatly dependent on the content of D-lactide. Crystallization in the cooling process was usually difficult to observe for PLA with high D-lactide content under fast cooling rate. There was no exothermic peak in pure PLA during cooling under the same conditions. The appearance of melt crystallization was apparently the nucleating effect induced by PEG/DBS gel. A comparison between the crystallization behaviors of PLA with different D/P gel systems and talc as nucleating agent is shown in Figure 4. For P-D/P1, P-D/P2, and P-D/P3 systems, the content of D/P1, D/P2, and D/P3 gel was the same (10%), but the content of DBS in the blend was 1.5%, 1%, and 0.5%, respectively. For all series of P-D/P samples, the thermal flow curves were similar. There was an exothermic peak in the cooling curve, and multiple melting peaks in the heating curve were observed. Cold crystallization during heating was hardly observed, which implies that the crystallization almost completes during cooling. Meanwhile, the effect of a frequently used nucleating agent, talc, on the crystallization of PLA was compared. It was seen that talc cannot promote the crystallization of PLA when cooling from melt, but could induce significant cold crystallization as compared to neat PLA during heating. This means that nucleating efficiency of talc was not high enough to induce crystallization of PLA during cooling. Such comparisons imply that the PEG/DBS gel system was a more efficient nucleating agent than talc for PLA. 1100



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Table 3. Nonisothermal Behaviors of PLA Blends at the 10 °C/min Heating and Cooling Rate cooling process

a b

heating process

samples

Tca(°C)

Tc, onseta(°C)

ΔHc(J/g)

t1/2(min)

Xcc(%)

Tm1(°C)

Tm2(°C)

Tcca(°C)

ΔHcc(J/g)

ΔHm (J/g)

Xcb(%)

PLA P-talc P-D/P1 P-D/P2 P-D/P3 LCB P-D/P1

− − 117 115 115 123

− − 133 131 131 131

− − 24.9 18.2 11.6 28.6

− − 1.87 1.53 1.37 0.85

− − 26.6 19.4 12.4 30.5

− − 143 142 139 −

153 150 151 149 148 152

114 122 − − − −

0.6 13.1 − − − −

1.2 21.4 26.4 20.0 13.4 31.7

0.6 8.9 − − − −

Tcc is the peak cold crystallization temperature. Tc is the peak melt crystallization temperature. Tc, onset is the onset melt crystallization temperature. Xc (%) = 100 × (−ΔHcc + ΔHm)/ΔHm0 with ΔHm0 = 93.7 J/g 33 for PLA. cXc (%) = 100 × ΔHc/ΔHm0

and DBS on the crystallization of PLA. Thermal behaviors of PD/P1, P-DBS, and P-PEG blend are shown in Figure 5a. The

contents of DBS in P-DBS blend and PEG in P-PEG blend were the same as that in P-D/P1 blend. The melt mixing condition was also the same as that of P-D/P1. A summary of crystallization temperatures, melting temperatures, and crystallinity is listed in Table 4. In the PLA-PEG system, as reported in the literature,33 there was no exothermic peaks on the cooling curve, but an evident cold crystallization peak appeared on the heating curve. It means that PEG alone as a plasticizer could speed up the crystallization of PLA by altering the chain mobility. However, crystallization rate of PLA in PEG systems was so slow that there was not enough time to form crystals in the cooling process. For P-DBS blend, there was a small, wide exothermic peak on the cooling curve which was similar to that of P-D/P1 blend. However, the peak crystallization temperature (Tc) of P-DBS was 103 °C, which was much lower than that of the P-D/P1 system. The degree of crystallization (Xc) is 8.4%, which was also much smaller than that of the P-D/P1 system. This means that the nucleating efficiency of pure DBS was much lower than that of PEG/DBS gel. The difference was related to the microstructures of DBS. It was known that DBS could self-assemble into fibrils and exhibit large-scale structural elements during cooling from the melt. It was observed that the average fibril diameter of pure DBS was about 1.4 μm, which was actually large bundles of nanofibrils.32 For PLA-DBS blend, the mixing temperature (180 °C) was not high enough to melt all the DBS fibrils. Melt mixing only breaks the DBS fibrils into short fibrils. The micrometer-scale DBS fibrils did not have sufficient interface to induce the nucleation of PLA, which results in the low efficiency of neat DBS. In contrast, DBS could form nanofibrils in PEG (Figure 2), which were expected to be much more effective in nucleation if they were well-kept during mixing. In fact, if PEG and DBS were mixed with PLA directly (P-DP1 sample), a completely different thermal behavior would be observed as compared to the P-D/P1 sample with premade DBS gel. The contents of DBS and PEG in P-D-P1 were the same as those in P-D/P1. The thermal behavior of P-D-P1 at the scanning rate of 10 °C/min is also shown in Figure 5a. For

Figure 5. The curves of different samples compared with P-D/P1 at 10 °C/min scanning rate: (a) P-D/P1, P-D/P1, P-PEG, and P-DBS; (b) P-D/P1, P-D/P1h, and P-D/P1L. Solid lines denote cooling processes, and dashed lines denote heating processes.

Table 4. Results from the Thermal Analysis of the P-D/P1, P-PEG, and P-DBS Samples cooling process

a

heating process

samples

Tc (°C)

Tc, onset(°C)

ΔHc(J/g)

Xcb(%)

Tm1(°C)

Tm2(°C)

Tcc(°C)

ΔHcc(J/g)

ΔHm (J/g)

Xca(%)

P-PEG P-D/P1 P-DBS P-D/P1L P-D/P1h

− 117 103 115 99

− 133 119 127 113

− 24.9 7.9 25.7 −

− 26.6 8.4 27.4 −

− 143 140 139 142

146 151 150 151 150

116 − − − 101

17.5 − − − 14.4

22.8 26.4 16.6 27.7 19.7

5.6 − − − 5.6

Xc (%) = 100 × (−ΔHcc + ΔHm)/ΔHm0, and ΔHm0 = 93.7 J/g bXc (%) = 100 × ΔHc/ΔHm0 1101



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of starlike or other long-chain branching PLA where the branch points of LCB-PLAs could play the role of nucleating agent. Considering that the self-assemble of DBS was also driven by strong hydrogen bonding, it was expected that its nucleating effect for PLA would be even better in branched PLA. The nonisothermal thermal flow under cooling and second heating with 10 °C/min for LCB P-D/P1 are also shown in Figure 4, and the transition temperatures and crystallinity are shown in Table 3. As expected, a strong and narrow exothermic peak was seen in the cooling curve. Although the crystallization onset temperature was close to that of linear PLA, the halfcrystallization time under nonisothermal condition (10 °C/ min) of LCB sample was less than 1 min, which was only half that of the linear sample (about 2 min). The crystallinity of the LCB sample was also higher than that of the linear sample. Moreover, only one melting peak was observed. The synergistic contributions due to nucleating effect of DBS nanofibrils, the plasticizing effect of PEG, and the hydrogen bonding interactions among LCB PLA and DBS nanofibrils caused the accelerated crystallization of PLA. 3.4. Crystallization Behaviors. The crystallization behaviors of PLA blends under nonisothermal and isothermal conditions were studied. Some studies suggested that scanning rate affected the nonisothermal thermal behaviors of PLA.39 Understanding the crystallization behaviors under different cooling rates, especially at high cooling rates, would help to guide the choice of processing conditions. Therefore, both PD/P1 and LCB P-D/P1 samples were scanned at different cooling rates and the corresponding second heating rates at 10 °C/min (Figure 6). It was found that the peak melt crystallization temperature Tc decreases when the cooling rate increases for both linear PLA blend and LCB PLA blend. It was interesting to find that the crystallization of PLA still happened even under cooling rate as fast as 40 °C/min. However, the effect of cooling rate on the melt crystallization temperature was different for linear PLA blend and LCB PLA blend. The dependence of Tc on the cooling rate is shown in Figure 7. Tc of linear PLA blend decreased from 121 °C under 5 °C/min to 102 °C under 40 °C/min, and the decrement was nearly 20 °C. In contrast, Tc of LCB PLA blend decreased from 124 °C under 5 °C/min to 116 °C under 40 °C/min, and the decrement was only 8 °C. It was obvious that the crystallization of LCB PLA blend was less sensitive to the cooling rate as compared to linear PLA blend. The crystallinity of the linear PLA system and LCB PLA system also decreased with the increase of cooling rate, and the trends in both blends were similar. Moreover, the melting behaviors after cooling were also dependent on the cooling rate. At slower cooling rates, the melting behavior was almost unaffected in both the linear PLA system and LCB PLA system. As the cooling rate increased, there appeared cold crystallization during the subsequent heating, and the melting temperature decreased greatly. This was ascribed to the incomplete crystallization during fast cooling. Once again, the melting temperature in the LCB PLA system was less affected by the cooling rate as compared to the linear PLA system. This manifested that the extra interactions between the PETA grafted LCB PLA and DBS fibrils were especially helpful for crystallization under fast cooling conditions. The kinetics of isothermal crystallization of PLA samples was also studied by DSC. The samples were cooled rapidly from 180 °C to the crystallization temperature. The relative crystallinity Xt at different crystallization times could be obtained from the integration of the heat flow. In this study,

P-D-P1, no exothermic peak appeared on the cooling curve, whereas an exothermic peak appeared on the heating curves. This was similar to those of P-PEG blend. Moreover, there appeared two melting peaks, which resembled that in the PDBS system. Therefore, for P-D-P1, plasticizers PEG and DBS microfibrils affected the crystallization independently. The addition of PEG in the PLA-DBS system did not help DBS form nanofibrils in PLA. Therefore, the self-assembly of DBS in the preformed PEG/DBS gel was a crucial step. The final question was whether DBS nanofibrils remain during melt mixing of PLA with PEG/DBS gel. The melting process of PEG/DBS gel (D/P1) starts from around 140 to 171 °C with the peak at 163 °C (Figure 2). The wide range of melting implies the wide distribution of nanofibril diameters. The mixing temperature was set as 180 °C, which was higher than the end of the melting point (171 °C) of D/P1 gel (Figure 2). However, the fast crystallization of PLA in the P-D/P1 system implied the presence of DBS nanofibrils under temperatures not far from the nominal melting point of PLA/DBS gel. To justify such conjecture, two additional P-D/ P1 blends were prepared under different mixing temperatures. They were denoted as P-D/P1L and P-D/P1h, which were melt mixed at 150 and 230 °C, respectively. 230 °C is higher than the melting temperature of DBS (220−225 °C), and all the nanofibrils either in DBS or in PEG/DBS gel disappear under such temperature. Compared with the 163 °C melting temperature of D/P1 gel, the nanofibrils in D/P1 gel should not be destroyed at the lower mixing temperature of 150 °C, while they could be destroyed completely at the higher mixing temperature 230 °C. Figure 5b shows the thermal flow curves of the three P-D/P1 blends. It was found that the crystallization and melting behaviors of P-D/P1L are almost the same as those of P-D/P1 blend, but the behaviors of P-D/P1h are quite different. These experimental results are also shown in Table 4. For P-D/P1L, crystallization temperature and melting temperature were close to those of P-D/P1, while the degree of crystallinity (Xc) increased slightly. For the P-D/P1h sample, there was a weak crystallization peak on the cooling curve but evident cold crystallization appeared on the second heating curve. The crystallinity of PLA in P-D/P1h was even lower than that of the P-DBS system, and the crystallization temperature during cooling was also lower. Considering the nucleating effect of DBS fibrils, the amount of effective DBS fibrils in P-DBS system was expected to be higher than that in the P-D/P1h system. This implied that the self-assembly of DBS in PLA into nanofibrils was quite difficult even in the presence of PEG. Therefore, the formation of DBS nanofibrils was the key factor for nucleation, and complete destruction of DBS nanofibrils requires mixing under temperatures much higher than the DSC melting point of PEG/DBS gel. 3.3. Nonisothermal Crystallization of LCB PLA with PEG/DBS Gel As Nucleating Agent. We have found pronounced cold crystallization in long-chain branching PLA prepared by reactive mixing through radical reaction with a multifunctional monomer (PETA).16 Unlike the usual LCB polymers, where the crystallization would be hindered because of the decreased mobility of branched chain, long-chain branching induced by PETA could speed up crystallization in polymers like PLA and polypropylene.2,34,35 This was ascribed to the possible aggregates formed because of the strong hydrogen bonding interaction between PETA molecules, which could be effective nucleating points for crystallization. In fact, some researchers17,36−38 also reported improved crystallization 1102



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associated with both nucleation and growth contributions. The Avrami parameters n and crystallization rate k could be obtained from the plots of ln[−ln(1 − Xt)] versus ln t, and the data are listed in Table 5. The data for P-PEG and P-talc Table 5. Avrami Parameters from the Thermal Analysis of PD/P Blends, P-talc and P-PEG Systems at Different Isothermal Crystallization Temperatures sample PLAa

P-D/P1

P-D/P2

P-D/P3

LCB P-D/P1

P-talc

Figure 6. The cooling curves (solid lines) at different scanning rate and the corresponding second heating curves (dashed lines) at 10 °C/ min for (a) P-D/P1 and (b) LCB P-D/P1.

P-PEG

Tc (°C)

n

K(T) (×105)

t1/2 (min)

110 115 120 125 110 115 120 125 110 115 120 125 110 115 120 125 125 128 130 132 110 115 120 110 115 120

4.5 4.5 4.5 4.5 3.6 3.9 3.5 3.5 3.5 3.5 3.8 3.6 3.5 3.6 3.3 3.5 3.4 3.3 3.5 3.3 4.3 4.1 4.2 3.0 3.0 3.0

30.9 13.8 4.17 0.59 77 566 29 221 800 246 1 842 297 18.6 10.2 898 155 13.6 3.96 49 772 11 851 3 662 638 84.3 194.6 285.3 99.4 30.7 67.4

5.66 6.70 8.75 13.55 0.97 1.25 3.64 5.01 2.80 4.85 8.81 11.60 3.52 5.57 13.70 16.30 0.56 0.85 1.20 2.06 4.64 4.10 3.67 8.76 12.80 10.25

a

The data are from isothermal cold crystallization by heating neat PLA rapidly from room temperature to the crystallization temperature Tc.

systems are also shown. The half-crystallization times of PLA in the presence of DBS/PEG gels were much smaller than that of pure PLA and PLA with PEG or talc, which illustrated the significant acceleration of crystallization by DBS nanofibrils. Moreover, an evident decrease of the half-crystallization time (t1/2) was observed in P-D/P1 as compared to P-D/P3, while t1/2 of P-D/P2 was between that of P-D/P1 and P-D/P3. The half-crystallization time was even smaller in the LCB PLA system filled with DBS/PEG gel. t1/2 was about 0.56 min at 125 °C, which was sufficiently fast for typical polymer processing. The average value of the Avrami exponent n was around 4.5 for neat PLA polymer, which was similar to those reported in the literature.18 For P-D/P systems, the Avrami exponent n became smaller, ranging from 3.3 to 3.6 in both linear PLA system and LCB PLA systems. The decrease of the Avrami exponent n upon adding DBS/PEG gel also manifested the heterogeneous nucleation effect due to DBS nanofibrils. Compared to the PLA blends, the value of n for the LCB PLA system was smaller with the existence of long-chain branched structure because the hydrogen bonding interactions among LCB PLA and DBS nanofibrils further improved the nucleation density. For the value ofthe crystallization rate constant k, it decreased with an increase in crystallization temperatures as shown in Table 5. Moreover, compared to P-D/P3 polymer, the value of k for P-

Figure 7. Melt crystallization temperature and melting temperature for P-D/P1 (solid symbols) and LCB P-D/P1 (empty symbols) system under different cooling rates.

there was no isothermal crystallization for neat PLA when it was cooled from melt in 30 min. The isothermal crystallization of pure PLA was performed by heating the sample from room temperature to the crystallization temperature. The Avrami equation was used to analyze the isothermal crystallization kinetics of PLA samples, which could be expressed as 1 − X t = exp( −kt n)

(1)

where Xt is the relative degree of crystallinity at time t, the exponent n a constant depending on the type of nucleation and the growth dimension, and k the crystallization rate constant 1103



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Figure 8. The morphology of spherulites at different times for neat PLA (A), P-D/P1 (B), P-D/P2 (C), P-D/P3 (D), LCB P-D/P1 (E), P-PEG (F), and P-talc (G) crystallized isothermally at 130 °C.

DBS/PEG gel significantly enhanced the crystallization. The difference in the kinetics of crystallization in PLA-P/D systems was definitely ascribed to the morphology and the amount of DBS nanofibrils as well as the amount of PEG in DBS/PEG gel.

D/P1 was greatly increased (∼100 times) at the same crystallization temperatures, while that of P-D/P2 was slightly increased (∼2 times), which means that the P-D/P1 had the highest crystallization rate. This indicated that the additional 1104



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synergistic effect between DBS nanofibrils and long-chain branching PLA. To exclude the possibility that the speedup in crystallization came from the change in the crystal forms, wide-angle X-ray diffraction (WAXD) was used to investigate the crystalline structure of PLA samples. For PLA systems filled with DBS/ PEG gels, the diffraction patterns were obtained from the samples that were quenched directly from the melt state, while for neat PLA, it was obtained from the sample that had been annealed for one hour at 110 °C. The results of WAXD patterns are shown in Figure 9. It was clear that PLA systems

There were two factors affecting the polymer crystallization. One was the nucleation density of spherulites and the other was the spherulitic growth rate. The half-crystallization time and the crystallization rate constant k gave only an overall evaluation of the crystallization speed. The spherulitic morphologies of the neat PLA, P-D/P1, P-D/P2, and P-D/P3 during isothermal crystallization at 130 °C were studied by polarized optical microscopy and are shown in Figure 8. The data for P-PEG and P-talc systems are also shown. First, it was obvious that there were more spherulites in PLA with DBS/PEG gel than in neat PLA. Specifically, after isothermal crystallization at 130 °C for 10 min, the nucleation density of P-D/P1 was about 1.9 × 106 cm−3, which was about 37 times greater than that of neat PLA (5.2 × 104 cm−3), while that of P-D/P2 was about 7.3 × 105 cm−3 and that of P-D/P3 was more than 3.6 × 105 cm−3. The difference in the nucleation density in PLA blends with different DBS/PEG gels was correlated with the amount of DBS nanofibrils. On the other hand, the nucleation density of LCB PLA-D/P1 was about 4.0 × 106 cm−3 under the same conditions, which was the highest in all samples. Second, the spherulitic growth rate was calculated from the time-dependent size of the spherulites. The spherulites of all samples showed a linear growth in the diameter with time. The spherulitic growth rate of PLA was 1.08 μm/min, while that for P-D/P1, P-D/P2, and P-D/P3 was 2.14, 2.29, and 2.56 μm/min, respectively. Apparently, adding DBS/PEG gel doubled the growth rate of spherulites in PLA, which was ascribed to the plasticizing effect of PEG in the gel. The spherulitic growth rate of PLA with gel is even higher than that of PLA/PEG blend (1.40 μm/min). The difference in the amount of PEG in DBS/PEG gel also explained the slight difference in the growth rate of spherulites in PLA blends filled with three different kinds of gel. In contrast, the growth rate of PLA spherulites in the LCB PLA-D/P1 system was about 1.50 μm/min, which was much smaller than that in the linear PLA system with the same DBS/PEG gel. The increased molecular weight and the long-chain branched structure actually decreased the chain mobility, which was unfavorable for the growth of spherulites. In summary, three kinds of comparisons have been made to elucidate the synergistic nucleating effects in PLA blends. The first comparison is shown in Figure 4, where different PLA samples were prepared under the same processing conditions. It shows that PEG/DBS gel can accelerate the crystallization of both linear PLA and LCB PLA as compared to the pure PLA and the frequently used nucleating agent (talc). Moreover, the nucleating effect is more obvious in linear PLA when there is more DBS in the gel. The nucleating effect of PEG/DBS gel in LCB PLA is even more significant than that in linear PLA. Second, several samples with different processing conditions were selected to elucidate the nucleating mechanism of PEG/ DBS gel. It is shown that PEG or DBS alone cannot induce evident melt crystallization (Figure 5a). Moreover, if PEG and DBS were added into PLA without forming a gel, there is still no melt crystallization during cooling (Figure 5b). Furthermore, the PEG/DBS gel fails to act as a nucleating agent when the processing temperature is as high as 230 °C, where selfassembled DBS nanofibrils melt during processing (Figure 5c). From these efforts, we can conclude that it is the DBS nanofibrils that act as the nucleating agent in PLA. Lastly, different nucleating effects of PEG/DBS gel in linear PLA and LCB PLA are compared (Figures 6 and 7). The increasing discrepancy as the cooling rate increases manifests the

Figure 9. WAXD patterns for different PLA samples.

filled with DBS/PEG gels exhibit strong diffraction peaks after direct cooling, which implied that PLA in these systems were well-crystallized even under quick quench. The peaks appeared at the 2θ values of 14.8°, 16.7°, 19.1°, and 22.3°, which were ascribed to the (010), (110/200), (203), and (015) reflection, respectively. These peaks at 2θ = 16.7° and 22.3° are the characteristic peaks of α-form PLA, and 2θ = 14.8° and 19.2° are the characteristic peak of β-form crystal.40 For neat PLA samples, the diffraction peaks were relatively weak even after annealing at 110 °C for 1 h. Only the (110/200) peak and the (203) peak were seen. Considering the difference in the crystallinity, it was believed that the crystal form of PLA did not change after adding DBS/PEG gel as nucleating agent. Therefore, the acceleration of crystallization upon adding DBS/PEG gel was the cooperative effect of DBS nanofibrils and PEG. The nucleating effect of DBS nanofibrils would increase the nucleation density substantially, whereas the plasticizing effect of PEG would increase the chain mobility, which favors the growth of spherulites. Both effects would contribute to the acceleration of the overall speed of crystallization. Considering DBS/PEG gels with different compositions, increase in the DBS content led to higher nucleation density but lower spherulite growth rate. It was apparent that the nucleating effect of DBS nanofibrils was dominant because the half-crystallization time for D/P1 gel-filled PLA was the shortest among the three gel systems. The even faster crystallization in LCB PLA-D/P1 system also manifested the importance of nucleation.

4. CONCLUSIONS Crystallization of PLA with high D-lactide content was very slow, which hinders its wide application. In this work, we suggested a new method to accelerate the crystallization of PLA. Evident crystallization during cooling even under high 1105



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cooling rate (40 °C/min) could be observed, and the halfcrystallization time at 125 °C was found to be about 0.5 min. The key point was the acceleration of nucleation by a nucleating agent, DBS/PEG gel. It was proven that the DBS nanofibrils formed in the gel, which could supply active nucleating sites for PLA when the gel is added in PLA. The gel needs to be prepared separately because DBS molecules are difficult to self-assemble directly to form nanofibrils in PLA melt. PEG in the gel, on the other hand, could act as a plasticizer to enhance the chain mobility of PLA. Further modification of PLA to form a long-chain branching structure with multifunctional monomer PETA could introduce extra interaction between PLA molecules and DBS nanofibrils, which makes the nucleation easier and faster than linear PLA system. Such methods could be readily implemented in the processing of PLA.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86 21 54743275. Fax: 86 21 54741297. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Basic Research Program of China (973Program) 2012CB025901 and the National Natural Science Foundation of China (21074072) for support. W.Y. is supported by the Program for New Century Excellent Talents in University and the SMC project of Shanghai Jiao Tong University.

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Accelerated Crystallization of Poly(lactic acid): Synergistic Effect of

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