Shear Flow and Carbon Nanotubes Synergistically Induced

Oct 16, 2012 - I&EC Process Design and Development · - I&EC Fundamentals .... Department of Chemistry, Stony Brook University, Stony Brook, New ... In...
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Shear Flow and Carbon Nanotubes Synergistically Induced Nonisothermal Crystallization of Poly(lactic acid) and Its Application in Injection Molding Hu Tang,† Jing-Bin Chen,† Yan Wang,† Jia-Zhuang Xu,† Benjamin S. Hsiao,‡ Gan-Ji Zhong,*,† and Zhong-Ming Li*,† †

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China ‡ Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States ABSTRACT: The effect of shear flow and carbon nanotubes (CNTs), separately and together, on nonisothermal crystallization of poly(lactic acid) (PLA) at a relatively large cooling rate was investigated by time-resolved synchrotron wide-angle X-ray diffraction (WAXD) and polarized optical microscope (POM). Unlike flexible-chain polymers such as polyethylene, and so on, whose crystallization kinetics are significantly accelerated by shear flow, neat PLA only exhibits an increase in onset crystallization temperature after experiencing a shear rate of 30 s−1, whereas both the nucleation density and ultimate crystallinity are not changed too much because PLA chains are intrinsically semirigid and have relatively short length. The breaking down of shear-induced nuclei into point-like precursors (or random coil) probably becomes increasingly active after shear stops. Very interestingly, a marked synergistic effect of shear flow and CNTs exists in enhancing crystallization of PLA, leading to a remarkable increase of nucleation density in PLA/CNT nanocomposite. This synergistic effect is ascribed to extra nuclei, which are formed by the anchoring effect of CNTs’ surfaces on the shear-induced nuclei and suppressing effect of CNTs on the relaxation of the shear-induced nuclei. Further, this interesting finding was deliberately applied to injection molding, aiming to improve the crystallinity of PLA products. As expected, a remarkable high crystallinity in the injection-molded PLA part has been achieved successfully by the combination of shear flow and CNTs, which offers a new method to fabricate PLA products with high crystallinity for specific applications.

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temperature of 115 °C.11 The onset crystallization temperature during nonisothermal crystallization of PLA can also be largely increased from 95 to 131 °C at a cooling rate of 5 °C/min.12 However, the actual processing of PLA is usually a nonisothermal process where an extremely large cooling rate exists (dozens or hundreds of °C/min) in order to improve production efficiency. At such cooling rate, addition of CNTs alone is not effective enough to achieve high crystallinity of PLA. Our previous study showed under quiescent nonisothermal conditions, even at a moderate cooling rate of 10 °C/min, only a crystallinity of less than 2% was obtained in PLA with addition of CNTs alone.11 An alternative approach to enhance crystallization is shear flow, which exists inevitably in polymer processing (e.g., extrusion, injection and blowing molding). The well-known flow-induced crystallization is evidenced to take place in various semicrystalline polymers such as polyethylene (PE) and polypropylene.15−17 A shear flow field can make polymer chains orient along the flow direction, resulting in plenty of row

INTRODUCTION Poly(lactic acid) (PLA) exhibits excellent performance in biocompatibility, biodegradability, renewability, and mechanical properties, thus allowing for wide potential applications in biomedical, agricultural, and packaging areas.1−4 Unfortunately, owing to its intrinsically slow crystallization kinetics, PLA products are usually amorphous, especially under actual processing conditions such as conventional injection molding and extrusion, where a large cooling rate exists, leading to some undesirable properties such as poor barrier property and thermal resistance.5,6 Usually, a postannealing process is employed to elevate crystallinity of PLA as high crystallinity is required,7,8 which, however, prolongs the processing cycle and thus increases the cost of production. Therefore, there still remains a huge challenge to promote crystallization of PLA under actual processing conditions. To enhance crystallization of PLA, nucleating agents are usually employed, among which carbon nanotubes (CNTs) are evidenced to be very effective.9−14 Our previous study indicated that CNTs have a strong nucleation effect on PLA even at an extremely low concentration of 0.08 wt %, where the half crystallization time of isothermal crystallization (t0.5) is greatly decreased from 23 to 7.2 min at the isothermal crystallization © 2012 American Chemical Society

Received: August 28, 2012 Revised: October 9, 2012 Published: October 16, 2012 3858

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Chinese Academy of Sciences R&D Center for Carbon Nanotubes (China). Ingeo 4032D PLA, a semicrystallinegrade PLA containing around 2% D-LA, was supplied by NatureWorks LLC (America). Its weight-averaged molecular weight and polydispersity are 9.96 × 104 g/mol and 1.67, respectively. Anhydrous ethanol (C2H5OH, AR grade) and dichloromethane (CH2Cl2, AR grade) were purchased from Chengdu Kelong Chemical Reagent Factory (China) and were used as received. Preparation of PLA/CNT Nanocomposite. CNTs and PLA were dried in vacuum before used at 80 °C for 12h and 3h, respectively. PLA masterbatch containing CNTs was prepared through solution coagulation method. The detailed experimental procedures are as follows: CNTs (1 g) were added to C2H5OH (250 mL), and the mixture was ultrasonicated for 1 h to obtain a uniform dispersed suspension. Meanwhile, PLA (19 g) was completely dissolved in CH2Cl2 (250 mL) by stirring for 1 h. By pouring the C2H5OH/CNT suspension into the C2H2Cl2 /PLA solution, coagulated material precipitated continuously. Thereafter, extra C2H5OH was poured into the mixture until no more coagulations precipitated. The coagulated PLA/CNT nanocomposite was transferred to Petri dishes, left overnight at room temperature, and then further dried in a vacuum oven for 3 days at 60 °C to remove residual solvent. The masterbatch was further melt compounded with fresh PLA using a corotating twin screw extruder with a barrel temperature of 170 °C and a rotation speed of 150 rpm to produce PLA/CNT nanocomposite with 0.1 wt % of CNTs. Neat PLA was also prepared under the same conditions for comparison. Characterization of Shear-Induced Nonisothermal Crystallization. Synchrotron WAXD experiments were carried out at the Advanced Polymers Beamline (X27C, λ =0.1371 nm) in the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), in order to investigate the shearinduced nonisothermal crystallization behavior of PLA/CNT nanocomposite. A MAR CCD detector (MAR-USA) with a resolution of 1024 × 1024 pixels (pixel size =158.44 μm) was used to acquire data. The samples for in situ WAXD measurements were prepared by compression-molding the asextruded pellets into 0.5 mm thick plates at 180 °C. A modified Linkam CSS-450 high-temperature shear stage was employed to precisely control shear flow field and thermal history of the samples. The detail experimental procedures were set as follows: (1) heating from room temperature to 200 °C at a rate of 30 °C/min; (2) holding at 200 °C for 180 s to erase any thermal history; (3) cooling down to 40 °C at a rate of 30 °C/ min and, at the same time, a steady shear of 30 s−1 was imposed on the PLA melt for 180 s and WAXD patterns were collected at 20s/frame during the whole nonisothermal crystallization process. Optical microscopy observation was also performed with an Olympus BX51 polarizing optical microscopy (Olympus Co., Tokyo, Japan) equipped with MicroPublisher 3.3 RTV CCD and Linkam CSS-450 high-temperature shear stage to observe the crystalline morphology of neat PLA and PLA/CNT nanocomposite. The gap between the two plates was 10 μm. The experimental procedures including temperature settings and shear profiles were the same as that in the synchrotron WAXD experiments. Injection Molding of PLA/CNT Nanocomposite. The dried material was first melt-injected into a dumbbell mold of the injection molding machine with a temperature profile of

nuclei and thereby enhance crystallization kinetics significantly.18−20 As an environmentally friendly polymer commercialized later, the flow-induced crystallization of PLA has only received limited attention compared with polyolefins such as PE.21,22 Previous work showed that the crystallization behavior of PLA under shear flow depends on the shear rate and crystallization temperature. The shear flow is beneficial for crystallization kinetics and formation of the more ordered αform crystals of PLA. Nevertheless, flow-induced crystallization of PLA was mainly investigated during isothermal crystallization based on a polarized optical microscope (POM) equipped with a shear stage, which could provide limited information directing its actual processing.23,24 The flowinduced nonisothermal crystallization of PLA at a high cooling rate, which is essential for its actual processing, is hardly investigated.25 The combined effect of shear flow and nucleating agents on polymer crystallization is an interesting research subject, since crystallization kinetics can be promoted distinctly by nucleating agents and shear flow individually. An early study by Lagasse et al.,26 a later study by Naudy et al.,27 and a recent study by D’Haese et al.28 reported that the acceleration of the crystallization kinetics with increasing shear rate was less remarkable for polymers containing nucleating agents or particles. These authors concluded that the contribution of nucleating agents or particles and shear flow to nucleation density was additive. By contrast, a synergistic effect existing in a polymer/nucleating agent (or filler) system was suggested by other groups,29−31 which was found experimentally in isotactic polypropylene (iPP)/aramid fiber composites,32 iPP/graphene nanosheet (or CNTs) nanocomposites,33,34 and isotactic poly(1-butene)/CNT nanocomposites.35 Our study on flowinduced crystallization of iPP in the presence of CNTs showed that the crystallization rate was increased about 40 times in comparison to that of quiescently crystallized pure iPP.34 Naturally, an interesting question arises, i.e., does this synergistic effect still work for nonisothermal crystallization of PLA? If so, taking advantage of this synergistic effect, we could enhance the crystallinity of PLA parts produced by actual processing technologies. However, compared to conventional polypropylene with flexible long chains, PLA chains are somewhat rigid, and its molecular weight is usually low, which makes PLA crystallize in quite a different way especially under flow.22 As a kind of biomacromolecule, shear-inducedcrystallization of PLA is a fundamental issue for its processing and applications, which, however, has been rarely studied. In the present work, we attempted to investigate the combined effect of CNTs and shear flow on the nonisothermal crystallization of PLA. Employing time-resolved wide-angle Xray diffraction (WAXD) and POM, we confirmed the synergistic effect of shear flow and CNTs on enhancing crystallization of PLA during nonisothermal crystallization. Furthermore, we applied this interesting synergistic effect to actual injection molding. A remarkably high crystallinity in the injection-molded PLA part has been achieved for the first time by the combination of shear flow and CNTs, which offers a novel approach to fabricate PLA products with high crystallinity for specific applications.

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EXPERIMENTAL SECTION Materials. CNTs are multiwalled carbon nanotubes of outer diameter (o.d.) 20−30 nm and length 10−30 μm, which were purchased from Chengdu Organic Chemicals Co., Ltd., the 3859

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Figure 1. Schematic of the positions of the samples for 2D-WAXD measurements (MD, the molding direction (i.e., flow direction); TD, the transverse direction; ND, the direction normal to the MD−TD plane). The unit is mm.

130−170 °C from hopper to nozzle. Subsequently, oscillation shear with a frequency of 0.3 Hz and pressure of 13 MPa provided by oscillatory shear injection-molding (OSIM) technology was imposed on the melt. The detailed description of OSIM technology was reported elsewhere.36 The primary feature of OSIM is that the hot melt is subjected to high pulse shear stress in the mold, which is provided by two pistons moved reciprocally at the same frequency during the packing stage. Generally, the OSIM could provide a peak shear rate of several reciprocal seconds or hundreds of reciprocal seconds in a single cycle. In this work, the shear rate is about 200 s−1. Conventional injection-molding (CIM) was also carried out under the same processing conditions (i.e., without oscillatory shear) for comparison. Crystal Structure Measurement of Injection-Molded Parts. Two-dimensional WAXD (2D-WAXD) was employed to characterize the crystalline structure of the injection-molded samples. The 2D-WAXD images were collected with an X-ray charge-coupled device (CCD) detector (Model Mar345, MAR Research Co. Ltd., Germany) in the U7B beamline at the National Synchrotron Radiation Laboratory, Hefei, China (wavelength λ=0.14809 nm). The samples for 2D-WAXD measurements were machined from the central part of the dumbbell samples (as shown in Figure 1) into a slice with the thickness of 1 mm and the width of 6 mm. Since injectionmolded samples usually show layered structure,37 the sample was characterized layer by layer along the transverse direction (TD, see Figure 1). The WAXD measurements were taken at the positions of ∼200 μm, ∼1500 μm, and ∼3000 μm away from the surface along the TD direction, representing the skin, intermediate, and core layer of the injection-molded samples, respectively. One-dimensional WAXD (1D-WAXD) profiles were obtained from circularly integrated intensities of 2D-WAXD patterns acquired. Subsequently, through deconvoluting the peaks of 1D-WAXD profiles, the overall crystallinity (Xc) was calculated by Xc =

where Acryst and Aamorp are the fitted areas of crystal and amorphous, respectively.

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∑ Acryst ∑ Acryst + ∑ A amorp

RESULTS

Shear Flow and CNTs Induced Nonisothermal Crystallization of PLA. Thanks to the high time resolution of synchrotron X-ray diffraction, we successfully traced the nonisothermal crystallization process of neat PLA and PLA/ CNT nanocomposite at a relatively high cooling rate of 30 °C/ min. The choice of such a high cooling rate was intended to match, to the greatest degree, the cooling condition in the actual processing. Accordingly, the results obtained would help us better understand the crystallization behavior and morphology formation of the PLA products during actual processing. As shown in Figure 2, the 2D-WAXD patterns at selected temperatures show the development of PLA crystallization with and without shear flow. Under the quiescent condition (Figure 2a), it is hard to observe the crystalline diffractions in the neat PLA due to its intrinsic low crystallization rate, which is in good accordance with the literature.5−8 The addition of CNTs significantly alters the crystallization behavior of PLA. The reflections of PLA crystals start to appear at about 110 °C, indicating that the strong nucleation ability of CNTs is still active at such high cooling rate (Figure 2b). Intriguingly, the onset crystallization temperature is greatly elevated when a shear flow is applied. As seen from Figure 2c,d, clear diffractions in both neat PLA and PLA/CNT nanocomposite emerge at about 150 °C, which is even 40 °C higher than that of the quiescently crystallized PLA/CNT nanocomposite. This result means the shear-induced nuclei could be generated at a much higher temperature than the CNT-induced nuclei during the nonisothermal process. Another phenomenon is that the diffractions in the sheared PLA (or PLA/CNT nanocomposite) is quite different from polyolefins such as PE,15,38 i.e., the crystal reflections are homogeneous, that is, almost no longrange oriented crystals of PLA form after a shear rate of 30 s−1 for 3 min. In order to gain insight into the effect of shear flow and CNTs on the crystalline structure of PLA during the nonisothermal crystallization process, the evolution of 1D-

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WAXD profiles with temperature was obtained by radially integrating 2D-WAXD patterns. As shown in Figure 3a, for quiescent nonisothermal crystallization of PLA, an extremely weak diffraction peak at 2θ = 16.4° appears first at ∼80 °C, corresponding to (200)/(110) reflections of α′ form of PLA,39 which has the same 103 helix chain conformation and orthorhombic unit cell as the α form, but bears a looser chain packing and less ordered packing of the side groups in the helical chains as proposed by Zhang, Pan, and Kawai.40−42 The α′ form of PLA is proved to form when the crystallization temperature (Tc) is below 100 °C, whereas the α form of PLA preferentially grows at Tc above 120 °C, and they coexist between 100 and 120 °C.43,44 Unexpectedly, for quiescent nonisothermal crystallization of PLA/CNT nanocomposite, two distinct diffraction peaks at 16.4° and 18.7°, indicative of (200)/(110) and (203) reflections of the α′ form rather than α form of PLA, are observed at a high temperature (∼110 °C) (Figure 3b). We believe that a fast packing rate due to the strong nucleation ability of CNTs favors the formation of the α′ form with looser chain packing when PLA crystallizes even at a higher temperature than 100 °C. When the shear flow is applied, the diffraction peaks shift to larger 2θ of 16.6° and 18.9° (Figure 3c,d). The diffraction peaks at 16.7 o and 19.1 o are admittedly assigned to (200)/(110) and (203) reflections of the α form of PLA, respectively.45 Therefore, it is reasonable to deduce that coexistence of the α′ and α forms (α′/α form) occurs when the shear flow is imposed on PLA or PLA/CNT melt. This could be attributed to the crystallization temperature

Figure 2. Selected 2D-WAXD patterns showing crystal growth during nonisothermal crystallization of (a) quiescently crystallized PLA, (b) quiescently crystallized PLA/CNT nanocomposite, (c) sheared PLA, and (d) sheared PLA/CNT nanocomposite. Cooling rate is −30 °C/ min, shear rate is 30 s−1 with a duration of 3 min, and the arrows indicate diffraction of (200)/(110) reflections of PLA crystals. Note that, the temperatures marked are the final temperatures when the signal collection is stopped.

Figure 3. 1D-WAXD curves showing crystal growth during nonisothermal crystallization of PLA and PLA/CNT obtained from circularly integrated intensities of the 2D-WAXD patterns in Figure 2. a−d stand for the same samples as in Figure 2. 3861

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crystallization studies, we might imagine that the shear-induced nuclei suffer strong relaxation to random coil at the same time at such a high temperature, which is also reflected by the relatively slow crystallization rate of sheared PLA (see the slope of curve for sheared PLA sample in Figure 4). Interestingly, when the shear flow and CNTs coexist, the crystallinity of PLA/CNT (∼19%) is higher than not only that of the sheared PLA and the quiescently crystallized PLA/CNT, respectively, but also the linear addition of the quiescent PLA/CNT (11%) and the sheared PLA (5%). These results definitely verify the pronounced synergetic effect of CNTs and shear flow on enhancing crystallization of PLA in the nonisothermal crystallization process. The final morphology of the nonisothermal crystallized neat PLA and PLA/CNT nanocomposite is presented in Figure 5. Almost no crystal is observed in the quiescently crystallized neat PLA (Figure 5a). By contrast, a large amount of spherulites are observed in the quiescently crystallized PLA/ CNT nanocomposite (Figure 5b). For the sheared neat PLA (Figure 5c), the spherulites and cylinder-like crystallites appear simultaneously, which is different from the typical cylindrical crystalline morphology obtained from shear-induced crystallization of flexible polymers such as polypropylene.46,47 This observation is in line with the in situ WAXD experiments, through which we can infer that the shear-induced nuclei of PLA suffer a quick relaxation and subsequently grow into isotropic spherulites. For the sheared PLA/CNT nanocomposite, more cylinder-like crystallites occupy the whole viewing area (Figure 5d). Therefore, from the POM observations, we can make a similar conclusion that, compared to the individual effect of shear flow and CNTs, the combination of shear and CNTs exhibits a more obvious effect on increasing the nucleation density during the nonisothermal crystallization process. Application of Shear Flow and CNTs Synergistically Induced Crystallization to Injection Molding of PLA. Insitu WAXD results demonstrate the distinct synergetic effect of shear flow and CNTs on enhancing nonisothermal crystal-

dependence of the crystalline structure of PLA, i.e., the shear flow could be in favor of the formation of the α form due to the increased onset crystallization temperature, or the formation of shear induced more ordered structure.23 The variation of crystallinity with temperature during cooling was evaluated from the integrated 1D-WAXD profiles to reveal the kinetics of nonisothermal crystallization of PLA. As shown in Figure 4, the crystallization rate of the quiescent PLA is quite

Figure 4. Crystallinity evolution of PLA and PLA/CNT nanocomposite with and without shear flow during the nonisothermal crystallization process.

slow, and the ultimate crystallinity is only ∼1%. After addition of 0.1 wt % CNTs, the crystallization rate of PLA becomes faster, and the ultimate crystallinity increases markedly to ∼11%. This relatively high crystallinity of PLA/CNT nanocomposite compared with the neat PLA is ascribed to the strong nucleation ability of CNTs and the elevated Tc, which allows more time for PLA crystals to grow during the nonisothermal crystallization process. Unexpectedly, the ultimate crystallinity of the sheared neat PLA is only 5%. Taking the steady shear flow of 30 s−1 for a duration of 3 min into account, which is a high shear intensity in usual

Figure 5. Morphology of PLA or PLA/CNT nanocomposite after the nonisothermal crystallization at a cooling rate of 30 °C/min. a−d stand for the same samples as in Figure 2. 3862

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noncrystalline core layer coexist. Differing from the isotropic crystals observed in the in situ WAXD experiments, the arc-like reflections of 2D-WAXD patters exist in the skin layer. This is possibly because the strong shear flow in the skin layer makes the molecular chains orient along the flow direction to a large extent. In contrast to the CIM sample of neat PLA, the reflections are also observed in the intermediate layer of the CIM sample of PLA/CNT nanocomposite, showing a thicker crystalline layer. This result indicates that the nucleation activity of CNTs does take effect during injection-molding. As for the OSIM sample of PLA, one might expect that the crystalline layer becomes thicker than that of the CIM sample of PLA since a reciprocal shear was supplied by OSIM as the melt was filled into the mold cavity. As seen in Figure 6c, however, a point-like diffraction could only be seen in the skin layer of the OSIM samples of PLA, i.e., the intermediate layer and core layer are amorphous, which verifies again that the quick relaxation of shear-induced PLA nuclei happens in the inner region where the temperature remains high for a long duration. Interestingly, when shear flow and CNTs coexist, strong reflections are observed over the whole PLA/CNT samples including the skin and core layers. This definitely confirms that the crystallinity of the injection-molded samples is strikingly enhanced by the synergistic effect of shear flow and CNTs. Figure 7 illustrates the integrated 1D-WAXD curves at different positions in the CIM and OSIM samples of PLA and PLA/CNT nanocomposite. In the skin layer of CIM sample of PLA, the most intense diffraction peaks occur at 2θ = 16.6° and 18.9°, corresponding to (200)/(110) and (203) reflections of α′/α form of PLA, respectively (Figure 7a). The formation of α′/α form might be ascribed to the counteraction effect of the high cooling rate and the high shear rate close to the cold mold on the crystalline modifications of PLA, i.e., the high cooling rate favors the formation of α′ form, while the shear flow tends

lization of PLA, and thus increasing the ultimate crystallinity of PLA. This raises our curiosity as to whether such synergistic effect could be realized in injection molding of PLA. Unfortunately, in conventional injection molding, it is hard to manipulate the shear flow of PLA melt in a mold cavity. There is nearly no appreciable shear flow as soon as the mold cavity is full of the polymer melt.48−50 In order to achieve a controllable flow of PLA melt, we employed a modified injection molding, i.e., so-called OSIM technology, to process PLA, in which a shear flow can be imposed on the polymer melt in the mold cavity until it fully solidifies. Figure 6 shows the 2D-WAXD

Figure 6. 2D-WAXD patterns at different positions of (a) CIM sample of neat PLA, (b) CIM sample of PLA/CNT nanocomposite, (c) OSIM sample of neat PLA, and (d) OSIM sample of PLA/CNT nanocomposite. The flow direction is vertical.

patterns at different positions of CIM and OSIM samples of neat PLA and PLA/CNT nanocomposite. The CIM samples of PLA and PLA/CNT nanocomposite exhibit a similar layered structure (Figure 6a,b), in which a crystalline skin layer and a

Figure 7. 1D-WAXD curves of injection-molded samples circularly integrated from 2D-WAXD patterns in Figure 6. a−d stand for the same samples as in Figure 6. 3863

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to induce α form.40,43,44 With addition of 0.1 wt % CNTs, although the crystalline layer becomes thicker, CNTs do not alter the crystalline structure of PLA since the 2θ values of diffraction peaks remain unchanged (Figure 7b). For the OSIM samples of PLA and PLA/CNT nanocomposite (Figure 7c,d), typical diffraction peaks of α form are observed at 2θ = 16.7° (i.e., (200)/(110)) and 19.1° (i.e., (203)), and some weak diffraction peaks at 2θ = 12.4°, 15.1° and 22.5°, indicative of (004)/(103), (010) and (015) reflections of α form of PLA, also become distinguished. These results shows that the strong shear flow applied by OSIM tends to induce a larger amount of the more ordered α form during nonisothermal crystallization compared to the in situ WAXD experiments. The crystallinity at different positions of the CIM and OSIM samples of neat PLA and PLA/CNT nanocomposite is listed in Table 1, aiming to quantify the combined effect of shear flow

only those polymer chains with the length above a critical value (M*) could be oriented in the flow direction,51 thus, the high molecular weight part mainly contributes to the formation of nuclei and the increase of the crystallization rate. The molecular weight of PLA available for general use (100K g/mol) is relatively low and it suffers hydrolysis reaction due to the ester groups in its main chain during melt annealing or cooling from the melt if it is not absolutely dried, which would further decreases its molecular weight. Consequently, only a very small proportion of PLA chains have a high molecular weight above M*, and can be oriented for nucleation. On the other hand, the oriented PLA chains tend to relax since the shear-induced rownuclei do not result from a thermodynamic process, which follows the power law decay. It is known that the chain rigidity strongly affects this relaxation process, and the exponent varies from 1/2 for flexible chains to 5/4 for rigid ones.52 PLA chains have ester groups in the main chains, allowing PLA to be regarded as a semirigid polymer; therefore, the shear-induced nuclei of semirigid PLA chains tend to break down into several point precursors (or random coil) easily compared to flexible polymer chains (see Figure 5c). In addition, its molecular weight between entanglements (Me), which also plays a crucial role in the balance between shear-induced nuclei formation and relaxation, is reported to be 4 times higher than that of PE.53 On the basis of these facts, for the neat PLA melt, the moderate shear intensity (several tens of s−1) adopted in the in situ WAXD experiments could only induce point-like nuclei with a low nuclei density. The subsequent growth of lamellar crystals from these point precursors cannot form oriented crystals, and the low nucleation density results in low ultimate crystallinity (∼5%, see Figure 4). The CNTs are more efficient in enhancing nucleation density than shear flow; however, this kind of nuclei only form at a larger supercooling, which is undesirable for crystal growth and thus for increase of crystallinity. As it is well-known, the enhancement of polymer crystallization is dependent on the nucleation and growth of crystallites. Basically, there are mainly two kinds of nucleating sites for sheared PLA/CNT nanocomposite. One is the heterogeneous nucleating sites provided by CNTs’ surfaces, and the other is the self-nucleating sites (or homogeneous nucleating sites) due to the effect of shear flow. Our results show that heterogeneous nucleation could only occur around 110 °C under quiescent conditions (Figure 4). Correspondingly, under the sheared condition, the nucleation at Tc above ∼110 °C should mainly originate from self-nucleating sites, since shear flow could further decrease the nucleation energy barrier of PLA than CNTs in the nonisothermal crystallization process, leading to generation of shear-induced nuclei at lower supercooling.38,54 What is interesting is that the nucleation density of the sheared PLA/CNT nanocomposite is much higher than that of the quiescent PLA/CNT nanocomposite and the sheared PLA, respectively, and even the sum of these two. It is reasonable to deduce that some extra nucleating sites originate from the interactions between CNTs and PLA chains. Generally, polymer chains could be absorbed on fillers’ surface (nucleating agents, fibers, particles, and CNTs etc.) due to interaction between the surface groups of additives and polymer chains, and subsequently be stretched along the flow field.55,56 As confirmed by our previous work,9,10 a strong −CH−π interaction between the PLA backbone and CNTs favors a surface-induced conformational order (SICO) process and thus accelerates crystallization kinetics of PLA during

Table 1. Crystallinity Estimated at Different Layers of CIM and OSIM Samples sample

skin

intermediate

core

CIM PLA CIM PLA/CNT OSIM PLA OSIM PLA/CNT

0.150 0.187 0.162 0.204

0 0.093 0.012 0.282

0 0 0.004 0.207

and CNTs on the crystallinity and its distribution of PLA in the injection-molded samples. For the CIM sample of PLA, the crystallinity in the skin layer reaches ∼15.0%, but drops sharply to zero in the intermediate and core layers. After addition of CNTs, the crystallinity in the skin layer reaches ∼19%, higher than that of the CIM sample of PLA. Attractively, due to the efficient nucleation activity of CNTs, the intermediate layer still exhibits a crystallinity of ∼9%. When the OSIM technology is applied, a slight increment of ∼1% in the crystallinity is obtained in each layer for neat PLA compared to the CIM sample of PLA. Only when both shear flow and CNTs are simultaneously applied could a high crystallinity (≥20%) be obtained in all three layers, and especially, the crystallinity of the intermediate layer even approaches to 28%, which is much higher than that of the other three samples. These results clearly demonstrate that the synergistic effect of shear flow and CNTs on enhancing crystallization of PLA is successfully realized in the injection molding.

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DISCUSSION Shear flow and CNTs are separately demonstrated to be effective in enhancing PLA crystallization.9−20 Our interesting finding is that the combination of shear flow and CNTs shows a distinct synergistic effect on enhancing nonisothermal crystallization kinetics of PLA even at a high cooling rate. For flowinduced polymer crystallization, it is generally believed that shear flow could induce orientation of polymer chains along the flow direction, and the oriented chains then assemble into parallel array and form the precursors of primary nuclei for crystallization.18−20 Under shear, PLA displays a high onset crystallization temperature (Figure 2), which is consistent with the shear-induced crystallization of other polymers such as PE.15,38 Nevertheless, considering the ultimate crystallinity of sheared PLA (∼5%), shear flow seems to have a very limited effect on increasing the nucleation density of PLA. The nucleation density is closely related to the balance of shearinduced nuclei formation and relaxation. At a given shear rate, 3864

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nanocomposite takes place, we successfully extend this synergistic effect to injection-molding of PLA products. Crystallization of semicrystalline polymers during injectionmolding is much more complicated than that in simple shear stage. The feature of crystalline morphology formed in injection-molded PLA could be helpful for a better understanding of difference between the shear-induced crystallization of PLA based on a simple shear stage and a real processing machine. Generally, during CIM processing, the hot polymer melt in vicinity of the cold mold walls experiences large cooling rate and intensive shearing (and elongational) flow due to fountain flow,50 and thus flow-induced crystallization could take place near the walls.59,60 Our results further reveal a thin crystalline PLA layer near the wall with relatively high crystallinity of ∼15% at such high cooling rate (∼100 °C/ min). In contrast, for the core region, the shear rate is low on the one hand, and a low cooling rate leads to relaxation of oriented chains for a longer time on the other, subsequently resulting in a limited amount of shear-induced nuclei in the core region. This is the reason why the overall crystallinity obtained in CIM samples of PLA is very low.8,21 With the presence of CNTs, the crystalline layer of the CIM sample becomes much thicker since CNTs could stabilize the shearinduced nuclei under low supercooling and also induce heterogeneous nucleation of PLA at relatively high supercooling. However, the crystallinity still declines to zero in the core region because only a limited amount of nuclei could be induced by the relatively low shear rate existing in the core region of CIM. By contrast, successive flow existent in OSIM could be used to realize high nucleation density especially in the core region, which provides an oscillation shear flow imposed on PLA melt until solidification. With such an effect, largely enhanced crystallinity in the OSIM sample of PLA/CNT with a crystallinity of more than 20% over the whole thickness has been achieved. It should be noted that there is still a thin crystalline layer in the OSIM sample of PLA (see Figure 6 and Table 1); the reason is that the shear-induced nuclei still have a chance to relax into a random coil due to the nature of the semirigid chain and short length. Meanwhile, almost pure α form with more ordered structure can be obtained by the intensive shear flow provided by OSIM technology. This novel approach to elevate the crystallinity of PLA and induce more ordered α form might have potential to enhance the heat resistance and mechanical properties of PLA, thus expanding its applications.23 More interestingly, the marked orientation indicates that oriented crystalline structure other than pointlike nuclei-induced spherulites could form because the intensity of shear flow is high enough during injection-molding (see Figure 6), and further investigation would be interesting both in academics and in industry.

isothermal crystallization. With such an anchoring interaction, the PLA chains are expected to be stretched more easily along the flow direction, giving rise to the extra nuclei. Additionally, as stated above, the sheared-induced nuclei of PLA tend to relax into point-like nuclei (or random coil); however, the relaxation of shear-induced nuclei could be suppressed to some extent due to the adsorption of additives surfaces.57,58 Here, we could safely conclude that the extra nucleating sites are attributed to the anchoring effect of CNTs on shear-induced crystallization of PLA and suppressing effect of CNTs on the relaxation of shear-induced nuclei. A schematic is provided for summarizing the mechanism of shear flow and CNTs' synergetically induced crystallization of PLA (see Figure 8). When a shear flow is applied to PLA melt

Figure 8. Schematic of enhanced shear-induced crystallization caused by CNTs in PLA melt: (a) amorphous PLA/CNT melt: PLA chains are disordered; (b) PLA chains are partially oriented toward the shear direction; (c) nucleation; (d) crystallization.

with the presence of CNTs, PLA chains (or segments) oriented toward the flow direction, where shear-induced nuclei are more likely to form in the vicinity of CNTs due to the anchoring effect. More importantly, these nuclei absorbed by CNTs are reluctant to relax compared to those far away from CNTs, which is the case of the sheared neat PLA. These effects cause the formation of extra nuclei compared to sheared PLA and quiescent PLA/CNT nanocomposite, and result in high nucleation density and thus high crystallinity. Furthermore, shear flow helps formation of more ordered α form of PLA during nonisothermal crystallization at a high cooling rate. Recently, Huang et al.23 also reported that the higher ordered α form could be formed even at an isothermal crystallization temperature lower than 100 °C under shear flow, especially at high shear rates. Our results verify that the onset temperature increase sharply from 90 to 150 °C when shear flow is imposed on PLA melt (see Figures 2−4). Considering this, it is easy to understand why the α/α′ form rather than the individual α′ one readily forms when shear flow is imposed on PLA and PLA/CNT nanocomposite in the nonisothermal process. Matching the conditions under which the synergistic effect of flow and CNTs on nonisothermal crystallization of PLA/CNT

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CONCLUSIONS We investigated shear-induced nonisothermal crystallization of PLA with the presence of CNTs at a high cooling rate by synchrotron in situ WAXD. It is clearly shown that the flowinduced crystallization happens in the nonisothermal crystallization of PLA, leading to a high onset crystallization temperature as well as crystallinity. When CNTs was added into PLA, a pronounced synergetic effect of CNTs and shear flow (other than simple additive effect) on enhancing crystallization of PLA in the nonisothermal crystallization process was observed, which is attributed to the extra nucleating sites produced by the interplay of shear flow and 3865

dx.doi.org/10.1021/bm3013617 | Biomacromolecules 2012, 13, 3858−3867

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CNTs, leading to high nucleation density for subsequent crystal growth. We believe that the extra nucleating sites originate from the anchoring effect of CNTs on shear-induced crystallization of PLA and suppressing effect of CNTs on the relaxation of shear-induced nuclei. Further, we extended this synergistic effect to injection molding successfully for the first time by OSIM. It is confirmed that the anchoring effect of CNTs mentioned above could be used to stabilize the shear-induced nuclei especially in the core region, leading to largely enhanced crystallinity in the OSIM sample of PLA/CNT nanocomposite with a crystallinity of more than 20% over the whole thickness. The mechanism for the synergistic effect of CNTs and shear flow on the crystallization of PLLA could be applicable to other PLLA nanocomposites, and this novel approach to elevate the crystallinity of PLA might have the potential to expand its application.

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AUTHOR INFORMATION

Corresponding Author

*Ph: +86-28-8540-0211; Fax: +86-28-8540-6866; e-mail: ganji. [email protected] (G.-J.Z.). Ph: +86-28-8540-6866; Fax: +8628-8540-6866; e-mail: [email protected] (Z.-M.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China (Grant No. 51120135002, 51121001, 51203104) and the Startup Fund of Sichuan University (Grant No. 2011SCU11072). The project was also partially supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (Grant No. 201014). We also would like to express sincere thanks to the National Synchrotron Radiation Laboratory (NSRL, Hefei, China), the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China), and the Brookhaven National Laboratory (USA), for the careful 2D-WAXD measurements.

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