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Low-Temperature Sintering of Stereocomplex-Type Polylactide Nascent Powder: Effect of Crystallinity Dongyu Bai, Huili Liu, Hongwei Bai,* Qin Zhang, and Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China ABSTRACT: Stereocomplex (SC) crystallization between highmolecular-weight poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) provides a promising route to substantially improve the properties of polylactide (PLA), but conventional melt processing of the SC-type PLA (SC-PLA) is nearly impossible primarily due to the poor crystallization memory effect as well as serious thermal degradation after complete melting of SC crystallites with high melting temperatures of above 220 °C. Recently, we reported an innovative low-temperature (180−210 °C) sintering technology for fabricating SC-PLA products from its nascent powder. Unfortunately, its practical application has been significantly hindered by an extremely high pressure of 1 GPa, which must be utilized to ensure good surface wetting of the densified powder particles. With this challenge in mind, herein, the role of powder crystallinity in the low-temperature sintering has been investigated. Interestingly, we first demonstrate that depressing powder crystallinity is favorable for the particle wetting under a much lower pressure during the densification stage of the nascent powders because the deformation of the powders becomes easier with the decrease in the fraction of rigid crystal network. Moreover, during the subsequent interface welding stage, more PLLA/PDLA chains could be involved in the interdiffusion and SC crystallization across particle interfaces, thus forming large amounts of new SC crystallites capable of tightly welding the interfaces. As a consequence, SC-PLA sheets with excellent heat resistance and mechanical properties have been successfully fabricated by sintering under a pressure of as low as 300 MPa. Overall, these fascinating findings not only provide new fundamental understandings on the role of initial crystallinity in the low-temperature sintering of SC-PLA powder but also indicate an avenue toward industrial-scale fabrication of SC-PLA products from low-crystallinity nascent powder using conventional polymer sintering equipment. engineering plastic.8−10 For instance, the highest heat distortion temperature (HDT) for highly crystalline PLA is only 100−120 °C.11,12 Therefore, a great deal of effort has been devoted to substantially enhancing these properties of PLA so as to meet the essential requirements for large-scale engineering applications. Very interestingly, PLA has two typical enantiomers, namely, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA). The past several decades have revealed that the two enantiomeric PLAs can pack tightly into the unit cell in a side-by-side manner and ultimately form stereocomplex (SC) crystallites with superb physicochemical properties.13−18 Because of the stronger interchain interactions and denser chain packing density than that in the common homocrystallites of enantiopure PLLA or PDLA, the SC crystallites possess superior heat resistance,16,19,20 higher strength and stiffness,21,22 better thermal stability,23 and enhanced hydrolysis resistance.24−26 Especially, the Tm of SC crystallites is as high as 220−230 °C, and thus the HDT of SC-type PLA (SC-PLA) can reach to 200 °C.19 These impressing attributes enable SC-PLA to compete with some

1. INTRODUCTION In recent decades, increasing resource crisis and environmental concerns have encouraged global scientific and industrial communities to develop bio-based and biodegradable polymers capable of replacing conventional petroleum-based polymers for numerous applications.1,2 Among these emerging replacements that can be produced from renewable resources (e.g., corn and sugar cane) and fully biodegradable in soil environments after use, polylactide (PLA) represents the most promising frontrunner with enormous application value and broad market potential due to its good biocompatibility, extraordinary transparency, useful mechanical properties, versatile processability, and significant drop of production cost.3−7 In particular, most physical properties of PLA including mechanical strength and modulus are comparable to those of petroleum-based engineering plastics such as polystyrene (PS) and poly(butylene terephthalate) (PBT). Nowadays, commercial PLA has been ubiquitously used for a variety of daily commodities such as packing materials, containers, textile fibers, agriculture films, and medical devices. However, the low melting temperature (Tm, ca. 150−180 °C) and potential hydrolytic degradation during service life make PLA unable to provide sufficient heat resistance and durability, which significantly hinder its further applications as an eco-friendly © XXXX American Chemical Society

Received: August 18, 2017 Revised: September 24, 2017

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Figure 1. Schematic illustration showing the microstructure evolution during low-temperature sintering of SC-PLA nascent powders with different crystallinities.

at the packing stage of OSIM. However, all melt-processing of PLLA/PDLA blends inevitably induces a serious thermal degradation of PLA chains,4,55 thereby leading to the performance deterioration of final products. Although much attention has been paid to the suppression of degradation during melt processing of PLA, no notable achievement has been attained so far.56−58 On the other hand, SC-PLA products with different forms can also be fabricated by solvent processing,59−63 but the processing efficiency is usually very low and the use of toxic organic solvents (e.g., chloroform) may cause great damage to natural environment and human health. Therefore, the fabrication of well-stereocomplexed HMW SCPLA products using conventional processing technologies remains a bottleneck issue to obtain good heat resistance and durability. Inspired by powder metallurgy, we recently achieved a significant progress in the fabrication of high-performance and transparent SC-PLA products with 100% SC crystallites through low-temperature (as low as 180−210 °C) sintering of HMW PLLA/PDLA blend powder, without degradation of PLA chains.64 The sintering contains two main steps, i.e., the densification of nascent powder under a sufficient pressure to facilitate the complete wetting of the powder particles and the subsequent welding of particle interfaces via interdiffusion and cocrystallization of enantiomeric PLA chain segments across the interfaces. The sintered SC-PLA products exhibit not only excellent heat resistance and hydrolysis stability but also good optical transparency, vividly indicating a promising route toward processing and application of SC-PLA. However, the current commercial application of this processing technology is nearly impossible mostly associated with the requirement of using an extremely high pressure of 1 GPa to ensure good surface wetting of the powder particles. The high-pressure densification process makes it hard to fabricate SC-PLA products using conventional polymer sintering equipment.65−67 Also, besides high energy consumption and safety risk, special high-pressure equipment and high-precision mold should be designed to impose such a high pressure on the powder particles at the densification step. More importantly, the

petroleum-based engineering plastics in many potential applications. The properties of SC-PLA are largely determined by the degree of stereocomplexation.14,17,20−22,24−26 The higher the stereocomplexation degree, the better the properties are. The conversion of thermoplastic polymers into useful products generally involves melt processing, such as melt blending and injection molding, and high molecular weight (>105 g/mol) is indispensable for favorable properties as well as good meltprocessability of SC-PLA. Unfortunately, both SC crystallites and homocrystallites are formed completely and simultaneously in the commercially available high-molecular-weight (HMW) PLLA/PDLA blends.14,17,20,27−31 Moreover, the relative content of the SC crystallites decreases greatly with increasing molecular weight probably owing to the higher kinetic barrier for SC crystallization. (If both the enantiomeric PLLA and PDLA must be fully involved in the SC crystallization, they would suffer from not only prolonged chain diffusion path but also restricted diffusion process as compared to homocrystallization.30) The restriction of available space at the fold surface on the folding of the enantiomers may be another reason for the difficulty in SC crystallization of HMW PLLA/PDLA blends.32 More importantly, the SC crystallites formed in the HMW SC-PLA have a poor melt memory effect to trigger exclusive SC crystallization when they are completely melted,20,33,34 so the formation of homocrystallites always prevails over that of SC crystallites in the melt-processed SCPLA products. Up to now, many strategies have been developed to significantly promote SC crystallization in the crystallization and processing of HMW PLLA/PDLA blends, such as stereoblock copolymerization,18,35 synthesis of PLAs with special chain topologies,36−40 addition of nucleating agents,41−47 use of plasticizers,48,49 introduction of compatibilizers,50−52 and thermal drawing.53 Very recently, Zhang et al.54 demonstrated a new technology to fabricate highly crystalline SC-PLA products with almost exclusive SC crystallites by using oscillation shear injection molding (OSIM) without introducing any additives. It is believed that an intense shear flow (ca. 103−104 s−1) imposed on the PLLA/PDLA blend melts can substantially enhance the SC crystallization during solidification B

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Figure 2. (a) DSC melting curves and (b) WAXD patterns of SC-PLA nascent powders prepared by melt-blending at different temperatures. The data of crystallinity estimated by DSC and WAXD are given in the figure profiles. hydraulic press machine (Chengdu Zhengxi Hydraulic Equipment Manufacturing Co., Ltd., China) equipped with a high-pressure apparatus of tungsten carbide, as described in our previous work.64 An electric heating collar was utilized to control the temperature of the sample cell. The pressure and sample temperature were monitored using an accurate pressure meter and a thermocouple, respectively. The sintering process contains two steps: (i) densification of the SCPLA powder at 160 °C (below the Tm,HC but well above the glass temperature) for 10 min under different pressures (300−700 MPa), allowing the surface wetting of the densified powder particles, and (ii) welding of the particle interfaces via chain diffusion and cocrystallization at 210 °C (above the Tm,HC but well below the Tm,SC) for 30 min under a pressure of 50 MPa in order to prevent cracks induced by the crystallization-induced shrinkage. High pressure could restrict the interdiffusion of PLLA/PDLA chains across particle interfaces due to the reduced chain mobility associated with the decrease of free volume. The SC-PLA disk products (24 mm in diameter and 0.5 mm in thickness) were obtained by slowly cooling the sintered sample to room temperature. Dumbbell-shaped specimens (10 mm in gauge length and 4 mm in width) used for mechanical tests were cut out from the disks. 2.4. Characterization. Differential scanning calorimeter (DSC) analysis was performed on a PerkinElmer pyris-1 DSC instrument (PerkinElmer Inc., USA) under a dry N2 atmosphere (20 mL/min). The specimen (ca. 5 mg) sealed in an aluminum pan was heated from 50 to 250 °C at a heating rate of 10 °C/min. The crystallinity for SC crystallites (XDSC c,SC ) was calculated based on the equation

fundamental understandings on the interdiffusion and SC crystallization across particle interfaces are still unclear. With these challenges in mind, we attempted to reveal the key factors controlling low-temperature sintering of SC-PLA powder, such as molecular weight of PLAs and crystallinity of SC-PLA powder. In this work, SC-PLA nascent powders with different crystallinities (32−47%) were prepared by simple melt blending at temperatures of 160−200 °C,68,69 and we demonstrate that the powder crystallinity plays an important role in the sintering of SC-PLA for the first time. On one hand, because the densification of the nascent powder is carried out at temperatures below the Tm of homocrystallites but well above the glass temperature, the deformation ability of the powders could increase significantly with the decrease in the fraction of rigid crystal network, and thus depressing powder crystallinity can facilitate the good particle wetting under a much lower pressure (Figure 1). On the other hand, the depressing crystallinity makes it possible for more PLLA/PDLA chains to participate in the interdiffusion and SC crystallization across particle interfaces and finally form large amounts of new SC crystallites capable of tightly welding the interfaces (Figure 1). Impressively, the high-performance and optically transparent SC-PLA sheets have been successfully fabricated by sintering under a very low pressure of 300 MPa. We believe that our findings could open a promising way toward large-scale fabrication of SC-PLA products from low-crystallinity nascent powder using conventional polymer sintering equipment, which is very meaningful for the industrial processing and commercial application of SC-PLA materials.

DSC Xc,SC =

ΔHm,SC ° ΔHm,SC

(1)

where ΔHm,SC is the melting enthalpy of SC-PLA and ΔH°m,SC is the melting enthalpy of perfect SC crystal (selected as 142 J/g14). Wide-angle X-ray diffraction (WAXD) measurement was conducted on an X’Pert pro MPD X-ray diffractometer (PANalytical B.V., Holland) with a Cu Kα radiation (40 kV, 40 mA). The specimen was scanned in the diffraction angle (2θ) range of 5°−35°. The content of SC crystallites (XWAXD c,SC ) was evaluated by comparing the characteristic peak area of SC crystallites with the total peak area. Morphological observation was carried out using an Inspect F scanning electron microscope (SEM) (FEI Company, USA) operating at 10 kV. Before the observation, the specimen surface was coated with a thin gold layer. Density measurement was carried out using the buoyancy method with distilled water as an immersion liquid. The density (ρ) was calculated according to the equation

2. EXPERIMENTAL SECTION 2.1. Materials. PLLA with a D-isomer content of 1.4%, a weightaverage molecular weight (Mw) of 1.7 × 105 g mol−1, and a polydispersity (PDI) of 1.7 was purchased from NatureWorks LLC, USA. PDLA (D-isomer content = 99.5%, Mw = 1.2 × 105 g mol−1, PDI = 1.6) was kindly supplied by Zhejiang Hisun Biomaterial Co., Ltd., China. Prior to use, both PLLA and PDLA pellets were vacuum-dried at 50 °C for at least one night. 2.2. Preparation of SC-PLA Powder. SC-PLA nascent powders with different crystallinities were prepared by melt blending of PLLA and PDLA (PLLA/PDLA = 50:50, wt/wt) in a Haake Rheomix 600 internal mixer (Germany) at temperatures of 160−200 °C and a rotor speed of 60 rpm for 5 min. For convenience, the obtained powders were denoted as SC-x, where x represents the blending temperature. 2.3. Low-Temperature Sintering of SC-PLA Powder. Lowtemperature sintering of SC-PLA powder was carried out using a

ρ= C

m1ρw m = v m1 − m2

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Figure 3. SEM micrographs showing the morphologies of the SC-PLA nascent powders: (a) SC-160, (b) SC-180, and (c) SC-200. where m1 and m2 are the weight of the specimen in air and that in water, respectively, ν is the specimen volume, and ρw is the density of water (1.0 g/cm3). The dissolution experiment was done by dipping SC-PLA products in chloroform (about 50 mg/mL) for 2 weeks, and then digital photos were taken. Dynamic mechanical analysis (DMA) was conducted on a Q800 analyzer (TA Instruments, USA) in the tensile mode from 0 to 250 °C at a heating rate of 3 °C/min. The oscillating strain and frequency are 10 μm and 1 Hz, respectively. Tensile properties were tested using a SANS universal tester (China) at a cross-head speed of 5.0 mm/min. The tests were carried out at room temperature, and the reported value was averaged from six independent bars for each sample.

Figure 3 presents the SEM micrographs of the SC-PLA nascent powders prepared by melt blending at various temperatures. Obviously, all SC-PLA powders are composed of the similar irregular particles with sizes ranging between 50 and 150 μm, but the particle surface becomes smooth with increasing blending temperature from 160 to 200 °C. Considering that a broad particle size distribution could be favorable for the densification of SC-PLA nascent powders due to the gap filling ability of the fine particles,70 the SC-PLA powders used in this work has not been fractionated from these irregular particles, but the effect of particle size distribution on the low-temperature sintering will be investigated in our future work. 3.2. Role of Crystallinity in the Densification of SCPLA Powder. The densification of the as-prepared SC-PLA nascent powders was carried out at 160 °C for 10 min under different pressures (between 300 and 700 MPa). The SEM micrographs of Figure 4 show that crystallinity has significant

3. RESULTS AND DISCUSSION 3.1. Preparation of SC-PLA Nascent Powders with Different Crystallinities. SC-PLA nascent powders with different crystallinities were prepared by melt blending of equimolar PLLA and PDLA at temperatures of 160−200 °C, where only SC crystallites can form under the drive of strong shear force but the homocrystallization can no longer occur.68,69 Figure 2 shows the DSC melting curves and WAXD patterns of the as-prepared SC-PLA powders. From Figure 2a, one can observe a single melting peak at around 215−220 °C in each DSC thermogram, clearly indicating the excusive formation of SC crystallites in these SC-PLA nascent powder. More interestingly, with the increase in the blending temperature from 160 to 200 °C, both the melting temperature (from 217.5 to 220.0 °C) and DSC crystallinity (from 32.1% to 46.7%) increase evidently possibly because of the enhanced molecular diffusion kinetic at the growth front of the SC crystallites. It should be mentioned that we have attempted to prepare SC-PLA powders with very low crystallinity by melt blending of PLLA and PDLA at temperatures below 160 °C (e.g., 155 °C), but the poor mixing makes them hard to form high-purity SC-PLA powders because the melting temperatures of both PLLA and PDLA are higher than 160 °C. The WAXD patterns further confirm the successful preparation of highpurity SC-PLA nascent powders with tunable crystallinities (WAXD crystallinity is between 29.4% and 39.6%). As shown in Figure 2b, the WAXD pattern of the SC-PLA powder prepared at 160 °C exhibits three typical characteristic diffraction peaks at around 11.8°, 20.6°, and 23.9°, ascribed to the (110), (300)/(030), and (220) planes of SC crystallites,14 and one tiny characteristic peak at around 16.6°, corresponding to the (200)/(110) plane of homocrystallites.14 Moreover, when the blending temperature increases up to 200 °C, the intensity of these characteristic diffraction peaks of SC crystallites increases gradually while that of homocrystallites decreases rapidly.

Figure 4. SEM micrographs showing the morphologies of the SC-PLA powders densified under different pressures. The densities of the densified powders are shown in the profiles.

effect on the surface contact and wetting of the powdery particles during the densification stage, although all particles keep their integrity due to the absence of chain interdiffusion across interfaces. For the low-crystallinity SC-PLA powders (e.g., SC-160 with a crystallinity of 32.1%), the surface wetting improves significantly with increasing densification pressure from 300 to 700 MPa (Figure 4a,d,g), and perfect wetting can be achieved in the densified SC-160 powder under 500 MPa D

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Macromolecules (Figure 4d), indicating a high level of plastic deformation which is necessary to rule out porosity. Density measurements also provide a direct evidence for the good surface wetting. The density (1.192 g/cm3) of the SC-160-500MPa powder is very close to that (1.199 g/cm3) of SC-160-700MPa powder. In contrast, even an extremely high pressure (i.e., 700 MPa) cannot ensure good surface wetting of the high-crystallinity SCPLA powders (e.g., SC-PLA-200 with a crystallinity of 46.7%) (Figure 4c) due to the existence of the rigid crystal network in the particles to prevent their high-level plastic deformation. Moreover, the density of the SC-200-700MPa powder is only 1.176 g/cm3, slightly lower than that (1.178 g/cm3) of the SC160-300MPa powder, suggesting the existence of some pores at particles interfaces. These results prove that depressing powder crystallinity is favorable for the particle wetting under a much lower pressure during the densification process. 3.3. Role of Crystallinity in the Interface Welding of Densified SC-PLA Powder. The welding of the particle interfaces via chain interdiffusion and subsequent SC crystallization was performed by sintering of the predensified SC-PLA powders at 210 °C for 30 min. Figure 5 presents the

Figure 6. Digital photos of sintered SC-PLA sheets after dipping in chloroform (50 mg/mL) for at least 2 weeks.

wetting makes it hard for the chain interdiffusion and the formation of new SC crystallites across the interfaces. Most importantly, it should be noted that both the SC-160-300MPa and SC-200-700MPa products share the same surface wetting and density (Figures 4g and 4c), but their interface welding strength is completely different. It implies that besides good surface wetting, powder crystallinity may play an important role in interface welding. Compared with the high-crystallinity SCPLA powders, the low-crystallinity ones contain much more crystallizable PLLA/PDLA chain sgments, and thus large amounts of SC crystallites could be formed across the interfaces to tightly weld the interfaces. Figure 7 presents the DSC melting curves of the obtained SC-PLA products. Obviously, only one strong melting peak of SC crystallites is observed in the DSC thermograms (Figure 7a−c), proving that the SC-PLA products with 100% SC crystallites have been fabricated. Moreover, both the surface wetting and powder crystallinity have a significant effect on the formation of new SC crystallites across the interfaces (Figure 7d). The better the surface wetting and the lower the powder crystallinity, the higher the content of the newly formed SC crystallites (as indicated by the higher crystallinity increments during the sintering of the nascent powders) is. In particular, the crystallinity of the SC-160-500MPa powder is dramatically increased from 32.1% to 46.1%, while that of the SC-200500MPa powder is only slightly increased from 46.7% to 51.7%. The difference in the ultimate crystallinity of SC-PLA products should be ascribed to the difference between the preparation condition of SC-PLA nascent powders and the formation condition of SC crystallites at the interfaces. During the melt blending of equimolar PLLA and PDLA, the shear flow could trigger complete SC crystallization by encouraging the intermolecular interactions between PLLA and PDLA chains.68,69 However, the formation of SC crystallites at the interfaces is carried out under quiescent crystallization condition. In this case, many crystallizable PLLA/PDLA chain segments are hard to crystallize.

Figure 5. Material appearance of the SC-PLA bars fabricated by powder densification under different pressures and subsequent sintering at 210 °C.

appearance of the SC-PLA bars fabricated by sintering of the powders predensified under different pressures (300−700 MPa). It is interesting to find that all SC-160 products display excellent transparency, suggesting that the particle interfaces should be tightly welded by the newly formed SC crystallites across the interfaces. However, for the SC-PLA products sintered from high-crystallinity powders, only the SC-200700MPa product exhibits such good transparency (Figure 5c). The poor transparency observed in the sintered SC-PLA products from powders predensified under lower pressures should be ascribed to the scattering effect of the unwelded interfaces due to the existence of some pores (as evidenced by the inferior surface wetting and lower density shown in Figure 4). In order to confirm the formation of SC crystallites at the interfaces and evaluate the interface welding strength of the densified SC-PLA powders with different crystallinities during sintering process, we delicately designed a dissolution experiment based on the fact that only amorphous phase and homocrystallites of SC-PLA products can be easily dissolved in chloroform which is a nonsolvent for SC crystallites. Some representative results are shown in Figure 6. As expected, all SC-160 products can remain their integrity after dipping in chloroform for at least 2 weeks, indicating that the particle interfaces has been tightly welded by the newly formed SC crystallites even in the case of the SC-160 powder predensified under 300 MPa (Figure 6a,d,g). However, for the SC-180 and SC-200 products, most SC-PLA particles are not connected together as a whole, and then only the fragments can be observed in the chloroform possibly because the poor surface E

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Figure 7. (a−c) DSC melting curves of sintered SC-PLA products and (d) the data of crystallinity of densified SC-PLA powders and their sintered products.

Figure 8. (a) DSC melting curves of annealed SC-160 (denoted as SC-160-A) powder and its sintered product and (b) digital photo of the densified SC-160-A powder.

strength can also be obtained at lower sintering temperatures (i.e., 190 and 200 °C) where no SC crystallites can be melted, clearly demonstrating that the favorable welding behavior of the SC-160 powder should be mainly ascribed to its low crystallinity rather than partial melting. Our previous work also reported that high-performance SC-PLA products can be obtained by sintering of SC-PLA powders at temperatures as low as 180 °C.64 The dynamic mechanical properties of the obtained SC-PLA products were measured with DMA, and some representative results are shown in Figure 9. Evidently, all the products sintered from high-crystallinity SC-PLA powders (including SC-180, SC-200, and SC-160-A) are broken or softened at a low temperature of around 170 °C (near the Tm of PLLA and PDLA homocrystallites) owing to the poor interface welding

In order to further demonstrate such important role of the powder crystallinity in interface welding, the SC-160 nascent powder was annealed at 200 °C for 1 h, and subsequently the SC-PLA product was fabricated by sintering the powder predensified under 700 MPa. Expectedly, with the increase of powder crystallinity from 32.1% to 42.9% (Figure 8a), it is not only difficult to ensure good surface wetting during the densification stage (Figure 8b) but also hard to form sufficient SC crystallites to tightly weld the powdery particles together (Figure 8a). In addition, one may argue that the partial melting of SC crystallites at 210 °C may contribute to the favorable interface welding of densified SC-160 powder. To rule out its contribution, we have investigated the effect of sintering temperature on the interface welding behavior of SC-160 powder (not shown here) and found that such strong welding F

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GPa, which are comparable to those of the SC-160-700MPa product.

4. CONCLUSIONS In summary, we have demonstrated that the initial crystallinity of SC-PLA powder plays an important role in the lowtemperature sintering. For the sintering of high-crystallinity SCPLA powder, the rigid crystal network can significantly prevent the wetting of powdery particles during the densification process due to the extremely low deformationability, and during the subsequent interface welding process, only very limited PLLA/PDLA chain segments can participate in the interdiffusion and SC crystallization at the interfaces between adjacent particles. However, depressing powder crystallinity not only facilitates the particle wetting under a much lower pressure but also benefits the formation of more new SC crystallites to tightly weld the particle interfaces. As a result, SC-PLA sheets with good heat resistance, mechanical properties, and optical transparence have been fabricated by the low-temperature sintering under a densification pressure of as low as 300 MPa. We believe that these interesting findings could substantially promote the large-scale fabrication of SC-PLA products from low-crystallinity nascent powder using conventional polymer sintering equipment.

Figure 9. Storage modulus vs temperature curves of SC-PLA products fabricated by powder densification under different pressures and subsequent sintering at 210 °C.

strength provided by trace amounts of the newly formed SC crystallites across the interfaces, in spite of the densification pressure (500 or 700 MPa). In contrast, for the products sintered from low-crystallinity SC-160 powder, the storage modulus starts to drop considerably at temperature above 210 °C (near the Tm of the newly formed SC crystallites), vividly indicating that the damaging temperature is dominated by the crystal structure at the particle interfaces, and large amounts of interface-localized SC crystallites can only be formed during the sintering of the low-crystallinity SC-PLA powders. More interestingly, the outstanding heat resistance can be obtained by sintering the low-crystallinity powders at a much lower densification pressure (e.g., 300 MPa), which is very meaningful for the industrial fabrication of high-performance SC-PLA materials. Figure 10 shows the tensile properties of some representative SC-PLA products. As expected, it is interesting to observe that

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AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax +86 28 85461795; e-mail [email protected], [email protected] (H.W.B.). *Tel/Fax +86 28 85461795; e-mail [email protected] (Q.F.). ORCID

Hongwei Bai: 0000-0003-4927-6422 Qiang Fu: 0000-0002-5191-3315 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 51673133 and 51421061). REFERENCES

(1) Reddy, M. M.; Vivekanandhan, S.; Misra, M.; Bhatia, S. K.; Mohanty, A. K. Biobased Plastics and Bionanocomposites: Current Status and Future Opportunities. Prog. Polym. Sci. 2013, 38 (10−11), 1653−1689. (2) Schneiderman, D. K.; Hillmyer, M. A. 50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers. Macromolecules 2017, 50 (10), 3733−3750. (3) Lee, C. T.; Huang, C. P.; Lee, Y. D. Biomimetic Porous Scaffolds Made from Poly(L-lactide)-g-Chondroitin Sulfate Blend with Poly(Llactide) for Cartilage Tissue Engineering. Biomacromolecules 2006, 7 (7), 2200−2209. (4) Lim, L. T.; Auras, R.; Rubino, M. Processing Technologies for Poly(lactic acid). Prog. Polym. Sci. 2008, 33 (8), 820−852. (5) Nampoothiri, K. M.; Nair, N. R.; John, R. P. An Overview of the Recent Developments in Polylactide (PLA) Research. Bioresour. Technol. 2010, 101 (22), 8493−8501. (6) Arias, V.; Hoglund, A.; Odelius, K.; Albertsson, A. C. Tuning the Degradation Profiles of Poly(L-lactide)-Based Materials through Miscibility. Biomacromolecules 2014, 15 (1), 391−402. (7) Muiruri, J. K.; Liu, S. L.; Teo, W. S.; Kong, J. H.; He, C. B. Highly Biodegradable and Tough Polylactic Acid-Cellulose Nanocrystal Composite. ACS Sustainable Chem. Eng. 2017, 5 (5), 3929−3937.

Figure 10. Tensile strength and Young’s modulus of SC-PLA products fabricated by powder densification under different pressures and subsequent sintering at 210 °C.

both the tensile strength and Young’s modulus of the SC-PLA products decrease significantly with increasing the crystallinity of nascent powders. Specially, the SC-160-300MPa product sintered from low-crystallinity powder exhibits a high tensile strength of 54.3 MPa and a superior Young’s modulus of 2.8 G

DOI: 10.1021/acs.macromol.7b01794 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01794 Macromolecules XXXX, XXX, XXX−XXX