Shanghai, China
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Zhang Z.-C.,Sichuan University | Sang Z.-H.,Sichuan University | Huang Y.-F.,Sichuan University | Ru J.-F.,Sichuan University | And 4 more authors.
ACS Sustainable Chemistry and Engineering | Year: 2017

Stereocomplex crystals (SCs) of polylactides (PLAs) with melting points over 220 °C show great potential to improve the heat deflection resistance of PLAs. However, it is still a challenge to fabricate PLA materials with high SC contents due to the requirement for high production efficiency and thus an extremely large cooling rate. In the present work, an upgraded injection molding method, i.e., oscillation shear injection molding (OSIM), was employed to impose intense shear flow on poly(L-lactide) (PLLA)/poly(D-lactide) (PDLA) samples. It is proved that even though a large cooling rate existed, the intense shear flow provided by OSIM induced higher crystallinities of the SCs and well-defined lamellar structure in comparison with conventional injection-molded ones, which subsequently resulted in a high Vicat softening temperature (close to 200 °C) and superb heat deflection resistance in boiling water. To clarify the mechanism of shear-induced SC formation, in situ characterization with precisely controlled parameters was done by performing rheological measurements. A more sensitive response of SC crystallization kinetics to shear flow is observed compared to that of shear-induced homocrystallites, which is attributed to shear-induced stereoselective interaction and the existence of a transiently cross-linking network built through hydrogen bonds in sheared PLLA/PDLA melts. These findings provide an effective method to prepare PLA samples with promising heat deflection resistance without introducing any extra component and reducing its environmental friendliness. ©2016 American Chemical Society.


Song Y.,CAS Beijing National Laboratory for Molecular | Zhang X.,Beijing Institute of Fashion Technology | Yin Y.,CAS Beijing National Laboratory for Molecular | Zhang C.,CAS Beijing National Laboratory for Molecular | And 5 more authors.
European Polymer Journal | Year: 2016

The isothermal crystallization behavior of a 1:1 poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) mixture under pulsed shear was investigated by in-situ polarized light optical microscopy (POM) and wide angle X-ray scattering (WAXS). The effects of shear rate and temperature were studied. The results showed that application of shear flow increased the nucleation density, crystallization rate and degree of orientation for both stereocomplex crystals (SC) and α crystals. Interestingly, when the crystallization temperature was below the melting temperature of α crystals, the phase content of SC increased with shear rate, while the α content displayed a bell-shaped curve with regard to shear rate. The structure formation in this system was closely related to the shear-induced nucleation, phase mixing and chain mobility. © 2015 Elsevier Ltd. All rights reserved.


Yin Y.,CAS Beijing National Laboratory for Molecular | Yin Y.,University of Chinese Academy of Sciences | Liu G.,CAS Beijing National Laboratory for Molecular | Song Y.,CAS Beijing National Laboratory for Molecular | And 7 more authors.
European Polymer Journal | Year: 2016

The formation of stereocomplex (SC) in enantiomeric poly(lactide)s via recrystallization of homocrystals was investigated by using simultaneous wide-angle and small-angle X-ray scattering. When the symmetric blend of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) was heated from the glassy state at a slow heating rate (5 °C/min), predominant homocrystals were formed during heating (cold crystallization). Melting of homocrystals and crystallization of stereocomplex occurred almost simultaneously. In this case, the diffusion of PLLA and PDLA chains played an important role in determining the maximum attainable SC crystallinity. The crystallization behavior during annealing at different temperatures (185 °C, 195 °C and 225 °C) and subsequent cooling was further investigated to understand the effectiveness of annealing on the formation of stereocomplex. The stereocomplex content was unchanged during annealing at 185 °C and kept constant during cooling to room temperature. The SC crystallinity increased slightly during annealing at 195 °C and subsequently cooling. On the other hand, the content of SC increased during annealing at 225 °C and increased remarkably during cooling. A possible mechanism considering the diffusion of PLLA and PDLA chains was proposed. © 2016 Elsevier Ltd


Song Y.,CAS Beijing National Laboratory for Molecular | Zhang X.,Beijing Institute of Fashion Technology | Yin Y.,CAS Beijing National Laboratory for Molecular | De Vos S.,Corbion Purac | And 4 more authors.
Polymer (United Kingdom) | Year: 2015

The non-isothermal crystallization behavior of poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blends under shear was investigated by in-situ polarized light optical microscopy (POM) and wide angle X-ray scattering (WAXS). Pulse shear for 5 seconds was applied to the melt during cooling. The effects of shear temperature and shear rate were systematically studied. It was found that the shear field could increase the nucleation density of stereocomplex crystal (SC), and accelerate the overall crystallization rate. The final crystallinity of SC increased to as high as ∼45% with a shear rate of 178 s-1 when the shear temperature was 150°C. The onset crystallization temperature of α crystals increased by ∼10°C. The final content of α crystals reduced with the increase of SC content. Multiple factors influence the crystallization of SC and α crystals, including the shear rate, shear temperature and the nucleating/confinement effect of SC on α crystals. © 2015 Elsevier Ltd.All rights reserved.


Xiong Z.,Beijing Institute of Fashion Technology | Xiong Z.,CAS Beijing National Laboratory for Molecular | Zhang X.,Beijing Institute of Fashion Technology | Wang R.,Beijing Institute of Fashion Technology | And 4 more authors.
Polymer (United Kingdom) | Year: 2015

An aryl amide derivative (TMB-5) was used to nucleate the poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) blend for the first time. The effect of TMB-5 nucleating agent on the crystallization behavior of PLLA/PDLA blend was systematically investigated. The selective nucleation of stereocomplex (sc) crystals by incorporation of TMB-5 was realized under appropriate crystallization conditions. During the cooling process, various morphology of TMB-5 due to its partial or complete dissolution in the PLLA/PDLA blend melt was observed. Furthermore, an sc crystals layer on the lateral surface of TMB-5 was found. Upon cooling a PLLA/PDLA blend from 260 °C at rates between 2 and 100 °C/min, 0.5 wt% addition of TMB-5 induced a single melting peak of sc crystals and no homocrystallization was observed in the subsequent DSC heating scan. The melting temperature of the sc crystals was found to decrease in the presence of TMB-5 from 230 to ca. 200 °C, which is still 20 °C higher than that of PLLA homopolymer. The results suggest that the supramolecular structure of the nucleating agent plays an important role in the crystallization of the PLLA/PDLA blend. The difference in solubility and recrystallization ability of TMB-5 in polymer melt results in the variation of crystallization behavior of PLLA/PDLA blend, which is correlated with the concentration of TMB-5 and crystallization conditions including the final melting temperature, annealing time and cooling rate. © 2015 Elsevier Ltd. All rights reserved.


Yin Y.,CAS Beijing National Laboratory for Molecular | Song Y.,CAS Beijing National Laboratory for Molecular | Xiong Z.,CAS Beijing National Laboratory for Molecular | Zhang X.,Beijing Institute of Fashion Technology | And 5 more authors.
Journal of Applied Polymer Science | Year: 2016

The effect of the final melting temperature (Tf) on the crystallization of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) was studied via a combination of differential scanning calorimetry, wide-angle X-ray scattering, polarized optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. We observed that a residual stereocomplex (SC) crystal induced the formation of SC crystals during cooling from a Tf (230°C) just above the melting peak of the SC crystals. On cooling from a Tf (240°C) just above the endset temperature of SC crystal melting [Tm(S)(E)], the possible order structure and the strong interchain interaction promoted the preferential crystallization of SC crystals; this enhanced the formation of α crystals. During cooling from a Tf (≥250°C) far above Tm(S)(E), the crystallization peaks of α and SC crystals converged. The FTIR results indicated that the residual SC crystals, possible ordered structure, and interchain interactions in the melt might have been the key factors for the different crystallization of PLLA/PDLA. © 2015 Wiley Periodicals, Inc.


Yin Y.,CAS Beijing National Laboratory for Molecular | Zhang X.,Beijing Institute of Fashion Technology | Song Y.,CAS Beijing National Laboratory for Molecular | De Vos S.,Corbion Purac | And 4 more authors.
Polymer (United Kingdom) | Year: 2015

This work examined the effect of nucleating agents on the strain-induced crystallization of poly(l-lactide) (PLLA). Talc and poly(d-lactide) (PDLA) were used as the nucleating agents of PLLA. Wide angle X-ray scattering (WAXS) revealed that the original injection molded specimens of nucleated PLLA blends were free of PLLA α crystal. The structural evolution during uniaxial stretching of neat and nucleated PLLA above the glass transition temperature was investigated by in-situ WAXS. It was observed that the nucleating agents significantly accelerated the crystallization of PLLA during stretching. The accelerating effect for the PLLA crystallization during stretching increased in the following order: PDLA/talc > PDLA > talc. In step-strain measurements with a high stretching speed, the effect of nucleating agents on the crystallization of PLLA showed a strain-dependence. The enhancement of crystallization by nucleating agents was remarkable at 200% strain. However, with 500% strain, the strain-induced crystallization was dominant and the effect of nucleating agents on crystallization of PLLA was not obvious. The results were analyzed considering the nucleating effect of nucleating agents on PLLA. © 2015 Elsevier Ltd. All rights reserved.


Huang W.,Ningbo Institute of Materials Technology and Engineering | Huang W.,Ningbo University | Luo X.-Z.,Ningbo Institute of Materials Technology and Engineering | Wang B.-J.,Ningbo Institute of Materials Technology and Engineering | And 6 more authors.
Macromolecular Chemistry and Physics | Year: 2015

Nanostructures of stereocomplex polylactide (sc-PLA) are obtained and studied in poly(l-lactide) (PLLA) doped with a low amount of poly(d-lactide) (PDLA) during successive melt-quenching, extrusion, spinning, and drawing processes corresponding to quiescent, shear flow, elongational flow, and tensioned annealing conditions, respectively. Nanogranules of predominantly sc-PLA are initially formed with rapid quenching in quiescent and shear flow, which developed into microspheres with slow quenching and uniform nanofibrils in elongational flow. While only amorphous or the α′-form PLLA is formed with the quenched melts and macroscopic fibers, the embedded nanogranules and nanofibrils are highly crystallized with the coexistence of sc-PLA and the α-crystals. A 1D coalescence of nascent sc-nuclei into nanofibrils in elongational flow is preliminarily proposed to explain the structure evolution and the minor reinforcement of the nanofibrils on the macroscopic fibers. Nanostructures of predominantly stereocomplex polylactide (sc-PLA), including nanogranules and nanofibrils, are obtained during direct melt process of poly(l-lactide) doped with poly(d-lactide). The nanostructures take diverse, but well-defined, morphologies and substructures, as governed by thermal and flow conditions. This opens up a facile and scalable route toward either all-PLA nanocomposites or nanofibrils and membranes of sc-PLA. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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