Temperature-dependent Crystallization and Phase Transition of Poly(L-lactic acid)/CO 2 Complex Crystals
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ARTICLE
POLYMER SCIENCE
https://doi.org/10.1007/s10118-021-2502-6 Chinese J. Polym. Sci.
Temperature-dependent Crystallization and Phase Transition of Poly(L-lactic acid)/CO2 Complex Crystals Ying Zhenga, Cai-Liang Zhanga,b, Yong-Zhong Baoa,b, Guo-Rong Shana,b, and Peng-Ju Pana,b* a State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China b Institute of Zhejiang University-Quzhou, Quzhou 324000, China
Abstract Semicrystalline polymers can crystallize in the unique crystalline polymorph and show different phase behaviors under the highpressure CO2 treatment. Understanding such unique crystallization and phase transition behavior is of fundamental importance for the CO2assisited processing of semicrystalline polymers. Herein, we investigated the polymorphic crystalline structure, phase transition, and structureproperty relationships of poly(L-lactic acid) (PLLA) treated by CO2 at different pressures (1−13 MPa) and crystallization temperatures (Tc’s, 10− 110 °C). PLLA crystallized in the PLLA/CO2 complex crystals under 7−13 MPa CO2 at Tc ≤ 50 °C but the common α crystals under the high-pressure CO2 at Tc ≥ 70 °C. Solid-state nuclear magnetic resonance analysis indicated that the PLLA/CO 2 complex crystals possessed weaker interactions between the PLLA chains than the common α crystals. The PLLA/CO2 complex crystals were metastable and transformed into the thermally stable α crystals via the solid-to-solid route during heating or annealing at the temperature above 50 °C. The complex crystals of PLLA produced at low Tc was more ductile than the α crystals due to the lower crystallinity and the plasticizing effect of CO2. Keywords Poly(lactic acid); CO2; Complex crystal; Phase transition Citation: Zheng, Y.; Zhang, C. L.; Bao, Y. Z.; Shan, G. R.; Pan, P. J. Temperature-dependent crystallization and phase transition of poly(L-lactic acid)/CO2 complex crystals. Chinese J. Polym. Sci. https://doi.org/10.1007/s10118-021-2502-6
INTRODUCTION As a potential substitute of the petroleum-based polymers, poly(L-lactide) (PLLA) derived from the plant-derived chemicals has attracted increasing interests in both the scientific and technological fields due to its good biocompatibility, biodegradability, processing ability, and mechanical property.[1,2] As a typical semicrystalline polymer, physical properties of PLLA depend strongly on the crystalline structure, morphology, and crystallinity (Xc).[3−6] PLLA is a representative polymorphic polymer that can grow in several crystal modifications such as α,[7] α' (or δ),[8,9] α",[10] β,[11,12] γ[13] forms, and mesophase.[14,15] In addition to these polymorphs, PLLA is also able to cocrystallize in the complex crystals with CO2[16,17] and some organic solvents[17−19] under the specific conditions. In the case of PLLA/CO2 complex crystals, the CO2 molecules are encapsulated in the cavity surrounded by four PLLA chains in the crystal unit cell.[16] Compressed and supercritical CO2 has been widely
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