High-Strain Shape Memory Behavior of PLA-PEG Multiblock Copolymers and Its Microstructural Origin

Shape memory properties of two thermoplastic multiblock copolymers composed of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) having different PEG-segment lengths of 6 and 11 kDa were studied. The performance as a shape memory polymer at high strain level (600%) and its interrelations with shape-programming conditions, molecular orientation, and microstructural changes are elucidated. A significant contribution of strain-induced crystallization of PLA segments to the improvement of temporary shape fixation was evidenced upon increasing draw ratio and/or shape-holding duration as well as programming temperature (within certain range) without largely sacrificing the shape recoverability. Series of microstructural characterizations reveal the occurrence of fibrillar-to-lamellar transformation upon shape recovery (at 60 degrees C) of the samples programmed at 40 degrees C, generating shish-kebab crystalline morphology. Such phenomenon is responsible for the high-strain shape memory effect of these materials. The unprecedented formation of shish-kebab structure at such relatively low temperature (instead of the melting temperature range) in solid state observed in these copolymers as well as their high-strain shape memory functionality would bestow the promising future for their practicability in diverse areas. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 241-256