Controlling orientation, polymorphism, and crystallinity in thin films of poly(lactic-acid) homopolymer and stereocomplex aligned by high temperature rubbing

Abstract

High temperature rubbing is a fast and low-cost method that can align semi-crystalline polymers such as poly(lactic acid) (PLA) by application of a shear-force via a rotating cylinder covered with a microfiber cloth on a thin film maintained at high temperature. A multi technique approach uncovers the role of rubbing temperature and film thickness on the PLA thin film structure. Both poly(l-lactic acid) (PLLA) and the stereo-complex (SC-PLA) can be readily aligned by high temperature rubbing. The rubbing temperature TR and the film thickness are the key parameters that control polymorphism, in-plane orientation, crystallinity and contact plane of the polymer crystals. Rubbing PLLA films in the range 80–140°C affords different aligned polymorphs (mesophase, α′, α′ + α mixtures and pure α phase) depending on the rubbing temperature. Pure α form of PLLA with 53% crystallinity is obtained at TR = 140°C whereas pure oriented crystalline SC-PLA films are obtained by rubbing at 200°C.