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

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.