Tacticity-Dependent Epitaxial Crystallization of Poly(<scp>l</scp>-lactic acid) on an Oriented Polyethylene Substrate
Li Li, Rui Xin, Huihui Li, Xiaoli Sun, Zhongjie Ren, Qigu Huang, Shouke Yan
Abstract
The influence of tacticity on the bulk crystallization and polyethylene (PE)-induced epitaxial crystallization of poly(l-lactic acid) (PLLA) was investigated. It was found that cold-crystallization of PLLA with different tacticities from the amorphous glassy state is easier than the melt-crystallization from the isotropic melt and the cold-crystallization of PLLA at the PE surface takes place earlier than in bulk. The time-lag between the PE-induced epitaxial crystallization and bulk cold-crystallization of PLLA increases with decreasing tacticity. For example, while the cold-crystallization of the PLLA sample with 95% tacticity in the bulk and at the PE surface starts almost simultaneously, the bulk cold-crystallization of PLLA with a tacticity of 89% starts at a time point where over 90% PLLA already crystallized epitaxially on the PE substrate. The almost simultaneous start of bulk and PE-induced cold-crystallization of high tacticity PLLA enables the creation of a cross-hatched lamellar structure, which is composed of PLLA lamellae oriented along the PE chain direction and tilted ca. 64° with respect to the PE chain direction. The big time-lag between PE-induced epitaxy and bulk cold-crystallization of low-tacticity PLLA results in the disappearance of tilted PLLA lamellae. This further confirms that the PLLA lamellae aligned in the chain direction of PE are caused by PE-induced epitaxial crystallization, whereas the tilt lamellae are caused by the homoepitaxy of PLLA based on the parallelism of the helical paths.