Investigation on the Influence of Fold Conformation on PLLA Lamellar Splaying by Film Crystallization in Supercritical CO2
Lamellar splaying is an important non-crystallographic branching phenomenon in the polymer spherulite growth. The common cognition on the origin of lamellar splaying is attributed to the repulsive action caused by cilia, but the recent researches using solid nuclear magnetic resonance indicate that there are no cilia in the amorphous phase between adjacent lamellae. In order to further understand the origination of lamellar splaying, we reported a method to prepare Poly(L-lactic acid) (PLLA) multilayer lamellae by film isothermal crystallization from melt state in supercritical CO2. We obtained the screw terrace crystals with three different surface amorphous conformations assembled from three PLLA samples with different molecular weights. According to the characterizations results of atomic force microscopy, transmission electron microscopy, nuclear magnetic resonance and X-ray diffraction, three kind of intercrystalline phases were determined as no-fold, tight fold, and loose fold, respectively. It was further found that the no-fold and tight fold rarely contribute to lamellar splaying of PLLA, but the loose fold on lamellar surface obviously contributes to lamellar splaying of PLLA. Furthermore, the molecular weight dependence and film thickness dependence of PLLA crystal form indicated that loose fold may be caused by the limit of in-plane entanglements in the lamellar thickening growth.