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