Texture Induced by Molecular Weight Dispersity: Polymorphism within Poly(L-lactic acid) Spherulites

Abstract

Poly(L-lactic acid) (PLLA) has drawn much attention due to its excellent medical and pharmaceutical applications for decades. As a semi-crystalline polymer, morphology and crystal structure of PLLA greatly determine its properties. Here, we demonstrate, for PLLA films, a non-conventional texture featuring two types of spherulites emerging in pairs to form a distinct nested structure where a small spherulite (~10 µm) is embedded in a large one (100 µm to 300 µm). In addition to the size, the molecular weight and polymorph are different in the large and small spherulites. Crystallographic α-form and relatively low molecular weight are identified in the large spherulites, while meta-stable α′-form and relatively high molecular weight in the small ones. These differences suggest that the polydisperse PLLA polymers fractionate during film formation and the high-molecular-weight fraction crystallizes into the small spherulites with meta-stable structure because of its complicated polymer entanglement and high viscosity. In contrast, the rest of polymers crystallize into the large spherulites with the thermodynamically stable polymorph. Furthermore, this texture exhibits accelerated PLLA degradation initiated from the small spherulites, which is distinct from the typical PLLA spherulites. Insights provided by this work may lead to new texture-properties relationship associated with polydispersity of molecular weight.