Kinematic Interplay of Homopolymer Crystallization with Stereocomplex Formation in Poly(L-lactic acid) / Poly(D-lactic acid) Blends Having Asymmetric Compositions 1. Effects of Molecular Weights of Polymers

Abstract

Poly(lactic acid) is a biobased polymer and a promising material to contribute sustainability. Many reports have been published to improve its mechanical and thermal properties. Among them, crystallization behaviors are one of the main topics. It is known that equimolar mixing of poly(L-lactic acid) / poly(D-lactic acid) (PLLA/PDLA) forms stereocomplex crystal (SC), in addition to the crystallization of these individual homopolymer moieties. In this study, effects of blend composition were investigated to understand the roles of the excess moiety in the PLLA/PDLA blends having asymmetric compositions. For this purpose, DSC (differential scanning calorimetry), POM (polarizing optical microscopy), and WAXD (wide-angle X-ray diffraction) measurements have been conducted. One of the significant results is a remarkably high amount formation of homocrystal (HC) in the 20/80 blend, which contributes the total degree of crystallinity (χ_total = χ_SC + χ_HC) where χ_SC and χ_HC denote the crystallinity of SC and HC, respectively. This further may contribute prevention of thermal contraction of the blend specimens at temperature below 100˚C when χ_total exceeds 50%. Since it is considered that the 50/50 blend is the best to attain the maximum crystallinity, the experimental result showing the maximum crystallinity of the 20/80 blend is surprising. Furthermore, for the opposed composition, i.e., the 80/20 blend, the total crystallinity is not so high. Namely, it was found that the dependencies of χ_total, χ_SC, χ_HC, and the growth rate of the spherulite as a function of the blend composition exhibited asymmetric features. This result suggests that the excess moiety in the biased blend (i.e., 20/80 vs. 80/20) did not play completely an identical role. It is speculated that the subtle difference in the molecular weights of PLLA and PDLA samples used in this study may even ascribe to the asymmetric tendency of the effects of the blend composition.