Quaternary stereocomplex formation of substituted poly(lactic acid)s, l- and d-configured poly(2-hydroxybutanoic acid)s and l- and d-configured poly(2-hydroxy-3-methylbutanoic acid)s

Abstract

Quaternary and binary blends of l- and d-configured poly(2-hydroxybutanoic acid)s [P(L-2HB) and P(D-2HB), respectively] and l- and d-configured poly(2-hydroxy-3-methylbutanoic acid)s [P(L-2H3MB) and P(D-2H3MB), respectively] were prepared by solution-casting to have fixed ratios of P(L-2HB)/P(D-2HB) = 50/50 and P(L-2H3MB)/P(D-2H3MB) = 50/50 and different poly(2-hydroxybutanoic acid) [P(2HB), i.e., P(L-2HB) + P(D-2HB)] contents of 0–100 mol% and their melt-crystallized samples were prepared. The crystallization behavior in the quaternary blends was investigated by wide-angle X-ray diffractometry (WAXD) and differential scanning calorimetry. For the WAXD profiles of all melt-crystallized blends, only stereocomplex crystalline diffractions were observed at 9–11° and 16–22°, irrespective of their P(2HB) content values. Importantly, the interplane distance and melting temperature of the melt-crystallized blends continuously decreased and increased, respectively, with an increase in P(2HB) content, whereas the stereocomplex crystallinity or melting enthalpy values were higher than or similar to those calculated from the crystallinity or melting enthalpy values of P(L-2HB)/P(D-2HB) and P(L-2H3MB)/P(D-2H3MB) blends wherein only binary stereocomplexes were formed. These findings indicate that quaternary stereocomplex of P(L-2HB), P(D-2HB), P(L-2H3MB), and P(D-2H3MB) is formed. Quaternary stereocomplex formation of optically active biodegradable poly(2-hydroxyalkanoic acid)s reported in the present study is expected to provide a novel versatile method for preparing biodegradable materials with a wide range of physical properties and biodegradability.