Stereocomplex crystallization and homo-crystallization of enantiomeric substituted poly(lactic acid)s, poly(2-hydroxy-3-methylbutanoic acid)s

Abstract

In both solution- and melt-crystallization of blends of poly(l-2-hydroxy-3-methylbutanoic acid) [P(L-2H3MB)] and poly(d-2-hydroxy-3-methylbutanoic acid) [P(D-2H3MB)], solely stereocomplex (SC) crystallites were formed without formation of homo-crystallites, excluding the melt-crystallization at crystallization temperature (Tc) = 100 °C, wherein both SC and homo-crystallization took place. In solution-crystallization of neat P(L-2H3MB) and P(D-2H3MB), the same type of homo-crystallites as reported earlier were formed. In melt-crystallization of the neat P(L-2H3MB) and P(D-2H3MB), for the Tc range of 0–150 °C, a new type of homo-crystallites having WAXD profiles similar to the α- or δ-form homo-crystallites of neat PLLA and PDLA were formed. The melting temperature (Tm) values of SC crystallites in the blend (189–193 °C) were slightly higher than those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB) (163–178 °C). The radial growth rate of spherulites (G) and the spherulitic number per unit area of SC crystallites in the blend were respectively much lower and higher than those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB). Despite not so high Tm and lower G of SC crystallites in the blend compared with those of homo-crystallites in the neat P(L-2H3MB) and P(D-2H3MB), the critical Tc of the blend above which the induction period for spherulite growth (ti) becomes positive was higher by 30 °C than those of the neat polymers. These results indicate that in the blend samples, the large side chains of P(L-2H3MB) and P(D-2H3MB) lowered the G of SC spherulites but facilitated the nucleation process, compared with those of the spheruites of homo-crystallites in the neat polymers.