Stereocomplex and individual crystallization behavior of symmetric or enantiomeric substituted Poly(lactic acid)s random copolymers with high crystallizabilities

Abstract

Stereocomplex (SC) crystallization is reported for the first time for the enantiomeric blends of random copolymers of l-configured poly(l-2-hydroxybutanoic acid-co-l-2-hydroxy-3-methylbutanoic acid) [P(L-2HB-co-L-2H3MB)] (49/51) and d-configured poly(d-2-hydroxybutanoic acid-co-d-2-hydroxy-3-methylbutanoic acid) [P(D-2HB-co-D-2H3MB)] (47/53) with high individual crystallizabilities. Unblended l-configured P(L-2HB-co-L-2H3MB) and d-configured P(D-2HB-co-D-2H3MB) formed individual crystallites through purification, solvent evaporation, and melt crystallization. Three types of individual crystallites (provisionally named as α-, β-, and γ-forms) were observed in the unblended copolymers depending on the crystallization methods. SC crystallites were formed in the blends through solvent evaporation and melt crystallization method. All types of monomer units cocrystallized in the individual and SC crystallites. The melting and equilibrium-melting temperatures of SC crystallites in the blends were 192.5–203.7 °C and 205.2 °C, respectively, which were significantly higher than the melting temperatures of individual crystallites in the unblended copolymers (61.6–133.1 °C). The maximum radial growth rate of SC crystallites (13.5 μm min−1) was 10 and 27 times higher than those of individual crystallites of the unblended copolymers (1.4 and 0.5 μm min−1), indicating that SC crystallization increased the crystallization rate. SC crystallization proceeded via regimes II and III. The copolymer crystallizability, monomer cocrystallizability, and crystallization rate of P(L-2HB-co-L-2H3MB) (49/51) and P(D-2HB-co-D-2H3MB) (47/53) are compared with those reported for the series of symmetric or enantiomeric polymer blends of poly(l-lactic acid-co-l-2-hydroxybutanoic acid) (56/44) and poly(d-lactic acid-co-d-2-hydroxybutanoic acid) (52/48) with lowindividualcrystallizabilities and poly(l-lactic acid-co-l-2-hydroxy-3-methylbutanoic acid) (47/53) and poly(d-lactic acid-co-d-2-hydroxy-3-methylbutanoic acid) (47/53) with noindividualcrystallizability.