Cocrystallization of monomer units of biobased and biodegradable Poly(l-lactic acid-co-glycolic acid) random copolymers

Abstract

Random copolymers of l-lactic acid (LLA) and glycolic acid (GA) [P(LLA-GA)] with 0–100 mol% LLA units in the polymers were synthesized and their crystallization behavior was investigated by two representative crystallization methods of precipitation or melt-crystallization. P(LLA-GA) copolymers with LLA unit contents in the ranges of 0–20 and 73–100 mol% were crystallizable during precipitation or melt-crystallization. Surprisingly, these crystallizable ranges are similar to those reported for P(LLA-GA) copolymers synthesized by ring-opening polymerization of cyclic dimers (l-lactide and glycolide), which should have monomer sequence lengths twice those of the P(LLA-GA) copolymers synthesized by polycondensation of the monomers (LLA and GA) in the present study and are expected to have wider crystallizable ranges. The GA units of the LLA-rich P(LLA-GA) copolymers were incorporated in a lattice of LLA unit segments, whereas the LLA units of the GA-rich P(LLA-GA) copolymers were incorporated in a lattice of GA unit segments. Even the incorporation of relatively small-sized GA units in relatively large-sized LLA unit segments strongly induced structural disorder in the crystalline lattice. The crystalline size and melting temperature of the P(LLA-GA) copolymers decreased dramatically as the LLA unit content deviated from 0 to 100 mol%.