Biobased copolymers composed of l -lactic acid and side-chain-substituted lactic acids: Synthesis, properties, and solid-state structure

Various biobased copolyesters composed of l-lactic acid (LLA) and side-chain-substituted lactic acids (SLAs) with high molecular weight and LLA-rich composition are synthesized by direct solution polycondensation. Glycolic acid, 2-hydroxyisobutyric acid, (S)-2-hydroxy-3-methylbutanoic acid (LVOH), and (S)-2-hydroxy-4-methylpentanoic acid (LLOH) are chosen as SLAs. Wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) reveal that the substituent size of the SLAs determines whether or not SLA units are included in crystallites of the LLA sequences. Unlike poly(l-lactic acid), tensile necking is seen in the copolymers with 2 mol% LVOH and with 6 mol% LLOH. Enzymatic degradation is accelerated with increasing SLA content. Especially for 6 mol% LLOH, the SLA introduction improves both the mechanical properties and the enzymatic degradability.