Crystalline morphology and thermal properties for random copolyesters of(R)-3-hydroxybutyric acid with different hydroxyalkanoic groups

Abstract

The solid-state structures and thermal properties of melt-crystallized films of random copolymers of (R)-3-hydroxybutyric acid (3HB) with different hydroxyalkanoic acids such as (R)-3-hydroxypentanoic acid (3HV), (R)-3-hydroxyhexanoic acid (3HH), medium-chain-length (R)-3-hydroxyalkanoic acids (mcl-3HA; C8-C12), 4-hydroxybutyric acid (4HB), and 6-hydroxyhexanoic acid (6HH) were characterized by means of small-angle X-ray scattering, differential scanning calorimetry, and optical microscopy. The randomly distributed second monomer units except for 3HV in copolyesters act as defects of P(3HB) crystal and are excluded from the P(3HB) crystalline lamellae. The lamellar thickness of copolymers decreased with an increase in either the main-chain or the side-chain carbon numbers of second monomer units. In addition, the growth rate of spherulites decreased with an increase in the carbon numbers of second monomer units for copolymers with an identical comonomer composition. These results indicate that the steric bulkiness of second monomer unit affects on the crystallization of 3HB segments in random copolyesters.