Co-crystallization phenomena in biosynthesized isotactic poly[(R)-lactate-co-(R)-2-hydroxybutyrate]s with various lactate unit ratios

Poly[(R)-lactate-co-(R)-2-hydroxybutyrate], which is biosynthesized as a random co-polyester by microbial organisms, with various ratios of lactic acid (LA) to hydroxybutyrate monomers has been isothermally crystallized to investigate its thermal properties and crystal structure changes. Differential scanning calorimetry on the isothermally crystallized samples detected endothermic peaks due to the melting of the crystals at all LA unit ratios. Melting temperatures of the copolymers increased from ca. 107 °C–146 °C as the LA unit ratio increased. Glass transition temperatures also increased from 24 °C to 44 °C with increasing LA unit ratio. A melting temperature was observed even at LA unit ratios around 50%. Wide-angle X-ray measurements also revealed that co-crystallization occurred at all samples, including at LA unit ratios of ca. 50%. Despite of “jumping” phenomena many co-crystallizable random copolymers having, the lattice constants changed “linearly” from resembling poly[(R)-lactate] to poly[(R)-2-hydroxybutyrate] depending on the LA unit ratio. This unique co-crystallization behavior is discussed in detail.