Thermal and Crystallization Properties of a Polyhydroxyalkanoate Binary Copolymer Containing 3-Hydroxybutyrate and 3-Hydroxy-2-Methylvalerate Units

Abstract

Polyhydroxyalkanoates (PHAs) are aliphatic polyesters synthesized intracellularly by microorganisms as a carbon-storage substance. Among the various PHAs, 3-hydroxybutyrate (3HB)-based copolymers are crystalline polymers widely used as biodegradable plastics. Recently, PHAs containing α-carbon-methylated monomers, such as 3-hydroxy-2-methylbutyrate (3H2MB) and 3-hydroxy-2-methylvalerate (3H2MV), have been synthesized and characterized. However, a binary copolymer of 3HB and 3H2MV, P(3HB-co-3H2MV), had not yet been synthesized, and its material properties had not been investigated. In this study, P(3HB-co-11 mol% 3H2MV) (PHBMV11) was synthesized with recombinant Escherichia coli LSBJ, using trans-2-methyl-2-pentenoic acid as the 3H2MV precursor. The thermal properties of PHBMV11 were characterized using differential scanning calorimetry (DSC), and the results were compared with those of P(3HB-co-12 mol% 3-hydroxyvalerate) (PHBV12) to explore the effect of α-carbon methylation in the comonomer unit. PHBMV11 exhibited a higher enthalpy of fusion during the DSC heating process and higher crystallization temperature during the DSC cooling process than those of PHBV12. The half-crystallization time of PHBMV11 was slightly longer than that of the P(3HB) homopolymer and much shorter than that of PHBV12 previously reported. The α-carbon methylation of the 3H2MV unit also has a positive effect on the crystallization of 3HB-based copolymers, as the 3H2MB unit demonstrated previously.