Biosynthesis of Poly(3-Hydroxybutyrate-co‑3-Hydroxyhexanoate) with a High 3-Hydroxyhexanoate Fraction and Low Molecular Weight for Polymer Blending

Abstract

Polyhydroxyalkanoates (PHAs) are aliphatic polyesters that are biosynthesized and accumulate in bacterial cells. Among PHAs, poly(3-hydroxybutyrate-co‑3-hydroxyhexanoate)s [P(3HB‑co‑3HHx)s] are known to be practical PHA copolymers with an appropriate degree of flexibility. In the biosynthesis of PHA, ethanol induces chain transfer (CT) reaction, resulting in the biosynthesis of low-molecular-weight PHA. In this study, P(3HB‑co‑3HHx)s were biosynthesized using recombinant Escherichia coli by feeding fatty acid(s) and ethanol as the 3HHx precursor and CT agent, respectively, to obtain polymers with high 3HHx fractions and low molecular weights. Two biosynthesized P(3HB‑co‑3HHx)s, whose 3HHx fractions were 49 and 86 mol% and weight average molecular weights were 1.5×104 and 2.8×104, respectively, were blended with P(3HB‑co‑5 mol% 3HHx) (PHBH5) as the base material. The blending properties were investigated to explore how the thermal properties can be modified. Thermal analysis by differential scanning calorimetry (DSC) showed that blends prepared by solution casting were obviously immiscible when 30 wt.% or more of biosynthesized polymer was added, even if its molecular weight was low. In the blended polymers, a lower crystallization effect compared to neat PHBH5 was observed without changing the melting temperature. However, crystallization was promoted in the blended material under specific blending conditions. Thus, blending is a simple method for producing polymer materials with altered thermal and microstructural properties