Effect of monomeric composition on the thermal, mechanical and crystalline properties of poly[(R)-lactate-co-(R)-3-hydroxybutyrate]

Poly[(R)-lactate-co-(R)-3-hydroxybutyrate] [P(LA-co-3HB)] is a biobased polyester with flexible properties efficiently synthesized by engineered Escherichia coli. Here, we aimed at optimizing the monomeric composition of the copolymer in terms of its flexibility, and elucidating structural features contributing to their mechanical properties. The LA fraction was successfully regulated in the range of 6–66 mol% by combination of metabolic and enzyme engineering approaches. The copolymers with higher LA fraction showed decreasing melting point from 160 to 125 °C, but increasing glass transition temperature from 7 to 27 °C. Crystallinity of the as-cast film, that was mainly attributed to the crystallization of 3HB unit, also decreased with the increasing LA fraction. Owing to the combined effect of these parameters, the highest elongation to break (approximately 400%), which is comparable to that of polyethylene, was obtained for the copolymers with middle range LA fraction (33 mol%). The high flexibility was maintained in most of the copolymers. Notably, P(21mol%LA-co-3HB) retained its high elongation at break (about 300%) even after storage under ambient condition for 5 months. These results demonstrate that introduction of LA units into the polymer chain effectively and stably inhibited the crystallization of 3HB units.