Improving the Mechanical Properties of Biodegradable Polyhydroxyalkanoates via PHA–PHA Block‐Copolymer Synthesis

Polyhydroxyalkanoates (PHAs) are materials of high interest due to being bacterially synthesized and biodegradable in a variety of environments. Yet the most commonly available PHAs still lack the mechanical properties needed to truly replace a wide range of nondegradable plastics. Poly(3‐hydroxybutyrate) (P3HB) and its copolymer poly(3‐hydroxybutyrate‐ co ‐3‐hydroxyvalerate) (PHBV) exhibit high tensile strength and modulus. However, their high crystallinity leads to a lack of toughness and especially elongation, resulting in a relatively stiff and brittle material. By contrast, the copolymerization product of 3‐hydroxybutyrate (3HB) and 4‐hydroxybutyrate (4HB)—poly(3‐hydroxybutyrate‐ co ‐4‐hydroxybutyrate) (P3HB4HB)—shows a considerable improvement in elongation at break at higher 4HB contents, but lacks tensile strength and toughness, leading to a ductile but weak material. By coupling these very different PHAs together as a block copolymer, we have produced novel materials with significantly improved toughness compared to the homopolymers as well as comparable blends thereof, with an elongation at break of 630% and a toughness of 52 MJ/m 3 , more than tripling previously reported results for PHA–PHA block‐copolymers. Such attractive properties, comparable to those of commercial nondegradable thin film plastics, point to the potential for use of these biodegradable block copolymers in film applications.