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/m³, 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.