Physicochemical and mechanical properties of mixed culture polyhydroxyalkanoate (PHBV)

The mechanical properties of poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) copolymers with 3HV unit content ranging from 12 mol% to 72 mol% were evaluated. Samples were produced using a naturally enriched mixed culture fed with acetic and propionic acids. A range of feeding strategies was used to explore the manipulation of comonomer composition and sequence distribution of the monomeric units in order to influence mechanical properties. GC/MS, 13C NMR and DSC analysis of the resulting solvent cast films revealed that the as-produced PHAs were a mixture of copolymers distinct in microstructure and/or composition. Atomic force microscopy demonstrated the formation of varying phase structures as well as surface morphologies. In spite of the range of microstructure or compositional variation that was achieved through feeding strategies, all samples but one exhibited mechanical properties similar to the homopolymer P(3HB). The one distinct copolymer, with 43 mol% 3HV content, showed significantly increased elongation to break, as assessed after 2 weeks of ageing. These results were attributed to the formation of different phase structures arising from the competition between cocrystallisation and phase segregation in blends of semicrystalline copolymers.