Synthesis and characterisation of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) multi-block copolymers comprising blocks of differing 3-hydroxyvalerate contents

Polyhydroxyalkanoates (PHAs) are attractive alternatives to commodity petroleum derived plastics, being bacterially derived, thermoprocessible and truly biodegradable under ambient conditions. However, the common short chain length PHAs - poly(3-hydroxybutyrate) (P3HB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) - are relatively stiff and brittle, and P3HB in particular has a narrow processing window. Block copolymer synthesis offers an opportunity to produce novel PHA materials comprised of different PHA phases, delivering a potential step-change improvement in PHA properties due to microphase separation. Therefore, this work demonstrates the synthesis and characterisation of novel high molecular weight PHBV-b-PHBV-b-PHBV block copolymers using isocyanate chemistry. High molecular weight, narrow compositional distribution, hydroxy-functionalised PHBV blocks of low (1 mol%) 3HV and high (67 mol%) 3HV contents were used as building blocks. The resulting block copolymers showed a very significant increase in elongation at break (to 76.0 %), as well as improved tensile toughness (to 15.0 MJ/m3) and tensile strength (to 18.5 MPa) compared to the starting materials or comparable blends. Based on the similarity in properties between the 1 mol% 3HV block copolymer products and their counterpart random copolymers, it is also shown that the introduction of urethane linkages had minimal effect on polymer properties. The morphologies of these novel PHBV materials were investigated, supporting the concept that the microphase separation of the different PHBV phases delivered these improved mechanical properties. This approach to PHA-b-PHA block copolymer synthesis offers an attractive generic platform for the production of novel biodegradable PHA materials for film and other applications.