Variation in melting point with molecular weight for hydroxybutyrate/hydroxyvalerate copolymers

Melting points are measured and equilibrium melting points calculated for hydroxybutyrate/hydroxyvalerate copolymers (PHB/HV) of varying molecular weight, containing 4 and 18% of hydroxyvalerate units. For low-HV copolymer, there is a significant reduction in melting point with molecular weight: values of equilibrium melting point in the range 176–188°C can be obtained from samples with molecular weight between 56000 and 220000. For the higher-HV copolymer, complex melting behaviour is observed, which suggests molecular-weight-dependent variations in the degree of exclusion of HV from the crystal lattice.