Viscoelastic relaxations and thermal properties of bacterial poly (3-hydroxybutyrate-co-3-hydroxyvalerate)and poly (3-hydroxybutyrate-co-4-hydroxybutyrate)

3-Hydroxybutyrate-3-hydroxyvalerate (3HB-3HV)as well as 3-hydroxybutyrate-4-hydroxybutyrate (3HB-4HB)copolyesters have been investigated by differential scanning calorimetry, thermogravimetric analysis and dynamic mechanical spectroscopy, over a wide range of compositions (0-95 mol percent 3HV; 0-82 mol percent 4HB). Both series of isolated copolyesters are partially crystalline at all compositions. Quenched samples show a glass transition that decreases linearly with increasing co-monomer molar fraction, more markedly when the co-monomer is 4HB. Above Tg, all copolyesters, rich in 3HB units, show a cold crystallization phenomenon followed by melting, while at the other end crystallization on heating is observed only in 3HB-3HV copolymers. The viscoelastic spectrum, strongly affected by thermal history, shows two relaxation regions; the glass transition, whose location depends on copolymer type and composition, and a secondary dispersion region at low temperatures (?130?80°C). The latter results from a water-related relaxation analogous to that of P(3HB)and, in 3HB-4HB copolymers, from another overlapping absorption peak centred at ?130°C, attributed to local motion of the methylene groups in the linear 4HB units.