STUDY OF THE CRYSTAL PHASE AND CRYSTALLIZATION RATE OF BACTERIAL POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE)

The crystal phase of random 3-hydroxybutyrate/3-hydroxyvalerate (3HB/3HV) copolyesters (3HV content 0-95 mol %) has been investigated by wide-angle X-ray scattering. Isodimorphism of the copolymer system is confirmed, P(3HB-co-3HV) crystallizing in either a P(3HB) or P(3HV) lattice for 3HV contents lower or higher than 40 mol %, respectively. P(3HB-co-41% 3HV) shows the coexistence of both crystal phases and corresponds to the pseudoeutectic composition. With an increasing amount of counits, both P(3HB) and P(3HV) crystal lattices change: the P(3HB) cell expands (changes in both the a and b parameters), and the P(3HV) cell slightly contracts (changes in the b parameter). All P(3HB-co-3HV) are highly crystalline (> 50%), but crystallinity develops at very different rates depending on composition: the slower crystallization rates are found close to the pseudoeutectic, where the spherulite radial growth rate is 4 orders of magnitude lower than that of P(3HB). Upon isothermal crystallization, 3HB/3HV copolyesters develop banded spherulites over rather wide ranges of crystallization temperature (T(c)). The band spacing is found to increase with increasing T(c).