C-13 NMR DETERMINATION OF THE DEGREE OF COCRYSTALLIZATION IN RANDOM COPOLYMERS OF POLY(BETA-HYDROXYBUTYRATE-CO-BETA-HYDROXYVALERATE)

A series of bacterially produced poly(beta-hydroxybutyrate-co-beta-hyd roxyvalerate) copolymers containing random distributions of 3-hydroxyb utyrate (3HB) and 3-hydroxyvalerate (3HV) were characterized by solid- state CPMAS C-13 NMR to determine the degree of cocrystallization of t he two components. The 3HV minor component was shown to be incorporate d into the poly(beta-hydroxybutyrate)-type crystalline phase over the 3HV composition range of 0-27 mol %. The ratio of the 3HV content in t he crystalline phase to the overall 3HV composition increases with inc reasing 3HV content, implying that it is easier to accommodate the lar ger 3HV residue in a crystalline region which already contains lattice imperfections. For samples with 3HV compositions of 21 and 27% the 3H V content in the crystalline phase was roughly 2/3 of the overall 3HV content. Asymmetric line broadening in the C-13 MMR spectra showed tha t the 3HV ethyl side branches assume multiple configurations when incl uded in the crystalline phase. For the 27% 3HV sample, two different c rystalline structures were detected-the poly(beta-hydroxybutyrate) (PH B) and poly(beta-hydroxyvalerate) (PHV) type lattice structures. The l atter crystal structure is present despite the fact that the overall 3 HV composition is below 40%, the composition at which the PHV-type cry stal structure is generally formed. Both crystal types are formed beca use the 3HV content in the noncrystalline phase is above the changeove r composition. The coexistence of PHV and PHB lattices suggests that t here may also be a change in crystal composition as a function of crys tallization time.