MEASUREMENT OF THE CRYSTALLINITY OF POLY(BETA-HYDROXYBUTYRATE-CO-BETA-HYDROXYVALERATE) COPOLYMERS BY INVERSE GAS-CHROMATOGRAPHY

Inverse gas chromatography (IGC) studies on semicrystalline polymers provide information supplemental to X-ray and differential scanning calorimetry (DSC) experiments. In this work, studies were made of a series of random copolymers of beta-hydroxybutyrate (HB) and beta-hydroxyvalerate (HV) ranging from 0-27% in HV content. While X-ray data indicated that the degree of crystallinity remained at about 60% throughout this composition range, DSC and density data indicated a substantial reduction. The IGC experiments confirmed the X-ray results, as did studies by Fourier transform infrared spectroscopy. It is concluded that, for polymers which exhibit isodimorphism (unit cocrystallization), the assumptions involved in calculating the degree of crystallinity for the copolymers from the density or heat of fusion of the pure homopolymers, poly(beta-hydroxybutyrate) and poly(beta-hydroxyvalerate), are incorrect. Cocrystallization of this type apparently introduces defects in the crystal lattice which reduce the heat of fusion without changing the degree of crystallinity.