THERMODYNAMICS OF THE MELTING-POINT DEPRESSION IN POLY(BETA-HYDROXYBUTYRATE-CO-BETA-HYDROXYVALERATE) COPOLYMERS

Both small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) data were used to establish relationships between the degree of crystallinity, melting temperature, and lamellar thickness for random copolymers of poly(beta-hydroxybutyrate-co-beta-hydroxyvalerate) (PHB/V). It was shown that the predictions of Sanchez and Eby that lamellar thickness increases with both cyrstallization temperature and comonomer content are correct. The Sanchez-Eby model for inclusion of both comonomer units within crystalline regions was used to describe the thermodynamics of the melting point depression in isodimorphic systems such as PHB/V and predictions from this model were contrasted with predictions from the Flory exclusion model. It was found that for isodimorphic systems the Flory exclusion model does not correctly predict the melting point depression, however, the Sanchez-Eby inclusion model does provide a good fit to the experimental data.