MISCIBILITY OF BACTERIAL POLY(3-HYDROXYBUTYRATE) WITH CELLULOSE ESTERS

Blends of bacterial poly(3-hydroxybutyrate) (P(3HB)) with cellulose acetate butyrate (CAB) and cellulose acetate propionate (CAP) were prepared by melt compounding. P(3HB)/CAB blends containing 5-50% P(3HB) and P(3HB)/CAP blends with 5-60% P(3HB) are transparent, stable homogeneous amorphous glasses, while blends with higher P(3HB) content are partially crystalline. When in the amorphous state, both P(3HB)/CAB and P(3HB)/CAP blends show a glass transition which regularly decreases with increasing P(3HB) content, in excellent agreement with the behavior predicted for totally miscible blends. Both dynamic mechanical (DMTA) and calorimetric (DSC) measurements show that P(3HB) and CAB can crystallize from the blends only at temperatures higher than the composition-dependent T(g). When crystallization is induced by thermal treatments, the melting temperature of the crystalline phase obtained depends on composition, as expected for miscible blends of crystallizable polymers. Besides the strongly composition dependent glass transition, another relaxation is observed, located in proximity to the T(g) of P(3HB) and slightly shifting to higher temperature with increasing CAB or CAP content. DSC measurements on melt-quenched blends containing more than 50% P(3HB) indicate contribution of both blend components to this glass transition process, on the basis of the very large specific heat increment observed. It is suggested that the two glass transitions are the manifestation of two mobilization processes coexisting in blends which appear in all respects to be single-phase, homogeneous mixtures.