EFFECT OF A LOW-MOLECULAR-WEIGHT PLASTICIZER ON THE THERMAL AND VISCOELASTIC PROPERTIES OF MISCIBLE BLENDS OF BACTERIAL POLY(3-HYDROXYBUTYRATE) WITH CELLULOSE-ACETATE BUTYRATE

Blends obtained by melt compounding poly(3-hydroxybutyrate) (PHB) with cellulose acetate butyrate (CAB, DS(Bu) = 2.50; DS(Ac) = 0.18) are found to be miscible over the whole composition range by both calorimetry (DSC) and dynamic mechanical spectroscopy (DMTA). In the range of PHB contents from 0 to 50% the blend glass transition temperature (T(g)) strongly depends on composition (in excellent agreement with the predictions of Wood's equation), while a much less substantial dependence is found when the amount of PHB exceeds 50 %. In the former composition range, in addition to the strongly composition-dependent T(g), another relaxation associated with mobilization of the low-T(g) component is observed at a lower temperature. The plasticizer di-n-butyl phthalate (DBP) is miscible in all proportions with both CAB and PHB. Analogous to the polymeric CAB/PHB blends, the two polymer/diluent systems investigated (CAB/DBP and PHB/DBP) show a dual dependence of T(g) on composition. In binary mixtures such behavior appears to be independent of the macromolecular or low molecular weight nature of the low-T(g) component. Addition of a fixed amount of DBP plasticizer to CAB/PHB blends with varying composition (PHB content from 0 to 100 %) causes a significant decrease of T(g) of the binary polymer blends; the T(g) depression is larger the higher, the amount of DBP in the ternary blend. Concomitant with the expected ''plasticizing'' effect on T(g), the presence of DBP also induces a decrease in the characteristic temperature of the additional low-temperature transition observed in CAB/PHB blends. In the ternary blends, the temperature of such a transition is a function of DBP content only, being independent of the relative amount of the two polymers (CAB and PHB).