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 fo und to be miscible over the whole composition range by both calorimetr y (DSC) and dynamic mechanical spectroscopy (DMTA). In the range of PH B contents from 0 to 50% the blend glass transition temperature (T(g)) strongly depends on composition (in excellent agreement with the pred ictions of Wood's equation), while a much less substantial dependence is found when the amount of PHB exceeds 50 %. In the former compositio n range, in addition to the strongly composition-dependent T(g), anoth er relaxation associated with mobilization of the low-T(g) component i s observed at a lower temperature. The plasticizer di-n-butyl phthalat e (DBP) is miscible in all proportions with both CAB and PHB. Analogou s to the polymeric CAB/PHB blends, the two polymer/diluent systems inv estigated (CAB/DBP and PHB/DBP) show a dual dependence of T(g) on comp osition. In binary mixtures such behavior appears to be independent of the macromolecular or low molecular weight nature of the low-T(g) com ponent. Addition of a fixed amount of DBP plasticizer to CAB/PHB blend s with varying composition (PHB content from 0 to 100 %) causes a sign ificant decrease of T(g) of the binary polymer blends; the T(g) depres sion is larger the higher, the amount of DBP in the ternary blend. Con comitant with the expected ''plasticizing'' effect on T(g), the presen ce 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 func tion of DBP content only, being independent of the relative amount of the two polymers (CAB and PHB).