POLYCARBONATE SAN COPOLYMER INTERACTION

Blends of polycarbonate (PC) with ABS (acrylonitrile-butadiene-styrene) materials have commercially useful properties without requiring a compatibilizer, which is necessary for most other commercial multiphase blends. It has been proposed that this is due to a favourable thermodynamic interaction between PC and the styrene-acrylonitrile (SAN) copolymer matrix of ABS. A variety of evidence suggests this interaction is optimum at about 25 wt% acrylonitrile, which is roughly the composition of the azeotrope and of most commercial materials. A critical molecular weight analysis of the phase behaviour of PC-SAN blends is presented here, which leads to a quantitative estimate of the interaction energy that is consistent with available facts. When the molecular weights of the PC and SAN components are reduced to about 3000 each, their blends become miscible and the observed phase behaviour allows calculation of interaction energies in terms of a binary interaction model using both the Flory-Huggins and the Sanchez-Lacombe equation of state theories. End-group effects were accounted for as necessary. The estimated interaction energy is a refinement of a previous estimate based on the phase behaviour of copolycarbonates and SAN copolymer blends. The current estimate allows prediction of interfacial tensions that are consistent with morphology observations for PC-SAN blends presented here and elsewhere. It also predicts that the extent of partial miscibility is very limited and should produce very small shifts in the glass transitions of the components when blended as observed here and by others for purified SAN materials. Prior claims to the contrary result from partitioning between the phases of oligomers in commercial SAN copolymers.