Co-continuous morphologies in polymer blends: a new model

Abstract

A model for phase continuity in polymer blends based on geometrical requirements for co-continuous structures has been developed. The minor phase of a fully co-continuous blend is considered as a dispersion of randomly oriented rods at their maximum packing density in the major phase. An existing empirical relation between this packing density and the aspect ratio of the rods, in conjunction with micro-rheological requirements for the existence of such rods, results in an equation describing the critical volume fraction of the minor phase for complete co-continuity as a function of the matrix viscosity, interfacial tension, shear rate and phase dimensions. This equation naturally leads to a range of compositions within which fully co-continuous structures can exist. It is demonstrated that existing relations between the volume fraction at phase inversion and the viscosity ratio of the blend components are not generally valid. Experimental evidence obtained by blending of PS and PE shows that the new model quantitatively predicts trends in the critical volume fraction as a function of the viscosities of both phases. An absolute prediction is not yet possible because the phase dimensions cannot a priori be predicted, but have to be measured.