Polymer mixing is a flexible method to produce materials with special properties, more economical that chemical synthesis. Most polymers are immiscible and their blends show heterogeneous morphologies. The type of morphology and the phase dimensions determine the mechanical and physical properties of the blend at large. The mixing of the components is done mostly in the melt state. The morphology is formed during melt processing and depends on the physical properties of the components, the interfacial tension, the viscosities, the volume fractions and the mixing conditions. The morphology can consist of one phase dispersed in domains in the other phase or of two continuous phases, one inside the other (co-continuous). From all possible morphologies, the latter gives a unique combination of the properties of the components. In the case, e.g., that one phase is conductive, then the continuity of this phase, even if it the minor, results in conductivity of the total.

Co-continuous morphologies are formed easily around the inversion point of the composition (e.g. at 50/50, when the viscosities are equal). However, the research project that we developed at Delft from 1994 to 1999 showed that this kind of morphologies is possible in a much broader range of composition. This range depends on the relative viscosities and the interfacial tension, but also on the mixing conditions. The research produced a model that relates these parameters with the type of morphology. Examples of applications in blends with controlled co-continuous morphology are the auto bumpers from PP/Nylon (e.g. FORD Taurus, USA) and parts of SMART (Cycoloy®), which is a blend of PC/SAN made by GEP.