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The interfacial thickness between immiscible polymers, polystyrene/polymethylmethacrylate (PS/PMMA) and PMMA/random copolymers of styrene and acrylonitrile (PMMA/SAN), was obtained by ellipsometry in the temperature range from 140 to 170 °C. Using these values, it is possible to calculate the polymer-polymer interaction parameter xab as well as the segmental interaction parameters x¡/j. It has been shown that all three segmental interaction parameters of the system PMMA/SAN, i.e. xs/mma, Xan/mma, and Xs/an, are decreasing with increasing temperature, but the polymer-polymer interaction parameters between PMMA and SAN in a certain copolymer composition range are increasing with temperature, leading to LCST (lower critical solution temperature) behavior. It has to be considered that the temperature dependence of segmental interaction parameters is governed by changes of interactions as well as free volume contributions, but in copolymer blends it raises the possibility of LCST behavior which is not driven by the free volume term. The experiments predict UCST (upper critical solution temperature) behavior in blends of low molecular weight PMMA and PS, which is in agreement with experimental findings. Furthermore, segmental interaction parameters and their temperature dependence for blends of random copolymers of styrene and methylmethacrylate (SMMA) and poly(vinyl methyl ether) (PVME) were estimated. The segmental interaction parameter xs/vme is increasing with increasing temperature, implying a free volume contribution to the well-known LCST behavior.

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