Studies on miscibility in polycarbonate/poly(styrene-co-acrylonitrile) blends by ellipsometry

Abstract

The interfacial thickness between immiscible polymers, bisphenol A polycarbonate (PC) and styrene–acrylonitrile random copolymers (SAN), was measured by ellipsometry as a function of temperature and acrylonitrile content in SAN, in order to study the miscibility between PC and SAN. Both the temperature and copolymer composition (AN content) dependence of the interfacial thickness in the equilibrium state indicated a maximum. Using the values of the interfacial thickness, it is possible to calculate the polymer–polymer interaction parameter χAB between PC and SAN. The χAB exhibited the lowest positive value at about 200°C for each copolymer composition due to the maximum of the interfacial thickness, i.e. at low temperature range, χAB decreased with increasing temperature, then χAB increased at high temperature range. This behaviour implies the possibility of the coexistent behaviour of upper and lower critical solution temperature (UCST and LCST) in the phase diagram of the mixture consisting of polymers with suitable molecular weight. This coexistent behaviour of UCST and LCST was confirmed by measuring cloud point curves of the mixtures having lower molecular weight PC. On the other hand, the copolymer composition dependence of χAB also has a minimum in a certain copolymer composition, which leads to the miscibility window behaviour. From these results, it was shown that there appeared to be a loop-type miscibility window (`miscibility egg') in the miscibility diagram of copolymer composition versus temperature of PC/SAN. From the calculation based on these results, it has also been shown that all three segmental interaction parameters χi/j of PC/SAN, i.e. χS/C, χAN/C and χS/AN, decreased with increasing temperature monotonically. It was predicted that the LCST behaviour in PC/SAN mixtures could occur due to the remarkable decrease of interaction χS/AN between comonomers in SAN at high temperature, without the free volume contribution.