Interpretation of a new interface-governed relaxation process in compatibilized polymer blends

We have analyzed the influence of different amounts wb, of two diblock copolymers, poly(styrene-b-methyl methacrylate) (sm blend series) and poly(cyclohexyl methacrylate-b-methyl methacrylate) (cm blend series), on the morphological and rheological characteristics of a blend containing w = 7.5 wt % polystyrene in poly(methyl methacrylate) matrix. The morphological analysis is based on the sphere size distribution function, which was determined from the image analysis of the transmission electron micrographs. Using this function and assuming that all block copolymers are located at the interface, the interfacial area per copolymer joint, Sigma, was calculated. From its hyperbolic dependence on w(bc) the value at the critical micelle concentration, Sigma(cmc), was found to be about 10 nm(2) for both systems. The rheological analysis reveals that in addition to the form relaxation process, well-known for polymer blends, a new relaxation process is observed for these systems. Its relaxation time, tau(beta), has been studied in dependence on the amount of added block copolymers. The observed phenomena for each blend series, i.e. constant blend viscosity, slight shift of the form relaxation times tau(1), and systematic shift of the interface governed relaxation time tau(beta) (tau(beta) > tau(1)), have been interpreted quantitatively. In contrast to tau(1), tau(beta) is less influenced by the interfacial tension but is mainly governed by an additional contribution, the interfacial shear modulus. Formulas were derived from an expanded version of the Palierne emulsion model which allows the determination of the proposed interfacial properties from rheological measurements. In general, the interfacial tension decreases with increasing amount of block copolymer, and the decrease is more pronounced for the cm blend series. The interfacial shear modulus increases during compatibilization from 0 to amounts which are in the range of 20-30% of the interfacial tension. The decrease of interfacial tension is in good agreement with predictions from Leibler's brush model extended by Dai et al. In conclusion, it was found that the Palierne model with an nonisotropical interfacial stress state is quantitatively correct to describe the observed phenomena for those blends.