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 cont aining w = 7.5 wt % polystyrene in poly(methyl methacrylate) matrix. T he morphological analysis is based on the sphere size distribution fun ction, which was determined from the image analysis of the transmissio n electron micrographs. Using this function and assuming that all bloc k copolymers are located at the interface, the interfacial area per co polymer joint, Sigma, was calculated. From its hyperbolic dependence o n w(bc) the value at the critical micelle concentration, Sigma(cmc), w as found to be about 10 nm(2) for both systems. The rheological analys is reveals that in addition to the form relaxation process, well-known for polymer blends, a new relaxation process is observed for these sy stems. Its relaxation time, tau(beta), has been studied in dependence on the amount of added block copolymers. The observed phenomena for ea ch blend series, i.e. constant blend viscosity, slight shift of the fo rm relaxation times tau(1), and systematic shift of the interface gove rned relaxation time tau(beta) (tau(beta) > tau(1)), have been interpr eted quantitatively. In contrast to tau(1), tau(beta) is less influenc ed by the interfacial tension but is mainly governed by an additional contribution, the interfacial shear modulus. Formulas were derived fro m 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 incre asing amount of block copolymer, and the decrease is more pronounced f or 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 a n nonisotropical interfacial stress state is quantitatively correct to describe the observed phenomena for those blends.