Viscosity and morphology of the two-phase system PDMS/P(DMS-ran-MPS)

Stationary state viscosities eta were measured at 50 degrees C for two-phas e blends consisting of poly(dimethylsiloxane) (PDMS) and poly(dimethylsilox ane-ran-methylphenyl-siloxane) (COP) at different compositions as a functio n of shear rate up to 100 s(-1). All mixtures exhibit shear-thinning behavi or in contrast to the pure components; the sensitivity of eta towards shear varies with the composition in a characteristic manner reflecting the morp hology of the blends. On the basis of these rheological results we infer th at the concept of a single composition of co-continuity should be replaced by a composition range of co-continuity. For blends consisting of droplets of one phase suspended in the matrix of the complementary phase pictures we re taken after rapid transfer from the shear cell (100 s(-1)) into the ligh t microscope. The average dimensions of the droplets are approximately 13 m u m (PDMS matrix) and 14 mu m (COP matrix); this observation is in contrast with the data calculated according to Wu [Wu, S., "Formation of dispersed phase in incompatible polymer blends: Interfacial and rheological effects," Polym. Eng. Sci. 27, 335-343 (1987)] from the interfacial tension, which w e have measured as a function of temperature, and from the viscosity ratios of the two phases. For the PDMS matrix this prediction (24 mu m) is reason able; however, if the matrix consists of COP, the theoretical result exceed s the measured dimension by approximately a factor of 14. Micrographs of a blend containing 60 vol% PDMS taken in situ at 1 s(-1) under stationary con ditions disclose thread-like structures. The correlation between the preval ence of certain morphologies and the extent of shear thinning is discussed. (C) 1999 The Society of Rheology. [S0148-6055(99)01204-3].