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

Abstract

Stationary state viscosities η were measured at 50 °C for two-phase blends consisting of poly(dimethylsiloxane) (PDMS) and poly(dimethylsiloxane-ran-methylphenyl-siloxane) (COP) at different compositions as a function of shear rate up to 100 s−1. All mixtures exhibit shear-thinning behavior in contrast to the pure components; the sensitivity of η towards shear varies with the composition in a characteristic manner reflecting the morphology of the blends. On the basis of these rheological results we infer that 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 were taken after rapid transfer from the shear cell (100 s−1) into the light microscope. The average dimensions of the droplets are approximately 13 μm (PDMS matrix) and 14 μm (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 we have measured as a function of temperature, and from the viscosity ratios of the two phases. For the PDMS matrix this prediction (24 μm) is reasonable; however, if the matrix consists of COP, the theoretical result exceeds 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 conditions disclose thread-like structures. The correlation between the prevalence of certain morphologies and the extent of shear thinning is discussed.