The Effect of Blending on Viscoelastic Properties for Polymer Melts

Abstract

The dynamic viscosity η' and rigidity G' of polystyrene and polypropylene in molten states have been measured by means of a concentric cylinder-type rheometer in the frequency range of about 0.001 to 0.5cps at different temperatures, in order to study the effect of blending on the properties of these substances. The frequency dependence curves of η' and G' at different temperatures can be superposed well according to the usual time-temperature superposition principle without correction for temperature and density. The viscosity data for the polystyrene blends in a wide range of frequency show that the viscosity curves for these blends, differing in their molecular weights from each other, join together at the angular frequencies (ω) higher than about 300, while the rigidity curves for the blends show much difference even at ω=300. This suggests that as a means to distinguish polymers measuring their viscosity alone is not adequate.
So far as the range of ω is not so high, log (η'/η0) (η0=zero-frequency viscosity) vs. log ω curves for the blends of two components differing only in molecular weight can be superposed into a master curve by shifting the curves for the blends having compositions differing from each other in the horizontal direction by log a_M. On the other hand, log G' vs. log ω curves can be superposed by shifting them first horizontally by log a_M and then vertically by log c_M. The shift factors a_M and c_M depend not only upon the molecular weight but also upon the molecular weight distribution of the blend. When molecular weight variation of the two components is small, log a_M and log c_M bear straight-line relationship between log(η₀₂/η_0b), where η₀₂ and η_0b are respectively the zero-frequency viscosity for the component chosen as the standard and the blends. In the case of polyethylene studied previously²⁾, log c_M was zero independent of the composition, and hence the slope of log a_M vs. log (η₀₂/η_0b) curve was zero. However, in the case of the blends of polystyrene and polypropylene studied here, where the molecular weight variatian of the two components is conspicuously large, the slope of similar straight lines becomes larger. The slopes of the straight lines for log a_M and log c_M plotted against log (η₀₂/η_0b) depend upon the difference between the molecular weights of the two components. Moreover, there can be found the following relation between a_M and c_M both experimentally and theoretically: c_M/a_M=η₀₂/η_0b.