Effect of flow geometry on the rheology of dispersed two-phase blends of polystyrene and poly(methyl methacrylate)

Abstract

The rheological properties of blends of poly(methyl methacrylate) (PMMA) and polystyrene (PS), forming two phases in the molten state, were measured using cone-and-plate and capillary rheometers. For the PS/PMMA blends, we have found that logarithmic plots of steady shear viscosity versus shear rate obtained by a cone-and-plate rheometer do not overlap those obtained by a capillary rheometer, whereas for the homopolymers PS and PMMA there is a good agreement between the two. This observation is explained in terms of the differences in the morphological states of the blends between the uniform shear flow in a cone-and-plate rheometer and the non-uniform shear flow in a capillary rheometer, which involves the entrance and exit effects. We have pointed out that, owing to the changes occurring in the shape of the dispersed droplets in the entrance region of a cylindrical die, the use of the Bagley plot to calculate shear stresses (thus shear viscosities) of dispersed two-phase polymer blends is of no rheological significance. We have also pointed out that the Cox-Merz rule does not hold for dispersed two-phase polymer blends, because the morphology of a dispersed two-phase blend in oscillatory shear flow would not be the same as that in steady shear flow. We have found that in the terminal region, logarithmic plots of dynamic storage modulus ( G′ ) versus dynamic loss modulus ( G″ ) give rise to curvature for the PS/PMMA blends but a straight line for the constituent components, leading us to conclude that such plots are very sensitive to variations in the morphological state of the blends.