Rheological behavior of polymer blends

Steady and oscillatory shearing flow properties of compatible and incompatible polymer blend systems were measured, using a cone-and-plate rheometer. The compatible blend systems investigated are blends of two low-density polyethylenes (LDPE) having different values of molecular weight and blends of poly(methyl methacrylate) (PMMA) with poly(vinylidene fluoride) (PVDF). The incompatible blend system investigated is a blend of poly(methyl methacrylate) (PMMA) with polystyrene (PS). It was found that (1) plots of first normal stress difference (τ₁₁ – τ₂₂) vs. shear stress (τ₁₂) and plots of storage modulus (G′) vs. loss modulus (G″) for the LDPE blends become independent of temperature and blend composition; (2) plots of τ₁₁ – τ₂₂ vs. τ₁₂, and G′ vs. G″ for the PMMA/PVDF blends become independent of temperature but dependent upon blend composition. It was found further that, for the incompatible PMMA/PS blends, the dependence of τ₁₁ – τ₂₂ on blend composition, when plotted against τ₁₂, is different from the dependence of G′ on blend composition, when plotted against G″. However, in both compatible and incompatible blend systems, plots of τ₁₁ – τ₂₂ vs. τ₁₂ and plots of G′ versus G″ are independent of temperature. The seemingly complicated composition-dependent rheological behavior of the incompatible blend system is explained with the aid of photomicrographs describing the state of dispersion.