Usually the shape of the glass transition dispersion in the mechanical or dielectric spectra of pure polymers is skewed toward higher frequencies. In miscible polymer blends not only is this peak broader than in pure polymers, the broadening is often asymmetric towards lower frequencies. Concentration fluctuations are an obvious source of the broadening; however, a simple distribution of relaxation times, corresponding to a distribution in local compositions, would not account for the reversal in the asymmetry of the dispersion. Intrinsic differences in component mobilities, as directly seen in solid state nuclear magnetic resonance experiments, also exert an influence on segmental relaxation in blends. The differences in the components' segmental relaxation behaviors are also manifested in the thermorheological complexity of miscible blends. An approach to blend dynamics based on the coupling model of relaxation can successfully describe the most prominent features.
