Theory of Polydispersity Effects on Polymer Rheology: Binary Distribution of Molecular Weights

Effects of polydispersity on rheological properties of entangled polymers are analyzed. A number of models of tube renewal are discussed and compared with each other and with experiments. A theory incorporating reptation, tube renewal, and fluctuations in the tube length into a general description of stress relaxation is developed. Dynamical moduli were calculated for monodispersed and bidispersed systems and compared with experiments. The product of viscosity and recoverable shear compliance is predicted to be of the order of the longest relaxation time of the higher molecular weight component with a weak dependence on the relative volume fraction of the binary mixture for a large enough concentration of longer molecules.