RELATING THE SHEAR-THINNING CURVE TO THE MOLECULAR-WEIGHT DISTRIBUTION IN LINEAR POLYMER MELTS

The double-reptation ansatz for stress relaxation in broadly polydispe rse melts of linear chains relates the molecular weight distribution p hi(n) and the dynamic viscosity eta(omega), and has been used to infer phi(n) from rheological data. I show that the microscopic theory of c onstraint release implies the double-reptation model is valid for prac tical distributions phi(n). I also provide a heuristic theoretical arg ument for the Cox-Merz empirical rule that the shapes of the functions etaomega and eta(gamma over dot) are similar, which allows the shear -thinning curve to be predicted. (C) 1996 Society of Rheology.