Effect of molecular weight distribution on the shear dependence of viscosity in polymer systems

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The viscosity of concentrated solutions of polystyrene were measured in steady shearing flow with a Weissenberg rheogoniometer. The polymers ranged in weight average molecular weight M̄w from 97,000 to 3,370,000 and in distribution breadth M̄w/M̄n from 1.05 to 3.1. The solvent was n‐butylbenzene, and the solutions ranged in concentration from 0.20 to 0.55 g./cc. These data were supplemented by measurements on bulk polystyrenes and polyethylenes with distribution breadths as high as M̄w/M̄n = 20. Master curves of reduced viscosity vs. reduced shear rate were calculated theoretically for each molecular weight distribution. The experimental viscosity data superimposed satisfactorily on these curves and furnished the viscosity at zero shear rate τ0 and an experimental time constant τ0 for each solution. The time constant paralleled closely the Rouse relaxation time τR in all systems. Residual variations in the ratio τ0/τR with molecular weight and concentration correlated uniquely with the product cM̄w. These results indicate that the viscosity behavior of a polymer can be predicted with reasonable accuracy from data on its molecular weight distribution.