TOWARD A QUANTITATIVE THEORY OF THE RHEOLOGY OF CONCENTRATED-SOLUTIONS OF STIFF POLYMERS

Predictions of the rheological properties of concentrated isotropic solutions of stiff polymers are made that improve on those obtained with the approximate Doi theory. Predictions of the shear-rate and concentration dependence of the viscosity-eta and the first and second normal stress differences N1 and N2 that avoid the closure approximations of Doi are obtained by using a Galerkin numerical solution for the orientation distribution function. The predictions for eta and N1, though they neglect the effects of polydispersity in molecular weight, are shown to be in qualitative agreement with recent measurements on solutions of poly-gamma-benzyl-L-glutamate (PBLG). The polydispersities of a series of commercial PBLG and PBDG samples are measured by gel permeation chromatography, and found to be broad. An attempt is made to account for the effects of polydispersity on the linear viscoelastic storage and loss moduli for PBLG solutions by using the Marrucci-Grizzuti extension of the Doi theory. The attempt fails. The most probable explanation for the failure is that the rotary diffusive motions of the shorter molecules in the mixture are drastically slowed by the matrix of surrounding longer molecules.