RHEOLOGICAL DIFFERENCES AMONG LIQUID-CRYSTALLINE POLYMERS .1. THE 1ST AND 2ND NORMAL STRESS DIFFERENCES OF PBG SOLUTIONS

In this paper and its sequel, we attempt to understand the rheological differences between lyotropic and thermotropic liquid-crystalline polymers by contrasting the rheology of three different liquid-crystalline polymers at concentrations ranging from moderate (12%-30%) to highly concentrated (30%-50%) to densely packed (50%-100%). This first paper presents the steady-state first and second normal stress differences N1 and N2 as functions of the shear rate gamma for solutions of poly(gamma-benzyl-glutamate) with molecular weight 238 000, in the solvent metacresol, at concentrations C ranging from 12.5% to 40% by weight. Predictions of N1 and N2 for this range of concentration are obtained from the Doi molecular theory for rod-like nematics, using both an approximate, and a nearly exact, method for solving the Doi equation. The predictions of N1, and to a lesser extent N2, agree qualitatively with measured values for all concentrations. In particular, the range of shear rates over which N1 is negative shifts upward with increasing concentration: if we define gamma(max) as the shear rate for which N1 reaches a positive maximum value N1max, then we find that theory agrees with experiment in that both gamma(max) and N1max increase monotonically with concentration C These increases occur because the strength of the nematic interaction increases with C, which implies that for high concentrations, N1 becomes negative only at high-shear rates.