RHEOOPTICAL STUDY OF ISOTROPIC SOLUTIONS OF STIFF POLYMERS

Stresses and birefringences are measured for isotropic solutions of poly(γ-benzyl L-glutamate) as functions of shear rate and time at concentrations ranging from dilute to concentrated. The Doi theory and other Landau–de Gennes theories for rigid molecules predict a pretransitional rise in the stress-optical ratio C—that is, the ratio of birefringence to stress—as the concentration approaches the value at which a first-order liquid-crystalline transition occurs. This predicted rise occurs because of critical slowing down as the concentration approaches the spinodal before being cutoff by the first-order transition. Our measurements of C for PBLG show a rise of this kind by a factor of 3.5. Kinetic rod-jamming effects in our data can be distinguished from the thermodynamic critical slowing down by combined use of stress and birefringence. The Doi theory also predicts a nonlinear coupling between the shearing field and the excluded-volume potential that causes curves of steady-state shear viscosity versus shear rate at different concentrations to cross, an effect seen in our measurements. We find violations of the stress-optical rule in that the measured values of C depend not only on molecular weight and concentration but also on shear rate and time after start-up and cessation of shearing. These latter dependencies can be explained by modest levels of polydispersity in molecular weight. © 1990, American Chemical Society. All rights reserved.