Rheo-optical Study of Isotropic Solutions of Stiff Polymers.

Stresses and birefringences are measured for isotropic solutions of poly-gamma-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 pre-transitional 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 cut off 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.