THE EFFECT OF CONCENTRATION, TEMPERATURE, AND MOLECULAR-WEIGHT ON THEDYNAMICS OF RIGID-ROD MOLECULES IN SEMIDILUTE SOLUTIONS

The dynamics of rigid-rod-like molecules are studied using rheo-optica l techniques. Measurements of flow birefringence as a function of shea r rate are utilized to understand the scaling behavior of rotational d iffusivity with respect to concentration and temperature. The concentr ation scaling exponent increases with increasing concentration and the scaling laws are valid in narrow concentration windows. The Doi-Edwar ds (DE) scaling law D-r similar to c(-2), holds at very high concentra tions (cL(3) > 150). The concentration scaling exponent decreases dram atically with increasing temperature at concentrations, cL(2)d > 1. Sc aling of rotational diffusivity, with respect to temperature and solve nt viscosity in the semidilute regime, does not follow the predictions of DE theory (and related caging ideas). On the contrary, a model pro posed by Fixman was found to explain both the temperature and concentr ation dependence of the rotational diffusivity. (C) 1998 John Wiley & Sons, Inc.