Initial Concentration Dependence of the Oscillatory Flow Birefringence Properties of Polystyrene and Poly(α-methylstyrene) Solutions

The concentration dependence of the oscillatory flow birefringence (OFB) properties of narrow-distribution linear polystyrene (M̄_w = 390 000) or poly(α-methylstyrene) (M̄_w = 400 000 and 5 000 000) Aroclor solutions has been obtained for concentrations in the range c[η] ≲ 3. These data are compared with the predictions of the Muthukumar and Freed treatment of the concentration dependence of bead-spring model relaxation times, the Wang-Zimm theory, and the infinite dilution bead-spring model (hydrodynamic interaction varied to duplicate the apparent transition from nearly non-free-draining to free-draining behavior with increasing concentration). The data obtained show that the relaxation time spectrum is affected markedly by concentration. The longest relaxation time is affected most and appears to exhibit a nearly exponential dependence on concentration for the solutions examined; the shortest relaxation times are almost unaffected. The Muthukumar and Freed predictions provide the best OFB frequency dependence curve shapes and absolute positions on the frequency axis and predict the observed frequency dependence for a surprisingly large range in concentration. At the highest concentrations the OFB properties show behavior indicative of enhanced separation of the longest relaxation times due to the onset of significant entanglement effects.