ELECTRIC BIREFRINGENCE MEASUREMENTS IN AQUEOUS POLYELECTROLYTE SOLUTIONS

Aqueous solutions of poly(sodium p-styrenesulfonate) with molecular weights 100 000, 400 000, and 1 200 000 were examined by the transient and dynamic electric birefringence method. On the basis of the results it is demonstrated that the investigated polyelectrolyte solutions have to be divided into four different concentration ranges (I-IV). In the dilute concentration range (range I) the buildup and decay of the electric birefringence proceed by a single process (tau-1). In this concentration range the interactions between the different molecules can be neglected, and the polyelectrolytes are present in a more or less stretched conformation. The birefringence in this concentration region is negative, and the molecules are oriented with their long axis in the direction of the electric field. The concentration range II starts at the concentration c1* where the rotational volumes of the rods begin to overlap. A second effect (tau-2) with opposite sign of the first effect is present together with the tau-1 process. The time constant-tau-2 of the second effect is longer than the time constant of the first effect. Its amplitude can also be larger than the amplitude of the first effect, and the total sign of the birefringence becomes positive. It is concluded that during this second process the macromolecules are oriented perpendicular to the electric field. At even higher concentrations (range III) a third effect (tau-3) with the same sign as the first effect becomes visible. This effect is likely due to the cooperative motion of an entangled transient network structure, which again can be oriented parallel to the electric field. In a very small concentration range the three processes can be observed simultaneously. At even higher concentrations the birefringence vanishes. This seems to be a consequence of the coiling of the macromolecules (range IV). The observed results on the polyelectrolytes are very similar to results that have been obtained previously on ionic surfactants that form rodlike micelles. The principal results on the two different systems are the same, but rodlike micelles possess an inherent stiffness in comparison to the investigated polyelectrolytes. The concentration-dependent flexibility of the polyelectrolytes causes some special features of the anomalous birefringence behavior. It can be concluded that all systems with charged anisometric particles show the anomaly of the electric birefringence.