Electro‐optical properties and molecular flexibility of polyelectrolytes in dilute solutions

Measurements of the electric birefringence of aqueous polyelectrolyte solutions in the very dilute regime ( ≈ 10 −5 g · mL −1 ) reveal that the macroions are fully extended. However, this rod‐like conformation is easily altered towards a more coiled conformation when a strong static electric field is applied, particularly in the case of very high molar‐mass polymers. With the application of rectangular dc pulses, as ordinarily used in Kerr‐effect measurements, the change of molecular shape interferes with the transient rise and decay of the birefringence signal, because the rotational mobility as well as the optical anisotropy of the macroions become time‐dependent. As a result, the mean relaxation time for the rise curve is considerably smaller than that of the field‐free decay, and the steady‐state birefringence is much lower than expected for the unperturbed macroions. The change of the molecular shape can be avoided if the orientation of the macroions is induced by high‐frequency ac pulses. This experimental technique is therefore suitable to obtain the unperturbed shape and size of the macroions. A molecular interpretation for the dependence of the birefringence function on the pattern of the electric field is proposed.