Low‐frequency and high‐frequency relaxations in dynamic electric birefringence of poly(γ‐benzyl‐<scp>L</scp>‐glutamate) in <i>m</i>‐cresol

Abstract

Abstract The dc component Δ n of the electric birefringence of poly(γ‐benzyl‐ L ‐glutamate) in m ‐cresol is measured under an ac electric field at frequencies from 0.5 Hz to 200 kHz for solutions covering the dilute and semidilute regions. The dispersion curve indicates that at low frequencies Δ n decreases with increasing frequency (low‐frequency relaxation). For high‐molecular‐weight polymers at high concentration, Δ n becomes negative at high frequency and its absolute value decreases with further increase in frequency (high‐frequency relaxation). A unified theory for the two relaxations is developed on the basis of a model in which, in the semidilute regime, the rodlike polymer is confined in a cage formed by neighboring polymers and the lifetime of the cage lies between relaxation times of the two relaxations. The low‐frequency relaxation is ascribed to end‐over‐end rotation of the polymer and the high‐frequency relaxation to the rotation within a limited angle in the cage. The dependences of relaxation parameters on polymer concentration and molecular weight are reasonably explained by the theory.