Electric Birefringence of Dilute Suspensions of Poly(ethylene oxide) Crystals in Ethylbenzene

Abstract

Suspensions of poly(ethylene oxide) (PEO) crystals in ethylbenzene were subjected to electric pulses, including those of rapidly reversed polarity, and to alternating fields. The time and frequency dependence of the resulting birefringence shows that crystal orientation arises from a slowly induced dipole moment, as concluded by Picot et al. from the frequency dependence of the birefringence. The effect of the finite rate of polarization on the time and frequency dependence of the birefringence is discussed in terms of an electric-relaxation time, estimated to be a few milliseconds, and the birefringence-relaxation time. The field-strength dependence of the steady-state birefringence was represented by an equation; derived by Shah, that applies to a disk-shaped particle in which a dipole moment is induced perpendicular to its unique axis. Thereby it was found that (α2 -α1)/L3≅2.4X10-11 F/m, where (α2-α1) is the excess electric polarizability and L is the effective length of the square lamellar crystals. This relation agrees semiquantitatively with that obtained from a treatment by O'Konski and Krause of the excess polarizability of a conducting spheroid in a fluid. The suspensions studied were prepared by the self-seeding technique from ethylbenzene solutions of a PEO-PPO-PEO triblock copolymer (Pluronic F127), where PPO denotes atactic poly(propylene oxide). The size of the crystals, which were essentially monodisperse, was varied between 0.87 and 2.65 μm. © 1978, American Chemical Society. All rights reserved.