Ionic effects on the electric field needed to orient dielectric lamellae

TL;DR

This study investigates how mobile ions influence the electric field required to reorient lamellar structures in dielectric materials, revealing that ion confinement and neutrality conditions significantly affect the reorientation process.

Contribution

It introduces a model showing how ion confinement and neutrality constraints alter the electric field needed for lamellae reorientation, with implications for diblock copolymer films.

Findings

Ions reduce the reorientation voltage most when they are confined to neutral A layers.

Charge neutrality in A layers lowers the energy barrier for perpendicular orientation.

Ion addition can sometimes stabilize the parallel lamellar configuration.

Abstract

We consider the effect of mobile ions on the applied potential needed to reorient a lamellar system of two different materials placed between two planar electrodes. The reorientation occurs from a configuration parallel to the electrodes favored by surface interactions to an orientation perpendicular to the electrodes favored by the electric field. The system consists of alternating A and B layers with different dielectric constants. The mobile ions are assumed to be insoluble in the B layers and hence confined to the A layers. We find that the ions reduce the needed voltage most strongly when they are constrained such that each A lamella is electrically neutral. In this case, a macroscopic separation of charge and its concomitant lowering of free energy, is attained only in the perpendicular orientation. When the ions are free to move between different A layers, such that charge neutrality is only required globally, their effect is smaller and depends upon the preferred surface interaction of the two materials. Under some conditions, the addition of ions can actually stabilize the parallel configuration. Our predictions are relevant to recent experiments conducted on lamellar phases of diblock copolymer films with ionic selective impurities.