Structural changes in block copolymers: coupling of electric field and mobile ions

TL;DR

This paper explores how dissociated ions in block copolymers under electric fields can induce structural phase transitions from insulating to conducting states, supported by theoretical predictions and experimental observations.

Contribution

It introduces a novel mechanism where free ions cause phase transitions in block copolymers under electric fields, expanding understanding of electro-responsive polymer morphologies.

Findings

Ions can induce phase transitions at lower electric fields (~10 V/μm).

Transition from bcc to hexagonal phase involves charge mobility along cylinders.

Experimental results confirm theoretical predictions of ion-induced morphological changes.

Abstract

We consider the coupling between an external electric field and dissociated ions embedded in anisotropic polarizable media such as block copolymers. We argue that the presence of such ions can induce strong morphological changes and even lead to a structural phase transition. We investigate, in particular, diblock copolymers in the body centered cubic (bcc) phase. In pure dielectric materials (no free charges), a dielectric breakdown is expected to occur for large enough electric fields, preempting any structural phase transition. On the other hand, dissociated ions can induce a phase transition for fields of about 10 V/$\mu$m or even lower. This transition is from an insulator state, where charges are preferentially localized inside bcc spherical domains, to a conducting state where charges can move along the long axis of oriented cylinders (forming a hexagonal phase). The cylinders diameter is small and is determined by the volume fraction of the minority copolymer component. The strength of this mechanism can be tuned by controlling the amount of free ions present as is observed by our experiments. These theoretical predictions support recent experimental findings on the bcc to hexagonal phase transition in copolymer systems.