Molecular theory of electrostatic collapse of dipolar polymer gels

TL;DR

This paper presents a molecular theory explaining how electrostatic interactions cause dipolar polymer gels to undergo a first-order collapse transition, with potential applications in stimuli-responsive materials.

Contribution

It introduces an analytical expression for the electrostatic free energy of dipolar gels within the random phase approximation, revealing the collapse mechanism.

Findings

Electrostatic interactions induce a first-order collapse transition.

The transition depends on dipole coupling and dipole-to-Kuhn length ratio.

Results are relevant for designing stimuli-responsive polymer systems.

Abstract

We develop a new quantitative molecular theory of liquid-phase dipolar polymer gels. We model monomer units of the polymer network as a couple of charged sites separated by a fluctuating distance. For the first time, within the random phase approximation, we have obtained an analytical expression for the electrostatic free energy of the dipolar gel. Depending on the coupling parameter of dipole-dipole interactions and the ratio of the dipole length to the subchain Kuhn length, we describe the gel collapse induced by electrostatic interactions in the good solvent regime as a first-order phase transition. This transition can be realized at reasonable physical parameters of the system (temperature, solvent dielectric constant, and dipole moment of monomer units). The obtained results could be potentially used in modern applications of stimuli-responsive polymer gels and microgels, such as drug delivery, nanoreactors, molecular uptake, coatings, superabsorbents, etc.