Interplay of internal stresses, electric stresses and surface diffusion in polymer films

TL;DR

This paper develops a comprehensive theory for destabilization in elastic polymer films, analyzing how internal stresses and electric fields induce surface undulations, and explores their combined effects and boundary condition influences.

Contribution

It introduces a unified framework that simultaneously considers stress-driven and electric field-driven instabilities in polymer films, including their interplay and boundary effects.

Findings

Surface undulation wavelength depends on stress and electric field strength.

Both destabilization mechanisms can coexist and influence each other.

Boundary conditions significantly affect instability onset and pattern formation.

Abstract

We investigate two destabilization mechanisms for elastic polymer films and put them into a general framework: first, instabilities due to in-plane stress and second due to an externally applied electric field normal to the film's free surface. As shown recently, polymer films are often stressed due to out-of-equilibrium fabrication processes as e.g. spin coating. Via an Asaro-Tiller-Grinfeld mechanism as known from solids, the system can decrease its energy by undulating its surface by surface diffusion of polymers and thereby relaxing stresses. On the other hand, application of an electric field is widely used experimentally to structure thin films: when the electric Maxwell surface stress overcomes surface tension and elastic restoring forces, the system undulates with a wavelength determined by the film thickness. We develop a theory taking into account both mechanisms simultaneously and discuss their interplay and the effects of the boundary conditions both at the substrate and the free surface.