Capillary leveling of stepped films with inhomogeneous molecular mobility

TL;DR

This study investigates how inhomogeneous molecular weight distributions in thin polymer films affect capillary leveling, revealing viscosity gradients' role in film dynamics and developing a predictive hydrodynamic model.

Contribution

It introduces a hydrodynamic model for thin films with in-plane viscosity gradients due to molecular weight variations, aligning well with experimental data.

Findings

Viscosity gradients influence film leveling behavior.

The model accurately predicts self-similar profile shapes.

Capillary velocity measurements agree with bulk rheology data.

Abstract

A homogeneous thin polymer film with a stepped height profile levels due to the presence of Laplace pressure gradients. Here we report on studies of polymeric samples with precisely controlled, spatially inhomogeneous molecular weight distributions. The viscosity of a polymer melt strongly depends on the chain length distribution; thus, we learn about thin-film hydrodynamics with viscosity gradients. These gradients are achieved by stacking two films with different molecular weights atop one another. After a sufficient time these samples can be well described as having one dimensional viscosity gradients in the plane of the film, with a uniform viscosity normal to the film. We develop a hydrodynamic model that accurately predicts the shape of the experimentally observed self-similar profiles. The model allows for the extraction of a capillary velocity, the ratio of the surface tension and the viscosity, in the system. The results are in excellent agreement with capillary velocity measurements of uniform mono- and bi-disperse stepped films and are consistent with bulk polymer rheology.