Wetting is of interest for many applications, including the engineering of phobic and/or anti-fouling coatings. A promising system for controlling wetting, are polymer brushes: coatings that consist of macromolecules that are end-grafted to a substrate at sufficiently high grafting densities that their free volumes overlap, such that they are forced to extend from the surface, and form a so-called ‘brush’ structure. In a nutshell, the mechanics of a polymer brush are governed by a competition between two phenomena: the entropic elasticity of a chain, which wants to contract the brush, and the excluded volume effects, which tends to extend the brush. As such, polymer brushes inherently experience a lateral stress, which is largest at the grafting plane, and smoothly reaches zero at the brush surface (in good solvent conditions), as shown by both self-consistent field theory and molecular dynamics. This brush-induced lateral stress can be so large that it has been shown to be able to significantly deform flexible substrates. Naturally, this has implications for the interfacial energy / tension of a polymer brush. In the naive application of the Kirkwood-Buff method (i.e. integrating the stress anisotropy over the entirety of the bulk of the brush), it typically results in a negative surface tension. However, wetting experiments and simulations where the surface tension is determined from contact angles using the Young-Dupré method, suggest that droplets do experience a positive surface tension, although one that appears nonetheless smaller than expected from integrating only the positive part of the stress anisotropy profile. Using coarse-grained molecular dynamics simulations of polymer brushes solvated by both liquids and vapours, we explore the link between negative stress anisotropy in brushes and their effective surface tension.

Stress anisotropy in polymer brushes and its implications on interfacial tension

ACS Spring 2022

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