Polymer brushes are a type of polymeric surface coating in which macromolecules are end-anchored to the surface at high densities. The resulting excluded volume interactions cause chains to extend away from the surface. Under good solvent conditions, brushes may absorb solvent, leading to swelling of the brush. This structure lends itself well to mechanical applications: a solvated brush resists compression due to its internal osmotic pressure, and can stabilize a lubricious solvent layer against squeeze-out. Hence, polymer brushes are of interest as potential surface functionalizations with extreme low friction or adhesion properties.<br/><br/>However, polymeric materials display a glass transition, in which they pass from a rigid solid state to a viscous or viscoelastic state. Naturally, this dramatically impacts the mechanical response of the material. While this is typically considered as a temperature-dependent process, it is well known that small molecules may serve as plasticizers, lowering the glass transition temperature of the polymer. The composition of a polymer brush is typically not homogeneous over its height. Rather, the concentration of polymer segments in a brush decays away from the grafting plane, and the outer layers of a solvated polymer brush are typically enriched in solvent. As a result, a polymer brush coating near its glass transition temperature could display a mixed structure, with a glassy bulk and a viscous surface layer.<br/><br/>We present molecular dynamics simulations in which we investigate the solvent-induced glass transitions of polymer brushes and their variation over the brush height, and compare to non-anchored polymer films.

Solvent-induced glass transitions in grafted and free polymer films

ACS Spring 2022

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