Utilizing vapor swelling of surface-initiated polymer brushes to develop quantitative measurements of brush thermodynamics and grafting density

The stimuli-responsive behavior of densely grafted polymer brushes makes them attractive for designing tunable coatings and thin film sensors, provided physical properties and thermodynamics of the brush are well characterized. This work investigates the swelling of highly extended poly(methyl methacrylate) polymer brushes in solvent vapors. All brushes have identical grafting density, but differing degrees of polymerization. Brushes were equilibrated in various saturated vapors, then characterized by X-ray reflectivity to measure thickness and scattering length density. One dimensional swelling normal to the surface is observed for all brushesolvent pairs; brushes swollen in good solvents generally exhibit decreasing polymer density along the thickness direction of the brush, while poor solvents swell the brush with minimal changes in the bulk brush density profiles relative to the dry state. Comparison of changes in brush height and film density upon swelling to existing mean-field models indicate that vapor solvation thermodynamics of polymer brush thin films is not explained by existing theory. Published by Elsevier Ltd.