Utilizing Vapor Swelling of Surface-Initiated Polymer Brushes to Develop Quantitative Measurements of Brush Thermodynamics and Grafting Density
Sara V. Orski, Richard J. Sheridan, Edwin P. Chan, Kathryn L. Beers
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 produce thickness and scattering length density measurements. One dimensional swelling normal to the surface is observed for all brush-solvent pairs; good solvents induce homogeneous swelling, while poor solvents yield heterogeneous brush density profiles. 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.