Tuning the Effective Viscosity of Polymer Films by Chemical Modification

We report adjustment of the dynamics of polystyrene (PS) films supported by oxide-covered silicon (SiOx) by a combination of ultraviolet ozone (UVO) modification and substrate surface modification. For all the specimens studied, greatly enhanced effective viscosity (η_eff) was found with the UVO modification when film thickness (h₀) was less than the radius of gyration (R_g) of the polymer. After etching the substrate with hydrogen fluoride, which removes the oxide layer, η_eff was much less enhanced. Surface analysis revealed that oxygenated groups were produced in the polymer near the film surface upon the UVO treatment. These groups diffused into the films during the dynamic measurement. If h₀ < R_g, the groups could interact with the hydroxyl groups on the SiOx substrate and caused strong enhancement in η_eff. When h₀ > R_g, the η_eff of the MW > 13.7 kg/mol films was bulk-like, suggesting bulk-like surface mobility of oxygenated PS. In contrast, the η_eff of the MW = 13.7 kg/mol films with h₀ > R_g was depressed in the same way as the pristine films resulting from enhancement in the surface mobility, which indicates that the thickness of the surface mobile layer is bigger than the R_g of those films.