UV-cross-linked poly(vinylpyridine) thin films as reversibly responsive surfaces

We report that UV-cross-linked poly(4-vinylpyridine) (P4VP) films acted as reversibly responsive coatings that controlled surface wettability and swelling toward external stimuli: solvent and pH. The polymer films were prepared simply by spin-coating a solution of P4VP followed by UV irradiation. These cross-linked films, when treated with chloroform, showed similar to 31% increase in film thickness whereas films extracted with methylene chloride or n-butanol exhibited a slight decrease. The increase in film thickness was due to the protonation of pyridyl groups by hydrogen chloride resulting from the photodegeneration of chloroform. The film expanded to minimize repulsion around the charged centers. This hypothesis was further confirmed by exposing the cross-linked film to hydrogen chloride vapor. The film expanded similar to 37% whereas no thickness increase was observed for films exposed to ammonia or methanol vapors. The extent of swelling was monitored in situ using a quartz crystal microbalance sensor. Large oscillation frequency shifts were detected when the UV-cross-linked P4VP film was exposed to acidic buffer solutions. The changes were rapid, and the effect was reversible.