PNNL

Abstract

The mechanism involved in the photochemical immobilization of poly(4-vinylphenol) (PVP) thin films was investigated. The films were fabricated by a simple procedure of UV irradiation and solvent extraction. A combination of ellipsometry, IR, and high-resolution x-ray photoelectron spectroscopy (XPS) were used to provide detailed and quantitative analysis of the composition of the photochemical reaction products. Upon irradiation at 260 nm, benzyl and phenoxy radicals are generated in the polymer. In the absence of oxygen, PVP films crosslink via the combination of the benzyl radicals or phenoxy radicals. At lower irradiation doses, the photochemical process was dominated by crosslinking of the polymer backbone via the combination of benzyl radicals. At higher exposure doses, phenoxy radicals were generated producing crosslinked quinoid structures. The concentration increased 2 with the irradiation time and thus the percentage of the crosslinked quinoid structures. No oxidation or degradation products were observed. In the presence of oxygen, additional reactions of oxidation and degradation occurred. At lower doses, oxidation at the benzyl position produced the ketone structure evidenced by the drastic increase the O content in the irradiated films. As the irradiation doses increased, further oxidation at the methylene position occurred, and in addition, volatile and degradation products were also generated. This photochemical process was successfully employed to fabricate patterned PVP structures. Photolithography at lower doses followed by solvent development created negative-tone PVP patterns. Irradiating the film at higher doses generated “pseudo” positive-tone patterns after the volatile products in the irradiated areas left the film.