Effect of UV-Ozone treatment on poly(dimethylsiloxane) membranes: Surface characterization and gas separation performance

A thin SiO_x selective surface layer was formed on a series of cross-linked poly(dimethylsiloxane) (PDMS) membranes by exposure to ultraviolet light at room temperature in the presence of ozone. The conversion of the cross-linked polysiloxane to SiOx was monitored by attenuated total reflectance Fourier transform infrared (ATRFTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) microanalysis, contact angle analysis, and atomic force microscopy (AFM). The conversion of the cross-linked polysiloxane to SiO_x increased with UV-ozone exposure time and cross-linking agent content, and the surface possesses highest conversion. The formation of a SiO_x layer increased surface roughness, but it decreased water contact angle. Gas permeation measurements on the UV-ozone exposure PDMS membranes documented interesting gas separation properties: The O₂ permeability of the cross-linked PDMS membrane before UV-ozone exposure was 777 barrer, and the O₂/N₂ selectivity was 1.9; after UV-ozone exposure, the permeability decreased to 127 barrer while the selectivity increased to 5.4. The free volume depth profile of the SiO _x layer was investigated by novel slow positron beam. The results show that free volume size increased with the depth, yet the degree of siloxane conversion to SiO_x does not affect the amount of free volume.