Overturn of Polar Groups on Polymer Surface

Abstract

Generally, good water wettability attained on hydrophobic polymer surfaces by exposing to a plasma discharge becomes poorer by aging in air and the contact angle gradually rises again. The following three explanations have been proposed as the reason contamination of the polymer surface, change in roughness of the surface, and overturn and migration of polar groups on the surface. In order to elucidate or examine which phenomenon is the most probable, we studied the wettability change of glow-discharged, hydrophobic polymer surfaces using a contact angle measurement with the sessile drop method. The most probable reason seems to be the overturn (e. g., for polyethylene) and the migration (e. g., for silicone) of the generated polar groups into the inside of the film, because the lowered wettability recovered again when the aged films were immersed in water for a prolonged duration.
n addition, the water wettability of poly(vinyl alcohol) (PVA) and cellulose hydrogels was studied by measuring advancing and receding contact angles by the inverted bubble method. A marked hysteresis of the contact angles was observed for PVA. This hysteresis is probably due to the overturn of the hydroxyl groups. On the contrary, no significant hysteresis was found for cellulose and both of the contact angles were as low as 13°. This fact was explained in terms of the chemical structure of the repeating unit of cellulose.