Dynamics of Polmeric Solid Surfaces Treated by Oxygen Plasma: Plasma-Induced Increases in Surface Molecular Mobility of Polystyrene

Abstract

The dynamic behavior of the oxygen-containing polar functional groups on the plasma-treated polymer surfaces has been studied in relation to the polymer-chain mobility with regard to the effect of the aging temperature and the crosslinking density of the polymer. The films of polystyrene (PS) were treated by O2plasma, and aged in dry nitrogen gas or distilled water at 25 and 75°C for 480 h. Second, the styrene–divinylbenzene (St–DVB) copolymers with different crosslinking densities were synthesized and aged in dry nitrogen for 480 h after the O2-plasma treatment. The low molecular-weight oxidation products formed by the plasma treatment were removed by washing the films with methanol, which made it possible to eliminate the effects of the products. The wettability of PS film acquired by the O2-plasma treatment became poorer during the aging in nitrogen. Especially the aging at 75°C in nitrogen led the contact angle of water close to that of the untreated PS, which suggested that the surface molecules of the treated PS were beyond the glass transition point even at the temperature. On the other hand, the restriction of the hydrophobic recovery by crosslinking was, contrary to the expectations, rather mild: The St–DVB copolymer with as much as 20 mol% DVB still increased the contact angles of water by 30° from the value immediately after the plasma treatment. These results suggested that the molecular mobility on the surface of polystyrene was increased by the O2-plasma treatment compared with that of the untreated state, which enhanced the migration of the polar functional groups introduced into the PS.