Polystyrene thin films treatment under DC pulsed discharges conditions in nitrogen-argon and oxygen-argon mixtures

This paper is the third of a trilogy [16,75] devoted to polystyrene thin film treatment under DC pulsed discharges conditions. In order to study the wettability variations ($\Delta \theta/\theta_i$) of the surface, N2-Ar or O2-Ar mixtures are used. The experimental conditions are close to the best running conditions deduced from [16,75]. The pressure is varied from 1 mbar to 10 mbar and the mixture proportion from 0% to 100%. The importance of metastable states is pointed out. Reaction processes in the plasma and on the polystyrene surface are proposed in order to explain the surface activation and the formation of chemical bonds with nitrogen or oxygen. The best running conditions are reached when the generator frequency equals 500 Hz for a pressure about 4 mbar in nitrogen and 10 mbar in oxygen, whatever the gas mixture composition is. The wettability is better with oxygen than with nitrogen and the best value is reached for a duration time shorter in oxygen than in nitrogen. The minimum of treatment time necessary to reach the $\Delta \theta/\theta_i$ plateau value depends on the mixture composition