Study of DC point-to-plane glow discharge in O-2 and Ar in relation to atatic polystyrene (aPS) wettability

摘要

Polystyrene thin films undergo treatment by cold plasma produced from DC point-to-plane low-pressure (4 mbar) discharge in O-2 and Ar. The surface tension highly increases (more than 70% for O-2 plasma and 60% for Ar plasma) by applying as weak as 200 mu A discharge mean current. The treatment time required for obtain the maximum wettability is double in the case of Ar (similar to 60 s). Important increase in surface polar component takes place while the dispersive component decreases, regardless of the gas used. Oxygen-functionalities (10-12%) graft the first 5 nm of surface and the surface polarity is decupled, in both gases. The main part (>99.8% for O-2 and >97% for Ar) of the treatment time corresponds to a relaxation phase of the discharge (post-discharge time) and, thus, the role of long lifetime excited particles (>3 eV) in the activation of the polymer surface is emphasized. An ionization front having velocity similar to 10(4)-10(6) m/s ( higher in O-2 than in Ar) crosses the gap starting from the anode and propagating towards the cathode. The arrival of this wave at the cathode region boosts the cathode secondary processes. The secondary electrons amplify the discharge current and trigger off the initiation of a new ionization front, stronger than the first one, which establishes the glow discharge regime. These fronts (weaker in Ar discharge) are suggested as the main mechanisms for the cold plasma production and active particles generation for the polymer treatment.