Influence of a DC, point-to-plane, low-pressure discharge in nitrogen on polystyrene thin films

Abstract

In the first part of this work the regime of the DC electrical discharge in nitrogen, in absence and in presence of polystyrene thin films in the cold plasma reactor, for a constant gap length d = 1 cm and for variable pressures (1 mbar < p < 10 mbar) is characterized, while the gas flow rate is maintained at Q = 1 seem. V-I curves underline that in any case a normal glow-like discharge is established. Experimental results bring out that the presence of the films in the volume of the produced plasma seems to modify the structure and some of the discharge characteristics, like the cathode secondary emission effects. V-I curves (I < 200 μA), discharge repetitive current impulses, discharge repetitive emitted light impulses along the discharge symmetry axis and photos of the discharge, in presence and in absence of the films, are presented. In the second part of this paper results of polystyrene thin films treatment under the above DC normal glow-like discharge regime in nitrogen for improving the wettability of the films are presented. The effect of discharge main parameters (pressure p and mean current I through the discharge) on the treatment is studied under certain experimental conditions. The wettability is characterized by contact angle measurements using deionized water drops and the evolution versus the treatment time t tr of the Δθ/θ i = (θi - θf)/θi ratio (θi: the contact angle before the treatment and θf: the contact angle after the treatment) is estimated. It is deduced that optimizing main parameters of the DC electrical discharge in nitrogen it is possible to obtain films with very good wettability (suggestively Δθ/θi = 0.951 corresponding to θ f = 4°, at p = 4 mbar, I = 200 μA, t tr ≅ 600 s). A rapid change of the films surface nature (0 s < t tr < 60 s, suggestively) seems to be followed by a slower one and finally by a saturation. The gas pressure and the discharge mean current seem to play a very important role for the treatment. The experimental results allow us to start a discussion about the possible role of some active particles produced in the plasma to the treatment of the films and especially the role of the neutrals and the metastables.