O^·, H^·, and ^·OH radical etching probability of polystyrene obtained for a radio frequency driven atmospheric pressure plasma jet

Atmospheric pressure plasma jets have great potential for the surface modification of polymers. In this work, the authors report on polystyrene etching by a radio frequency driven atmospheric pressure plasma jet with a focus on the role of $H⋅$⁠, $O⋅$⁠, and $⋅OH$ radicals in this process. The absolute flux of $H⋅$⁠, $O⋅$⁠, and $⋅OH$ radicals reaching the surface of the polymer was determined by a comsol multiphysics reacting fluid dynamics model incorporating detailed transport phenomena in the boundary layer near the substrate. The simulated results of $H⋅$ and $⋅OH$ densities in the jet effluent were experimentally verified by two-photon absorption laser induced fluorescence and laser induced fluorescence, respectively. The carbon atom removal flux from the polystyrene surface was taken from previously reported measurements using the same plasma source. The authors show that the boundary layer effects in the interfacial region above the substrate can have a significant impact on the calculated etching probabilities. The reaction probability (⁠$β$⁠) has a significant uncertainty although a variation of 2 orders of magnitude in $β$ leads to uncertainties of approximately 1 order of magnitude variation in the determined etching probability. The etching probability of polystyrene by $⋅OH$ radicals was confirmed to be at least an order of magnitude larger than the polystyrene etching probability by $O⋅$ radicals. The authors also confirmed the weak polystyrene etching probability by $H⋅$ radicals. The model suggests that the presence of a 30 ppm $O2$ impurity can lead to the production of $⋅OH$ radicals in the far effluent of the $Ar+1%H2$ plasma jet close to the substrate at sufficient densities to enable effective etching.