Morphology of Polymer Film Coatings Produced in a Barrier Gas Discharge at Atmospheric Pressure

Abstract

The technique of deposition of polymer films from the barrier gas discharge plasma on top of dielectric substrates is developed. The film precursor is a monomer material highly dispersed in a transport plasma gas. The plasma formation of the continuous polymer film can be divided into two stages. The first stage is deposition of liquid monomer droplets with subsequent polymerization on the dielectric barrier surface resulting in growth of a discontinuous film. It is found that the ratio of the height of droplets to their lateral size is almost constant and for polystyrene it is equal to ~0.01. The second stage is expansion of the droplets into islands and their coalescence into a continuous polymer film on the dielectric barrier surface. The polymer coating thickness and the amount of cross bonds essentially depend on the current density and concentration of the monomer in the transport discharge gas. A continuous polymer film, which is not contaminated with the monomer destruction products, can be obtained in the current density range of 7–25 mA/cm². Experiments with several monomers, like methylmethacrylate, styrene and acrylonitrile, have shown that the growth rate is maximal for monomers with oxygen-free molecules. At the same time, a higher growth rate provides low cross-bond coatings, whereas to get high density of the cross-linked bonds one has to utilize low deposition rates. The minimal thickness at which continuity of the film is achieved increases when the monomer concentration in the plasma rises. In general, the coating thickness depends linearly on the discharge current density; the particular figures depend on the type and concentration of the monomer. The technological parameters are established and given for the three aforementioned monomers.