Hydrophobic recovery of polydimethylsiloxane after exposure to partial discharges as a function of crosslink density

Polydimethylsiloxanes with different crosslink densities were exposed to corona discharges or GHz air plasma and studied by contact angle measurements, X-ray photoelectron spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. The degree of surface oxidation increased with increasing exposure time with a limiting depth of 100–150 nm. Surface oxidation was faster in more highly crosslinked polymers. Within the oxidised layer, a brittle, microporous silica-like layer with a minimum organic silicone content of 40% gradually developed with increasing exposure time. The strain at which the silica-like layer cracked decreased with increasing dose of corona or air plasma. The hydrophobic recovery following the corona/air plasma exposure occurred at a slow rate by diffusion of oligomers through the microporous but uncracked silica-like layer or at a much higher rate by transport of oligomers through cracks in the silica-like layer.