Corona-discharge-induced hydrophobicity loss and recovery of silicones

Abstract

The effect of corona discharge on the hydrophobicity of polydimethylsiloxane (PDMS) used as high-voltage (HV) insulation and the subsequent recovery of hydrophobicity when exposure ceases is now a well-known phenomenon. Surface characterization studies have established that corona exposure in the laboratory forms a brittle, wettable, very thin silica-like layer on the surface of most PDMS elastomers, consistent with similar effects from radio-frequency plasma treatments. However, there is still considerable debate regarding the relative importance of the two major hydrophobic recovery mechanisms postulated, diffusion of low molecular weight (LMW) material and reorientation or overturn of treated chains in the surface region. This study used extracted PDMS and also PDMS with deliberately added LMW PDMS fluid. Contact angle and X-ray photoelectron spectroscopy (XPS) were used to assess the effect of corona discharge voltage variation and aging at different temperatures. In all cases the familiar highly cross-linked silica-like layer is formed. Such a structure cannot readily reorient between hydrophilic and hydrophobic states implying that diffusion of LMW components is the key mechanism of hydrophobic recovery. These latest studies of PDMS systems, solvent extracted to be free of diffusible species, also shows hydrophobic recovery suggesting that LMW PDMS might be produced in-situ by depolymerization.