Hydrophobicity recovery of polydimethylsiloxane after repeated exposure to corona discharges. Influence of crosslink density

Crosslinked polydimethylsiloxane, with two different crosslink densities (M¯_c), have been repeatedly exposed to corona discharges. Between each exposure the specimens were allowed to rest for at least 1000 h allowing recovery of hydrophobicity. The material with the dense network (M¯_c: 700 g·mol^-1) was, according to XPS, more oxidized compared to the less crosslinked material (M¯_c: 12000 g·mol^-1). The material with the dense network gradually decreased the rate of recovery of hydrophobicity with increasing dose of corona whereas it was only moderately retarded in the less crosslinked material. Scanning electron microscopy showed the formation of surface cracks in both materials after a certain dose of corona. The material with the dense network exhibited thinner (<1 μm), more narrow cracks, whereas the less crosslinked material exhibited wider (>5 μm), more curved surface cracks. In the less crosslinked material, new cracks were initiated at the bottom of older cracks. Thus by further oxidation of previously unoxidized PDMS in the bottom of the cracks, new cracks were formed. It is suggested that the build-up and the fracture of the silica-like surface layer influence the transport of low molar mass siloxanes to the surface, and the rate of recovery of hydrophobicity