Hydrophobic Recovery of Plasma-Treated Polydimethylsiloxane

Abstract Plasma treatment of silicone surfaces is a useful, environmentally-sound method of increasing wettability to improve adhesion. A thin, wettable silica-like layer is produced with various plasma gases such as argon, helium, oxygen and nitrogen. However, in each case the surfaces gradually recover their hydrophobicity. The silica-like layer is brittle and microcracking is evident at more severe levels of plasma treatment. The onset of cracking is a function of plasma gas, RF power, pressure and treatment time. Scanning electron microscopy has been used to characterize the cracks. The hydrophobic recovery has been monitored by water contact angle changes. It occurs with both cracked and uncracked treated surfaces. There is an initial jump in hydrophobicity at the onset of cracking. Thereafter, the recovery of both cracked and uncracked surfaces broadly parallels each other with virtually complete recovery of original hydrophobicity within one week. These effects can be accounted for by rapid surface diffusion of low molecular weight material out of fresh cracks followed by slower bulk diffusion through the polymer matrix. Significant differences in recovery rates are also evident between different plasma gases.