HYDROPHOBICITY RECOVERY OF POLYDIMETHYLSILOXANE AFTER EXPOSURE TO CORONA DISCHARGES

A high-temperature-vulcanized polydimethylsiloxane (PDMS) elastomer ha s been subjected to corona discharges for different periods of time in dry air. The loss and recovery of hydrophobicity of the surface have been characterized by contact angle measurements. Immediately after ex posure to corona discharges, samples showed a low surface hydrophobici ty and, on storage in dry air, a continuous increase in hydrophobicity finally approaching the hydrophobicity of the unexposed material. The activation energy of the hydrophobicity recovery was two to four time s greater than the activation energy of the diffusivity of low molar m ass PDMS in PDMS elastomers, indicating that the diffusivity propertie s of the oxidized surface layer were different from that of the bulk. PDMS elastomers quenched in liquid nitrogen or subjected to small mech anical deformation (<1% strain) after exposure to corona discharges fo r 1 h or more recovered their hydrophobicity faster than untouched spe cimens kept under identical conditions. X-ray photoelectron spectrosco py confirmed the early formation of a silica-like surface layer, with a thickness of at least 10-12 nm. The atomic composition of the oxidiz ed surface layer remained essentially unchanged after the first hour o f corona discharges. It is suggested that the silica-like surface laye r delayed the recovery of hydrophobicity by inhibiting the transport o f low molar mass PDMS to the surface. It is also suggested that therma lly or purely mechanically induced stresses lead to a cracking of the brittle silica-rich layer and that this in turn facilitates the transp ort of low molar mass PDMS to the surface and to a more rapid recovery of the hydrophobicity. Data obtained by reflection infrared spectrosc opy assessing the outermost micrometer, confirmed the oxidation and th e formation of hydroxyl groups at a progressively higher concentration with increasing exposure time of corona discharges. (C) 1998 Elsevier Science Ltd. All rights reserved.