Plasma-oxidized polystyrene: Wetting properties and surface reconstruction

The surface of oxygen-plasma-treated polystyrene (PSox) was investigated us ing X-ray photoelectron spectroscopy (XPS), streaming potential measurement s and a dynamic study of the wetting properties at different pH (Wilhelmy p late method). The PSox surface is functionalized with various oxygen-contai ning groups, including carboxyl functions, and must be viewed as covered by a poly-electrolyte which swells depending on pH. The wetting hysteresis, i ts evolution upon repeated cycles and the influence of pH are controlled by the dissolution of functionalized fragments and the retention of water upo n emersion; the retained water may evaporate progressively and allow macrom olecule compaction and/or reorientation. Modification of the PSox surface u pon aging in dry atmosphere, humid atmosphere, and water was studied using XPS and dynamic wetting measurements. Aging in water provoked the dissoluti on of PSox macromolecular chains, as indicated by adsorption of released fr agments on a check PS sample placed nearby. However, the concentration of f unctionalized molecules at the surface of water-aged PSox was still suffici ent to allow swelling at pH 5.6 and 11.0. Hydrophobicity recovery was faste r in humid air (R. H. 95%) compared to dry air (R. H. 5%), due to the plast icizing effect of water. Hydrophobicity recovery upon aging in air was reve rsed quickly by immersion at pH 5.6 or 11.0, due to deprotonation and swell ing.