Plasma-Oxidized Polystyrene:  Wetting Properties and Surface Reconstruction

Abstract

The surface of oxygen-plasma-treated polystyrene (PSox) was investigated using X-ray photoelectron spectroscopy (XPS), streaming potential measurements and a dynamic study of the wetting properties at different pH (Wilhelmy plate method). The PSox surface is functionalized with various oxygen-containing groups, including carboxyl functions, and must be viewed as covered by a polyelectrolyte which swells depending on pH. The wetting hysteresis, its evolution upon repeated cycles and the influence of pH are controlled by the dissolution of functionalized fragments and the retention of water upon emersion; the retained water may evaporate progressively and allow macromolecule compaction and/or reorientation. Modification of the PSox surface upon aging in dry atmosphere, humid atmosphere, and water was studied using XPS and dynamic wetting measurements. Aging in water provoked the dissolution of PSox macromolecular chains, as indicated by adsorption of released fragments on a check PS sample placed nearby. However, the concentration of functionalized molecules at the surface of water-aged PSox was still sufficient to allow swelling at pH 5.6 and 11.0. Hydrophobicity recovery was faster in humid air (R. H. 95%) compared to dry air (R. H. 5%), due to the plasticizing effect of water. Hydrophobicity recovery upon aging in air was reversed quickly by immersion at pH 5.6 or 11.0, due to deprotonation and swelling.

Effects of Surface Modification and Moisture on the Rates of Charge Transfer between Metals and Organic Materials

“…Figure 6 shows plots of the advancing contact angles of drops of water on surfaces of PS ox as a function of the pH of the drop. In accord with previous studies, 30 cos θ a increases with increasing pH for surfaces presenting appreciable concentrations of acidic oxygen-containing species (>25 atom % oxygen; the concentration of acidic groups on the surface of "unoxidized" PS (<1%) was too low to detect using this method).…”

“…15,25,26 Oxygen-containing species are present even on native surfaces of "unoxidized" PS (not UVO oxidized) because surface oxidation occurs naturally for samples aging under ambient laboratory conditions; 29 native PS petri dishes have typically ∼1 atom % oxygen on their surface. 30,31 The oxidation of PS is restricted to its surface because the kinetics of oxidation of the surface of PS is much faster than that of the bulk as a consequence of the limited depth of penetration of UV light and the limited diffusion of ozone into the bulk. 28 Using measurements of contact angles, we correlated the concentrations N ox 's of oxygen-containing species on the surfaces of PS ox to the times of oxidation t ox 's (Figure 4a).…”

Conditioning materials with biomacromolecules: Composition of the adlayer and influence on cleanability

“…Table 2 gives the surface elemental composition determined by XPS on the residue obtained after evaporation of drops of dextran and BSA solution, on bare substrates, on conditioned substrates, rinsed or not, on quartz powder collected from a suspension in water, and on quartz powder conditioned by suspension in a dextran or BSA solution, and rinsed or not. The presence of oxygen, oxidized carbon and silicon at the surface of non-conditioned polystyrene may explain that the water contact angle (Figure 3) was lower than expected (Dupont-Gillain et al, 2000). Figure 4 presents representative O 1s, N 1s and C 1s peaks recorded with the siliceous solids.…”

Oxygen and nitrogen plasma hydrophilization and hydrophobic recovery of polymers

“…On dry storage, the principal reasons for the recovery have been identified as being the reorientation of the surface layer, or the migration of the polymer chains from the bulk of the polymer to the surface or from the surface to the bulk. 19,[25][26][27][28][29][30] If the samples are stored or washed in a solvent, polymer fragments can also leave the surface by dissolving into the solvent. 19,25,31 The plasma hydrophilization and hydrophobic recovery have been studied for many polymers, in both mechanistic and application oriented studies.…”