SURFACE FORCES BETWEEN PLASMA POLYMER-FILMS

The surface properties of two different plasma polymer coatings were investigated by means of ESCA, wetting studies, and surface force measurements. The plasma polymer films formed from hexamethyl-disiloxane (HMDSO) monomers were found to be very smooth. This made it possible to measure the attractive van der Waals interaction between HMDSO-coated surfaces in air. When the HMDSO-coated surfaces were immersed in water, they initially appeared strongly hydrophobic with an advancing and receding contact angle of 109-degrees and 98-degrees, respectively. Nevertheless, no long range ''hydrophobic'' interaction was observed. The HMDSO coating became slightly more hydrophilic when kept in an aqueous phase for a prolonged time, indicating migration/reorientation of polar groups to the surface. Surface force measurements in water clearly demonstrated the appearance of electrostatic charges and extending polymer tails as a result of a prolonged exposure to water. Plasma polymer coatings Of C2F4 or C2F6 deposited on top of a plasma polymer of CH4 were also studied. The forces between such a polymer and a glass surface are on the first approach in air characteristic of a van der Waals force. However, the attraction increases as the surfaces are brought together repeatedly. The increase in the long range attraction is a result of contact electrification due to electron transfer from glass to the fluorocarbon surface. It was found that in aqueous environment the C2F4 plasma polymer surface is significantly less smooth and less stable than the HMDSO plasma polymer surface.