Influence of gas pressure on the plasma reaction on polyethersulphone

Different gases (Ar, H-2, N-2 and O-2) were used for plasma treatments of polyethersulphone (PES) performed in an electron cyclotron resonance (ECR) plasma. The effect of the gas pressure on the chemical modification of the PES surface during plasma treatment was investigated by X-ray photoelectron spectroscopy (XPS). The experimental set-up of the plasma chamber connected via an ultra high vacuum transfer to a surface analysis system allows to measure well-defined surface modifications. The experiments have shown that the plasma-polymer interaction in the topmost surface region of the polymer increase with decreasing gas pressure. At low gas pressures (p < 10(-3) mbar) the plasma attacks first the polar bonds of the sulphonic group, this results in the reduction to sulphide of all sulphonic groups (-SO2- --> -S-) in the study state. This effect decreases progressively for higher gas pressures(p > 10(-3) mbar) and is no more observable at p = 10(-1) mbar. In addition to the amount of sulfide formation also the time scale of sulfide formation varies with the gas pressure. The time constant for the sulfide formation change from tenth of seconds to about one minute for 10(-4) to 10(-2) mbar gas pressure, respectively. The thickness of the surface layer in which sulphide is formed depends on the pressure and is at least equal to the XPS probing depth of about 100 Angstrom for p < 10(-3) mbar. The incorporation of new chemical functionalities by reactive gas plasma treatments is similar for all gas pressures. Differences were observed in the amount of adsorbed species at the top most surface layer. The higher the gas pressure is, the higher the content of adsorbed species. The results are discussed in function of the flux phi(i), and the kinetic energy E(kin) of the ions interacting with the polymer surface.