Low-energy ion scattering (LEIS) study of the ageing of oxygen-plasma-treated high-density polyethylene (HDPE) surfaces

Static low-energy ion scattering (LEIS or ISS) and x-ray photoelectron spectroscopy (XPS) are used to study the surface of high-density polyethylene (HDPE) that has been modified by exposing it to a `cold' oxygen plasma. The HDPE surface composition is monitored by both techniques as a function of time after the plasma treatment. With LEIS, large differences in ageing are observed between a sample that is exposed to air after the plasma treatment and a sample that is not exposed to air, but rather kept under vacuum. The hydrophobic recovery of the sample exposed to air is very large, whereas no recovery is observed on the other sample. The XPS measurements also show a decrease in oxygen concentration as a function of time for the sample that was exposed to air, but not as steep as in the LEIS measurements. The differences between the air- and vacuum-exposed samples are explained in terms of differences in composition and cross-linking in the surface region. Static LEIS seems to be a promising method to study the ageing of plasma-treated polymers.