ION-SCATTERING BY POLYMER SURFACES - ELASTIC AND INELASTIC EFFECTS

The use of low energy ion scattering as a probe for the chemical composition and the molecular arrangement of polymer surfaces is reviewed. The ability of ISS to detect preferential orientation of functional groups at the polymers surface is presented. However, from the examination of the published ISS data, it appears that the correlation between the surface composition, as seen by ISS, and the chemical composition of polymers is quantitatively very poor. The possible origins of this discrepancy are discussed. They mainly belong to experimental factors and the limited knowledge of both the physics of ion-polymer interaction and the unique dynamical nature of polymer surfaces. Moreover, the high sensitivity of polymers to ion irradiation leads to artifacts that have to be minimized by adopting the so-called ''static'' analysis conditions in order to study the intrinsic surface composition of these materials. ISS has a very low sensitivity for carbon and its intensity seems independent of the polymer nature. This effect is discussed in detail, taking into account the experimental factors, the elastic shadowing related to the structure and the He+ ion neutralization. In that way, the energy dependence of the scattering ion yields on the carbon and oxygen atoms of polyethylene terephthalate (PET) has been calculated and measured for the two He isotopes ions. The comparison between simulated and experimental results leads to the definite conclusion that a preferential neutralization of the ions occurs on the carbon atoms, as it could be expected from the examination of the polymer valence band structure. All these observations open new perspectives for the interpretation of the ISS data obtained on polymers. At the end, the potentialities of time of flight direct recoil spectroscopy (TOF-DRS) are also presented as a new promising development of ionic spectrometries for the analysis of the polymer surface structure.