Chemical selectivity and energy transfer mechanisms in the radiation-induced modification of polyethersulphone

The mechanism of radiation-induced degradation of PES has been studied in a great detail by using XPS technique. PES films have been irradiated both with 6 keV Ar and 3 keV e(-) beams. The Ar irradiation is assumed to be representative of the processes which strictly depend on collisional energy loss, while the electron irradiation puts in evidence mainly the processes related to electronic energy loss. In particular, three basic chemical reactions have been followed by XPS: 1) the reduction of the sulphonyl groups (-SO2-) to sulphidic-like groups (-S-); 2) the elimination of sulphur-containing groups; 3) the formation of new oxygen-containing groups, as ether, hydroxyl or carbonyl groups. Differences are observed when irradiating with Ar and with e(-) projectiles both for the rate of evolution of the different species and for the quantitative trends of modification. The rates of the reactions 1) and 2) are found to depend essentially on the total deposited energy. The trend and the rate of reaction 3) depend dramatically on the energy transfer mechanism. The key factor determining the sensitivity to the energy deposition mechanisms seems to be the inherent chemical selectivity of the involved reactions. In particular, this means that some types of reactions are sensitive to the energy transfer mechanism due to their selectivity (as it is the case of the ''oxygen attachment'' reactions), while others will depend only on the total deposited energy (as the sulphur loss or the sulphonyl reduction) due to the availability of many concurrent pathways producing a random succession of chemical events yielding an unique product.