An esca investigation of low energy electron beam modification and formation of polymers

Abstract

Physical and chemical aspects of the interaction of low energy electrons with organic polymers and polymer precursors have been investigated using X-ray photoelectron spectroscopy (XPS or ESCA). Electron mean free paths have been determined over a substantial kinetic energy range using TiKα and CuLα excitation sources employing the substrate overlayer technique for poly (paraxylylene) films on gold substrates. The ability of the Ti source to probe deep lying core levels in conjunction with the use of a CuLα source enabled a square root dependence of mean free path on kinetic energy to be formulated over the energy range 0.5 → 4.4 keV such that at 4.4 keV mean free path is ~4OÅ. In view of technological interest in the radiation modification of polymers the interaction of low energy electron beams with fluoropolymers, specifically PTFE and PVF(_2), has been investigated. In both cases defluorination of the surface regions has been shown to take place which increases with radiation dose and dose rate leading to the formation of an unsaturated carbonaceous network. The reaction is, however, independent of electron beam energy. In the initial states of irradiation inhomogeneities in the surface region occur in the case of PTFE but not PVF(_2), but the depth of modification in both cases being beyond the ESCA sampling depth using MgKα radiation. The irradiation of condensed fluorobenzenes results in the formation of films of involatile polymeric material whose composition is a function of starting material and shows greater defluorination with applied dose. Under controlled conditions the films closely resemble those obtained by plasma polymerisation illustrating the similarity in electron beam and plasma processes. As an extension to this idea MNDO-SCF-MO calculations have been carried out on the structural isomers of the perfluorodiazabenzenes in an attempt to interpret previous experimental observations relating to their plasma polymerisation, which may also be relevant to their electron beam polymerisation.