The application of esca to structure, bonding and reactivity of some polymer systems, with particular reference to surface modifications

Abstract

X-ray photoelectron spectroscopy (ESCA) is used to study structure bonding and reactivity of polymeric materials. Particular emphasis is placed on the investigation of surface modification of polymers, on the monomolecular depth scale, effect by Inductively coupled radlofrequency glow discharges excited in a variety of inert gases and oxygen. Low energy satellite structures, deriving from shake up transitions accompanying direct core ionization are identified in the spectra of unsaturated systems and demonstrated to be characteristic of the polymer structure and sensitive to substituent effects. The shake up transitions are essentially localized within a given pendant group or backbone feature and are exploited on a semi-quantitative basis, as a monitor of the extent of unsaturation in the surface of the polymer, providing a valuable extra level of information. The equilibrium static charge consequent upon the various electron fluxes arriving at and leaving the sample during the ESCA experiment has been correlated to polymer structure within a depth scale of the same order as electron mean free paths and is shown to be worthwhile studying in its own right. The effect of sample charging with respect to energy referencing employing extraneous hydrocarbon contamination in the spectrometer is shown to be small. Shake up and charging phenomena along with several other facets of the ESCA experiment including angular dependence of relative and absolute signal intensities, analytical depth profiling, difference spectroscopy and Madelung charge potential calculations, as well as the primary sources of data (absolute and relative binding energies, and relative peak intensities) have been employed in an extensive investigation into the surface modification of polymers by means of Inert gas and oxygen plasmas. For the inert gas plasmas the rates for direct and radiative energy transfer processes are determined within the framework of a kinetic model, and are shown to have a strong dependence on the sustaining gas, the power, the pressure and the plasma configuration. The depth to which direct energy transfer processes are important is determined to range from about one monolayer for krypton to three monolayers for helium. Radiative energy transfer to the bulk polymer is best effected by neon and some aspects of the vacuum ultraviolet radiation emitted from the plasmas are also presented. Modification by the oxygen containing plasmas is demonstrated to be extensive but confined within approximately one monolayer, in the initial stages. The rate and extent of oxidation is a strong function of polymer structure for pure oxygen plasmas (r.f. low power, low pressure) and is thought to be initiated by a crosslinking mechanism. This is affirmed by comparison with plasmas excited in helium/oxygen mixtures.