The application of ESCA to structure and bonding in polymer surfaces with particular reference to glow discharge polymerization

Abstract

x-ray photoelectron spectroscopy is used to study structure, bonding and reactivity of materials, in particular polymeric materials, especially those prepared by 'glow discharge techniques'. The glow discharge polymers have been prepared predominantly in radiofrequency inductively coupled gas plasmas using fluorine containing monomers. Films of known thickness have been prepared in situ by plasma techniques from 1,1-difluoroethylene and the structure investigated by means of ESCA. Electron mean free paths have been measured at kinetic energies corresponding to photoemission from the F(_1s) (563 eV), C(_1s) (967 eV), Au(_4f7/2) (1170 eV) and F(_2s) (1220 eV) and found to be 7 ± 3Å, 10 ± 3Å, 17 ± 4Å and 25 ± 7Å respectively. Polymers produced by plasma techniques from three isomeric difluoroethylene monomers are examined by ESCA. The polymer produced from 1,1 difluoroethylene is discussed in some detail and compared with those for the two 1,2 difluoroethylene isomers. The kinetics of deposition and structures of the resultant plasma polymers are compared. Charge distribution for both the neutral molecules and their radical cations are calculated and a possible reaction scheme involving fluoroacetylene is outlined. This work is extended to include the results of polymerizations in microwave discharges. The perfluorocyclohexa compounds have been polymerized by plasmatechniques and the polymer studied in detail as a function of power input to the plasma and the operating pressure. The rates of deposition parallel the first ionization potentials of the monomers and are: C(_6)F(_6) > C(_6)F(_8) (1,3) ~ C(_6)F(_8) (1,4) > C(_6)F(_10) ~ C(_6)F(_12). C(_1s) spectra of the resultant polymer are shown to reflect the nature of the starting monomer. Through investigations of polymer deposited in non-glow regions reactive CF(_2) containing species are shown to be important. Fluorine incorporation into the polymer decreases with increasing working pressure and is explained in terms of an increase in molecular rearrangements at lower pressures. Preliminary studies of the vacuum U.V. transmission properties of a number of polymers show that all the systems investigated absorb strongly below 1700Å and the relative degree of absorption is: polystyrene > polyparaxylylene ~ polyvinylchloride > plasma polymerized C(_6)F(_6) > plasma polymerized C(_6)F(_12). A number of paint systems have been investigated by ESCA and the broad changes in surface chemistry monitored for weathering periods of 1, 3 and 6 months. Oxidation is shown to be an important process and erosion becomes important with long exposures. The relative sensitivities to degradation by weathering are made apparent. Selected polymers weathered for a fixed time are studied and oxidation is shown to be important. In some cases a mode of degradation is suggested and compared with other data. Differences in rates of modification are shown to be : polysulphone > polyphenylether > nylon 6,6 > low density polyethylene. Oxidation penetrates beyond the immediate surface except in the case of L.D.P.E. The angular dependence using a fixed X-ray and analyser arrangement for horizontally inhomogeneous samples is discussed and shown to be complex but reproducible. The changes in effective sampling area contribute to this. The angular dependent behaviour for vertically inhomogeneous samples is discussed and it is shown that for thin films (<20Å) the angular behaviour is distinct from that observed for a homogeneous sample. The angular behaviour of a composite probe with 18 electrically isolated points is described. The behaviour of off-axis points is complex and the area sampled on the probe tip is not symmetric.