An experimental study of thin film physisorption and chemisorption
The physical adsorption of a liquid film and the chemisorption of a solid polymer film were investigated from macroscopic and microscopic viewpoints respectively. The vapour adsorption and subsequent formation of thin equilibrium hexane films (<30AA ) on a mica substrate (initially in ultra high vacuum) was examined. The fundamental Van der Waals intermolecular forces which determine the measured film thickness are neatly envisaged by the continuum theory due to Lifshitz. A theoretically derived adsorption isotherm was found to underestimate that measured experimentally by up to 100% on the freshly (air) cleaved substrate. Heat treatment cleaning prior to adsorption led to better agreement. The film thickness measurements were made with a novel design of return path ellipsometer. The incident and reflected laser beams both use the same set of optics and are also transmitted through an identical point in the chamber window. Its advantages, particularly for use in ultra high vacuum environments, are discussed. Investigation of deposited films of 4,4-oxydianline (ODA) and 1,2,3,5 benzenetetracarboxylic anhydride (PMDA) on a polycrystalline silver substrate were made using X-ray photoelectron spectroscopy. Co-deposition followed by heating led to polymerization and the formation of thin (~11AA ) and thermally stable (T<450°C) polyimide films. Both PMDA and ODA films chemisorb on the clean surface under partial fragmentation. Adhesion of the polymer film involves chemical bonding to these fragments. This also demonstrates that sufficiently thin polymerized films can be prepared and applied to fundamental studies of adhesion.