The use of XPS to examine the interaction of poly(acrylic acid) with oxidised metal substrates

Abstract

Abstract As part of a wider investigation into the use of high resolution (monochromated Al Kα) XPS to probe the interfacial reactions responsible for adhesion, thin films of poly(acrylic acid) have been studied on a variety of different substrates. It is shown that sample degradation, induced by photoelectrons from the metallic substrate, occurs in all cases. This is recognised by the appearance of a component in the C 1s spectrum 1.7 eV removed from the main aliphatic C 1s line. The intensity of this component is shown to scale with the total photoelectron cross-section of the metallic substrate. Through careful peak fitting of the high resolution C 1s spectra three different types of specific interaction between the PAA molecule and the oxidised substrates have been identified: (i) hydrogen bonding of the PAA molecule with strongly acidic sites; (ii) the formation of a carboxylate anion on interaction with weakly basic sites; (iii) nucleophilic attack of the acyl carbon by strongly basic sites. This, in principle, provides a direct estimation of the density of the different classes of acido-basic sites on the oxidised substrates.