Plasma co-polymerisation of two strongly interacting monomers: Acrylic acid and allylamine

Plasma co-polymerisation presents an exciting opportunity to produce coatings with mixtures of functional species that are difficult to achieve from single monomer systems. Unfortunately, the complexity of reactions in the plasma environment can make it difficult to predict and characterise the coating chemistries. In this study, X-ray photoelectron (XP) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy have been used to examine coatings prepared by the co-polymerisation of allylamine and acrylic acid. As expected, a mixture of chemical groups including amines, nitriles, amides and acids were identified. Significantly, the copolymers showed strong evidence of protonated amine (based on the N 1 s XP spectra) and carboxylate anions (based on the NEXAFS O K edge); species that were not evident in coatings produced from the individual parent monomers. The results suggest that as the two monomers mix in the gas phase, allylammonium acrylate salts form and may act as a third monomer during the plasma polymerisation. The results also indicated that the addition of acrylic acid monomer suppressed the dehydrogenation of amines to nitriles and increasing the proportion of nitrogen as amine species at the interface. We speculate that these coatings will exhibit zwitterionic characteristics in aqueous solution and that they may find utility in the isoelectric focussing and separation of proteins from solution.