Ultra-Thin Plasma Polymer Films as a Novel Class of Adhesion Promoters: A Critical Review

Several reaction mechanisms have been proposed for formation of plasma polymers, such as monomer fragmentation followed by poly-recombination into randomly structured and crosslinked films; fragmentation, accompanied by the formation of acetylene or other film-forming intermediates and deposition of polystyrene-like material; plasma-initiation of radical chain-growth polymerization; and ion-molecule reactions, as well as ionic chain-growth polymerization. The bulk structure of plasma polymers was found to be more or less randomly formed, far from that of classic polymers. Nevertheless, the retention of functional groups in the plasma polymer is higher than the retention of monomer structure in repeat units of polymer backbones. Such functional groups bearing plasma polymers were found to be suitable as adhesion promoting layers in polymer composites. These functional groups are essential for covalent bonding to substrate and coating. Additional introduction of aliphatic spacers could flexibilize the interface between plasma polymer covered substrate and coating. Thus, maximum adhesion in composites and laminates was produced. Several examples are given where plasma polymers alone or linked with spacers have been shown to be effective as adhesion promoter.