Interface/interphase engineering of polymers for adhesion enhancement: Part I. Review of micromechanical aspects of polymer interface reinforcement through surface grafted molecular brushes

This article reviews the theoretical principles of macromolecular design of interfaces between glassy polymers as well as those between rigid substrates and elastomers for maximizing adhesion and fracture performance of bonded assemblies. According to contemporary theories, macromolecular "connector molecules" grafted onto solid polymer surfaces effectively improve adhesion and fracture performance of interfaces between polymers by improving the interactions with adjacent materials through one or both of the following mechanisms: (1) interpenetration into adjacent polymeric phase, and (2) chemical reaction/crosslinking with the adjacent material. It is shown that the effectiveness of the interface reinforcement by surface-grafted connector molecules depends on the following factors: surface density of grafted molecules, length of individual chains of grafted molecules, and optimum surface density in relation to the length of connector molecules. The influence of the above-mentioned physico-chemical parameters of molecular brushes on the interphase-interface reinforcement is discussed and quantified by contemporary theories. Also, the optimum conditions for maximum adhesion enhancement are specified and verified by a range of experimental examples. Part II of this article demonstrates a novel and relatively simple, industry-feasible technology for surface grafting connector molecules and engineering of interface/interphase systems, which is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theories, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.