THE INTERPHASE IN ADHESION - BRIDGING THE GAP

This paper addresses the origin, structure and properties of the inter phase in adhesion. By interphase is meant that interfacial region betw een bulk adhesive and bulk adherend over which the local density displ ays a spatial gradient. In such a region all the local thermodynamic p roperties, including the mechanical properties, will be a function of distance from the surface. The question of how large these variations may be both in terms of magnitude and range is clearly a matter of som e import in adhesive technology. The term ''local'' is a key considera tion because meaningful discussion of the interphase can hardly be con templated without explicit recognition that matter in the interphase s hould be regarded properly as existing in a thermodynamically metastab le small system of low dimensionality. The results of a number of our numerical and theoretical studies, namely molecular dynamics simulatio n of realistic interfaces and nonlinear dynamical analysis, have been used to investigate the nature of materials close to surfaces. It has been found that the form and range of the density profile are indeed s ensitive to the chemical nature of the surface but it has not been pos sible to rationalize the existence of the macroscopic long-range inter phase on the basis of the usual type of spatial correlation. Recognizi ng that many adhesive bonds are formed using a reactive resin system w e have investigated the effects of nonlinearities in the reaction kine tics on the structure and morphology of the interphase. When these fac tors are taken into account, a theoretical description of the interpha se is possible in which the coupling between the various distance scal es arises in a very natural fashion.