Adsorbed polymers in colloidal systems: From statics to dynamics

Abstract

When (homo-) polymers adsorb from solution onto a surface, forming reversible, physical bonds with the surface, a diffuse structure results which can be characterized in several ways. On the level of individual molecules one may distinguish between sequences of monomer units attached to the surface (‘trains’), parts which start from the surface and return there (‘loops’) and parts where one end dangles freely in solution (‘tails’). On the level of the adsorbed layer as a whole one is interested in the density of monomer units as a function of distance from the surface (the so-called profile), the (total) adsorbed mass, and the thickness of the layer. Of course all these properties depend on the molar mass of the polymer, its structure and its interaction with the surface and with the solvent. The static (equilibrium) situation has been studied in depth over the past 20 years, both experimentally and theoretically. A particularly important step forwards was the theoretical work of Scheutjens et al. who developed an elegant and exact method to calculate adsorbed polymer properties on the basis of a lattice model. Together with results from modern experimental techniques (e.g., small angle neutron scattering, photo-correlation spectroscopy) a fairly consistent picture is now established and work is directed towards more complicated systems such as copolymers and polyelectrolytes. The kinetics of the adsorption process however, is much less understood and the same holds for the dynamics of adsorbed chains. Although it was long suspected that slow rearrangement processes occur in adsorbed layers, it was shown only recently that a freshly prepared polymer layer thins in the course of time as long as the surface is unsaturated. This has important consequences for the process of flocculation (aggregation) of colloidal particles by adsorbed polymer. Such aggregation will namely occur only if the polymer molecules can bridge the gap between two particles in close proximity (so-called ‘bridging’ flocculation). If the particles repel each other (e.g., by virtue of their electrical charge) they may not approach closely enough for bridging to occur. Big polymer molecules can however bridge a gap of 100 Å or more, provided they have not yet had time to rearrange into a flat conformation. Hence, only if adsorption occurs rapidly enough to compete successfully with the rearrangement process, there is the possibility of floc formation. Recent attempts to get more direct information on the kinetics of adsorption, desorption and exchange will be reviewed. © 1991 The Society of Polymer Science, Japan. All Rights Reserved.