Measurement of Forces Between Surfaces in Polymer Fluids

The physical origins of the forces exerted among polymer molecules include those arising from sources familiar to small molecules (van der Waals, electrostatic, hydrogen bonding, and others) as well as those that can be traced directly to the chain-like character of the polymer. Macromolecules multiply the effects of interactions among their constituent monomers so that a small intersegmental interaction can produce a large intermolecular effect. This is a key factor in polymer adsorption on solids, interaction of polymer molecules with solvents, and interface formation in polymers via thermodynamic phase separation. Chain connectivity and rotational isomerism give a random (or self-avoiding) walk character to macro­ molecular configurations ( 1 ). Distortion of this "natural" population of conformations by the application of perturbations, such as surface poten­ tials, mechanical strain, or electromagnetic fields, produce "elastic" or confinement forces arising from the tendency to maximize the randomness of the conformational distribution. Macromolecular fluids, as they are viscous and frequently highly entangled, can also exhibit nonequilibrium forces, which arise from incomplete adaptation to changes in their environ­ ment. Such forces can be very slow to relax (2).