Non-equilibrium interaction forces between adsorbed polymer layers

The non-equilibrium forces of interaction between an 11 mu m radius sphere of zirconia and a flat polished zirconia plate in the presence of adsorbed polyvinyl pyrrolidone (M(w) = 40000) have been investigated. The force-distance data were collected using an atomic force microscope (AFM) configured to operate as a force balance. Specific effects related to the velocity at which the surfaces were brought together and separated were observed. For the compression cycles the energy of interaction at any given interaction distance was seen to be higher at a higher rate of collision. These effects are explained from a consideration of polymer relaxation times and increased effective rigidity of the chains at higher scan rates causing the increased repulsive behaviour. For the decompression cycles, a longer time of contact between the surfaces before decompression was seen to lead to the formation of bridging interactions. Faster scan rates which led to shorter surface contact times resulted in the absence of any energy minima indicative of adhesive bridging links.