Interactions between poly(ethylene oxide) layers adsorbed to glass surfaces probed by using a modified atomic force microscope

We have investigated the adsorption of 56 000 molecular weight poly(ethylene oxide) in an aqueous system (good solvent) to glass using a development of the atomic force microscope technique. A glass particle is glued to a silicon cantilever to give a particle probe surface forces apparatus. The design of this custom built machine is discussed with reference to the particular problems inherent to the investigation. The data presented describe the evolution of the adsorbed polymer layer with time and the changes resulting from only allowing one surface to adsorb polymer. We also examine the change of the layer conformation with repeated compressions. Scans are carried out at close to Brownian collision rates and energies. The results are discussed in the light of previous surface force apparatus work. The development of the layer is clearly tracked from an initially thin coverage up to a stable equilibrium layer of some 90 nm. The ''equilibrium'' thickness is greater than those reported on the surface force apparatus. This is due to the increased resolution of the current apparatus, which enables energies as small as 0.5 mu J m(-2) to be measured. At partial coverages of polymer on approach of the surfaces, a weak attraction is occasionally observed due to bridging of the polymer between the two surfaces. On separation a strong adhesion is noted. The lack of consistent strong attractions on approach of the surfaces is due to the relatively rapid rate of approach of the two surfaces, which does not allow sufficient time for the polymer to bridge between the surfaces and bring about an attraction. At full coverages of polymer, repulsive interactions at all surface separations are observed. However following many rapid approaches and separations at such coverages, attractive interactions mag be observed, indicating that the structure of the adsorbed layer is changing and being disrupted with time. The results the therefore demonstrate physically important interactions that would not be easily observed by any other force sensing technique.