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(et hylene oxide) in an aqueous system (good solvent) to glass using a dev elopment 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. Th e 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 w ith repeated compressions. Scans are carried out at close to Brownian collision rates and energies. The results are discussed in the light o f previous surface force apparatus work. The development of the layer is clearly tracked from an initially thin coverage up to a stable equi librium 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 po lymer on approach of the surfaces, a weak attraction is occasionally o bserved due to bridging of the polymer between the two surfaces. On se paration a strong adhesion is noted. The lack of consistent strong att ractions on approach of the surfaces is due to the relatively rapid ra te of approach of the two surfaces, which does not allow sufficient ti me for the polymer to bridge between the surfaces and bring about an a ttraction. At full coverages of polymer, repulsive interactions at all surface separations are observed. However following many rapid approa ches and separations at such coverages, attractive interactions mag be observed, indicating that the structure of the adsorbed layer is chan ging and being disrupted with time. The results the therefore demonstr ate physically important interactions that would not be easily observe d by any other force sensing technique.