Microinterferometric study of the structure, interfacial potential, and viscoelastic properties of polyelectrolyte multilayer films on a planar substrate

Thin films of polyelectrolyte multilayers composed of alternating layers of poly(styrene sulfonate) and poly(allylamine hydrochloride) were characterized in terms of their structure, surface potential, and viscosity, by monitoring the Brownian height fluctuations of colloidal beads placed on the films using dual wavelength-reflection interference contrast microscopy (DW-RICM). Special attention was directed toward characterizing the surface properties of the probe beads. The effect of coating the beads with differently charged polymers and of having differently charged polymers as the topmost layer of the film was investigated. It was found that, unless the coated beads and the topmost layer were similarly charged, the beads were present at a lower height than expected and did not exhibit any Brownian fluctuations. In the case of well-fluctuating beads, the film thickness obtained by DW-RICM agreed well with that obtained by optical waveguide lightmode spectroscopy. The height fluctuations of the beads were analyzed to yield information about the bead/film interaction potential and the viscosity of the films. Both the viscosity and the stiffness of the potential decrease as the film thickness increases. We also report an unexpected power law dependence of the stiffness of the bead/substrate interaction potential on the height of the beads.