Disjoining Pressure and Swelling Dynamics of Thin Adsorbed Polymer Films under Controlled Hydration Conditions

In this paper we study the static properties and the dynamical swelling behavior of thin spin-coated layers of natural polysaccharides on Si/SiO 2 substrates: the neutral and slightly branched dextran and the negatively charged, linear hyaluronic acid, by ellipsometric observation of the film swelling in a humidity-controlled environmental chamber. Equilibrium swelling ratios and disjoining pressures are measured as a function of the relative humidity of the surrounding atmosphere, which was varied from 4% up to 98%, with the former corresponding to intermolecular separation distances smaller than 6 Å. With increasing water uptake the disjoining pressure curves exhibit a crossover from hard-core repulsion to a regime dominated by hydration forces, which were recently recognized to dominate the interaction of polysaccharides at close distances. On the basis of the static measurements, the kinetics of swelling of the adsorbed polymers were determined by a sudden application of an osmotic stress. The kinetics of film thickening after stepwise increase of the humidity is discussed in terms of a simple diffusion model (generalized Maxwell−Stefan approximation). From this analysis we obtained an effective mesh size in the dry polymer film and a power law relating the mesh size to the volume fraction in the expanding network. We further demonstrate the possibility of controlling swelling behavior of laterally microstructured polymer layers by relative humidity and of measuring it locally by a recently developed quantitative imaging microellipsometer.