In this paper we study the static properties and the dynamical swelling behavior of thin spin-coated layers of natural polysaccharides on Si/SiO2 substrates: the neutral and slightly branched dextran and the negatively charged, linear hyaluronic acid, by ellipsometric observation of the film swelling in a humiditycontrolled 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.

In this study phase transitions and thermodynamic properties of monolayers of short poly(ethylene glycol) chains (abbreviated as EG) covalently attached to lipids were determined by analyzing pressurearea isotherms at three different temperatures by using a film balance. The EG chain lengths were varied systematically between N ) 3 and N ) 15 repeating EG units. For the two longest EG chains (N ) 12 and N ) 15) a new synthesis is described. For short chains (N < 9) the monolayer phase transition is determined by the alkyl chain moiety of the headgroup, while for N g 9 the typical behavior of lipopolymers is observed. For the fluid-gel phase transition the entropy and the corresponding latent heat were determined for 3, 6, and 9 EG lipids. In the second part the lipids were transferred to hydrophilic silicon oxide substrates by the Langmuir-Blodgett technique and characterized by their equilibrium swelling behavior under controlled humidity by using ellipsometry. In agreement with the monolayer experiments, we find a "polymer brush"-like behavior already at chain lengths of N g 12 despite the fact that the "statistical" limit N . 1 is hardly fulfilled. For degrees of polymerization of N ) 3 and N ) 6 EG units, relative small swelling ratios F are observed due to a "rigid rod"-like behavior. Between N ) 6 and N ) 9 repeating units an intermediate swelling behavior is found.

We report on the design of an imaging microellipsometer enabling the generation of maps of the two ellipsometric angles Δ and Ψ. Areas of 60×200 μm2 are imaged at a rate of 1–2 images per minute. By working at angles (45°) much smaller than the Brewster angle (≈73° for Si/SiO2/air) a lateral resolution of 3 μm and a height resolution of 5 Å is achieved. The performance is demonstrated by thickness measurement of a laterally structured polymer film and a transient thickness measurement of dewetting fluid film of n-hexadecane on a Si/SiO2 wafer.

In this paper, we study wetting and dewetting of hydrated biopolymer layers mediating cell-cell and cell-tissue contacts, called the extracellular matrix and cell surface glycocalix, by the combination of various physical techniques. Here, the sum of the net effects of the various interfacial forces, which is referred to as the disjoining pressure, is used as a semi-quantitative measure to describe the thermodynamics of hydrated interlayers. The disjoining pressure can be measured by applying external forces to maintain the equilibrium distance between two parallel surfaces (in biology, two neighbouring plasma membranes). Using artificial models of the extracellular matrix and glycocalix, we describe stable cell-cell contacts in terms of the wetting (or spreading) of complex fluids on polymer surfaces. In fact, the adjustment of the wetting interaction via thin hydrating layers enables us to transform three-dimensional cell membranes into quasi-two-dimensional films on macroscopically large surfaces. Fine-tuning of local wetting conditions at the interface further allows for the selective wetting of native cell membranes on microstructured polysaccharide films, which has a large potential for individual detection of biological functions in confined geometries.

No abstract