The ability of superhydrophobic surfaces to stay dry, self-clean and avoid biofouling is attractive for applications in biotechnology, medicine and heat transfer 1-10 . It requires that water droplets placed on superhydrophobic surfaces have large apparent contact angles (θ* > 150°) and low roll-off angles (θroll-off < 10°), realized with surfaces having low-surface-energy chemistry as well as micro-or nanoscale surface roughness that minimizes liquid-solid contact 11-17 . But rough surfaces where liquid contacts only a small

Atomistic molecular dynamics simulations of a chemically realistic model of atactic short-chain polystyrene between gold surfaces (111) and positron annihilation lifetime spectroscopy experiments on similar polystyrene thin films on gold were performed. Results from both approaches show that the free volume voids in the film have a slightly smaller average size than in bulk polystyrene. In agreement to that the existence of an interphase of higher density at the polymer−solid substrate interface is shown both by the simulation as well as in the experiment. The average shape of the voids is similar in the bulk and the film. ■ INTRODUCTIONPolymer−solid interfaces are at the center of intensive research due to their importance in polymer coatings, hybrid materials, lubrication, adhesion, etc. The polymer properties close to this interface are of paramount importance for the performance of these composite systems. From the viewpoint of fundamental science, the confinement imposed by a solid substrate on the polymer should affect the polymer's glass transition temperature, and this has led to a number of studies.Various properties of polymers confined in thin films, be it supported or free-standing, differ from their bulk properties. In the case of supported thin films, an interphase between the substrate and the bulk phase of the polymer is postulated, and the width of this interphase layer has been the focus of many studies. Confinement effects on polystyrene thin films and their implications for the polymer glassy dynamics were studied by molecular dynamics (MD) simulation. 1 For freestanding polymer films, the glass transition temperature in the boundary layer near the gas phase was found to be lower than in the bulk. 2 On a completely smooth, structureless, solid substrate modeled by a truncated 9−3 Lennard-Jones potential three layers with different density and glass transition behavior substrate, middle, surfacehave been shown in MD simulations. 3 MD simulations of polystyrene on gold, 4,5 polybutadiene on graphite, 6 and polyethylene on graphite 7 have shown the existence of a higher-density interface layer of a width of about 2 nm. The dynamical properties of polymers near an interface have also been probed using MD simulations. 4,6,8 Experimental work about polymers at the interface claim the existence of e.g. a "dead layer" or "Guiselin brushes" 9 of a few nanometers at the interface as permanently attached polymer chains where even washing with a good solvent cannot remove this layer again. Using gold nanoparticles as markers in a X-ray photon correlation (XPCS) experiment, 10 an irreversibly adsorbed layer with a surface reduced viscosity (compared to bulk viscosity) layer altering the dynamics up to a distance of approximately 20 nm from the substrate was postulated as interpretation of their results. With the recently developed technique resonance enhanced dynamic light scattering (REDLS), experiments on polybutadiene on gold 11 showed a slowing-down in dynamics, and hence an increase in viscosity goi...

No abstract

The dynamics of thin, liquid polybutadiene films on solid substrates at temperatures far above the glass transition temperature T(g) was studied by Resonance Enhanced Dynamic Light Scattering. The capillary wave dynamics is the stronger suppressed by the substrate the thinner the film. We find a molecular weight independent film-thickness below which the dynamics change dramatically--the viscosity increases by orders of magnitude. This change is not related to 3R(g) as postulated in theory and claimed in some experimental findings but rather to a fixed distance from the solid interface. Part of our observations we attribute to a, compared to bulk polymer, less mobile viscoelastic layer adjacent to the substrate, and part to a more mobile layer at the liquid-gas interface. Thus, the overall behavior of the dynamics can be explained by a "three layer" model, the third layer having bulk behavior in between the above two layers.