Wetting and dewetting behavior of polymers on surfaces

It is evident that wettability is important for various areas such as coating applications, adhesion, detergency, lubrication and other applications in which liquids are applied directly to solid surfaces. The phenomenon of dewetting also occurs frequently in our everyday life and for a variety of systems. For example, everybody has already observed how a water film drying on a windscreen or the retraction of paint from an oily (dirty) surface. On the basis of such experiments, studying (de)wetting can then lead to the determination of interfacial properties and their changes in real time and in situ.

Polymers are often used as model liquids in wetting experiments because they have a small vapor pressure and both the viscosity and the molecular weight can easily be controlled by the degree of polymerization. In a number of systems it was observed that the wettability of the surface is determined by the ability of the liquid to penetrate into a polymer network. Such is the case of a polymer melt which is put into contact with a surface covered by a densely grafted polymer brush.

When the melt and the brush are of identical chemical structure, e. g. polystyrene, entropic considerations limit the penetration of the melt chains into the brush, leading to dewetting. This phenomenon is known as autophobicity. As origins for autophobicity a few possibilities, probably occuring simultaneously, were proposed: packing or steric effects; orientation or dipolar effects.

The aim of the project is to investigate the dewetting behaviour as a function of the architecture of the surface bound layer as well as the molecular weight of both the attached and the free polymer.

Figure 1: Free molecules that penetrate the attached brush cause stretching of the brush molecules. If the loss in configurational entropy of the bound chains exceeds the gain in mixing entropy of the two layers dewetting takes place.

Using this approach we attached a variety of different polymers with very different properties, such as hydrophobic (polystyrene, polymethyl methacrylate) and hydrophilic polymers (e.g. polyethyloxazoline), very flexible (polydimethylsiloxane) and stiff polymers (poly-p-phenylenes) as well as polyolefine derivatives (polyethylene-co-norbornene). In addition, the approach can also be used to synthesize surface attached polymer networks by simultaneous crosslinking of photoreactive groups within the polymer (e.g. pendant anthracene units) and surface coupling via benzophenone monolayers [3].

References
1) S. Dietrich, C. Domb (ed.) and J. L. Leibowitz (ed.), Phase Transitions and Critical Phenomena, Vol. 12, Academic Press 1988, San Diego; Chapter 1: Wetting Phenomena.
2) P. G. de Gennes, Wetting: statics and dynamics, Rev. Mod. Phys. 1985, 3, 827-863.
3) G. H. Findenegg and S. Herminghaus, Wetting: statics and dynamics, Curr. Opinion Coll. Interface Sci. 1997, 2, 301-307.