Wetting of polymer liquids: Monte Carlo simulations and self-consistent fieldcalculations

Abstract

Using Monte Carlo simulations and self-consistent field (SCF) theory we study thesurface and interface properties of a coarse grained off-lattice model. In thesimulations we employ the grand canonical ensemble together with a reweightingscheme in order to measure surface and interface free energies and discuss variousmethods for accurately locating the wetting transition. In the SCF theory, weuse a partial enumeration scheme to incorporate single-chain propertieson all length scales and use a weighted density functional for the excessfree energy. The results of various forms of the density functional arecompared quantitatively to the simulation results. For the theory to beaccurate, it is important to decompose the free energy functional into arepulsive and an attractive part, with different approximations for the twoparts.Measuring the effective interface potential for our coarse grained model we exploreroutes for controlling the equilibrium wetting properties. (i) Coating of thesubstrate by an oxide layer gives rise to a subtle interplay between short-rangeand long-range forces, which may stabilize a film of mesoscopic thickness or resultin the formation of nano-droplets. (ii) Coating the substrate with a polymerbrush, we observe second-order wetting transitions at intermediate graftingdensities, while the wetting transition is of first order at low and high graftingdensities. In the latter limit, polymers of the same chemical structure asthe brush do not wet the surface (autophobicity). (iii) Surface pattern(stripes) might give rise to unusual adsorption properties, which are relatedto morphological transitions. We relate our findings to experiments anddiscuss perspectives and limitations of the computational methods.