Thickness-dependent autophobic dewetting of thin polymer films on coated substrates

We demonstrate that the wetting behavior of a thin liquid film, poly(4-bromostyrene) (PBrS), on top of a solid substrate may be effectively controlled with the insertion of a secondary liquid film, poly(4-vinyl pyridine) (P4VP), underneath the primary film. This secondary film remains stable under all conditions, and can be viewed as an extension of the substrate itself. On the basis of results from X-ray standing waves generated via total external reflection from an X-ray mirror, time-of-flight secondary ion mass spectroscopy, optical microscopy, and atomic force microscopy, we construct the full Helmholtz free energy versus PBrS thickness curve using existing theories that account for both long- and short-range interactions. The form of the free energy curve, which contains an inflection point and an absolute minimum at a nonzero PBrS thickness, accurately reflects our observation that thick PBrS films undergo autophobic dewetting on top of the stable P4VP, while sufficiently thin PBrS films remain stable. The thickness of the autophobic wetting layer is controlled by the range of the repulsive interaction between the film and the substrate, and is found to be ∼4 nm for the PBrS/P4VP interface.