  Polymer brushes, coatings consisting of densely grafted macromolecules,
experience an intrinsic lateral compressive pressure, originating from chain
elasticity and excluded volume interactions. This lateral pressure complicates
a proper definition of the interface and, thereby, the determination and
interpretation of the interfacial tension and its relation to the wetting
behavior of brushes. Here, we study the link between grafting-induced
compressive lateral pressure in polymer brushes, interfacial tension, and brush
wettability using coarse-grained molecular dynamics simulations. We focus on
grafting densities and polymer-liquid affinities such that the polymer and
liquid do not tend to mix. For these systems, a central result is that the
liquid contact angle is independent of the grafting density, which implies that
the grafting-induced lateral compressive pressure in the brush does not
influence its wettability. Although the definition of brush interfacial
tensions is complicated by the grafting-induced pressure, the difference in
interfacial tension between wet and dry brushes is perfectly well-defined. We
confirm explicitly from Young's law that this difference offers an accurate
description of the brush wettability. We then explore a method to isolate the
grafting-induced contribution to the lateral pressure, assuming the interfacial
tension to be independent of grafting density. This scenario indeed allows to
disentangle interfacial and grafting effects for a broad range of parameters,
except close to the mixing point. We separately discuss the latter case in the
light of autophobic dewetting.
