Wetting of a short chain liquid on a brush: First-order and critical wetting transitions

We investigate the wetting behaviour of short chains on a surface covered with a brush of end-grafted chains of the same architecture by a combination of self-consistent field calculations and liquid-state theory. The surface interacts with the monomers via (non-retarded) van der Waals interactions of strength A . At low grafting densities, we find first-order wetting transitions. The value of the effective Hamaker constant A wet > 0, at which the transition occurs, decreases and the strength of the first-order transition becomes weaker as we increase the grafting density. In an intermediate range of grafting densities, we encounter second-order wetting transitions at a vanishing Hamaker constant A wet = 0. The second-order transition is preceded by a first-order transition between a thin and a thick liquid layer ("frustrated" complete wet state) at negative values of A . This line of first-order transition terminates in a critical point. Upon increasing the grafting density further, we encounter a tricritical point, beyond which the wetting transition is again of first order and occurs at A wet > 0. At these high grafting densities, the brush expels the free chains (autophobicity).