Wettability of End-Grafted Polymer Brush by Chemically Identical Polymer Films

Previous experiments on chemically homogeneous polymer melt-brush systems have found large deviations from theory. By using X-ray reflectivity and atomic force microscopy, we find that the equilibrium structure of these systems is complex, composed of dewetted droplets of the constituent polymers residing on a residual film of the end-grafted brush swollen by the free melt chains. When the molecular weight of the melt or the grafting density of the brush is decreased, the thickness of the residual film increases and the contact angle decreases, both indicating improved wettability. At the same time, the profile of the dewetted droplets exhibits a progressively longer and gentler tail at the ends. But a wetting-dewetting transition is never observed. By comparing the measurements to recent results of the self-consistent field theory, good quantitative agreement is found. On the other hand, Leibler's analytical theory-which has been used almost exclusively for comparison with experiment-provides a poor description.