Wetting transition in a polymer brush: Polymer droplet coexisting with twofilm thicknesses

We have systematic experimental observations that just below the wetting tr ansition of a polystyrene (PS) melt onto a substrate covered by a poly-4-vi nylpyridine-PS (PVP/PS) di-block copolymer there can be a macroscopic dropl et and two coexisting layer thicknesses of PS, In addition to the usual mac roscopic droplet and a microscopically thin film at the substrate (typical for partial wetting), a second mesoscopically thick film is selected. This film is a polymer brush of order 10 nm thick swollen with free PS chains. T he presence of this extra mesoscopically thick film proves that the wetting behavior near the wetting transition is richer than predicted by the class ical wetting theory. The disjoining potential profile must be a function wi th two local minima near the surface instead of just one. We have seen the selection of this mesoscopic film thickness in the dewetting behavior of an unstable spin-coated film of PS on the brushed interface. The self-consist ent field theory applied to this problem shows that a mesoscopically thick film may be trapped in a brush: there is a prewetting transition in combina tion with partial wetting.