Wetting of mesoscopic soft cylinders: structure and layering transitions

The wetting of soft mesoscopic long-chain particles is studied. As a model system, a cylindrical brush with poly(vinyl)pyridine side chains on the water surface is characterized by isotherms and x-ray reflectivity. The forces from the two planar interfaces and the intra- and interparticle interactions are all of comparable magnitude. Two layering transitions occur, one from the monolayer to the double layer, the next to a homogeneous multilayer. The hard wall from which layering starts is the smooth polymer/air interface. Indeed, they particles in the top layer of both the double- and the multilayer have their cylinder axis parallel to the surface and are laterally compressed. In contrast, the polymer/water interface is diffuse due to brush swelling. Generally, the long-chain particles adjacent to the respective interfaces do not maintain their circular diameters. The thickness of the monolayer can be varied by a factor 3.5, up to 53 A. An additional phase transition occurs within the monolayer, which is attributed to a change of the side chains from a flattened to a compressed state at constant volume. Atmoic force microscope images of the monolayer transferred onto a solid indicate local cylinder alignment.