Bending or buckling: Compression-induced phase transition in a semi-flexible polymer brush

Molecular-dynamics simulations are presented for systems of densely grafted semiflexible macromolecules grafted to a planar non-adsorbing substrate, studying the case where the persistence length of the polymers is of the same order as their contour length so that the polymer brush may exhibit nematic order. We focus our attention on the case where the first bond must orient perpendicularly to the substrate (so the structure resembles a "Fakir's bed" for short chains and a "polymer bristle" for longer chains). When such layers are exposed to uniform compression, the pressure vs. distance relationship exhibits two stages: i) for very small compression the chains exhibit "buckling" yet maintain their average orientation perpendicular to the surface. In this stage the pressure rises rapidly, and the components of the last bond vectors in the plane parallel to the compressing piston remain randomly oriented. ii) For larger compression, the pressure decreases after a slight overshot, and then stays constant before the pressure starts to slowly rise again. In this stage, the bond vector components (of the bonds adjacent to the compressing piston) exhibit a symmetry breaking, XY-model-like orientational order develops, which then determines the orientation characterizing the collective bending of the whole chains. Surprisingly, the resilient response of stiff polymer brushes to pressure turns out to be much weaker than that of ordinary brushes made of totally flexible polymer chains. Copyright (C) EPLA, 2013