Critical adsorption of a single macromolecule in polymer brushes

The adsorption of long flexible macromolecules by polymer brush-coated surfaces is studied by molecular dynamics simulations and by calculations using density functional and self-consistent field theories. The case of repulsive interactions between the substrate surface and the monomers of both the brush polymers and the extra chains that can get absorbed into the brush is considered. Under good solvent conditions, critical absorption can occur, if the interaction between the monomers of the brush polymers and the extra chain is (weakly) attractive. It is shown that it is possible to map out the details of the critical absorption transition, if the chain length and/or the grafting density of the brush polymers are varied. In this way both the strength and the range of the effective adsorption potential of the substrate surface can be controlled. However, it is found that in the general case there is no straightforward mapping of the present problem to the simpler problem of polymer adsorption in a square well potential, in contrast to suggestions in the literature. In particular, it is found that the fraction of monomers of the long free chain that is absorbed in the brush shows a nonmonotonic variation with the grafting density; i.e. from dense brushes free chains are again expelled from their interior. For strong attraction the free chain then gets adsorbed at the brush-solution interface.