Surface Properties of Bottlebrush Polymer Thin Films

Bottlebrush polymer thin films may be attractive for the preparation of antifouling and/or stimuli-responsive surface coatings due to the high grafting density and conformational flexibility of polymeric side chains, but bottlebrush polymer thin films have not been previously reported and their surface properties are unknown. Herein, we report a study of the surface properties of mixed bottlebrush polymer (MBBPs) films. MBBPs with hydrophobic polystyrene (PS) and hydrophilic poly(ethylene glycol) (PEG) side chains are synthesized using a grafting-through ring-opening metathesis polymerization (ROMP) approach. Stimuli-responsive MBBPs films are prepared by spin-casting a solution of MBBPs onto a solid surface, and the resulting film morphology and surface properties are characterized using atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), water contact angle measurements, and X-ray photoelectron spectroscopy (XPS). The water contact angles of MBBPs films decrease or increase upon exposure of the MBBPs films to selective solvents methanol or cyclohexane, respectively. This contact angle change is dependent on the length of the PEG side chain; longer PEG side chains result in greater contact angle changes with solvent exposure. Consistent with water contact angle measurements, XPS indicates enrichment of PEG or PS chains at the film surface after exposure of the MBBPs film to methanol or cyclohexane solvent vapors, respectively. Finally, it is demonstrated that bottlebrush polymer films can be stabilized by the addition of a radical cross-linker and irradiation with UV light. This work demonstrates that bottlebrush polymers enable the preparation of stimuli-responsive, brush-like polymeric coatings using simple solution processing methods.