Comparing simulated and synthesized polymer brush profiles

Herein, the conformation of planar polymer brushes of varying chain length and grafting density is investigated, comparing experiments with molecular dynamics (MD) simulations. The grafting densities investigated cover a wide range: from the mushroom regime to a dense brush. Experimentally, poly(ethylene glycol)methylethermethacrylate (MN = 300) based polymer brushes were synthesized by atom transfer radical polymerization. The conformation was characterized by neutron reflectometry. At room temperature, below the lower critical solution temperature, the extracted polymer density profiles reveal a stretched conformation that becomes more pronounced with increasing grafting density. The presented MD simulations employ a new approach to "synthesize" polymer chains from the grafting surface. Chains can only grow when a free monomer approaches an active chain-end, analogous to the experimental synthesis. The resulting polymer brushes exhibit a more stretched polymer conformation, differing significantly from the usual monodisperse case. The thereby induced intrinsic polydispersity increases with increasing grafting density due to chain crowding during growth. For the first time, experimentally acquired polymer volume fraction profiles, in a good solvent, are matched to those of MD-simulated brushes by comparing grafting density lengths. Normalized profiles show very good agreement between experiments and simulations, even reproducing the extent of stretching.