Covalently cross-linked poly(acrylamide) brushes on gold with tunable mechanical properties via surface-initiated atom transfer radical polymerization

Versatile surface coatings, which exhibit anti-fouling properties and allow one to tune surface functionality and the state of internal (macro)molecular organization, are of central importance as platforms for the study of cell–surface interactions. To enable studies of various cell characteristics, such including cell migration, proliferation and adhesion, we investigated how the mechanical properties of poly(acrylamide) (PAAm) brushes on gold can be controlled by the varying the cross-link density. Brushes and covalently cross-linked hydrogel brushes were synthesized using self-assembled monolayers (SAMs) of ω-mercaptoundecyl bromoisobutyrate (MUBiB) on gold to initiate the surface-initiated atom transfer radical polymerization (SI-ATRP) of AAm with different fractions of (N,N′-methylenebis(acrylamide)) bisAAm in the feed. The polymer layers prepared with varied polymerization times or cross-linking densities were characterized by water contact angle measurements, grazing incidence reflection FTIR spectroscopy, ellipsometry and atomic force microscopy (AFM). It was found that during PAAm brush synthesis the film thickness increased markedly in early stages of the reaction, whereas after 60 min a plateau of approximately 50 nm (dry) film thickness was reached. The introduction of bisAAm into the feed resulted in reduced film thicknesses. AFM force–displacement measurements revealed that the variation of cross-linking density of polymer brush films influenced both the surface morphology and the films’ nanomechanical properties. The stiffness of the polymer brush films increased with increasing fraction of bisAAm, while the swelling ratio decreased. Hence the elastic modulus of the cross-linked PAAm brushes, as determined by AFM nanoindentation, could be varied between 20 and 700 kPa, which will find application in the study of cell–surface interactions.