Modulating the Swelling Behavior of Polymer Brushes via Interfacial Chemistry

Polymer brushes composed of densely grafted end-tethered polymer chains are often used in the solvent-swollen state, for example, as boundary lubricants or non-fouling surface coatings. For a given polymer brush in a specific solvent system, grafting density and polymer molecular weight are the principal structural parameters to tune swelling behavior. This report presents substrate surface chemistry as an additional parameter to control the swelling properties of polymer brushes generated via surface-initiated atom transfer radical polymerization (SI-ATRP). To uncover the impact of substrate surface chemistry, hydrophobic poly(tert-butyl methacrylate) (PtBMA), and hydrophilic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were grafted from substrates, which presented dipeptide and alkyl spacer tethered ATRP initiators and displayed water contact angles ranging from 48°-78°. Swelling of PtBMA brushes in a good solvent (THF) was found to be governed by polymer-solvent interactions, and substrate effects are negligible. In mixed solvents with a minority component that is a non-solvent for the polymer, in contrast, substrate surface chemistry does influence swelling, and was found to correlate with the polarity of the dipeptide ATRP initiator presenting substrate. Results of the analysis of the swelling behavior of hydrophilic brushes in water also point toward an impact of substrate surface chemistry.