Combined effects of pH, salt concentration, and ionizable monomer fraction on the swelling behavior and hysteresis of weak polyelectrolyte brushes

Weak polyelectrolyte brushes (PEBs) are highly tunable pH-responsive coatings, with applications in separations and biological industries. Weak PEBs exhibit significant changes in swelling in response to changes in their charge state, which is regulated by ion exchange between the bulk solution and the brush layer and can be tuned by changing pH and salt concentration. It has been shown, however, that the swelling response of weak PEBs to changes in pH is hysteretic. Therefore, a more thorough understanding of the physical properties that give rise to this hysteretic effect is required to realize the full potential of weak PEBs as pH-responsive coatings. Here, we investigate the combined effects of ionizable monomer fraction (25% to 100%), pH cycling (pH 3 through 10) and salt concentration (1 mM to 1 M) on the swelling behavior and hysteresis in weak basic PEBs using in situ ellipsometry. PEBs consist of random copolymers of basic 2-(dimethylamino)ethyl acrylate (DMAEA) (pKa = 8.4) and neutral 2-hydroxyethyl acrylate (HEA), synthesized by SI-CuCRP. We observe that the swelling and extent of hysteresis in weak PEBs can be controlled by altering both ionizable monomer fraction and salt concentration. Hysteresis generally increases with ionizable monomer fraction, whereas the effects of salt are non-monotonous. The non-monotonic effect of salt concentration is consistent with the change from the osmotic to the salted brush regime as salt concentration is increased.