Effect of ionic strength and salt identity on swelling behavior of weak polybasic brushes with various charge fractions

Charged polymer brushes offer enhanced properties compared to neutral brushes due to the intra- and interchain interactions between charged monomers. These interactions can be tuned through the charged monomer fraction and by modulating environmental conditions such as solution pH and ionic strength. Here, we examine the effect of salt concentration and identity on the conformational pH-response of weak polybasic brushes with various charged monomer fractions using in-situ ellipsometry and streaming zeta potential. Our model system consists of random copolymers of weakly charged 2-(dimethylamino)ethyl acrylate (DMAEA) and neutrally charged 2-hydroxyethyl acrylate (HEA). We synthesize polymer brushes with charge fractions ranging from f = 1 to 0 at constant grafting density using surface-initiated copper(0) controlled radical polymerization (SI-CuCRP). Brushes exhibit hysteretic memory behavior in pH-induced swelling and collapse transitions. The extent of hysteresis, however, depends on both charge fraction and the solution ionic strength. The brush isoelectric point varies with the monomer charge fraction, suggesting that the extent of electrostatic interactions on polyelectrolyte brushes affects their physical and chemical properties.