Effects of the Ionizable Monomer Fraction on the Swelling Behavior of Weak Polyelectrolyte Brushes

Through a combination of in situ ellipsometry and streaming zeta potential measurements, we investigated the swelling behavior of weak polybasic brushes as a function of pH and the fraction of ionizable monomers f. The fraction of the ionizable monomer (2-(dimethylamino)-ethyl acrylate, DMAEA) was precisely tuned by mixing a neutral hydrophilic monomer (2-hydroxyethyl acrylate, HEA) from f = 1.00 to f = 0.00. All charge containing fractions show pH-responsive behavior. At acidic pH, the brushes are within the strongly charged osmotic brush regime, and the swelling ratio increases with increasing f, qualitatively agreeing with the scaling laws for polyelectrolyte brushes (PEBs). Moreover, as the fraction of DMAEA is increased, the transition pH (pH*) shifts to more basic values, indicating that ionization becomes more favorable with increasing f. In the intermediate pH range, all charge-containing fractions show hysteretic behavior, where the pH* values in the backward direction shift to more acidic values, and the extent of hysteresis increases with f. Streaming zeta potential measurements are in good agreement with the observed brush swelling behavior at the minimum and maximum pH ranges for all fractions. However, the isoelectric point (pI) values are consistently larger than the pH* values with no appreciable hysteresis present in the zeta potential measurement cycles. We propose the hydrophobic periphery mechanism as the source of hysteresis in the swelling experiments and hypothesize that the origin of the disparity between pI and pH* is the probing length scale differences between the two methods. These findings elucidate the effects of charge on the swelling behavior and charge state of PEBs, which can lead to tailored pH-responsive surfaces for many applications.