Charge State of Weak Polyelectrolyte Brushes Determines Swelling and Hysteretic Behavior

Polyelectrolyte brushes (PEBs) consist of charged polymers end-tethered to impermeable supports and are commonly used to modify the interfacial properties of multicomponent materials. Controlling the interfacial response requires understanding how structural and environmental parameters influence brush conformation, which is vital for applications requiring responsive behavior, such as separations, drug release, sensing, and actuation. These parameters include the polymer molecular weight distribution as well as the type and spacing of charges along the polymer. Despite their importance for myriad applications the swelling behavior of polyelectrolyte brushes in varying salt and pH is not fully understood. In this work, we show how the hysteretic pH response of weak PEBs is influenced by their charge state, which we adjust by varying the ionizable monomer fraction, solution pH, and ionic strength. In the strongly charged state at pH 3, the swelling behavior of weak PEBs qualitatively aligns with the theoretical predictions for strong PEBs, whereas under moderate to weakly charged states follows theoretical predictions for weak PEBs. However, the scaling exponents of the brush height with salt deviate from theoretical predictions in both the osmotic and salted regimes. Notably, the acid dissociation constant pKa of the brushes shifts towards more basic pH values as the salt concentration increases, differing significantly between the protonation and deprotonation directions, which contributes to the observed hysteretic behavior.