Controlling the collapse/swelling transition in charged hydrogels

In this paper, we investigate systematically the parameters controlling the swelling transition of charged hydrogels. Combining theoretical analysis and experimental investigations we characterize the transition pH and the width of the transition as a function of the hydrogel crosslink density (i.e. subchain molecular weight) and the solution salt concentration. We also investigate the effect of pendant polyethylene glycol (PEG) side chains on the swelling transition. Quite surprisingly, we find that (1) the degree of swelling varies non-monotonically with the solution salt concentration, first increasing and then decreasing with salinity, (2) even in the limit of high salt concentrations, where electrostatics are expected to play a minor role, we find that the degree of swelling is quite sensitive to the suspension pH, namely, to the gel degree of charging, (3) in the limit of long subchains the transition pH depends only on the chemistry of the gel charges, and the transition is quite sharp, (4) the transition pH shifts with decreasing subchain length and/or with decreasing salt concentration, and the width of the transition increases and (5) the overall degree of swelling increases significantly upon the incorporation of pendent PEG chains, while the transition pH remains unaffected and the width of the transition narrows significantly.