The swelling capacity (S) of poly(N‐vinylimidazole) (PVI) hydrogels in aqueous acid solutions was measured as a function of the pH, the nature of the acid, and the polymer network structure. In acid solutions, imidazole groups become protonated and the swelling capacity increases to reach a maximum corresponding to a degree of protonation equivalent to the Manning limit charge density. The variable controlling the degree of protonation, and therefore the swelling capacity, is the mass of dry polymer immersed in a given volume of solution, which was called the gel effective concentration. Divalent counteranions reduce significantly S with respect to the same ionic strength and degree of protonation, obtained with monovalent anions, while non‐complexant divalent cations have no specific influence on the PVI swelling capacity. Results were discussed in terms of the effective charge density caused by counterion condensation on protonated PVI, the non‐covalent crosslink density caused by ionic interactions with divalent anions, and the non‐Gaussian character of chains between crosslinks.
Swelling capacity of two PVI samples in aqueous solutions of HCl with two different gel effective concentrations: 10−2 M and 2.7 × 10−2 M .
