Regulated the Swelling Properties of Poly(acrylamide‐co‐acrylic acid) Hydrogel by Changing the Charge Density

Abstract Hydrogels can swell without dissolving in water and are widely used in water treatment and biomedicine. The swelling rate of hydrogels is closely related to the network structure and charge density of hydrogels, but there are few detailed studies in this area. In this work, poly(acrylamide‐ co ‐acrylic acid) anionic hydrogel with acrylic acid (AA) as anionic monomer is prepared. By changing the monomer ratio, or changing the pH value of hydrogels, the internal charge density can be adjusted, and then the swelling rate can be controlled. With the increase of AA monomer content, the content of carboxylate in hydrogel increases, resulting in higher charge density. A high charge density corresponds to a high swelling rate. On the other hand, the swelling rate is closely related to the pH value, which is because the concentration of proton affects the ionization of carboxylic acid, which leads to the change of charge density. It is worth noting when pH increases to 10, the charge shielding effect of the counter‐ion (Na + ) gradually increases, thus limiting the swelling of the hydrogel and reducing the swelling rate. This work has reference value for the practical application of hydrogels.