Superswelling of Hydrogels Based on the Copolymer of Acrylamide and Methacrylic Acid

Weakly crosslinked hydrogels of copolymers of acrylamide and methacrylic acid are synthesized by radical polymerization in aqueous solution in a wide range of comonomer ratios. It is found that gels containing 20–40 mol % methacrylic acid are characterized by superswelling that exceeds the degree of swelling of homopolymer hydrogels by tens of times. The observed degree of swelling reaches 830, corresponding to an almost completely stretched conformation of subchains between network junctions. For dried hydrogel samples, the equilibrium degree of swelling is also maximal in the mentioned composition range; however, its maximum value is lower than the equilibrium degree of swelling for the initial samples. The possible reason behind the superswelling of hydrogels containing 20–40% units of methacrylic acid is an increase in ionization of network subchains. This hypothesis is proved by an increased sensibility of hydrogels of this composition to a change in the pH of a medium and the lowest values of the effective acidity constant determined by potentiometric titration for methacrylic acid units. The values of partial heat capacity of a gel polymer network are determined by calorimetry. These values are significantly higher than the heat capacities typical of vinyl polymers. This is evidence that the hydrophobic type of hydration predominates in the copolymers of acrylamide and methacrylic acid. The partial heat capacity reaches its maximum within the copolymer composition range corresponding to superswelling.