Analysis of the swelling dynamics of cross-linked P(N-iPAAm-co-MAA) copolymers and their homopolymers under acidic medium.: A kinetics interpretation of the overshooting effect

Dynamic swelling curves of copolymer hydrogels based on N-isopropylacrylamide (N-iPAAm) and methacrylic acid (MAA) under acidic medium exhibit a remarkable overshoot. This effect may be interpreted as the consequence of a swelling-deswelling process. The fundamental origin of this process is attributed to a reorganization of their own gel structures due to the hydrogen bond formation among the amide groups of N-iPAAm and the carboxyl groups of MAA in a hydrophobic environment. This fact, which may be considered as a cooperative physical cross-linking, leads to water expelling during the dynamic swelling measurements. A quantitative model of the gel swelling kinetics, coupled with the release of water from the gel, has been developed, which is in very good agreement with the experimental data. Some other overshooting phenomena from the literature may be also explained on the basis of a cooperative physical cross-linking process due to intermolecular forces among functional group moieties, promoted by the solvent. The influence of the previous soaking pH on the overshooting has been proved by using samples with controlled swelling history.