A series of hydrogels were synthesized by graft cross-link copolymerization of carboxy methylcellulose (CMC) and acrylic acid (AA) using N,N′-methylene-bis-(acrylamide) as a cross-linker. The swelling kinetics of the hydrogels in different buffer solutions were studied, and the kinetic curves in acidic medium exhibited a remarkable overshooting effect, viz. the gels firstly swell to a maximum value followed by a gradual deswelling until the equilibrium. The phenomenon was attributed to a cooperative physical cross-linking caused by the hydrogen bond formation between the carboxyl groups of the hydrogels. The effect of pH of buffer solution on overshooting effect is more remarkable than that of the composition of hydrogels. The former reason is that the total molar fraction of carboxyl groups does not change with the molar ratio of the units of carboxy methylcellulose (CMC) and acrylic acid (AA) in hydrogels, thus the degree of physical cross-linking is little or not changed by the composition of hydrogels in the case of the same molar fraction of cross-linkers; while the latter is that the pH change of buffer solution can cause the remarkable change in the extent of protonization of carboxyl groups, consequently resulting in a remarkable change of the degree of physical cross-linking. The swelling processes of the hydrogels under acidic medium follow a quantitative model proposed by E. Díez-Peña et al, and the theoretical curves are in very good agreement with the experimental data. While in pH≥5.0 buffer solutions, the overshoot phenomenon does not appear, and this is attributed to the physical cross-linking not being formed between the ionized carboxyl groups in the media. The swelling processes follow Schott second-order swelling kinetics.

Key words: hydrogel, overshooting effect, cooperative physical cross-linking, swelling kinetics