Swelling behavior of chemically cross-linked poly(N-IPAAm-allylglycine) hydrogels: Effects of NaCl and pH

The influence of NaCl and pH on the swelling behavior of a series of chemically cross-linked ionic poly(N-IPAAm-allylglycine) hydrogels in aqueous solutions was investigated at 298 K. The hydrogels were synthesized by free radical polymerization using methylenebisacrylamide as crosslinker. For all investigated hydrogels, the mass fraction of polymerizable substances was constant (0.1). The hydrogels differ in the amount of crosslinker (mole fraction: 0.02 and 0.05, respectively) and in the amount of the comonomer allylglycine (mole fraction: from 0.05 to 0.2). The experimental results for the degree of swelling in aqueous salt solutions of NaCl exhibit two transitions: the first (second) transition occurs at salt mass fractions between about 10−6 and 10−4 (≈10−2). The first transition reduces the degree of swelling by about 20%. That transition is caused by the screening of the charged amino and carboxylic groups of allylglycine by the salt ions. After that transition the gel still contains much water. In the second transition the gel collapses like non-ionic N-IPAAm – hydrogels. The swelling of the ionic hydrogels is also influenced by the pH of the aqueous solution. The degree of swelling reveals a minimum at 4 < pH < 8. In that region most allylglycine comonomers are present as zwitterions, i.e., the otherwise ionic hydrogel behaves like a non-ionic gel and the degree of swelling is low. A previously developed thermodynamic model was applied to describe the swelling behavior. The model is able to reproduce the experimental data quantitatively when some model parameters are adjusted to selected experimental results for the degree of swelling.