Volume-phase transitions of ionized <i>N</i>-isopropylacrylamide gels

Equilibrium swelling curves of N-isopropylacrylamide (NIPA) gel and a series of its ionized counterparts were measured as a function of temperature. Nonionic NIPA gels underwent a sharp yet continuous volume change, whereas incorporation of a small amount of ionizable groups into the gel network drives the transition toward a discontinuous one. The critical ionic concentration and polymer network density at which the get undergoes a critical phase transition were determined. The results were analyzed using a mean field theory. Discrepancy between experimental data and theory is nontrivial, and may require the formulation of a more elaborate theory.