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

TLDR

The study measured equilibrium swelling curves of NIPA gels and ionized variants across temperature, identified the critical ionic concentration and polymer density for phase transition, and analyzed the results with a mean‑field theory. Nonionic NIPA gels exhibit a sharp continuous volume change, but adding ionizable groups makes the transition discontinuous, and the observed discrepancy between experiment and mean‑field theory suggests a more elaborate theory is needed.

Abstract

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.