Reentrant phase transition of N-isopropylacrylamide gels in mixed solvents

Abstract

Reentrant volume–phase transitions are observed in N‐isopropyoacrylamide gels in the methanol–water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed‐loop instability phase boundary having both upper and lower critical points. The closed‐loop phase boundary depends on temperature and diminishes to a point at approximately 0 °C. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol–water interaction by presence of polymer network. In the case of ethanol–water mixtures, there appear two closed‐loop phase boundaries, whose physico‐chemical basis are not yet clear.