Swelling of n-isopropyl acrylamide hydrogels in water and aqueous solutions of ethanol and acetone

Hydrogels are three-dimensional networks of hydrophilic polymer chains with properties in between liquids and solids. One of the most interesting features is their capability to swell as well as to shrink depending on their surroundings. This paper deals with experimental investigations of the swelling behavior of nonionic synthetic hydrogels of n-isopropyl acrylamide. The swelling of some hydrogels differing in, e.g. the cross-linking density and the length of the polymer chains, in aqueous solutions of ethanol as well as of acetone was investigated at 298 K. The experimental results are presented and discussed also with respect to the phenomenon of "multiple phase transitions". Typically, the amount of absorbed material depends on the composition of the solvent showing a considerable decrease at around 20-40 mass% of the organic solvent component. In aqueous solutions of ethanol, some hydrogels reveal a phase transition from a swollen into a shrunken state at a certain ethanol concentration. "Multiple phase transitions" (i.e. several transitions at different ethanol concentrations) were also observed, but they proved to be caused by the kinetics of the swelling/shrinking process which can result in heterogeneous gel structures. The equilibrium properties are correlated with a thermodynamic model which combines an expression for the Gibbs energy of a liquid with an expression for the Helmholtz energy of an elastic network. The compositions of the coexisting phases (gel phase and surrounding liquid solution) were also determined. As long as the gel absorbs a large amount of solvent, there are only small differences between the solvent concentrations in the gel and in the surrounding liquid. When the gel is in a shrunken state, there are (small) differences in the composition which are reliably predicted. (C) 2003 Elsevier B.V. All rights reserved.