Comparison of different approaches to describe the thermotropic volume phase transition of smart microgels

Abstract

The description of gel swelling by Flory and Rehner using the original Flory–Huggins interaction parameter for the polymer–solvent interaction cannot be applied to most smart microgels. Here, we compare descriptions of the swelling curves of such microgels using series expansions of the Flory–Huggins parameter chi with the results of Hill-like equation for chi. We study N-isopropyl-acrylamide particles at different concentrations of the cross-linker N,N-methylenebisacrylamide. The hydrodynamic radius R_{mathrm {H}} of the microgel particles is determined using photon correlation spectroscopy. The fits with the series expansion of chi nicely follow the experimental data. However, already with the first-order series expansion, the computed Theta temperatures are not physically reasonable. Moreover, the physical meaning of the parameters of the series expansion is not clear. The Hill-like equation, which we recently introduced, yields a good description of all measured microgel swelling curves and provides physically meaningful parameters. For instance, the Hill parameter nu corresponds to the number of water molecules per network chain cooperatively leaving the chain at the volume phase transition.Graphical abstractDifferent approaches to model the Flory-Huggins interaction parameter are explored and compared with respect to the quality of the fit of microgel swelling curves.