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

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-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.

Automated summary

The comparison of different methods for describing the swelling curves of smart microgels shows that the Hill-like equation provides physically meaningful parameters, unlike the series expansion of the Flory-Huggins parameter chi.