Rheological study of the sol-gel transition of hybrid gels

As the temperature decreases, thermally sensitive spherical poly(N-isopropylacrylamide) (PNIPAM) microgels with a lower critical solution temperature (LCST similar to 32 degreesC) swell in a dispersion, leading to a possible volume-concentration induced sol-gel transition if the microgel concentration is sufficiently high. In such a formed gel, polymer chains inside each microgel were chemically cross-linked, but individual microgels were physically close-packed into a three-dimensional network. Such hybrid gels can be used as model systems for studying the sol-gel transition, which avoids several problems, such as chain entanglements, phase separation, and vitrification, normally occurring in a gelling process. The sol-gel transition of such hybrid gels was studied by the change of viscoelastic modulus G' and G. As expected, the sol-gel transition depends on the polymer concentration, frequency, and shear stress. The gelation point could be roughly estimated by the method of Winter and Chambon. Our results showed that the temperature dependence of G and tan delta (=G/G') had a minimum, which corresponded to the sol-gel transition, providing a better way to determine the sol-gel transition temperature.