Sol-gel transition in polyacrylamide gels by using a fluorescence technique

Pyranine (8-hydroxypyrene-1, 3,6-trisulfonic acid, trisodium salt) was used as a floroprobe for monitoring the solution free-radical cross-linking polymerization of acrylamide (AAm) to test the universality class of the sol-gel transition. The spectra of the pyranines shift to shorter wavelengths during the polymerization of AAm. The intensity of the shifted spectra monitors the sol-gel transition, without disturbing the system mechanically. Observations around the critical point show that there are three regimes of AAm concentration in which the gel fraction exponent and the weight-average degree of polymerization exponent differ drastically. They agree best with the static percolation results for higher AAm concentration, but cross over from percolation to mean-field values when AAm concentration is lower than 2 M.