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

Abstract

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.