Direct test of the critical exponents at the sol-gel transition.

The steady state fluorescence technique was used to study the sol-gel transition for the solution-free radical cross-linking polymerization of acrylamide (AAm), with N,N'-methylenebis (acrylamide) as cross linker in the presence of ammonium persulfate as an initiator. Pyranine (8-hydroxypyrene-1, 3,6-trisulfonic acid, trisodium salt) is used as a fluoroprobe for monitoring the polymerization. Pyranine molecules start to bind to acrylamide polymer chains upon the initiation of the polymerization, thus the spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allows one to monitor the sol-gel transition, without disturbing the system mechanically, and to test the universality of the sol-gel transition as a function of some kinetic parameters such as polymer concentration, cross-linker concentration, and temperature. Observations around the critical point show that there are three regimes for AAm concentration in which the exponents differ drastically. The gel fraction exponent beta and the weight average degree of polymerization exponent gamma agree best with the static percolation results for higher acrylamide concentrations above 1M, but they cross over from percolation to mean-field (Flory-Stockmayer) values when the AAm concentration is lower than 2M. For very low polymer concentrations, below which the system can not form the gel, the exponents differ considerably from both the percolation and the mean-field values.