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

The sol-gel transition for the solution-free radical cross-linking polymerization of acrylamide was studied using the steady-state fluorescence technique. Pyranine molecules start to bind to acrylamide polymer chains upon the initiation of the polymerization. Fluorescence spectra from the bonded pyranines allows one to monitor the sol-gel transition, without disturbing the system mechanically. The results indicate that for very low polymer concentrations, the exponents differ considerably from both the percolation and the mean-field values.