Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study

Abstract

Recent observations on phase transitions around the critical point showedthat the critical exponents differed drastically from percolation results and classicalresults for pure hydroge l systems, depending on monomer concentration. In additionto pure hydrog els, the sol-gel phas e transition during radical crosslinking copoly-merization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studiedby using the steady state ?uorescence (SSF) technique. N,N0-methylenebis(acrylamide)(BIS) and ammonium persulfate (APS) were used as crosslinker and initiator,respectively. Pyranine (trisodium 8-hydroxypyrene-1,3,6-trisulfonate acid, HPTS)was added as a ?uoroprobe for monitoring the polymerization. It was observedthat pyranine binds to AAm and NIPA ch ains on the initiation of the polymerization,thus the ?uorescence spectra of the bonded pyranines shift to shorter wa velengths.Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phasetransition without disturbing the system mechanically and to test the universality ofthe sol-gel transition as a function of polym er concentration ratios. The observationsaround the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent bobeyed the percolation result