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

Abstract

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.

Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study

“…Our recent observations around the critical point showed that the critical exponents did not obey the same universality class for neat AAm [13] and NIPA [14] hydrogels, while bulk gels [15][16][17][18] were modeled by percolation picture. In neat AAm hydrogels, it was shown that the critical exponents agree best with the percolation results for higher AAm concentrations above 1 M, but they crossover from percolation to classical (mean-field or Flory-Stockmayer) values when the AAm concentration goes from 2 M down to 1 M, and the present classical values below 1 M. Further down below 0.5 M, however, no gelation was observed.…”

“…In neat AAm hydrogels, it was shown that the critical exponents agree best with the percolation results for higher AAm concentrations above 1 M, but they crossover from percolation to classical (mean-field or Flory-Stockmayer) values when the AAm concentration goes from 2 M down to 1 M, and the present classical values below 1 M. Further down below 0.5 M, however, no gelation was observed. [13] The gel fraction (above the gel point) and average cluster size (below the gel point) exponents and the gel points (for 1 M PAAm hydrogel for various BIS contents) are summarized in Table 2.…”

Conductivity Percolation of Carbon Nanotubes in Polyacrylamide Gels

“…As one can see from Figure 6 the intensity of the short-wavelength peak (380 nm-peak) increases during the course of polymerization, meanwhile as the polymerization progresses the maxima of the short wavelength-peak shifts to some higher wavelengths from 380 nm to 427nm (Kaya et al, 2004). Here at first the shift from 512 nm to 380 nm in the emission spectra due to a C-O ether bond formation between the hydroxyl oxygen of 3sPyOH and a terminal C-atom of the growing AAm chain.…”

“…Above t gel , if the intensity from finite clusters distributed through the infinite network I ct is subtracted from the maximum fluorescence intensity, then, the corrected intensity I`-I ct measures solely the gel fraction G, the fraction of the monomers that belong to the macroscopic network. In summary, we have the following relations, (Kaya et al, 2004) '…”

“…Therefore, we monitored the fluorescence spectra in relatively large periods of time and plotted the intensity I em corresponding to the maxima of the spectra as a function of time. These data were used to evaluate the critical behavior of the sol-gel phase transition (Kaya et al, 2004). For the determination of the gel points, t gel , each experiment was repeated at the same experimental conditions, and the gel points were produced by dilatometric technique (Okay et al, 1999).…”