Electrochemical measurements for studying the polymerization process and critical point behaviors of hydrogels

We propose a new technique based on electrochemical measurements for studying the critical point behaviors of the sol–gel transition of acrylamide–N,N′‐methylene bisacrylamide hydrogels. In this technique, no chemical activator is used for accelerating the polymerization reaction. However, a potential difference is applied by means of silver and calomel electrodes placed in the reaction mixture. The silver electrode begins to be ionized and loses its electrons. The free radicals, ^?O₃S? O^?, H^?, and ^?OH, form on the silver electrode via persulfate dissociation. The polymerization is initiated by means of these free radicals. The current measured during the gelation processes passes through a maximum (a Gaussian‐like behavior) and varies linearly with the reaction time during linear polymerization. All the parameters (the monomer, initiator, and crosslinker concentrations, the applied voltage, and the stirring rate of the reacting mixture) affecting the current have been studied in detail. We show that the maxima appearing in the current–time plots correspond to the gelation thresholds, the so‐called sol–gel transition points. We also analytically prove that the current monitors the weight‐average degree of polymerization (DP_w) and the gel fraction (G) below and above the threshold, respectively. The scaling behaviors of DP_w and G have been tested near the gelation thresholds, and we have observed that the critical exponents γ and β, defined for DP_w and G, agree with the predictions by mean‐field theory. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009