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, ^-O ₃S-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 volt- age, 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.