On the kinetics of monomer incorporation into polyacrylamide gels, as investigated by capillary zone electrophoresis

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Abstract
The kinetics of monomer incorporation into a polyacrylamide gel have been studied in a photopolymerization system comprising 100 μM methylene blue in presence of a red-ox system, 1 mM sodium toluenesulfinate (reducer) and 50 μM diphenyliodonium chloride (oxidizer). A precise assessment of gel point (pc) was obtained in a droplet chamber, in which argon was gently bubbled with a fused silica capillary into the reaction mixture. At pc, 50% (±3) acrylamide was incorporated into the matrix, vs. 80% (±4) N,N′-methylenebisacrylamide. This incorporation level remained the same when polymerized in the 2–36°C temperature range. Incorporation continued almost linearly for acrylamide up to 80% conversion. The reaction was continued up to 55 min (at 2°C), at which point bisacrylamide had been essentially consumed (>99.5% incorporation) and acrylamide had reacted (95 %). At 2°C, after gelation, the gel became progressively turbid (the Tyndall effect plateauing at 50 min), but it remained fully transparent if, at the gel point, reaction was continued at 50°C. The consumption of the pendant double bonds of Bis followed the progression of turbidity. It is concluded that, by gelation at 2°C, the nascent chains form clusters held together by hydrogen bonds (melting point at 28°C); such clusters are subsequently “Frozen” in the three-dimensional space as the pendant double bonds in the chains react progressively. Such turbid matrices are more porous and less elastic than when the gel is polymerized at 50°C. This process is similar to the “lateral aggregation” occurring when gels are formed in presence of a polymer in solution (e.g. 10 KDa polyethylene glycol; Righetti et al., Electrophoresis 1992, 13, 587–594).