Photopolymerization of polyacrylamide gels with methylene blue

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Abstract
Photopolymerization of polyacrylamide gels in the presence of methylene blue (100 μM) and a redox couple (1 mM sodium toluenesulfinate, a reducer, and 50 μM dipheriyliodonium chloride, an oxidizer) has been investigated. The gel point, i.e. the time needed for onset of gelation upon illumination, has been found to lengthen progressively at lower temperatures and at lower light intensities. If the three catalysts are progressively diluted, the gel point does not vary for a threefold dilution, but gelation is greatly hampered below a 1:5 dilution of the three effectors. Photobleaching has been assessed as a function of liquid layer thickness (from 0.5 to 2 mm), of a progressive dye dilution (down to a fourfold dilution) and as a function of temperature. A maximum of elastic modulus is located in correspondence to a minimum of permeability (both situated at 5% cross-linker). It is found that methylene blue-activated polymerization produces polyacrylamide gels with elastic properties which are higher than in persulfate-activated gels, so far the most popular matrices for electrokinetic separations. Due to the ease of preparation, the full control of all experimental parameters, and the lack of oxidizing power of this catalyst system (as opposed to the strong oxidation power of persulfate catalysis), methylene blue catalysis is advocated as a valid alternative to other redox systems.