Influence of the initiator system on the spatial inhomogeneity in acrylamide-based hydrogels

The effect of the initiator system used in the gel preparation on the spatial inhomogeneity in poly(acrylamide) (PAAm) and poly(N,N- dimethylacrylamide) (PDMA) hydrogels was investigated by static light scattering and elasticity measurements. The hydrogels were prepared by free-radical crosslinking copolymerization of the monomers acrylamide (AAm) or N,N-dimethylacrylamide (DMA) with N,N′-methylenebisacrylamide as a crosslinker. Two different redox-initiator systems, ammonium persulfate (APS)-N,N,N′,N′-tetramethylethylenediamine (TEMED) and APS-sodium metabisulfite (SPS), were used to initiate the gelation reactions. Compared to the APS-TEMED redox pair, no significant scattered light intensity rise was observed during the crosslinking polymerization reactions initiated by the APS-SPS system. It was found that both PAAm and PDMA gels are much more homogeneous when the APS-SPS redox pair was used as the initiator. The results are explained by the formation of shorter primary chains as well as the delay of the gel point in APS-SPS initiated gel formation reactions.