Polymerization kinetics of polyacrylamide gels I. Effect of different cross‐linkers

Polymerization kinetics of polyacrylamide gels, cross‐linked with the following: N,N′‐methylenebisacrylamide (Bis), N,N′‐bisacrylylcystamine (BAC), N,N′‐ diallyltartardiamide (DATD), N,N′(1,2‐dihydroxyethylene) bisacrylamide (DHEBA) and ethylene diacrylate (EDA), have been studied. The gels are polymerized directly in a spectrophotometer quartz cuvette and the kinetics followed at 283 nm (disappearance of the double bond) or at 600 nm (Tyndall effect due to turbidity of Bis, DHEBA and BAC gels). The order of reactivity of the various cross‐links appears to be: Bis ≅ DHEBA > EDA ≅ BAC ≫ DATD. The last cross‐link, (DATD), was found to actually be an inhibitor of gel polymerization, leading to highly unpolymerized gels, especially at high %C values. Bis and DHEBA gels, at 3 to 5 %C levels, are fully polymerized within 30 min at room temperature, while EDA and BAC gels, in the same %C range, require at least 3h. All cross‐links, when used above 10 %C, display quite slow polymerization kinetics, requiring overnight reaction for good conversion of monomers into polymer chains. When polymerizing liquid linear polyacrylamide (without cross‐link), an initial concentration of > 5% should be used and the reaction continued overnight, at room temperature or higher, for acceptable polymerization efficiency. On the basis of these data, a new structural model for highly cross‐linked gels is proposed.