Influence of polymerization initiation rate on permeability of aqueous polyacrylamide gels

It was shown by Weiss and Silberberg in 1975 that polyacrylamide gels prepared by copolymerization of acrylamide and N,N′‐methylene bis‐acrylamide in water are heterogeneous. A two‐phase model developed in 1979 satisfactorily accounted for the permeability of the gels. These gels had been prepared under standard conditions of initiator and activator concentrations and involved precooling of the reaction mixture to 0°C with completion of the reaction at room temperature, a thermal history not very suitable for kinetic analysis. A procedure was, therefore, devised where gels are prepared at room temperature throughout. Much better correlations are then obtained, and it can be shown that the initiation is controlled by the initiator (ammonium persulfate) concentration times the activator (N,N,N′,N′‐tetramethylethylene diamine) concentration raised to the 1.5 power. These data, discussed in terms of the two‐phase model, show that the initiator concentration directly determines the number of crosslinks in the more dilute, permeability‐determining, gel phase. The faster the initiation, the less permeable a gel results. An increase of permeability with overall degree of crosslinking at constant monomer concentration is a feature only of gels prepared at sufficiently low polymerization initiation rates.