Effect of varying preparing-concentration on the equilibrium swelling of polyacrylamide gels

In order to develop the realistic model of the network structure of chemically crosslinked gels, polyacrylamide gels were prepared by radical copolymerization varying systematically and widely the concentration of pre-gel solution. The equilibrium swelling-ratio as a function of the concentration of pre-gel solution was determined. Two different kinds of behavior were observed. The swelling ratio of the gel prepared in dilute pre-gel solution diverges as the preparing concentration decreases toward the gelation point. This suggests that such a gel has the nature of fractals in its network structure and the sizes of mesh are inhomogeneous. For the gel prepared in concentrated pre-gel solution, the swelling ratio decreases as the preparing concentration increases, contrary to the prediction of the blub model. It suggests that such a gel contains a topological entanglement among the subchains of the network; the entangled points play an important role rather than the chemically crosslinked points in the swelling behavior. A new experimental formula that represents both kinds of behavior was presented. A fitting analysis was performed with it. The parameters obtained by the fitting were able to scale all the experimental data and make them collapsed into the master curve of the swelling behavior. The two regions of the different behavior were presented on the regional map of the preparing condition. The network structures in the different regions were discussed. (C) 2000 Elsevier Science B.V. All rights reserved.