Rheological monitoring of polyacrylamide gelation: Importance of cross-link density and temperature

Dynamic shear oscillation measurements at small strains are used to characterize the polymerization process in situ and the viscoelastic properties of cross-linked polyacrylamide hydrogels. Hydrogels are synthesized by free-radical redox polymerization of acrylamide (8 wt %) for different concentrations of cross-linker, N,N'-methylenebis(acrylamide) (BIS), at different temperatures. Both elastic modulus G' and viscous modulus G" are measured in real time during the gelation which takes place directly between parallel rheometer plates. The elastic modulus G' remains constant with frequency, G'(omega) approximate to cte, and is significantly larger than G"(omega), characteristic of a well-developed cross-linked polymer network. Temperature scanning of the elastic modulus shows that G'(T) is a linear relationship with a proportionality value that depends on the polymerization temperature T-pol. This observation is in agreement with the classical theory of rubberlike elasticity, i.e., G' = n(e)RT where n(e) is the active network links density. The results confirm that the final G' and G" are sensitive to the cross-linker concentration as well as the polymerization temperature. Moreover, G' follows a linear progression over a large range of BIS concentration. For a given acrylamide monomer concentration, there exists an optimal bis(acrylamide) cross-linker concentration and an optimal polymerization temperature which give rise to an "ideal" hydrogel, i.e., exhibiting a maximal elasticity.