Rheological Monitoring of Polyacrylamide Gelation: Importance of Cross-Link Density and Temperature

Abstract

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 ‘ ( ω ) ≈ cte, and is significantly larger than G ‘ ‘ ( ω ), 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.