Rheological properties of poly(acrylamide)-bentonite composite hydrogels

Dynamic and transient rheological properties were measured for a series of hydrogel composites whose microstructure has been reported previously. These hydrogels, composed of radiation-crosslinked polyacrylamide and bentonite clay particles acting as polymer-absorbing mechanical crosslink sites, were prepared in the range 50–95% water. Dynamic storage and loss moduli (G′, G′′) were obtained at several strain amplitudes, over a range of frequency (ω) from 10-2 to 102 rad/s. Step strains produced stress peaks and decays interpreted in terms of the stress relaxation modulus, carried to 104 s. Rheological complications with possible slip, yielding, and nonlinearities were avoided but are discussed in detail. Rubberlike rheology was exhibited in general, and G′ found to depend exponentially on solids content, with parameters only weakly dependent on ω. A practical measure of gel strength, defined in terms of the size of a water-containing cube that is mechanically stable, is used to demonstrate that these gels have considerable strength, with even a 1-m cube stable at 80% water.