Abstract
Fracture stress is the key parameter relating the laboratory evaluations of bulk gels to their plugging efficiency. By the in-situ gelling method, the complete deformation curves of polyacrylamide (PAM) gels are successfully obtained. We observe the strain hardening behavior from all the interested gels, which exhibits an exponential relation between the apparent modulus and the strain. Based on the exponential equation, the fracture stress is factorized into three rheological parameters, i.e., the plateau modulus, degree of stiffening, and fracture strain. In this article, we focus on the concentration dependence on these mechanical properties of PAM gels. It is observed that the plateau modulus of unentangled gels is dominated by the crosslinking density, and the entangled gels exhibit a scaling law with exponent of 2.2-2.3 between the plateau modulus and polymer concentration. In general, the degree of stiffening would be advanced by increasing the crosslinker concentration or decreasing the polymer concentration, while the fracture strain is reduced with both of them. In addition, these variations would be significantly exacerbated if the polymer concentration exceeds the entanglement concentration. As a result, the contribution of gellant concentration to the fracture stress of gels is weakened due to entanglement. Around the entanglement concentration, the fracture stress-polymer concentration curves of the organic gels are continuous but the increasing rates are decreased, while the inorganic gels experience a sudden reduction of fracture stress. Finally, we briefly discuss the fingering-rupture process of the bulk gels, and find the rupture pressure gradient is in linear relation with the fracture stress.
NSTL
Elsevier