Abstract
The influence of the entanglement on the mechanical properties of partially hydrolyzed polyacrylamide gels is investigated. In the entangled gels, a power law, relating the plateau modulus and the polymer concentration, with an exponent of 2.3 is dominant. By contrast, the exponent of the power law is determined by the crosslinking density in the unentangled gels. On basis of the in-situ gelling method, the strain hardening characteristic of partially hydrolyzed polyacrylamide gels was observed. The strain hardening behavior is found to be controllable by the component of gels, but is insensitive to the frequency. In general, gels with higher polymer concentration exhibit a weaker tendency of stiffening and tend to fracture at lower strain. However, concentration dependence of the non-linear rheology and fracture properties is discontinuous around the entanglement concentration. The underlying mechanism is assumed to be relevant with the conversion of the crosslinking type, which is promoted from inter-crosslinking to intra-crosslinking due to entanglements. As a result, the fracture stress experiences a significant reduction as the polymer concentration exceeds the entanglements concentration. This article points out the significance of entanglements on the fracture toughness of polymer gels, and we hope it would provide suggestions for further studies.
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