Effect of cross-linking methods on stress relaxation of PVA/PAM-co-PAA-based hydrogels

Both physical and chemical cross-linking methods can improve the mechanical properties of hydrogel by increasing the cross-link density. In this study, a series of polyvinyl alcohol/polyacrylamide-co-polyacrylic acid (PVA/PAM-co-PAA)-based hydrogels were prepared by using Fe3+ as physical cross-linking agent and N,N'-methylenebisacrylamide (MBA) as chemical cross-linking agent. The stress relaxation experiments of PVA/PAM-co-PAA-based hydrogels and nonlinear fitting analysis were performed to study the effects of cross-linking methods on stress relaxation behavior of hydrogels. Furthermore, the four-element generalized Maxwell model succeeded in simulating stress relaxation behavior of hydrogels. When Fe3+ was used as the cross-linking agent of Fe3+/PVA/PAM-co-PAA hydrogel, it took a long time of 199.9 s when the structural units began to move. However, when MBA was used as the cross-linking agent of PVA/PAM-MBA-PAA hydrogel, the time was shortened significantly. Compared with PVA/PAM-co-PAA hydrogel, the initial and equilibrium stress of Fe3+/PVA/PAM-co-PAA and PVA/PAM-MBA-PAA hydrogel significantly increased. When Fe3+ and MBA were used as the cross-linking agent of Fe3+/PVA/PAM-MBA-PAA hydrogel together, the network structure became looser. The initial and equilibrium stress were 0.04 and 0.015 MPa, respectively.