Rheological properties of tough hydrogels based on an associating polymer with permanent and transient crosslinks: Effects of crosslinking density

We studied the rheological and mechanical properties of a series of poly(vinyl alcohol) dual crosslink hydrogels, permanently crosslinked by glutaraldehyde and transiently crosslinked by borate ions, at various crosslinking densities. The contribution of the chemical and physical crosslinks to the dynamic moduli was first determined with linear oscillatory tests, and the additivity of the two contributions was validated. The single relaxation time of the system was interpreted as the physical bond breaking time which can be slowed down by reassociation of the open physical bonds before full relaxation. By comparing the Rouse relaxation time of the strand between two adjacent crosslinks with the healing time of the physical bonds, an increase in the relaxation time with decreasing density of physical crosslinks was explained. In all cases, the extensibility and stress at break of the gels were found to increase with the addition of physical crosslinks, but the extensibility increased the most for the smallest amount of chemical and physical crosslink densities while the stress at break increased the most for the most physically crosslinked system with a low chemical crosslink density. This strongly suggests that the toughening mechanism due to the presence of transient bonds can only be active on a network with a low density of chemical crosslinks. In large strain experiments, the tensile stress could be separated into a strain-dependent and a time-dependent term for all the crosslinking densities studied, and master curves of the reduced stress as a function of time were obtained. This showed that the chemical and physical crosslinks independently and additively contribute to the viscoelasticity of the dual crosslink gel.