Rheological Behavior of Tough PVP-<i>in Situ</i>-PAAm Hydrogels Physically Cross-Linked by Cooperative Hydrogen Bonding

Abstract

Rheology studies were performed on tough PVP-<i>in situ</i>-PAAm hydrogels physically cross-linked by cooperative hydrogen bonding to understand their viscoelastic response and, hence, the interactions and microstructure. The viscoelasticity of the PVP-<i>in situ</i>-PAAm hydrogels was strongly affected by the monomer ratio (<i>C</i>_AAm/<i>C</i>_VP). Hydrogels prepared with a high monomer ratio exhibited weak time, temperature and frequency dependence of the viscoelastic properties, similar to those of chemically cross-linked hydrogels. The storage modulus (<i>G</i>′) of the gels was much greater than the loss moduli (<i>G</i>″) and low loss factor (tan δ < ∼ 0.1), which indicated that they were solid-like, and mostly elastic. These supramolecular gels exhibited a strain- and <i>C</i>_AAm/<i>C</i>_VP-dependent reversible gel (solid) to viscoelastic liquid transition due to the dynamic nature of the cooperative hydrogen bonds. That transition also coincided with the onset of nonlinear viscoelastic behavior. The addition of a low molecular weight compound, urea, that competes for hydrogen bonding sites weakens the gel by decreasing the effective cross-link density or weakening the intermolecular hydrogen bonding