Elongational rheology of NIPAM-based hydrogels

Hydrogels of different composition based on the copolymerization of N-isopropyl acrylamide and surfmers of different chemical structure were tested in elongation using Hencky/real definitions for stress, strain, and strain rate, offering a more scientific insight into the effect of deformation on the properties. In a range between ε̇=10 and 0.01 s^-1, the material properties are independent of strain rate and show a very clear strain hardening with a "brittle" sudden fracture. The addition of surfmer increases the strain at break ε_H^max and at the same time leads to a failure of hyperelastic models. The samples can be stretched up to Hencky strains ε_H^max between 0.6 and 2.5, depending on the molecular structure, yielding linear Young's moduli E₀ between 2,700 and 39,000 Pa. The strain-rate independence indicates an ideal rubberlike behavior and fracture in a brittle-like fashion. The resulting stress at break σ_H^max can be correlated with ε_H^max and E₀ as well as with the solid molar mass between the cross-linking points M_c^solids, derived from E₀.