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 and 0.01 s, 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 and at the same time leads to a failure of hyperelastic models. The samples can be stretched up to Hencky strains between 0.6 and 2.5, depending on the molecular structure, yielding linear Young's moduli E 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 can be correlated with and as well as with the solid molar mass between the cross-linking points , derived from .