Fig. 4. (a) Evolution, with the aging time upon a tensile-unloading (to zero stretch) cycle, of the hysteresis loop re-formed from the cycle for a poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogel of maximum mechanical properties (tensile strength, elongation at break and toughness) having a mole ratio of the PVA- to Na alginate repeat units of 60/1 and Ca-dicarboxylate ionic cross-links of the minor dispersed alginate networks dramatically denser than the hydrogen bond cross-links of the major global PVA network. To conduct the experiments, five fresh parallel specimens of the gel first undergo a tensile-unloading cycle of 1.0 mm mm⁻¹ stretch at a crosshead speed of 50 mm min⁻¹ at room temperature (RT) (∼25 °C), then stand in a large amount of CaCl₂ aqueous solution of 1.0 mol L⁻¹ (i.e. age at the sacrifice of neither their water content nor cross-link morphology) at RT for increasing times of 0, 2, 10, 25 and 60 min, respectively, and are finally subjected to the tensile-unloading cycle again. Since the five fresh parallel specimens exhibit similar hysteresis loop profiles to each other, only the hysteresis loop of the fresh specimen to be subsequently aged for 0 min is shown as the initial one (i.e. control) for analysis; and, exceptionally, the hysteresis loop of the gel on 0 min of aging is realised by the tensile-unloading cycle run in situ immediately after the control, with the specimen kept gripped as it is and actually not immersed into the CaCl₂ solution for aging. (b) Percent recoveries of the Young's (i.e. tensile elastic) modulus, E, and dissipated energy, U_hys, towards those of the control as functions of the aging time for the PVA/alginate DN hydrogel of the maximum mechanical properties, where the E of all the loops (including the control) was read as the slope at the onset (i.e. non-zero stress) point of their tensile curve and the U_hys estimated as their area from (a).