Fig. 1. Stress, σ vs. stretch, ε curves, obtained by uniaxial tensile testing at room temperature (RT) (∼25 °C) at a crosshead speed of 50 mm min⁻¹, of poly(vinyl alcohol) (PVA)/alginate sequential, hydrogen-bonded-ionic dual-physical double-network (DN) hydrogels having mole ratios of the PVA- to sodium alginate (SA) repeat units of 20/1, 30/1, 40/1, 50/1, 60/1, 70/1 and 80/1, compared with those of their corresponding PVA hydrogen-bonded and alginate ionic single-network (SN) hydrogels. Both the DN-gels PVA networks and the PVA SN gel are formed by hydrogen bonding of an ∼13–20 wt% of PVA aqueous solution upon its freeze–thaw (RT–−25 °C–RT) cycle of being frozen at −25 °C for 20 h and then thawed back at RT for at least 4 h, followed by its immersion into a large amount of Ca²⁺ aqueous solution of 1.0 mol L⁻¹ at RT for at least 8 h, thus showing similar compositions (i.e. water contents) and morphologies (i.e. crosslink structures and -densities) to each other. Both the DN-gels alginate networks and the alginate SN gel are constructed through ionic cross-linking of an ∼1–3 wt% of SA aqueous solution with the large amount of Ca²⁺ immersion solution of 1.0 mol L⁻¹ at RT for at least 8 h, hence displaying analogous compositions and morphologies to each other.