Ultra-Tough and Super-Swelling Poly(vinyl alcohol)/Poly(AAm-co-AA Sodium Salts) Double Network Hydrogels

The development of hydrogels and their application in various fields, such as in medical dilators and scaffolds for cartridge regeneration, are restricted by their low modulus because of the trade-off between the swelling ratio and mechanical properties. In this study, a new three-step strategy was developed to increase the mechanical strength as well as the swelling ratio of hydrogels. In stage I, polyacrylamide (PAAm) was cross-linked in the presence of poly(vinyl alcohol) (PVA). Acrylamide was selected as it can be easily polymerized in air, and PVA was selected as it is soluble only in hot water. In stage II, a crystallized PVA/cross-linked PAAm dual network was obtained by an acid treatment followed by drying. In stage III, the cross-linked PAAm in the dual network was converted into a polyelectrolyte, poly(AAm-co-acrylic acid sodium salts), by hydrolysis to increase the swelling ratio of the hydrogel. In this manner, stage III afforded excellent mechanical properties owing to the phase-separated PVA and remarkable swelling because of the extended chain conformation of poly(AAm-co-acrylic acid sodium salts) that is connected to the PVA through co-crystallization or esterification with the poly(AAm-co-acrylic acid salts) chain. In addition, by simply stretching the hydrogel, anisotropic swelling can be achieved, which allows controlling the compressive modulus along the swelling direction.