Tough and stretchy double-network hydrogels based on in situ interpenetration of polyacrylamide and physically cross-linked polyurethane

Using super-tough thermoplastic polyurethane (HTPU) hydrogel and chemically cross-linked polyacrylamide (PAAm) as the first and second network, HTPU/PAAm double-network (DN) hydrogels are synthesized by a one-step radical polymerization in this research. The effects of the mass ratio of the two networks (HTPU and PAAm) and cross-linking density of the PAAm on the mechanical and morphological properties of the DN hydrogels are discussed. The as-prepared DN hydrogels can be stretched beyond 30 times their initial length, and their work of extension reaches a high value of 19.14 MJ/m3. With the introduction of super-tough HTPU as first network, the toughness of the DN hydrogel is increased by ~ 2600% and strength is increased by ~ 1200%, compared to that of the pure PAAm hydrogels. The stretched hydrogels also exhibit good recoverability at 100% extension strain without any hysteresis because of the good elasticity of the HTPU networks. The scanning electron microscopy observation shows that the DN hydrogels exhibit highly porous honeycomb-like structures, dependent on the composition ratio between the HTPU and PAAm. Strong hydrogel fibers with a modulus of 2.17 GPa and strength of 60 MPa can be prepared by drying super-tough DN hydrogels under stretching, holding a prospect in a wound dressing or other medical textile applications.