Fabrication of tough hydrogels composites from photo-responsive polymers to show double network effect.

Abstract

Inspired by the toughening mechanism of double-network (DN) gels, tough hydrogel composites with a sandwich structure were fabricated from photo-responsive polymers. By copolymerization of hydrophilic monomers, 2-ureidoethyl methacrylate (UM), and photo-responsive hydrophobic monomers, (2-nitrobenzyloxycarbonylaminoethyl methacrylate, NBOC) at high concentration, physical hydrogels that are soft and highly stretchable are formed due to the hydrophobic associations of NBOC, serving as dynamic crosslinkers. By UV irradiation, the physical crosslinking switches into chemical crosslinking, and the soft physical hydrogels transform to rigid and less stretchable chemical hydrogels. By UV curing the surface layers of the physical hydrogels, we prepared hydrogels composites having sandwiched structure with two rigid outer layers and a soft inner layer. The molecular-level continuous interfaces and matched swelling ratio between the layers ensure the macro-scale hydrogels composites high strength and toughness with a DN gel effect. The outer layers fracture preferentially at deformation, playing a role as like the 1st network of a DN gel, while the inner layer maintains the integrity, playing a role resemble to the 2nd network. The evolution of the fracture morphology of the rigid layers give useful insight on the internal fracture process of DN gels.