Freezing Molecular Orientation under Stretch for High Mechanical Strength but Anisotropic Hydrogels

The poor mechanical strength of hydrogels has largely limited their wide applications, and improving hydrogels' mechanical strength is a hot and important topic in the hydrogel research field. Although many successful strategies have been proposed to improve hydrogels' mechanical strength during the past decades, a hydrogel with a tensile stress surpassing dozens of mega Pascal is desirable, yet still a big challenge. To address this issue, the Fe(3+) -mediated physical crosslinking formed under stretch conditions was employed in a chemically crosslinked poly (acrylamide-co-acrylic acid) network to achieve a dual-crosslinked hydrogel. The expected molecular orientation occurs under stretch and allows the maximumu chelating interaction between pendant carboxylic anions and Fe(3+) and molecules conformation being frozen, leading to the mechanical strength improving dramatically. As a result, an unprecedentedly high mechanical strength, but anisotropic dual-crosslinked hydrogel was obtained. By optimizing the experimental parameters, the nominal tensile stress along pre-stretching direction can reach as high as ≈40 MPa with elastic modulus of ≈40 MPa at large strain (>200%). In addition, the molecular orientation also leads to big difference of mechanical performance between parallel and perpendicular direction.