A highly aligned X-shaped hydrogel fiber via cooperative roles of amorphous and crystalline network mediated by Hofmeister effect

Achieving high alignment of polymer chains is highly desirable for hydrogels as structural materials, but challengeable. Here, based on Hofmeister effect, a cooperative regulation strategy simultaneously using salting-in and salting-out salts (CSS) is proposed to fabricate well aligned X-shaped hydrogel fibers. First, the freezeresistant and salting-in salt depresses the growth of ice, inducing an amorphous network for freeze-thawed hydrogel. Combined with further densified polymer chains, extension ability of raw hydrogel reaches maximum extent and thus makes large prestretching strain available during stretching orientation. Second, a salting-out salt effectively fixes the aligned structures via triggering the approach of polymer chains, accompanied with enhanced crystallinity and H-bonds. Notably, the salting-out effect induces symmetrical diffusion of water, spontaneously generating a X-shaped cross-section with skin-core structure. These cooperative roles of amorphous and crystalline network mediated by Hofmeister effect also provide possibility for twisted hydrogel fibers with greatly improved load-bearing capacity.