H-bond/ionic coordination switching for fabrication of highly oriented cellulose hydrogels

Abstract

Various strategies for preparing highly oriented architectures in hydrogels remain challenging as the regulation of macromolecular chains is often restricted by electrostatic, hydrogen bond and other interactions, and a single strategy is not capable of breaking the equilibrium limits. In this study, a highly aligned cellulose hydrogel was fabricated with high birefringence (∼6.4 × 10⁻³) and high water content (∼72%) by alternately immersing the hydrogel in water and CaCl₂ solution. By a H₂O/Ca²⁺ exchange, the dominant interaction of cellulose molecules switched between H-bond and ionic coordination, so as to achieve continuous regulation of orientation. Moreover, using flexibly programmable exchange strategies, bilayer, gradient anisotropic, opposite anisotropic and adjacent anisotropic cellulose hydrogels were fabricated with precise control of local orientation. Diverse self-shaping gels including rings, hollow rectangles, hollow triangles and elastic springs can be prepared on variable surfaces without any inverted molds. This principle may provide new ideas and inspirations for constructing highly oriented structures and develop new avenues to produce versatile stimulus-responsive anisotropic materials.