Solvation-Controlled Elastification and Shape-Recovery of Cellulose Nanocrystal-Based Aerogels

Aerogels based on rod-like cellulose nanocrystals (CNCs) have been used in anisotropic materials, adsorbents and sensors, whereas they also suffer a low elasticity, leading to hard handling/processing in practical applications. Inspired by the sea cucumber, which transits from rigid to flexible when its cross-link network of collagen fibers is weakened by stiparin inhibitor, we cross-linked the CNCs with flexible poly ethylene glycol (PEG) to prepare an aerogel owning variable mechanical properties in different environments. This aerogel not only had a chemical-bond cross-link network, but also an H-bond one, which could be easily weakened by water. The results showed that the obtained CNC/PEG aerogel owned a high modulus of 0.80 MPa in a dry state and transited to an elastic state (modulus is 0.87 kPa) in a wet state. In the dry state, the shape change of the CNC/PEG aerogel could not recover when the strain was over 10%, when in the wet state the shape change could be reversible. Interestingly, the irreversible strain in the dry state could further transit to reversible in the wet state, and the wet aerogel could then transit back to rigid after freeze-drying. The mechanism study proved that this recovery came from the solvation-controlled weakening of the H-bond network between PEG and CNC. This work offered a simple but useful design of stimulation-response aerogels that can conduce to an elastification and shape recovery.