A Green and Iridescent Composite of Cellulose Nanocrystals with Wide Solvent Resistance and Strong Mechanical Properties

Cellulose is the most abundant plant-based natural material. As a green and multifunctional material, the chiral nematic films from self-assembled cellulose nanocrystals (CNCs) have drawn considerable attention in recent years. Although the swellability in water and other solvents has been used to develop CNCs-based moisture sensors, CNCs films with a wide range of solvent resistance will greatly expand their practical applications. In this study, we demonstrated that the iridescent and hydrophilic CNCs films have been changed to films with excellent water resistance, even the strongly acidic or basic aqueous solutions, and various organic solvents via co-assembly with oxidized starch (OS) and tannic acid (TA). The composite films retain their structural integrity and preserve their vivid structural color after being immersed in water for 24 h. Moreover, the CNCs/OS/TA composite film with the weight ratio of CNCs and OS of 85:15 and the weight percentage of TA 8 wt % shows a tensile strength of up to 70.89 +/- 3.21 MPa and the maximum Young's modulus to 18.67 +/- 1.25 GPa, while, in the wet state, those values are still as high as 58.60 MPa and 13.60 GPa, respectively. The hydrogen-bond-based cross-linking via tannic acid endows CNCs films with an extraordinary solvents resistance and a significant increase in mechanical properties, which opens up a wide range of applications for CNCs-based iridescent films in harsh conditions.

Automated summary

In this study, iridescent and hydrophilic CNCs films were transformed into films with excellent water resistance and resistance to strongly acidic or basic aqueous solutions and various organic solvents by co-assembling with oxidized starch (OS) and tannic acid (TA). The composite films maintained structural integrity and vibrant color even after immersion in water for 24 hours. The cross-linking via hydrogen bonds with tannic acid significantly improved solvent resistance and mechanical properties of CNCs films, expanding their applications in harsh conditions.