Reversible Surface Engineering of Cellulose Elementary Fibrils: From Ultralong Nanocelluloses to Advanced Cellulosic Materials.

Cellulose nanofibrils (CNFs) are supramolecular assemblies of cellulose chains that provide outstanding mechanical support and structural functions for cellulosic organisms. However, traditional chemical pretreatments and mechanical defibrillation of natural cellulose produce irreversible surface functionalization and adverse effects of morphology of the CNFs, respectively, which limit the utilization of CNFs in nanoassembly and surface functionalization. Herein, this work presents a facile and energetically efficient surface engineering strategy to completely exfoliate cellulose elementary fibrils from various bioresources, which provides CNFs with ultrahigh aspect ratios (≈1400) and reversible surface. During the mild process of swelling and esterification, the crystallinity and the morphology of the elementary fibrils are retained, resulting in high yields (98%) with low energy consumption (12.4 kJ g-1). In particular, on the CNF surface, the surface hydroxyl groups are restored by removal of the carboxyl moieties via saponification, which offers a significant opportunity for reconstitution of stronger hydrogen bonding interfaces. Therefore, the resultant CNFs can be used as sustainable building blocks for construction of multidimensional advanced cellulosic materials, e.g., 1D filaments, 2D films, and 3D aerogels. The proposed surface engineering strategy provides a new platform for fully utilizing the characteristics of the cellulose elementary fibrils in the development of high-performance cellulosic materials.