Controlled defibrillation of rice straw cellulose and self-assembly of cellulose nanofibrils into highly crystalline fibrous materials

Abstract

Coupled chemical–mechanical defibrillation and self-assembly processes have been successfully established for creating super-fine (125–497 nm wide) highly crystalline (63.2–71.5% CrI) cellulose Iβ fibrous materials from rice straw cellulose. Under the optimized TEMPO mediated oxidation with 5 mmol g⁻¹ NaClO/cellulose followed by 30 min mechanical blending, highly uniform (2.09 nm wide, 1.52 nm thick, up to 1 μm long) cellulose nanofibrils (CNFs) were efficiently derived at an impressive 96.8% yield and contained 1.29 mmol surface carboxyls per g of cellulose or 0.21 COOH/anhydroglucose (AG), representing 70.9% surface C6 primary hydroxyl to carboxyl conversion. Rapid freezing of aqueous CNF suspensions in liquid nitrogen and freeze-drying induced self-assembly of these nanofibrils into white fluffy fibrous materials via ice-crystal templating. The self-assembled fiber morphologies showed a strong dependence on CNF morphologies and extent of surface carboxylation. CNFs with 37.3% conversion of surface carboxyls assembled into 125 nm wide fibers, whereas wider fibers (327 nm and 497 nm) were assembled from the smaller CNFs and more carboxylated (51.5% and 70.9%, respectively) surfaces. This robust defibrillation–assembly approach offers new versatile and scalable alternatives to fabricate super-fine cellulose fibers with a highly crystalline cellulose Iβ structure from the by-product of the largest cereal crop in the world.