Low-energy synthesis of individualized pH-responsive cationic cellulose nanofibers and chitin nanocrystals by mechanochemistry and aging

Cellulose and chitin nanomaterials are promising sustainable materials that exhibit attractive mechanical, optical, thermal, and chemical properties. Cellulose nanofibers (CNFs) have found applications to the field of packaging, reinforced composite or biomedical applications. Introducing charged functional groups onto these nanomaterials is a proven strategy to improve their dispersibility and processability, as well as their properties, such as adsorption capacity. The use of high energy defibrillators has remained necessary to access CNFs despite the introduction of surface charges prior to increase the efficiency of nanomaterial extraction. To date, there is no known synthesis of cationic CNFs (CCNFs) that is both energy efficient in the defibrillation, and chemically efficient in material modification. Herein we report a strategy to access CCNFs directly from once-dried wood pulp through mechanochemical and aging-based nucleophilic substitution, followed by a short sonication. This treatment introduces pH-responsive cationic diethylethylamine (DEEA) groups with a degree of substitution (DS) as high as 0.80 (amine content of 3.29 mmol g^-1) without the use of excess reagents. The combination of short mechanochemical treatment (10 min), with aging (3 h) and sonication (5 min) allows rapid access to high quality, 2-nm-wide, 1-μm-long CCNFs with high crystallinity of 56.6% and high <i>ζ</i>-potential of 68.10 ± 1.43 mV from sheets of pulp. The method was also applied to powder microcrystalline cellulose and chitin, to afford cationic nanocrystals of cellulose and chitin.