One-Pot Water-Based Hydrophobic Surface Modification of Cellulose Nanocrystals Using Plant Polyphenols

An environmentally friendly procedure for the surface modification of cellulose nanocrystals (CNCs) in water is presented. Tannic acid (TA), a plant polyphenol, acts as the primer when mixed with CNCs in suspension, which are then reacted with decylamine (DA), the hydrophobe. Schiff base formation/Michael-type addition covalently attaches primary amines with long alkyl tails to CNC-TA, increasing the particle hydrophobicity (contact angle shift from 21 to 74°). After modification, the CNC-TA-DA particles in the water phase separate, allowing for easy collection of modified material. The dried product is readily redispersible in toluene and other organic solvents, as demonstrated by turbidity measurements, dynamic light scattering, optical microscopy, and liquid crystal self-assembly behavior. Electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, solid-state ¹³C NMR, and X-ray diffraction support the successful surface modification and indicate that CNC particle morphology is retained. The modified CNCs have a slightly decreased onset of thermal degradation (ca. 10 °C lower) compared with that of unmodified CNCs. We believe that this surface modification strategy presents a scalable, simple, and green approach to the production of hydrophobic biobased nanoparticles which may lend themselves as reinforcing agents in nonpolar polymer composites or stabilizers and rheological modifiers in nonaqueous liquid formulated products.