Green One-Pot Synthesis of Surface Hydrophobized Cellulose Nanocrystals in Aqueous Medium

The surface hydrophobization of cellulose nanocrystals (CNCs) by fatty acids (FAs), biodiesel, or plant oils was conducted via a green process using an organic solvent less one-pot method. In the process, an aqueous lactic acid syrup served as a reactive solvent to provide a stable and well-dispersed water suspension of CNC and participate in esterification reactions, producing an intermediate product of polylactic acid (PLA) oligomer grafted CNCs (CNC-g-PLA). This reactive solvent and intermediate product system allows for an in situ solvent exchange from water to lactic acid without prior drying of the CNC and a subsequent efficient esterification reaction of CNC with carboxylic acids or esters having a long hydrocarbon chain (FAs). Another advantage of the proposed process is the possibility to reuse the reagents in the subsequent reaction in order to reduce the production cost. The grafting efficiency of the CNC-g-PLA-FA samples covalently grafted with PLA and FAs were characterized by Fourier transform infrared spectroscopy (FT-1R), elemental analysis (EA), and C-13 nuclear magnetic resonance spectroscopy (ss-NMR). The change of dimensions and degree of crystallinity of the modified CNC was evaluated by dynamic light scattering (DLS) and X-ray diffraction (XRD). Different organic solvents were used here to investigate the relative hydrophobicity of the modified CNCs comparing the transmittance of their suspensions in the ultraviolet and visible (UV-vis) spectrometer, and evaluating their hydrodynamic radius by dynamic light scattering (DLS) techniques. Grafting of renewable materials on the surface of CNCs was developed by polyesterification that is capable of being environmentally friendly and mass-produced without any organic solvents or toxic reagents.