Versatile Surface Modification of Cellulose Fibers and Cellulose Nanocrystals through Modular Triazinyl Chemistry

The ability to tune the interfacial and functional properties of cellulose nanomaterials has been identified as a critical step for the full utilization of nanocellulose in the development of new materials. Here, we use triazine chemistry in a modular approach to install various functionalities and chemistries onto cellulose fibers and cellulose nanocrystals (CNCs). The surface modification is demonstrated in aqueous and organic media. Octadecyl, monoallyl-PEG, benzyl, and propargyl triazinyl derivatives were grafted onto cellulose/CNCs via aromatic nucleophilic substitution in the presence of base as hydrochloric acid scavenger. The covalent nature and degree of substitution of grafted aliphatic, polymeric, alkyne chains, and aromatic rings were characterized through Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, elemental analysis, and thermogravimetric analysis. In addition, AFM and DLS analysis showed minimal change in the geometry and individualized character of CNCs after surface modification. X-ray diffraction analysis confirmed that the modification happened only at the CNC surface, while the bulk crystalline core remained unmodified. Modified cellulose/CNCs showed hydrophilic or hydrophobic properties depending on the grafted functionality, which resulted in stable colloidal suspensions of CNCs in polar and nonpolar organic solvents. Furthermore, the reactive nature of propargyl-modified cellulose was demonstrated by the successful grafting of an azido-fluorescein dye via copper-catalyzed Huisgen 1,3-dipolar cycloaddition. The triazinyl chemistry thus presents a versatile route for tuning the interfacial properties of nanocellulose, with the possibility of postmodification for applications that require the conjugation of molecules onto cellulose through bio-orthogonal chemistries.