A comparative guide to controlled hydrophobization of cellulose nanocrystals via surface esterification

Surface esterification methods of cellulose nanocrystals (CNC) using acid anhydrides, acid chlorides, acid catalyzed carboxylic acids, and 1'1-carbonyldiimidazole (CDI) activated carboxylic acids were evaluated with acetyl-, hexanoyl-, dodecanoyl-, oleoyl-, and methacryloyl-functionalization. Their grafting efficiency was investigated using Fourier-transform infrared spectroscopy and C-13 solid state NMR spectroscopy. Acid anhydride and CDI were found to be the most applicable reagents to graft short and long chain aliphatic carbons, respectively. The preservation of structural morphology and crystallinity of grafted CNCs were confirmed using transmission electron microscopy and X-ray diffraction. The hydrophobicity of grafted CNCs was evaluated by dispersing them in organic solvents with different Hansen's solubility parameters. The dispersibility of grafted CNCs in organic solvents was improved by using never-dried CNCs as source materials and keep CNCs wet in their washing solvents after grafting, thus increasing the solvency range to disperse CNCs.