Preparation, characterization and acetylation of cellulose nanocrystal allomorphs

Acetylation of cellulose nanocrystals (CNCs) is one of the most available routes to enhance hydrophobicity of CNCs. This study investigated the impact of CNC allomorphs on the acetylation. CNC samples with different allomorphs (cellulose I, II and III) were prepared and surface modified by acetylation in heterogeneous system. Then, CNCs and acetylated CNCs (ACNCs) were characterized by solid-state CP/MAS C-13 NMR spectroscopy, X-ray diffraction, atomic force microscope and thermogravimetric analysis. It was found that all obtained ACNC allomorphs kept their original intact crystal forms and rod-like structure. The degree of substitution of ACNC I was highest and up to 1.98, followed by ACNC III', and ACNC II' was lowest which only came to 0.32 after the reaction of 3 h. In the process of acetylation, the crystallinities of CNC allomorphs decreased, in which CNC II dropped least and CNC III dropped most. Both allomorphic transformation and acetylation improved the thermal stability of native CNC I. This comparative study about CNC allomorphs can offer a new sight for their morphologies and internal properties and also guide for applications like composite materials.