Amidation of carboxy groups in TEMPO-oxidized cellulose for improving surface hydrophobization and thermal stability of TEMPO-CNCs

Abstract

Surface-hydrophobized cellulose nanomaterials (CNs) with high thermal degradation points are required for preparing various materials, such as epoxy nanocomposites, which possess high mechanical strength, optical transparency, and thermal stability. Amidation of carboxy groups in CNs is one possible chemical modification for hydrophilic CNs that contain abundant carboxy groups. However, achieving efficient amidation of high ratios of carboxy groups in CNs is highly challenging for industrial applications. In this study, carboxy group-containing fibrous wood pulp was subjected to amidation in heterogeneous solid/liquid systems to prepare products with high amidation ratios and high yields, while implementing cost-effective isolation and purification processes. Consequently, a partially acid-hydrolyzed wood pulp with abundant carboxy groups was first prepared. Subsequently, 88 % and 91 % of the carboxy groups in the pulp were successfully amidated using polyalkylene glycols-NH₂ and octylamine, respectively. This was achieved by utilizing 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride and N-methylmorpholine as the condensation reagent and activator, respectively, in N,N-dimethylformamide (DMF) at approximately 23 °C for 16 h. The thermal degradation point increased from 224 °C for the acid-hydrolyzed pulp to over 250 °C after amidation. The amidated pulps were then converted into transparent dispersions, consisting of amidated cellulose nanocrystals, by homogenization in an epoxy monomer/DMF mixture using high-pressure homogenization.