Improvement of nanodispersibility of oven-dried TEMPO-oxidized celluloses in water

Abstract

Never-dried 2,2,6,6,-tetramethylpiperidinyl-1-oxyl (TEMPO)-oxidized wood celluloses (TOCs) with carboxylate contents of 1.0–1.7 mmol/g can be converted to TEMPO-oxidized cellulose nanofibrils (TOCNs) with homogeneous widths of ≈3 nm by mechanical disintegration treatment in water. However, oven-dried TOCs have low nanodispersibility in water. In this study, when TOC was reduced with NaBH4 under suitable conditions, almost all C6-aldehydes and C2/C3 ketones present in the TOCs were converted to hydroxyl groups. The NaBH4-reduced TOCs had sufficiently high nanodispersibility (similar to that of never-dried TOCs) in water even after oven drying at 105 °C for 3 h. Thus, interfibrillar hemiacetal linkages are probably formed between hydroxyl groups and either C6-aldehydes or C2/C3 ketones present on the crystalline microfibril surfaces of TOCs during heating, resulting in low nanodispersibility. The heat-induced discoloration of TOCs can also be avoided by using NaBH4-reduced TOCs. Heating of the oven-dried TOCs in water at 80 °C for 0.5–3 h improves their nanodispersibility, presumably because the interfibrillar hemiacetal linkages formed by oven drying are broken during the hot-water treatment. A semi-quantitative method to determine C6-aldehydes and C2/C3 ketones present in dried TOCs is proposed, in which the specific UV absorbance spectra of TOCN films heated at 105 °C for 3 h are used. The presence of C6-aldehydes and C2/C3 ketones in TOCs can be determined by UV absorption peaks of the TOCN films at ≈260 and ≈290 nm, respectively.