Preparation of Cellulose Nanofibrils by Multi-Site Regioselective Oxidation

Abstract

Cellulose nanofibrils (CNFs) are promising sustainable materials that can be applied to nanocomposites, as well as medical and life-sciences devices. However, methods for the preparation of these important materials are energy intensive because heating and mechanical disintegration are required to produce cellulose fibers below 100 nm in size. In this study, CNFs were prepared through the multi-site regioselective oxidation of cellulose with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and periodate at room temperature (20–25°C), without any mechanical-disintegration treatment. Transmission electron microscopy (TEM) revealed that the CNFs had the average widths of 14.1, 55.4, and 81.9 nm for three different treatments. Fourier-transform infrared spectroscopy revealed that carboxyl groups were created on the surfaces of the microfibrils, while X-ray diffraction studies showed that the cellulose I structure was maintained after oxidation, and that the cellulose nanofibril crystallinity index exceeded 70%. These results demonstrate that CNFs can be prepared by multi-site regioselective oxidation at room temperature in the absence of mechanical disintegration. In addition, a model was developed to calculate the total content of carboxylate and aldehyde groups of CNFs prepared by the TEMPO mediate oxidation, the periodate oxidation, and the multi-site regioselective oxidation methods based on the particle width determined by TEM. The calculated values of the model were in good agreement with the total content (experimental value) of carboxylate and aldehyde groups of CNFs prepared by the TEMPO-mediated oxidation and the multi-site regioselective oxidation methods. However, the model was not valid for CNFs prepared by the periodate oxidation method.