Chemical modification of cellulose by TEMPO-mediated oxidation

点击翻译按钮获取中文摘要

Details about chemical modifications of native cellulose by 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO)-mediated oxidation were studied in this project. Carboxylate and aldehyde groups introduced into native celluloses by the TEMPO-mediated oxidation were determined in terms of oxidation conditions such as reaction time and the amount of sodium hypochloride added as a co-oxidant to the cellulose/water suspensions. Linter cellulose was used to begin with as the starting native cellulose. Microscopic observation showed that the original fibrous morphology of linter cellulose was unchanged before and after the TEMPO-mediated oxidation, and recovery ratios of the TEMPO-oxidized linter cellulose were almost 100% by filtration. Carboxylate and aldehyde contents in the TEMPO-oxidized linter cellulose reached to 35 times and 1000 times as much as those of the original linter cellulose, respectively. Crystallinity and crystal size of the native cellulose I structure of the original lint … More er cellulose were unchanged even after the TEMPO-mediated oxidation, which was proved by X-ray diffraction and solid-state ^<13>C-NMR analyses of the oxidized products. Thus, TEMPO-mediated oxidation allows to introduce carboxylate and aldehyde groups only on the surface of cellulose microfibrils quite efficiently, maintaining the microfibril structure of native cellulose. Such surface-modified celluloses prepared by the TEMPO-mediated oxidation showed specific adsorption behavior of metal ions. Especially, carboxylate ions in the TEMPO-oxidized cellulose form metal salt structures at molar ratio of 1:1 with either calcium, silver or lead ion. When the TEMPO-mediated oxidation was applied to wood chemical pulp, bacterial cellulose, tunicate cellulose, ramie cellulose and wood holocellulose, each cellulose had characteristic carboxylate and aldehyde contents even under the same TEMPO-mediated oxidation conditions, depending on the original cellulose used. Particularly, we have succeeded in preparation of transparent cellulose microfibril dispersion gels by TEMPO-mediated oxidation under quite mild conditions followed by mechanical treatments. These transparent gels consisted of independently-dispersed cellulose microfibrils with nano-size width in water, differing from the conventional cellulose microcrystals dispersed in water or cellulose derivatives molecularly dissolved in water, and thus behaved quite high viscosities even at low consistencies. These new cellulose nano-materials dispersed in water are expected to show some unique functionalities. Less