Crystallization of Cellulose Fiber by NO₂ Oxidation

Abstract

Under sufficiently drying condition when ramie was oxidized by NO₂ gas, it was found that its crystallinity and its crystallite size in a [101] direction increase with increasing degree of oxidation after the first stage in which the crystallinity decreses. At the degree of oxidation of 250 mmol-COOH/100 g, the crystallinity becomes about 1.3 times of its original ramie and the same is true for its crystallite size. However, from this point, both turn out to decrease with increasing degree of oxidation.
When the NO₂ oxycellulose was converted into CA²⁺ or Ag+ salt, the crystallinity and the crystallite size decreased with increasing degree of oxidation. By desalting with dilute aq. HCI solution, they return to have the values of the original oxycellulose.
In our previous paper, it was suggested that the molecular group in the (101) plane of cellulose is a structure unit, the planes are connected together by intermolecular hydrogen bond participating in the -OH at C, and forming a crystallite, and an amorphous part shows the (101) plane lattice structure having disorder in the regularlity between the planes.
NO₂ oxidizes selectively the -OH at CB in cellulose. It was confirmed that the stereochemical arrangement of CB, oxidized to -COOH, does not differ from that in the crystallite of cellulose. Therefore, it was considered that the -COOH in the NO₂ oxycellulose incorporated the (101) planes in amorphous part into the crystallite of cellulose in terms of its high ability of intermolecular hydrogen bond, and brougnt about the increase in the crystallinity and the crystallite size in the [101] direction. These effects are eliminated in its salt. The hypothesis was very available for the interpretation of the characteristic behaviors of the NO₂ oxycellulose which are dependent on the large resistance to esterification and to the solubility of the ester, on the high solubility in a alkali solution and on the regeneration in high yield from the solution. The HI04-HCl02 oxycellulose in which -OH at C_2C and C, position are oxidized, does not show the behavior like NO₂ oxycellulose.