Quantitative X-Ray Investigations on the Crystallinity of Cellulose Fibers. A Background Analysis

Abstract

A quantitative investigation of the intensity of scattered radiation in a series of native and regenerated cellulosic objects, including bacterial cellulose and fibers transformed into cellulose IV, is offered. The technique used involves the use of copper radiation monochromatized by reflection, specimens with randomized orientation of equal size and density, rotation of the plane film, and measurement of primary beam intensity by means of a device introduced by Goppel. The procedure of evaluation consists of measuring integrated intensities of the crystalline peaks and that of the diffuse background. After correction of the latter for radiation scattered by air and for the background components due to Compton radiation and thermal scattering, as derived from exposures of sugar crystals and diamond, the diffuse background is represented by a curve exhibiting a flat maximum. The height of this maximum was assumed to be a correct relative measure of the disordered portion and the total intensity of the coherently diffracted radiation of the crystalline peaks as a relative measure of the crystalline portion. Within the series of native specimens the ordered and disordered fractions are found to be equal in cotton, cotton linters, ramie, and flax fibers. In woodpulp the disordered fraction is slightly higher and in bacterial cellulose considerably higher. Within a series of rayons, including slightly and highly orientated viscose rayons, Cellophane, Lilienfeld rayon, and cuprammonium rayon, the ordered and disordered fractions were also found to be equal. Slightly less disordered substance is found in Fortisan, in mercerized ramie and in specimens where cellulose II has been transformed in cellulose IV by a heat treatment. It is shown that the relative proportions of the fractions of disordered substance, for all objects investigated, are in satisfactory quantitative agreement with those deduced from the sorptive capacity for water vapor. The absolute percentage of the crystalline portion calculated from the experimental data is 70±2 percent for the native fibers and 39±3 percent for the rayons. These figures are in reasonable agreement with those previously estimated from sorption isotherms (68 percent and 35 percent), from density determinations (60 percent and 25 percent), from birefringence and x-ray orientation measurements in regenerated fibers (≤40 percent for rayon), and from recrystallization of amorphous cellulose powder (∼35 percent for rayon). The observed relative intensities of the principal crystalline interferences give rise to a discussion of the limited scope of the concept ``crystallinity'' in cellulose fibers, and it is argued that the concept ``degree of lateral order'' may be preferable. Comparison of observed relative line intensities with the theoretically calculated ones, reveals another marked difference between native and regenerated fibers, pointing to the fact that certain states of lateral order prevalent in the latter may be absent in the former.

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