ARS | Publication request: Cellulose polymorphy, crystallite size, and the segal crystallinity index

Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2012
Publication Date: N/A

Interpretive Summary: Cotton fibers and other plant materials are composed of small fibrils of cellulose. These fibrils have a substantially crystalline component, and an important point in the understanding of structure – function relationships is knowledge of the extent of crystallinity. A simple evaluation of the crystallinity is provided by the Segal Crystallinity Index (CI) that was developed nearly 50 years ago by ARS scientists in New Orleans. The CI has been very widely used but sometimes inappropriately. The present paper shows how the CI has very different values for different crystalline forms despite having no differences in crystal size or other aspect of crystallinity except for the particular crystalline form. The paper is intended primarily for scientists characterizing cellulose and those trying to make use of crystallinity information.

Technical Abstract: The X-ray diffraction-based Segal Crystallinity Index (CI) was calculated for simulated different sizes of crystallites for cellulose I' and II. The Mercury software was used, and different crystallite sizes were based on different input peak widths at half of the maximum peak intensity (pwhm). The two cellulose polymorphs, I' and II, gave different CIs despite having the same pwhm values and perfect periodicity. The higher CIs for cellulose II were attributed to a greater distance between the major peaks that are closest to the recommended 2-' value for assessing the amorphous content. That results in less peak overlap at the recommended 2 ' value. Patterns calculated with simulated preferred orientation had somewhat higher CIs for cellulose I', whereas there was very little effect on the CIs for cellulose II.