Cellulose polymorphy, crystallite size, and the Segal Crystallinity Index

The X-ray diffraction-based Segal Crystallinity Index (CI) was calculated for simulated different sizes of crystallites for cellulose I beta 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 beta 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-theta value for assessing the amorphous content. That results in less peak overlap at the recommended 2-theta value. Patterns calculated with simulated preferred orientation had somewhat higher CIs for cellulose I beta, whereas there was very little effect on the CIs for cellulose II.