The Segal Crystallinity Index As It Relates to Crystallite Size

Cotton fibers are composed of crystals of cellulose that yield a diffraction pattern, although fibers from varying sources and histories are said to have different degrees of crystallinity. There are many methods to assess this crystallinity. One of the most popular is the Segal Crystallinity Index (CI) that is based on X-ray powder diffraction. The CI is simply calculated by observing the height of the main peak and the intensity at the lowest point between the peaks. Cellulose crystals have sizes in the range that can be determined by analysis of the diffraction peak widths, and conversely, patterns can be calculated for crystals of a particular size. Calculated patterns for otherwise perfect crystals of different sizes show a strong but non-linear correlation with the Segal CI. These calculated patterns resemble the observed patterns to a substantial degree, and it appears that much of the previously observed non-crystalline component is simply due to crystals of small size.