Submitted to: Carbohydrate Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2015
Publication Date: 1/1/2016
Citation: Nam, S., French, A.D., Condon, B.D., Concha, M.C. 2016. Segal crystallinity index revisited by the simulation of x-ray diffraction patterns of cotton cellulose IB and cellulose II. Carbohydrate Polymers. 135:1-9.
Interpretive Summary: Due to its simplicity, the Segal crystallinity index (CI) has been used widely to measure the relative crystalline (or amorphous) fraction in cellulose I' and II materials, but its accuracy is controversial. The simulation of the X-ray diffraction patterns of control and mercerized cotton celluloses suggested some aspects that should be considered in measuring or interpreting the Segal CI values: (a) a pressed powder type of sample was recommended by comparison with a nonwoven fabric type, because the preferred orientation suppressed the (012) and (102) peaks of cellulose I' located near the Segal amorphous location; (b) the Segal method may not be appropriate for partially mercerized samples, because the (1-10) and (110) peaks of cellulose I' remained near the Segal amorphous location of cellulose II; (c) the Segal CI depended on FWHM (i.e., crystal size) and cellulose polymorph; and (d) the Segal method underestimated the amorphous fraction for cellulose II, because its Segal amorphous location failed to measure the major intensity of amorphous cellulose. These results are expected to be informative in obtaining unbiased CI values of cellulose materials after physicochemical or biological treatments.
Technical Abstract: The Segal method estimates the amorphous fraction of cellulose IB materials simply based on intensity at 18o 20 in an X-ray diffraction pattern and was extended to cellulose II using 16o 2O intensity. To address the dependency of Segal amorphous intensity on crystal size, cellulose polymorph, and the degree of polymorphic conversion, we simulated the diffraction patterns of cotton celluloses (IB and II) and compared the simulated amorphous fractions with the Segal values. The diffraction patterns of control and mercerized cottons, respectively, were simulated with perfect crystals of cellulose IB (1.54' FWHM) and cellulose II (2.30o FWHM) as well as 10% and 35% amorphous celluloses. Their Segal amorphous fractions were 15% and 31%, respectively. The higher Segal amorphous fraction for control cotton was attributed to the peak overlap. Although the amorphous fraction was set in the simulation, the peak overlap induced by the increase of FWHM further enhanced the Segal amorphous intensity of cellulose IB. For cellulose II, the effect of peak overlap was smaller; however the lower reflection of the amorphous cellulose scattering in its Segal amorphous location resulted in smaller Segal amorphous fractions. Despite this underestimation, the relatively good agreement of the Segal method with the simulation for mercerized cotton was attributed to the incomplete conversion to cellulose II. The (1-10) and (110) peaks of cellulose IB remained near the Segal amorphous location of cellulose II for blends of control and mercerized cotton fibers.