Increment in evolution of cellulose crystallinity analysis
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EDITORIAL
Increment in evolution of cellulose crystallinity analysis Alfred D. French
Ó Springer Nature B.V. 2020
Crystallinity analysis is important for practical reasons and related research can offer information on the nature of amorphous cellulose. Two papers in this issue mark a transition in general understanding of cellulose crystallinity analysis. First, a brief review of diffraction crystallinity methods. Several diffraction methods are used to analyze cellulose crystallinity (Thygesen et al. 2005; Park et al. 2010). The most prevalent, and by far the simplest, is the Segal peak height method (Segal et al. 1959). It had 4871 citations as of this writing, despite frequent use with no attribution or with only citations of secondary publications. Another approach, peak deconvolution, is more effort to carry out and to attribute. Perhaps Hermans and Weidinger (1948) were first to suggest that the area under diffraction peaks be divided by the total area. At present, conventional peak deconvolution involves curve fitting to the observed pattern with the individual visible peaks plus a very broad, but simple, e.g., Gaussian, peak for the amorphous material. Typically, general purpose curve-fitting software is used. A third method (Rietveld 1969 (16,400 citations); Young 1995) is used for general molecular structure determination of powders as well as occasionally for cellulose crystallinity. The Rietveld method also A. D. French (&) New Orleans, USA e-mail: [email protected]
optimizes variables to fit a diffraction pattern, but it uses all of the diffraction peaks. Unlike peak deconvolution, the Rietveld method includes the smaller peaks that are lost in what appears to be the background or amorphous scattering. These smaller peaks can be visualized by calculating a diffraction pattern for an unrealistically large (100 nm) model cellulose crystal. Paul Scherrer (1918) showed that peaks are sharp when crystals are large, and broad when crystals are small. When the sharp and separated calculated peaks are broadened to mimic the peaks that arise from model crystals of a size similar to most cellulosic samples (a few nanometers), it appears that much of the intensity formerly attributed to ‘‘background’’ or ‘‘amorphous scatter’’ is just the overlapped intensity from adjacent crystalline peaks (French 2014). The interpretation that the intensity between peaks results from peak overlap, particularly in the region that Segal attributed to only amorphous intensity, casts doubt on the Segal method as a ‘‘crystallinity’’ determination (French and Santiago Cintro´n 2013). It also disqualifies a fourth method, ‘‘amorphous subtraction’’ (Thygesen et al. 2005). The Rietveld method uses the x-, y-, and zcoordinates of the atoms in the crystal structure unit cell such as from Nishiyama et al. (2002) to calculate a diffraction pattern for an ideal crystalline powder. Experimental patterns from cellulose are never ideal, so Rietveld analyses can compensate for the varied u
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