Researchers routinely want to know the crystallinity of cellulose samples. Powder X-ray diffraction (PXRD) is a way to study crystallinity because diffraction is intrinsically based on periodically repeated atoms, i.e., crystals. Other techniques typically measure signals from certain groups of atoms that are associated with crystals than from the crystals themselves. Park et al. describe three PXRD analyses. However, given the overlap of diffraction peak tails, the amorphous subtraction and popular Segal peak height methods have a fatal flaw: substantial intensity from crystalline material occurs in the areas ascribed to amorphous scattering. This was shown by our calculated PXRD patterns with different peak widths that are inversely related to crystallite size. Park et al.’s third method, peak deconvolution, considers peak overlap but most deconvolutions do not consider all the smaller peaks, and peak size and shape are not tied to known diffraction physics. The information content in cellulose PXRD patterns limits how much can be learned. One way to maximize the information is to use the known crystal structures for the different polymorphs of cellulose to calculate the diffraction patterns and fit those calculated patterns to the observed experimental data. The Rietveld method embodies that strategy. Essentially, it is curve fitting software that modifies peaks that arise from the input crystal structure in ways restricted to known diffraction physics. The lecture will demonstrate Lutterotti's general purpose Rietveld program Maud for cellulose. Such programs have many options. However, analyses can be done quickly once the most likely variables are known. Follow-up with a spreadsheet is also useful.

Simplified studies of cellulose crystallinity with a Rietveld program

253rd National Meeting (2017)

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