According to the International Union of Crystallography, “material is a crystal if it has essentially a sharp diffraction pattern. The word essentially means that most of the intensity of the diffraction is concentrated in relatively sharp Bragg peaks, besides the always present diffuse scattering.” Therefore, non-crystals do not give diffraction patterns containing Bragg peaks. However, cellulose samples give diffraction peaks of intermediate width at half height (pwhm) ranging from 0.6° for algal cellulose to 3.5° or so from wood cellulose. Much of that range can be assigned to crystallite sizes of more than 10 nm down to 2.3 nm, respectively, that can be observed by electron or atomic force microscopy. Smaller crystals have higher surface areas and there is less restriction on positions of surface atoms. That can lead to indications of amorphous matter by various spectroscopic methods, even if the diffraction pattern is not changed noticeably from one whose surface atoms are in the same relative positions as those in the interior. Still, some samples require consideration of contributions from material that scatters very diffusely. Although the Segal method is familiar and easy to use, it does not distinguish between small crystals and larger ones that have contributions from both well-organized molecules and from material that does not produce relatively sharp Bragg peaks. The calculated diffraction pattern for cellulose II with a pwhm of 9° is similar to the experimental pattern for of ball-milled cellulose, one kind of amorphous cellulose. Calculated amorphous patterns can be mixed in a spreadsheet with patterns for crystalline cellulose to find a pattern by trial-and-error that best agrees with experiment. Similarly, patterns can be calculated by mixing calculated patterns for cellulose I, II, and amorphous to mimic partially mercerized cellulose. The quality of fit will depend on the quality of the experimental pattern, especially regarding avoidance of preferred orientation.

Increased understanding of cellulose crystallinity

251st National Meeting (2016)

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