Submitted to: Cellulose
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2013
Publication Date: 8/25/2013
Publication URL: https://handle.nal.usda.gov/10113/58661
Citation: French, A.D. 2013. Idealized powder diffraction patterns for cellulose polymorphs . Cellulose. 21:885-896 DOI: 10.1007/s10570-013-0030-4.
Interpretive Summary: Cotton and indeed, most of biomass, is composed of the molecule cellulose that is found in the form of fibrils that contain small crystals. X-ray diffraction studies of cellulose are carried out for two main purposes. The first is to determine the arrangement of the cellulose molecules within these crystals, and therefore to help understand the properties of the cellulose. The second reason to perform X-ray studies is to determine which crystal form is present and the extent of the crystalline material compared to the remaining cellulose which is considered to be non-crystalline. The latter studies are often done by workers without extensive background in crystallography, and the variations in nomenclature used over the past 80 years have been confusing. This paper provides a more detailed understanding of the X-ray pattern and urges adoption of the same conventions. This should be helpful to the many cellulose researchers who rely on powder diffraction data to help them understand their samples.
Technical Abstract: Cellulose samples are routinely analyzed by X-ray diffraction to determine their crystal type (polymorph) and crystallinity. However, the connection is seldom made between those efforts and the crystal structures of cellulose that have been determined with synchrotron X-radiation and neutron diffraction over the past decade or so. In part, this desirable connection is thwarted by the use of different conventions for description of the unit cells of the crystal structures. In the present work, powder diffraction patterns from cellulose I', I', II and IIII were calculated based on the published atomic coordinates and unit cell dimensions contained in ‘crystal information files’ (.cif). The calculations used peak widths at half height of both 0.1 and 1.5° 2', providing highly resolved indications of the contributions of each contributing reflection to the observable diffraction peaks as well as intensity profiles that more closely resembled practical cellulose samples. The effects of preferred orientation are also included. A modified .cif file is provided for cellulose I' that has c as the fiber axis, as is a cellulose III file created from the published coordinates. Miller indices that conform to the convention with c as the fiber axis are provided for each contributing peak. Adoption of this convention is urged. The diffraction intensities, output by the Mercury program from the Cambridge Crystallographic Data Centre, can be added in varying proportions using a spreadsheet program to simulate patterns such as those from partially mercerized cellulose or various composites.