Submitted to: Cotton Fiber: Physics, Chemistry and Biology
Publication Type: Book / Chapter
Publication Acceptance Date: 9/18/2018
Publication Date: 11/10/2018
Citation: French, A.D., Kim, H.J. 2018. Cotton fiber structure. Cotton Fiber: Physics, Chemistry and Biology. pp. 13-40. https://doi.org/10.1007/978-3-030-00871-0_2.
Interpretive Summary: This chapter covers the recent understanding of crystal and molecular structure from upland cotton fibers. Crystalline cellulose structure of cotton fiber are characterized using spectroscopic methods and X-ray diffraction. Crystallite size and crystallinity are determined by various methods including Rietveld refinement, peak height, amorphous subtraction, and deconvolution methods. Strength and weakness of the methods are described as well.
Technical Abstract: This chapter covered the crystal and molecular structure as well as some aspects of the supramolecular structure. Reference is made to some work initiated more than 80 years ago that is still on the verge of yielding a sufficient understanding. The summary of various dimensions could have been written many years ago, but our modern listing could be helpful. One finding herein that is potentially transformative for the cotton literature is that the layers of secondary wall at any given point along the fiber length may not be progressively changing. Another transformative point may be that the amount of scattered intensity that comes from a component of the fiber that resembles ball-milled cotton is very small. Relatively new efforts that should have fruitful results include further work with single fiber diffraction that can more clearly connect the convolutions with the changing microfiber orientation to help understand the secondary cell wall development. The extra resolution provided by synchrotron diffraction on bundles of cotton fiber can be effective for determining the effective crystallite dimensions. Also, the experimentally observed variable elongation for different samples could depend on the extent of misalignment of the microfibrils with the fiber axis. Finally, the ability to simulate the powder diffraction pattern for cotton samples can help to understand the experimentally determined powder patterns. The question becomes “what must be done to an ideal sample crystal to make its diffraction pattern resemble the experimental pattern?” In this writer’s experience, the presented Rietveld refinement of Texas Marker 1 cotton gave exceptional agreement between the observed and calculated patterns. The final variables refined were those related to the shape anisotropy clearly observed in the experimental fiber patterns. That refinement fine-tuned the agreement between the observed and calculated data, but the values of the added variables were not totally convincing. Issues of the background and other factors are not completely resolved. Yet, it is clear that the current popular methods (peak height, amorphous subtraction, and deconvolution) have substantial flaws, starting with the faulty assignment of amorphous contributions based on what appears to be overlap of the tails of the diffraction peaks. That overlap is very dependent on the crystallite size. Size effects can also be a factor in various experimental studies of crystallinity, such as deuteration or spectroscopy.