Fibre Diffraction

Natural and synthetic polymers are long chain molecules made up of repeating units. The number of repeats is usually very large, and could be up to 20,000 or more. Moreover, natural fibres tend to have helical symmetry, where the repeat units arrange themselves in a continuous helix with an irrational repeat ratio, which can only be approximated by factors like '213 repeats in 71 turns'. Crystalline material made of fibres will never have exact symmetry to produce single crystal-type diffraction intensities. Diffraction is observed in quasi-continuous smears arranged in layer lines, with no clear separation between one diffraction order and another.

Fibre diffraction patterns can be deconvoluted mathematically to produce diffraction amplitudes similar to single crystal data. Various arrangements of the known model of the repeat unit can then be tested for correctness.

The degree of crystallinity in a synthetic fibre is a quick and qualitative result that can be obtained readily with the fibre diffraction facility. Quantitative results, like the structure of a filamentous virus, require extensive modelling after data acquisition.

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