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X-Ray Diffraction Click on the green arrow to return to the previous page
Information about the structure of materials can be determined from the scattering patterns produced when X-rays are diffracted by the regular crystal lattices within the materials and the analytical technique makes use of Bragg's law. X-ray diffraction data provides a means of identifying materials because the data is unique to a particular chemical structure. Different types of data can be generated depending on how the material is presented to the X-ray beam. Line spectra comprising a series of peaks is the most common form and the relative intensities of the peaks is characteristic for a given chemical structure as the data is collected over a range of diffraction angles (Equatorial scans). Orientation of the crystallites can also be determined by rotation of the material at a fixed diffracting angle (Azimuthal scans).
Instead of using an X-ray diffractometer to collect data,a different experimental arrangement allows images of the diffraction pattern to be recorded and the periodicities in the pattern and breadth and spacing of any arcs is again characteristic of the diffracting structure. Large crystallites give sharp intense diffraction peaks whereas tiny crystallites, common in many polymers, give very broad diffraction peaks so there is a reciprocal relationship between crystallite size and peak width. XRD is routinely used to identify minerals in geological samples and such samples defract X-rays efficiently and are not damaged by the X-rays. Polymers on the otherhand have structures which change over time because they are often comprised of very tiny crystallites of long chain molecules dispersed within an amorphous matrix. Over time the crystallinity can change in polymers and their properties also. Obtaining images of diffraction patterns requires long exposure times of many hours because the small crystallites diffract the X-rays very weakly. For some polymers there is a need to understand structural arrangements that develope as the material is formed from the melt or by wet spinning and this can only be acheived using a very intense source of high energy X-rays, such as is obtained in a sychrotron, where beams of electrons are accelerated to velocites very close to the speed of light and then allowed to hit a metal target. The rapid deceleration of the electrons produce exceptionally intense X-rays which can then be used to probe structures with patterns that develope in minutes that would otherwise take hours using commercial X-ray sources.