Experiment 21 - X-ray diffraction

Abstract

X-rays diffraction (XRD) is a powerful tool in the study of the structure of polymers. The wavelengths of X-rays are often comparable to the interatomic distance in crystals. XRD and scattering experiments involve placing the sample in the path of a monochromatized X-ray beam of low divergence. The scattered X-rays from the regularly placed atoms interfere with each other, giving strong diffraction signals in particular directions. Two primary diffraction methods commonly used to study polymers include wide angle X-ray scattering (WAXS) and small angle X-ray scattering (SAXS). Over the years, WAXS technique is extensively used to study the structural properties of polymers. The value of angles used in WAXS is from 5° to 120°. WAXS is also used to provide information on the number of repeat units per turn in helical structures that are typical of linear polymers, the length of the repeat unit along the fiber axis, and the degree of orientation. The value of angles used in SAXS is from 1° to 5°. SAXS is useful in detecting larger periodicities in a structure.