Small-angle X-ray diffraction of Kevlar using synchrotron radiation

A synchrotron source of X-rays has been used to obtain high-resoultion small-angle X-ray scattering (SAXS) patterns from commercial poly(p-phenylene terephthalamide) (Kevlar) fibres. In Kevlar 49, detailed analysis of the equatorial streak, often called the ‘void scattering’, shows that it comes from the crystals in the fibre and not from any voids. The scattering objects producing the SAXS pattern have the same length, width and orientation distribution as the crystals. In Kevlar 149, there is less scatter overall, a bimodal distribution of sizes and orientation, and an off-axis streak at ±25° to the equator. Similar off-axis small-angle scattering has recently been seen in other rigid-rod polymer fibres, but the fibre structure producing it is not understood.