Thermal- and stress-induced lattice distortions in a single Kevlar49 fibre studied by microfocus X-ray diffraction

Understanding the macroscopic physical and mechanical properties of poly(p-phenylene terephthalamide) (PpTA) fibres as a function of temperature requires an understanding of how temperature influences its microscopic structure. This study investigates lattice distortions in single PpTA fibres using the high brilliance of a synchrotron radiation microbeam. Lattice distortions are studied over a temperature range of 110–350 K and the influence of tensile deformation is also considered. The results reveal linear thermal expansion behaviour for all unit cell axes, in general agreement with literature. Expansion/contraction is greatest along the [100] direction whilst being reduced along [010] by inter-chain hydrogen bonding. During macroscopic deformation, longitudinal crystal strain dominates with respect to axial lattice distortions induced by temperature changes. There is only a small change in the [100] coefficient of thermal expansion, with the [010] and [001] directions being largely unaffected.