On the morphology and structure of poly(p-phenylene terephthalamide) single crystals and spherulites

Abstract

Abstract Poly(p-phenylene terephthalamide) (PPTA) single crystals and negative spherulites obtained from concentrated sulfuric acid solution were studied by polarizing microscopy, transmission electron microscopy, electron diffraction, and infrared spectroscopy techniques. Electron microscopy shows that the single crystals with nearly pyramidal shape are composed of rectangular platelets which are packed in parallel with the same orientation. The long edge of the platelet is parallel to [avec]∗, while the macromolecular chains are located in the plane of the platelet and perpendicular to the long edge. The width of the platelet is dependent on the molecular weight. The radius direction of PPTA negative spherulites is [avec]∗. Electron diffraction patterns indicate that the unit cell parameters of PPTA single crystals and spherulites are not the same as those of Kevlar fibers. For the former c = 11.5 Å, while for the latter c = 12.9 Å. This indicates that the molecular conformation of PPTA molecules in the single crystals and spherulites is different from the conformation of PPTA molecules in the fibers. Infrared absorption spectra show large shifts of the amide I and amide II bands between the two conformations.