Preparation of solution-grown lozenge-shaped poly(p-phenylene terephthalamide) single crystals and their structural stabilization by heat treatment

In this study, the preparation of poly (p-phenylene terephthalamide) (PPTA) single crystals was examined using crystallization from dilute solutions in concentrated sulfuric acid. Lozenge-shaped PPTA single crystals were successfully prepared using a self-seeding method with a low degree of supercooling, and they consisted of modification I crystals. The a-axis direction of the crystal corresponded to the long diagonal direction of the rhombus, the b-axis direction with the short diagonal direction, and the PPTA molecular chain direction (the c-axis direction) with the crystal's thickness direction. In addition, the PPTA single crystals had a (110) growth plane, where the thickness of each single crystal was approximately equal to the molecular chain length of the PPTA. Upon heat treatment of the PPTA single crystals, the symmetry changed from P1a1 to the more stable P11n. In addition, the heat treatment caused a difference in the density of each symmetric crystal, resulting in crack formation along the b-axis direction, which is the hydrogen-bonding direction. However, the heat treatment did not change the thickness of the PPTA single crystals. Conversely, the isothermal crystallization of the PPTA caused progression in the crystallization only under a high degree of supercooling, thus yielding plate-like PPTA crystals that consisted of modification II crystals. In these plate-like PPTA crystals, the length corresponded to the crystal a-axis direction, and the electron diffraction pattern was broad. Furthermore, the equilibrium dissolution temperature of the PPTA single crystals was discussed.