Electron diffraction and molecular modelling investigation of the crystal structure of poly (para-phenylene terephthalamide) form II

Microsingle crystals of poly(para-phenylene terephthalamide), abbreviated PPTA, were obtained by water-vapor induced precipitation of PPTA rigid-flexible copolymers in sulfuric acid solution. Electron diffraction was performed and yielded hk0 patterns. The crystal structure of a model compound, N, N′ para (4-methoxycarbonyl) phenylene dibenzamide, abbreviated 3ARB-COOMe, is also reported. Its structure is compatible with that of the polymer, in which aromatic rings are all coplanar and form a 30° angle with amide groups. A reinvestigation of form II, based on electron diffraction data and molecular modeling simulations, is reported. The proposed space group is P112, and corresponds to a parallel chain arrangement, with terephthaloyl groups facing one another, and carbonyl groups pointing in opposite directions along the b-axis in adjacent hydrogen-bonded sheets. Chain conformation is the same as in form I, as previously established. The 3ARB-COOMe conformation is suggested as a possible polymorph or as defects in the crystal phase.