X-ray diffraction study of poly(p-phenylene terephthalamide) fibres

On the basis of an X-ray diffraction study, a model is proposed for the crystal and molecular structure of poly(p-phenylene terephthalamide). The monoclinic (pseudo-orthorhombic) unit cell [$\text{a = 7·87}\text{A}\text{?}\text{, b = 5·18}\text{A}\text{?}\text{, c (}\text{fibre axis}\text{) = 12·9}\text{A}\text{?}\text{and}\text{γ = 90°}$] possesses Pn or P2₁/n space-group symmetry. Two molecular chains pass through the cell, one through the centre and the other through a corner. Approximate values for the orientation angles between the phenylene planes and the amide planes are 38° for the p-phenylene diamine segment and -30° for the terephthalic segment. Hydrogen bonds are formed between adjacent chains lying in the (100) plane. The conformation of the chain is primarily governed by competitive intramolecular interactions between the conjugated groups.