Crystal Structure of Poly(2-cyano-1,4-phenylene terephthalamide)

Abstract

The crystal structure of poly(2-cyano-1,4-phenylene terephthalamide) (CN-PPTA) has a monoclinic (metrically orthorhombic) unit cell with dimensions a = 9.21 Å, b = 5.08 Å, and c = 12.9 Å, containing monomer repeats of two chains. The space group is P21/n, and the two chains in the unit cell are related by both the n-glide plane and the 21 screw axis. The calculated density is 1.447 g/mL, which is close to the observed density of 1.420 g/mL. A number of models were considered to accommodate the random 50/50 −CN substitution at 2- and 3-positions of the p-phenylenediamine segment. The data favor a structure in which 3-CN-substituted phenylenes are rotated by 180° (about their 1,4-axes), so that effectively we have a mixture of 2- and 5-CN groups, which allows the substituents on adjacent chains to interleave. This structure would in fact be compatible with 2,5-disubstitution: random monosubstitution was modeled by 50% occupancy for the atoms of the −CN groups. LALS refinement led to a structure free of steric hindrance, with a crystallographic R value was 0.27. The phenylene−amide torsion angles are 50° for the p-phenylenediamine segment and −30° for the terephthalic segment, values which are similar to those seen for Kevlar. The amide plane is rotated from the bc plane of the unit cell by 11°. The density is lower than that observed for Kevlar (1.50 g/mL) due mainly to the 50% vacancies at the CN sites. These vacancies probably facilitate penetration of the lattice by common polar organic molecules, leading to higher solubility.