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, 6 = 5.08 A, and c = 12.9 Å, containing monomer repeats of two chains. The space group is P2 1/n, and the two chains in the unit cell are related by both the n-glide plane and the 2 1 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 be 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.
Original language | English |
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Pages (from-to) | 3713-3718 |
Number of pages | 6 |
Journal | Macromolecules |
Volume | 38 |
Issue number | 9 |
DOIs | |
State | Published - 3 May 2005 |