Structural Studies of Polyaramid Fibers: Solid-State NMR and First-Principles Modeling

Abstract

Aramid fibers are of practical interest due to their high tensile strength, high elastic modulus, low elongation at breakage, and thermomechanical stability. Here we combine high-resolution solid-state NMR and density functional theory (DFT) calculations to gain insight into the details of the molecular packing of p-phenylene terephthalamides (PPTA). On the basis of the four models discussed thus far in the literature, we create a family of 16 possible structures. Calculations relate 1H and 13C chemical shifts obtained from experiments to structural aspects. Nucleus independent chemical shift (NICS) calculations show that ring currents and σ–π interactions as well as hydrogen bonding influence the chemical shifts on the rings. We obtain an unambiguous assignment, which differs from the literature data for carbon, for all resonances relating to the repeating unit of PPTA and obtain new insights into the possible packings of the PPTA units within the unit cell.