Molecular Organization of Poly(p-phenyleneterephthalamide) in Sulfuric Acid: An NMR Study

Abstract

Solutions prepared by dissolving synthetic poly(p-phenyleneterephthalamide) (PPTA) in 99.8% H2SO4 were analyzed using natural abundance NMR methods as a function of the polymer concentration, molecular weight, and temperature. Concentration and molecular weight-driven transitions between isotropic, nematic, and solid-like phases could be clearly distinguished from the 13C NMR spectra of the solute and from 1H NMR spectra of the solvent. The 13C solute NMR spectra point toward a distribution in the order parameter of the liquid crystalline director and could be quantitatively reproduced using 13C shielding tensor elements measured by solid NMR in polycrystalline PPTA. Thermodynamic parameters for the nematic ⇄ isotropic equilibrium were obtained from the temperature dependence of the liquid crystalline 13C NMR spectra, and 2D NMR methods were employed to retrieve information about the kinetics of PPTA and H2SO4 migration between isotropic and nematic domains. The results obtained from these spectroscopic s...