A variable-director C13 NMR analysis of lyotropic aramide solutions

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Abstract
The order and dynamics of two aromatic polyamides in their lyotropic phases were investigated with the aid of variable-director nuclear magnetic resonance (NMR). In these experiments polymers were dissolved in concentrated sulfuric acid and allowed to equilibrate inside the main NMR magnetic field B0 to yield macroscopically-aligned liquid crystalline solutions. These ordered fluids were then rotated away from equilibrium for brief periods of time, and their natural abundance C13 NMR spectra collected as a function of different angles between the liquid crystalline director and B0. The resulting spectra showed peaks shifting as well as broadening as a function of the director's orientation, variations that were also found to be concentration- and temperature-dependent. All such changes could be successfully accounted for on the basis of an exchange model involving molecular reorientations of the polymer chains that are occurring in the intermediate NMR time scale. Based on this assumption, the experimental line shapes could be used to extract a detailed description of the macromolecular order and dynamics in these fluids. The former appeared substantially high, and not very different from the one characterizing order in commercial extruded aramide fibers. The latter enabled an estimation of the hydrodynamic radii adopted by the macromolecules in their mesophases, which ended up in close agreement with dimensions recently reported on the basis of small-angle neutron scattering analyses.