Abstract
The use of two-dimensional isotropic-anisotropic correlation spectroscopy for the analysis of orientational alignment in solids is presented. The theoretical background and advantages of this natural-abundance 13C NMR method of measurement are discussed, and demonstrated with a series of powder and single-crystal variable-angle correlation spectroscopy (VACSY) experiments on model systems. The technique is subsequently employed to analyze the orientational distributions of three polymer fibers: Kevlar ® 29, Kevlar ® 49 and Kevlar ® 149. Using complementary two-dimensional NMR data recorded on synthetic samples of poly( p-phenyleneterephthalamide), the precursor of Kevlar ®, it was found that these commercial fibers possess molecules distributed over a very narrow orientational range with respect to the macroscopic director. The widths measured for these director distribution arrangements were (12 ± 1.5) ° for Kevlar ® 29, (15 ± 1.5) ° for Kevlar ® 49, and (8 ± 1.5) ° for Kevlar ® 149. These figures compare well with previous results obtained for non-commercial fiber samples derived from the same polymer.
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