ORIENTATIONAL ALIGNMENT IN SOLIDS FROM BIDIMENSIONAL ISOTROPIC-ANISOTROPIC NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY - APPLICATIONS TO THE ANALYSIS OF ARAMIDE FIBERS

The use of two-dimensional isotropic-anisotropic correlation spectrosc opy for the analysis of orientational alignment in solids is presented . The theoretical background and advantages of this natural-abundance C-13 NMR method of measurement are discussed, and demonstrated with a series of powder and single-crystal variable-angle correlation spectro scopy (VACSY) experiments on model systems. The technique is subsequen tly employed to analyze the orientational distributions of three polym er fibers: Kevlar(R) 29, Kevlar(R) 49 and Kevlar(R) 149. Using complem entary two-dimensional NMR data recorded on synthetic samples of poly( p-phenyleneterephthalamide), the precursor of Kevlar(R), it was found that these commercial fibers possess molecules distributed over a ver y narrow orientational range with respect to the macroscopic director. The widths measured for these director distribution arrangements were (12+/-1.5)degrees for Kevlar(R) 29, (15+/-1.5)degrees for Kevlar(R) 4 9, and (8+/-1.5)degrees for Kevlar(R) 149. These figures compare well with previous results obtained for non-commercial fiber samples derive d from the same polymer. (C) 1997 Elsevier Science B.V.