2-DIMENSIONAL NMR-STUDY OF SLOW PHENYL-FLIPS IN LIQUID-CRYSTALLINE SIDE GROUP POLYMERS

Glass-forming liquid-crystalline group polymers were investigated by H -2 2D exchange NMR. In oriented systems, the exchange pattern of a sin gle 2D spectrum yields the orientational distribution function with at least as high precision as a full rotation pattern of 1D spectra. In addition, important information about the geometry of the phenyl-flip motion is obtained. The 2D lineshapes are particularly sensitive towar ds possible slow motion of the flip axis which could be envisaged to f luctuate within the orientational distribution of the mesogenic group. However, the experimental spectra for a frozen nematic and a frozen s mectic polysiloxane with an orientational distribution of width +/- 18 .5-degrees and +/- 12.5-degrees, respectively show no indication of th e corresponding features, proving that the flip axis is fixed within /- 3-degrees even after a large number of flips.