MOLECULAR ORDER IN THE NEMATIC MELT OF A SEMIFLEXIBLE POLYETHER BY DEUTERON NMR

The conformations and orientational order of decamethylene spacer unit s in the nematic melt of a semiflexible polyether, based on -(4-hydrox y-4'-biphenyl)-2-(4-hydroxyphenyl)butane and 1,10-dibromodecane-d(20), have been studied by deuteron NMR methods. The quadrupole echo spectr um obtained immediately after rapid cooling to the nematic melt phase around 20 K below the isotropic-nematic transition yields a motionally averaged Fake pattern scaled by a factor of 1/3 relative to that obta ined at low temperature in the rigid solid. This result indicates that all the C-D vectors of the decamethylene units, while undergoing rapi d reorientational motions in this nematic melt, always maintain the sa me orientational angle of ca. 70 degrees to the local nematic director axis. This condition is satisfied by a model where the decamethylene spacers adopt highly extended alternate trans conformations with nearl y perfect orientational order with respect to the local nematic direct or. Slow cooling through the isotropic-nematic phase transition result s in an oriented sample and its spectrum is characterized by an asymme trically broadened doublet at all temperatures, exhibiting the same qu adrupolar splitting for all the five distinct H-2 's. The magnitude of the splitting increases with increasing supercooling and reaches a ma ximum of about 85 kHz while still in the fast motional limit. These de utron NMR patterns for the oriented samples indicate that the alternat e trans conformational order remains nearly unchanged at all temperatu res, but the orientational order continues to increase with increasing supercooling and reaches a nearly perfect order at Delta T similar to 20 K, These results are in good agreement with recent theoretical pre dictions (D. Y. Yoon and P. J. Flory, MRS Symp. Proc. 134, 11 (1989)) that the polymer chains adopt nearly fully extended conformations in t he nematic melt. The asymmetric broadening of the doubler can be expla ined by assuming slow fluctuations and the resulting distribution in l ocal nematic director orientations with respect to the magnetic field direction.