Molecular Order in the Nematic Melt of a Semiflexible Polyether by Deuteron NMR

Abstract

The conformations and orientational order of decamethylene spacer units in the nematic melt of a semiflexible polyether, based on 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)butane and 1,10-dibromodecane-d₂₀, have been studied by deuteron NMR methods. The quadrupole echo spectrum obtained immediately after rapid cooling to the nematic melt phase around 20 K below the isotropic-nematic transition yields a motionally averaged Pake pattern scaled by a factor of 1/3 relative to that obtained at low temperature in the rigid solid. This result indicates that all the C-D vectors of the decamethylene units, while undergoing rapid reorientational motions in this nematic melt, always maintain the same orientational angle of ca. 70° to the local nematic director axis. This condition is satisfied by a model where the decamethylene spacers adopt highly extended alternate trans conformations with nearly perfect orientational order with respect to the local nematic director. Slow cooling through the isotropic-nematic phase transition results in an oriented sample and its spectrum is characterized by an asymmetrically broadened doublet at all temperatures, exhibiting the same quadrupolar splitting for all the five distinct ²Hp 's. The magnitude of the splitting increases with increasing supercooling and reaches a maximum of about 85 kHz while still in the fast motional limit. These deutron NMR patterns for the oriented samples indicate that the alternate trans conformational order remains nearly unchanged at all temperatures, but the orientational order continues to increase with increasing supercooling and reaches a nearly perfect order at ΔT ∼ 20 K. These results are in good agreement with recent theoretical predictions (D. Y. Yoon and P. J. Flory, MRS Symp. Proc. 134, 11 (1989)) that the polymer chains adopt nearly fully extended conformations in the nematic melt. The asymmetric broadening of the doublet can be explained by assuming slow fluctuations and the resulting distribution in local nematic director orientations with respect to the magnetic field direction.