NMR Study of Nematic Order of Semiflexible Thermotropic Polymers

Abstract

Chain conformations and orientational order parameters in the nematic state of the deuterium-labeled thermotropic polyester (C10 D20OC6H4COOC6H46C10D20OC6H4OOCC6H4O)x are determined by deuterium (D) NMR and H NMR methods. The D NMR spectra show one quadrupole splitting in the nematic state, indicating that only a specific set of chain conformations are admitted for C10D20 spacer groups. They are found to comprise those conformers that place every second bond starting from the OCD2 attached to the rigid (phenylene) unit almost exclusively in the trans state. From the magnitudes of quadrupole splittings, the orientational order parameters of chain segments, the alignment axis of which virtually coincides with the axis of rigid (phenylene) units for these conformers, are found to fall around 0.8 and extrapolate to ca. 0.75 at the nematic-isotropic transition. Separate determination of orientational order parameters of the rigid aromatic units from H NMR dipolar splittings yields the order parameters in excellent agreement with those derived from D NMR results, and thus validates our interpretation of D NMR spectra in terms of the specific conformational order. The conformational order thus elucidated by NMR study therefore supports the previous conclusion of Yoon and Bruckner, who proposed that the poly (methylene) spacers assume highly extended configurations in the nematic state upon considering the enthalpies and the entropies of isotropic-nematic transitions of thermotropic polymers comprising rigid groups connected by poly(methylenes). © 1985, American Chemical Society. All rights reserved.