Order Determinations in Liquid Crystals by Dynamic Director NMR Spectroscopy

Autor: Veronica Frydman, Lucio Frydman, Min Zhou
Rok vydání: 1998
Předmět:
Zdroj: Journal of the American Chemical Society. 120:2178-2179
ISSN: 1520-5126
0002-7863
Popis: ReceiVed August 22, 1997 ReVised Manuscript ReceiVed December 8, 1997 Liquid-crystalline polymers (LCPs) constitute an important class of macromolecules with numerous applications as ultrastrong materials and nonlinear optical devices.1 A defining characteristic of these polymers is their ability to organize into aligned domains when molten or dissolved in suitable solvents and preserve this order upon freezing or coagulation. NMR can play an important role in the design of LCP-based materials by enabling the quantification of macromolecular order in their fluid phases.2 So far studies of this kind have involved deuterium,3 a quadrupolar probe which although sensitive demands site-specific isotope enrichment. Recently we have shown that order determinations in synthetic LCPs can also be reliably executed by analyzing the anisotropic displacements observed via natural abundance 13C NMR,4 a simple approach that provides simultaneous information for all inequivalent sites in the monomer. Unfortunately, as the chemical complexity of a polymer grows, such 13C NMR experiments become impractical due to the difficulty to ascribe each nematic resonance to a particular chemical site. The present Communication describes a new spectroscopic approach for bypassing these limitations, based on a combination of NMR measurements and discrete reorientations of the nematic director achieved by mechanical means. The type of spectral difficulties that may be encountered in natural abundance LCP studies are illustrated in Figure 1 on poly(pentamethylene-diphenoxyterephthalate) (1), a main-chain thermotropic polyester first synthesized by Lenz and co-workers that shares several features of interest with commercial analogues.5 13C resonances appearing in both the high-resolution solid state and the molten isotropic NMR spectra of 1 can be readily assigned to individual sites based on standard substituent chemical shifts. NMR spectra recorded between the melting and clearing points, however, present resonances significantly shifted from these δiso frequencies due to the onset of liquid crystallinity. Although these liquid crystal displacements ∆δ ) δlc δiso carry valuable information about the sites order parameters, the ambiguities that arise upon attempting to establish the chemical origin of the nematic peaks (and thus their ∆δ values) preclude their reliable use in order analyses. Assigning 13C NMR spectra for low molecular weight nematics becomes feasible with the aid of rapid sample spinning at an angle â, as this enables the discrimination of individual isotropic and anisotropic chemical shift contributions.6 Although reminiscent of coherent averaging, the goal of nematic variable-angle-spinning (VAS) is not modulating spin interactions but achieving a deviation between the orientations of the director D and the external field Bo. Once this is accomplished rapid rotational diffusion discriminates between anisotropic and isotropic interactions, scaling the former by P2(cosâ) ) (3cos2â-1)/2 while leaving the latter unaffected. Despite the success of VAS procedures toward analyses of low molecular weight nematics,6 their application to structural polymers such as 1 faces a number of complications. These include the limited range of â angles that can be explored by virtue of the D randomization usually occurring when â > 54.7° and of irradiation constrains associated to conventional VAS assemblies, the inherently large 13C line widths characterizing LCPs, the relatively extreme temperatures or pressures at which nematic phases arise for structural thermotropics, and the high viscosities of these fluids. In view of these complications, we decided to explore an alternative route capable of discriminating isoand anisotropic 13C shifts over an extended angular range while avoiding the spinning of the liquid crystal altogether. Our scheme exploits the relatively long director relaxation times and high viscosities of LCPs, to achieve a scaling of the spin anisotropies with the aid of discrete D reorientations in and out of its equilibrium position. Events in these experiments thus begin with an initial relaxation delay during which both spins and nematic are allowed to equilibrate in Bo; this is followed by an NMR pulse sequence applied in synchrony with well defined D reorientations implemented with the aid of a stepping motor and concluded with a return of the sample to its initial orientation in preparation for a new scan (Figure 2A). The relaxation time of the nematic imposes a limit to the duration that D may remain away from equilibrium, but for most synthetic LCPs as well as for numerous biological liquid crystals and monomeric smectics these times exceed 10-1 s and are consequently compatible with a majority of NMR pulse sequences. In fact for numerous LCPs these (1) (a) Samulski, E. T. Physics Today 1982, 35, 40. (b) Liquid Crystal Polymers; Gordon, M., Cantou, H., Eds.; 1984; Vol. 59-61. (c) Recent AdVances in Liquid Crystalline Polymers; Chapoy, L. L., Ed.; Elsevier: London, 1985. (d) Liquid Crystalline Polymer Systems; Isagev, A. I., Kyu, T., Cheng, S. Z. D., Eds.; American Chemical Society: Washington, 1996. (2) NMR of Liquid Crystals; Emsley, J. W., Ed.; Reidel Publishers: Dordrecht, 1985. (3) (a) Samulski, E. T.; Gauthier, M. M.; Blumstein, R. B.; Blumstein, A. Macromolecules 1984, 17, 479. (b) Mueller, K.; Hisgen, B.; Ringsdorf, H.; Lenz, R. W.; Kothe, G. Chapter 13 in ref 1c. (c) Abe, A.; Yamazaki, T. Macromolecules 1989, 22, 2138. (d) Fan, S. M.; Luckhurst, G. R.; Picken, S. J. J. Chem. Phys. 1994, 101, 3255. (4) (a) Zhou, M.; Frydman, V.; Frydman, L. J. Phys. Chem. 1996, 100, 19280. (b) Zhou, M.; Frydman, V.; Frydman, L. Macromolecules 1997, 30, 5416. (5) (a) Jin, J.-I.; Antoun, S.; Ober, C.; Lenz, R. W. British Polym. J. 1980, 132. (b) Ober, C. K.; Jin, J.-I.; Lenz, R. W. AdV. Polym. Sci. 1984, 59, 103. (6) (a) Emsley, J. W.; Lindon, J. C.; Luckhurst, G. R.; Shaw, D. Chem. Phys. Lett. 1973, 19, 345. (b) Courtieu, J.; Alderman, D. W.; Grant, D. M. J. Am. Chem. Soc. 1981, 103, 6783. (c) Courtieu, J.; Alderman, D. W.; Grant, D. M.; Bayle, J. P. J. Chem. Phys. 1982, 77, 723. (d) Courtieu, J.; Bayle, J. P.; Fung, B. M. Progress NMR Spectrosc. 1994, 26, 141. Figure 1. 13C NMR spectra recorded for 1 in the solid phase (A), in its isotropic melt (B), and as a nematic (C). The polyester was prepared by condensing under N2 4,4′-dihydroxy-1,6-diphenoxypentane with terephthaloyl chloride in tetrachloroethane/pyridine5a and characterized by elemental analysis, polarized microscopy, and DSC. NMR measurements were carried out at 7.1 T using a laboratory-built spectrometer, a dynamicangle-spinning probe built around a 5 mm Doty stator for the solid experiments, and a high-temperature fixed-solenoid probe incorporating a sample container coupled to a stepping motor for the fluid state acquisitions. The solid measurements employed cross polarization, dipolar decoupling (70 kHz), and sample spinning at 7 kHz; fluid spectra were collected using 5 μs π/2 excitations, NOE, and WALTZ-16 decoupling. Differences between the intensities of protonated and nonprotonated 13C resonances in the solid and fluid phases originate from distortions introduced by the NOE. 2178 J. Am. Chem. Soc. 1998, 120, 2178-2179
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