Liquid-crystal spinning method

Chachaty and co-workers [8.20, 8.37, 8.38] were first to describe correlated internal motions in alkyl chains of surfactant molecules that form lyotropic liquid crystals. The last section described an extension of the master equation method of Wittebort and Szabo [8.4] to treat spin relaxation of deuterons on a chain undergoing trans-gauche jump rotations in liquid crystals. This method was also followed by Chachaty et al. to deal with spin relaxation of nuclei in surfactants. However, they assumed that the conformational changes occur by trans-gauche isomerization about one bond at a time. In their spectral density calculations (see Section 8.3.1), they used a transition rate matrix that was constructed from the jump rate Wi, W2, and Ws about each bond. Since W3 is much smaller than Wi and W2, the time scale of internal motions was practically governed by Wi and W2 of each C-C bond. Since... 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

Table 4-3 lists various recoupling techniques, which have been applied to liquid crystals. The methods are eompared in terms of heteronuclear dipolar scaling factor, y-encoding property, sensitivity to frequency offsets and chemical shift anisotropy of S (rare) and I (abundant) spins, tolerance to rf inhomogeneity, accomplishment of active homonuclear proton decoupling, length of rf cycle time (which limits the maximum spectral width in dipolar dimension), and requirement of rotor... 

Recent solid state NMR studies of liquid crystalline materials are surveyed. The review deals first with some background information in order to facilitate discussions on various NMR (13C, ll, 21 , I9F etc.) works to be followed. This includes the following spin Hamiltonians, spin relaxation theory, and a survey of recent solid state NMR methods (mainly 13C) for liquid crystals on the one hand, while on the other hand molecular ordering of mesogens and motional models for liquid crystals. NMR studies done since 1997 on both solutes and solvent molecules are discussed. For the latter, thermotropic and lyotropic liquid crystals are included with an emphasis on newly discovered liquid crystalline materials. For the solute studies, both small molecules and weakly ordered biomolecules are briefly surveyed. 

A still unresolved question is whether the anisotropies observed in the nematic phase are equivalent to those in the gas, liquid, or solid states. For acetylene the spin-rotation measurements42 along withab initio calculations of ad (B. R. Appleman and C. W. Kern, unpublished data) yield a value for Ao of 18 1 ppm that is not quite within the experimental uncertainty of the liquid crystal value. It still appears, however, that, for small or moderate-sized proton anisotropies, the large uncertainties associated with the use of liquid crystal measurements make this method one of doubtful reliability. 

The liquid crystallinity of poly(3-alkylthiophene) has been pointed out [121]. Enhancement of crystallinity by molecular alignment in a liquid crystal phase is prominent in poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b] thiophene) exhibiting a lamellar mesophase. When a thin film of the polymer fabricated by the spin-coating method is annealed at 100 °C, domain size remarkably extends from several tens of nanometers to a several micrometers, maintaining molecular alignment within domains because of the thermal motion of the polymer chains. The hole mobility of the thin film transistors based on this polymer reached 0.7 cm2 V-1 s 1 [122]. 

Jeener and Broekaert introduced, in 1%7, a three-pulse B,(r) sequence to measure the relaxation time Tm of the dipolar order of / = 1 spin systems in the presence of a conventional high Zeeman field, Bq, which is based on the decay time of the so-called Jeener echo . It was later extended by Spiess and Kemp-Harper and Wimperis to study in a similar way the quadrupolar order in / a 1 systems. The appearance of a Jeener echo depends upon the existence of interactions that are not averaged out by molecular motions on the considered time scale. The method has become of great importance in recent relaxation studies, in particular of liquid crystals because, in standard spin relaxation theories, it provides a power l means to separate and analyse the spectral densities / v) and /2) j. i4,is,2025 ggg... 

From the positions and the number of lines observed in the spectra of solutes dissolved in a liquid-crystal solvent it is possible to determine their bond angles, relative bond lengths, and the signs of spin-spin coupling constants. In some cases chemical shielding anisotropies may be determined, and the method appears to offer another technique for investigating solvent-solute interactions. Several reviews of the area have now been published. " ... 

Locci et al. describe a method to use xenon to monitor chemical transformations, which they describe as the Spin-Spy methodology. Xenon was added to a solution of a- and p-D-glucose and the change in concentration of these speeies was monitored via the Xe chemical shift as equilibrium concentrations of the interconverting sugars were reached over a period of 300 min. The xenon chemieal shift difference in a 1 M concentration of these two species is approximately 1.3 ppm. Xenon s interaction with liquid crystal environments has also been reported over the past several years." This area was reviewed extensively by Jokisaari in 1994. 

Kevlar fibers [22] were the first application of liquid crystals in polymers. A special technique had to be developed spin fibers because they were too stiff to be processed by the known fiber spinning methods. The straight stiff nature of these chains however confers exceptional strength on the final Kevlar fiber. 

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