Investigation of Molecular Motions in Smectic A and C TBBA by NMR Relaxation Dispersion

Mugele, Th.; Gräf, Volker; Wölfel, W.; Noack, F.

doi:10.1007/978-3-642-67848-6_21

Cited by 4 publications

(3 citation statements)

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“…4 This assertion was generally checked to be consistent with our results. By translational diffusion mechanism, or self diffusion, we mean the modulation of the dipole-dipole interaction between protons located on different molecules, as a consequence of the particle motion, usually thermally activated, from one site to another.…”

Section: Relaxation Modelssupporting

confidence: 88%

Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication

Bonetto

Anoardo

2004

The Journal of Chemical Physics

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Proton field-cycling nuclear magnetic resonance relaxometry is used to study the spin-lattice relaxation dispersion of selected standard smectic A liquid crystals at different temperatures. Relaxation features at both, in the presence and absence of a monochromatic ultrasonic field are considered. We show that the laboratory-frame spin-lattice relaxation time is mainly governed by translational diffusion. Order director fluctuations (ODF) are less important while rotational diffusion seems to be only relevant near the clearing point. Our study suggests that sonication enhances the ODF contribution in the SmA mesophase. Within the framework of the approach we have outlined, different features associated with the ODF mechanism can be investigated.

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Section: Relaxation Modelssupporting

confidence: 88%

Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication

Bonetto

Anoardo

2004

The Journal of Chemical Physics

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“…26 The relaxation in the tilted smectic phases, composed of nonchiral molecules, has been up to now only marginally included in the papers describing the relaxation in liquid crystals. 27,33,34 No definite conclusion on the frequency dependence of T 1 Ϫ1 in the kHz regime has been given. In some cases it was ascribed to nematiclike director fluctuations and in others to undulations as in the smectic A phase.…”

Section: Introductionmentioning

confidence: 99%

Molecular dynamics in tilted bilayer smectic phases: A proton nuclear magnetic resonance relaxometry study

Carvalho

Sebastião²,

Ribeiro³

et al. 2001

The Journal of Chemical Physics

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A proton nuclear magnetic resonance ͑NMR͒ relaxation study of molecular dynamics in the liquid crystal 4-octylphenyl 2-chloro-4-͑4-cyanobenzoyloxy͒benzoate ͑DB 8 Cl͒ is presented. DB 8 Cl molecules possess a strong polar terminal group and form, in addition to the nematic phase, three different smectic phases: bilayer smectic A, bilayer smectic C, and anticliniclike smectic C phase. The proton spin-lattice relaxation times were measured in all mesophases over a broad frequency range of six decades by applying conventional and fast field-cycling NMR techniques. The parameters obtained in the analysis of the experimental data give quantitative information on molecular motions, particularly for the tilted smectic phases of DB 8 Cl. In contrast to former conjectures, we found that the low-frequency relaxation in the bilayer smectic C phases results from director fluctuations about the layer normal, which occur without distortion of the layers, and from layer undulations, similar to those in the smectic A phase. In the low-temperature bilayer smectic C phase, a considerable slowing-down of molecular translational diffusion is observed. It confirms indirectly the anticlinic character of this mesophase. Measurements of angular dependence of the relaxation times at 60 MHz support the conclusions obtained from the frequency dispersion data.

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“…Larmor frequency dependence of the proton relaxation time T, in nematic and smectic-A TBBA[23]. Top: nematic phase with square-root dispersion profile according to equation (1 a).…”

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confidence: 99%

Relaxation dispersion and zero-field spectroscopy of thermotropic and lyotropic liquid crystals by fast field-cycling N.M.R.

1988

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Systematic field-cycling measurements of the TI relaxation dispersion in numerous nematic liquid crystals (azoxybenzenes, Schiff's bases, biphenyls, phenyl-cyclohexanes, cyclo-cyclo-hexanes) confirm our previous observations obtained for PAA and MBBA that order fluctuations of the nematic director are a significant relaxation contribution only at low Lamor frequencies v, i.e. far below the usual megahertz range. Their significance is demonstrated most convincingly by the characteristic square-root dispersion law, TI -v''~, which occurs in the kilohertz range and which completely disappears above the nematic-isotropic phase transition. The strength of the collective relaxation mechanism varies by more than two orders of magnitude in the sequence (selection) PAA-d,, PAA, PAA-d,, PAB, OCB7, MBBA, CB7, PCH7, MBBA-d6, MBBA-dl, and CCH7. This finding can be understood almost quantitatively by the widely differing separations and orientations of proton pairs on the molecules, together with the different viscoelastic parameters of the nematogens. In addition, the underlying slow molecular reorientations have been observed in MBBA and PAA by intensity changes of the zero-field spectra, which are absent for high-field measurements. Similarly, smectic type order fluctuations in layered liquid crystal structures prove to be an effective relaxation mechanism only at low Lamor frequencies. This has been verified by the related linear relaxation dispersion profile, TI -v', for both thermotropic systems (TBBA, CI2-AA) and lamellar lyotropic mixtures (e.g. potassium laurate in water and phospholipids in water). Our results concerning the time scale of the TI -vli2 and TI -vl regime do not agree with conclusions drawn from conventional high-field techniques. Introduction: Why field-cycling measurements of order and order fluctuations in liquid crystals? Measurements of the molecular order in liquid crystals by the line splittings of proton, deuteron or carbon N.M.R. spectra are now a standard procedure. The basic idea of this analysis is well-known since the early sixties from the pioneering work of Lippmann et al. [l] and of Saupe et al. [2]. However looking closer at the details of the ordering still reveals many unsolved questions, in particular with respect to the time scale or fluctuation of the order; in other words details of the related collective molecular reorientations are rather unclear. In 1969 Pincus [3] and Blinc et al. [4] predicted independently that a special kind of order fluctuation, now called order fluctuation of the nematic director (OFD or OFN), should lead to a characteristic square-root dependence of the longitudinal proton relaxation time TI on the Larmor frequency v (= 4 2~) .The various modes of the fluctuation with a distribution of wave-vectors involve a broad spectrum of

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Investigation of Molecular Motions in Smectic A and C TBBA by NMR Relaxation Dispersion

Cited by 4 publications

References 1 publication

Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication

Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication

Molecular dynamics in tilted bilayer smectic phases: A proton nuclear magnetic resonance relaxometry study

Relaxation dispersion and zero-field spectroscopy of thermotropic and lyotropic liquid crystals by fast field-cycling N.M.R.

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