Proton Spin-Lattice Relaxation in Liquid Crystals

Doane, J. William; Visintainer, J. J.

doi:10.1103/physrevlett.23.1421

Cited by 67 publications

(14 citation statements)

References 7 publications

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“…(6)(7)(8) The frequency dependence of the spin-lattice relaxation time T, given by (2) is found to be in good agreement with experiments, except a t very low frequencies, where this :formula cannot be valid because it gives infinitely high relaxation rates. On the contrary, the temperature dependence of T, predicted hy the expression above does not a t ell follow the experimental data.…”

Section: Introductionsupporting

confidence: 60%

Thermal Fluctuations and Proton Spin-lattice Relaxation in Nematic Liquid Crystals

Martins

1971

Molecular Crystals and Liquid Crystals

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Measurements of the spin-lattice relaxation time T , in both nematic and isotropic phases of normal (PAA) and methyl-deuterated (PAA-CD,) p-azoxyanisole are reported and discussed. For both materials, T , varies linearly with temperature in the isotropic phase, just above the transition. In order to explain this behaviour two processes are discussed: diffusion and thermal fluctuations in the local order parameter. It. is shown crudely that diffusion, in this region, is to be expected as a " correlated " process. Recent measurements of D are criticized withir! this context, but no definite conclusion can be drawn. A simple theoretical approach of the fluctuations in the order parameter is proposed which accounts for the observed behaviour of T , ( T ) . In the nematic phase of PAA-CD,, T , does not depend on temperature.Our results show the relative importance of short range phenomena and allow us to question the meaningfulness of earlier discussions of experimental data on the unique basis of Pincus's formula.The frequency dependence is the same as for PAA.

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Section: Introductionsupporting

confidence: 60%

Thermal Fluctuations and Proton Spin-lattice Relaxation in Nematic Liquid Crystals

Martins

1971

Molecular Crystals and Liquid Crystals

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“…Almost all early measurements believed to have found the crucial v1I2 regime at megahertz Lamor frequencies using standard N.M.R. spectrometers [16]. But it can easily be shown that a squareroot law fit over a narrow frequency range of 1 or 2 decades is almost meaningless due to the presence of other relaxation processes.…”

Section: T Relaxation Dispersion In Thermotropic and Lyotropic Liquimentioning

confidence: 97%

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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“…Accompanied with the crystal-nematic phase transition, T 1 decreased rapidly to about 10 ms. In the liquid crystal phase, T 1 decreased critically with increasing temperature and the [6,[18][19][20][21]. Fig.…”

Section: H Nmrmentioning

confidence: 94%

2H and 13C NMR studies of molecular orientation and dynamics in liquid crystal 6BA

Suzuki¹,

Uta²,

Ida³

et al. 2010

Journal of Physics and Chemistry of Solids

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Proton Spin-Lattice Relaxation in Liquid Crystals

Abstract: Measurements of spin-lattice relaxation in nematic and smectic liquid crystals are contrasted to recent theoretical predictions for T^ in these phases.

Cited by 67 publications

References 7 publications

Thermal Fluctuations and Proton Spin-lattice Relaxation in Nematic Liquid Crystals

Thermal Fluctuations and Proton Spin-lattice Relaxation in Nematic Liquid Crystals

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

2H and 13C NMR studies of molecular orientation and dynamics in liquid crystal 6BA

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