Anisotropic Diffusion in a Nematic Liquid Crystal— An Electric Field PFG NMR Approach

Holstein, Peter; Bender, Michael L.; Galvosas, Petrik; Geschke, D.; Kärger, Jörg

doi:10.1006/jmre.2000.2028

Cited by 15 publications

(14 citation statements)

References 8 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Self-diffusion studies by NMR have been performed by a number of groups. [61][62][63][64] In the isotropic phase, Dvinskikh and Furo report Arrhenius-type behavior with an activation energy of ∼33 kJ/mol. 61 Slightly above T NI , the isotropic self-diffusion constant was found to be approximately 5.3 × 10 −10 m 2 /s, roughly 1 ns to diffuse 1 Å.…”

Section: The Nature Of Spectral Diffusionmentioning

confidence: 99%

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

Sokolowsky

Bailey

Fayer

2014

The Journal of Chemical Physics

View full text Add to dashboard Cite

The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.

show abstract

Section: The Nature Of Spectral Diffusionmentioning

confidence: 99%

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

Sokolowsky

Bailey

Fayer

2014

The Journal of Chemical Physics

View full text Add to dashboard Cite

show abstract

“…Several modifications of the PFG NMR method including nematic and solid echoes [15,21], dipolar decoupling [15,21,22], magicangle sample orientation [15,23], and a combination of multipulse dipolar decoupling in addition to the stimulated echo [17,24] were suggested to overcome the above-mentioned difficulties. The most accurate value of the diffusion coefficient in liquid crystals was obtained by using a gradient slice selection, stimulated echoes and a multipulse homonuclear decoupling [16,17,25].…”

Section: Introductionmentioning

confidence: 98%

Diffusion studies in confined nematic liquid crystals by MAS PFG NMR

Romanova

Grinberg

Pampel

et al. 2009

Journal of Magnetic Resonance

Self Cite

View full text Add to dashboard Cite

“…With this in mind, we're preparing an experimental setup that exploits an electric field as orientation means of the liquid crystal that will allow the determination of DðhÞ at different h angles in the nematic phase also. A similar approach has been already used by Holstein et al, to orient the 5CB nematic phase at the magic angle in respect to the static magnetic field [26]. In that case however the work was limited to determine the Dð54:7 Þ coefficient in the nematic phase at one temperature.…”

Section: Discussionmentioning

confidence: 89%

Translational Self-Diffusion Measurements and Analysis of a Calamitic Smectogen

Cifelli

2007

Molecular Crystals and Liquid Crystals

View full text Add to dashboard Cite

Translational self-diffusion measurements have been performed in the mesophases of the p-hexyloxybenzylidene-p'-fluoroaniline (FAB.OC6) liquid crystal, by means of a static fringe field gradient approach. The method provides an accurate enough data set of the diffusion tensor as a function of temperature in the nematic and smectic A phase to be used for accurate analysis exploiting well established selfdiffusion models. Also measurements in the smectic B phase have been carried out and the results obtained seem to be consistent with a picture where the out of plane diffusion is strongly inhibited ð< 10 À15 m 2 =sÞ and only a slow in plane diffusion ð$ 10 À13 m 2 =sÞ is present.

show abstract

Anisotropic Diffusion in a Nematic Liquid Crystal— An Electric Field PFG NMR Approach

Cited by 15 publications

References 8 publications

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

Diffusion studies in confined nematic liquid crystals by MAS PFG NMR

Translational Self-Diffusion Measurements and Analysis of a Calamitic Smectogen

Contact Info

Product

Resources

About