A mode coupling theory description of the short- and long-time dynamics of nematogens in the isotropic phase

Li, Jie; Cang, Hu; Andersen, Hans Christian; Fayer, M. D.

doi:10.1063/1.2145679

Cited by 38 publications

(93 citation statements)

References 36 publications

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“…For nematic liquid crystals, a modified MCT schematic model was used to describe the orientational dynamics of nematogens in the isotropic phase. 49 κ was found to increase with decreasing temperature and diverged at the nematic-isotropic phase transition. For organic van der Waals liquids, the increase in κ is responsible for the appearance of the boson peak as the temperature approaches and drops below T c .…”

Section: Resultsmentioning

confidence: 96%

Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis

et al. 2006

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Optically heterodyne-detected optical Kerr effect experiments are applied to study the orientational dynamics of the supercooled ionic organic liquids N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) and 1-ethyl-3-methylimidazolium tosylate (EMImTOS). The orientational dynamics are complex with relaxation involving several power law decays followed by a final exponential decay. A mode coupling theory (MCT) schematic model, the Sjögren model, was able to reproduce the PMPIm data very successfully over a wide range of times from 1 ps to hundreds of ns for all temperatures studied. Over the temperature range from room temperature down to the critical temperature T c of 231 K, the OHD-OKE signal of PMPIm is characterized by the intermediate power law t -1.00(0.04 at short times, a von Schweidler power law t -0.51(0.03 at intermediate times, and a highly temperature-dependent exponential (R relaxation) at long times. This form of the decay is identical to the form observed previously for a large number of organic van der Waals liquids. MCT analysis indicates that the theory can explain the experimental data very well for a range of temperatures above T c , but as might be expected, there are some deviations from the theoretical modeling at temperatures close to T c . For EMImTOS, the orientational dynamics were studied on the ps time scale in the deeply supercooled region near its glass transition temperature. The orientational relaxation of EMImTOS clearly displays the feature associated with the boson peak at ∼2 ps, which is the first time domain evidence of the boson peak in ionic organic liquids. Overall, all the dynamical features observed earlier for organic van der Waals liquids using the same experimental technique are also observed for organic ionic liquids.

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Section: Resultsmentioning

confidence: 96%

Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis

et al. 2006

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“…The analogy between the relaxation in supercooled liquids and that in the isotropic phase near the I-N phase transition has been discussed only in recent literature [7,11,33]. Even the present simple model shows dynamic signatures which are well-known in supercooled liquids.…”

Section: Discussionmentioning

confidence: 99%

Power law relaxation and glassy dynamics in Lebwohl-Lasher model near the isotropic-nematic phase transition

2006

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Orientational dynamics in a liquid crystalline system near the isotropic-nematic (I-N) phase transition is studied using Molecular Dynamics simulations of the well-known Lebwohl-Lasher (LL) model. As the I-N transition temperature is approached from the isotropic side, we find that the decay of the orientational time correlation functions (OTCF) slows down noticeably, giving rise to a power law decay at intermediate timescales. The angular velocity time correlation function also exhibits a rather pronounced power law decay near the I-N boundary. In the mean squared angular displacement at comparable timescales, we observe the emergence of a subdiffusive regime which is followed by a superdiffusive regime before the onset of the long-time diffusive behavior. We observe signature of dynamical heterogeneity through pronounced non-Gaussian behavior in orientational motion particularly at lower temperatures. This behavior closely resembles what is usually observed in supercooled liquids. We obtain the free energy as a function of orientational order parameter by the use of transition matrix Monte Carlo method. The free energy surface is flat for the system considered here and the barrier between isotropic and nematic phases is vanishingly small for this weakly first-order phase transition, hence allowing large scale, collective and correlated orientational density fluctuations. This might be responsible for the observed power law decay of the OTCFs.

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“…The endpoint of the fit varied from 2.9 × 10 5 ps for samples at 309.7 K to 7.5 × 10 3 ps at 349.2 K. Like previous experiments, the exponents of the power laws were found to be temperature independent. 17,18,20 The data were fit again holding these parameters constant at their average, and the quality of the fits did not change, but the number of adjustable parameters and the error bars on the exponential decay times were reduced.…”

Section: B Optical Heterodyne Detected-optical Kerr Effect Spectroscopymentioning

confidence: 99%

“…Numerous techniques including dynamic light scattering 8,10,15,16 and optical heterodyne detected-optical Kerr effect (OHD-OKE) spectroscopy 10,11,13,14,17,18 can measure the orientational relaxation associated with the bulk nematogen liquid in the isotropic phase. To determine the influence of orientational correlation on molecules within a pseudonematic domain, alternate techniques are required.…”

Section: Introductionmentioning

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

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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.

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A mode coupling theory description of the short- and long-time dynamics of nematogens in the isotropic phase

Cited by 38 publications

References 36 publications

Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis

Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis

Power law relaxation and glassy dynamics in Lebwohl-Lasher model near the isotropic-nematic phase transition

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

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