Liquid crystals dielectric relaxation

The dramatic slowing down of molecular motions is seen explicitly in a vast area of different probes of liquid local structures. Slow motion is evident in viscosity, dielectric relaxation, frequency-dependent ionic conductance, and in the speed of crystallization itself. In all cases, the temperature dependence of the generic relaxation time obeys to a reasonable, but not perfect, approximation the empirical Vogel-Fulcher law ... 

Existence of a high degree of orientational freedom is the most characteristic feature of the plastic crystalline state. We can visualize three types of rotational motions in crystals free rotation, rotational diffusion and jump reorientation. Free rotation is possible when interactions are weak, and this situation would not be applicable to plastic crystals. In classical rotational diffusion (proposed by Debye to explain dielectric relaxation in liquids), orientational motion of molecules is expected to follow a diffusion equation described by an Einstein-type relation. This type of diffusion is not known to be applicable to plastic crystals. What would be more appropriate to consider in the case of plastic crystals is collision-interrupted molecular rotation. 

Liquids are difficult to model because, on the one hand, many-body interactions are complicated on the other hand, liquids lack the symmetry of crystals which makes many-body systems tractable [364, 376, 94]. No rigorous solutions currently exist for the many-body problem of the liquid state. Yet the molecular properties of liquids are important for example, most chemistry involves solutions of one kind or another. Significant advances have recently been made through the use of spectroscopy (i.e., infrared, Raman, neutron scattering, nuclear magnetic resonance, dielectric relaxation, etc.) and associated time correlation functions of molecular properties. 

Our calculations show that to achieve good accuracy with Eqs. (4.233) and (4.234) in a wide range of temperature and frequency variations, it is necessary to retain at least five (odd k= 1, 3,..., 9) lower modes of the spectrum. We remark that the first three relaxational modes have once been evaluated both numerically [109] and analytically [82] in studies of dielectric relaxation in nematic liquid crystals, where the forms of the potential and of the basic equation coincide with those given by our Eqs. (4.224) and (4.225), respectively. 

A measurement of the Kerr relaxation times in succinoni-trile(SN)as a function of temperature is shown in Fig. 2. The Kerr relaxation times measured show the effect of temperature on the rotational motion of the SN molecules as they undergo a change from the liquid to the plastic crystal phase. The data obtained from the Kerr gate measurement is shown along with a best fit curve from depolarized Rayleigh scattering (dotted line), and a best fit curve from dielectric relaxation measure-... 

The VFT behavior of supercooled glycerol is well known from studies of liquid and supercooled glycerol [3,186-190], while the Arrhenius dependence of the dielectric relaxation time is more relevant for crystals. For example, the temperature dependence of the dielectric relaxation time of ice I also obey the Arrhenius law with the activation energy about 60 kJ moF1 [198,199]. 

Relaxation functions for fractal random walks are fundamental in the kinetics of complex systems such as liquid crystals, amorphous semiconductors and polymers, glass forming liquids, and so on [73]. Relaxation in these systems may deviate considerably from the exponential (Debye) pattern. An important task in dielectric relaxation of complex systems is to extend [74,75] the Debye theory of relaxation of polar molecules to fractional dynamics, so that empirical decay functions for example, the stretched exponential of Williams and Watts [76] may be justified in terms of continuous-time random walks. 

Pentyl-4 -cyanobiphenyl and 4-octyl-4 -cyanobiphenyl liquid crystals (LCs) confined in molecular sieves of MCM-41 and cloverite types are studied in a wide temperature range by dielectric spectroscopy, thermal analysis and in-situ FTIR spectroscopy. The phase transitions of the bulk LCs cannot be detected when confined in MCM-41 sieve. A relaxational process occurs due to the molecular motions in the layer at the pore walls the temperature dependence of the characteristic frequency obeys a Vogel-Fulcher-Tamman law associated to a glassy state. In the cloverite cages, the LCs keep the phase transitions of the bulk but shifted. Interactions between Lewis and Brdnsted sites and the LC molecules are monitored by IR spectroscopy. DTA measurements put also in evidence strong guest-host interactions. 

Dielectric relaxation and dielectric losses of pure liquids, ionic solutions, solids, polymers and colloids will be discussed. Effect of electrolytes, relaxation of defects within crystals lattices, adsorbed phases, interfacial relaxation, space charge polarization, and the Maxwell-Wagner effect will be analyzed. Next, a brief overview of... 

In equations (5)-(8), i is the molecule s moment of Inertia, v the flow velocity, K is the appropriate elastic constant, e the dielectric anisotropy, 8 is the angle between the optical field and the nematic liquid crystal director axis y the viscosity coefficient, the tensorial order parameter (for isotropic phase), the optical electric field, T the nematic-isotropic phase transition temperature, S the order parameter (for liquid-crystal phase), the thermal conductivity, a the absorption constant, pj the density, C the specific heat, B the bulk modulus, v, the velocity of sound, y the electrostrictive coefficient. Table 1 summarizes these optical nonlinearities, their magnitudes and typical relaxation time constants. Also included in Table 1 is the extraordinary large optical nonlinearity we recently observed in excited dye-molecules doped liquid... 

Carius Hans-Eckart, Schonhals Andreas, Guigner Delphine, Sterzynski Tomasz, Brostow Witold. (1996). Dielectric and Mechanical Relaxation in the Blends of a Polymer Liquid Crystal with Polycarbonate. Macwmolecules, 29(14), 5017-5025. 

Broadband dielectric spectroscopy enables one to analyse the dynamics of polar groups in polymeric systems. Due to its broad frequency range of more than 10 decades a manifold of different molecular fluctuations can be studied from the dynamic glass transition (spanning already more than 10 decades in times) to secondary relaxations. Additionally one finds in chiral liquid crystals cooperative processes like soft-and Goldstone modes. 

Dielectric relaxation spectroscopy is widely used to study molecular dynamics of conventional and liquid crystal polymers. Since the mesogenic groups of a side-chain liquid crystal polymer contain strong permanent dipoles, the technique may be utilized to study the reorientation and, as Attard et have shown, the level of... 

Dielectric relaxation spectroscopy can monitor molecular and collective modes for motion of liquid crystalline molecules. In ferrolectric liquid crystals based on chiral tilted smectic phases the complex dielectric permittivity e has, in addition to molecular orientational modes, two contributions from the director fluctuations. 

The glass transition (Tg) of the amorphous PVDF regions is in the range of -40 to -30°C, depending upon the sample and test method. Other sub-Tg transitions have been studied recently by dielectric relaxation spectroscopy (108). These studies also indicate correlations with other techniques and identify a 50°C molecular chain transition as probably related to the amorphous region at the surfaces of crystals (109). Permeation characteristics are very sensitive to these transitions as well as the usual environmental parameters (110). Water molecules trapped in the amorphous regions are monomeric, not associated and clustered as in the liquid state (111). 

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