Liquid crystals dielectric spectroscopy

The extension of continuum models to complex environments is further analyzed by Ferrarini and Corni Frediani, respectively. In the first contribution the use of PCM models in anisotropic dielectric media such as liquid crystals is presented in relation to the calculation of response properties and spectroscopies. In the second contribution, PCM formulations to account for gas-liquid or liquid-liquid interfaces, as well for the presence of a meso- or nano-scopic metal body, are presented. In the case of molecular systems close to metal bodies, particular attention is devoted to the description of the surface enhanced effects on their spectroscopic properties. 

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. 

Additional information on ferroelectric and electroclinic switching can be obtained with broadband dielectric spectroscopy. It appears that the molecular dynamics of FLCPs are comparable to those of low molecular weight compounds [67]. However, the experimental observations are made more difficult for FLCPs than for low molecular weight SmC liquid crystals due to the conductivity contribution which takes place at frequencies below 10" Hz and to the difficulty to get a macroscopically well-aligned sample. 

P. Ben Ishai, D. Libster, A. Aserin, N. Garti, Y. Feldman, Molecular interactions in lyotropic reverse hexagonal liquid crystals a dielectric spectroscopy study. J. Phys. Chem. B 113, 12639-12647 (2009)... 

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. 

Kremer, E. (1997), Broadband dielectric spectroscopy on collective and molecular dynamics in ferroelectric liquid crystals, in Dielectric Spectroscopy of Polymeric Materials—Eundamental and Applications, Runt, J. P. and Eitzgerald, J. J., eds., ACS, Washington, DC, pp. 423 44. 

Mellinger A (2003) Dielectric resonance spectroscopy a versatile tool in the quest for better piezoelectric polymers. IEEE Trans Dielectr Electr Insul 10 842-861 Meyer RB (1969) Piezoelectric effects in liquid crystals. Phys Rev Lett 22 918-921 Newnham RE (2005) Properties of materials anisotropy, symmetry, stmeture. Oxford University Press, Oxford/New York... 

The polarization reversal measurement is a large signal method requiring full switching of the liquid crystal, and therefore cannot be expected to coincide with the much more precise results from the low signal dielectric relaxation spectroscopy measurements. Basically, a Yq> value from the polarization reversal technique involves spurious contributions of elastic effects due to the... 

It may be mentioned that deuterium NMR and dielectric spectroscopy studies [24, 25] on columnar liquid crystals indicate the existence of rotational motions of the molecules or groups of molecules about the column axes. 

Lucchetta DE, Criante L, Francescangeli O, Simraii E (2004) light amplification by dye-doped holographic polymer dispersed liquid crystals. Appl Phys Lett 84 4893-4895 Lvovich VF (2012) bnpedance spectroscopy applications to electrochcamcal and dielectric phenomena. Wiley, New Jersey... 

Schlosser E, Schonhals A (1989) Recent development in dielectric relaxation spectroscopy of polymer. Colloid Polym Sci 267 963-969 Shanks lA (1977) Liquid crystal devices. US Pateait 4,048,358... 

Shenhar R, Norsten TB, Rotello VM (2005) Polymer-mediated nanoparticle assembly structural control and applications. Adv Mater 17(6) 657-669 Sinha G, Glorieux C, Thoen J (2004) Broadband dielectric spectroscopy study of molecular dynamics in the glass-forming liquid crystal isopentylcyanobiphenyl dispersed with aerosils. Phys Rev E 69 031707-031712... 

The orientational dynamics of liquid crystals in the nematic phase has been widely investigated using NMR and dielectric relaxation spectroscopy. Here, we briefly discuss the applications of the two techniques and present the experimental data concerning a few liquid crystals. 

It is evident from these studies that dielectric relaxation spectroscopy provides a direct and unambiguous method for studying the molecular dynamics and alignnient properties of liquid crystalline side chain polymers. It has the distinct advantage over the NMR, ESR and DSC methods that a wide frequency range (10 to 10 Hz) can be covered at each temperature in the liquid crystal, biphasic and isotropic ranges of these anisotropic materials. 

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