Broadband dielectric spectroscopy of a nematic liquid crystal in benzene

Abstract

Broadband dielectric spectroscopy has been used to analyze the temperature, frequency, and concentration dependences of the molecular dynamics of a nematic liquid crystal (5CB) mixed with the nonpolar solvent benzene. Differential scanning calorimetry measurement has been also performed to confirm the phase transitions of 5CB/benzene mixtures. The phase transition temperatures (crystalline to isotropic phases) thus obtained have been described very accurately from the temperature-dependent relaxation strength, the relaxation time, and the symmetric shape parameter of the relaxation function obtained from the fitting procedure. Two relaxation processes reflecting overall rotations around the short and long molecular axes are observed in both the nematic and isotropic phases. In the crystalline phase, the former process with the longer relaxation time disappeared, and latter process with shorter relaxation time shows a discontinuity at the freezing temperature. The relaxation process with shorter relaxation time obtained in the crystalline phase is larger than that obtained in the nematic phase because of the large restrictions in the crystalline phase. For the first time, we have precisely explained the molecular mechanism and structure of liquid crystalline materials as a function of concentration, temperature, and frequency.

Anti-Parallel Dimer Formation of 4-Cyano-4’-Alkyl Biphenyls in Isotropic Benzene Solution - Seeds of Liquid Crystalline Phases -

“…In the case of neat 5CB in nematic LC state at 25 ºC, a dielectric relaxation time of the longer relaxation mode, t 2 ~ 20 ns, 24) well agrees with the fluorescence lifetime of excimers and anti-parallel dimeric association was not evaluated directly from dielectric spectra, but e 0 and the assumption of a constant K d value independent of c.…”

“…A rotational process around On the other hand, Kundu et al 24) reported dielectric spectra of Bz solutions of 5CB in both isotropic and LC regimes over a wide w range and they decomposed the spectra into two Debye-type relaxation modes with two sets of relaxation parameters, t j and e j ( j = 1 and 2) up to 2.6 M (70 wt%). They claimed that 5CB molecules have two kinds of DR processes related rotational motions around a longer and shorter molecular axes.…”

“…They claimed that 5CB molecules have two kinds of DR processes related rotational motions around a longer and shorter molecular axes. 24) According to their interpretation, 5CB molecules possesses a small permanent dipole moment perpendicular to the longer molecular axis because of presence of flexible pentyl group, and rotational motion around the longer molecular axis is dielectrically active.…”

Does transparent nematic phase exist in 5CB/DDAB/water microemulsions? From the viewpoint of temperature dependent dielectric spectroscopy

“…Similar phenomena were also observed in ADLC and 5CB/benzene mixtures. 42 When T < T IN , ε increases while ε iso decreases obviously as temperature decreases. An increase of g factor cannot give rise to such a big change; this result thereby indicates a diminishing of the isotropic phase and meanwhile a growing of the nematic phase.…”

“…It is known that the 5CB molecules tend to form antiparallel pairs. 41,42 The addition of water droplets may break some of the antiparallel pairs and thus increase the g factor. With more water droplets added, more antiparallel pairs will be broken; therefore, g factor increases with increasing φ m .…”