Isotropic Phase Of Nematogens Research Articles

Universal Scaling of Alpha Relaxation in Complex Liquids

Abstract A plot is given, showing the result of a scaling analysis of dielectric loss curves containing, apart from low molecular glass former data (glycerol, dibutyl phtalate), also loss curves of the following liquid crystalline materials, mostly in the iso­ tropic phase: 4-(2-methylbutyl)-4'cyanobiphenyl (5*CB, supercooled isotropic phase), 4-cyano-4-n-alkyl biphenyls (nematogens 5CB and 8CB, isotropic phase), 4-(4-cyano-4-butylcyclohexyl)-4'-octylbiphenyl (laterally substituted nema-togen, isotropic phase), and 4-n-alkyl-4'-isothiocyanatobiphe-nyl (5 and 10 BT, isotropic and SmE phases). The plot applies the scaling formula originally proposed for glassforming, super­ cooled liquids [Dendzik et al.7]. The result supports the recent suggestion that dielectric relaxation in the isotropic phase of nematogens may show some features typical for "glassy" mate­ rials.

The pretransitional anomaly of dielectric permittivity in the isotropicphase of nematogens and in the homogeneous phase of criticalsolutions

The pretransitional behaviour of static dielectric permittivity in theisotropic phase of nematogens and in the homogeneous phase of a weaklyoff-critical binary solution is compared. The pretransitional anomalies aredescribed by analogous relations, with critical exponentsϕ = 1-α≈0.50 and ϕ = 1-α≈0.89, respectively. Testswere conducted for pressure paths approaching the virtual critical point. Thespecific-heat-like dependences of derivatives of dielectric permittivities inboth systems have been shown. This result agrees with the dependence suggestedfor critical mixtures by Mistura (1973 J. Chem. Phys. 59 4563). Results obtainedpoint to the validity of the recently formulated hypothesis on the fluid-likecritical behaviour of the isotropic-nematic transition (Mukherjee P K et al 1995 Phys. Rev. E 51 4570, Mukherjee P K 1998 J. Phys.: Condens. Matter 10 9191).

Quasi-Critical Behaviour of “Linear” and “Nonlinear” Dielectric Permittivity in the Isotropic Phase of Nematogens

Results are presented of dielectric permittivity and nonlinear dielectric effect (NDE) studies in the isotropic phase of n-pentylcyanobiphenyl (5CB), Both properties can be described by relations identical to those applied in critical binary solutions. For the isothermal, pressure and isobaric, temperature paths approaching the nematic clearing point pretransitional anomalies of dielectric permittivity and NDE are described by the same relations with critical exponents α ≈ 0.5 and γ = 1, respectively This substantiates the postulate of the isomorphism of critical phenomena in the isotropic phase of nematogens. For tested “linear” and “nonlinear” changes of dielectric permittivity the same values of the discontinuity of the isotropic — nematic phase transition was obtained.

Quasicritical behavior of dielectric permittivity in the isotropic phase of n-hexyl-cyanobiphenyl in a large range of temperatures and pressures.

Results of studies of the dielectric permittivity and nonlinear dielectric effect (NDE) in the isotropic phase of n-hexyl-cyanobiphenyl are presented. The data reported cover both pressure and temperature dependence. Measurements under atmospheric pressure were carried out in a wide range, up to 100 K from the clearing temperature. The application of a weak measurement frequency (f=67 kHz) resulted in a negligible influence of relaxation processes on NDE results. For both temperature and pressure paths, the dielectric permittivity and the NDE exhibited strong pretransitional effects, described by "critical" exponents phi =1-alpha approximately 0.5 and gamma=1, respectively. Scaling expressions for the pretransitional behavior of the NDE and of the dielectric permittivity have been proposed. The relationship between pretransitional effects in the isotropic phase of nematogens and in critical solutions has also been discussed.

Critical behavior of dielectric permittivity in the isotropic phase of nematogens.

It has been shown that the temperature behavior of dielectric permittivity ($\ensuremath{\varepsilon}$) in the isotropic phase of nematogens can be described in the same way as in critical binary solutions. Hence, using a relation with the critical exponent $\ensuremath{\varphi}=1\ensuremath{-}\ensuremath{\alpha}=0.5\ifmmode\pm\else\textpm\fi{}0.03$ it was possible to portray the results of $\ensuremath{\varepsilon}(T)$ measurements in the isotropic phase of 5-heptyl-2-(4\ensuremath{'}-cyanobiphenyl)-pyrimidine and 4,4-$n$-octylcyanobiphenyl. The influence of the position of the permanent dipole moment on the results was tested by additional measurements in $n$-($p$-methoxybenzylidene)-${p}^{\ensuremath{'}}$-butylaniline. It also has been shown that a fluidlike analogy can be applied to the nonlinear dielectric effect (NDE), which describes changes of dielectric permittivity induced by a strong electric field. Measurements were conducted for the lowest frequency used in NDE studies ($f=67$ kHz), so the condition $\ensuremath{\tau}f\ensuremath{\ll}1$ (with $\ensuremath{\tau}$ the relaxation time) was always fulfilled. Values of discontinuities of the isotropicnematic phase transition from both the analysis of $\ensuremath{\varepsilon}(T)$ measurements and NDE studies are in good agreement.

The equivalence of density and field variables for the pretransitional effect in the isotropic phase of nematogens

Abstract The nonlinear dielectric effect (NDE) is described with reference to the concentrational, isothermal approach to the weakly discontinuous phase transition point from the isotropic to the nematic phase in MBBA (n-4-methoxybenzylidene-4-butylaniline)-benzene solution (nematogen-nonmesogenic component). It was ascertained that the dependence of the pretransitional effect as a function of benzene concentration (‘density’ variable) may be described, as with temperature or pressure (‘field’ variables) studies in pure MBBA, by the mean-field theory.

Classical behaviour of the non-linear dielectric effect in the isotropic phase of nematogens

Abstract Results are presented of temperature and pressure studies of the non-linear dielectric effect (NDE) in the isotropic phase of MBBA (N-4-methyoxybenzylidene-4′-n-butylaniline) and EBBA (N-4-ethoxybenzylidene-4′-n-butylaniline). In both nematogens the pretransitional increase of the NDE is described by the classical relation obtained on the basis of the Landau-de Gennes model. The value of the ratio of amplitudes describing the pretransitional effect of MBBA and EBBA shows a dependence on the measurement frequency of the NDE (a few MHz). In nematogens with a large positive anisotropy of the electric permittivity, change in the measurement frequency even leads to a change in sign of the NDE on approaching the clearing temperature.

Kerr effect and the nonlinear dielectric effect on approaching the critical consolute point.

For the exponent \ensuremath{\psi}, which describes a critical anomaly of the nonlinear dielectric effect (NDE) and the electro-optic Kerr effect (EKE) in critical solutions, theoretical models predict ${\mathrm{\ensuremath{\psi}}}_{\mathrm{NDE}}$=${\mathrm{\ensuremath{\psi}}}_{\mathrm{EKE}}$=${\mathrm{\ensuremath{\psi}}}_{\mathrm{theor}}$=\ensuremath{\gamma}-2\ensuremath{\beta}. An experiment gave ${\mathrm{\ensuremath{\psi}}}_{\mathrm{NDE}}$ (\ensuremath{\simeq}0.37)${\mathrm{\ensuremath{\psi}}}_{\mathrm{theor}}$ (\ensuremath{\simeq}0.59)${\mathrm{\ensuremath{\psi}}}_{\mathrm{EKE}}$ (\ensuremath{\simeq}0.85). For all other experimental results, however, the agreement with the theory is good. In this paper it is shown that also both of the above relations may be valid. This result is based on the assumption that the NDE and the EKE exhibit a mixture of both a nonclassical and a classical type of behavior of critical fluctuations. Additionally, a relation was found between the semimicroscopic models for critical solutions and the phenomenological Landau--de Gennes model for the pretransitional effect of the NDE and the EKE in the isotropic phase of nematogens.

Kerr-Effect in the Isotropic Liquid Phase of Nematogens†

Abstract The existence of strong fluctuations of the orientational order in the isotropic phase of nematics determines the nature of electro-optical effects near to Tc the temperature of the phase transition of the isotropic liquid into the liquid crystalline state. Electric birefringence in the isotropic phase of liquid crystals differing in the value and direction of the dipole moment of the molecules was investigated. The relaxation phenomena manifest using a radio frequency sinusoidal electric field in the isotropic phase of nematogens in the vicinity of Tc are discussed. Two ranges of dispersion of the Kerr effect were detected experimentally: a low frequency range (f = 105 to 106 Hz) and a high frequency range (for f> 106 Hz) The first range has been related to supermolecular relaxation processes and characterizes the rotational mobility of the fluctuation ‘swarm’ (i.e., the nucleus of the mesophase) in the isotropic phase. The second range was due to the relaxation of the dipole orientation of the ...

Direct Observation of Picosecond Reorientation of Molecules in the Isotropic Phases of Nematogens

Abstract We present here an experimental investigation of individual molecular movements inside the isotropic phase of nematogens, by means of Optical Kerr EKect techniques. They give rise to a fast response of amplitude C, in the picosecond range, which is evidenced. Its time constant is evaluated and compared to that of molecules prefigurating liquid crystals but showing no nematic phase. Moreover, we present in the case of p-methoxybenzylidene-p-n-butylanili (MBBA), the first direct measurement of the ratio CF/Cs, where C, is the amplitude of the well known slow response, induced by the order parameter fluctuations.

Fast picosecond reorientation in the isotropic phase of nematogens

We report the first observation of a fast optical Kerr signal given by nematogens in the isotropic phase, induced by intense mode-locked laser waves. This signal can be attributed to the individual orientational relaxation of molecules inside the highly correlated domains. The relative contribution of this fast relaxation process can be directly measured.

Fast picosecond reorientation in the isotropic phase of nematogens

Abstract We report the first observation of a fast optical Kerr signal given by nematogens in the isotropic phase, induced by intense mode-locked laser waves. This signal can be attributed to the individual orientational relaxation of molecules inside the highly correlated domains. The relative contribution of this fast relaxation process can be directly measured.