Critical behavior of dielectric permittivity in the isotropic phase of nematogens

The Dominance of Pretransitional Effects in Liquid Crystal-Based Nanocolloids: Nematogenic 4-methoxybenzylidene-4'-butylaniline with Transverse Permanent Dipole Moment and BaTiO3 Nanoparticles

The report presents static, low-frequency, and dynamic dielectric properties in the isotropic liquid, nematic, and solid phases of MBBA and related nanocolloids with paraelectric BaTiO3 nanoparticles (spherical, d = 50 nm). MBBA (4-methoxybenzylidene-4'-butylaniline) is a liquid crystalline compound with a permanent dipole moment transverse to the long molecular axis. The distortions-sensitive analysis of the dielectric constant revealed its hidden pretransitional anomaly, strongly influenced by the addition of nanoparticles. The evolution of the dielectric constant in the nematic phase shows the split into two regions, with the crossover coinciding with the standard melting temperature. The 'universal' exponential-type behavior of the low-frequency contribution to the real part of the dielectric permittivity is found. The critical-like pretransitional behavior in the solid phase is also evidenced. This is explained by linking the Lipovsky model to the Mossotti catastrophe concept under quasi-negative pressure conditions. The explicit preference for the 'critical-like' evolution of the apparent activation enthalpy is worth stressing for dynamics. Finally, the long-range, 'critical-like' behavior of the dissipation factor (D = tgδ), covering the isotropic liquid and nematic phases, is shown.

The impact of ionic contribution to dielectric permittivity in 11CB liquid crystal and its colloids with BaTiO3 nanoparticles

The report shows the temperature behavior of the real part of dielectric permittivity in the static (dielectric constant) and low-frequency (LF) domains in bulk samples of 11CB and its BaTiO₃-based nanocolloids. The study covers the isotropic liquid (I), nematic (N), smectic A (SmA), and solid crystal (Cr) phases. For each phase, the dominance of pretransitional fluctuations, significantly moderated by nanoparticles, is shown. The authors consider separate focuses on the dielectric constant [Formula: see text] evolution in the static domain, yielding mainly response from permanent dipole moment and its arrangement, and in the low-frequency (LF) domain [Formula: see text] (where [Formula: see text] is for the real part of dielectric permittivity in the LF domain), which is associated solely with ionic-related polarization mechanisms. All of these led to new experimental evidence concerning I-N, N-SmA, and SmA-solid transitions, focusing on the strength and extent of pretransitional effects, critical exponents, and phase transitions discontinuities. The strong evidence for pretransitional effects near the SmA-Cr transition is notable, particularly regarding [Formula: see text]. Studies are supplemented by the discussion of DC electric conductivity-a parameter also related to the LF domain. Finally, the validity of the relation [Formula: see text] (where f stands for frequency, and A is a constant parameter), often used for discussing dielectric spectra in LC compound and its nanocolloids in the LF domain, is examined.

The fluctuation-driven dielectric properties of liquid crystalline 8OCB and its nanocolloids

Results of broadband dielectric spectroscopy studies in liquid crystalline octyloxycyanobiphenyl (8OCB) and its colloids with BaTiO₃ nanoparticles (paraelectric, diameter d = 50 nm) are presented. Studies were carried out in isotropic liquid, nematic, smectic A and solid crystalline phases. They are supported by derivative-based and distortion-sensitive analyses, revealing a set of universal scaling patterns for temperature evolution of static and dynamic dielectric properties. All these yielded evidence for a pretransitional fluctuation impact on the dielectric constant, primary relaxation time, loss curve maximum and translational-orientational decoupling associated with anomalous values of the fractional Debye-Stokes-Einstein (DSE) exponent 0.2 < S < 2.2. The evidence for critical like changes in a distribution of relaxation times is shown. For mesophases, a tiny addition of nanoparticles causes permanent orientation of LC molecules, leading even to a 16% increase of the dielectric constant above a maximal value in pure 8OCB. A split of two regions, dominated by pre-isotropic and pre-smectic fluctuations, is evidenced. Model explanations for detected phenomena, particularly in the isotropic liquid phase, are presented. New evidence for a premelting effect in the solid phase is also shown.

Broadband-dielectric-spectroscopy study of molecular dynamics in a mixture of itraconazole and glycerol in glassy, smectic-A, and isotropic phases

Itraconazole (ITZ) is a thermotropic liquid crystal that exhibits isotropic, nematic, and smectic phases on cooling towards the glass transition upon melting. Over the years, new aspects regarding the liquid-crystalline ordering of this antifungal drug were systematically revealed. It has been shown recently that the temperature range of individual mesophases in ITZ can be modified by adding a small amount of glycerol (GLY). Moreover, above the critical concentration of 5% w/w, a smectic to nematic transition can be avoided. Here we go one step further, and we used broadband dielectric spectroscopy to investigate the new phase behavior of the ITZ-GLY mixture (5% w/w). To confirm the phase transformations of the ITZ-GLY mixture, differential scanning calorimetry was also employed. The analysis of molecular dynamics of the ITZ-GLY mixture in the glassy and isotropic phases revealed features similar to those observed for neat ITZ. Two relaxation processes were identified in the smectic-A phase, with similar temperature dependence, most likely related to the fast rotations around the long axis of a molecule. Additionally, the derivative analysis revealed another low-frequency process hidden under DC conductivity ascribed to the slow rotations about a short axis. We will show that the differences in the molecular organization in the smectic-A and isotropic phases leave a clear fingerprint on the temperature behavior of relaxation times and other dielectric parameters, such as DC conductivity and dielectric strength, for which a pretransition effect has been detected.

Pretransitional Effects of the Isotropic Liquid-Plastic Crystal Transition

We report on strong pretransitional effects across the isotropic liquid-plastic crystal melting temperature in linear and nonlinear dielectric response. Studies were carried out for cyclooctanol (C8H16O) in the unprecedented range of temperatures 120 K < T < 345 K. Such pretransitional effects have not yet been reported in any plastic crystals. Results include the discovery of the experimental manifestation of the Mossotti Catastrophe behavior, so far considered only as a hypothetical paradox. The model interpretations of experimental findings are proposed. We compare the observed pretransitional behavior with the one observed in octyloxycyanobiphenyl (8OCB), typical liquid crystal (LC), displaying a reversed sequence of phase transitions in orientational and translational degrees of order on varying temperature. Furthermore, in its nematic phase, we demonstrate first-ever observed temperature-driven crossover between regions dominated by isotropic liquid and smectic A pretransitional fluctuations. We propose a pioneering minimal model describing plastic crystal phase behavior where we mimic derivation of classical Landau-de Gennes-Ginzburg modeling of Isotropic-Nematic-Smectic A LC phase behavior.

Dynamics and Pretransitional Effects in C60 Fullerene Nanoparticles and Liquid Crystalline Dodecylcyanobiphenyl (12CB) Hybrid System

The report shows the strong impact of fullerene C60 nanoparticles on phase transitions and complex dynamics of rod-like liquid crystal dodecylcyanobiphenyl (12CB), within the limit of small concentrations. Studies were carried out using broadband dielectric spectroscopy (BDS) via the analysis of temperature dependences of the dielectric constant, the maximum of the primary loss curve, and relaxation times. They revealed a strong impact of nanoparticles, leading to a ~20% change of dielectric constant even at x = 0.05% of C60 fullerene. The application of the derivative-based and distortion-sensitive analysis showed that pretransitional effects dominate in the isotropic liquid phase up to 65 K above the clearing temperature and in the whole Smectic A mesophase. The impact of nanoparticles on the pretransitional anomaly appearance is notable for the smectic-solid phase transition. The fragility-based analysis of relaxation times revealed the universal pattern of its temperature changes, associated with scaling via the "mixed" ("activated" and "critical") relation. Phase behavior and dynamics of tested systems are discussed within the extended Landau-de Gennes-Ginzburg mesoscopic approach.

‘Quasi-Tricritical’ and Glassy Dielectric Properties of a Nematic Liquid Crystalline Material

Results of dielectric studies in the nematic and isotropic liquid phases of pentylcyanobiphenyl (5CB), a model rod-like liquid crystalline material, are shown. They are based on the discussion of the evolution of dielectric constant ( ε ), its changes under the strong electric field (nonlinear dielectric effect, NDE), and finally, the primary relaxation time. It is shown that changes in ε T and NDE are entirely dominated by the impact of pretransitional fluctuations (pre-nematic and pre-isotropic, respectively) which are associated with the weakly discontinuous character of the isotropic–nematic phase transition. This influence also extends for the low-frequency, ionic species dominated, region. Notable, that the derivative-based and distortions sensitive analysis revealed the tricritical nature of the I-N transition. Although the glass transition in 5CB occurs in the deeply supercooled state at T g ≈ − 68 ° C , the dynamics (changes of the primary relaxation time) follow a previtreous pattern both in the isotropic and in the nematic phase. Finally, the discussion of the ’molecular’ vs. ‘quasi-critical’ characterizations of the isotropic and nematic phases is presented. It shows the evident prevalence of the ‘quasi-critical-picture’, which offers the consistent temperature parameterization in the total tested temperature range.

Nanoparticle-controlled glassy dynamics in nematogen-based nanocolloids

Results of broad-band dielectric spectroscopy studies in liquid crystal (pentylcyanobiphenyl, 5CB)-based nanocolloids are presented. They reveal the strong impact of BaTiO_{3} nanoparticles on dynamics and uniaxial ordering. Studies were carried out in an extreme range of temperatures (∼150 K), including the supercooled nematic phase. For the latter, the unique "pretransitional" effect for dielectric constant on approaching solid state is reported. The distortion-sensitive analysis revealed super-Arrhenius dynamics but associated with critical-like behavior. In the isotropic phase, translational-orientational decoupling, unusual for the high temperature dynamic domain, was detected. It can be directly link to heterogeneities-prenematic fluctuations. The model linking the classical Landau-de Gennes approach with Imry-Ma arguments has been developed to discuss experimental results.

Impact of ferroelectric and superparaelectric nanoparticles on phase transitions and dynamics in nematic liquid crystals

Results of broadband dielectric spectroscopy (BDS) studies of pure liquid crystalline (4-pentyloxy-4-biphenylcarbonitryle) 5OCB and its nanocolloids with BaTiO_{3} nanoparticles (NPs) under varying pressure and temperature are presented. The notable impact of NPs on phase transitions and dynamics was found. Particularly strong impact on pretransitional behavior was observed for relatively low concentrations of NPs, which can be related to the NPs-induced disorder. There are also notable differences between pressure and temperature paths of studies for nanocomposites, absent for the pure LC compound. For instance, tests focused on the translational orientational decoupling via the fractional Debye-Stokes-Einstein relation yielded S=0.71 and S=0.3 for the temperature and pressure paths, respectively: S=1 is for the complete coupling. The possible theoretical frame of observed phenomena is also proposed.

Impact of BaTiO(3) nanoparticles on pretransitional effects in liquid crystalline dodecylcyanobiphenyl

The pretransitional behavior of dodecylcyanobiphenyl (12CB) (isotropic-smectic-A-solid mesomorphism) with d=50nmBaTiO(3) nanoparticles (NPs) linked to the cubic phase was monitored via temperature studies of dielectric constant. Tests were carried out in the isotropic, liquid crystal mesomorphic, and solid phases. For each phase transition the same value of the critical exponent α∼0.5 was obtained, including nanocolloids. All phase transitions show the weakly discontinuous nature. The temperature metric of the discontinuity ΔT notably decreases when adding nanoparticles. The addition of nanoparticles first decreases the dielectric constant by approximately 50% in comparison with pure 12CB, but already for a concentration ∼x=0.4% NP an increase over 50% takes place. It is notable that for the latter concentration unique hallmarks of the pretransitional effect emerge also for the solid-mesophase transition. All these indicate the important impact of nanoparticles on multimolecular mesoscale fluctuations.

Effect of hockey-stick-shaped molecules on the critical behavior at the nematic to isotropic and smectic-A to nematic phase transitions in octylcyanobiphenyl

In the field of soft matter research, the characteristic behavior of both nematic-isotropic (N-I) and smectic-A nematic(Sm-A N) phase transitions has gained considerable attention due to their several attractive features. In this work, a high-resolution measurement of optical birefringence (Δn) has been performed to probe the critical behavior at the N-I and Sm-A N phase transitions in a binary system comprising the rodlike octylcyanobiphenyl and a laterally methyl substituted hockey-stick-shaped mesogen, 4-(3-n-decyloxy-2-methyl-phenyliminomethyl)phenyl 4-n-dodecyloxycinnamate. For the investigated mixtures, the critical exponent β related to the limiting behavior of the nematic order parameter close to the N-I phase transition has come out to be in good conformity with the tricritical hypothesis. Moreover, the yielded effective critical exponents (α', β', γ') characterizing the critical fluctuation near the Sm-A N phase transition have appeared to be nonuniversal in nature. With increasing hockey-stick-shaped dopant concentration, the Sm-A N phase transition demonstrates a strong tendency to be driven towards a first-order nature. Such a behavior has been accounted for by considering a modification of the effective intermolecular interactions and hence the related coupling between the nematic and smectic order parameters, caused by the introduction of the angular mesogenic molecules.

Distortion-sensitive insight into the pretransitional behavior of 4-n-octyloxy-4'-cyanobiphenyl (8OCB)

Results of studies of the static and dynamic dielectric properties in rod-like 4-n-octyloxy-4'-cyanobiphenyl (8OCB) with isotropic (I)-nematic (N)-smectic A (SmA)-crystal (Cr) mesomorphism, combined with measurements of the low-frequency nonlinear dielectric effect and heat capacity are presented. The analysis is supported by the derivative-based and distortion-sensitive transformation of experimental data. Evidence for the I-N and N-SmA pretransitional anomalies, indicating the influence of tricritical behavior, is shown. It has also been found that neither the N phase nor the SmA phase are uniform and hallmarks of fluid-fluid crossovers can be detected. The dynamics, tested via the evolution of the primary relaxation time, is clearly non-Arrhenius and described via τ(T) = τ(c)(T-T(C))(-φ). In the immediate vicinity of the I-N transition a novel anomaly has been found: Δτ is proportional to 1/(T - T*), where T* is the temperature of the virtual continuous transition and Δτ is the excess over the 'background behavior'. Experimental results are confronted with the comprehensive Landau-de Gennes theory based modeling.

Does the characteristic value of the discontinuity of the isotropic-mesophase transition in n-cyanobiphenyls exist?

Results of the extended Landau-de Gennes model analysis and experimental studies of the isotropic-nematic (I-N) and isotropic-smectic-A (I-SmA) phase transitions in rod-like liquid crystalline n-alkylcyanobiphenyls are presented. Experiments were carried out as a function of temperature and pressure using the static dielectric permittivity and its 'nonlinear' (strong electric field related) counterpart-the low-frequency nonlinear dielectric effect. Precise estimations of the values of the discontinuity of the isotropic-mesophase transitions (ΔT) for nCB from n = 3-14 have been obtained. It is suggested that for each nCB a unique, characteristic minimal value of ΔT, associated with the I-N-SmA triple point, exists. For 'shorter' nCBs it can be hidden in the negative pressures domain. The possibility of the extension of the 'melting curve' into the negative pressures region as well as the appearance of the 'melting inversion' at high enough pressures is indicated.

Tricritical-like behavior of the nonlinear optical refraction at the nematic-isotropic transition in the E7 thermotropic liquid crystal

We use Z-scan technique to investigate the nonlinear optical response of the thermotropic liquid crystal E7 in the neighborhood of the nematic-isotropic phase transition. The analysis of the data for the nonlinear optical birefringence is compatible with an effective critical exponent of the order parameter, β = 0.28 ± 0.03, which is close to the classical value, β = 0.25 , for a tricritical point. The nonlinear optical absorption in the nematic range depends on the geometrical configuration of the nematic director with respect to the polarization beam, and vanishes in the isotropic phase.

Glassy dynamics in the isotropic phase of a smectogenic liquid crystalline compound

The temperature evolution of the primary relaxation time in the isotropic phase of 4-cyano-4'-tetradecylbiphenyl (14CB) above the isotropic-smectic A (I-SmA) transition is discussed. Based on the enthalpy space and distortion-sensitive analysis, the prevalence of the mode coupling theory (MCT) "critical" and "glassy" dynamics is shown. The obtained singular dependence is related to the MCT critical temperature located approximately 48 K below the clearing (I-SmA) temperature. However, a weak but detectable distortion in the immediate vicinity of the transition occurs. It is also shown that the value of the fragile strength coefficient D(T) is characteristic of a very fragile glassy liquid whereas the steepness index m is typical of a strong one. Both magnitudes anomalously change on approaching the I-SmA phase transition. The static permittivity shows the pretransitional effect linked to the temperature of the hypothetical continuous phase transition located approximately 10.2 K below the I-SmA transition.

Pretransitional behaviour in the vicinity of the isotropic-nematic transition of strongly polar compounds

The isotropic-nematic transition, being weakly first order, exhibits pretransitional effects signifying the appearance of the nematic-like regions in the isotropic phase. In the isotropic phase, strongly polar liquid crystals, such as the popular alkyl and alkoxy cyano biphenyl behave in a non-standard fashion: whereas far away from the transition the dielectric constant ε(iso) has a 1/T dependence (a feature also commonly seen in polar liquids), on approaching the nematic phase the trend reverses resulting in a maximum in ε(iso), at a temperature slightly above the transition, an effect explained on the basis of short-range correlations with an antiparallel association of the neighbouring molecules. Recently, there has been a revival in studies on this behaviour to possibly associate it with the order of transition. Here we report dielectric measurements carried in the vicinity of this transition for a number of compounds having different molecular structures including a bent core system, but with a common feature that the molecules possess a strong terminal polar group, nitro in one case and cyano in the rest. Surprisingly, the convex shape of the thermal variation of ε(iso) was more an exception than the rule. In materials that exhibit such an anomaly we find a linear correlation between δε = (ε(peak)-ε(IN))/ε(IN) and δT = T(peak)-T(IN), where ε(peak) is the maximum value of the dielectric constant in the isotropic phase, ε(IN) the value at the transition, and T(peak) and T(IN) the corresponding temperatures.

Critical Behavior and Scaling Relationships at the SmAd−N and N−I Transitions in Nonyloxycyanobiphenyl (9OCB)

Different kind of measurements were performed on the liquid crystal nonyloxycyanobiphenyl (9OCB) to carry out a study of the molecular dynamics in the smectic A (SmA), nematic (N), and isotropic (I) phases as well as an exhaustive analysis of both the SmA-to-N and N-to-I phase transitions. For the dynamic study, broadband dielectric spectroscopy (102 to 1.8 x 109 Hz) was used. Two orientations (parallel and perpendicular) of the molecular director with regard to the probing electric field were investigated. From this study, the static dielectric permittivity was obtained in both alignments and, in addition, the molecular motions that contribute to each one were discussed. The static dielectric data together with specific heat and volumetric determinations were analyzed, proving that both phase transitions are weakly first order, displaying a nearly tricritical behavior. However, the width of metastable regions seems to be dependent on the physical magnitude, although specific heat and volumetric determinations allow for comparable results. It should be noticed that the temperature derivative of the static dielectric permittivity, specific heat, and isobaric thermal expansion coefficient data derived from volumetric determinations are related to each other by scaling relationships.

Molecular self-organization in cylindrical nanocavities

We studied molecular organization in cylindrical nanocavities using liquid crystals. NMR analysis shows high surface-induced ordering way above the bulk critical temperature. The surface-order evolution reveals replacement of the isotropic phase by a paranematic phase and surface-induced disordering in the nematic phase. Due to strong surface potential and nanoconfinement, complete wetting and continuous evolution of the surface-order parameter are observed through the nematic-paranematic transition. As we show, the counter-intuitive absence of complete phase transition at the interface while an abrupt phase transition was measured in the averaged order parameter is in good agreement with established theories.

Pretransitional behavior in the isotropic phase of a nematic liquid crystal with the transverse permanent dipole moment

The results presented give the evidence for the quasicritical, pretransitional behavior of dielectric properties in the isotropic phase of a rodlike nematic liquid crystal with the transverse permanent dipole moment. Studies were conducted in 2-cyano-4-pentylbiphenyl 4-(trans-4-pentylcyclohexyl) benzoate, focusing on the static-and ionic-dominated low-frequency (LF) regions. For the static dielectric permittivity [epsilon(')(T)] the application of the derivative analysis revealed the pretransitional anomaly associated with the specific heat exponent alpha approximately 0.5. For the LF domain the contribution to epsilon(')(T) from residual ionic impurities follows a linear temperature dependence on approaching the isotropic-nematic (I-N) transition. This dependence and pretransitional anomalies of electric conductivity and dielectric modulus can be associated with the influence of prenematic fluctuations. "Linear" dielectric studies were supported by the static nonlinear dielectric effect measurements, which delivered reliable estimations of the temperature of the hypothetical continuous phase transition T(*) and the discontinuity of the I-N transition DeltaT approximately 1.7 K.

Dielectric and viscous properties of mesogenic n-nonylcyanobiphenyl (9CB)

Static and dynamic linear dielectric properties and shear viscosity of freely flowing 9CB in the isotropic (I), nematic (N) and smectic A (S(A)) phases were investigated. Pretransitional effects observed in the vicinity of the I to N phase transition manifest themselves as anomalies in the temperature dependence of the static permittivity, the rotational diffusion exponent and the activation energy of molecules rotating around their short axis. As a presmectic effect, a jump in the temperature dependence of the shear viscosity of freely flowing nematic 9CB is observed at about 0.5 °C before the transition to the S(A) phase.

Heterogeneity-related dynamics in isotropic n-pentylcyanobiphenyl

The evolution of the dielectric relaxation time tau(T) and its strong electric field related counterpart tau(fluct).(T) in the isotropic phase of rodlike liquid crystalline n-pentylcyanobiphenyl (5CB) is discussed. For the tau(fluct).(T) a strong pretransitional increase on approaching the virtual critical point located at deltaT = 1.1 K below isotropic nematic clearing temperature T(IN) is shown. The linearized derivative analysis applied to test the evolution of tau(T) revealed a sequence of Vogel-Fulcher-Tammann based dynamical domains, for which the fragility increases enormously on approaching T(IN). The similar analysis focused on testing the "critical" mode-coupling theory behavior showed superior features of such description. A strong asymmetric broadening of the dielectric loss curves is also discussed.

Complex dynamics of supercooling n-butylcyanobiphenyl (4CB)

Results of broadband dielectric spectroscopy (BDS) studies, for a remarkable temperature range (130 K), in supercooled rodlike liquid crystalline n-butylcyanobiphenyl (4CB) are shown. They are supplemented by static nonlinear dielectric effect (NDE) measurements in 4CB and 3CB (n-propylcyanobiphenyl), giving unequivocal estimations of the hypothetical isotropic-nematic (I-N) continuous phase transition temperature T* and of the discontinuity of this transition deltaT. The distribution of relaxation times becomes strongly non-Debye on approaching the I-N transition and follows the Debye pattern for T > T(I-N) + 75 K. The temperature evolution of relaxation times [tau(T)] and dc conductivity [sigma(T)] remain non-Arrhenius in the tested temperature range. Their descriptions via the Vogel-Fulcher-Tammann (VFT), Cohen-Grest (CG), and mode coupling theory (MCT) based relations are discussed. The derivative analysis of tau(T) and sigma(T) experimental data enabled the detection of dynamic crossovers and the insight into the behavior of the apparent activation energy which follows the relations E(A)(tau),E(A)(sigma) (sigma) is proportional to (T/T-T0)2, with a small distortion in the immediate vicinity of the I-N transition. The glassy dynamics coexist with the critical-like behavior of temperature dependences of the static dielectric permittivity, maximum of the loss curves and strong pretransitional NDE pretransitional anomaly. These results indicate the significant role of prenematic fluctuations-heterogeneities in the isotropic, fluidlike, surrounding regarding both static and dynamic properties.

Pretransitional critical-like behavior of dielectric permittivity in mixtures of mesomorphic and nonmesomorphic compounds

Static dielectric measurements are performed in the temperature range of the isotropic-nematic phase transition of binary mixtures of mesomorphic n-heptylcyanobiphenyl (7CB) and nonmesomorphic: (a) n-heptylcyanophenyl (7CP) and (b) carbon tetrachloride (CCl4) , the solutes of different molecular shape and polarity. In the whole studied range of the solutes mole fraction (x) (x(max) approximately equal to 0.17) , the critical-like temperature behavior of the permittivity in the vicinity of the transition from the isotropic phase to the two-phase (nematic+isotropic) region can be well described with a critical exponent close to 0.5, as in a pure 7CB, indicating the tricritical nature of the transitions. It seems to be important that the fitting-determined temperatures of the virtual second-order transition in the solutions, T*, i.e., the temperature limit of the thermodynamic stability of supercooled isotropic phase, correspond well to the experimentally observed low-temperature limits of the two-phase isotropic+nematic region.

Static and dynamic dielectric behavior of mesogenic compounds of different polarity in the vicinity of the isotropic to nematic phase transition

Mesogenic compounds belonging to the two well-known -cyanophenyl, and -isothiocyanatophenyl homologous series, which distinctly differ in the molecular polarity (-C identical with N, 5D; -N=C=S, 2.5D), show an essential difference in the pretransitional dielectric behavior in the vicinity of the isotropic to nematic (I-N) phase transition. Taking into account the results presented in Phys. Rev. E 67, 041705 (2003), the features of the I-N transition observed for the less polar mesogens are characteristic for the first order phase transition, whereas in the case of the strongly polar ones, the I-N transition is undoubtedly close to the second order.

Static and dynamic dielectric properties of strongly polar liquids in the vicinity of first order and weakly first order phase transitions

The paper presents the results of measurements of the linear dielectric properties of the compounds from the homologous series of alkylcyanobiphenyls (C(n)H(2n+1)PhPhCN, nCB) in the vicinity of the first order transition (from the isotropic liquid to the crystalline phase) of nonmesogenic nCB's (n=2-4) and the weakly first order transition (from the isotropic liquid to the nematic phase) of 5CB. The experimental method for the separation of the critical part of the static permittivity derivative and the activation energy for rotation of the mesogenic molecules, in the vicinity of weakly first order phase transition, is proposed. It is shown that the critical temperature dependence of the permittivity and the activation energy can be described with a function of (T-T*)(-alpha) type, with the same values of the temperature of virtual transition of the second order (T*) and the critical exponent (alpha).

Reorientational dynamics of the pseudonematic domains studied with nonlinear dielectric spectroscopy

Nonlinear dielectric spectroscopy was used for studies of the reorientational dynamics of the pseudonematic domains in the isotropic phase of the mesogenic substance in the vicinity of the isotropic to nematic phase transition. The results were interpreted in the frame of the Landau-de Gennes theory.

Dynamics of the nonlinear dielectric properties of a nematogenic compound dissolved in a nonpolar medium

The nonlinear dielectric spectra of 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (C6H13CyHx Ph NCS, 6CHBT) in benzene solutions were recorded in the whole concentration range up to an appearance of the two-phase isotropic and nematic region (0.82 mole fraction of 6CHBT, at 25 degrees C). Two electric fields: (i) the field of high strength (110 kV/m) and low frequency (85 Hz) causing the dielectric nonlinearity, and (ii) the probing field of low strength and high frequency (100 kHz-3 GHz), were used. At about of 0.2 mole fraction of 6CHBT the nonlinear dielectric increment becomes positive. With increasing of 6CHBT concentration the dynamics of the increment shows a critical-like behavior, which can be interpreted in terms of the reorientation of the growing pseudonematic domains in the solutions studied.

Critical anomaly of dielectric permittivity for the temperature and pressure paths on approaching the critical consolute point

The experimental results of isothermal pressure dielectric permittivity epsilon studies in a critical mixture characterized by a negative shift of critical temperature induced by pressure (dT(C)/dP<0) are presented. The critical effect is portrayed by the same relation as in previous epsilon(T) and epsilon(P) studies, with the critical exponent alpha=0.12+/-0.03. The advantage of pressure studies is the negligible influence of the correction-to-scaling term and the low-frequency Maxwell-Wagner effect. This conclusion is supported by the distortion-sensitive derivative analysis of the experimental data. In contrast to previous epsilon(P) studies, carried out in mixtures with dT(C)/dP>0, the critical effect manifests by the bending-up behavior near the critical point. It is suggested that signs of the critical amplitudes of epsilon(P) and epsilon(T) anomalies may be related to the excess volume V(E) and the excess enthalpy H(E), respectively.

Fluidlike behavior of dielectric permittivity in a wide range of temperature above and below the nematic-isotropic transition

Singular behavior of the static dielectric permittivity of n-alkyloxycyanobiphenyls (CnH2n+1O-Ph-Ph-C [triple bond] N, n=6, 7) was studied above and below the nematic clearing point (T(I-N)). On approaching the clearing point, the evolution of principal components of the nematic permittivity tensor, epsilon(parallel) and epsilon(perpendicular), is described by the order parameter exponent beta approximately 0.25. The mean value of the nematic permittivity epsilon(mean)=(epsilon(parallel)+2epsilon(perpendicular))/3 exhibits a singular behavior similar to that observed in the isotropic phase and that for the diameter of the coexistence curve in binary mixtures. The derivative of experimental data d(epsilon)iso(T)/dT and d(epsilon)mean(T)/dT shows the specific-heat-like behavior with universal exponents alpha=alpha' approximately 0.5. Results obtained confirm the hypothesis of the fluidlike, pseudospinodal, and tricritical behavior of the isotropic to nematic phase transition. [A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)].

Pretransitional behavior of dielectric permittivity on approaching a clearing point in a mixture of nematogens with antagonistic configurations of dipoles

The results of studies of static dielectric permittivity epsilon(T) in the isotropic phase of 4-cyano-4-pentylalkylbiphenyl and n-p-methoxybenzylidene-p'-butylaniline (5CB-MBBA) mixtures are presented. 5CB and MBBA are nematogens with antagonistic permanent dipole moments configurations. An increase in MBBA concentration strongly decreases the pretransitional effect. However, a derivative analysis detected the existence of a pretransitional anomaly even if its weakness made a straightforward epsilon(T) fit impossible. The obtained anomalies can be well portrayed by fluidlike equations, isomorphic to that applied in critical binary mixtures. Every time the same value of the specific heat critical exponent alpha approximately 0.5 was obtained. A preliminary discussion of the influence of the permanent dipole configuration on the pretransitional behavior of dielectric permittivity was also possible.

Quasicritical behavior of the low-frequency dielectric permittivity in the isotropic phase of liquid crystalline materials

Results presented give evidence of the existence of quasicritical, fluidlike behavior in the isotropic phase of 4-cyano-4-pentyl-biphenyl (5CB) for frequencies ranging from the static to the ionic-dominated [low-frequency (LF)] region. Despite the boost of dielectric permittivity on lowering the frequency below 1 kHz, values of the isotropic-nematic transition discontinuity (approximately 1.1 K) and the critical exponent alpha (approximately 0.5) remain constant. It is shown that the contribution from residual ionic impurities is a linear function of temperature in the critical, prenematic fluctuation-dominated region. The validity of the fluidlike and critical behavior for LF dielectric permittivity confirmed results of a derivative analysis of the experimental data: d(epsilon)/dT proportional to (T-T*)(-alpha), originally proposed for critical mixtures. Results of a preliminary test in the isotropic phase of 4-decyl-4'-isothiocyanatobiphenyl (10BT), on approaching the smectic-E phase, may indicate a general validity of results obtained.

Phase transitions from the isotropic liquid to liquid crystalline mesophases studied by linear and nonlinear static dielectric permittivity

Results of studies of static dielectric permittivity (epsilon) and nonlinear dielectric effect (NDE) in the isotropic phase of 4-n-4'-isothiocyanatobiphenyl (nBT) homologous series from n = 2 to n = 10 exhibiting the isotropic-smectic E (I-SmE) transition, are presented. They are compared with results of similar studies in 4-cyano-4-n-alkylbiphenyls (nCB) from n = 4 to n = 12. In this homologous series isotropic-nematic (I-N) and isotropic-smectic-A (I-Sm-A) transitions take place. Despite significant differences between N, Sm-A, and Sm-E phases the same pretransitional behavior of epsilon and NDE in the isotropic phase, described by critical exponents gamma = 1 and alpha = 0.5, was found. It has been shown that when the length of the alkyl chain of a compound increases the discontinuity of the transition drops in nBT and rises in nCB. The influence of pressure on the discontinuity is also discussed.

Dynamics of critical fluctuations in a binary mixture of limited miscibility under a strong electric field

Experimental investigations of relaxation after switching off the strong electric field in a nitrobenzene-dodecane mixture are presented. Studies were conducted for mixtures of critical and noncritical concentrations using the time-resolved nonlinear dielectric effect. The decays obtained can be portrayed by means of the stretched exponential function with the value of the exponent in agreement with the dynamic droplet model predictions. It has been shown that experimental decays exhibit a universal scaling behavior. The relaxation time (scaling factor) shows a power behavior with the exponent y approximately 1.2 for the critical mixture and y-->1 for the noncritical one. These values are much smaller than theoretically predicted y=1.8-1.9. Based on the assumption that a strong electric field induces in the mixture a quasinematic structure with semiclassical critical properties, a quantitative explanation of this difference is proposed.

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.