Pretransitional Behavior Research Articles

Pretransitional Effects in Martensitic Transformations

The martensitic transition in shape memory alloys has an evident first-order character. Nevertheless, pretransitional effects are observed. The situation is illustrated with experimental results (neutron diffraction and ultrasonic measurements) in different shape-memory materials such as Cu-based systems, Ni-Al alloys and the ferromagnetic Ni 2 MnGa Heusler compound. In all cases it is shown that pretransitional behaviour is associated with low restoring forces for specific vibrational modes related to the path which lead the system towards the martensitic phase. Within the framework of the Landau theory, the existence of a first-order transition is explained by the anharmonic coupling of the incipiently unstable exitations to a secondary field.

Order Parameter Theory for Switching in Antiferroelectric Liquid Crystals

In order to model the field induced switching of AFLC materials we have employed a bilayer order parameter theory which includes changes in the distribution of molecules around the smectic cone. The free energy is a function of the azimuthal director angle and the molecular order about this director in the odd and even layers. By minimizing the free energy the effective optic axis may be calculated as a function of applied voltage. This theory is able to produce not only hysteretic tristate switching typical of AFLC materials but also the pre-transitional electroclinic-like behaviour at low field strengths and the frequency dependence of the double hysteresis loop.

The Pre-Transitional Effect in Antiferroelectric Liquid Crystals: a Comparison between Theory and Experiment

We have investigated, experimentally and theoretically, the low field (pre-transitional) behaviour of an AFLC material. By measuring the light transmission through a device between crossed polarizers, as a function of both orientation and applied voltage, we determine the variation of both the effective optic axis tilt angle φ and the optical anisotropy Δn as a function of voltage. In order to model this behaviour we use an order parameter theory based on changes to the distribution of molecules within the AFLC layers rather than the soft mode description. With this model we have been able to describe the observed tilt and anisotropy behaviour.

Liquid crystal acrylate-based networks: polymer backbone–LC order interaction

New synthetic pathways to poly- and monodomain acrylate-based networks via gamma-irradiation and chemical crosslinking are described. Strain-induced orientation transitions and phase behavior are studied by X-ray scattering, differential scanning calorimetry and optical microscopy. Analysis of the observed phenomena is given in terms of crosslinking conditions, network topology, structure and spatial distribution of crosslinks. The pretransitional behavior and the observed shifts in clearing point Tn–i are in good agreement with theoretical predictions. The reorientation process in monodomain networks is regulated via strain gradients generated by external mechanical field. The variation in geometrical shape of the samples gives an opportunity to control and realize two different reorientation mechanisms: uniform director rotation and reorientation via stripe domains. Gamma-irradiation is found to be a powerful tool for fixation of any current distribution of poly- and monodomain regions within the LC film. ‘Macroscopic memory’ of the information recorded on the films is observed and discussed.

Transient Electro-Optic Kerr Effect in Liquid Crystalline Isothiocyanates

Measurements are reported for the first time on the D.C. Kerr response and the pre-transitional behaviour of two liquid crystalline isothiocyanates 4-(4 ′ -n-pentyl phenyl)-phenyl isothiocyanate (5NCS) and 4-(4 ′ -n-heptyl phenyl)-phenyl isothiocyanate (7NCS) at temperatures above the smectic-isotropic transition temperatures. The Kerr law has been verified and the real part of the third order nonlinear susceptibilities (χ (3) ) above the smectic-isotropic transition are determined for the two liquid crystalline isothiocyanates from the electro-optic Kerr effect (EOKE) experiments. The variation of susceptibility with temperature has also been studied. The results have been compared with the cyanobiphenyls which have been widely studied and are known for their giant optical non-linearities.

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.

Enhanced optical nonlinearity by photoinduced molecular orientation in absorbing liquids

We present a theoretical and experimental study of the molecular orientation induced in a light-absorbing liquid of rodlike molecules by a laser pulse that explains the large enhancement of anisotropic optical Kerr nonlinearity observed in liquids in which a small amount of dye has been dissolved. Our model is based on the assumption that a molecule changes significantly its rotational kinetic and equilibrium behavior when it is electronically excited by the absorption of a photon. The anisotropy of the absorption probability then results in a corresponding orientational anisotropy of the molecules involved in electronic transitions, which in turn orient the whole liquid via intermolecular interactions. To test our model, we performed nanosecond pump-and-probe measurements of nonlinear birefringence at varying pump pulse energy and temperature. In our experiments the host was a liquid-crystal mixture in its isotropic phase and the dye was an anthraquinone derivative. Both the decrease in the nonlinearity enhancement observed for increasing pulse energies and the pretransitional behavior occurring for temperatures approaching the isotropic-nematic transition point are in good agreement with the predictions of our model. The contribution of secondary effects related to light absorption and light-induced heating has been also taken into account in order to properly compare the nonlinearity of an absorbing liquid with that of a transparent one.

Fluctuation-induced structural forces in nematic films

In a brief review of the theory of pseudo-Casimir interaction in nematic liquid crystals, we describe long- and short-range fluctuation-induced forces in these materials and discuss the role of the anchoring strength in this context. We also analyze the pretransitional behavior of the interaction in the vicinity of the clearing point.

Influence of measurement frequency on the pretransitional behaviour of the no-linear dielectric effect in the isotropic phase of liquid crystalline materials

Results are presented of the non-linear dielectric effect (NDE) as a function of temperature (T) in the isotropic phase on approaching the nematic phase for the liquid crystalline materials 4-cyano-4-hexylbiphenyl (6CB), 5-heptyl-2-(4-cyanophenyl)pyrimidine smectic A phase of 4-cyano-4-decylbiphenyl (10CB). For low measurement frequencies (f) the NDE 1(T) exhibits linear behaviour from the clearing temperature (T) up to about C TC50 K. Increase of f causes an increased deviation from this dependence near T C. The influence of the measurement frequency can be associated with the link between the system time-scale, and the measurement time-scale defined by the relaxation time of pretransitional processes (tau),(f1). A quantitative agreement with de Gennes model NDE 1(T) analysis and with the results of time- and frequency-resolved the relation derived from the Landauof Kerr effect studies is shown. A possible influence of the intensity of the weak measurement field on the pretransitional effect in the immediate vicinity of TC is discussed. Similiarities between pretransitional behaviour in the isotropic phase of the nematogens and in the homogeneous phase of a critical binary solution are considered. (HCPP) and the

Thickness dependence of polarization and response characteristics in thin FLC films

The effects of cell thickness on the pretransitional behaviour of ferroelectric liquid crystals are studied experimentally and theoretically for thin cells. The temperature and field dependences of the tilt angle, polarization, polarization to tilt angle ratio, dielectric permeability and relaxation frequency are considered in detail in the framework of a simple model for the surface tension. Apparent values of the phenomenological coefficients in the free energy expansion and some bulk and surface material parameters are estimated.

Nature of the Blue-Phase-III–isotropic critical point: An analogy with the liquid-gas transition

The analogy with the liquid-gas critical point is analyzed to clarify the nature of the pretransitional behavior of physical properties in the vicinity of the Blue-Phase-III--isotropic transition in chiral liquid crystalline systems. The analogy is unusual: temperature serves as the ordering field and entropy plays the role of the order parameter. Both mean field and parametric equations of state are formulated in terms of scaling fields. The scaling fields are linear combinations of the physical fields, which are temperature and chirality. It is shown that mixing of the physical field variables naturally leads to a strong asymmetry with respect to the transition temperature in the behavior of the physical properties that cannot be described by simple power laws. While the mean field theory gives a good description of the experimental data, the scaling theory, if one incorporates mixing of the field variables, gives even better agreement with the experimental data, placing this transition in the same universality class as the three-dimensional Ising model.

Thermal properties of lithium sulphate

The thermal conductivity and diffusivity of lithium sulphate have been measured simultaneously, using the transient plane source technique over the temperature range 300 - 900 K. The thermal conductivity decreases slowly up to about 640 K, whereupon a distinct rise occurs, indicating the onset of a pre-transitional behaviour, which causes a continuous growth of the conductivity up to the structural phase transition at 851 K, whereupon a very sharp increase occurs. A similar behaviour has been observed for the thermal diffusivity, for which a very sharp dip occurs at the transition point due to the exceptionally large transition enthalpy. The pre-transitional behaviour of heat transport is associated with the librational disorder of the sulphate anions known from Raman scattering studies of both phases (and neutron scattering from the cubic phase), whereas the translational disorder of lithium cations is of hardly any importance. It is thus possible to link the `paddle-wheel' concept of ion migration in the cubic phase to the enhancement of heat transport observed in the `pre-transition' region, as well as to the large difference in heat-transport rates between the monoclinic and cubic phases.

High pressure optical studies of the chirality and phase behaviour of liquid crystalline blue phases

The blue phase (BP) polymorphism of chiral S -(+)-4-(2-methylbutyl)phenyl-4-decyloxy benzoate (CE6) and S -(+)-4-(2-methylbutyl)phenyl-4-dodecyloxy benzoate (CE7) was studied at elevated transition pressures up to 280MPa using optical activity measurements. The increased density causes an increase in phase chirality which in the case of CE6 manifests itself in the disappearance of BP II (BP I-BP II-BP III triple point) and in the case of CE7 in the appearance of BP II. At atmospheric pressure, CE6 exhibits BP I, BP II and BP III, while CE7 only possesses BP I. This pressure behaviour of CE6 and CE7 is contrary to that of cholesteryl nonanoate, which has been investigated previously. This result is particularly interesting when discussed in connection with the effect of elongation of the terminal n alkylcarboxy chain of the cholesteryl n -alkanoates and of the terminal n -alkyloxy chain in the homologous series to which CE6 and CE7 belong. In the first case the phase chirality increases; in the second case it decreases. Since CE6 shows an obvious pretransitional optical activity in its isotropic liquid phase, pretransition and BP phase behaviour could be compared with each other at elevated transition pressures and temperatures. For those transition pressures or temperatures where anomalies in the BP phase behaviour occur, anomalies in the pretransition behaviour are also observed.

Pretransitional behaviour of nematogenic mixtures 2H NMR investigation

T. W. Cheung, S. Fan, G. R. Luckhurst and D. L. Turner, J. Chem. Soc., Faraday Trans., 1997, 93, 3099 DOI: 10.1039/A702282D

Pretransitional Behavior in Zincblende HgTe under High Pressure and Temperature

AbstractThe equation of state of mercury telluride, HgTe, has been measured under pressure in its zincblende and cinnabar phases up to 800K, by X‐ray diffraction under synchrotron radiation in a large volume Paris‐Edinburgh cell. The thermal expansion coefficient decreases to zero around 1.1 GPa and then becomes negative up to the first‐order phase transition to the high‐pressure cinnabar phase, around 1.4 GPa. This behavior is related to the increase with temperature of the nonlinearity in the pressure dependence of the bulk modulus. It indicates an overall softening of the phonon branches of zincblende HgTe prior to the phase transition which characterizes a second‐order pretransitional behavior.

Melting of phospholipid tubules.

The melting of lipid-based microcylinders (tubules) has been investigated for systems with single and multiple bilayer walls using high field, magnetic birefringence, and precision microcalorimetry. The pretransitional behavior of both the magnetic birefringence and the specific heat is very different in tubules with a single bilayer wall from that of tubules with multiple bilayers.

Dendrimeric Liquid Crystals: Isotropic−Nematic Pretransitional Behavior

Kerr measurements, ellipsometry, and quasielastic light scattering have been used to probe nematic fluctuations and nematic wetting at an isotropic liquid crystal−substrate interface in four generations of liquid crystalline monodendrons and dendrimers. We find that the pretransitional behavior is qualitatively similar to that of low molecular weight liquid crystals, exhibiting a Landau-like divergence of the relaxation time and a logarithmic divergence of the optical retardation on approaching the nematic−isotropic phase transition from above. Quantitatively, we find that the transition temperatures of the monodendrons are typically about 0.5 K above the supercooling limit of the isotropic phase and that the orientational viscosities may be fit to a power law in molecular weight Mn, where the exponent ≤ 1.0.

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Abstract Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4′-butylaniline (MBBA) and 4-n-pentyl-4′-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.

Electro-Optic Kerr Relaxation Studiesin a Nematic Liquid Crystal 4-(Trans-4'-n-Pentylcyclohexyl)Benzonitrile (PCH-5)

In this paper we report the variation of the Electro-optic Kerr relaxation time (τ) with temperature at and above the nematic to isotropic phase transition ( T NI ) in a polar nematic Liquid crystal 4-(trans-4'-n Pentylcyclohexyl) Benzonitrile (PCH-5). The measurements of τ and the Electro-optic Kerr constant ( B ) have been made using an indigeneously built square pulse generator with a fall time of 45 nanoseconds. The variations of B and τ with temperature have been found to follow the predictions of the Landau-de Gennes theory for pretransitional behaviour in Liquid crystals.

Concentrational effects in anisotropic artificial Kerr media.

We report the results of experimental and theoretical studies of the laser-induced birefrigence of concentrated suspensions of shaped, anisotropic artificial Kerr media. The systems we investigate experimentally are aqueous suspensions of polytetrafluoroethylene (PTFE). The observed slowing down of the relaxation of the birefringence signal is in agreement with pretransitional behavior near the isotropic-nematic phase transition. However, in order to explain the relatively small increase of the induced birefringence with an increase in concentration, and the double exponential character of relaxation, one should include the possible deviation of the optical axis from the geometrical, or long, axis of the PTFE particles.