Pretransitional Behavior Research Articles

Dielectric pre-transitional behavior in the vicinity of isotropic–blue phase III transition

Pre-transitional effects have been observed from the study of static dielectric permittivity () and its temperature derivatives () near the isotropic to blue phase-III (I–BPIII) transition in a highly polar T-shaped liquid crystalline material having a stable BPIII phase which persists over a comparatively wide range of temperature (∼8°C). The pre-transitional behavior near I–BPIII transition is found to be very similar to that observed in isotropic–nematic (I–N) transition. Landau–de Genes and Maier–Saupe approaches, used earlier to describe anomalous behavior of εS near the I–N transition, have been implemented to investigate the present pre-transitional behavior near the I–BPIII transition. The value of critical exponent (α) for the I–BPIII transition is found to be close to that of the reported I–N transition. Temperature-dependent behavior of and have also been interpreted in terms of field-induced increments of basic thermodynamic quantities, namely internal energy, entropy, and Helmholtz free energy.

Investigation of the Kerr effect and third-order susceptibility constants in a nematic liquid crystal

In this paper, the static Kerr effect was investigated for the W-1680 at temperatures above the nematic–isotropic transition temperature and compared with 9CHBT. We experimentally measured the electro-optical Kerr constant and the pre-transitional behavior. The Kerr law has been confirmed and the variation of the inverse Kerr constant, with a temperature above the smectic–isotropic transition temperatures, were determined. The linear dependence of (T − T*) − 1 on the Kerr constant is found to be in good agreement with the predications of the Landau–De Gennes model. Additionally, the temperature dependence of third order susceptibility has been studied. This value for these samples will reduce when the temperature increases.

Nematic electroclinic effect in a carbon-nanotube-doped achiral liquid crystal

A small quantity of carbon nanotubes (CNTs) dispersed in an achiral liquid crystal (LC) matrix transmits chirality a short distance into the LC, and the LC+CNT mixture is found to exhibit a bulklike electroclinic effect in the nematic phase. The magnitude of the effect increases rapidly on cooling, showing significant pretransitional behavior on approaching the nematic-smectic-A transition temperature (T(NA)) from above. The variation of the electroclinic coefficient is negligible over the frequency range 100 Hz to 100 kHz in the in the nematic phase well above T(NA) and in the smectic-A phase, whereas the electroclinic coefficient falls off significantly with increasing frequency just above T(NA).

Relatedness with teachers and peers during early adolescence: An integrated variable-oriented and person-oriented approach

The primary aims of the present longitudinal study were to examine (a) patterns of early transition relatedness with teachers and peers in 6th grade, (b) whether pre-transition behavior in 5th grade predicted early transition relatedness in 6th grade, and (c) how unique indicators of early transition relatedness with teachers and peers and patterns of early transition relatedness were associated with school adjustment among 383 rural, lower- to middle-class, White youth. Results suggest that behavioral characteristics in elementary school may contribute to early transition patterns of relatedness with teachers and peers in middle school. Findings also indicate that having a pattern of poor relationships with the primary social partners in the school context is negatively associated with adjustment above and beyond the independent indicators of relatedness. Implications for school practice are discussed.

Carbon nanotube-induced chirality in an achiral liquid crystal

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal (LC), and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the LC’s smectic-A phase, which showed significant pretransitional behavior on approaching the smectic-A–smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the LC.

PH Sensitive Polymer Nanoparticles: Effect of Hydrophobicity on Self-Assembly

The influence of hydrophobicity on formation, stability, and size of pH-responsive methacryloylated oligopeptide-based polymer nanoparticles has been studied by dynamic light scattering (DLS), transmission electron microscopy (Cryo-TEM), and NMR. Different polyanions/surfactant systems have been studied at constant polymer concentration and within a broad range of surfactant concentrations. The two newly synthesized pH-sensitive hydrophobic polyanions, poly(N(ω)-methacryloyl glycyl-L-leucine) and poly(N(ω)-methacryloyl glycyl-L-phenylalanyl-L-leucinyl-glycine), and three nonionic surfactants (Brij97, Brij98, and Brij700) have been investigated. The surfactants were different in the length of hydrophilic poly(ethylene oxide) (PEO) chain. In surfactant-free solution at basic pH, the polyanions form hydrophobic domains. In the presence of a surfactant, our results prove the complex formation at high pH between the nonionic surfactant and the polyelectrolyte; a pearl-necklace structure is formed. At low pH below critical pH (pH(tr)), reversible nanoscale structures occur in solutions for all systems. The detailed mechanism of the formation of pH-sensitive nanoparticles from polymer-surfactant complex with varying pH is established. Our results suggest that the polymer hydrophobicity is of primary importance in pretransitional behavior of the complex. Once preliminary nanoparticle nuclei are formed, the hydrophobicity of the polymer plays a minor role on further behavior of formed nanostructures. The subsequent transformation of nanoparticles is determined by the surfactant hydrophilicity, the length of hydrophilic tail that prevents further aggregation due to steric repulsions.

Electro-optical Kerr effect in the isotropic phase of the two antiferroelectric liquid crystal mixtures

We have experimentally studied for the first time the electro-optical Kerr effect and the pre-transitional behavior in the isotropic phase of two antiferroelectric liquid crystal mixtures, W-232 and W-204D, composed of rod-like ester molecules exhibiting the direct smectic-A to isotropic (SmA-I) phase transition. The Kerr law has been confirmed for the two compounds and the variation of inverse Kerr constant with temperature above the smectic–isotropic transition temperatures were determined. Both the mixtures with very broad antiferroelectric phase around room temperature have similar sequence of the phases (i.e., Cr-SmC*A-SmC*-SmA-I). Although, the pre-transitional behavior is usually complex in the isotropic phase of the chiral smectic liquid crystal compounds, the investigated compounds showed a similar behavior compared to that of nematic–isotropic behavior.

High-temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle

SUMMARY Mineralogical models suggest that low velocity zones of the upper mantle may be accounted for by elasticity of dry rocks along geotherms away from the mid-oceanic ridges (MOR) while, closer to MOR, anelasticity plays a significant role in reducing shear wave velocities VS and generating attenuation. We investigate the potential influence of elastic softening, precursor of high-temperature phase transition in orthopyroxenes, on the seismic properties of the upper mantle. In situ Brillouin and Raman spectroscopy were used to evidence pre-transitional behaviour at high temperature in natural San Carlos orthopyroxene. Pre-transitional behaviour induces a large softening of the acoustic and low frequency modes, resulting in a large anharmonic decrease of sound velocities, similar to that observed in the MgSiO3 end-member orthoenstatite. The high-temperature high-pressure phase diagram of enstatite is revised to account for the new phase transition, and a simple model is developed to evaluate elastic softening effects on the upper-mantle seismic properties, whose results depend much on the as yet poorly constrained pressure and compositional dependence of the phase transition boundary. Within the tested range of parameters, OPx softening is likely to affect the seismic properties of mantle rocks at depths shallower than 80 km in hot regions. Thus elastic softening of pyroxene is unlikely to affect the LVZ or continental lithospheric mantle unless the transition temperature is drastically reduced by incorporation of aluminium in orthopyroxene. It will contribute to a decrease of VS near MOR, hot spots and evolved continental rifts, where it can explain part of the non-linear high-temperature decrease of VS in the lithospheric oceanic mantle. It will affect the magnitude of anelastic effects and the nature of the likely mechanisms of attenuation required to match seismological observations.

Investigations of phase transitions in liquid crystals by means of adiabatic scanning calorimetry

High-resolution calorimetric techniques have substantially contributed in characterising and understanding the delicate thermal behaviour near many phase transitions in liquid crystals. In this paper we describe a high-resolution adiabatic scanning calorimetric technique that has proven to be an important tool in discriminating between first-order and second-order phase transitions in addition to rendering high-resolution information on fluctuations-induced pretransitional specific heat capacity behaviour. The capabilities of adiabatic scanning calorimetry are illustrated with experimental results for the isotropic to nematic and the isotropic to smectic A transitions for a series of alkylcyanobiphenyl compounds. For the nematic to smectic A transition results are presented for pure compounds and mixtures of liquid crystals as well as on the effects of added non-mesogenic solutes and nanoparticles. For chiral molecules results for phase transitions involving blue phases and twist grain boundary phases are considered.

Dynamic Light Scattering Study of Concentrated Triblock Copolymer Micellar Solutions under Pressure

The results from a dynamic light scattering study of concentrated aqueous systems of polyethylene oxide−polypropylene oxide−polyethylene oxide triblock copolymer (PEO−PPO−PEO, Pluronic F108) are reported at pressures to 2500 bar and temperatures from 0 to 110 °C. The pressure−temperature (P−T) phase behavior for this micellar system exhibits several phase transformations at high concentrations including a transition from a liquid to a gel phase and a subsequent re-entrant transition from the gel phase to a liquid phase with increasing temperature. The gel phase region is bounded at low temperatures by a liquid-to-gel phase transition curve that increases in pressure with increasing temperature, eventually exhibits a maximum in pressure, and then at high temperatures rapidly decreases as the temperature is further increased. The high temperature branch of this curve represents a gel-melting transition. Aqueous F108 samples are doped with latex probe nanoparticles to facilitate the characterization of the local viscosity and also to serve as a reference for the measurement of the light intensity scattered by the Pluronic systems. Both the intrinsic collective dynamics of the Pluronic systems and the diffusive dynamics of the nanoparticle probes are characterized. The diffusive dynamics of the nanoparticle probes reveal the nonmonotonic character of viscosity that results from phase transformations dependent on pressure and temperature. The micellar system scattering intensity, obtained from DLS data through comparison with the light intensity scattered by the probe nanoparticles, increases with heating indicating critical pretransitional behavior of concentration fluctuations on approach to the cloud-point boundary over a broad temperature interval that spans the gel phase temperature range.

Pretransitional behaviour in the vicinity of the isotropic–nematic transition of strongly polarcompounds

The isotropic–nematic transition, being weakly first order, exhibits pretransitional effectssignifying the appearance of the nematic-like regions in the isotropic phase. In the isotropicphase, strongly polar liquid crystals, such as the popular alkyl and alkoxy cyano biphenylbehave in a non-standard fashion: whereas far away from the transition the dielectric constantεiso hasa 1/T dependence (a feature also commonly seen in polar liquids), on approachingthe nematic phase the trend reverses resulting in a maximum inεiso, at a temperature slightly above the transition, an effect explained on the basisof short-range correlations with an antiparallel association of the neighbouringmolecules. Recently, there has been a revival in studies on this behaviour to possiblyassociate it with the order of transition. Here we report dielectric measurementscarried in the vicinity of this transition for a number of compounds having differentmolecular structures including a bent core system, but with a common featurethat the molecules possess a strong terminal polar group, nitro in one case andcyano in the rest. Surprisingly, the convex shape of the thermal variation ofεiso wasmore an exception than the rule. In materials that exhibit such an anomaly we find a linear correlationbetween δε = (εpeak−εIN)/εIN and δT = Tpeak−TIN,where εpeak is the maximum value of the dielectric constant in the isotropic phase,εIN the value at thetransition, and Tpeak and TIN the corresponding temperatures.

Diverging elasticity and director uniformation in a nanopatterned cell near the nematic–smectic-Aphase transition

The stylus of an atomic force microscope is used to scribe herringbone patterns of various wavelengths into a polyimide-coated substrate. The patterns serve as a template for alignment of the liquid crystal octyloxycyanobiphenyl and impose a bend distortion in the liquid crystal in the vicinity of the herringbone apexes. The pretransitional behavior of the liquid crystal is observed by polarized microscopy as it is cooled through the nematic-smectic- A phase transition, facilitating direct visualization of the extrapolation length, which is related to the trade-off between elastic and anchoring forces. Just above the phase transition temperature the expulsion of bend deformation is observed and is shown to be in good quantitative agreement with continuum theory. Very close to the transition temperature a weak threshold behavior is observed, wherein the smectic- A phase forms a monodomain for short period herringbones, but breaks into multiple domains when the patterned period is large.

Singular Pretransitional Behavior of the Electric Field-Dependent Part of the Thermodynamic Quantities of Strongly Polar Mesogenic Liquids in the Isotropic Phase

A new interpretation of the pretransitional temperature behavior of the static dielectric permittivity and its derivative in terms of the basic thermodynamic quantities: the internal energy, the entropy, and the Helmholtz free energy is presented. It was shown that in the case of strongly polar mesogenic compounds (as the homologous series of alkylcyanobiphenyls) the electric field-induced increments of both the internal energy and the entropy exhibit a critical-like temperature dependence in the vicinity of transition from the isotropic liquid to the nematic phase. As a most important pretransitional effect, it is shown that at a temperature of about 10 degrees before the phase transition, an electric field applied to the isotropic liquids induces an increase in the entropy (∆S > 0) instead of its decrease, as observed far from the phase transition or for the non-mesogenic dipolar liquids.

Pre-transitional temperature behaviour of the shear viscosity of freely flowing thermotropic liquid crystals†

This article presents a review of experimental results on the shear viscosity of freely flowing nematic liquid crystals and the mixtures exhibiting a re-entrant nematic phase. Peculiarities in the viscosity temperature behaviour in the vicinity of the nematic to smectic A phase transition are discussed. †This article is dedicated to Professor Jerzy A. Janik on his 80th birthday.

Pretransitional effects near the smectic-A–smectic-C*phase transition of hydrophilic and hydrophobic aerosil networks dispersed in ferroelectric liquid crystals

A detailed x-ray scattering and high-resolution ac calorimetric study has been carried out near the smectic-A to chiral smectic-C phase transition of liquid-crystal compounds 4-(2-methyl butyl) phenyl 4-n-octylbiphenyl-4-carboxylate (CE8) and p-( n-decyloxy) benzylidene-p-amino-(2-methylbutyl) cinnamate (DOBAMBC) confined in hydrophilic and hydrophobic aerosil nanoparticle networks. The character of the transition, which is mean field near a tricritical point in bulk, is changed dramatically with an increase of aerosil-induced disorder. X-ray measurements revealed pretransitional behavior and compression of the smectic layers, phenomena that are strongly pronounced in high aerosil concentrations. A theoretical model that takes into account the interplay of relevant mechanisms is proposed to explain the observed phenomena. The effect of chirality on the interaction of liquid crystals with aerosils is discussed.

Phase Morphologies and Viscoelastic Relaxation Behaviors for an LCST‐Type Polymer Blend Composed of Poly(methyl methacrylate) and Poly[(α‐methyl styrene)‐co‐acrylonitrile

AbstractSummary: Small amplitude oscillatory shear rheology is employed to investigate the linear viscoelastic behavior of the LCST‐type PMMA/α‐MSAN polymer blends as a function of annealing temperature and time. The results reveal that when temperature approaches the separation temperature, the blends exhibit some characteristic of complex thermorheological behavior, such as a shoulder in the dynamic storage modulus G′ or the linear relaxation modulus G(t), the appearance of a loss tangent (tan δ) peak and the additional relaxation in the relaxation spectrum H(τ). All of these can be attributed to the enhanced concentration fluctuations near the phase boundary. The anomalous pretransitional behavior can be quantified to yield the binodal temperature from the inflexion of variation and the spinodal temperature on the basis of the mean field theory. Furthermore, the linear viscoelastic properties of the phase‐separated PMMA/α‐MSAN (80/20) blends can be described well by either Palierne's or Bousmina's emulsion model in virtue of the calculated $\alpha /\bar R_v$ value in the whole frequency region.Morphology images of 80/20 PMMA/α‐MSAN blends annealed for 4 h at 180 °C observed from phase contrast microscope.magnified imageMorphology images of 80/20 PMMA/α‐MSAN blends annealed for 4 h at 180 °C observed from phase contrast microscope.

Optical and electro-optical derivation of the pretransitional behavior of orientational and shear viscosities in the isotropic phase of liquid crystals

The pretransitional equilibrium properties of isotropic liquids in the proximity of the isotropic-nematic phase transition are well known and successfully modeled. Much less is known about the dynamic behavior, and in particular about the pretransitional viscosity. In this work we combine two techniques [dynamic light scattering (DLS) and electric birefringence spectroscopy (EBS)] offering complementary insights into both static and dynamic pretransitional behavior of the homologous nCB family (n-alkyl cyanobiphenyl). EBS explores the single molecule flipping dynamics retarded by a paranematic potential barrier and enables extracting the associated transport coefficient, which is found to be of Arrhenius type in the whole temperature range explored. DLS reflects the collective dynamics of correlated domains and depends on the viscous damping of the orientational order. Such a viscosity displays Arrhenius behavior only sufficiently far from the transition temperature, with deviations growing as a power law of the appropriate reduced temperature with exponents around 0.1.

Electro-optic characterization of novel tolane-based nematic liquid crystals

Four new tolane-based liquid crystals, 4′-pentyl-, 4′-hexyl-, and 4′-heptyl-3-fluoro-4-isothiocyanatotolane and 4′-hexyloxy-3′-fluoro-4-isothiocyanatotolane have been investigated for the first time to determine their electro-optical behavior and third order non-linearity by the static Kerr effect method. The temperature dependence of electric Kerr constant in the isotropic phase and the pretransitional behavior, related to order parameter fluctuations, have been investigated for four new high birefringence nematic liquid crystals, laterally fluorine substituted isothiocyanatotolanes in the isotropic phase. All the compounds had a positive and large Kerr constant which increased with decreasing temperature. The Landau–De Gennes model was obeyed for these compounds.

Kerr effect of low melting nematic liquid crystals with negative dielectric anisotropy

The two low melting nematic liquid crystals, 2‐chloro‐4‐heptylphenyl 4‐pentylbicyclo[2,2,2]octane‐1‐carboxylate (7CP5BOC) and 2‐chloro‐4‐heptylphenyl 4‐heptylbicyclo[2,2,2]octane‐1‐carboxylate (7CP7BOC) have been investigated to determine their electro‐optical behaviour and third order non‐linearity by the static Kerr effect method. Both liquid crystals are laterally substituted by a single chlorine atom located close to the ester linking group. The temperature dependence of the electric Kerr constant in the isotropic phase and the pretransitional behaviour have been investigated for these low birefringence nematic liquid crystals in the isotropic phase. Both the compounds, with negative dielectric anisotropy, have a positive Kerr constant. The Landau–de Gennes model was obeyed for these compounds.

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.