Pretransitional Behavior Research Articles

Pretransitional behaviour of a nematic liquid crystal in the isotropic phase

Polarized and depolarized light scattering measurements have been performed from a nematic liquid crystalline material in the isotropic region, approaching the nematic phase transition. A critical static scattering intensity and a critical slowing-down of the orientational fluctuations were observed in a temperature range of about 10 o C above the phase transition which followed a (T-T c *-γ dependence, but with an exponent slightly smaller than unity. The angular dependence of the polarized static scattering intensity differed in behaviour from other liquid crystals by the occurrence of a strong excess scattering at small scattering angles. These observations give evidence for spatial structural heterogeneities, spherical in shape and optically isotropic, in a bath of a nematic liquid crystalline system. This conclusion is confirmed by dynamic light scattering where in addition a slow mode appeared in the time correlation function which results from random migration of these heterogeneities

Composition fluctuation induced depolarized Rayleigh scattering from diblock copolymer melts

Dynamic light scattering in the depolarized geometry has been used to investigate the pretransitional behavior of diblock copolymer melts near the order–disorder transition (ODT). Besides the segmental orientational dynamics, an additional slow relaxation process contributes to the dynamic depolarized intensity. This new relaxation mode exhibits an almost exponential shape, a wave vector (q) independent relaxation rate, and a strongly temperature dependent amplitude that compares with the temperature dependence of the maximum of the static structure factor at q*, measured with small-angle x-ray scattering. An account for the characteristics of this new relaxation process is based on the coupling between orientation and order parameter fluctuations. The depolarized intensity is due to orientation and/or extension of the copolymer chains in the pretransitional state above the ODT, with the relevant length scale of the slow orientation fluctuations being close to 2π/q*.

Anomalies of the temperature dependence of electrostriction in blue phases

We investigated the field-induced shift of the blue phase diffraction bands. The temperature dependence of the electrostriction tensor components is determined for the first time. In the blue phase BPI at constant field strength ${\bf E}/\!\!/~[001]$, a change of the temperature leads to a change of both the unit cell volume and its shape. The deformation of the unit cell displays a pretransitional behaviour on approaching the BPI↦BPII phase transition temperature.

Pre-Transitional Behavior in Cubic Potassium Niobate

ABSTRACTPre-transitional behavior prior to the onset of the cubic-to-tetragonal phase transformation in KnbO3 has been studied with synchrotron x-ray scattering at NSLS, Brookhaven National Lab. Measurements of the Thermal Diffuse Scattering (TDS), Grazing Incidence X-ray Scattering (GIXS), and precursor Bragg scattering have been performed at a series of temperatures on a single crystal sample in the cubic phase (Tc = 405 °C). The TDS was measured in the (001), (011) and (111) cubic planes. The GIXS was measured above, at and below the critical scattering angle of the sample. Based on localized soft mode theory, dynamic tetragonal fluctuations may be expected prior to the onset of the phase transition initiated by defects or surfaces. The GIXS data indicate that the tetragonal phase is apparent prior to the phase transition of the bulk - - consistent with the notion that nucleation originates from potent defect sites during a martensitic-like displacive transformation. The TDS intensity distribution is disk-shaped; arising from a localized, low energy transverse optic mode in the [100] directions extending out to the Brillouin zone boundary (reduced wave vector, q =0.5). Scans made in the (011) planes show TDS intensity extending out to a reduced wave vector of q ˜ 0.2. Preliminary analysis indicates that a low energy transverse optic mode extends in the [011] direction as well. The TDS intensity decreases as the transition temperature is approached from above. Comparison of the TDS results with inelastic neutron scattering data will be made.

Laser-Induced Pretransitional Ordering And Nonlinear Optical Properties Of Rigid-Rod Polymer Suspensions

ABSTRACTWe investigate the pretransitional behavior in laser-induced alignment of rigid rod-like polytetraflouroethylene (PTFE) suspensions. Using a laser-induced birefringence experiment, we measure both the orientational order parameter and the orientational relaxation time. We find that both increase as the volume fraction of colloidal particles approaches the critical value for the isotropic-nematic phase transition. Experimental results are compared with theory which takes into account the possibility of a first-order phase transition induced by a laser electric field.

Transient Kerr response in a nematic liquid crystal

Measurements are reported on the DC Kerr response and the pre-transitional behaviour of a negative nematic liquid crystal-p-ethoxy-benzylidine-p-butylaniline (EBBA)-at temperatures above the nematic-isotropic transition temperature. The observed second-order pre-transitional temperature T* from the measurements is found to be within 1 degrees C below the first-order nematic-isotropic transition temperature. The observed dependence of the Kerr constant on (T-T*)-1 is found to be in good agreement with the predictions of the Landau-de Gennes model. The pre-transitional temperatures have also been investigated in p-ethoxy benzylidine p-butyl aniline-p-methoxy benzylidene p-n-butyl aniline (EBBA-MBBA) liquid crystal mixtures.

Optical Studies of Pretransitional Surface Ordering and Disordering in Liquid Crystals

Abstract The Landau-deGennes theory predicts interesting pretransitional surface behavior at the interface between a liquid crystal and a smooth or rough substrate. In the former case, we propose a simple form for the liquid crystal-substrate interaction which yields enhanced surface ordering in the nematic and a uniaxial-biaxial transition in the isotropic phase. For the experimentally accessible range of liquid crystal-substrate interaction strengths, the experimentally observed pretransitional behavior in the isotropic phase is in excellent agreement with the theory. For the case of a rough substrate, pretransitional surface disordering in the nematic is predicted and measured directly by evanescent-wave ellipsometry, while pretransitional random-planar surface ordering occurs in the isotropic phase.

An Investigation of Pre-Transitional Effects in a Thin Liquid Crystal Layer via the Excitation of ps Mixed Optic Modes

Abstract The pre-transitional Kerr effect in K18 is examined using a novel technique. Mixed polarisation grating coupled guided modes are excited within the liquid crystal layer, these are probed for small changes in position using a differential technique which resolves induced birefringences to an accuracy of 5 × 10−8. The experiment is performed at several different temperatures in order to characterize this pre-transitional behaviour, and from this, one of the Landau expansion coefficients can be found.

Transient kerr effect in nematic liquid crystal mixtures

Measurements are reported on dc Kerr response and the pretransitional behavior in (p-ethoxybenzylidine)-p- butylaniline (EBBA) - (p-methoxybenzylidine)-p-butyl aniline (MBBA) liquid crystal mixtures at temperatures above nematic to isotropic transition temperature. The observed dependence of Kerr constant on (T-T☆) −1 is found to be in good agreement with the predections of Landau-de Gennes model.

Nematic-isotropic pretransitional behaviour in dimers with odd and even spacer lengths

Abstract Magnetic field induced optical birefringence measurements in the isotropic phase are reported for two members of a series of phenyl benzoate dimers: one having an odd number of methylene units in the spacer and one having an even number. The discontinuity at the nematic-isotropic transition, as determined by the difference between the first order phase transition temperature TNI and the supercooling limit of the isotropic phase T∗, is approximately 8 ± 3 times larger for the even species than for the odd. In addition, the product of the Cotton-Mouton coefficient dΔn/dH2 and T–T∗ is the same for both materials. In terms of both steric and molecular field theories, the results indicate that the orientations of the terminal mesogenic groups are essentially uncorrected in the isotropic phase.

Measurements of electric conductivity and reorientation times for the cesium perfluorooctanoate-D2O micellar system.

Measurements of the electric conductivity and the relaxation times for reorientation in magnetic fields are presented for the cesium perfluorooctanoate (CsPFO)--${\mathrm{D}}_{2}$O micellar system, as a function of temperature and concentration. Five sample concentrations are presented, namely 41%, 48%, 50%, 55%, and 60% of CsPFO in ${\mathrm{D}}_{2}$O by weight. At these concentrations, the system shows a nematic liquid crystal, between a lower-temperature lamellar smectic and a higher-temperature isotropic micellar phase. At the transition from the nematic to lamellar phase, the principal conductivities change rather continuously, and do not show any marked features. The measurements show that at the same reduced temperature, the structure of the nematic phase becomes more anisotropic and the relaxation times become longer with decreasing CsPFO concentration in the system, suggesting that at the same reduced temperature, the size of the nematic micelles increases with decreasing CsPFO concentration. Comparison of the conductivity anisotropy data with values calculated from structural models allows estimates of the aspect ratio of the micelles and the fraction of the area of the smectic planes that is covered by water. The behavior of the relaxation times at the nematic-to-lamellar transition shows dependence on the concentration. The relaxation times at the nematic-to-lamellar transition for the four lower concentrations diverge; the critical exponents in order of increasing CsPFO concentration are 0.71, 0.76, 0.77, and 0.85. The 60% sample does not show any pretransitional behavior at this transition.

Light scattering and magnetic birefringence measurements at the isotropic-nematic and nematic-reentrant-isotropic transitions in a lyotropic nematic liquid crystal.

The pretransitional behavior in the isotropic and reentrant-isotropic phases near a uniaxial nematic phase in the potassium laurate--decanol--${\mathrm{D}}_{2}$O system has been studied by light scattering and magnetically induced birefringence. The light-scattering measurements show mean-field behavior at both isotropic transitions and strong angular dependences from which a bare correlation length of 22 \AA{} has been determined for both isotropic phases. The magnetic-field effects, however, depart from mean-field behavior, and, in conjunction with the light-scattering data, reveal a temperature-dependent magnetic anisotropy and furnish evidence for a change in micellar shape.

Brillouin and ultrasonic studies of phase transitions in KA1F4and K0.94Rb0.06A1F4

Abstract The elastic behaviour of KA1F4 has been studied above the martensitic-like first order (P4/mbm - P21/m) transition at T24 = 260 K : Brillouin and ultrasonic results show significant pretransitional softenings of C33 and of C44 while the other stiffness constants are not affected. In K0.94Rb0.06AlF4, an intermediate monoclinic phase appears between T23 = 205 K and T34 = 175 K. The C33 pretransitional behaviour observed in the pure compound well extrapolates down to T23. Below T23, domain walls prevent ultrasonic measurements whereas around T23 the Brillouin spectra exhibit anomalies in C11, C22 and more drastically in C33. These variations are discussed within the framework of a Landau development of the free energy.

Density studies on various smectic liquid crystals

Abstract Density measurements as a function of temperature for four homologues of the 5-n-alkyl-2-(4-n-alkyloxy-phenyl)-pyrimidines (PYP nOm) which exhibit nematic, smectic A and smectic C phases are reported. Furthermore 1-butyl-c-4-(4′-octyl-biphenyl-4-yl)-r-cyclo-hexan- carbonitrile (NCB84) is studied; this has additionally a smectic G phase. From these data the thermal expansion coefficients are calculated. Comparing PYP 907 and PYP 709, differing in their exchanged alkyl chains, we observe in the smectic A and the smectic C phase a distinctly lower density for PYP 709 whereas their densities nearly agree in the isotropic phase. The pyrimidines PYP 709 and PYP 808 exhibit a continuous volume change on crossing the smectic A-smectic C transition which differs dramatically from PYP 909 which shows a small volume jump. Furthermore a binary mixture of PYP 708 and PYP 706 is analysed which shows only a nematic and a smectic C phase. The associated phase transition is probably first order revealing nearly no pretransitional behaviour. The smectic A-smectic C transition of NCB84 seems to be second order exhibiting a continuous change of volume across the transition whereas the smectic C-smectic G transition shows a volume discontinuity and is first order. In order to induce ferroelectric smectic C∗ phases all smectic C materials were doped with a chiral pyrimidine dopant. Astonishingly the thermal expansion coefficient across the smectic A-smectic C∗ transition is influenced by the dopant in a very different way.

Measurements of density and expansion coefficient for the nematic, lamellar smectic, and isotropic phase of micellar systems

Measurements of the density and expansion coefficient as a function of temperature are presented for two micellar systems, namely, decylammonium chloride/NH4Cl/water, and cesium perfluorooctanoate/water. The measurements cover the lamellar smectic, the nematic, and the isotropic micellar solution. The measurements show that away from the phase transitions, the density decreases smoothly with increasing temperature. At the nematic-to-isotropic transition the density shows steplike features, indicating a change in the micellar structure at the transition, with denser packing of the amphiphiles in the micelles of the isotropic phase. A similar change is observed in the lamellar to nematic transition, however, the detailed behavior at this transition showed some dependence on the history of the sample. The expansion coefficients decrease at the transitions. Considerable pretransitional behavior is observed about the lamellar-to-nematic transition.

Calorimetric study of smectic polymorphism in octyloxyphenyl-nitrobenzoyloxy benzoate + decyloxyphenyl-nitrobenzoyloxy benzoate mixtures.

Binary mixtures of octyloxyphenyl-nitrobenzoyloxy benzoate (${\mathrm{DB}}_{8}$${\mathrm{ONO}}_{2}$) and decycloxyphenyl-nitrobenzoyloxy benzoate (${\mathrm{DB}}_{10}$${\mathrm{ONO}}_{2}$) exhibit a rich variety of smectic polymorphism. An ac calorimetric study has been carried out on six mixtures with mole percent X of the decyl homolog between 51 and 57, a range that includes the Sm-${A}_{d}$--${N}_{r}$--Sm-${A}_{1}$ point at X\ensuremath{\simeq}56. The sample with X=57 exhibits a first-order direct Sm-${A}_{d}$--Sm-${A}_{1}$ transition, while the sample with X=51.3 exhibits two transitions---a second-order Sm-${A}_{d}$--${N}_{r}$ transition with a very small excess heat-capacity peak and a tricritical ${N}_{r}$--Sm-${A}_{1}$ transition with a large heat-capacity peak. For samples with intermediate compositions, only weakly first-order ${N}_{r}$--Sm-${A}_{1}$ heat-capacity peaks are observed in the vicinity of the Sm-${A}_{d}$--${N}_{r}$--Sm-${A}_{1}$ point. All six investigated samples exhibit the Sm-${A}_{1}$--Sm-C\ifmmode \tilde{}\else \~{}\fi{}--Sm-${C}_{2}$ phase sequence at lower temperatures. The Sm-${A}_{1}$--Sm-C\ifmmode \tilde{}\else \~{}\fi{} transition is a weakly first-order phase transition with pretransitional excess heat capacity below the transition temperature. In contrast, the Sm-C\ifmmode \tilde{}\else \~{}\fi{}--Sm-${C}_{2}$ transition is a weakly first-order inverted Landau transition with no pretransitional behavior in the Sm-${C}_{2}$ phase below the transition but a large mean-field excess heat capacity observed in the Sm-C\ifmmode \tilde{}\else \~{}\fi{} phase above the transition.

ChemInform Abstract: Pretransitional Behavior in Liquid Crystals. The Roles of NMR Spectroscopy and Molecular Field Theory

ChemInform Abstract: Pretransitional Behavior in Liquid Crystals. The Roles of NMR Spectroscopy and Molecular Field Theory

Pretransitional behaviour in nematogens composed of flexible molecules

Certain properties of the isotropic phases of nematogenic liquid crystals are observed to diverge as the transition to the nematic phase is approached. Here we develop a molecular field theory for the divergence or bifurcation temperature, T*, of nematogens composed of flexible molecules. The theory is based on an extension of the Marcelja model and results in a relatively simple expression for the divergence temperature because of the low orientational order. The molecular factors which influence T* are considered in detail. The theory is used to predict the variation in the divergence temperature along two homologous series. In one, the 4-n-alkyloxy-4′-cyanobiphenyls, there is a single flexible alkyl chain attached to a mesogenic group while in the other series, the α-ω-bis(4,4′-cyanobiphenyloxy)alkanes, the alkyl chain links two mesogenic groups. The theory predicts a significantly larger alternation in T* and T*/T NI for the series of dimers than for the conventional nematogens. These predictions are in agreement with results for a main chain liquid-crystalline polymer and have subsequently been confirmed for the dimers.

Phase transitions and rheology of aramid solutions

Abstract Clearing temperatures of solutions of poly(4-4′-benzanilidylene-terephthalamide) in concentrated sulphuric acid are presented as a function of polymer concentration and average molecular weight. The orientational order parameter is obtained from birefringence measurements. The experimental results are explained by a mean-field type theory similar to the Maier-Saupe model for thermotropic liquid crystals. Molecular flexibility, concentration and molecular weight are taken into account by using simple scaling factors. The birefringence induced by shear flow in an isotropic solution of poly(para-phenylene-terephthalamide) shows a strong pretransitional behaviour. This indicates the occurrence of a flow-induced phase transition.

Light-scattering study in nematic-smectic-A-smectic-C multicritical mixtures.

Light scattering in the bend, splay, and bend-twist modes has been studied in two binary liquid-crystal systems exhibiting the nematic\char21{}smectic-A\char21{}smectic-C multicritical phase diagram. The results on the behavior of the longitudinal correlation length above the nematic\char21{}smectic-A transition are found to be in agreement with x-ray scattering data. The tilt susceptibility above the smectic-A\char21{}smectic-C transition is consistent with a simple mean-field description. The pretransitional behavior of the bend and splay elastic constants above the nematic\char21{}smectic-C transition in a pure material is found to follow the predictions of the Chen-Lubensky model. Near the multicritical point, the temperature dependence of the light-scattering intensity exhibits an anomalous minimum above the nematic\char21{}smectic-C transition in the bend and bend-twist modes but not in the splay mode. This behavior is not described by any existing model.