Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz-13 MHz) has been used to analyse the frequency, temperature and bias-field dependences of the molecular dynamics of a very high-spontaneous-polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature-dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X-mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*-SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*-SmA phase transition was revealed.     

Dielectric spectroscopy of a smectic liquid crystal

Complex dielectric spectroscopy (frequency range 5 Hz–13 MHz) has been used to analyse the frequency, temperature and bias‐field dependences of the molecular dynamics of a very high‐spontaneous‐polarization ferroelectric liquid crystalline material exhibiting SmA, SmC* and unknown SmX smectic phases. Different smectic phase transition temperatures have been observed from the study of the temperature dependence of the dielectric strength and the relaxation frequency. The phase transition temperatures (crystalline to isotropic phases) have also been described very accurately from the temperature‐dependent symmetric and asymmetric shape parameters of the relaxation function and also the dc conductivity. In a planar aligned cell, two symmetric modes (Goldstone mode and domain mode) have been observed in both the SmX and SmC* phases. One asymmetric mode (X‐mode) observed in the SmC* and SmA phases could be related to the interaction of dipoles of the ferroelectric liquid crystals being affected by the surface of the cell. The soft mode, which usually appears very close to the SmC*–SmA phase transition was not observed until the bias field was applied. The second order nature of the SmC*–SmA phase transition was revealed.     

Short range order in the isotropic phase of antiferroelectric liquid crystals

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C^*_A, and smectic Q. In the other system the phase sequence is smectic C^*, smectic C^*_A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results.     

First observation of double-twisted helical structure in a banana-shaped achiral molecule

The double-twisted helical structure was confirmed for the first time for the smectic phase of a banana-shaped molecular system from observation of the microscopic pattern. Optically polarized microscopic textures and phase transitions of 1,3-phenylene bis\[4-(3-chloro-4- n-octyloxyphenyliminomethyl) benzoate (PBCOB) and related compounds are described. The chloro-substituent introduced into the 3-position of the Schiff's base moiety of the banana-shaped molecule destabilized the smectic phase and enhanced new mesomorphic properties. The mesophases exhibited by the octyloxy homologue of the chloro-substituted compound were of the smectic phases, SmX1 and SmX2. The mixing behaviour between chloro-substituted and non-chloro-substituted compounds resulted in the appearance of a wide miscibility gap and enlargement of the SmX1 phase which can be seen in a lateral chloro-substituted compound.     

First observation of double-twisted helical structure in a banana-shaped achiral molecule

The double-twisted helical structure was confirmed for the first time for the smectic phase of a banana-shaped molecular system from observation of the microscopic pattern. Optically polarized microscopic textures and phase transitions of 1,3-phenylene bis\[4-(3-chloro-4- n-octyloxyphenyliminomethyl) benzoate (PBCOB) and related compounds are described. The chloro-substituent introduced into the 3-position of the Schiff's base moiety of the banana-shaped molecule destabilized the smectic phase and enhanced new mesomorphic properties. The mesophases exhibited by the octyloxy homologue of the chloro-substituted compound were of the smectic phases, SmX1 and SmX2. The mixing behaviour between chloro-substituted and non-chloro-substituted compounds resulted in the appearance of a wide miscibility gap and enlargement of the SmX1 phase which can be seen in a lateral chloro-substituted compound.     

High pressure study of the cholesteric/smectic C* phase transition of 4-n-hexyloxyphenyl-4'-(2'-methylbutyl)biphenyl-4-carboxylate (CE3)

The cholesteric (Ch)/smectic C* (S_c*) phase transition of CE3 has been studied up to 2kbar and 115°C by a light reflection method. In contrast to the cholesteric/smectic A phase transition, which can cross over from first to second order at elevated pressure, the Ch/S_c* transition of CE3 was found to remain first order. This result is in agreement with most theoretical predictions. The pretransformation on the cholesteric side of the Ch/S_c* transition is influenced only weakly by increasing pressure. Because of the high viscosity of the smectic C* phase corresponding results could not be obtained on the smectic side of the transition.     

Determination of elastic constants of a binary system (7CPB+9.CN) showing nematic,induced smectic Ad and re-entrant nematic phases

The temperature variation of the splay and bend elastic constants of a binary system exhibiting nematic-induced smectic A_d and re-entrant nematic phases measured by electric field-induced Freedericksz transition method has been reported. As bend deformation is not permitted in the smectic A phase, bend elastic constant (K₃₃) could only be determined in the nematic and re-entrant nematic phases. In both the nematic phases, the splay elastic constant has the same order of magnitude and does not show any pretransitional effect. However, in the induced smectic A_d phase, the value of K₁₁ is about one to two orders higher than that in the nematic phases. The bend elastic constant shows a strong pretransitional effect near the nematic–smectic and smectic–re-entrant nematic phase transitions. The influence of the presence of the induced smectic phase is observed even in those mixtures which have no induced smectic phases. As the smectic phase is approached, the ratio K₃₃/K₁₁ increases rapidly and diverges to infinity.     

Short range order in the isotropic phase of antiferroelectric liquid crystals

Optical activity measurements in the isotropic phase of two antiferroelectric liquid crystal systems in which the chirality can be varied reveal unusual behaviour of the short range order. In one system the phase sequence as the chirality is increased is smectic A, smectic C^* _A, and smectic Q. In the other system the phase sequence is smectic C^*, smectic C^* _A, and smectic Q as the chirality is increased. The short range order of the isotropic phase behaves similarly for these systems, showing mean field behaviour at low chirality and far above the phase transition, but deviating from this behaviour significantly as the chirality is increased and the phase transition is approached. These optical activity results indicate how different is the short range order in the isotropic phase for these antiferroelectric liquid crystal systems and demonstrates the crucial role played by chirality. Past theoretical work that includes smecticlike fluctuations in the calculation of short range order in the isotropic phase is capable of qualitatively explaining these results.     

Effect of spontaneous polarization on smectic C*-smectic A* phase transition temperature and the thickness dependence of the spontaneous polarization of ferroelectric liquid crystal

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical     

Pressure-induced re-entrant cholesteric phase behaviour of non-polar liquid crystals

In a previous paper we reported the existence of a pressure-induced re-entrant cholesteric phase in mixtures of non-polar liquid crystals. Now the influence of the mixing ratio on this behaviour has been studied up to 3000 bar and 190°C and the phase boundaries based on light reflection measurements have been confirmed by transmission and texture observations in a diamond anvil cell. Additional thermodynamic investigations show that when the cholesteric/smectic A phase transition line possesses a maximum temperature the pretransition enthalpy and volume disappear.     

Nature of smectic A*-C* phase transitions in a series of ferroelectric liquid crystals with little smectic layer shrinkage

The smectic layer spacing of two homologous series of ferroelectric liquid crystal compounds was characterized by small-angle x-ray diffraction and different degrees of smectic layer shrinkage on cooling from the SmA* into the SmC* phase were observed. The smectic A*-smectic C* phase transition was further studied by measuring the thermal and electric field effects on the optical tilt angle and the electric polarization. With decreasing length of the alkyl terminal chain the phase transition changes from tricritical exhibiting high layer shrinkage to a pure second-order transition with almost no layer shrinkage. This is explained by the increased one-dimensional translational order of the smectic layers, which seems to promote the "de Vries"-type [Mol. Cryst. Liq. Cryst. 41, 27 (1977)] smectic A*-C* phase transition with no or little layer shrinkage.     

Ferroelectric Liquid Crystals: Synthesis and Thermal Behavior of Optically Active,Three‐Ring Schiff Bases and Salicylaldimines

The chiral ferroelectric smectic C (SmC*) phase, characterized by a helical superstructure, has been well exploited in developing high‐resolution microdisplays that have been effectively employed in the fabrication of a wide varieties of portable devices. Although, an overwhelming number of optically active (chiral) liquid crystals (LCs) exhibiting a SmC* phase have been designed and synthesized, the search for new systems continues so as to realize mesogens capable of meeting technical necessities and specifications for their end‐use. In continuation of our research work in this direction, herein we report the design, synthesis, and thermal behavior of twenty new optically active, three‐ring calamitic LCs belonging to four series. The first two series comprise five pairs of enantiomeric Schiff bases whereas the other two series are composed of five pairs of enantiomeric salicylaldimines. In each pair of optical isomers, the configuration of a chiral center in one stereoisomer is opposite to that of the analogous center in the other isomer as they are derived from (3 S)‐3,7‐dimethyloctyloxy and (3 R)‐3,7‐dimethyloctyloxy tails. To probe the structure–property correlations in each series, the length of the n‐alkoxy tail situated at the other end of the mesogens has been varied from n‐octyloxy to n‐dodecyloxy. The measurement of optical activity of these chiral mesogens was carried out by recording their specific rotations. As expected, enantiomers rotate plane polarized light in the opposite direction but by the same magnitude. The thermal behavior of the compounds was established by using a combination of optical polarizing microscopy, differential scanning calorimetry, and powder X‐ray diffraction. These complementary techniques demonstrate the existence of the expected, thermodynamically stable, chiral smectic C (SmC*) LC phase besides blue phase I/II (BPI or BPII) and chiral nematic (N*) phase. However, as noted in our previous analogous study, the vast majority of the Schiff bases show an additional metastable, unfamiliar smectic (SmX) phase just below the SmC* phase. Notably, the SmC* phase persists over the temperature range ≈80–115 °C. Two mesogens chosen each from Schiff bases and salicylaldimines were investigated for their electrical switching behavior. The study reveals the ferroelectric switching characteristics of the SmC* phase featuring the spontaneous polarization (P_S) in the range 69–96 nC cm^?2. The helical twist sense of the SmC* phase as well as the N* phase formed by a pair of enantiomeric Schiff bases and salicylaldimines has been established with the help of circular dichroism (CD) spectroscopic technique. As expected, the SmC* and the N* phase of a pair of enantiomers showed mirror image CD signals. Most importantly, the reversal of helical handedness from left to right and vice versa has been evidenced during the N* to SmC* phase transition, implying that the screw sense of the helical array of the N* phase and the SmC* phase of an enantiomer is opposite.     

The effect of molecular chirality on the incidence of twisted smectic A phases

Four chiral chloro-esters were synthesized in order to examine systematically the effect that molecular chirality has on the temperature range and incidence of the twisted smectic A phase or the twist grain boundary phase as it has become known. We find that when the motion of the chiral centre is restricted by rotational hindrance, thereby increasing the chirality of the system, the twisted S_A phase range is increased. At low chirality the twisted S_A phase disappears altogether. Furthermore, it is shown that the transition temperatures of the chiral compounds are lower than their racemic analogues.     

Density studies on various smectic liquid crystals

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.     

Effect of the Magnetic Field on the Supramolecular Structure of Chiral Smectic C Phases: 2H NMR Studies

We present a theoretical and experimental ²H NMR study of the effect of external magnetic fields on the supramolecular organization of chiral smectic liquid‐crystalline mesophases, such as SmC* and re‐entrant SmC*. Three experimental cases in which the supramolecular helical structure of the smectic C* phase is unwound by a magnetic field (H), parallel to the helical axes of this phase, are discussed in detail. Unwinding of the helical structure is described by using a theoretical model based on the Landau‐de Gennes theory, which allows us to explain the transition temperatures among the SmA, SmC*, and uSmC* phases. The energy‐density behavior in the vicinity of the transitions and the value of the critical magnetic field H_C for unwinding the helical structure are discussed by applying this model to three ferroelectric smectogens ( MBHB , 11EB1M7 , ZLL7/* ), which are studied by ²H NMR spectroscopy at different magnetic fields (from 2.4 to 9.4 Tesla). Furthermore, the tilt angle of the three smectogens in the SmC* phase has been directly evaluated, for the first time, by comparing the quadrupolar splittings at different magnetic fields. In one case, ²H NMR angular measurements are used to obtain the tilt angle in the re‐entrant smectic C phase.     

Effect of alkyl chain length in the terminal ester group on mesomorphic properties of new chiral lactic acid derivatives

Two series of new liquid crystalline lactic acid derivatives with a terminal ester group have been synthesised. The effect of this ester unit and the length of its alkyl chain on the mesomorphic and dielectric properties of the compounds exhibiting a broad temperature range of chiral smectic phases have been studied. We found that the mesomorphic behaviour and phase transition temperatures are strongly affected by the molecular architecture. Depending on the alkyl chain length in the terminal ester unit, the studied materials exhibited paraelectric smectic A*, ferroelectric tilted smectic C* and antiferroelectric smectic C_A* phases over a broad temperature range. The physical properties of the compounds have been studied by optical polarising microscopy, differential scanning calorimetry, electro-optic measurements, small-angle X-ray scattering and dielectric spectroscopy. Furthermore, the homologues with short terminal alkyl chains showed a very small layer shrinkage at the transition from the orthogonal SmA* to the tilted SmC* phase, which is a characteristic feature of ‘de Vries-type’ behaviour.     

Differences between smectic homo- and co-polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the 'de Vries' type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the 'de Vries' type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC-elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Differences between smectic homo‐ and co‐polysiloxanes as a consequence of microphase separation

This paper compares smectic phases formed from LC‐homo‐ and LC‐co‐polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo‐ and co‐polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC‐elastomers formed from homo‐ and co‐polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers.     

Amplitude and phase fluctuation modes in the smectic Cα* phase of an antiferroelectric compound

Dielectric measurements have been carried out on the chiral smectic C_α (SmC_α*) phase of a MHPOBC analogous compound. Two relaxation modes have been observed in this phase for planar orientation of the molecules. One process has been observed at frequency lower than that of the soft mode of the chiral smectic A (SmA*) phase. This relaxation process is connected with the helicity of the SmC_α* phase. In the high‐frequency region, another relaxation process has been observed in the SmC_α* phase for which bias field dependence is similar to that of the soft mode at the SmA*–SmC* phase transition. The experimental observations are in agreement with a recently proposed dielectric theory for the SmC_α* phase and theoretical dielectric results obtained by numerical simulations. Thus, we report here experimental verification of two theoretically predicted dielectric modes in the SmC_α* phase.     

De Vries smectic A phase formed by a liquid crystal side chain copolymer? A 13C NMR study

We study the orientation and order parameter of a liquid crystalline random side chain copolymer by ¹³C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A–smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X‐ray and elastic characterizations of structurally similar materials.