Sub‐millisecond transient vertical chevron formation during the electroclinic effect in the smectic A∗ phase

X‐ray diffraction studies are carried out in order to probe the smectic layer structure in liquid crystal devices filled with FLC mixture SCE8 and AFLC mixture CS4001, at a temperature just above the SmA?–SmC? phase transtion. The data gathered are time‐resolved in synchronization with a bipolar voltage pulse applied across the device. The layers are observed to move dynamically and reversibly with voltage application and removal, giving evidence for temporary vertical chevron formation due to the electroclinic effect on a timescale consistent with this phenomenon.     

Generation of frustrated liquid crystal phases by mixing an achiral nematic-smectic-C mesogen with an antiferroelectric chiral smectic liquid crystal

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmC(a)(*) phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB(*)) phase, two or three blue phases, the untilted SmA(*) phase, as well as all three chiral smectic-C-type "subphases," SmC(alpha)(*), SmC(beta)(*), and SmC(gamma)(*). The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB(*) at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC(*) exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmC(a)(*) transition or a temperature range of the SmC(beta)(*) subphase of about 50 K.     

X-ray diffraction analysis of the layer structure in a ferroelectric liquid crystal with a chiral nematic-chiral smectic C phase transition

A half-V-shaped switching ferroelectric liquid crystal (FLC) is a promising candidate for fast response displays. In the half-V FLC display, a liquid crystal with a chiral nematic-chiral smectic C phase transition is used, and the smectic layer is formed by cooling from N* to SmC* with an applied d.c. field. We studied the layer structure by means of X-ray measurements for two axes (ω and χ). By using a point-focused X-ray tube and optimizing the slit width, we succeeded in the two-axis measurement with a commercial X-ray system. The ω-χ profile of the half-V FLC showed two broad peaks in an arc-shaped high-intensity area. Our interpretation of this result is that the major part of the layer consists of a tilted-bookshelf structure and that the minor part consists of a near-bookshelf structure. Since optical microscopy observations on the half-V FLC cells showed a stripe-shaped texture, we consider that the coexistence of the tilted-bookshelf and the near-bookshelf structures forms the stripe-shaped patterns. The radius of the arc-shaped high-intensity area was nearly equal to the molecular tilt angle. This result can explain why the half-V FLC showed a desirable black appearance in spite of the stripe-shaped texture.     

FTIR spectroscopy of smectic elastomer films under lateral strain

Polarized Fourier transform infrared (FTIR) spectroscopy is used to study the strain-induced compression of molecular layers in oriented smectic liquid crystal elastomer films. A reversible change of the smectic layer thickness in SmA and SmC* films in response to external strain was revealed earlier by optical reflectometry and X-ray measurements. However, these methods cannot probe the mechanism of layer compression on a molecular level. Polarized FTIR spectra show that the induced mesogenic tilt, one of the possible mechanisms, is too small to provide the dominating contribution to the layer shrinkage. The FTIR absorbance spectra of stretched samples are also evidence that there are no significant changes of the order parameter. Apparently, layer compression is achieved by a certain interpenetration of neighbouring layers, and/or compression of the interstitial backbone and spacer layers.     

CdSe quantum dot-dispersed DOBAMBC: an electro-optical study

Cadmium selenide quantum dot (CdSe QD) has been used as a dopant in ferroelectric liquid crystal (FLC) 2-methylbutyl 4-(4-decyloxybenzylideneamino) cinnamate (DOBAMBC). Effect of CdSe QD in DOBAMBC on its different electro-optical (E-O) properties has been studied in the SmC* phase. The optical micrographs recorded for the pure and composite material are showing good dispersion of QDs in the FLC matrix. Micrographs of unaligned sample cell revealed that CdSe QDs induce homeotropic alignment of FLC molecules. An appreciable change in the value of E-O parameters like tilt angle, spontaneous polarisation and response time with shifting of SmA–SmC* phase transition temperature has been observed for CdSe QD–DOBAMBC composite. The observed properties of composite system have been discussed on the basis of surface properties of QDs in FLC system.     

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.     

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.     

Orientational order in smectic liquid-crystalline phases of amphiphilic diols

The thermotropic smectic phases of amphiphilic 2-(trans-4-n-alkylcyclohexyl)-propane-1,3-diols were investigated by means of small- and wide-angle x-ray scattering and values of the smectic (bi-)layer spacing, the orientational order parameters P(2) and P(4), the orientational distribution function as well as the intralayer correlation length were extracted from the scattering profiles. The results for the octyl homolog indicate that these smectic phases combine a very high degree of smectic one-dimensional-translational order with remarkably low orientational order, the order parameter of which (P(2) approximately 0.56) is far below those values typically found in nonamphiphilic smectics. This combination, quite exceptional in thermotropic smectics, most likely originates from the intermolecular hydrogen bonding between the terminal diol groups which seems to be the specific driving force in the formation of the thermotropic smectic structure in these amphiphiles and leads to a type of microphase segregation. Even in the absence of a solvent, the liquid-crystalline ordering of the amphiphilic mesogens comes close to the structure of the so-called neat soaps, found in lyotropic liquid crystals.     

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.     

X-ray diffraction analysis of the layer structure in a ferroelectric liquid crystal with a chiral nematic–chiral smectic C phase transition

A half-V-shaped switching ferroelectric liquid crystal (FLC) is a promising candidate for fast response displays. In the half-V FLC display, a liquid crystal with a chiral nematic–chiral smectic C phase transition is used, and the smectic layer is formed by cooling from N* to SmC* with an applied d.c. field. We studied the layer structure by means of X-ray measurements for two axes (ω and χ). By using a point-focused X-ray tube and optimizing the slit width, we succeeded in the two-axis measurement with a commercial X-ray system. The ω–χ profile of the half-V FLC showed two broad peaks in an arc-shaped high-intensity area. Our interpretation of this result is that the major part of the layer consists of a tilted-bookshelf structure and that the minor part consists of a near-bookshelf structure. Since optical microscopy observations on the half-V FLC cells showed a stripe-shaped texture, we consider that the coexistence of the tilted-bookshelf and the near-bookshelf structures forms the stripe-shaped patterns. The radius of the arc-shaped high-intensity area was nearly equal to the molecular tilt angle. This result can explain why the half-V FLC showed a desirable black appearance in spite of the stripe-shaped texture.     

Systematic study of the chiral smectic phases of a fluorinated compound

ABSTRACT

Physical properties of the partially fluorinated compound 3F5FPhF, with hockey stick-like molecules, were studied by complementary methods. Apart from the already reported paraelectric SmA*, ferroelectric SmC* and antiferroelectric SmC*_A phases, the presence of the smectic C*_α subphase in the phase sequence was proved by differential scanning calorimetry, polarising optical microscopy, electro-optic and dielectric spectroscopy methods. The temperature dependence of the smectic layer thickness and correlation length of the lateral short-range order was determined by X-ray diffraction. Based on dielectric measurements three relaxation processes were revealed in the antiferroelectric SmC*_A phase (two collective: P_L, P_H and one molecular: s-process), two collective ones (Goldstone and soft modes) were found both in the ferroelectric SmC* phase and SmC*_α subphase while one relaxation process (soft mode) in the paraelectric SmA* phase. The results were compared with that obtained for other structurally similar compounds, and it was shown that even addition of one methylene group to the side chain influences much on the physical properties.     

Ferroelectric polysiloxane liquid crystals with 'de Vries'-type smectic A*-smectic C* transitions

We report preliminary results of optical and small angle X-ray scattering (SAXS) experiments on the smectic A*-smectic C* transition in two ferroelectric liquid crystalline polysiloxanes. Although the optical tilt angle in the SmC* phases reaches values up to 30°, temperature-dependent SAXS measurements clearly reveal that the smectic layer spacing is basically conserved during the A*-C* transition as well as in the subsequent C* phase. Connected with the A*-C* transition we further observed a significant increase in birefringence, hence reflecting an increase of orientational order. The practical absence of layer shrinkage and the enhanced orientational ordering are consistent with the de Vries diffuse cone model of smectic A-smectic C transitions.     

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.     

Polymerization of polymer/ferroelectric liquid crystal composites formed with branched liquid crystalline bismethacrylates

Polymerization and phase behaviour of a branched liquid crystalline bismethacrylate in a ferroelectric liquid crystal (FLC) were characterized. Addition of the monomer increases the temperature range of the smectic A phase, and, at relatively low concentrations of monomer, the temperature range increases to more than 10 times that observed in the neat FLC. Other phases such as the smectic C* phase are no longer exhibited as the monomer interferes with the inherent tilt of the FLC molecules. After polymerization, the polymer network phase separates and the phase transition temperatures return to values close to those of the FLC. The monomer also shows a high degree of orientational order before polymerization, which is retained to a large extent after polymerization. The order in the polymer network results in considerable birefringence at temperatures well above the clearing point of the pure FLC. This behaviour is induced by the order of the polymer network and interactions of the FLC molecules with the network which prevent a fraction of the FLC molecules from exhibiting typical phase behaviour. In the formation of the branched liquid crystalline bismethacrylates/FLC composites, a steady increase is observed in the polymerization rate as the polymerization temperature increases and the order of the FLC phase decreases, a behaviour significantly different from that observed in other monomer/FLC mixtures for which the polymerization rate increases as the order of the FLC phase increases. Additionally, at appropriate polymerization temperatures around the clearing point, polymerization driven endothermic phase transitions may be observed.     

Use of a gelator in a ferroelectric liquid crystal: pitch compensation and nanofibres

A new azobenzene-containing gelator for liquid crystals, AG2, was synthesized and used to prepare a ferroelectric liquid crystal (FLC) gel. The FLC gel shows interesting features. On cooling from the isotropic phase into the N * phase, the dissolved AG2 acts as a chiral dopant and has a compensation effect on the helical pitch of the N * phase. With 0.5 wt% of AG2 in the FLC host, a homogeneous alignment of the FLC molecules is formed in the N * phase, ensuring the bulk alignment in the SmC * phase, even on quenching the mixture from the isotropic phase. This alignment under fast cooling contrasts sharply with the slow cooling rate required for the alignment of a pure FLC. After formation of the bulk alignment, the aggregation of AG2 occurs in the lower temperature SmA or SmC * phases, and the gelator molecules self-assemble into nanometer-sized fibres (about 100nm diameter) that are aligned and located between the smectic layers. As the gelator is microphase-separated from the FLC in the SmC * phase, it exerts little disruption on the electro-optic properties of the FLC cell.     

Dynamics of the smectic layer reorientation of ferroelectric liquid crystals

Detailed experimental results of a systematic investigation of the dynamics of the in-plane smectic layer reorientation in SmC* ferroelectric liquid crystals on application of different types of asymmetric electric fields are reported. The reversible reorientation of smectic layers is characterized as a function of field asymmetry, electric field amplitude, frequency, cell gap and temperature. On the basis of the observed behaviour we discuss a phenomenological interpretation of the smectic layer reorientation in terms of dominant influences director switching, convection due to ionic motion and liquid crystal substrate interactions which limit the rotation to the amount of twice the tilt angle.     

A bent-core dopant-induced smectic A* twist state

The phase behaviour of a commercial calamitic ferroelectric liquid crystal mixture, doped with different mesogenic and non-mesogenic bent-core molecules was investigated through polarising microscopy, optical measurements and quenched growth. A twisted smectic structure, similar but not equivalent to a twist grain boundary (TGB) phase, and absent in the neat FLC mixture, was verified. The twisted smectic state can only be observed on cooling and its stability depends on the rate of temperature decrease, which indicates a kinetically governed behaviour. Further, the growth dynamics of the low temperature uniform SmA* bookshelf structure is dominated by viscosity instead of free energy density, as would be expected for a true thermodynamic phase transition. The investigations signify the chiral induction capability of achiral, bent-core dopant molecules and we believe that the observed behaviour represents the onset of TGB formation at very large pitch. It can thus give valuable information for the fundamental physical understanding of twist grain boundary phase formation.     

Tilted supramolecular structures of amphiphilic glycolipids in straight-core smectic C liquid crystals

Optical microscopic and the electric polarization studies show that amphiphilic glycolipids form tilted supramolecular structures in straight-core tilted smectic liquid crystals. Glycolipid and chiral smectic C (SmC) structures show spontaneous polarization while glycolipid and non-chiral SmC structures show no spontaneous polarization. We deduce from these observations that glycolipid molecules in chiral SmC structures nano-segregate in tilted double layers sandwiched between layers of straight-core liquid crystal molecules. In non-chiral SmC structures, tilted glycolipid molecules are nano-segregated in the layers of straight-core liquid crystal molecules. This indicates that, in contrast to the currently accepted interdigitated model, amphiphilic glycolipids may already have a tilted structure.     

Achiral diacrylate with an anticlinic smectic phase

We present an achiral diacrylate that shows a phase transition from the synclinic smectic C phase (SmC) to the anticlinic smectic C (SmCalt). This last phase has been previously reported only for swallow-tailed or dimeric compounds. Our studies using differential scanning calorimetry, X-ray diffraction, polarizing optical microscopy and broad band dielectric spectroscopy suggest the existence of this mesophase in this bifunctional smectogen.     

On the phase sequence of antiferroelectric liquid crystals and its relation to orientational and translational order

The substance MHPOBC is the oldest and still most important reference antiferroelectric liquid crystal (AFLC). There is still considerable controversy concerning the correct phase designations for this material and, in particular, about the presence or absence of SmC* in its phase sequence. By means of dielectric spectroscopy and polarizing microscopy, we show that whereas the pure compound lacks the SmC* phase, this phase rapidly replaces the SmC*_β subphase through the reduced purity resulting from temperature-induced chemical degradation which is hard to avoid under standard experimental conditions. X-ray investigations furthermore show that this change in phase sequence is coupled to a decrease in translational order. This explains the large variations in the reported phase sequence and electro-optic behaviour of MHPOBC, in particular concerning the SmC*β phase which has been said to exhibit ferro-, ferri- as well as antiferroelectric properties. It is likely that the sensitivity of the AFLC phase sequence to sample purity is a general property of AFLC materials. We discuss the importance of optical and chemical purity as well as tilt and spontaneous polarization for the observed phase sequence and propose that one of the key features determining the existence of the different tilted structures is the antagonism between orientational (nematic) and translational (smectic) order. The decreased smectic order (increased layer interdigitation) imposed by chemical impurities promotes the synclinic SmC* phase at the cost of the AFLC phases SmC*_α, SmC*_β, SmC*_γ and SmC*_a. We also propose that the SmA* phase in FLC and AFLC materials may actually have a somewhat different character and, depending on its microstructure, some of the tilted phases can be expected to appear or not to appear in the phase sequence. AFLC materials exhibiting a direct SmA* -SmC*_a transition are found to be typical ‘de Vries smectics’, with very high orientational disorder in the SmA* phase. Finally, we discuss the fact that SmC*_β and SmC*_γ have two superposed helical superstructures and explain the observation that the handedness of the large scale helix may very well change sign, while the handedness on the unit cell level is preserved.