Smectic Phase

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Conductivity Measurements and Thermodynamical Response of Optical-Birefringence of Reentrant Smectic Phase of Liquid Crystalline Materials

Conductivity Measurements and Thermodynamical Response of Optical-Birefringence of Reentrant Smectic Phase of Liquid Crystalline Materials

For the purpose of optical texture studies, the given mixtures of the samples was sandwiched between a slide and a cover glass and then the optical textures were observed using a Leitz polarizing microscope in conjunction with a hot stage. When given mixtures shows the existence of different liquid crystalline induced phases such as cholesteric, SmA, SmC, ReSmA, SmC* and SmE phases: that have been obtained at different concentrations of given multi-component systems are at different temperatures sequentially when they cooled from their isotropic melt. The mixture with different concentrations ranging from 5% to 50% of 10OBAC in 7AB7+CN has been considered for the experimental studies. When the specimen of 35 % of 10OBAC in 7AB7+CN is cooled from their isotropic liquid phase, it exhibits I–Cho–SmA–SmC-ReSmA-SmC*–SmE–K phases sequentially. While the sample is cooled from isotropic liquid phase, the genesis of nucleation starts in the form of small bubbles growing radially, which form a fingerprint pattern of cholesteric phase with large values of pitch [5-7], is shown in Figure 1(a). On further cooling the specimen, the cholesteric phases are slowly changed over to a well-defined focal conic fan-shaped texture, which is the characteristic of SmA phase and is shown in Figure 1(b). The SmA phase is unstable and then changes over to the schlieren texture of SmC phase as shown in Figure 1(c), and then this phase is also not energetically stable, which changes over to ReSmA [8] phase. On further cooling the specimen, this phase changes over to SmC∗ phase, which exhibits radial fringes on the fans of focal conic textures, they are the characteristic of chiral SmC ∗ phase, sequentially this phase changes over to SmE phase and then this phase remains stable till
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Octa­decane 1,18 diol

Octa­decane 1,18 diol

Crystal structures of long-chain compounds such as n-alkanes (Müller, 1928) and n-higher primary alcohols (e.g. Watanabe, 1961; Seto, 1962), have been studied by many researchers from the viewpoint of basic polymer science. According to those results, the compounds have a simple straight hydrocarbon chain as a skeleton and the molecular shape can be regarded as a rod-like, which is one of the typical features of liquid crystal molecules. In addition, some long-chain compounds construct a layer structure in the crystal state, which is similar to that of a smectic phase of liquid crystals. Therefore, these compounds have been studied from a structural point of view as models for smectic liquid crystals.
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Ferroelectric ordering in chiral smectic- C* liquid crystals determined by nonchiral intermolecular interactions

Ferroelectric ordering in chiral smectic- C* liquid crystals determined by nonchiral intermolecular interactions

a chiral biaxial molecule represented as a rigid rod with two ’lateral groups’ and a permanent dipole perpendicu- lar to the molecular plane as shown in Fig. 4a. Note that the lateral groups make the molecule biaxial while the chirality is determined by the transverse dipole. With- out this dipole, the molecule possesses a mirror plane and thus it is nonchiral. Now let us assume that the lat- eral groups have a tendency to point in the direction of the region between two adjacent smectic layers. One can readily see that in the smectic A phase (i.e. without any tilt) the two orientations of such a molecule, which cor- respond to opposite directions of the transverse dipole µ are energetically equivalent. Thus the macroscopic po- larization in the untilted smectic phase should vanish. In contrast, in the tilted phase the balance between two opposite directions of the transverse molecular dipole is violated because the the molecular orientation A is more favorable than the orientation B (see Fig.2b). As a re- sult, the average molecular dipole does not vanish, and a macroscopic polarization appears in the direction per- pendicular to the tilt plane. Now one has to clarify how this type of ordering corresponds to the general macro- scopic description presented above.
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Molecular simulation of chevrons in confined smectic liquid crystals

Molecular simulation of chevrons in confined smectic liquid crystals

formed in each half of the simulation box. A slow cooling of the system into the smectic phase was judged inappropriate since the GB model has very little temperature dependence in its smectic layer spacing. Rather, the method used to induce the system to form a chevron was to quench it into the smectic phase from a point close to the nematic-smectic transition line, the expectation being that tilted layers seeded at each surface would grow and meet in the middle to form a chevron tip. The conditions for the simulation were chosen, from the phase diagram for this parameterisation [17], to be a system quenched from T = 0.95 to T = 0.85.
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Sequence of Four Orthogonal Smectic Phases in an Achiral Bent Core Liquid Crystal: Evidence for the SmAP(alpha) Phase

Sequence of Four Orthogonal Smectic Phases in an Achiral Bent Core Liquid Crystal: Evidence for the SmAP(alpha) Phase

application in the next generation of fast switching displays [11,12] as well as to advancing the understanding of their internal structures and establishing molecular structure- property relationships [13]. Recently, another randomized polar smectic phase, assigned as SmAP AR , has been ob- served in a bent-core compound containing a 3-aminophe- nol-derived central unit [8]. It was suggested that this phase consists of randomly aligned domains of local antiferro- electric order and its stability was attributed to a nonsym- metric molecular architecture containing intermolecular hydrogen bonding.
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Electric field dependent dielectric response in the de Vries type smectic  A* phase possessing local orientational order with nanoscale correlation length

Electric field dependent dielectric response in the de Vries type smectic A* phase possessing local orientational order with nanoscale correlation length

maximum of around 1300 molecules at 28 ° C. Thus, the in- teraction of a molecular dipole and the applied electric field does not involve one molecule, but a maximum of around 1300 molecules cooperatively respond to the applied electric field. Hence, we note that the molecules in the de Vries–type SmA * phase possess a local order even without the applica- tion of electric field. A similar type of behavior has been observed in a nontilted smectic phase of an asymmetric bent- core liquid crystalline compound 关30兴. The number of mol- ecules possessing the local order depends upon the tempera- ture. Normally, we need a large electric field to orient randomly distributed dipoles under the action of thermal en- ergy. But, in the case of de Vries–type SmA * , because of the existing local order of the molecules, a relatively low electric field is sufficient to induce a large polarization. For lower temperatures, the number of molecules possessing the local order is higher, as a result, less electric field is needed to induce a large polarization. As a result, we observe higher ⌬ ⑀ and lower f max for lower temperatures. By inserting the mea-
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Structure, Photochromism and Liquid Crystal  Properties of 1 Alkyl 2 (Arylazo)Imidazoles  (Raai CnH2n+1, n (Even) = 10   22)

Structure, Photochromism and Liquid Crystal Properties of 1 Alkyl 2 (Arylazo)Imidazoles (Raai CnH2n+1, n (Even) = 10 22)

at 58˚C (5a) and 65˚C (7a) respectively. The crystal and nematic phases are under thermal equilibrium. During cooling from isotropic phase, 5a shows strongly fluctu- ating textures of the N phase and has been transformed to textures in which “Schlieren” regions can be seen (Fig- ure 7). Homeotropic and homogenous oriented domains cause this coexistence, which permits the assignment of the smectic phase in compound 7a. The transition tem- perature and associated enthalpies are Cr → N 37.4˚C (15.9 KJ·mol −1 ) → Iso 58˚C (0.5 KJ·mol −1 /heating cycle)
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Annihilation of edge dislocations in smectic A liquid crystals

Annihilation of edge dislocations in smectic A liquid crystals

conventional smectic phase a climb of a dislocation is much easier than a glide because a glide necessitates layer breaking. Therefore a relevant characteristic time scale in a layer plane direction and perpendicular to it are expected to be apparently different. The classical theory [4] predicts that the interaction force in the x-direction between dislocations can become repulsive for ∆z 6= 0 for a sufficiently small separation L. Therefore for the case that gliding is apparently less probable than climbing a pair of edge dislocations is expected to get caught in a metastable state at a separation L = L(∆z). The system would then remain in this state until a fluctuation triggers a gliding event enabling annihilation into the defectless state. In order to simulate this case with our model an anisotropic symmetry allowed dissipation term (see Eq.(5)) including different viscosities in a layer plane and perpendicular to it should be introduced, which is the focus of our future work.
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Polymer stabilization of the smectic C-alpha* liquid crystal phase—Over tenfold thermal stabilization by confining networks of photo-polymerized reactive mesogens

Polymer stabilization of the smectic C-alpha* liquid crystal phase—Over tenfold thermal stabilization by confining networks of photo-polymerized reactive mesogens

stability was determined by the current reversal method. SEM imaging was used to interpret the mechanism of the phase stabilization and revealed that the most stable SmC a * phase mixtures featured an extensive network among and across layers of the smectic phase. 27 However, more experi- ments and tests (including detailed x-ray experiments) of this and related SmC a * systems are needed to continue improve

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An investigation of the second order nonlinear optical properties of self assembled organic materials

An investigation of the second order nonlinear optical properties of self assembled organic materials

with its transverse methyl group, which has poor coefficients due to the lack of polarisable electrons. Probably the next largest effect on the values is the ability of the material to form a stable and well aligned structure for measurement This factor is very difficult to eliminate or account for in calculations, and so makes comparisons between two molecules (e.g. SJ54 & SJlO l) difficult It is however, a factor which must be taken into account when evaluating materials for potential device applications and so is useful to note. Strong influences on the alignment quality are the pitch length of the smectic C helix, the type of liquid crystal phases above the smectic C phase, and the width of the smectic C phase. Materials with short pitches tend to 'wind up' easily, making it difficult for the surface forces to align the molecules in a uniform structure. This may be compensated by mixing with other materials which have an opposite winding sense. It was found from Pg and SHG measurements that mixing molecules with a standard host (SCE13 Racemic) had a detrimental effect on the polar order and the value of p beyond the simple number density factor. It was believed the high concentration of host molecules disrupted the hindered rotation system, thus reducing the value of P and IT. Miscibility studies between two or more materials of the same type e.g. materials from table 4.1 may improve the smectic C phase without destroying the hindered rotation of the molecules, or the number density if both are chiral.
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Spontaneous helix formation in non chiral bent core liquid crystals with fast linear electro optic effect

Spontaneous helix formation in non chiral bent core liquid crystals with fast linear electro optic effect

AFLC) 3 . A new class of compounds with ferroelectric and antiferroelectric LCs is built from achiral bent-core (BC) molecules and has engendered great scientific interest in recent years 4 due to a range of fascinating phenomena arising from the interplay of polarity and chirality 5–7 . Unlike rod-like LCs, the BC compounds, even being achiral, may exhibit spontaneous polarization in the orthogonal (SmA–like) 8 and tilted (SmC–like) smectic phases 4,9 . In contrast to orthogonal BC phases 10,11 , electro-optical effects can possibly be used for applications in tilted smectic phases of BCLC have not so far been reported, although electro-optical switching has been observed 4,9,12 . A major reason is the impossibility to align these BC SmC phases. Besides these technological relevant aspects, spontaneous emergence of chirality in the tilted smectic phases of achiral BCLC is of prime general scientific importance 13,14 . Chirality in the LC phases of BCLC results from the combination of tilt and polar order in the smectic phases (Supplementary Fig. 1) 9 and was found for the so-called dark conglomerate phases 5,13 , representing strongly distorted smectic phases with sponge-like structure 15 or formed by helical nano-filaments 16 and nano-size crystallites 17 . However, formation of smectic phases with helical superstructure having a helix axis parallel to the layer normal was not yet observed in the LC phases of any achiral BC mesogen 5,6,18 .
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Conoscopy of chiral smectic liquid crystal cells

Conoscopy of chiral smectic liquid crystal cells

method adopting more or less the same procedure has fre- quently been used by a number of authors [2–8]. In those papers, the cells were considered as single-biaxial plates. The cells of achiral SmC and SmA phases in reality are single biaxial plates, and the tilt angle and biaxiality are reasonably defined. However, the cell containing the chi- ral smectic in its various phases is not a single biaxial plate, and strictly speaking, the biaxiality cannot be de- fined in the cell having a helical structure. We pose the fundamental question: What is the meaning of the biaxi- ality obtained using conoscopy in chiral smectic phases? In chiral smectic phases, each layer has its own refractive index ellipsoid, and the directions of the principal axes gradually vary from layer to layer. Hence, the biaxiality and the apparent tilt angle estimated from conoscopic experiments can only represent the average properties of the various layers.
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Behaviour of a shear-wave at a solid-smectic interface

Behaviour of a shear-wave at a solid-smectic interface

and refracted waves in terms of the problem’s physical parameters. In particular, the refracted wave number q, which characterises the attenuation of the wave in the SmA case, was provided in terms of the parameters characterising the solid and the smectic by equation (3.16). Further, expressions for the amplitudes of these at normal incidence were derived in terms of the incident wave amplitude and these parameters, with (5.2) and (5.3), lead to expressions for the reflected and refracted waves, respectively. For the purpose of a qualitative comparison, we derived analogous terms via the results of Gill and Leslie [2], who performed calculations for the identical experiment for a sample of SmC, utilising the LSN theory for SmC. It is readily seen that, at normal incidence, the behaviour of the two phases is qualitatively the same, with the refracted wave amplitudes showing a departure in behaviour from the approximate expressions given in equations (5.6) and (5.7) as ω increases beyond the critical value 10 10 Hz. Before ω attains this value, the aforementioned expressions provide a very accurate approximation to the respective exact expressions for the refracted wave amplitudes given in (5.3) and (5.5).
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On the biaxiality of smectic C and ferroelectric liquid crystals

On the biaxiality of smectic C and ferroelectric liquid crystals

Unlike AC stabilisation, if the polarity of the applied field opposes the spontaneous polarisation reorientation occurs towards the opposite side of the cone, as shown in figure 11. The resulting high gradient close to the chevron interface will eventually be sufficient to cause the director to swap discontinuously from one allowed state to the other through the formation and movement of a domain wall. After removal of the field, the director relaxes back to a TDP but with the opposite sign to the original state. Crossing the energy barrier between the bistable states requires the director to move between the two allowed orientations at the chevron interface. This cannot occur by change in orientation φ C alone and must involve compression of the smectic layers.
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Discovery of a strain stabilised smectic electronic order in LiFeAs

Discovery of a strain stabilised smectic electronic order in LiFeAs

To assess whether the modulation of the charge density is associated with a characteristic energy scale, and its influence on superconductivity, we have acquired a spectroscopic map to study the electronic states across the modulation. Figure 2d, e shows the topography and a differential conductance map g(x, V) = dI/dV (x, V). The map exhibits a strong modulation of the height of the superconducting coherence peaks, which can be more clearly seen from spectra taken on top of the charge modulation and between the maxima in Fig. 2f. Most notably, the spectra show an additional feature at 12 mV which is modulated with opposite phase compared to the coherence peaks. In addition, a weak feature can be seen at −18 mV. To extract the characteristic energy scale of the charge modulation, we analyse the amplitude of the modulation in the ratio of the differential to the total conductivity l(x, V) = g(x, V)/(I(x, V)/V), a quantity for which the set point effect due to variation in the tip-sample distance is suppressed if the tunneling matrix element is only weakly energy dependent and which can be taken as a representative of the density of states ρ(x, V) 24,25 . From the above, we calculate at each bias voltage the spatial variance of l(x, V), denoted σ 2 (l(x, V)), to determine what the characteristic energy scale of the charge modulation is. In Fig. 2g we show the variance in l(x, V) of the stripe modulation, as a function of bias voltage, as well as its wave vector. The variance exhibits a clear maximum at +11 mV and −16 mV, at slightly smaller energies than the maxima in g(V) seen in Fig. 2f. The wave vector stays practically constant within the energy range investigated here, confirming that it stems from a static charge modulation rather than quasi-particle interference, which would lead to a dispersion of the modulation. From the contrast inversion seen in topographic images and the character- istic energy scale of the modulation we observe in the differential conductance g(x, V) (or l(x, V)), we attribute the modulated phase to the formation of a CDW, with formation of a partial gap between +11mV and -16mV.
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Effective long range interlayer interactions and electric field induced subphases in ferrielectric liquid crystals

Effective long range interlayer interactions and electric field induced subphases in ferrielectric liquid crystals

electric-field range. In the electric-field–temperature E-T phase diagram, characteristic sigmoid-shaped birefringence contours are expected to be observed in the neighborhood of subsequent decreasing and increasing. Although in different materials Sandhya et al. actually observed the sigmoid-shaped contours in the MHPOCBC-MHPOOCBC binary mixture system as given in Figs. 2(i)–2(k) of Ref. [4]. They referred to the emergence of several field-induced subphases, but did not suggest such large unit cells of 12- and 15-layer periodicity shown in Fig. 4. Their microscopic short-pitch distorted helical structures have not yet been verified experimentally by using polarized RXRS experiments.
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Orientational Behavior of Liquid-Crystalline Polymers with Amide Groups

Orientational Behavior of Liquid-Crystalline Polymers with Amide Groups

The influence of hydrogen bonding between the secondary amide groups was shown by the temperature-variable IR spectra of PAADH. Moreover, the very sharp outer reflection in the wide-angle X-ray scattering exhibited the formation of hexatic packing within the SmB layer. The hexatic packing was stabilized through hydrogen bonds between the secondary amide groups. Hydrogen bonding can lead to the formation of smectic phases with enhanced thermal stability.

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Optical Birefringence Studies on Cholesteric and Induced Smectic Phases of Ternary Mixture of Liquid Crystalline Materials

Optical Birefringence Studies on Cholesteric and Induced Smectic Phases of Ternary Mixture of Liquid Crystalline Materials

Molecular orientations of optical textures exhibited by the samples were observed and recorded using the Gippon-Japan-polarizing microscope in conjunction with hot stage. The specimen was taken in the form of thin film and sandwiched between the slide and cover glass. The concentrations from 10% to 80% of ternary mixture of CO in (Naol+GAA) have been considered for the experimental studies. When the specimen of 50% CO in (Naol+GAA) is cooled from its isotropic melt and hence it exhibits cholesteric, SmA, SmC and SmG phases sequentially. While the sample is cooled from its isotropic phase, the genesis of nucleation starts in the form of small bubbles growing radially, which are identified as spherulitic textures of cholesteric phase. On further cooling the specimen, the texture slowly transform to focal conic fan texture of SmA phase in which the molecules are arranged in layers and the texture is as shown in Figure1(a). On further cooling the specimen, the unstable SmA phase changes over to schlieren texture of SmC phase and it as shown in Figure1(b). Sequentially on cooling the specimen, SmC phase slowly changes over to SmG phase and then it remains stable at room temperature [8].
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Theory of the intermediate tilted smectic phases and their helical rotation

Theory of the intermediate tilted smectic phases and their helical rotation

from 1, and thus the remaining terms in Eq. 共6兲 do not par- ticipate in ⌬b, although they dominate b. Now the only ques- tion arises as why the last term in Eq. 共6兲 关or in Eq. 共5兲兴 is positive? This term consists both of the energy and the en- tropy contributions, but its positive sign is determined only by the entropy contribution that dominates in this term, whereas the energy participates mostly in the first term of Eq. 共5兲 which does not contribute into ⌬b Thus, at least in our theory, the de Vries’s phase is a pure product of the entropy effects in the presence of the biaxial nonpolar order- ing, that seems to be consistent with the “chaotic” character of this phase. It arises when the energy contribution is ex- tremely small due to a small tilt angle.
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The effect of confinement on the stability of field induced states and on supercooling in antiferro ferroelectric phase transitions in chiral smectic liquid crystals

The effect of confinement on the stability of field induced states and on supercooling in antiferro ferroelectric phase transitions in chiral smectic liquid crystals

and when ␦ changes from 0° to 5°, the apparent tilt angle changes by only 0.2° for the three-layered structure. On con- sidering that the measured apparent tilt angles for 共D-4兲 and 共D-2兲 are approximately 12°–13°, and 20°–21°, respectively. 共D-4兲 state is assigned to be the field induced three-layer structure and 共D-2兲 and/or 共U-3兲 state is assigned to the four- layered structure. A slight difference between the simulated values of 共 15.2° and 19.2° 兲 against the measured ones 共 12.5° and 20.5° 兲 for the two states might arise from the existence of a deformed helical structure for 共 D-4 兲 共 three-layered struc- ture 兲 and the coexistence of the ferrielectric and the ferro- electric states for 共 D-2 兲 共 four-layered structure 兲 . From our observations it is now clear that the field induced phase tran- sition is also accompanied by the solitary wave propagation, and hence the basic mechanism of the field induced transi- tion is the same as that of the temperature driven one. At the same time, the two field induced ferrielectric states are ob- served at intermediate fields between the antiferroelectric and the field induced ferroelectric states. Note that 12OF1M7 is found to have stable temperature induced three-layered and four-layered subphases that exist in the intermediate temperature range between the SmC A * and SmC * phases. 6 The observation of a four-layered field induced state is addi- tional to that reported by Jaradat et al., 16 as they only re- ported the field induced three-layered structure. This result is based on a comparison of the observed and simulated tilt angles using the three- and four-layered structures. Our ob- servations on texture also clearly show that there exist at least two apparent intermediate field induced states too. The sequence of the field induced ferrielectric states is found to be the same as the temperature driven phases. The field in- duced phase sequence is antiferroelectric-three-layered-four- layered-ferroelectric, and in the temperature driven 共induced兲 phase sequence is SmC A * -SmC A * 共1 / 3兲 共three-layered兲- SmC A * 共1 /2兲 共four-layered兲-SmC * . Note that the field induced polar state with four layers is also seen indirectly from a change in the slope of the normalized coefficient for the py- roelectric response at ⬃0.5 seen in Fig. 4 curves 共b兲, 共c兲, and 共d兲 of Ref. 6.
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