Nematic-smectic Phase Research Articles

A molecular field approach to pressure-induced phase transitions in liquid crystals: Smectic–nematic transition

Since a rigorous microscopic treatment of a nematic fluid system based on a pairwise interaction potential is immensely complex, we had introduced a simple mean field potential, which was a modification of the Maier–Saupe potential in a previous paper [S. DasGupta et al., “Pressure-induced phase transitions in liquid crystals: A molecular field approach,” Phys. Rev. E 98, 022701 (2018)]. Building upon that, here we have modified that potential to take into account the various aspects of a smectic A–nematic phase transition. In particular, we have studied the dependence of the phase transition on the coupling coefficient between the nematic and smectic order parameters, which in turn depends on the length of alkyl chain, variation of density, entropy, and specific heat. Detailed investigation on the coupling parameter shows the existence of a smectic A–nematic–isotropic triple point as well as a tricritical point where the smectic–nematic phase transition changes its nature from the second to the first order. It is also seen that the application of pressure can result in the appearance of a nematic phase.

Smectic and nematic phase modulations and transitions under electron beam in Tb2Cu0.83Pd0.17O4

Understanding the structural origin of the functionality in cuprates has attracted tremendous attention over several decades. In particular, probing distortions in the Cu-O bonding is of great importance for exploring the coupling between the charge and the lattice, a key mechanism for superconductivity and other functionality in correlated materials. Here we study a superlattice modulation in the ${\mathrm{Tb}}_{2}{\mathrm{Cu}}_{0.83}{\mathrm{Pd}}_{0.17}{\mathrm{O}}_{4}$ ``214'' material, which possesses no superconductivity itself but has the parent structure of the ${R}_{2}{\mathrm{CuO}}_{4}$ (R = a rare-earth element) superconducting cuprate group. Using transmission electron microscopy (TEM), we find that this superlattice modulation is formed by Cu ion displacements in a direction perpendicular to the Cu-O planes. The superlattice modulation undergoes a reversible electronic smectic-nematic phase transition under electron-beam illumination. With the help of in situ TEM results, our findings imply that the superlattice modulation in this material arises from spatially modulated charge ordering at the Cu sites.

Stress-strain and thermomechanical characterization of nematic to smectic A transition in a strongly-crosslinked bimesogenic liquid crystal elastomer

Bimesogenic liquid crystal elastomers, composed with controlled concentrations of two mesogenic constituents, specifically a nematogen and a smectogen, exhibit either enhanced nematic or smectic structure. Here we present a thorough examination of thermomechanical and elastic properties of such a system at high crosslinking density, with ensuing disordered smectic structure which prevents the observation of smectic-nematic phase transition by means of thermomechanical response measurements. Yet, the onset of local smectic order is clearly observable in stress-strain measurements via an anomaly in the elastic modulus. Breakdown of the local smectic structure is detected at higher mechanical loads. Bimesogen composition – temperature phase diagram is as well constructed, consistent with the one obtained by complementary experimental techniques. Stress-strain temperature dependence measurements can thus serve as efficient and simple phase diagram characterization method that is able to detect formation of smectic clusters in strongly crosslinked and thus highly disordered smecto-nematic liquid crystal elastomers.

Communication: Optically induced decoupling of the smectic-A and nematic order parameters at the AN phase transition in phototropic liquid crystals.

In the present work, by means of the photopyroelectric calorimetric investigations in liquid crystal/naphtopyran (NP) molecule mixtures, the effect of the photoisomerization of NP by UV light on the critical behavior of specific heat could be studied over the smectic-nematic phase transition. It is shown that the UV light irradiation of the sample can shift the critical exponent from a non-universal value typical of the non-irradiated mixture to one close to that of the 3D-XY universality class, predicted for such a transition. This witnesses the optically triggered reduction of the coupling between the smectic and nematic order parameters responsible for the deviation from the XY-like behavior. A tentative explanation is provided based on the weakening of the order parameter coupling caused by the NP molecules entering the smectic layers following the UV light illumination.

Nuclear magnetic resonance study of the smectic-cholesteric phase transition in a dimesogenic liquid crystal

Nuclear magnetic resonance (NMR) spectroscopy has provided a unique opportunity to study the characteristic smectic-A to chiral nematic phase transition in a dimesogenic liquid crystal (“KI-5S”). The order parameters in the liquid crystalline phases were obtained from the 2H NMR quadrupole splitting and 13C NMR chemical shift measurements, manifesting a first-order smectic-nematic phase 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 Ad 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 Ad 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.

Critical behavior of dielectric permittivity in the vicinity of nematic–isotropic and smectic–nematic phase transitions in smectogenic binary mixtures

We report the phase diagram of a binary mixture composed of compounds, one having NCS terminal group (5DBT, showing smectic A1 phase) and the other with CN terminal group (10OCB, showing smectic Ad phase), exhibiting induced nematic phase in a certain concentration range (0.05<x5DBT<0.95). Measurement of static dielectric parameters on this binary system within the whole mesomorphic range is being reported. Pretransitional behavior at the nematic–isotropic and the smectic A–nematic phase transitions has been studied by measuring the static dielectric parameters in the entire mesomorphic range. Precise determination of discontinuity (ΔT) and the critical exponent (α) has been carried out for the nematic–isotropic phase transition. Effective molecular dipole moment and the angle of inclination with respect to the molecular long axis have been determined. High resolution density measurement is also carried out for one concentration x5DBT=0.301 and from the molar volume so obtained, the discontinuity (ΔT) of the transition and the critical exponent are measured. Additionally, the nature of the N–SmA phase transition has been studied for a few mixtures from dielectric anisotropy measurements to explore the critical behavior of the N–SmA phase transition.

Supramolecular self-assembly of a novel hydrogen-bonded cholesteric liquid crystal exhibiting macromolecular behaviour

The mesomorphic, thermoptic and glass-forming properties of 4-[6-((cholesteryloxy) carbonyl)oxy hexyloxy] benzoic acid (Ch-BA) have been investigated as a novel supramolecular hydrogen-bonded cholesteric mesogen. Fourier transform infrared and 1H nuclear magnetic resonance studies have confirmed the chemical structure and the hydrogen-bond formation between the mesogens. According to polarising optical microscope observations, the compound exhibited smectic and chiral nematic phases. Differential scanning calorimetry indicated an unexpected glass transition (T g) around 32°C and a liquid crystalline region between 32 and 122°C, in which the cholesteric phase appeared at 80°C. As a result of the glass formation, samples of Ch-BA which were rapidly cooled below the T g were found to preserve the long-range ordering of the liquid crystalline state and retained the iridescent colours of the cholesteric phase. These results led to the conclusion that the formation of identical dimers by intermolecular hydrogen-bonding of the terminal carboxylic acids accompanying the lateral packing of the rigid cores, built a trimeric arrangement and this was responsible for the macromolecular behaviour of Ch-BA, despite its relatively simple structure and low molecular weight.

Rotational Viscosity Measurement of a Binary Mixture Showing Both Induced Smectic and Re-Entrant Nematic Phases

The rotational viscosity has been measured in a binary system of two polar nematic compounds, exhibiting nematic – smectic Ad – re-entrant nematic phase sequence. In both the nematic and re-entrant nematic phases, the rotational viscosity values are about 0.5 Pa s and show small temperature dependence. However, in the smectic Ad phase, the rotational viscosity values initially increase with decrease in temperature to attain a maximum near the middle of the smectic phase and then decreases with further decrease in temperature. The activation energies associated with rotational viscosity for the nematic phase are distinctly higher than those obtained for the re-entrant nematic phase.

Refractive index and orientational order parameter of a polar–polar binary system showing induced smectic Ad and re-entrant nematic phases

Refractive index measurements have been done on a polar–polar binary system, 4-cyanophenyl [4′(4″-n-heptylphenyl)] benzoate (7CPB) + 4-cyanophenyl-4-nonylbenzoate (9.CN), exhibiting nematic–smectic Ad–re-entrant nematic phase sequence using thin prism technique. From refractive index as well as density data, the polarizability anisotropy in the mesomorphic state has been calculated from the standard Vuks model and the orientational order parameter (⟨P 2⟩) values have been determined. By measuring the refractive indices and density in the solid phase, the validity of the Haller's extrapolation method for systems having higher order phases has been verified. The temperature variation of the density, birefringence, and ⟨P 2⟩ values at the nematic–smectic Ad and smectic Ad–re-entrant nematic transitions for all the mixtures are found to be continuous. The experimental ⟨P 2⟩ values have been compared with our previous X-ray data and also those calculated from the Luckhurst and Timimi model.

Orientational order parameters of a binary mixture showing both induced smectic and re-entrant nematic phases

The phase diagram of a new binary system of two polar nematic compounds, 4-cyanophenyl [4′ (4″-n-heptylphenyl)] benzoate(7CPB) and 4-cyanophenyl 4-nonylbenzoate (9.CN), shows the presence of both the induced smectic and re-entrant nematic phases in a certain concentration range 0.4 < x 9.CN < 0.87. The results of X-ray diffraction and density measurements are reported here. Orientational order parameters have been determined in the induced smectic, normal nematic and re-entrant nematic phases from X-ray diffraction measurements. The order parameters have been fitted to those calculated from modified McMillan's theory as proposed by Luckhurst and Timimi. The agreement is found to be closer to the modified model. The experimental variation of the layer thickness with mole fraction of 9.CN has also been explained fairly well by assuming the presence of two types of homo dimers (7CPB + 7CPB, 9.CN + 9.CN) and one type of hetero dimer (7CPB + 9.CN).

Erratum: Fluctuating stripes in strongly correlated electron systems and the nematic-smectic quantum phase transition [Phys. Rev. B78, 085124 (2008)

Erratum: Fluctuating stripes in strongly correlated electron systems and the nematic-smectic quantum phase transition [Phys. Rev. B<b>78</b>, 085124 (2008)

Mesomorphic phases of main‐chain liquid‐crystalline polymers with pendent sulfonic acid groups

AbstractSome main‐chain liquid‐crystalline polymers were synthesized with pendent sulfonic acid groups (0–6 wt %) on the polymer backbone. The solution viscosity behavior, the chemical structures, and the liquid‐crystalline (LC) properties were characterized with Fourier transform infrared spectroscopy, elemental analysis, 1H‐NMR, differential scanning calorimetry, polarizing optical microscopy, and X‐ray measurements. All the polymers displayed a smectic mesophase with a mesogenic range of 142–155°C and good thermal stability. With an increasing concentration of sulfonic acid groups in the polymer systems, some of the polymers exhibited two kinds of mesophases, including smectic and nematic phases. The glass‐transition temperatures and isotropic‐transition temperature were elevated slightly, whereas the temperature of the smectic–nematic phase transition changed a little. For polymers with a few sulfonated benzene groups, the regular mesogen/soft matrix structures led to a smectic structure because of microdomains between the mesogenic moieties and the soft main‐chain matrix, whereas for polymers with an increased number of sulfonic groups, the ionic groups were distributed in both the soft matrix and the LC microphase. Although the increased ionic interactions between mesogens led to more crystalline phases, the strong electrostatic interactions in the ionic group/mesogen/soft matrix systems may have been tangled in domains. Therefore, they disturbed the regular mesogen/soft matrix smectic phase structure, leading to the appearance of a smectic–nematic transition. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Electroclinic effect around a smectic A-chiral nematic phase transition

We present measurements of the dynamics of the electroclinic effect around a smectic Achiral nematic transition. The phenomenon has been studied between 1 kHz and 1 MHz, extending by one order of magnitude the frequency range of previous studies. The results have unambiguously allowed us to distinguish two additive mechanisms in the generation of the optical tilt. A model to explain the physical origin of both mechanisms is presented. The first mechanism (fast) is the only bulk process intrinsic to the material and behaves normally at the transition, in the sense that both the magnitude of the tilt as well as the relaxation time have the expected temperature dependence. On the other hand, the second mechanism (slow) is not properly an electroclinic effect, but a surface-mediated effect driven by elastic forces. This explains the apparent anomalous behaviour of the phenomenon reported in the literature one decade ago.

Electroclinic effect around a smectic A-chiral nematic phase transition

We present measurements of the dynamics of the electroclinic effect around a smectic Achiral nematic transition. The phenomenon has been studied between 1 kHz and 1 MHz, extending by one order of magnitude the frequency range of previous studies. The results have unambiguously allowed us to distinguish two additive mechanisms in the generation of the optical tilt. A model to explain the physical origin of both mechanisms is presented. The first mechanism (fast) is the only bulk process intrinsic to the material and behaves normally at the transition, in the sense that both the magnitude of the tilt as well as the relaxation time have the expected temperature dependence. On the other hand, the second mechanism (slow) is not properly an electroclinic effect, but a surface-mediated effect driven by elastic forces. This explains the apparent anomalous behaviour of the phenomenon reported in the literature one decade ago.

Computer simulation of diffuse X-ray scattering by an aligned smectic C-like nematic phase

We report a new method for the quantitative analysis of diffuse X-ray scattering by some disordered smectic phases. Computer simulations of diffuse X-ray scattering are compared with actual diffraction patterns of a series of oriented liquid crystalline polymers TDI-C m C n . The simulation shows that both Bragg peaks (spots) and equidistant streaks (diffuse scatterings) are diffracted by one smectic C-like structure in which each of the molecular chains is displaced from its mean position by a random distance Δ z , along the chain axis (z). This is a first type of disorder. By assuming a Gaussian distribution of the disordered displacements, the mean value of Δ z has been determined from the position and intensity of the streaks and Bragg peaks for the polymers.

CRITICAL FLUCTUATIONS AND QUENCHED DISORDERED TWO-DIMENSIONAL CHARGE STRIPES IN LA5/3SR1/3NIO4

Using high-resolution x-ray scattering, we have demonstrated the existance of quenched disordered charge stripes in a single crystal of La 5/3 Sr 1/3 NiO 4 at low temperatures. Above the second-order transition critical scattering was observed due to fluctuations into the charge stripe phase. The charge stripes are shown to be two dimensional in nature both by measurements of their correlation lengths (ξ a ≈185 Å, ξ b ≈400 Å, ξ c ≈25 Å.) and by the critical exponents of the charge stripe transition. The charge stripes are disordered and that the length scale of the disorder is quenched. Detailed measurements of the correlation lengths at higher temperatures, just below the charge stripe melting temperature, display an anisotropic broadening indicative of an order-disorder transition. We ascribe these results to a smectic-nematic phase transition at approximately 225 K. Such electronic quantum liquid-crystal phase transitions have been theoretically predicted before, but these results provide the first experimental evidence for them in charge stripe nickelates.

Smectic–nematic phase transition as wrinkling transition in a stack of membranes

We point out that the smectic–nematic phase transition may considered as a transition of a stack of membranes in 2+ ϵ dimensions, in which the layers become so wrinkled that they interpenetrate each other are no longer distinguishable.

Disorder effects in low concentration dispersions of small quartz spheres in cyanobiphenyl liquid crystals

A photopyroelectric study of the specific heat and the thermal conductivity of small quartz sphere (aerosil) dispersions in heptylcyanobiphenyl (7CB) and octylcyanobiphenyl (8CB) liquid crystals with a concentration rho(s)=0.005 g cm(-3) is reported. The thermal conductivity data show that, even at this low concentration, the elastic strain introduced in the liquid crystal by the aerosil particle is not completely annealed at the nematic-isotropic (NI) phase transition. In 8CB, annealing has been found at the smectic-nematic phase transition, while in the case of 7CB it occurs in the nematic phase also. Moreover, the depression of the NI transition temperature has been found to be much smaller than the one predicted by a random field model in which quenched disorder simply affects the average order of the sample and no effects of the elastic strain are considered.

Effect of polymer architecture on self-diffusion of LC polymers

Two LC side-group poly(methacrylates) were synthesized, and their melt dynamics were compared with each other and a third, main-chain side-group combined LC polymer. A new route was developed for the synthesis of the poly(methacrylate) polymers which readily converts relatively inexpensive perdeuteromethyl methacrylate to other methacrylate monomers. Self-diffusion data was obtained through the use of forward recoil spectrometry, while modulus and viscosity data were measured using rotational rheometers in oscillatory shear. Diffusion coefficients and complex viscosity were compared to previous experiments on liquid crystal polymers of similar architecture to determine the effect of side-group interdigitation and chain packing on center of mass movement. The decyl terminated LC side-group polymer possessed an interdigitated smectic phase and a sharp discontinuity in the self-diffusion behavior at the clearing transition. In contrast, the self-diffusion behavior of the methyl terminated LC side-group polymer, which possessed head-to-head side-group packing, was seemingly unaffected by the smectic-nematic and nematic-isotropic phase transitions. The self-diffusion coefficients of both polymers were relatively insensitive to the apparent glass transition. The presence of moderately fast sub-T g chain motion was supported by rheological measurements that provided further evidence of considerable molecular motion below T g . The complex phase behavior of the combined main-chain side-group polymer heavily influenced both the self-diffusion and rheological behavior. Differences between the self-diffusion and viscosity data of the main-chain side-group polymer could be interpreted in terms of the defect structure.