Smectic Phase Transition Research Articles

Calorimetric, optical and dielectric measurements on two Schiff's based liquid crystalline materials exhibiting multiple phase transitions.

We report high-resolution calorimetric, optical and dielectric studies on two Schiff's based liquid crystalline materials, 4O.5 and 5O.5, which exhibit multiple phase transitions. The study goes beyond the commonly studied isotropic-to-nematic (I-N) and nematic-to-smectic A (N-SmA) phase transitions and explores higher-order smectic phase transitions. The critical exponent, α values, for different phase transitions has been explored. For the I-N, N-SmA, SmA-SmC, SmC-SmF, SmF-CrG and SmB-CrG transitions, α values close to 0.5 and amplitude ratios A-/A+ ~ 1.6 and D-/D+ ~ 1 indicate a first-order nature. Notably, for the SmA-SmB transition, although α (≈ 0.64) is larger than the tricritical value of 0.5, A-/A+ ~ 1.2 and D-/D+ ~ 1, and the study reports an order parameter critical exponent (β) of 0.26 ± 0.002, which supports a tricritical nature for this transition. Importantly, the ratios of A-/A+ and D-/D+ are found to have similar values across all three-measurement methods, indicating consistency and agreement among these methods.

Cholesteric liquid crystal phase variations within nanotubes: an in-depth analysis of the influences of twist radius and molecular pitch

ABSTRACT This study investigates the self-assembly and phase transitions of cholesteric liquid crystals (CLCs) when confined in nanotubes, specifically focusing on the variations in their molecular structures. A dissipative particle dynamics method was used to investigate how the changes in helix radius ( r LC ) and pitch ( P ) affect the behaviour of the LCs. For pitches of 24, 12, and 6, the orientation order parameter ( P 2 ) showed distinct trends during cooling. Smaller helix radii resulted in isotropic to nematic to smectic phase transitions, whereas larger radii resulted in a novel smectic phase with spiral structures (Sm s ). Circumferential local-order parameters ( P 2 r and P 2 z ) were introduced to better characterise the phases, and a disordered spiral-structure phase (Sm s 2 ) with local perturbations was observed. The transitions between isotropic, nematic, and smectic phases, highlighting the influence of the pitch and helix radius, were observed from the phase diagrams. Smaller pitches and radii prevented the formation of stable structures; however, specific combinations induced characteristic spiral structures in the smectic phase. These results provide new insights into the self-assembly of confined CLCs, which have potential applications in the design of photonic devices and LC-based sensors. This study advances our understanding of phase control in quasi-one-dimensional systems and highlights the importance of molecular variations in liquid crystal behaviour.

Improving crystallinity and ordering of PBTTT by inhibiting nematic to smectic phase transition via rapid cooling

The liquid crystalline (LC) phase can promote the long-range ordering of conjugated polymer films. Therefore, the effects of different single LC phases and the coexistence of multiple LC phases on film morphology deserve further study. In this work, we found that distinct film morphologies can be obtained by undergoing different LC transition processes. It was demonstrated that Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) undergoes a high-temperature nematic LC I phase and a low-temperature smectic LC Ⅱ phase by optical images, in-situ UV–vis spectroscopy and Fourier transform infrared spectroscopy (FTIR). When the PBTTT is rapidly cooled from the melt to solid thin film at 130 °C/min, the films crystallize from LC I directly and keep the nematic-like texture. When the melt is slowly cooled at 1 °C/min, the films crystallize after transforming from LC I to LC II and retain the smectic-like texture. The former films exhibit 24.5 times higher crystallinity than the latter. Meantime, the lamellar packing d100 distance increases from 21.06 Å to 23.85 Å, and the intercrystallite distance increases from 9 nm to 25 nm, respectively. The restraint chain mobility in the highly ordered smectic phase and the nanoconfinement effect induced by phase separation between LC I and LC II are responsible for the reduced crystalline order in the films experienced by LC I and LC II. This work emphasizes the importance of LC phases and rational annealing programs on morphology design.

Controlling the Phase Behavior and Reflection of Main-Chain Cholesteric Oligomers Using a Smectic Monomer.

Cholesteric liquid crystals (CLCs) are a significant class of temperature-responsive photonic materials that have the ability to selectively reflect light of a specific wavelength. However, the fabrication of main-chain CLC oligomers with dramatic reflection band variation upon varying the temperatures remains a challenge. Here, a feasible method for improving and controlling the responsiveness of main-chain cholesteric liquid crystal oligomers by the incorporation of a smectic monomer is reported. The smectic monomer strengthens the smectic character of the oligomers and enhances the magnitude of the change of the pitch as a function of temperature upon approaching the cholesteric–smectic phase transition temperature. The central wavelength of the reflection band can be easily modified by mixing in an additional chiral dopant. This promising method will open the door to the preparation of temperature-responsive photonic devices with excellent responsiveness.

Volatility Transition from Short‐Term to Long‐Term Photonic Transistor Memory by Using Smectic Liquid Crystalline Molecules as a Floating Gate

AbstractRod‐like molecules with liquid crystalline phase transitions show highly ordered stacking and orientation, which is promising as the floating gate electret in high‐performance photonic field‐effect transistor (FET) memory. In this work, a series of rod‐like molecules, alkyl‐dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (Cn‐DNTT), are investigated with a smectic liquid crystalline phase transition. The photonic FET memory featuring these rod‐like molecules is correlated with their morphology and crystallographic properties. A volatility transition from short−term to long−term memory behavior is found by utilizing the smectic liquid crystalline phase transitions (SmX, X = H, K, E) to orient the stacking in the floating gate. Accordingly, the annealed 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C10‐DNTT) polycrystalline film with the optimized alkyl chain length achieved preferential end‐on orientation associated with the herringbone‐type stacking to store charges during photowriting, along with effective charge tunneling for the trapped charges during electrical erasing. Therefore, the resultant photonic FET memory provided a superior ON/OFF current ratio of up to 105 with retention of 104 after 104 s. The results demonstrate that rod‐like molecules with smectic liquid crystalline phase and highly ordered orientation have great potentials for developing high‐performance photonic FET memory.

Thermal- and photo-responsive liquid crystalline elastomers fabricated using tung oil-based azobenzene

Azobenzene-based liquid crystalline elastomers with dynamic ester bonds exhibit high thermal- and photo-responsive properties. However, there are a few studies that focus on photo-induced and thermal-induced behaviors separately. Herein, novel thermal- and photo-responsive LCEs containing dynamic ester bonds were prepared from 4,4’-diglycidyloxy-azobenzene, sebacic acid and tung oil-based azobenzene, which acted as a side chain. The dynamic transesterification reactions on the network structure were confirmed through stress relaxation. The photo-induced and thermal-induced behaviors were also investigated. The monodomain LCEs film was obtained by uniaxial stretching at a temperature above the smectic–isotropic phase transition temperature (Ti ∼82 °C). Due to the breakage and formation of dynamic ester bonds and the order-disorder smectic phase transition caused by heat and the trans-cis isomerization of azobenzene mesogens, the thermal-induced stress of the monodomain LCEs film reached ∼3 MPa. The LCEs film also exhibited thermal-induced shape-memory properties. Investigation of its photo-responsive behavior further demonstrated that the monodomain LCEs film generated stress of ∼1.1 MPa while exhibited photo-induced deformation due to the trans-cis isomerization of azobenzene mesogens upon irradiation with 365 nm UV light. Finally, we observed that the monodomain LCEs film could undergo quickly photo-induced deformation and could be processed into a photo-driven gripper.

Incommensurate smectic phase in close proximity to the high-Tc superconductor FeSe/SrTiO3

Superconductivity is significantly enhanced in monolayer FeSe grown on SrTiO3, but not for multilayer films, in which large strength of nematicity develops. However, the link between the high-transition temperature superconductivity in monolayer and the correlation related nematicity in multilayer FeSe films is not well understood. Here, we use low-temperature scanning tunneling microscopy to study few-layer FeSe thin films grown by molecular beam epitaxy. We observe an incommensurate long-range smectic phase, which solely appears in bilayer FeSe films. The smectic order still locally exists and gradually fades away with increasing film thickness, while it suddenly vanishes in monolayer FeSe, indicative of an abrupt smectic phase transition. Surface alkali-metal doping can suppress the smectic phase and induce high-Tc superconductivity in bilayer FeSe. Our observations provide evidence that the monolayer FeSe is in close proximity to the smectic phase, and its superconductivity is likely enhanced by this electronic instability as well.

Optical and thermal characterization of thermotropic hydrogen bonded liquid crystal mixture

A novel intermolecular hydrogen-bonded liquid crystal mixture (HBLC) has been synthesized from non-mesogenic itaconic acid (IA) and mesogenic 4-undecyloxybenzoic acid (11OBA). The formation of hydrogen bond has been analyzed by using spectroscopic studies. Polarizing optical microscope (POM) confirms the existence of liquid crystal phase corresponding to the transition temperature. LC transition temperature, enthalpy values, stability factors, and thermal width have been calculated using thermal analyzer. Band gap energies are evaluated by using ultraviolet visible and photoluminescence spectroscopy studies. The resultant POM and DSC studies reveal the present mixture enclose nematic and smectic mesogenic phase transitions. Induced thermoluminescence with parachromic variations are observed and represented as novel detection in the present HBLC mixture. This synthesized mixture is more suitable for HD display devices.

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.

Influence of ZnO nanoparticles on the polarization, dielectric and electro-optic behaviour in the smectic C* and hexatic I* phases

Here we report the results of the dielectric and polarization measurements in the smectic and the tilted hexatic phases of a liquid crystal-nano composite consisting of commercially available ferroelectric liquid crystal (FLC) doped with the ZnO nanoparticles (NPs). It has been observed that the inclusion of NPs not only decreases in the transition temperature but also dielectric strength of the Goldstone mode without altering the associated relaxation frequency. An additional relaxation observed for the FLC-ZnO nanocomposite, but not present in the pure material is attributed to the manifestation of the primary director mode. The observed results are analyzed using the generalized Landau model to understand the effect of the coupling between the polarization and optical tilt angle. While there is no change in either the magnitude of the tilt or the polarization across the orthogonal to tilted smectic phase transitions, clear changes are seen for both the parameter across the transition to the hexatic phase. The FLC-ZnO nanocomposite exhibits decreased rotational viscosity and a faster response time, a feature attractive from an application point of view.

The analysis of temperature effect on frequency dependent dielectric parameters of semifluorinated liquid crystalline biphenyl carboxylic acid

In this study, the dielectric properties of the liquid crystal 4–(7,7,8,8,9,9,10,10,10–nonafluorodecyloxy)biphenyl–4′–carboxylic acid (NBCA) exhibiting a Smectic C mesophase have been analyzed. NBCA has two critical temperatures which are 194 °C for the crystal phase to the smectic phase transition, and 269 °C for the smectic phase to the isotropic phase transition. Impedance spectroscopy measurements were made at regular intervals at different temperatures which includes these transitions. By using capacitance method, the complex dielectric constants have been calculated for each temperature, and the real part of dielectric constants have been fitted to the Cole-Cole equation. In addition, ac conductivity mechanisms of the NBCA at different temperatures and different phases and also activation energies which belongs to some selected frequencies have been determined by the imaginary part of dielectric constants.

Photopyroelectric Calorimetry Investigations of 8CB Liquid Crystal–Microemulsion System

In this work, the photopyroelectric technique has been used to investigate the phase transitions in a liquid crystal microemulsion by combining the simultaneous high temperature resolution thermal diffusivity measurements and optical polarization microscopy observations. It has been found that, during the conversion from the isotropic phase into the nematic one, the micelles are expelled from the nematic domains and remain confined in islands of isotropic material which survive down to the smectic temperature range. A hysteresis in the thermal diffusivity profiles between heating and cooling run over the isotropic–nematic transition temperature range has been observed which has been ascribed to the different micelles distribution into the sample volume during cooling and heating runs. Finally, the almost bulk-like behavior of the thermal diffusivity over the nematic–smectic phase transition confirms that a significant fraction of the micelles are expelled during the nucleation of the nematic phase.

Investigation of nematic to smectic phase transition and dynamical properties of strongly confined semiflexible polymers using Langevin dynamics.

We investigated the nematic to smectic phase transition for strongly confined semiflexible polymer solutions in slit-like confinements using GPU-accelerated Langevin dynamics. We characterized the phase transitions from the nematic to smectic phases for semi-flexible polymer solutions as the polymer density increased. The dependence for the lyotropic nematic to smectic transition can be collapsed by scaling exponents between 0.2 and 0.3. The smectic C phase is found for all the cases with the polymer orientation director tilted with respect to smectic layer lateral alignment. As the chain rigidity increases, the transition density decreases for systems in which the polymer persistence length (P) to slit height (H) ratios are 1.25, 2.5, 3.75, 5 and 25. We also characterized the polymer dynamics for the isotropic-nematic-smectic transitions. The overall polymer diffusivity decreased steadily as the polymer density increased. We observed anomalous polymer diffusion along the nematic director near the isotropic-nematic transition, similar to previously reported behavior for nematic-forming ellipsoids. Polymer diffusivity decreased sharply by two orders of magnitude upon the nematic-smectic transition.

Possible nematic to smectic phase transition in a two-dimensional electron gas at half-filling

Liquid crystalline phases of matter permeate nature and technology, with examples ranging from cell membranes to liquid-crystal displays. Remarkably, electronic liquid-crystal phases can exist in two-dimensional electron systems (2DES) at half Landau-level filling in the quantum Hall regime. Theory has predicted the existence of a liquid-crystal smectic phase that breaks both rotational and translational symmetries. However, previous experiments in 2DES are most consistent with an anisotropic nematic phase breaking only rotational symmetry. Here we report three transport phenomena at half-filling in ultra-low disorder 2DES: a non-monotonic temperature dependence of the sample resistance, dramatic onset of large time-dependent resistance fluctuations, and a sharp feature in the differential resistance suggestive of depinning. These data suggest that a sequence of symmetry-breaking phase transitions occurs as temperature is lowered: first a transition from an isotropic liquid to a nematic phase and finally to a liquid-crystal smectic phase.

Observation of induced new smectic phase in supramolecular hydrogen bonded liquid crystals between mesogenic and non-mesogenic aliphatic compounds

ABSTRACTPresent supramolecular hydrogen bonded liquid crystal (HBLC) complex is obtained between mesogenic 4-nonyloxybenzoic acid (9OBA) and non-mesogenic citric acid (CA). A phase transition temperature and enthalpy values are calculated by differential scanning calorimetry (DSC) and phase behaviour are identified by polarizing optical microscope (POM). The liquid crystal parameters and structure are characterized by POM, DSC, FT-IR, NMR and optical tilt angle studies. The DSC and POM reveals the smectic and nematic phase transitions in the present complex. The molecular structure of HBLC complex is elucidated by Proton Nuclear Magnetic Resonance (1H NMR). The noteworthy in the present complex is the nematic and Smectic C phase is separated by a Smectic X phase which shows the finger print textures. The Smectic X phase is estimated by varying the mole ratio of the CA+9OBA from 1:1 to 1:3. Molecular tilt with respect to the director the Smectic X and Smectic C phase data are fitted to a power law equation in the optical tilt angle studies and it follows the Mean Field Theory. The 1:2 mole ratio of CA+9OBA shows monotropic Smectic C phase. Another creditable observation is that while the mole ratio of 9OBA mesogenic compound increases the nematic range also increases in the present HBLC complex.

PAb03 スメクティックA-ネマティック相転移点近傍での粘性と電気粘性効果

PAb03 スメクティックA-ネマティック相転移点近傍での粘性と電気粘性効果

Reversibility of temperature driven discrete layer-by-layer formation of dioctyl-benzothieno-benzothiophene films.

Film forming properties of semiconducting organic molecules comprising alkyl-chains combined with an aromatic unit have a decisive impact on possible applications in organic electronics. In particular, knowledge on the film formation process in terms of wetting or dewetting, and the precise control of these processes, is of high importance. In the present work, the subtle effect of temperature on the morphology and structure of dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films deposited on silica surfaces by spin coating is investigated in situ via X-ray diffraction techniques and atomic force microscopy. Depending on temperature, bulk C8-BTBT exhibits a crystalline, a smectic A and an isotropic phase. Heating of thin C8-BTBT layers at temperatures below the smectic phase transition temperature leads to a strong dewetting of the films. Upon approaching the smectic phase transition, the molecules start to rewet the surface in the form of discrete monolayers with a defined number of monolayers being present at a given temperature. The wetting process and layer formation is well defined and thermally stable at a given temperature. On cooling the reverse effect is observed and dewetting occurs. This demonstrates the full reversibility of the film formation behavior and reveals that the layering process is defined by an equilibrium thermodynamic state, rather than by kinetic effects.

New ferroelectric and antiferroelectric liquid crystals studied by complementary methods

ABSTRACTFour new synthesised liquid crystalline compounds belonging to the homologous series of fluorinated biphenyl benzoate esters have been studied to compare their dielectric and electrooptic properties. Three of the studied compounds exhibited ferro- and antiferroelectric phases while one of them exhibited only one liquid crystalline phase – ferroelectric SmC*. No paraelectric phase was detected and straight transition between isotropic liquid and ferroelectric phases was observed for all studied compounds. Tilt angle for all of the studied compounds was equal to ca. 45ᵒ in the liquid crystalline phases, except temperature range close to the isotropic liquid–ferroelectric smectic phase transition. Temperature dependences of helical pitch, spontaneous polarisation and switching time have been determined. Based on XRD results, temperature dependence of the layer thickness has also been found. Only one relaxation process has been revealed in the ferroelectric as well as antiferroelectric phases, even the bias field up to 8 V/µm was applied. The dielectric and electrooptic data are discussed based on the mean-field theory predictions.

Critical behavior at the isotropic to nematic, nematic to smectic-A and smectic-A to smectic-C phase transitions in a pyrimidine liquid crystal compound

High-resolution optical birefringence (∆n) measurement of a pyrimidine liquid crystal compound having nematic, smectic-A and smectic-C phases are reported. The high-resolution ∆n data are rather successful in assessing the critical anomaly at different phase transitions in the investigated compound with a reasonably good accuracy. The critical exponent β, describing the limiting behavior of the nematic order parameter close to the isotropic–nematic (I–N) phase transition, is found to be in good agreement with the tricritical hypothesis. The critical behavior at the nematic–smectic-A (N–Sm-A) and the smectic-A–smectic-C (Sm-A–Sm-C) phase transitions has been explored with the aid of a differential quotient extracted from the ∆n values. The yielded effective critical exponent α′ is appeared to be nearly tricritical in nature for the N–Sm-A phase transition. For the Sm-A–Sm-C phase transition, α′ exhibits a weak dependence on the fit range and assumes tricritical value for large temperature range considered, which again is found to be diminished slightly with reduction in the temperature range. Related critical amplitude quotient and corrections-to-scaling quotient are found to display deviations from the theoretical models. Such behavior signals the appearance of a non-Landau character for the orthogonal to tilted smectic phase transition in the investigated compound.

On the phase transitions of 8CB/Sn2P2S6 liquid crystal nanocolloids.

Using differential scanning calorimetry measurements, the influence of Sn2P2S6 ferroelectric nanoparticles on the phase transition temperatures of the 8CB liquid crystal is studied. The spontaneous polarization, ionic and anchoring effects are discussed. For low concentration of dopant, the global effect leads to a decrease and an increase of the nematic-isotropic and the smectic A-nematic phase transition temperatures, respectively. For high concentrations, due to aggregates formation, the predominant anchoring effect induces a decrease of the both phase transition temperatures.