Liquid Crystal Phase Transition Research Articles

The influence of the azo chromophore structure and quantum dots on the photoinduced phase transition in a nematic liquid crystal

ABSTRACT We present an analysis of the ‘order-disorder’ phase transition into the liquid crystal matrix containing photochromic molecules with a small amount of semiconductor quantum dots (QDs). The factors studied are the influence of the chemical structure of azochromophores introduced in a liquid crystal (LC) on the phase behaviour and dielectric properties of the resulting composites as well as the life-time of the isotropic phase after photoinduced phase transition. We have chosen two azochromophores with different melting points Tm and degrees of branching. The branched structure of one chromophore promotes an increase in the life-time of Z-conformation and the corresponding stabilisation of the isotropic phase. The main factor influencing the increase in the life-time is a weaker intermolecular interaction between the LC matrix and the chromophore molecules. QDs in small amounts (0.5 and 1 wt.%) have a significantly smaller impact on the phase transition.

Climbing Plant‐Inspired Multi‐Responsive Biomimetic Actuator with Transitioning Complex Surfaces

AbstractBionic robots have great potential in healthcare, rescue, and surveillance. However, most soft actuators can only realize one or two deformation modes, which largely limit the application in complex environments. This study develops electric/Infrared (IR) light/magnetic multi‐field coupling soft actuators by combining liquid crystal elastomers (LCE), liquid metal (LM) and magnetic Ecoflex (Mecoflex). Originated from the synergistic effect of the differed thermal expansion and the liquid crystal phase transition, the actuator can realize outstanding large deformations (bending angle > 300°) at ultra‐low voltages (1.0 V). In addition, by designing the molecular orientation in the LCE layer and programming the magnetization in the Mecoflex layer, the bending or helical bending deformations can be controlled under electric/magnetic and IR light/magnetic coupling actuation. Based on the complex deformation capability, bionic climbing plants and two kinds of quadrupedal robots are developed. With the structural design of the quadrupedal robots, they can flexibly switch deformations to pass through complex environments by controlling the externally applied coupling fields, which demonstrates their broad potential in practical applications.

Liquid crystal phase behavior of oxalated cellulose nanocrystal and optical films with controllable structural color induced by centripetal force

Cellulose nanocrystal (CNC) is a sustainable bio-nanomaterial. The distinctive left-handed polarization properties render cellulose nanocrystal a promising candidate for optical film. Due to eco-friendliness, reliability, mildness and simplicity, the oxalate hydrolysis method stands out among various preparation methods for CNC. This study delved into the liquid crystal phase behavior of oxalated cellulose nanocrystal derived from pulp, and discovered the influences of CNC concentration and pH on suspension stability and phase transition, and evaluated its optical properties. The results demonstrated that oxalated CNC presented two different liquid crystal phases, the nematic phase and the cholesteric phase. The stability mechanism of CNC suspension and the regulatory principle of the liquid crystal phase transition were revealed. A novel CNC film-forming technology, the multilayer spin-coating technique, was developed for cellulose nanocrystal optical films. Driven by centrifugal force, cellulose nanocrystals were induced to self-assembly and formed the optical film with circular dichroism and structural color. This simple and efficient film-forming technology promised rapid processing (1 h) and controllable film structure and optical properties compared to traditional technologies. This work provided a theoretical understanding and practical prospects for integrating oxalated cellulose nanocrystal into sustainable advanced optical film materials.

Liquid crystal droplets formation and stabilization during phase transition process

Abstract The study of phase transition processes in liquid crystals (LCs) remains challenging. Most thermotropic LCs exhibit a narrow temperature range and a rapid phase transition from the isotropic (ISO) to the nematic (N) phase, which make it difficult to capture and manipulate the phase transition process. In this study, we observed the evolution of small droplets during the ISO–N phase transition in ferroelectric nematic (NF) LC RM734. After doping with metal nanoparticles (NPs), the temperature range of the phase transition broadened, and the droplets formed during the phase transition remained stable, with their diameter increasing linearly with temperature. In addition, droplets doped with NPs can be well controlled by an external electric field. This discovery not only aids in understanding the fundamental mechanisms of LC phase transitions but also provides a simple alternative method for preparing droplets, which is potentially valuable for applications in optoelectronic devices and sensors.

Helical Hybrid Nanostructure Based on Chiral M13 Bacteriophage via Evaporation-Induced Three-Dimensional Process.

The use of naturally sourced organic materials with chirality, such as the M13 bacteriophage, holds intriguing implications, especially in the field of nanotechnology. The chirality properties of bacteriophages have been demonstrated through numerous studies, particularly in the analysis of liquid crystal phase transitions, developing specific applications. However, exploring the utilization of the M13 bacteriophage as a template for creating chiral nanostructures for optics and sensor applications comes with significant challenges. In this study, the chirality of the M13 bacteriophage was leveraged as a valuable tool for generating helical hybrid structures by combining it with nanoparticles through an evaporation-induced three-dimensional (3D) printing process. Utilizing on the self-assembly property of the M13 bacteriophage, metal nanoparticles were organized into a helical chain under the influence of the M13 bacteriophage at the meniscus interface. External parameters, including nanoparticle shape, the ratio between the bacteriophage and nanoparticles, and pulling speed, were demonstrated as crucial factors affecting the fabrication of helical nanostructures. This study aimed to explore the potential of chiral nanostructure fabrication by utilizing the chirality of the M13 bacteriophage and manipulating external parameters to control the properties of the resulting hybrid structures.

Magnetic field induced isotropic ferrofluid to ferronematic phase transition in thermotropic liquid crystal

Experiments on liquid crystals doped with ferromagnetic nanoparticles predicted the isotropic ferrofluid to ferronematic phase transition. The magnetic field induced isotropic ferrofluid to ferronematic phase transition in thermotropic liquid crystal has been studied theoretically. Theoretical model is based on Flory–Huggins theory of isotropic mixing and Landau-de Gennes theory. The impact of ferromagnetic nanoparticles and magnetic field on the isotropic ferrofluid to ferronematic phase transition and transition temperature has been discussed. Theoretical results predict the magnetic field induced critical point of the isotropic ferrofluid to ferronematic phase transition. The temperature and volume fraction of ferromagnetic nanoparticles dependence of the magnetic birefringence and Cotton-Mouton constant are calculated.

Difluorinated tolane-based photoluminescent liquid crystals with imidazolium salt-terminated flexible chains: Thermophysical and photophysical properties

Photoluminescent liquid crystal (PLLC) molecules have attracted significant interest because they exhibit both photoluminescent (PL) and liquid crystal (LC) properties within a single-composition molecular system. In the quest to develop PLLC molecules, researchers have explored ionic compounds incorporating ionic moieties in flexible chains or mesogenic structures. Notable examples include disk-shaped columnar PLLCs and smectic (Sm) PLLCs, which exhibit distinct layers of ionic moieties, counterions, and luminescent mesogens. Ionic LC molecules exhibiting the Sm phase at low temperatures and whose phase transition temperatures can be modulated by tuning the ionic moiety show promise as PLLCs. However, reports on Sm PLLC molecules remain scarce. Based on previous findings that difluorinated tolanes can emit fluorescence in the crystalline (Cr) state, we explored their potential as luminescent mesogens for the development of efficient Sm PLLC compounds. In this study, we designed a rod-shaped compound featuring a difluorinated tolane skeleton as a luminescent mesogen, a decyleneoxy chain for stabilizing the LC phase, and an imidazolium ion as an ionic moiety at the chain end to induce the smectic A (SmA) phase. Upon investigating their thermophysical properties, all compounds demonstrated thermotropic LC attributes and contained a SmA phase. Notably, the melting temperature decreased with increasing anion volume. Photophysical property evaluation revealed that the compounds displayed blue photoluminescence in both dilute solutions and Cr states, with no counteranion-dependent variations in photoluminescence observed in the solution. However, the photoluminescence behavior in the Cr-state specimens varies with different counteranions, demonstrating an increased photoluminescence wavelength with increasing anion volume. Furthermore, the Cr ⇄ LC phase transition altered the photoluminescence behavior, highlighting the potential of our molecular design strategy for developing temperature-responsive PL materials. These findings provide valuable insights for designing new PLLC molecules that can serve as effective fluorescent sensors across a range of temperatures, including room temperature.

Exploring Colloidal Phase Transitions of Imogolite Nanotubes by Evaporation Induced Self‐Assembly in Levitation

AbstractEvaporation‐induced self‐assembly (EISA) is a versatile method for generating organized superstructures from colloidal particles, offering diverse design possibilities through the manipulation of colloid size, shape, substrate nature, and environmental conditions. While some work highlighted the potential of EISA to investigate phase transitions of inorganic liquid crystals, the influence of sample environment to determine their phase diagrams is often overlooked. In this work, the self‐assembly of lyotropic liquid crystals is compared by EISA on substrates, and by acoustic levitation (absence of substrate). The focus is on imogolite nanotubes, a model colloidal system of 1D charged objects, due to their tunable morphology and rich liquid‐crystalline phase behavior. It demonstrates the feasibility to obtain phase transitions in levitating droplets and on soft hydrophobic substrates, whereas self‐assembly is limited on rigid hydrophilic supports. Moreover, the aspect ratio of the nanotubes proves to be a pivotal factor, influencing both transitions and the resulting materials shape and surface. Besides material shaping, acoustic levitation emerges as a promising method for studying phase transitions by EISA, toward the rapid establishment of phase diagrams from diluted to highly concentrated states using a limited volume of sample.

Isothermal Phase Transitions in Liquid Crystals Driven by Dynamic Covalent Chemistry.

The dynamic nature of calamitic liquid crystals is exploited to perform isothermal phase transitions driven by dynamic covalent chemistry. For this purpose, nematic (N) arrays based on aldehyde 1 were treated with different amines (A-E) in an on-surface process, which resulted in different isothermal phase transitions. These phase transformations were caused by in situ imination reactions and are dependent on the nature of the added amine. Transitions from the N to crystal (1A, 1E), isotropic (1B), and smectic (Sm) (1C, 1D) phases were achieved, while the resulting materials feature thermotropic liquid crystal behavior. A sequential transformation from the N 1 to the Sm 1C and then to the N 1B was achieved by coupling an imination to a transimination processes and adjusting the temperature. All of these processes were well characterized by microscopic, spectroscopic, and X-ray techniques, unlocking not only the constitutional but also the structural aspects of the phase transitions. This work provides new insights into designing constitutionally and structurally adaptable liquid crystal systems, paving the way toward the conception of programable evolutive pathways and adaptive materials.

Isothermal Phase Transitions in Liquid Crystals Driven by Dynamic Covalent Chemistry

AbstractThe dynamic nature of calamitic liquid crystals is exploited to perform isothermal phase transitions driven by dynamic covalent chemistry. For this purpose, nematic (N) arrays based on aldehyde 1 were treated with different amines (A–E) in an on‐surface process, which resulted in different isothermal phase transitions. These phase transformations were caused by in situ imination reactions and are dependent on the nature of the added amine. Transitions from the N to crystal (1A, 1E), isotropic (1B), and smectic (Sm) (1C, 1D) phases were achieved, while the resulting materials feature thermotropic liquid crystal behavior. A sequential transformation from the N 1 to the Sm 1C and then to the N 1B was achieved by coupling an imination to a transimination processes and adjusting the temperature. All of these processes were well characterized by microscopic, spectroscopic, and X‐ray techniques, unlocking not only the constitutional but also the structural aspects of the phase transitions. This work provides new insights into designing constitutionally and structurally adaptable liquid crystal systems, paving the way toward the conception of programable evolutive pathways and adaptive materials.

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.

Effect of varying concentration of dichroic dye on textural, electro-optical, phase transition and band gap studies of zinc oxide nanoparticles induced vertically aligned liquid crystals

Vertically aligned liquid crystal (VALC) perpetually boosts the advancement for display devices and optoelectronics. Herein, initially, the nanoparticles (NPs) induced VALC (nVALC) cell was prepared via doping of 0.3 (wt./wt.%) of zinc oxide NPs in nematic liquid crystal (LC). After that, a series of orange azo dichroic dye doped nVALC cells were prepared with varying concentrations of dye such as 0.0625, 0.125, 0.25 and 0.5 wt.%. The effect of dye concentrations on LC textures, phase transition, electro-optical and band gap properties were investigated, as well as resulted in a significantly reduced value of the threshold and operating voltages as a function of dye concentrations. The contrast ratio was also enhanced up to 0.25 wt.% dye doped nVALC cells. Additionally, the transition temperature from nematic to isotropic phase and band gap studies were also performed. The present work provides improved properties of nVALC systems for next-generation display technologies.

Selective phase transition of cholesteric blue phase I induced by pattern-controlled Grandjean–Cano disclinations

This study investigates the influence of Grandjean–Cano disclinations on the cholesteric-blue phase I (Ch-BPI) phase transition, revealing the pivotal role of defects in the phase transition of liquid crystals (LCs). We prepared the disclinations by wedge cells or parallel cells with an alignment pattern and demonstrated that the disclinations induce a phase transition in Ch-BPI. Furthermore, we demonstrated the control of the BPI appearance areas by utilizing this property. This defect-induced transition technique not only provides a novel method for controlling BPI orientation, but also offers insights into soft material crystal growth, suggesting potential applications in new BPLC-based device development.

Hysteresis Elimination for an Anisotropic Liquid-Crystal Model via Molecule Design and Replica-Exchange Optimization.

The phenomenon of hysteresis in simulations, in which a system's current state is correlated to previous states and inhibits the transition to a more stable phase, may often lead to misleading results in physical chemistry. In this study, in addition to the replica exchange method (REM), a novel approach was taken by combining an evolution strategy based on the evolutionary principles of nature to predict phase transitions for the Hess-Su liquid-crystal model. In this model, an anisotropy term is added to the simple 6-12 Lennard-Jones model to intuitively reproduce the behavior of liquid crystals. We first applied the pressure-temperature REM to the Hess-Su model and optimized the replica spacing for the energy distribution to gain the maximum advantage from the REM. We then used the same approach as for the Hamiltonian REM, seeking to optimize the replica spacing in the same way. Based on both results, we attempted to predict this coarse-grained liquid-crystal model's exact phase transition point. In the Hamiltonian REM, replicas were prepared with different molecular aspect ratios corresponding to the values of the anisotropy terms in the potential function. The Hess-Su liquid-crystal model, which undergoes a direct transition from the nematic to the solid phase without going through a smectic phase, is a challenging research target for understanding phase transitions. Despite the tremendous computational difficulty in overcoming the strong hysteresis present in this system, our method could predict the phase transition point clearly and significantly reduce the extent of hysteresis. Our approach is beneficial when simulating more complex systems and, above all, shows great potential for more accurate and efficient phase transition predictions in the field of molecular simulation in the future.

Colloidal phase behavior of high aspect ratio clay nanotubes in symmetric and asymmetric electrolytes

HypothesisImogolite nanotubes (INTs) are unique anisometric particles with monodisperse nanometric diameters. Aluminogermanate double-walled INTs (Ge-DWINTs) are obtained with variable aspect ratios by controlling the synthesis conditions. It thus appears as an interesting model system to investigate how aspect ratio and ionic valence influence the colloidal behavior of highly anisometric rods. ExperimentsThe nanotubes were synthesized by hydrothermal treatment for 5 or 20 days to modify the aspect ratio while the electrostatic interactions were investigated by comparing the colloidal stability in symmetric and asymmetric electrolytes. The phase behavior and their related microstructure were determined by optical observations and small-angle X-ray scattering measurements, coupled with interparticle distance modelling. FindingsWe revealed that colloidal suspensions of Ge-DWINTs prepared in NaCl are guided by repulsive double layer forces, undergoing different liquid crystal phase transitions before stiffen into a glass-like state. We found that the microstructure can be rationalized by taking into account the anisometric nature of the particles. By contrast, dispersions prepared with asymmetric electrolytes are governed by strong attractive forces and thus form space-filling gels containing large nanotubes aggregates.

Thickness‐Dependent Bandgap and Atomic Structure in Elemental Tellurium Films

Elemental tellurium electrical switch, relying on a transient crystal–liquid–crystal phase transition, has recently been proposed as a promising selector candidate for the next‐generation 3D high‐density memory, bridging performance gap in today's computer. Further miniaturization of the switch cell to increase memory density strongly depends on the scalability of the tellurium film, which, however, has not been experimentally studied. Herein, the tellurium films are prepared with the thickness downscaled from 400 to 2 nm and a significant increase is found in the bandgap from 0.29 to 0.91 eV, as predicted by ab initio molecular dynamics. Interestingly, the as‐deposited tellurium films with a thickness above 3 nm are in the crystalline trigonal phase, whereas 2 nm thick film suddenly becomes amorphous, observed by both Raman and transmission electron microscopies. In this finding, since the leakage current of the elemental tellurium switch is determined by both the Schottky barrier between tellurium/electrode interface and the bandgap of the tellurium film, a reduction in leakage current is predicted with further miniaturization.

Dynamic Shape Change of Liquid Crystal Polymer Based on An Order‐Order Phase Transition

AbstractLiquid crystal actuators conventionally undergo shape changes across an order‐disorder phase transition between liquid crystal (LC) and isotropic phases. In this study, we introduce an innovative Liquid Crystal Polymer (LCP) actuator harnessing an order‐order LC phase transition mechanism. The LCP film is easily stretchable within the LC phase, facilitated by the π–π stacking of phenyl groups serving as robust physical crosslinking points, and thereby transforms to a stable monodomain structure. The resultant monodomain LCP actuator shows a distinctive reversible dynamic shape change, exhibiting extension followed by contraction along the LC director on cooling. The extension is propelled by the reversible smectic C to smectic A phase transition, and the contraction is attributed to the re‐entry to the smectic C phase from smectic A phase. Thermal annealing temperature determines this peculiar dynamic shape change, which occurs during both heating and cooling processes. This pivotal attribute finds manifestation in gripper and flower‐shaped actuators, adeptly executing grabbing and releasing as well as blooming and closure motions within a single thermal stimulation. In essence, our study introduces an innovative approach to the realm of LCP actuators, ushering in a new avenue for the design and fabrication of versatile and dynamically responsive LCP actuators.

Dynamic Shape Change of Liquid Crystal Polymer Based on An Order-Order Phase Transition.

Liquid crystal actuators conventionally undergo shape changes across an order-disorder phase transition between liquid crystal (LC) and isotropic phases. In this study, we introduce an innovative Liquid Crystal Polymer (LCP) actuator harnessing an order-order LC phase transition mechanism. The LCP film is easily stretchable within the LC phase, facilitated by the π-π stacking of phenyl groups serving as robust physical crosslinking points, and thereby transforms to a stable monodomain structure. The resultant monodomain LCP actuator shows a distinctive reversible dynamic shape change, exhibiting extension followed by contraction along the LC director on cooling. The extension is propelled by the reversible smectic C to smectic A phase transition, and the contraction is attributed to the re-entry to the smectic C phase from smectic A phase. Thermal annealing temperature determines this peculiar dynamic shape change, which occurs during both heating and cooling processes. This pivotal attribute finds manifestation in gripper and flower-shaped actuators, adeptly executing grabbing and releasing as well as blooming and closure motions within a single thermal stimulation. In essence, our study introduces an innovative approach to the realm of LCP actuators, ushering in a new avenue for the design and fabrication of versatile and dynamically responsive LCP actuators.

Regulating Crystallinity Mismatch Between Donor and Acceptor to Improve Exciton/Charge Transport in Efficient Organic Solar Cells.

Achieving a more balanced charge transport by morphological control is crucial in reducing bimolecular and trap-assisted recombination and enhancing the critical parameters for efficient organic solar cells (OSCs). Hence, a facile strategy is proposed to reduce the crystallinity difference between donor and acceptor by incorporating a novel multifunctional liquid crystal small molecule (LCSM) BDTPF4-C6 into the binary blend. BDTPF4-C6 is the first LCSM based on a tetrafluorobenzene unit and features a low liquid crystal phase transition temperature and strong self-assembly ability, conducive to regulating the active layer morphology. When BDTPF4-C6 is introduced as a guest molecule into the PM6 : Y6 binary, it exhibits better compatibility with the donor PM6 and primarily resides within the PM6 phase because of the similarity-intermiscibility principle. Moreover, systematic studies revealed that BDTPF4-C6 could be used as a seeding agent for PM6 to enhance its crystallinity, thereby forming a more balanced and favourable charge transport with suppressed charge recombination. Intriguingly, dual Förster resonance energy transfer was observed between the guest molecule and the host donor and acceptor, resulting in an improved current density. This study demonstrates a facile approach to balance the charge mobilities and offers new insights into boosting the efficiency of single-junction OSCs beyond 20 %.

Regulating Crystallinity Mismatch Between Donor and Acceptor to Improve Exciton/Charge Transport in Efficient Organic Solar Cells

AbstractAchieving a more balanced charge transport by morphological control is crucial in reducing bimolecular and trap‐assisted recombination and enhancing the critical parameters for efficient organic solar cells (OSCs). Hence, a facile strategy is proposed to reduce the crystallinity difference between donor and acceptor by incorporating a novel multifunctional liquid crystal small molecule (LCSM) BDTPF4‐C6 into the binary blend. BDTPF4‐C6 is the first LCSM based on a tetrafluorobenzene unit and features a low liquid crystal phase transition temperature and strong self‐assembly ability, conducive to regulating the active layer morphology. When BDTPF4‐C6 is introduced as a guest molecule into the PM6 : Y6 binary, it exhibits better compatibility with the donor PM6 and primarily resides within the PM6 phase because of the similarity‐intermiscibility principle. Moreover, systematic studies revealed that BDTPF4‐C6 could be used as a seeding agent for PM6 to enhance its crystallinity, thereby forming a more balanced and favourable charge transport with suppressed charge recombination. Intriguingly, dual Förster resonance energy transfer was observed between the guest molecule and the host donor and acceptor, resulting in an improved current density. This study demonstrates a facile approach to balance the charge mobilities and offers new insights into boosting the efficiency of single‐junction OSCs beyond 20 %.