Search Results (153)

Using reactive atomistic molecular dynamics, we simulate the network formation and bulk properties of chemically identical liquid crystal elastomers (LCEs) and isotropic elastomers. The nematic elastomer is from a family of materials that have been shown to be auxetic at a molecular level. The network orientational order parameters and glass transition temperatures measured from our simulations are in strong agreement with experimental data. We reproduce, in silico, the magnitude and onset of strain-induced nematic order in isotropic simulations. Application of uniaxial strain to nematic LCE simulations causes biaxial order to emerge, as has been seen experimentally for these auxetic LCEs. At strains of ~1.0, the director reorients to be parallel to the applied strain, again as seen experimentally. The simulations shed light on the strain-induced order at a molecular level and allow insight into the individual contributions of the side-groups and crosslinker. Further, the agreement between our simulations and experimental data opens new possibilities in the computational design of high-molecular-weight liquid crystals, especially where an understanding of the properties under mechanical actuation is desired. Moreover, the simulation methodology we describe will be applicable to other combinations of orientational and/or positional order (e.g., smectics, cubics). Full article

This study investigates the temperature-driven transmission switching behavior of our proposed smart glass, which utilizes a biphasic liquid crystal system under continuous application of a distinctive homeotropic (H) state voltage (V_H). By ascertaining V_H at temperatures near the phase transition point, the minimum voltage required to sustain the H state in the smectic A* (SmA*) phase is identified. Interestingly, this minimum V_H is unable to induce the H state in the chiral nematic (N*) phase, thereby maintaining a low-transmission scattering state; i.e., the focal conic (FC) state. This empowers passive, bidirectional optical switching between the transparent H state (in the SmA* phase) and the scattering FC state (in the N* phase) in an unaligned liquid crystal cell. This work employs two dissimilar chiral dopants, R811/S811 and CB7CB/R5011, both capable of inducing the SmA* phase. Neither resulting cell system underwent surface orientation treatment, and a black dye was incorporated to enhance the contrast ratio. The results indicate that the more efficacious CB7CB/R5011 system achieves a contrast ratio of 17 between the transparent and scattering states, with a corresponding haze level of 78%. To further reduce energy consumption, the experimental framework was transitioned from a continuous-voltage to a variable-voltage mode, giving rise to an increased haze level of 88%. The proposed switching scheme holds promise for diverse applications, notably in smart windows and light shutters. Full article

This study reports on the preparation and comprehensive characterisation of new brominated hydrogen-bonded liquid crystalline (HBLC) materials. Two distinct series of supramolecular complexes were prepared by hydrogen-bond formation between 3-bromo-4-pentyloxybenzoic acid as the proton donor and non-fluorinated and fluorinated azopyridines with variable terminal chains as proton acceptors. The successful formation of a hydrogen bond was confirmed by FTIR spectroscopy. The impact of alkyl chain length and fluorination on the mesomorphic properties of the HBLCs was systematically investigated. The molecular self-assembly was thoroughly examined using polarised optical microscopy (POM) and differential scanning calorimetry (DSC), revealing the presence of smectic C (SmC), smectic A (SmA), and nematic (N) phases, with thermal stability being highly dependent on the molecular architecture. Notably, the introduction of fluorine atoms significantly influenced the phase transition temperatures and the overall mesophase range. Using bromine as a lateral substituent induces the formation of SmC phases in these HBLCs, a feature absent in their non-brominated analogues. Further structural insights were obtained through X-ray diffraction (XRD) investigations, confirming the nature of the observed LC phases. Additionally, the photo-responsive characteristics of these HBLCs were explored via UV-Vis spectroscopy, demonstrating their ability to undergo reversible photoisomerisation upon light irradiation. These findings underscore the critical role of precise molecular design in tailoring the properties of HBLCs for potential applications such as optical storage devices. Full article

The Landau–de Gennes model is one of the most significant fundamental frameworks in The Physics of Liquid Crystals and Soft Matter Physics. It is validated by the universal parameterisation of the Cotton–Mouton effect, the Kerr effect, and light scattering in the isotropic phase of nematogens. However, as early as 1974, de Gennes identified the first two puzzling problems of this model. Over the following decades, this list has expanded. This report presents the first comprehensive analysis of these issues, with the explicit experimental reference. It focuses on the hardly coherently discussed pretransitional changes in the dielectric constant and the extension in a strong electric field, specifically the nonlinear dielectric effect (NDE). Notably, there are uniquely different pretransitional forms of pretransitional effects, depending on molecular structural features such as permanent dipole moment loci or a steric hindrance. It is tested for 5CB, 5*CB, and MBBA: nematogenic liquid crystalline materials that differ in the above features. The obtained specific pretransitional effects and the evidence for the essential importance of the interplay between observation and pretransition fluctuations time scales led to a new coherent, model-based explanation of all the discussed problems, which cannot be explained within the canonical Landau–de Gennes model. Full article

The experimental study has been carried out using advanced computer vision methods in order to visualize the moment of excitation and further propagation of a non stationary isotropic domain in a hybrid aligned nematic (HAN) microsized volume under the effect of a laser beam focused on a bounding liquid crystal surface. It has been shown that, when the laser power exceeds a certain threshold value, in bulk of the HAN microvolume, an isotropic circular domain is formed. We also observed a structure of alternating concentric rings around the isotropic circular region, which increases with distance from the center of the isotropic domain. The formation of a sequence of rings in a polarizing microscopic image indicates the formation of a complex topology of the director field in the HAN cell under study. The following evolution of the texture can be represented by two modes. Firstly, the “fast” heating mode, which is responsible for the formation and explosive expansion of an isotropic zone in bulk of the HAN microvolume with characteristic time ${\tau }_1$ due to a laser spot heating on the upper indium tin oxide (ITO) layer. Secondly, the “slow” heating mode, when an isotropic zone and concentric rings slowly expand with characteristic time ${\tau }_2$ mainly due to the finite thermoconductivity of ITO layer. When the laser power significantly exceeds the threshold value, damped oscillations of the isotropic domain are observed. We also introduced the metrics that allows quantitatively estimate the behavior of texture observed. The results obtained form an experimental basis for further investigation of thermomechanical force appearing in the LC system with coupled gradients of temperature and director fields. Full article

We conduct extensive Monte Carlo simulations to investigate the factors that control the isotropic-to-nematic transition of hard colloidal polymers in bulk and under various conditions of confinement. Utilizing a highly idealized model, polymers are represented as linear chains of tangent hard spheres of uniform length, whose stiffness is controlled by a bending potential leading to rod-like configurations. Confinement is realized through the presence of flat, parallel, and impenetrable walls in one, two, or three dimensions while periodic boundary conditions are applied on the unconstrained dimensions. All simulations are performed through the Simu-D software, composed of conventional and advanced, chain-connectivity-altering Monte Carlo algorithms. We explore in detail how distinct factors, including chain length, stiffness, confinement, and packing density affect the isotropic-to-nematic transition exhibited by the polymer chains and identify with high precision the concentration range where this phase change takes place as a function of the applied conditions. Full article

We report on the preparation and characterization of a new family of hydrogen-bonded nematogenic liquid crystalline dimers. The dimers are supramolecular complexes that consist of a benzoic acid derivative, acting as the proton donor, featuring a spacer with seven methylene groups and a terminal decyloxy chain, paired with an azopyridine derivative as the proton acceptor. The latter was either fluorinated or nonfluorinated with variable alkoxy chain length. The formation of a hydrogen bond between the individual components was confirmed using FTIR and ¹H NMR spectroscopy. All supramolecules were investigated for their liquid crystalline behaviour via a polarized optical microscope (POM) and differential scanning calorimetry (DSC). All materials exhibit enantiotropic nematic phases as confirmed by X-ray diffraction (XRD) and POM investigations. The nematic phase range depends strongly on the degree and position of fluorine atoms. Additionally, the supramolecules demonstrated a rapid and reversible transition between the liquid crystal phase and the isotropic liquid state because of trans-cis photoisomerization upon light irradiation. Therefore, this study presents a straightforward approach to design photo-responsive nematic materials, which could be of interest for nonlinear optics applications. Full article

Fluorescent liquid crystals (LCs) have attracted considerable interest owing to their unique combination of fluidity, anisotropy, and intrinsic emission. However, most reported fluorescent LCs exhibit high phase transition temperatures and/or smectic phases, limiting their practical applications. To address this, we designed and synthesized a series of 2,1,3-benzothiadiazole (BTD)-based fluorescent nematic liquid crystals incorporating donor (D) or acceptor (A) groups to form D–A–D or D–A–A structures. Most of the synthesized derivatives exhibited supercooled nematic phases at room temperature. They composed various functional groups, such as secondary alkylamine, branched alkyl chain, and trifluoroacetyl groups, which are rarely used in calamitic nematic LCs. Notably, dimethylamine- and carbonyl-substituted derivatives exhibited relatively high fluorescence quantum yields (Φ_fl) in both solid and mesophase states, demonstrating their potential as efficient fluorescent materials. Our findings underscore the versatility of BTD-based mesogenic skeletons for designing room-temperature fluorescent nematic LCs with various functional groups. These materials offer promising opportunities for next-generation display technologies, optical sensors, and photonic applications. Full article

In this study, we employed density functional theory (DFT), a standard method in quantum chemistry, to investigate the structural intricacies of thioether-linked naphthalene-based liquid-crystal dimers. The theoretical analysis included the calculation of the molecular bend angle, a crucial factor influencing the formation of the twist–bend nematic (N_TB) phase, as well as other molecular parameters such as transition dipole moments, bond lengths, and bond energies. These calculations allowed for the determination of the probable conformations and the computation of their vibrational spectra, which are essential for interpreting experimental spectra. Connecting these insights, we identified stable conformations and observed differences in the spectra between the conventional nematic (N) and N_TB phases. The combined DFT calculations and infrared absorbance measurements allowed us to investigate the structure and intermolecular interactions of molecules in the N and N_TB phases of the dimers. Notably, significant changes in average absorbance were detected in the experimental spectra in the N_TB phase. During the transition from the N phase to the N_TB phase, a clear decrease in absorbance for longitudinal dipoles and an increase for transverse dipoles were observed. This phenomenon suggests that longitudinal dipoles are antiparallel, while transverse dipoles are parallel. To verify the influence of nearest-neighbor interactions, DFT calculations were conducted on a system comprising several neighboring molecules. Full article

This report discusses the impact of nanoparticles on glass-forming systems composed of a liquid crystalline (LC) mixture E7 and paraelectric BaTiO₃ particles ($d\approx 50 \mathrm{n}\mathrm{m}$, globular), tested via broadband dielectric spectroscopy. In the isotropic phase, critical changes in the dielectric constant are shown. They are related to the weakly discontinuous nature of the isotropic–nematic transition. In the nematic phase, two primary relaxation times/processes and DC electric conductivity are considered, down to the glass temperature $T_g$. The prevalence of portrayals via the ‘double exponential’ MYEGA equation and the critical & activated Drozd-Rzoska relation for dynamic properties are shown. For the primary loss curve, critical-like changes of its maximum (peak) are evidenced: ${\epsilon }_{peak}^{″}\propto 1/\left(T-T_g^{*}\right)$ for $T_g<T<\left(T_g+25 \mathrm{K}\right)$, where $T_g^{*}<T_g$ denotes the extrapolated singular temperature. Dielectric constant monitoring revealed the permanent arrangement of rod-like LC molecules by nanoparticles’ endogenic impact in the nematic phase. The heuristic model regarding this unique behavior is presented. It considers a hypothetical link between the glass transition and a hidden near-critical discontinuous phase transition, uniquely avoiding a symmetry change. The uniaxiality of LC molecules enables the detection of critical-like features when approaching the glass transition, hypothetically associated with a specific ‘amorphous’ phase transition. Full article

This study investigates the interaction between thermochromic liquid crystal (TLC) inks and bacterial nanocellulose (BNC), emphasizing their compatibility for smart packaging applications. While the chiral nematic structure of TLC pigments dictates their dynamic colour changing behaviour, this research focuses on how TLC inks interact with BNC, a biodegradable and eco-friendly substrate. This study examines material compatibility, colorimetric properties, and the influence of substrate characteristics on the thermally induced colour transitions of TLC inks. Screen printing was employed to deposit TLC inks onto BNC-based films and black uncoated paper, followed by spectrometric analysis to evaluate the temperature-dependent colour response. The results indicate that BNC serves as a promising platform for TLC ink integration, enhancing its potential for intelligent packaging and indicator systems. These findings contribute to the advancement of sustainable, responsive materials for next-generation smart packaging solutions. Full article

This study investigates the liquid crystalline behavior, photophysical properties, and metal ion sensing capabilities of a pyrene-based imine dimer (DPyH9). The compound exhibits monotropic nematic mesophase behavior, with a glass transition at 43 °C, as confirmed by polarized light microscopy (PLM) and differential scanning calorimetry (DSC). Its photophysical properties, including UV-vis absorption, solvatochromic fluorescence, and acidochromism, observed through spectral shifts upon HCl addition, were systematically analyzed. Notably, DPyH9 displayed selective metal ion sensing capabilities towards Sn²⁺ and Cu²⁺ with binding constants of 4.51 × 10⁶ M⁻¹ and 4.03 × 10⁷ M⁻¹ and detection limits of 1.61 × 10⁻⁵ M (Sn²⁺) and 4.73 × 10⁻⁵ M (Cu²⁺). Fluorescence titrations revealed distinct responses: Sn²⁺ induced an initial quenching and an enhancement at higher concentrations, while Cu²⁺ caused significant fluorescence quenching. These results therefore highlight DPyH9 as a potential candidate for sensing applications and optoelectronic devices. Full article

Focal conics are nice geometric structures of liquid crystal phases which present periodicity such as smectic phase, cholesteric phase, etc. Here, we focus on focal conic domains (FCD) in smectic A liquid crystal. During a phase transition from smectic A to nematic, these FCDs evolve as the eccentricity increases until they completely disappear. Here, we present experimental observations of this phenomenon, along with a modeling approach that allows for an interestingly precise description of the phenomenon, revealing, in particular, a coefficient that seems to exhibit the behavior of a critical exponent. Full article

We report the first homologous series of methylene-linked cyanobiphenyl- and cyanoterphenyl-based liquid crystal (LC) dimers (CBnCT). To induce the heliconical twist-bend nematic (N_TB) phase through bent molecular shapes, the CBnCT homologs have an odd-numbered flexible alkylene spacer (n) ranging from 1 to 17. Polarized optical microscopy and differential scanning calorimetry are used to identify phases and analyze the phase-transition behavior. Except for n = 1, all the CBnCT homologs exhibit the conventional nematic (N) and N_TB phases. The CBnCT dimers with n = 3 and 5 show a monotropic N_TB phase, while those with n = 7, 9, 11, 13, 15, and 17 demonstrate an enantiotropic N_TB phase below the conventional N phase temperature. The N_TB phases of the CBnCT dimers (n = 7, 9, and 11) remain stable down to room temperature and vitrify without crystallization. Compared with cyanobiphenyl-based LC dimer homologs (CBnCB), the CBnCT dimers show significantly broader N and N_TB phase temperature ranges with higher isotropic and N_TB–N phase-transition temperatures. The N_TB phase temperature ranges of CBnCT (n = 7, 9, 11, and 13) are over 100 °C. Additionally, more CBnCT homologs exhibit the enantiotropic N_TB phase than the CBnCB ones. These enhancements result from increased π-conjugation and asymmetric molecular structures. Furthermore, CB9CT exhibits higher birefringence than CB9CB owing to its longer π-conjugated terphenyl moiety. Full article

The switching properties of nematic liquid crystals under electrical and mechanical stresses play a fundamental role in the design and fabrication of electro-optical devices. Depending on the stress applied to a nematic texture confined in a pi-cell, different nematic configurations are allowed inside the cell, while the induced distortion is relaxed by means of growing biaxial domains which can end with the order reconstruction phenomenon, a transition connecting two topologically different nematic textures which can occur in different regions of the pi-cell. Due to the different space and time scales involved, modelling in the frame of the Landau–de Gennes order tensor theory is mandatory to correctly describe the fast-switching mechanisms involved, while from a computational point of view, sophisticated numerical techniques are required to grasp tiny and fast features which can be predicted by the mathematical modelling. In this paper, we review the results obtained from the mathematical and numerical modelling of a 5CB liquid crystal confined in a pi-cell performed by using a numerical technique based on the equidistribution principle, tailored for the description of a complex physical system in which fast switching phenomena are coupled with strong distortions. After a recap on the underneath theory and on the numerical method, we focus on the switching properties of the nematic material when subjected to variable mechanical and electrical stresses in both symmetric and asymmetric conditions. Full article