Liquid Crystal Phase Transition Research Articles

Phase Transition of Mobile Potts Model for Liquid Crystals

Phase transitions in liquid crystals have been intensively studied during the last 40 years due to their numerous daily technological applications. We study by Monte-Carlo simulations the phase transitions in liquid crystals as function of molecule concentration and temperature. The molecule are represented by a 6-state Potts model on a simple cubic lattice, where each crystalline direction is assigned to a Potts value. Molecules can go from a lattice site to another one. We take into account an attractive interaction between nearest neighboring molecules. At low temperature, molecules form an ordered solid phase. As the temperature increases, molecules at the surface are detached from the solid to go to the empty space. Depending on the ratio of the solid phase volume to the whole volume, namely the concentration, we can have first order transition or second order transition and even, for the dilute case, no transition at all. The physical mechanism of each kind of transitions are analyzed.

Isotropic-to-nematic phase transition of liquid crystals confined in nanoemulsion droplets

We fabricated liquid crystalline nanoemulsions (LCNEs) by introducing low molecular weight liquid crystals (LMWLCs) into the core of nanoemulsions, and investigated the phase transition behavior of LMWLCs in the core part with the various weight ratios of LMWLCs to surfactants. The polarized dynamic light scattering measurement was performed to estimate the radii of LCNEs, and it is found that their radii can be controlled by the weight ratio of LMLCs to surfactant polymers. In the depolarized light scattering, it was revealed that the order of the isotropic-nematic phase transition behavior changes from the first order to biased second order with decreasing radius of LCNEs because of the three-dimensional confinement effect surrounded by an anchoring surface.

Near infrared light-driven liquid crystal phase transition enabled by hydrophobic mesogen grafted plasmonic gold nanorods.

Light-driven phase transition in liquid crystals is a fascinating endeavour from both scientific and technological points of view. Here we demonstrate the proof-of-principle that the photothermal effect of organo-soluble plasmonic gold nanorods can introduce the phase transition of thermotropic liquid crystals upon near infrared laser irradiation. Interestingly, the reverse process occurs when the laser is switched off.

Sequential phase transformation of propeller-like C3-symmetric liquid crystals from a helical to ordered to disordered hexagonal columnar structure.

In this paper, we report thermally induced intercolumnar phase transitions of C3-symmetric liquid crystals (LCs) bearing a triazole-based propeller-like aromatic mesogen. Since the constituting aromatic rings are conjugated through rotatable single bonds, the mesogenic shape is tuneable depending on the degree of conformational motion. Molecule 1 with ninefold octyl peripheries shows a hexagonal columnar liquid crystalline phase transition from ordered mesogenic stacking to disordered mesogenic stacking upon heating. On the other hand, molecule 2 with sixfold octyl peripheries displays a helical hexagonal columnar phase with the P6/mmm space group at ambient temperature as well as the ordered and disordered hexagonal columnar phases at higher temperatures. The intracolumnar helical order can be understood by an interdigitated stacking of the propeller-like mesogens along the columnar axis and the optimized space-filling. Notably, all the intercolumnar phase transformations in this study are revealed as second-order transitions. The thermodynamic nature agrees well with the fact that the conformational motions of the C3-symmetric aromatic mesogen change abruptly with each columnar transition.

Optically binary liquid crystalline blue phases induced by one-armed cholesterol-linked azobenzene molecules.

A series of one-armed cholesterol-linked azobenzene molecules named CholXAzo with different spacers were synthesized, in which Chol6Azo was found to have induced blue phases (BPs) with a concentration of 4.0 wt%. Under irradiation of 385 nm UV light with a density of 15.0 mW cm(-2), photo-responsive behaviour of the 4.0 wt% Chol6Azo doped sample named B3 shows a sensitive temperature dependence, which means that at 38.0 °C a phase transition from BPs to the isotropic phase is induced; however, at 33.0 °C, this phase transition does not take place. Results from the research show that the optically binary phase transition behaviour of B3 is sensitive to the isomerization degree of Chol6Azo, which is closely related to the stability of the BP structure and there is a critical isomerization degree of 13.7% for the phase transition of the B3 liquid crystals. Further POM observation shows that the liquid crystal samples doped with different concentrations of Chol6Azo have an increasing transition temperature for photo-induced phase transition from the BP to the isotropic phase along with the increasing concentration of Chol6Azo, which are found to have the same changing tendency with phase transition temperature from the isotropic phase to BPs and a phase diagram is made to map the optically binary behaviour of Chol6Azo doped blue phase liquid crystals. At last, a simple pattern with the BP and the isotropic phase arranged at an interval was made in this optically binary liquid crystalline blue phase under a suitable photomask.

Relaxation dynamics of liposomes in an aqueous solution.

The gel-liquid crystal phase transition has been studied by the temperature and frequency dependent dielectric relaxation behavior of liposomes in an aqueous solution (40 g L(-1) DPPC-water mixture). Four relaxation processes were observed in the frequency range from 40 Hz to 30 GHz which were ascribed to different molecular mechanisms, related to the structural units of the system. The gel-liquid crystal phase transition was also described very accurately from the temperature-dependent dielectric relaxation strength, relaxation time and symmetric shape parameter of the relaxation functions obtained from the fitting procedure. Relaxation process 3, obtained from the dielectric fitting procedure, was confirmed by dielectric modulus analysis. A comparison of the lipid membrane with non-biological systems like liquid crystals was performed. It was determined that the lipid membrane has a ferroelectric liquid crystal like behavior. Process 3 is comparable to the soft mode relaxation process observed in ferroelectric liquid crystals which was detected close to the smectic-C*-smectic-A phase transition. Differential scanning calorimetry was also used to confirm the gel-liquid crystal phase transition of this mixture.

Crossover from a Kosterlitz-Thouless phase transition to a discontinuous phase transition in two-dimensional liquid crystals.

Liquid crystals in two dimensions do not support long-range nematic order, but a quasinematic phase where the orientational correlations decay algebraically is possible. The transition from the isotropic to the quasinematic phase can be continuous and of the Kosterlitz-Thouless type, or it can be first order. We report here on a liquid-crystal model where the nature of the isotropic to quasinematic transition can be tuned via a single parameter p in the pair potential. For p<p(t), the transition is of the Kosterlitz-Thouless type, while for p>p(t), it is first order. Precisely at p=p(t), there is a tricritical point where, in addition to the orientational correlations, also the positional correlations decay algebraically. The tricritical behavior is analyzed in detail, including an accurate estimate of p(t). The results follow from extensive Monte Carlo simulations combined with a finite-size scaling analysis. Paramount in the analysis is a scheme to facilitate the extrapolation of simulation data in parameters that are not necessarily field variables (in this case, the parameter p), the details of which are also provided. This scheme provides a simple and powerful alternative for situations where standard histogram reweighting cannot be applied.

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy.

The isotropic phase of nematogenic liquid crystals has nanometer length scale domains with pseudonematic ordering. As the isotropic to nematic phase transition temperature (TNI) is approached from above, the orientational correlation length, ξ, of the pseudonematic domains grows as (T - T(*))(-1/2), where T(*) is 0.5-1 K below TNI. The orientational relaxation, which is a collective property of the pseudonematic domains, was measured with optical heterodyne detected-optical Kerr effect (OHD-OKE). The orientational relaxation obeys Landau-de Gennes theory, as has been shown previously. To examine the environmental evolution experienced by molecules in the pseudonematic domains, two-dimensional infrared (2D IR) vibrational echo experiments on the CN stretching mode of the non-perturbative vibrational probes 4-pentyl-4(')-selenocyanobiphenyl (5SeCB) and 4-pentyl-4(')-thiocyanobiphenyl (5SCB) in the nematogen 4-cyano-4(')-pentylbiphenyl (5CB) were performed. The 2D IR experiments measure spectral diffusion, which is caused by structural fluctuations that couple to the CN vibrational frequency. Temperature dependent studies were performed just above TNI, where the correlation length of pseudonematic domains is large and changing rapidly with temperature. These studies were compared to 2D IR experiments on 4-pentylbiphenyl (5B), a non-mesogenic liquid that is very similar in structure to 5CB. The time constants of spectral diffusion in 5CB and 5B are practically identical at temperatures ≥5 K above TNI. As the temperature is lowered, spectral diffusion in 5B slows gradually. However, the time constants for spectral diffusion in 5CB slow dramatically and diverge as T(*) is approached. This divergence has temperature dependence proportional to (T - T(*))(-1/2), precisely the same as seen for the correlation length of pseudonematic domains, but different from the observed orientational relaxation times, which are given by the Landau-de Gennes theory. The data and previous results show that spectral diffusion in 5CB has no contributions from orientational relaxation, and the structural dynamics responsible for the spectral diffusion are likely a result of density fluctuations. The results suggest that the correlation length of the density fluctuations is diverging with the same temperature dependence as the pseudonematic domain correlation length, ξ. The isotropic-nematic phase transition in liquid crystals is described in the context of the slowing of orientational relaxation associated with divergent growth of the orientational correlation length. The results presented here show that there is another divergent dynamical process, likely associated with density fluctuations.

Isotropic–nematic phase transition in aqueous sepiolite suspensions

Aqueous suspensions of sepiolite clay rods in water tend to form gels on increase of concentration. Here it is shown how addition of a small amount (0.1% of the clay mass) of a common stabiliser for clay suspensions, sodium polyacrylate, can allow the observation of an isotropic–nematic liquid crystal phase transition. This transition was found to move to higher clay concentrations upon adding NaCl, with samples containing 10-3M salt or above only displaying a gel phase. Even samples that initially formed liquid crystals had a tendency to form gels after several weeks, possibly due to Mg2+ ions leaching from the clay mineral.

A new approach to study interaction parameters in cyanobiphenyl liquid crystal binary systems

The phase transition of heptylcyanobiphenyl 7CB and pentylcyanobiphenyl 5CB liquid crystals was investigated using the differential scanning calorimetry DSC technique. Then, the phase transition of different compositions of 7CB/5CB binary mixture was studied to determine the eutectic point. The phase diagram of mentioned binary system in 7CB mole fraction of 0.45 at T=273.45K is in good agreement with that of predicted from Schroder–van Laar equation. The thermodynamic excess functions and interaction parameters were calculated to describe the phase transition physically using the non-random mixing for the first time. The P∗ randomicity parameter was used to describe the phase transitions of C–N and N–I in which a small amount of P∗ shows a non-random identity of C–N phase transition. Contrarily, the P∗ is greater in N–I phase transition showing a random mixing process.

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.

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.

Photopyroelectric Calorimetry for the Thermal and Optical Study Over Phase Transitions

The capabilities of an upgraded photopyroelectric calorimetric setup to perform simultaneous evaluations of thermal and optical parameters for a more comprehensive evaluation of several phase transition studies are reported. It has been applied to the study of the nematic–isotropic phase transition of an 8CB liquid crystal hosted in a network of silica nanoparticles, of the sol–gel transition in hydrated parchment fibers and of the nematic–isotropic and the smecticA–nematic transitions of 8CB liquid crystal with inclusions of photochromic molecules.

Phase Transitions in Liquid Crystal Doped with Magnetic Particles of Different Shapes in Combined Electric and Magnetic Fields

We have studied the influence of electric and magnetic fields on the orientational structure of ferronematics based on a thermotropic nematic 4-trans-4\(^{\prime }\)-n-hexylcyclohexyl-isothiocyanato-benzene (6CHBT). The 6CHBT liquid crystal has been dissolved in phenyl isothiocyanate and doped with rod-like or chain-like magnetic particles. In such a mixture, the phase transition from an isotropic to a nematic phase is via a droplet state, i.e., coexistence of nematic and isotropic phases. The obtained results showed that a combination of the electric and magnetic fields can change the character of a phase transition from the isotropic to the nematic phase via the droplet state in such systems. Moreover, magneto-dielectric measurements of structural transitions showed the magnetic field induced a shift of the phase transition temperature from the isotropic to the droplet state.

Ferrofluid to ferronematic phase transition in lyotropic liquid crystals

We present a mean-field analysis of the phase transitions of ferronematic in lyotropic liquid crystals. The model free energy is written in terms of the orientational order parameter, magnetization and micellar concentration. We find four different phases: (i) isotropic ferrofluid, (ii) isotropic micellar ferrofluid, (iii) lyotropic paranematic and (iv) lyotropic ferronematic. We put special emphasis on the phase transition from isotropic micellar ferrofluid to ferronematic phase. It was observed from the theoretical calculations that the transition from the isotropic micellar ferrofluid to lyotropic ferronematic phase is weakly first order. The temperature and concentration dependence of the magnetic birefringence and Cotton–Mouton constant are calculated.

On the size dependences of the crystal-liquid phase transition parameters

A comparatively simple method is proposed for calculating the size dependences of the latent heat, volume jumps, and the interfacial surface energy for the crystal-liquid phase transition proceeding from the results of experiments (real or computer) on determining size dependences of the melting point and the crystallization temperature. The method is tested on the basis of computer simulation data for copper, gold, aluminum, and nickel nanoparticles and experimental data for tin.

A luminescent liquid crystal with multistimuli tunable emission colors based on different molecular packing structures

A novel luminescent liquid crystal, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile) (PDPA), is reported, which shows green, yellow and orange colors depending on the self-assembled structure. The luminescence of PDPA can vary between those three different colors invoked by mechanical shearing and thermal annealing. Moreover, an excellent liquid crystal phase transition of PDPA is observed using polarized optical microscopy (POM) and differential scanning calorimetry (DSC) analysis. Finally, a prototype device is fabricated that demonstrates rewritable fluorescence optical recording with multistimuli luminescence tuning.

Disulfide‐centered star‐shaped polypeptide‐PEO block copolymers for reduction‐triggered drug release

ABSTRACTDisulfide‐centered star‐shaped poly(ε‐benzyloxycarbonyl‐l‐lysine)‐b‐poly(ethylene oxide) block copolymers (i.e., A2B4 type Cy‐PZlys‐b‐PEO) were synthesized by the combination of ring‐opening polymerization and thiol‐yne chemistry. Their molecular structures and physical properties were characterized in detail by FTIR, 1H NMR, gel permeation chromatography, differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized optical microscope. Despite mainly exhibiting an α‐helix conformation, the inner PZlys blocks within copolymers greatly prohibited the crystallinity of the outer PEO blocks and presented a liquid crystal phase transition behavior in solid state. These block copolymers Cy‐PZlys‐b‐PEO self‐assembled into nearly spherical micelles in aqueous solution, which had a hydrophobic disulfide‐centered PZlys core surrounded by a hydrophilic PEO corona. As monitored by means of DLS and TEM, these micelles were progressively reduced to smaller micelles in 10 mM 1,4‐dithiothreitol at 37 °C and finally became ones with a half size, demonstrating a reduction‐sensitivity. Despite a good drug‐loading property, the DOX‐loaded micelles of Cy‐PZlys‐b‐PEO exhibited a reduction‐triggered drug release profile with an improved burst‐release behavior compared with the linear counterpart. Importantly, this work provides a versatile strategy for the synthesis of the disulfide‐centered star‐shaped polypeptide block copolymers potential for intracellular glutathione‐triggered drug delivery systems. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2000–2010

Chiral smectic transition phases appearing near the electric-field-induced phase transition observed by resonant microbeam x-ray scattering.

The electric-field-induced phase transition of a chiral liquid crystal containing Br revealed a transition phase between the three-layer periodicity ferrielectric phase and the synclinic ferroelectric phase in the electric field versus temperature phase diagram. Resonant x-ray scattering from the transition phase showed a diffuse streak or spotty weak reflections, which were composed of strong m/3-order (where m = 1 and 2) reflections and other weak peaks. The spotty reflections were found to be related to a 12-layer periodicity phase with a weak contribution from the 15-layer periodicity. An x-ray intensity analysis based on the Ising model suggested that the 12-layer periodicity phase was composed of two three-layer ferrielectric blocks and six synclinic layers. This model indicated that, in the transition phase, the three-layer ferrielectric molecular configuration gradually changed to the synclinic configuration. The diffuse streak appearing around m/3-order reflections near the field-induced transition from the four-layer periodicity phase to the synclinic ferroelectric phase is also discussed.

Molecular Theory of Phase Separation in Nematic Liquid Crystals Doped with Spherical Nanoparticles

A molecular-statistical theory is developed, which enables one to describe the nematic-isotropic phase transition in liquid crystals doped with spherical nanoparticles taking into account the effects of phase separation. It has been shown that in the case of strong interaction between nanoparticles and mesogenic molecules the nematic nanocomposite possesses a number of unexpected properties. In particular, the nematic-isotropic co-existence region appears to be very broad, and the system either undergoes a direct transition from the isotropic phase into the phase-separated state, or undergoes the transition into the homogeneous nematic phase first and then phase-separates at a lower temperature. Phase separation does not occur at all if the concentration of the nanoparticles is sufficiently low, and in some cases it takes place only within a finite region of nanoparticle concentration. A number of temperature-concentration phase diagrams is presented and the molar fractions of nanoparticles in the co-existing isotropic and nematic phases are calculated numerically as functions of temperature.