Molecular-statistical theory of elasticity in nematic liquid crystals composed of polar and nonpolar molecules.

A molecular-statistical theory of the orientational elasticity of nematic liquid crystals has been developed employing the orientational deformation tensor which describes the rotation of the director. An explicit expression for the general elasticity tensor of the nematic phase has been obtained and the Frank elastic constants are expressed in terms of the three independent parameters of this tensor. Explicit expressions for the Frank elastic constants have been derived in the molecular field approximation in terms of the orientational order parameters and the corresponding coefficients of expansion of the intermolecular potential in spherical invariants. Frank elastic constants have been calculated numerically for nematic liquid crystals composed of both polar and nonpolar molecules together with the orientational order parameters using the classical Gay-Berne model interaction potential and the two of its popular modifications. The polarity of the uniaxial molecular shape has been directly introduced into the model potential by modifying the distance of closest approach. The elastic constants are presented as functions of temperature for different values of the molecular elongation, the anisotropy of the potential well and the molecular shape polarity. It has been shown that the elastic constants are much more sensitive to the details of the intermolecular interaction potential in comparison with the orientational order parameters. In particular, a relatively weak polarity of the molecular shape may result in an unusual decrease of the splay constant K_{11} which may vanish at some temperature leading to the instability of the homogeneous nematic phase. This may represent a mechanism of the formation of the splay-bend phase.

Molecular theory of the tilting transition and computer simulations of the tilted lamellar phase of rod-coil diblock copolymers.

Symmetric rod-coil diblock copolymers have been simulated using the method of dissipative particle dynamics in the broad range of the Flory-Huggins parameter. It has been found that the tilted lamellar phase appears to be the most stable one at strong segregation. The rod-coil copolymer tilt angle and orientational order parameters have been determined as functions of the segregation strength. The density functional theory of rod-coil diblock copolymers has been generalized to the case of the tilted lamellar phase and used to study the stability of the orthogonal lamellar phase with respect to tilt. The orthogonal phase indeed appears to be unstable in the broad region of the parameter space in the case of relatively strong segregation. It has also been shown that the transition into the tilted lamellar phase is determined by a strong coupling between two independent tilt order parameters.

Molecular theory of liquid-crystal ordering in rod-coil diblock copolymers.

Molecular-statistical theory of rod-coil diblock copolymers is proposed using the general density functional approach which enables one to consider the cases of both weak and strong segregation. The free energy of the system is expressed as a functional of the phase-space densities of rod and coil monomers, which depend on the orientational and translational order parameters. Temperature-concentration phase diagrams are obtained and the profiles of all order parameters are calculated numerically by minimizing the polymer free energy. The lamellar phase is shown to possess strong orientational order which is partially induced by the phase structural anisotropy. The enhanced stability of the lamellar phase is determined by a combination of the microphase separation effects and the emergence of long-range smectic order.

Phase behavior and orientational ordering in block copolymers doped with anisotropic nanoparticles.

A molecular field theory and coarse-grained computer simulations with dissipative particle dynamics have been used to study the spontaneous orientational ordering of anisotropic nanoparticles in the lamellar and hexagonal phases of diblock copolymers and the effect of nanoparticles on the phase behavior of these systems. Both the molecular theory and computer simulations indicate that strongly anisotropic nanoparticles are ordered orientationally mainly in the boundary region between the domains and the nematic order parameter possesses opposite signs in adjacent domains. The orientational order is induced by the boundary and by the interaction between nanoparticles and the monomer units in different domains. In simulations, sufficiently long and strongly selective nanoparticles are ordered also inside the domains. The nematic order parameter and local concentration profiles of nanoparticles have been calculated numerically using the model of a nanoparticle with two interaction centers and also determined using the results of computer simulations. A number of phase diagrams have been obtained which illustrate the effect of nanoparticle selectivity and molar fraction of the stability ranges of various phases. Different morphologies have been identified by analyzing the static structure factor and a phase diagram has been constructed in coordinates' nanoparticle concentration-copolymer composition. Orientational ordering of even a small fraction of nanoparticles may result in a significant increase of the dielectric anisotropy of a polymer nanocomposite, which is important for various applications.

Ordering of ferromagnetic nanoparticles in nematic liquid crystals.

Dispersions of magnetic nanoparticles in a nematic liquid crystal were investigated as magnetic fields were applied in three different boundary condition geometries: (i) planar substrates and B⊥n, (ii) planar substrates and B‖n, and (iii) homeotropic substrates and B⊥n. Particle chaining is observed when a magnetic field is applied, with a periodicity perpendicular to the chains. Furthermore, linear chains are observed for the magnetic field applied perpendicular to the director, while zigzag chains are formed when the magnetic field direction is parallel to the director field. This is attributed to a change from a dipolar defect configuration around dispersed nanoparticles, to a quadrupolar one, i.e. the change from satellite to Saturn-ring defects. This effect is largely independent of the sample thickness. The dynamic development of the chain length, as well as their two-dimensional order parameter was studied in all cases. Chain lengths increased rapidly until saturation at approximately l = 30 µm after a time of about t = 10 s. Similarly, the chain order parameters increased until saturation between S = 0.8-0.9, independent of sample geometry.

Effect of polar intermolecular interactions on the elastic constants of bent-core nematics and the origin of the twist-bend phase.

A molecular theory of both elastic constants and the flexoelectric coefficients of bent-core nematic liquid crystals has been developed taking into account dipole-dipole interactions as well as polar interactions determined by the bent molecular shape. It has been shown that if polar interactions are neglected, the elastic constants are increasing monotonically with the decreasing temperature. On the other hand, dipolar interactions between bent-core molecules may result in a dramatic increase of the bend flexocoefficient. As a result, the flexoelectric contribution to the bend elastic constant increases significantly, and the bend elastic constant appears to be very small throughout the nematic range and may vanish at a certain temperature. This temperature may then be identified as a temperature of the elastic instability of the bent-core nematic phase which induces a transition into the modulated phases with bend deformations like recently reported twist-bend phase. The temperature variation of the elastic constants is qualitatively similar to the typical experimental data for bent-core nematics.

[Melatonin enhances antitumor effect of doxorubicin on a model of transplantable Ehrlich tumor in female sHR mice].

A combined antitumor effect of a single injection of doxorubicin and the indole hormone melatonin was investigated in the model of transplantable Ehrlich carcinoma in female SHR mice. Animals received melatonin either subcutaneous 10 mg/ kg in the evening or with water 10 mg/l at night (from 20 pm to 8 am) for 3 weeks. The results of an experiment showed that combined use of doxorubicin and melatonin both subcutaneous and with water leads to statistically significant inhibition of tumor growth compared with control and doxorubicin alone.

[Prospects of use of antidiabetic biguanides for cancer prevention and treatment: results of preclinical studies].

The critical analysis of preclinical testing of anticarcinogenic and antitumor activity of biguanides presented in this paper. Experiments have been conducted using in total more than 20 models of carcinogenesis including models of spontaneous , chemically- , radiation- and virus-induced carcinogenesis, as well as carcinoigenesis induced by special fat diets and by genetic modification in rodents. Cancer preventive effect of buiguanides has been studied in relation to total tumor incidence and to 17 target organs in animals of 3 species, including 25 various strains of mice, 4 strains of rats and 1 strain of hamsters using various routs of administration and doses. In the majority of cases (86%) the exposure to biguanides leads to inhibition of carcinogenesis. In 14% of the cases inhibitory effect of the drugs was not observed, however there was no any case of stimulation of carcinogenesis by antidiabetic biguanides., Metformin suppressed tumor growth in the majority of in vitro studies conducted in 46 different cell lines originated from malignant tumors of 15 localization as well as in athymic mice with xenografts of 31 tumor lines. It was concluded that there are sufficient experimental evidences of anticarcinogenic and antitumor effects of antidiabetic biguanides revealed in a number of models of induced and spontaneous carcinogenesis.

Effect of nanoparticle chain formation on dielectric anisotropy of nematic composites.

A general theory of the dielectric constant of nematic liquid crystal mixtures is presented including the particular case of nematics doped with polar nanoparticles. The results are used to estimate the contribution of chains of polar nanoparticles to the static dielectric anisotropy and birefringence of the nematic composite taking into account contributions from chains of different lengths. The dependence of the dielectric anisotropy on the dipolar interaction strength is considered in detail and it is shown that formation of polar chains of nanoparticles enables one to explain a significant increase of the dielectric constant of the composite as observed experimentally.

Molecular theory of proper ferroelectricity in bent-core liquid crystals.

Antiferro- and ferro-electric ordering has been discovered in orthogonal smectic phases composed of nonchiral bent-core molecules. These systems are the only proper fluid ferroelectrics confirmed experimentally so far. We consider a molecular theory of proper ferroelectric ordering in isotropic, nematic and smectic A phases and conclude that the delicate balance between the tendencies for local parallel and antiparallel ordering of molecular electric and steric dipoles is strongly shifted in restricted geometries. This is a reason why dipolar ordering is more likely to occur within a smectic layer. We derive model interaction potentials for polar bent-core molecules and present the results of the mean-field theory of ferroelectric ordering in the orthogonal smectic phase taking into account also the molecular biaxiality. Order parameter profiles have been calculated numerically and phase diagrams are presented which enable one to analyze the relative importance of dipole-dipole interaction and intermolecular attraction modulated by polar bent-core molecular shape.

Unified molecular field theory of nematic, smectic-A, and smectic-C phases.

A unified mean-field molecular theory of nematic (N(U)), smectic A (SmA), and smectic C (SmC) liquid crystal phases, composed of uniaxial nonpolar molecules, is developed taking into account the variation of all orientational and translational order parameters in these phases. Numerical results, obtained by direct global minimization of the free energy, are presented in the form of three typical phase diagrams of different topology. Temperature variation of the relevant order parameters in different sequences of phases is analyzed for various cross sections of the phase diagrams. The present model enables one to reproduce all possible sequences of phase transitions between the given phases including isotropic (Iso)-N(U)-SmA-SmC, Iso-N(U)-SmC, Iso-SmA-SmC, and Iso-SmC. The properties of the NAC point, where the N(U), SmA, and SmC structures coexist, are considered in detail and the shape of the phase diagram in the vicinity of the NAC point is compared with existing experimental data.

[Markers of effectiveness of preoperative taxane-based chemotherapy for locally advanced breast cancer].

The absolute sensitivity signs of breast cancer to the drug have not yet been developed. Data from clinical trials on the study of experimental laboratory predictive markers of chemosensitivity: TOP2alpha (topoisomerase 2-alpha), beta-tubulin (subunit of dimeric protein tubulin), and BRCA1 (breast cancer 1) are contradictory and not numerous. Analysis of the results by the end of the clinical trial will allow examining the correlation between the effectiveness of preoperative taxane-chemotherapy and the level of experimental and standard molecular markets that is important for development of algorithm of treatment tactics for patients with locally advanced breast cancer.

Orientational order parameters of a de Vries-type ferroelectric liquid crystal obtained by polarized Raman spectroscopy and x-ray diffraction.

The orientational order parameters (P{2}) and (P{4}) of the ferroelectric, de Vries-type liquid crystal 9HL have been determined in the SmA and SmC phases by means of polarized Raman spectroscopy, and in the SmA phase using x-ray diffraction. Quantum density functional theory predicts Raman spectra for 9HL that are in good agreement with the observations and indicates that the strong Raman band probed in the experiment corresponds to the uniaxial, coupled vibration of the three phenyl rings along the molecular long axis. The magnitudes of the orientational order parameters obtained in the Raman and x-ray experiments differ dramatically from each other, a discrepancy that is resolved by considering that the two techniques probe the orientational distributions of different molecular axes. We have developed a systematic procedure in which we calculate the angle between these axes and rescale the orientational order parameters obtained from x-ray scattering with results that are then in good agreement with the Raman data. At least in the case of 9HL, the results obtained by both techniques support a "sugar loaf" orientational distribution in the SmA phase with no qualitative difference to conventional smectics A. The role of individual molecular fragments in promoting de Vries-type behavior is considered.

Molecular theory of smectic ordering in liquid crystals with nanoscale segregation of different molecular fragments.

A molecular statistical theory of the smectic A phase is developed taking into account specific interactions between different molecular fragments which enables one to describe different microscopic scenario of the transition into the smectic phase. The effects of nanoscale segregation are described using molecular models with different combinations of attractive and repulsive sites. These models have been used to calculate numerically coefficients in the mean filed potential as functions of molecular model parameters and the period of the smectic structure. The same coefficients are calculated also for a conventional smectic with standard Gay-Berne interaction potential which does not promote the segregation. The free energy is minimized numerically to calculate the order parameters of the smectic A phases and to study the nature of the smectic transition in both systems. It has been found that in conventional materials the smectic order can be stabilized only when the orientational order is sufficiently high, In contrast, in materials with nanosegregation the smectic order develops mainly in the form of the orientational-translational wave while the nematic order parameter remains relatively small. Microscopic mechanisms of smectic ordering in both systems are discussed in detail, and the results for smectic order parameters are compared with experimental data for materials of various molecular structure.

Order parameter dependence of the viscosity coefficients of a biaxial nematic liquid crystal.

We derive expressions for the order parameter dependence of the viscosity coefficients of a biaxial nematic liquid crystal by comparing its dissipation function expressed in terms of directors with that expressed in terms of order tensors. The results enable us to identify the dominant flow viscosity coefficients and to compare their temperature variation according to their dependence on the dominant scalar order parameters. By considering different orientations of an external field, we identify three characteristic switching times corresponding to three rotational viscosities, and we estimate the ratio of the switching times of the primary and the secondary directors.

Molecular model of biaxial ordering in nematic liquid crystals composed of flat molecules with four mesogenic groups.

Relative stability of uniaxial and biaxial nematic phases is analyzed in a model nematic liquid crystal composed of flat molecules of C2h symmetry with four mesogenic groups rigidly linked to the same center. The generalized effective quadrupole mean-field potential is proposed and its constants are evaluated numerically for the pair intermolecular potential based on Gay-Berne interaction between mesogenic groups. The dependencies of the constants on molecular shape parameters are systematically analyzed. Order parameters of the uniaxial and biaxial nematic phases are evaluated by direct minimization of the free energy at different temperatures. The corresponding phase diagrams are obtained enabling one to study the effects of molecular model parameters on the stability regions of uniaxial and biaxial phases. The results are used to clarify the nature of experimentally observed biaxial ordering in nematic liquid crystals composed of tetrapode molecules with the same symmetry.

Ferroelectric ordering in chiral smectic-C* liquid crystals determined by nonchiral intermolecular interactions.

General microscopic mechanism of ferroelectric ordering in chiral smectic-C* liquid crystals is considered. It is shown that if the mesogenic molecules have a sufficiently low symmetry, the spontaneous polarization is proportional to one of the biaxial vector order parameters of the smectic-C phase. This order parameter may be determined by intermolecular interactions which are not sensitive to molecular chirality. At the same time, the polarization is also proportional to a pseudoscalar parameter which vanishes if the molecules are nonchiral. The general statistical theory of ferroelectric ordering is illustrated by two particular models. The first model is based on electrostatic quadrupole-quadrupole interactions, and it enables one to obtain explicit analytical expressions for the spontaneous polarization. In the second model, the molecular chirality and polarity are determined by a pair of off-center nonparallel dipoles. For this case, the spontaneous polarization is calculated numerically as a function of temperature. The theory provides a more general interpretation of the previous approaches including the classical Boulder model.

Molecular models for ferroelectric liquid crystals with conventional and anomalously weak layer contraction.

A molecular theory of the ferroelectric smectic C* phase has been developed using the simple model of a chiral molecule composed of a uniaxial core and a pair of off-center nonparallel dipoles which determine molecular chirality and polarity. The interaction between uniaxial cores is modeled by a rather general effective potential which can be used to describe smectic materials with both conventional and anomalously weak layer contraction in the smectic C* phase. Spontaneous polarization, tilt, and layer spacing are calculated numerically as functions of temperature, and it is shown that the variation of the polarization generally deviates from that of the tilt angle. It is shown that this deviation is more pronounced in smectic materials tilting with low layer contraction which corresponds to existing experimental data. The model has been used to reproduce qualitatively the experimental data for polarization, tilt and layer spacing for two similar mixtures exhibiting conventional and anomalously weak layer contraction. The polarization and the tilt are also calculated in the case when the smectic A-smectic C* transition is characterized by the biaxial primary order parameter.

Molecular theory of layer contraction in smectic liquid crystals.

The period of the layered structure in smectic A and smectic C liquid crystal phases has been calculated numerically by direct minimization of the mean-field free energy which takes into account the interaction between molecules in adjacent smectic layers. The smectic layer spacing is calculated for two systems characterized by conventional and anomalously weak layer contraction in the smectic C phase. It is then compared with the simple estimate based on the average projection of the molecular long axis on the smectic layer normal. For both systems, temperature variation of the average molecular projection is qualitatively similar to that of the calculated layer spacing although certain quantitative deviations exist.

Molecular model for de Vries type smectic- A -smectic- C phase transition in liquid crystals.

We develop both phenomenological and molecular-statistical theory of smectic- A -smectic- C phase transition with anomalously weak smectic layer contraction. Using a general mean-field molecular model, we demonstrate that a relatively simple interaction potential suffices to describe the transition both in conventional and de Vries type smectics. The theoretical results are in excellent agreement with experimental data. The approach can be used to describe tilting transitions in other soft matter systems.