聚｛11-[(4′-正庚氧基-4-联苯基)羰基]氧-1-十一炔｝的相结构与相转变

采用示差扫描量热法(DSC)、一维(1D)、二维(2D)广角X-射线衍射(WAXD)和偏光显微镜(PLM)等研究手段对聚{11-[(4′-正庚氧基-4-联苯基)羰基]氧-1-十一炔}(PA-9,7)的本体相转变和相结构进行研究,并采用分子动力学方法对相结构进行模拟.结果表明,样品的相转变为近晶B相(SmB)近晶A相(SmA)各向同性态(Iso).在近晶B相中,侧链在层状结构中排列成具有六次对称性的准长程有序结构。     

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.     

Structure and internal dynamics of a side chain liquid crystalline polymer in various phases by molecular dynamics simulations: a step towards coarse graining

Side chain liquid crystalline polymer with relatively long spacer was modeled on a semiatomistic level and studied in different liquid crystalline phases with the aid of molecular dynamics simulations. Well equilibrated isotropic, polydomain smectic and monodomain smectic phases were studied for their structural and dynamic properties. Particular emphasis was given to the analysis on a coarse-grained level, where backbones, side chains, and mesogens were considered in terms of their equivalent ellipsoids. The authors found that the liquid crystalline phase had a minor influence on the metrics of these objects but affected essentially their translational and orientational order. In the monodomain smectic phase, mesogens, backbones, and side chains are confined spatially. Their diffusion and shape dynamics are frozen along the mesogen director (the one-dimensional solidification) and the reorientation times increase by one to one-and-half orders of magnitude. In this phase, besides obvious orientational order of mesogens and side chains, a stable detectable order of the backbones was also observed. The backbone director is confined in the plane perpendicular to the mesogen director and constantly changes its orientation within this plane. The backbone diffusion in these planes is of the same range as in the polydomain smectic phase at the same temperature. A detailed analysis of the process of field-induced growth of the smectic phase was performed. The study revealed properties of liquid crystalline polymers that may enable their future fully coarse-grained modeling.     

Mesoscale model of polymer melt structure: self-consistent mapping of molecular correlations to coarse-grained potentials

Development and application of coarse-graining methods to condensed phases of macromolecules is an active area of research. Multiscale modeling of polymeric systems using coarse-graining methods presents unique challenges. Here we apply a coarse-graining method that self-consistently maps structural correlations from detailed molecular dynamics (MD) simulations of alkane oligomers onto coarse-grained potentials using a combination of MD and inverse Monte Carlo methods. Once derived, the coarse-grained potentials allow computationally efficient sampling of ensemble of conformations of significantly longer polyethylene chains. Conformational properties derived from coarse-grained simulations are in excellent agreement with experiments. The level of coarse graining provides a control over the balance of computational efficiency and retention of chemical identity of the underlying polymeric system. Challenges to extension and application of this and similar structure-based coarse-graining methods to model dynamics and phase behavior in polymeric systems are briefly discussed.     

Shape dependence in the formation of condensed phases exhibited by disubstituted sucrose esters

We report on the self-organizing properties of sucrose esters that are di-(1',6', 1',6, and 6,6')-substituted with aliphatic chains of identical or different chain lengths and levels of saturation. For the materials possessing two saturated aliphatic chains, the compounds exhibited thermotropic lamellar smectic A phases. A remarkable new phase transition was observed for the di-octadecanoyl homologue in which one smectic A phase transformed into another with a continuous change in layer spacing, but with a discontinuous change in the correlation length. The incorporation of long cis-unsaturated chains led to increased cross-sectional areas of the chains relative to the sucrose head groups and, hence, columnar phases were observed.     

Mesophase formation in a system of top-shaped hard molecules: density functional theory and Monte Carlo simulation

We present the phase diagram of a system of mesogenic top-shaped molecules based on the Parsons-Lee density functional theory and Monte Carlo simulation. The molecules are modeled as a hard spherocylinder with a hard sphere embedded in its center. The stability of five different phases is studied, namely, isotropic, nematic, smectic A, smectic C, and columnar phases. The positionally ordered phases are investigated only for the case of parallel alignment. It is found that the central spherical unit destabilizes the nematic with respect to the isotropic phase, while increasing the length of the cylinder has the opposite effect. Also, the central hard sphere has a strong destabilizing effect on the smectic A phase, due the inefficient packing of the molecules into layers. For large hard sphere units the smectic A phase is completely replaced by a smectic C structure. The columnar phase is first stabilized with increasing diameter of the central unit, but for very large hard sphere units it becomes less stable again. The density functional results are in good agreement with the simulations.     

Conformations produced by interactions of the side chains in poly(silylenemethylene) with the repeating sequence [Si(CH3)R?CH2]x [R = ?O(CH2)3OC6H4C6H5]

A coarse-grained model has been developed for asymmetrically substituted poly(silylenemethylene)s in which the side chain is a flexible spacer terminated by a biphenyl unit. Each monomer unit is represented by four coarse-grained beads that interact via a Lennard–Jones potential and are subject to the first- and second-order interactions deduced from the atomistically detailed model. Metropolis Monte Carlo simulations were performed for isolated syndiotactic, isotactic, and atactic chains. Snapshots from the equilibrated coarse-grained chain on the discrete space of a high coordination lattice were reverse-mapped to atomistically detailed structures in continuous space. At 373 K, the chains were disordered independent of the stereochemical composition. The occupancy of bond pairs depended on the stereochemical composition, with the trans-gauche (tg) sequence being favored by the isotactic chain. When the simulation was performed with the backbone constrained to specific periodic structures, the g helix was the lowest energy structure for either the atactic or isotactic chains. For the syndiotactic chain, the g and gt helices were favored. The appearance of the g helix as the favored periodic structure of the isolated chain was consistent with the chain conformation reported previously for the smectic phase of this polymer in the bulk state. The g helix was disrupted when the backbone was allowed to access nonhelical conformations, even though these conformations may have been slightly higher in energy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 886–896, 2005     

Computer Simulation Studies of Anisotropic Systems XVI. The Smectic E-Smectic B Transition

We have investigated the smectic E-smectic B transition with the aid of a model smectogen whose properties have been calculated using the Monte Carlo technique of computer simulation. The lath-like mesogenic molecules are defined to lie in a plane with their centres on a triangular lattice and with their long axes orthogonal to the smectic layer. The quadrupolar interaction, restricted to nearest neighbours, is assumed to be responsible for the herring-bone arrangement of the molecular short axes, characteristic of the smectic E phase. The computer simulations have been employed to evaluate both thermodynamic and structural parameters as a function of temperature. The model smectogen is found to exhibit a continuous transition at which the long range herring-bone structure of the smectic E phase is destroyed only to be replaced by the analogous short range structure of the smectic B. Where possible the results simulated for the model smectogen are compared with the behaviour of real mesogens and the predictions of molecular field theories for the transition. In particular, contact is made with X-ray diffraction studies of the two phases by using optical techniques to generate the diffraction patterns associated with configurations produced by the simulation. The model is found to be in good accord with experiment but the molecular field prediction of the smectic E-smectic B transition temperature is shown to be poor.     

Atomistic molecular-dynamics simulations of the size and shape of polyethylene in hexane at infinite dilution

Parameters characteristic of size and shape of single polyethylene chains consisting of 15-60 monomer units dissolved in hexane are calculated by use of molecular-dynamics simulations based on a fully atomistic representation of the system. Results are compared with corresponding calculations in vacuum as well as Monte Carlo simulations of coarse-grained chains. The major concern of the study is a careful check of actual limits and possibilities of atomistic simulations of global properties of polymers. As expected such simulations are still restricted to rather small chain lengths but are already large enough to obey the characteristics of polymer coils.     

Influence of bonded-phase coverage in reversed-phase liquid chromatography via molecular simulation I. Effects on chain conformation and interfacial properties

Particle-based Monte Carlo simulations were employed to examine the effects of bonding density on molecular structure in reversed-phase liquid chromatography. Octadecylsilane stationary phases with five different bonding densities (1.6, 2.3, 2.9, 3.5, and 4.2 mumol/m(2)) in contact with a water/methanol (50/50 mol%) mobile phase were simulated at a temperature of 323 K. The simulations indicate that the alkyl chains become more aligned and form a more uniform alkyl layer as coverage is increased. However, this does not imply that the chains are highly ordered (e.g., all-trans conformation or uniform tilt angle), but rather exhibit a broad distribution of conformations and tilt angles at all bonding densities. At lower densities, significant amounts of the silica surface are exposed leading to an enhanced wetting of the stationary phase. At high densities, the solvent is nearly excluded from the bonded phase and persists only near the residual silanols. An enrichment in the methanol concentration and a disruption in the mobile phase's hydrogen bond network are observed at the interface as bonding density is increased.     

Effect of counter-ion on the thermotropic liquid crystal behaviour of bis(alkyl)-tris(imidazolium salt) compounds

Recently, new thermotropic ionic liquid crystals (LCs) with a hexyl-linked tris(imidazolium bromide) core and two terminal alkyl chains were synthesised and characterised. To explore the effect of different counter-ions on the LC behaviour of this system, derivatives with BF₄^? and Tf₂N^? counter-ions were prepared and analysed. Five of the BF₄^? derivatives were found to exhibit thermotropic LC behaviour. The 12-, 14- and 16-carbon tail BF₄^? compounds form SmA phases. The 18- and 20-carbon tail homologues form what appears to be a smectic phase but are weakly mesogenic and harder to characterise. Only two of the Tf₂N^? derivatives exhibited mesogenic behaviour. The 18-carbon tail Tf₂N^? compound forms an as-yet unidentified, highly periodic smectic phase with positional order while the 20-carbon tail homologue forms a periodic SmA phase. The Tf₂N^? mesogens have much lower clearing points even though their LC phases have more order than the Br^? and BF₄^? mesogens. X-ray diffraction showed that these mesogens have different amounts of tail interdigitation between the smectic layers depending on the counter-ion present. Atomistic molecular dynamics simulations indicated that counter-ion size plays an important role in defining the density of the ionic region, which in turn affects the amount of interdigitation in the smectic phases.     

Monte Carlo studies of isomers, structures, and properties in benzene-cyclohexane clusters: computation strategy and application to the dimer and trimer, (C6H6)(C6H12)n, n = 1-2

Low-temperature isomeric energies, structures, and properties of benzene-cyclohexane clusters are investigated via Monte Carlo simulations. The Monte Carlo strategy is first documented and then applied to (C(6)H(6))(C(6)H(12)) and (C(6)H(6))(C(6)H(12))(2) using four different potential energy surfaces. Results identify a single parallel-displaced dimer isomer. MP2 optimizations and frequency calculations support the Monte Carlo dimer structure and identify the van der Waals mode observed in vibronic spectra. Caloric simulations identify two temperatures where structural transitions occur and imply an experimental temperature below 10 K for dimers in cold supersonic expansions. The (C(6)H(6))(C(6)H(12))(2) studies identify eight independent trimer isomers: three form parallel-stacked (sandwich) arrangements with the two cyclohexane moieties related through a D(6)(h) transformation. The remaining five trimer isomers are trigonal, with no overall symmetry. Caloric studies indicate that the sandwich and trigonal isomeric classes coexist independently below 60 K, consistent with trimer vibronic spectra that contain two independent van der Waals progressions.     

Stability of nematic and smectic phases in rod-like mesogens with orientation-dependent attractive interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.     

Quantum Monte Carlo simulations of selected ammonia clusters (n = 2-5): isotope effects on the ground state of typical hydrogen bonded systems

Variational Monte Carlo, diffusion Monte Carlo, and stereographic projection path integral simulations are performed on eight selected species from the (NH(3))(n), (ND(3))(n), (NH(2)D)(n), and (NH(3))(n-1)(ND(3)) clusters. Each monomer is treated as a rigid body with the rotation spaces mapped by the stereographic projection coordinates. We compare the energy obtained from path integral simulations at several low temperatures with those obtained by diffusion Monte Carlo, for two dimers, and we find that at 4 K, the fully deuterated dimer energy is in excellent agreement with the ground state energy of the same. The ground state wavefunction for the (NH(3))(2-5) clusters is predominantly localized in the global minimum of the potential energy. In all simulations of mixed isotopic substitutions, we find that the heavier isotope is almost exclusively the participant in the hydrogen bond.     

Novel bent-shaped liquid crystalline compounds II. Synthesis and properties of the 1,3-bis4-[4-(4-alkyloxybenzoyloxy)benzylidene]aminophenoxypropan-2-ols

A series of new symmetric dimer compounds was synthesized, constaining 2-hydroxy-1,3-dioxypropylene as the central linkage and terminal alkyl chains with different lengths. The chemical structures of the liquid crystal dimers (2ES-n) were examined by FTIR and ¹H NMR spectroscopy. Their mesomorphism, thermodynamic properties and optical textures were investigated by differential scanning calorimetry, polarizing optical microscope and X-ray diffraction. For homologues with terminal propyloxy and butyloxy chains, no liquid crystalline phase was observed. Homologues with pentyloxy and hexyloxy terminal chains showed nematic phases, while those with heptyloxy, octyloxy, nonyloxy and decyloxy terminal chains displayed nematic phases and smectic phases. The results confirmed that the liquid crystalline phase changes from nematic to smectic as the terminal chain length increases.     

Computer simulation of chiral liquid crystal phases. I. The polymorphism of the chiral Gay-Berne fluid

We report the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential. Using Monte Carlo (MC) simulations in the NVT ensemble, the chirality-temperature plane of the phase diagram was studied at different points by a variation of the chirality parameter c describing the strength of the chiral potential. Additionally to the well-known isotropic, nematic and smectic phases of the Gay-Berne fluid, we localized regions of cholesteric phase. For large values of the chirality parameter we also observed blue phases. Furthermore, when starting from a cholesteric phase and decreasing the temperature at constant c, we obtained a phase region showing characteristics of the recently discovered helical smectic A* phase. All phases have been characterized by correlation functions, order parameters, and visual representations of selected configurations. All results of the simulation are limited by the small system size of N = 256 molecules and the use of periodic boundary conditions.     

Dielectric relaxations in bilayer structures of comb-like copolymers: Perfluoralkyl and liquid-crystalline side chains

Abstract : A series of comb-like copolymers and homopolymers containing different liquid-crystalline and perfluoralkyl side chains was studied by frequency and temperature dependent dielectric measurements. The structure and the phase behaviour of the systems were characterized by X-ray scattering and differential scanning calorimetry. The dielectric relaxation spectra of these polymers reflect the molecular mobility in bilayer structures formed by the mesogenic or crystalline side chains. By changing temperature it was possible to investigate the molecular motions in the different phases of the copolymers (smectic-crystalline, smectic-isotropic, and isotropic). The homopolymer containing perfluoralkyl side chains and all copolymers show a β-relaxation at low temperatures, which is assigned to local molecular motions. Above the glass transition temperature, all samples exhibit a segmental (α) relaxation with WLF-like temperature dependence in the activation plots. For the polymers forming LC structures only one relaxation process was detected in all phases, i.e. in the smectic, nematic, and isotropic structure or, in case of the copolymers, in the smectic-crystalline and smectic-isotropic double structures. This process was attributed always to the segmental motions, assisted by tumbling motions of the mesogens. The phase transitions are clearly indicated by discontinuities in the dielectric relaxation times and changes in the relaxation strength Δϵ. The dipole reorientations of the mesogens seem to be more restricted by the crystalline layers in the copolymers as by the smectic order of the LC homopolymers.     

Thermotropic semi-rigid copoly(imide-carbonate)s composed of 3,4,3,4-p-terphenyltetracarboxdi-imide and 3,4,3',4'-biphenyltetracarboxdi-imide rings

New semi-rigid copoly(imide-carbonate)s composed of 3,4,3",4"-p-terphenyltetracarboxdi-imide and 3,4,3',4'-biphenyltetracarboxdi-imide units and neighbouring homologous penta- and hexa-methylene spacer chains were prepared by melt polycondensation; the relationships between polymer structure and liquid crystalline (LC) properties are discussed. Differential scanning calorimetry measurements, polarizing microscope observations, miscibility tests and variable temperature X-ray analyses suggest that the 3,4,3",4"-p-terphenyltetracarboxdi-imide-rich copolymers form thermotropic LC nematic and smectic phases, but the 3,4,3',4'-biphenyltetracarboxdi-imide-rich copolymers are amorphous and have no LC melts. Therefore, the presence of 3,4,3",4"-p-terphenyltetracarboxdi-imide units confers good mesogenic properties.     

Synthesis and studies of some 4-substituted phenyl-4'-azopyridine-containing hydrogen-bonded supramolecular mesogens

Novel azopyridine-containing supramolecular liquid crystalline (LC) materials built via 1 : 1-heterointermolecular hydrogen bonding between some 4-substituted phenyl-4'-azopyridines and 4- n -alkyloxybenzoic acids are reported. These hydrogen-bonded LC complexes exhibit well defined nematic, smectic A and smectic C phases over wide ranges of temperature depending upon the number of carbon atoms present in the alkyl chains. The formation of pure LC materials on 1 : 1-complexation could be confirmed from the phase diagrams of the binary mixtures, which clearly indicated a melting maximum for the 50 mol % mixture as well as the presence of two eutectic points on either side of this mixture.