Photoinduced alignment of polymerisable liquid crystals on photoreactive polymers containing 2,6-bis(benzylidene)-1-cyclohexanone units in the main chain

Photoreactive polymers containing 2,6-bis(benzylidene)-1-cyclohexanone (bisBC) units were synthesised and investigated as a photoalignment layer for polymerisable liquid crystals (PLCs) and liquid crystalline polymers (LCPs). The liquid crystalline materials were aligned homogeneously on the photoalignment layers in a wide range of irradiation dose of linearly polarised UV light (LPUVL). Specifically, for the photoalignment layer baked at 80°C, order parameters of the liquid crystalline materials were low due to the disturbance of oriented-photoreactive polymer caused by the contact with the solvent of liquid crystalline materials. However, the liquid crystalline materials were aligned homogeneously even at low irradiation doses on the thermally cured photoalignment layer baked at 180°C. In addition, the liquid crystalline materials were aligned perpendicular to the LPUVL electric field. The alignment mechanism is discussed by comparing the retardation of photoalignment layer with anisotropic polarisabilities of model molecules calculated by density functional theory (DFT). It is suggested that the liquid crystalline materials aligned along the unreacted chromophores in the photoreactive polymer.     

柔性高分子/小分子液晶混合物的自洽场理论

发展了柔性高分子/小分子液晶混合物连续自洽场理论，将小分子液晶模型化 为取向与位置无关的单体分子，小分子液晶间存在各向异性的Maier-Saupe相互作 用，该理论可还原成高分子和各向同性小分子组成的Flory-Huggins溶液理论和纯 液晶的Maier-Squpe液晶理论，通过数值解自洽场方程组，还将理论用于研究柔性 高分子/小分子液晶混合物相分离开界面性质，得到的结果与用Helfand格子界面理 论和MOnte Carlo模拟的结果一致。     

Morphology development of main‐chain liquid crystalline polymer fibers during solvent evaporation

A nonequilibrium thermodynamic approach has been developed for describing the emergence of fiber morphologies from a liquid crystalline polymer solution undergoing solvent evaporation, including fibrillar structures, concentric rings, and spiral structures. We utilized Matsuyama–Kato free energy for main‐chain liquid crystalline polymer (MCLCP) solutions, which is an extension of Maier–Saupe theory for nematic ordering and incorporates a chain‐stiffening, combined with Flory‐Huggins free energy of mixing. Temporal evolution of the concentration and nematic order parameters pertaining to the above free energy density of liquid crystalline polymer solution was simulated in the context of time‐dependent Ginzburg–Landau theory coupled with the solvent evaporation rate equation under the quasi‐steady state assumption. The emerged morphological patterns are discussed in relation to the phase diagram of the MCLCP solution and the rate of solvent evaporation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 429–435, 2007     

Network morphology and electro-optical characterisations of epoxy-based polymer stabilised liquid crystals

ABSTRACT

In this paper, by utilising the photoinitiated cationic polymerisation of liquid crystalline epoxide in LC solvents, the epoxy-based polymer stabilised liquid crystals (EPSLCs) were fabricated. The effects of the liquid crystalline epoxide content, UV intensity, polymerisation temperature, and initiator content on the network morphology of liquid crystalline epoxy polymers (LCEPs) and electro-optical characterisations of EPSLC samples were studied. The results suggested that the preparation conditions had a dominant effect on the morphology of the LCEP and the void size in the EPSLC sample. The increase of liquid crystalline epoxide content, UV intensity, and initiator content induced that the density of LCEP network became larger and the void size decreased, which resulted in that the stabilising effect from the LCEP network on the mesogenic solvent molecules was strengthened. Conversely, the increase of polymerisation temperature induced that the stabilising effect from the LCEP network on the LC molecules was weakened.     

Side chain liquid crystalline elastomers II. Thermal properties and orientational behaviour of the LCE based on unsaturated copolyesters

The effect of crosslinking density on the phase behaviour of smectic liquid crystalline networks was studied; the results showed that crosslinking in their smectic phases can greatly enhance the stability of the liquid crystalline phase. The higher the crosslinking density, the higher the smectic-isotropic transition temperature. The mechanically-induced orientation was studied by polarized FTIR spectroscopy. The smectic liquid crystalline network could be oriented parallel to the mechanical field at higher draw ratio λ, while at lower λ the mesogenic groups are oriented perpendicular to the field for the networks with higher crosslinking density. The observed mechanically-induced orientation is interpreted by a proposed mechanism.     

Synthesis and properties of photochromic liquid‐crystalline copolymers containing both spironaphthoxazine and cholesteryl groups

Photochromic liquid‐crystalline copolymers consisting of a photochromic monomeric unit containing both a spironaphthoxazine group and an undecamethylene spacer, and a liquid‐crystalline monomeric unit containing both a cholesteryl group and a decamethylene spacer were prepared to investigate the effect of the thermal properties of the photochromic monomeric unit on the mesomorphic order of the side chain of the related copolymers. The photochromic liquid‐crystalline copolymers containing a photochromic liquid‐crystalline monomeric unit showed only a smectic phase. On the other hand, the photochromic liquid‐crystalline copolymers containing a photochromic non‐liquid‐crystalline monomeric unit showed a chiral nematic phase (cholesteric phase). The photochromic chiral nematic liquid‐crystalline copolymer containing 14 mol % photochromic monomeric unit reflected visible light around 104 °C. To lower the temperature range of reflection of visible light, cholesteryl oleyl carbonate was used as a chiral nematic plasticizer for the photochromic chiral liquid‐crystalline polymer systems. Photo‐induced pitch change of the mixture by means of UV irradiation was investigated and it was concluded that the pitch change observed under UV irradiation was mainly induced by thermal effect in the case of our system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 887–894, 2000     

Waveguide spectroscopic characterization of 3D anisotropies in conventionally photooriented and annealed films of liquid crystalline and amorphous azobenzene polymers

Films of azobenzene-containing polymers were photooriented in the glassy state in such a way that the azobenzene side groups were oriented preferably perpendicular to the electric field vector. In the case of liquid crystalline polymers ,the photoinduced anisotropies generated in the glassy state were modified by thermotropic self-organization due to annealing above T(g). The conventional photoorientation process results in an oblate order. The changes in photoinduced anisotropies brought about by annealing in the liquid crystalline phase were investigated quantitatively for the first time by us for different polymer compositions and experimental conditions. Different biaxial and homeotropic orders result for liquid crystalline polymers, depending on the experimental conditions. Different polymer structures are compared and the influence of the interfaces is investigated. Orientational gradients can be induced by irradiation or annealing and are for the first time determined by the WKB (Wentzel-Kramers-Brillouin) method.     

Mesomorphic properties of liquid crystalline polysiloxanes and elastomers based on a tri-vinyl crosslinker

A series of crosslinked liquid crystalline polymers and the corresponding uncrosslinked liquid crystalline polymers were prepared by graft copolymerization; their liquid crystalline properties were characterized by DSC, POM and X-ray measurements. The results show that the crosslinking obtained in the isotropic state leads to a reduction of the clearing point (T_c) of the crosslinked polymers, as compared with the corresponding uncrosslinked polymers. The crosslinked polymers with low crosslinking density (P₁-P₇) exhibit nematic mesogenic phases, as do the uncrosslinked polymers. In contrast, a high crosslinking density leads to the crosslinked polymers P₈ and P₉ losing their thermotropic liquid crystalline phases; they instead exhibit stress-induced orientation.     

Liquid crystalline triblock copolymers. Mechanical behaviour and orientation under uniaxial strain

The stress‐strain and orientation behaviour of side‐chain liquid crystalline(SLCP) ABA triblock copolymers with a backbone of polystyrene‐block‐1,2‐polybutadiene‐block‐polystyrene and a cyanobiphenyl mesogen in the side chain was investigated in dependence of molecular weight. The polymer shows the behaviour of a thermoplastic liquid crystalline elastomer(TPLCE) in the nematic phase in a region between the glass transitions of the polystyrene block and the SLCP. The ultimate properties and E‐modulus is lower than for conventional thermoplastic elastomers. Under uniaxial strain liquid crystalline order perpendicular to the direction of strain is induced.     

On the determination of order parameters for homogeneous and twisted nematic liquid crystals from Raman spectroscopy

Traditional approaches to the use of Raman spectroscopy as an aid to the determination of local order parameters in liquid crystalline materials have employed polarizations of the excitation source and/or the analyser which are orthogonal to the liquid crystalline director. The present paper describes a Raman study, which seeks to take advantage of the additional information available from examining the complete range of orientations of the director in relation to these polarization directions. A theory is developed which shows how it is possible to use this additional information to derive more reliable values of the P₂ and P₄ local order parameters in homogeneous and twisted nematic liquid crystal cells.     

Interface induced crystal structures of dioctyl-terthiophene thin films

Temperature dependent structural and morphological investigations on semiconducting dioctyl-terthiophene (DOTT) thin films prepared on silica surfaces reveals the coexistence of surface induce order and distinct crystalline/liquid crystalline bulk polymorphs. X-ray diffraction and scanning force microscopy measurements indicate that at room temperature two polymorphs are present: the surface induced phase grows directly on the silica interface and the bulk phase on top. At elevated temperatures the long-range order gradually decreases, and the crystal G (340 K), smectic F (348 K), and smectic C (360 K) phases are observed. Indexation of diffraction peaks reveals that an up-right standing conformation of DOTT molecules is present within all phases. A temperature stable interfacial layer close to the silica-DOTT interface acts as template for the formation of the different phases. Rapid cooling of the DOTT sample from the smectic C phase to room temperature results in freezing into a metastable crystalline state with an intermediated unit cell between the room temperature crystalline phase and the smectic C phase. The understanding of such interfacial induced phases in thin semiconducting liquid crystal films allows tuning of crystallographic and therefore physical properties within organic thin films.     

Determination of 13C chemical shift anisotropy tensors and molecular order of 4-hexyloxybenzoic acid

4-Alkoxy benzoic acids belong to an important class of thermotropic liquid crystals that are structurally simple and often used as starting materials for many novel mesogens. 4-Hexyloxybenzoic acid (HBA) is a homologue of the same series and exhibits an enantiotropic nematic phase. As this molecule could serve as an ideal model compound, high resolution (13)C NMR studies of HBA in solution, solid, and liquid crystalline phases have been undertaken. In the solid state, two-dimensional separation of undistorted powder patterns by effortless recoupling (2D SUPER) experiments have been carried out to estimate the magnitude of the components of the chemical shift anisotropy (CSA) tensor of all the aromatic carbons. These values have been used subsequently for calculating the orientational order parameters in the liquid crystalline phase. The CSA values computed by density functional theory (DFT) calculations showed good agreement with the 2D SUPER values. Additionally, (13)C-(1)H dipolar couplings in the nematic phase have been determined by separated local field (SLF) spectroscopy at various temperatures and were used for computing the order parameters, which compared well with those calculated by using the chemical shifts. It is anticipated that the CSA values determined for HBA would be useful for the assignment of carbon chemical shifts and for the study of order and dynamics of structurally similar novel mesogens in their nematic phases.     

Theoretical and vibrational spectroscopic analysis of the CO stretching mode of cholesteryl alkanoates: the particular case of the cholesteryl acetate

Structural and vibrational properties of the CO stretching bond of cholesteryl acetate and related steroids are investigated theoretically and by Micro-Raman spectroscopy. In this work, an analysis of the CO stretching mode for the cholesteryl acetate is presented. Experimental results in crystalline, isotropic liquid and liquid crystal phases are compared with quantum chemical calculations using semi empirical hamiltonians (AM1 and PM3) and the density functional theory. The calculations were performed on isolated molecules with different conformations as found on previous investigations giving strong evidence of their existence. Calculated frequencies are found to be very close to experiments and suggest the possible existence of the predicted conformers.     

Electric field effect on the nematic-isotropic phase transition

Pulsed electric fields are used to study the influence of a strong field on the nematic-isotropic phase transition for cyanobiphenyl and stilbene-type liquid crystals. Deviation of the electric field-induced optical anisotropy from the Kerr law is observed and it is shown that such electric fields can shift the transition temperature substantially. The induced birefringence and the shift of the transition temperature are measured as a function of the electric field strength. The results are explained qualitatively in the context of the Landau-De Gennes theory with two order parameters. The coefficients of the phenomenological theory are calculated using the simple density functional theory of polar nematics developed in this paper and the results for the shift of the transition temperature are compared with experiment results.     

A theory of adsorbate melting near the surfaces of adsorbents and in slit-shaped pores

Molecular principles of a theory of adsorbate melting near the open surfaces of adsorbents in the frozen state or in slit-shaped pores are discussed. The states of liquid and crystalline adsorbates are described in terms of a single molecular approach (the lattice gas model). The crystalline state is described using the concept of quasi-average distributions, for which the degeneracy of the density distribution function in the plane of the adsorbent is eliminated. Equations for the chemical potential of the adsorbate in defective crystals and vapor-liquid systems are derived with allowance for their vibrational motion, making it possible to calculate the concentration profile of the substance at the planar interface between two solid phases, between a solid and a liquid, and inside a slit-shaped pore.     

Optical studies of a liquid crystalline compound 6O.6

A detailed optical study for a Schiff's base liquid crystalline nO.m compound (6O.6) is reported in this article. The phase transition temperatures for the compound were investigated using a polarising microscopy and differential scanning calorimetry (DSC). Refractive index studies were conducted using the thin prism method with the 633 nm line from a He–Ne laser. The order parameters of the compound were calculated from the refractive index data. Order parameters calculated by two methods, namely, direct extrapolation method and modified Vuks’ method were found to be in close agreement. The normalised polarisabilities for extraordinary and ordinary rays and their ratio for the sample were also calculated. The order parameters are found to be reasonably high and the trend shown by normalised polarisabilities substantiates the correctness of our results. From the methods we adopted, the order parameters and the normalised polarisabilities could be determined directly from the refractive index measurements without doing the density studies. Both these methods are found to be highly suitable for the calculation of order parameters of liquid crystalline compounds having high clearing temperature where density studies are difficult and hazardous.     

Effect of charge density gradient on characteristics of electric double layer for salt melts

The equilibrium conditions are analyzed for a spatially inhomogeneous ionic liquid using the density functional theory with allowance made for the second order gradient corrections. Solutions for the distribution of potential and charge density in the electric double layer at the ionic liquid/vapor interface are obtained using a parameterized total density profile normal to the surface. It is shown that taking into account the effects of the charge density gradient in the theory results in the appearance of damped oscillations of the charge density near the surface, while the double layer localized on the surface is reduced.     

An Empirically Optimized Classical Force-Field for Molecular Simulations of 2,4,6-Trinitrotoluene (TNT) and 2,4-Dinitrotoluene (DNT)

An empirical classical all-atom specific force-field for use in molecular dynamics simulations (MD) has been developed to reproduce the experimental densities and structures of trinitrotoluene (TNT) in its crystalline and liquid phases at six different temperatures, as well as its enthalpies of sublimation and fusion. The average structural parameters and partial charges were obtained from density functional theory optimizations of single molecules at the B3LYP/6-311+G** level. The other constants for the potential were adjusted in order to obtain a classical force-field, which is able to reproduce the aforementioned properties for TNT with a high degree of accuracy. This force-field was also found to predict closely the experimental densities and structures of 2,4-dinitrotoluene (2,4-DNT) in its crystalline and liquid phases as well as its enthalpy of sublimation. It was a bit less successful for its enthalpy of fusion, but it still remained reasonable, and the model mechanical properties were of the right order of magnitude. As such, this fairly simple force-field can be used for MD simulations of both TNT and 2,4-DNT nitroaromatic compounds.     

Theoretical simulation of thermally induced phase separation in a main‐chain liquid‐crystalline polymer solution

Phase diagrams of main‐chain liquid‐crystalline polymer (MCLCP) solutions have been calculated self‐consistently on the basis of a simple addition of the Flory–Huggins free energy for isotropic mixing, the Maier–Saupe free energy for nematic ordering, and the Flory free energy for chain rigidity of the MCLCP backbone. The calculated phase diagram is an upper critical solution type overlapping with the nematic–isotropic transition. The phase diagram consists of liquid–liquid, liquid–nematic, and pure nematic regions. Subsequently, the dynamics of thermally induced phase separation and morphology development have been investigated by the incorporation of the combined free energy density into the coupled time‐dependent Ginzburg–Landau (model C) equations, which involve conserved compositional and nonconserved orientational order parameters. The numerical calculations reveal a variety of the morphological patterns arising from the competition between liquid–liquid phase separation and nematic ordering of the liquid‐crystalline polymer. Of particular interest is the observation of an inflection in the growth dynamic curve, which may be attributed to the nematic ordering of the MCLCP component, which leads to the breakdown of the interconnected domains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 913–926, 2003     

光致取向调控偶氮苯侧链液晶聚合物的阻隔特性研究

研究了液晶分子的排列方式对聚合物膜阻隔特性的影响,采用473 nm线偏振光照无定形偶氮液晶聚合物,使其介晶基元发生从无序到有序的取向排列.用膜透射率变化和锥光干涉图表征了分子的取向,其锥光干涉图为粗黑十字,说明在线偏振光下作用下液晶分子取向形成了单相畴沿面内排列的有序态.用金属表面氧化法进一步研究了取向态聚合物膜的阻隔...