Molecular Orientational Order Research Articles

Changing liquid crystal elastomer ordering with light – a route to opto-mechanically responsive materials

Long, flexible chains with rods and spacers give nematic polymer melts. Crosslinking yields highly extensible nematic elastomers. Molecular fluidity and orientational order are retained. Uniquely, these elastomers change shape drastically when orientational order is lost with temperature since chains lose their elongation. Reversible changes of 400% are easy to achieve. de Gennes envisaged this coupling between macroscopic strain and nematic (and other) liquid crystalline order. Dye molecular rods are excited by photons into bent conformations. Bent rods depress orientational order and large thermal shape changes will be duplicated by light. Equally, rods may rotate away from the optical electric vector to avoid bending, thus also leading to mechanical strain. Mechanical recovery follows decay back to the molecular ground state. We sketch how these effects arise, explain how even polydomain networks deform under polarised and unpolarised light, and present new results on how nuclear magnetic resonance can reveal details of polydomain optical response. We explore non-linear absorption (photo-bleaching) leading to mechanical response where, by Beer's law, little light should theoretically penetrate. In actuation, light penetrates quickly, is easy to deliver remotely, both excites and stimulates decay, and offers polarisation control over mechanics. Non-uniform director fields also control response. We illustrate novel photo-mechanical effects.

Deuterium NMR spectra of a monodomain nematic: Angular dependence of the linewidths

Deuterium NMR spectroscopy is widely employed to investigate the static and dynamic director alignment for low molar mass nematics contained in a thin sandwich cell. When the director is not uniformly aligned the appearance of the deuterium NMR spectra, recorded in the measurements, reflect the form of the director distribution. This occurs because the observed spectrum is a weighted sum of the spectra from all director orientations in the sample. In order to obtain the form of the director distribution function by simulating the spectra it is necessary to know how the spectral linewidths vary with the director orientation. As well as being of practical relevance, the angular dependence of the linewidths is of fundamental interest for nematic behaviour because it is related to factors such as molecular rotation, director fluctuations and orientational order. Here we explore ways to vary the director orientation, as a monodomain, using an electric field and employ this to measure the angular dependence of the NMR linewidths for the nematic phase of 4-α,α-d 2-pentyl-4′-cyanobiphenyl as a function of temperature.

Raman Imaging of Nematic and Smectic Liquid Crystals

In this article, we demonstrate that confocal Raman microscopy can be utilized to image the director fields of liquid crystal molecules, revealing the spatial patterns of the molecular orientational order both in lateral and axial directions. Here we report on the polarized confocal Raman imaging of the electric field-aligned uniaxial nematic liquid crystal and the in-depth director profiling of the focal conic domains of smectic A liquid crystal. The C≡N stretching vibrational band [ν(CN) = 2228 cm−1] and the C–C stretch of aromatic rings [ν(CC) = 1606 cm−1] are used to probe the spatial director orientations of 4-pentyl-4′-cyanobiphenyl (5CB) and 4-octyl-4′-cyanobiphenyl (8CB).

Thermal hysteresis of the dielectric susceptibility of solid oxygen in the audio frequency range

High sensitivity measurements of the dielectric constant of solid oxygen are reported for $4<T<54\text{ }\text{K}$. The results show unexpectedly large hysteresis effects for the temperature dependence of the dielectric constant in the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases of oxygen on thermal cycling below 44 K. The behavior is compared to that observed for solid ${\text{N}}_{2}\text{-Ar}$ mixtures where the geometrical frustration of the molecular orientational ordering leads to pronounced memory effects. In contrast to solid ${\text{N}}_{2}\text{-Ar}$ where the effects of frustration and disorder combine to form an orientational glass, there is no disorder present for pure ${\text{O}}_{2}$ and the hystereses are attributed to the strong frustration of the interactions.

Real-Time Sensing of the Thermal Diffusivity for Dynamic Control of Anisotropic Heat Conduction of Liquid Crystals

Molecular orientational order can be used to characterize the anisotropic behavior in mechanical, optical, and thermophysical properties. The creation of appropriate molecular orientation has the potential for producing a novel material or thermal switching device, which can control anisotropic heat conduction. Liquid crystals, which are widely used in display elements, have anisotropy not only in their optical, but also in their thermophysical properties, under given molecular orientational alignment conditions; this material can be a variable device with anisotropic heat conduction by controlling the molecular alignment. In the present study, a real-time sensing system for thermal diffusivity using the forced Rayleigh scattering (FRS) method was developed to investigate the transient behavior in the thermal anisotropy of nematic liquid crystals. This technique can be used to measure the in-plane thermal diffusivity perpendicular to the transient thermal grating created by interfering pulsed laser beams, and the thermal anisotropy of the sample can be determined using this non-contact method. The present FRS system can provide continuous measurements of the thermal diffusivity with subsecond time resolution, allowing evaluation of the dynamic behavior of anisotropy in the thermal diffusivity even during a transient process. In this article, the anisotropy of the in-plane thermal diffusivity of 4-4′-pentyl-4-biphenylcarbonitrile (5CB) with molecular alignment induced by either a rubbed substrate or an electric field has been measured. Also, the time evolution of the anisotropic thermal diffusivity in real-time under a dynamically controlled external electric field has been measured. The experimental results demonstrate the capability of dynamic anisotropic control of heat conduction by molecular alignment variations.

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.

高分子科学最近の進歩 刺激応答性液晶エラストマーの新展開

This article reviews the recent progress of liquid crystal elastomers (LCE) with a wide variety of stimulus-responsive properties that stem from the strong coupling of macroscopic shape (distortion) and molecular orientational order. We introduce the responses of nematic, smectic and cholesteric elastomers to temperature change, electric fields and mechanical stresses. The unique stimulus-responsive properties of LCEs provide a possibility of the applications to soft actuators and sensors as well as new interesting issues in the physics of polymers and liquid crystals.

Spatial variation in the molecular tilt orientational order within the solid domains of phase-separated, mixed dialkylphosphatidylcholine monolayers

The miscibility of the solid-phase-forming distearoylphosphatidylcholine (DSPC) and the fluid-phase-forming dilauroylphosphatidylcholine (DLPC) at the air/water interface was investigated by the Langmuir film balance. Surface pressure–area isotherms suggest that mixtures containing 25.0–62.5-mol% DLPC (range of composition investigated) are phase-separated. The lateral structure of the DSPC/DLPC monolayers was imaged by Brewster angle microscopy (BAM) as a function of the surface pressure. Quasi-circular condensed domains appeared at pressures between 0 and 0.5 mN m −1, and these structures were already fully developed at ∼1 mN m −1. Further compression of the monolayers above 1 mN m −1 merely brought the domains closer together. The mixed monolayers consisted of solid domains of DSPC, ∼3–20 μm in size, in a fluid matrix of DLPC. BAM and the phase contrast mode of intermittent-contact atomic force microscopy (AFM) revealed that the quasi-circular DSPC domains are divided into segments of different reflectivities (BAM) or phase shift (AFM) that arise from abrupt changes in the long-range orientational order of the tilted hydrocarbon chains. The DSPC domains in DSPC/DLPC internally exhibited star and cardioid textures that were heretofore only reported for single-component lipid monolayers in the phase coexistence region.

Molecular ordering in a biaxial smectic-A phase studied by scanning transmission X-ray microscopy (STXM)

Results of STXM investigations of a binary mixture (-TNF = 2 : 1; SmA(b) 140 M 180 Iso) known to form a SmA(b) phase [T. Hegmann, J. Kain, S. Diele, G. Pelzl and C. Tschierske, Angew. Chem. Int. Ed., 2001, 40, 887] are presented. Near edge X-ray absorption fine spectra (NEXAFS) of the -TNF board-like aggregates, in particular the intensity of the low energy peaks associated with aromatic ring pi* orbitals (284.5-286.5 eV), show that the molecular plane of these aggregates is very sensitive to the relative orientation of electric field vector E of linearly polarized light, which is used to determine the molecular orientation in the LC phase. The observed strong in-plane dichroic signal suggests the predominant orientation of the -TNF aggregates to be along the smectic layer normal as well as long-range ordering of the in-plane molecular orientation (biaxiality). Orientational maps derived from series of measurements at different sample rotation angles around the specimen normal clearly show a Schlieren-type texture, and permit a detailed examination of exclusive +/-(1/2) disclination theoretically predicted for the SmA(b) phase.

Effect of self-assembled monolayer film order on nanofriction

Self-assembled monolayers have increasingly been explored as potential protective films in devices against friction and adhesion. However, detailed characterization of the monolayer film structure is difficult. This article utilizes a combination of near edge x-ray absorption fine structure (NEXAFS) spectroscopy and Fourier transform infrared (FTIR) spectroscopy to determine the film structure in order to explain the observed nanofriction measurement results. A series of n-alkyltrichlorosilane self-assembled monolayer films with various chain lengths (C5–C30) was prepared on silicon (100) surfaces. Nanofriction measurements were conducted using an atomic force microscope. Results showed that the lowest friction was obtained with a C12 film with higher friction values observed for C5 and C30 films. To explain these observations, the x-ray absorption technique NEXAFS was used to quantitatively measure the surface molecular orientation (order) of these films. It was observed that C12, C16, and C18 films were highly ordered with a molecular orientation of the carbon backbone nearly perpendicular to the surface. C5 and C30 films were less oriented and C10 film showed partial orientation. FTIR spectra suggested that these films possessed different degrees of order. This combination of molecular orientation and order supports and confirms that nanofriction results were heavily influenced by the order and structure of these films.

Mechanical manipulation of molecular lattice parameters in smectic elastomers

Smectic liquid crystalline elastomers (SLCE) represent unique materials that combine a 1-D molecular lattice arrangement and orientational order with rubber-elasticity mediated by a polymer network. Such materials may exhibit large thermo-mechanical, opto-mechanical and electro-mechanical effects, due to the coupling of macroscopic sample geometry and microscopic structural features. It is shown that the molecular layer dimensions in the smectic phases can be influenced reversibly by macroscopic strain of the material. We present a microscopic model on the basis of experimental results obtained by mechanical dilatation measurements, optical interferometry, X-ray scattering, (13)C NMR, FTIR and polarizing microscopy data. The model gives an explanation of the controversial results obtained in different types of smectic elastomers.

Control of formation and molecular orientation of J-aggregates in Langmuir–Blodgett films of mixed merocyanine dyes

We have fabricated Langmuir–Blodgett (LB) films of mixed-merocyanine dyes, DS and its methyl-substituted analog (6-Me-DS), diluted with arachidic acid (AA), expressed as [6-Me-DS] 1− X [DS] X [AA] 2, in order to control the J-aggregation of dye molecules. For most of X -values, the red-shifted J-band peak appeared in the optical absorption spectra with its peak position depending on the mixing ratio of the dyes. In particular, J-band anomalously disappears around X = 0.9 , suggesting the dominance of monomer state. We have also characterized the in-plane preferential orientation of the dye molecules along the dipping direction in the mixed-dye films by the orientation-dependent ESR signals and polarized optical absorption spectra. The optical dichroic ratios were semiquantitatively reproduced by the angular distribution function of the molecular orientation determined by the ESR spectrum simulation. The in-plane orientation of the dye molecules was clearly observed for DS, 6-Me-DS and X = 0.2 films, whereas it was nearly random for X = 0.5 film, suggesting the possibility to control the molecular orientational order of dye molecules by employing mixed-dye system.

Trisialoganglioside GT1b Prevents Increase in Sperm Membrane Molecular Ordering Induced by In Vitro Lipid Peroxidation

The effect of various types of gangliosides, the sialic acid-containing glycosphingolipids, on human sperm membrane during lipid peroxidation induced by Fe(2+)/ascorbate ions was investigated. The monosialoganglioside (GM1), disialogangliosides (GD1a and GD1b), and trisialoganglioside (GT1b) were examined at a concentration of 100 microM, which was above their respective critical micellar concentrations. Lipid peroxidation was determined by quantification of malondialdehyde (MDA) concentration. The molecular orientational order in the membrane was assessed by fluorescence spectroscopy and electron paramagnetic resonance spectroscopy. Both approaches revealed a significant increase in membrane rigidity following oxidation, which correlated with an increase in the MDA level. The preincubation of spermatozoa with GM1 and GD1a did not have any effect on induced lipid peroxidation. In the presence of GD1b and GT1b, a reduced formation of MDA and a decrease in membrane rigidity was detected. The inhibitory effect of GT1b micelles toward membrane oxidation damage was found to be greater than that of GD1b. In conclusion, a direct relationship between the reduced content of the accumulated MDA and the longer preservation of the native-like membrane molecular ordering during sperm oxidation in the presence of GT1b suggests its protective effect. This phenomenon could be due to the specific GT1b conformation and its negative surface potential.

Excess properties of the binary mixtures of methylcyclohexane + alkanes (C 6 to C 12) at T = 298.15 K to T = 308.15 K

Experimental values of density, viscosity, and refractive index at T = (298.15, 303.15, and 308.15) K while the speed of sound at T = 298.15 K in the binary mixtures of methylcyclohexane with n-hexane, n-heptane, n-octane, n-nonane, n-decane, n-dodecane, and iso-octane are presented over the entire mole fraction range of the binary mixtures. Using these data, excess molar volume, deviations in viscosity, molar refraction, speed of sound, and isentropic compressibility are calculated. All the computed quantities are fitted to Redlich and Kister equation to derive the coefficients and estimate the standard error values. Such a study on model calculations in addition to presentation of experimental data on binary mixtures are useful to understand the mixing behaviour of liquids in terms of molecular interactions and orientational order–disorder effects.

Orientational order and distribution function in nematic 1-(4-trans hexylcyclohexyl)-4-[(2-4-isothiocyanato phenyl)ethyl] benzene

The refractive indices (n e, n o) and density have been measured as a function of temperature in the liquid-crystalline and liquid phases of nematic 1-(4-trans hexylcyclohexyl)-4-[(2-4-isothiocyanato phenyl)ethyl] benzene. The molecular polarizability and orientational order parameter have been computed using the Neugebauer anisotropic local field model. The distribution function f(β), and hence the higher order parameter ⟨P 4⟩, and nematic pair-potential have been computed at different temperatures in the nematic phase.

Orientational Ordering of a Bent-Core Mesogen by Two-Dimensional 13C NMR Spectroscopy

Various two-dimensional (2D) NMR techniques are reported on a bent-core mesogen 4,6-dichloro-1,3-phenylenebis[4'-(9-decenyloxy)-1,1'-biphenyl] carboxylate in its nematic and solid phases in order to unambiguously assign its carbon-13 NMR spectrum. The (13)C chemical shifts from the molecular core were studied as a function of temperature to extract its molecular geometry and orientational order tensor. To this end, the chemical shift anisotropy tensors of some carbon sites were measured in the solid state of this mesogen using a recent method called the separation of undistorted powder patterns by effortless recoupling (SUPER). The average bending angle subtended by the two arms of the bent-core structure is determined to be 148.7 degrees. The C-H dipolar couplings obtained from the separated local field (SLF) experiment for the aromatic rings are used to find the local order parameter tensors.

Study of molecular orientational order in the Lagmuir monolayerExperiment and model calculation

In experiment Maxwell’s displacement current (MDC) flows through the structure containing a metal electrode, an air gap insulating layer and the Langmuir monolayer on a water surface. MDC experimental results obtained during a lateral compression of the monolayer are analysed by a model based on the analogy with a non-ideal gas of polar molecules, assuming the Boltzmann statistics of orientational distribution. The theoretical model is presented for a monolayer of thin rod-like molecules (stearic acid) as well as for non-ideal thick amphiphilic units of substituted oligo(3-alkylthiophenes) and compared with experimental data.

Study of molecular orientational order in the Lagmuir monolayer: Experiment and model calculation

In experiment Maxwell’s displacement current (MDC) flows through the structure containing a metal electrode, an air gap insulating layer and the Langmuir monolayer on a water surface. MDC experimental results obtained during a lateral compression of the monolayer are analysed by a model based on the analogy with a non-ideal gas of polar molecules, assuming the Boltzmann statistics of orientational distribution. The theoretical model is presented for a monolayer of thin rod-like molecules (stearic acid) as well as for non-ideal thick amphiphilic units of substituted oligo(3-alkylthiophenes) and compared with experimental data.

Poling of liquid crystals in thin films.

We examine the effect of a strong DC electric field on the molecular orientational order and the nonlinear optical response of liquid crystals in thin films. We compare the results of second-harmonic generation measurements with the predictions of two models, one assuming that the dipoles carried by the molecules have no interactions (the isotropic model), and the other assuming that the dipoles evolve in a Maier-Saupe orienting field responsible for the liquid-crystalline order (the Maier-Saupe model). In both cases, we take into account the effect of surfaces and confinement on the behavior of the molecules. We find that the molecular dipoles behave as predicted by the isotropic model, but that their reorientation is correlated in such a way that the apparent dipole moment of the reorienting units is one order of magnitude larger than the molecular dipole moment.

Influence of Embedded Protons on the Structure of Ice Microcrystals. Computer Simulation

Ice microcrystals of 40 molecules with embedded proton and placed in the external electric field were simulated by the Monte Carlo method in connection with the problem of the destruction of the ozone layer in the stratosphere. The proton field does not disturb the cluster crystalline state, whereas the electric field of crystal defects on the ice surface can affect essentially the microcrystal melting and destroy the structural transition. The melting of microcrystals is mainly reduced to the distortion of orientational molecular order.