The temperature behaviour of a ferroelectric smectic mixture and the detection of ferroelectric switching by NMR

The orientational order of a ferroelectric mixture has been measured by proton NMR. The alignment of the mesogenic units is reflected in the splitting of the NMR signal into a doublet. Up to a cell thickness of 200 μm, it was possible to produce well-oriented layers in the bookshelf geometry by magnetic orientation of the substances in the B₀-field. The angular dependences of the line width of the NMR signal on the tilt angle of the director have been calculated. A ferroelectric switching was detected by measuring the angular dependence of the line width in the switched state. The tilt angle and the orientational order parameter S of the bookshelf samples were estimated at various temperatures.     

Influence of silver nanoparticles on the order parameter of liquid crystalline polymers

New polymer nematic nanocomposites are prepared containing 1.43-4.64 wt % of silver nanoparticles whose mean dimensions are 2-4 nm. According to ²H NMR spectroscopic measurements, on increasing the content of metallic nanoparticles, the orientational order parameter S_zz of the nematic phase shown by the nanocomposites increases.     

A 2D solid-state NMR experiment to resolve overlapping aromatic resonances of thiophene-based nematogens

In this study, we demonstrate the feasibility of resolving overlapping 13C chemical shift spectral lines of aromatic rings in a thiophene-based nematogen in the mesophase using a 2D PITANSEMA solid-state NMR method. This technique provided the information about chemical shift values as well as dipolar couplings that are used for determining the orientational order parameter. Large C-H dipolar coupling values measured for thiophene in contrast to phenyl rings suggest that the heterocyclic ring is not part of the molecular axis. Using the order parameter, we determined the orientation of C-H vectors of the thiophene ring. We believe that the 2D solid-state NMR can be extended to other types of liquid crystalline materials such as the banana-based mesogens for determining the orientational order and bent angle.     

Orientational ordering of a ferroelectric liquid crystal with high spontaneous polarization

The orientational ordering of the liquid crystal, 3M2CPHOB ((2S, 3S)-3-methyl-2-chloropentanoic acid-4',4'-hexyloxybiphenyl ester) in its smectic A and chiral smectic C phases has been studied by carbon-13 2D N.M.R. The technique used is separated local-field spectroscopy, which produces a first-order splitting for each carbon signal, from which carbon-proton dipolar constants can be determined. The order parameters for different molecular segments of the liquid crystal can then be calculated. The C-H bond order parameter at the chiral centre with a chlorine atom is unusually large, which is a strong indication of hindered rotation of the molecular segment at the chiral centre. The carbon-13 chemical shifts of 3M2CPHOB were also measured as a function of temperature, and the relation between the chemical shifts and the order parameters is discussed.     

2H NMR Spectroscopic Investigation of para-Nitroazobenzene Liquid Crystals

Ｔｈｅｕｎｉｑｕｅｐｒｏｐｅｒｔｙｏｆｔｈｅａｚｏｂｅｎｚｅｎｅｇｒｏｕｐｉｓｉｔｓｃｉｓ ｔｒａｎｓｉｓｏｍｅｒｉｚａｔｉｏｎｃａｕｓｅｄｂｙｂｅｉｎｇｉｒｒａｄｉａｔｅｄｗｉｔｈａｐｐｒｏｐｒｉａｔｅＵＶｌｉｇｈｔ(ｓｕｃｈａｓ 356ｎｍｗｈｉｃｈｄｅｐｅｎｄｓｏｎｔｈｅＵＶ Ｖｉｓｉｂｌｅａｂ ｓｏｒｐｔｉｏｎｏｆｔｈｅｃｏｍｐｏｕｎｄ)ａｎｄｔｈｕｓｍｏｎｏｍｅｒｉｃｏｒｐｏｌｙｍｅｒｉｃｌｉｑｕｉｄｃｒｙｓｔａｌｓｃｏｎｔａｉｎｉｎｇａｚｏｂｅｎ ｚｅｎｅｇｒｏｕｐｓｓｈｏｗ ｐｒｏｍｉｓｅｆ…     

Orientational ordering of liquid crystals containing a difluoro-substituted phenyl ring

The orientational ordering of several liquid crystals containing a difluorosubstituted phenyl ring has been studied through the use of C-13 NMR. The fluorinated phenyl ring of these liquid crystals have C_s symmetry, so three order parameters are required to completely describe the ordering of this ring. All three of these order parameters have been calculated from carbon-fluorine dipolar coupling constants obtained from the carbon-fluorine splittings in the C-13 NMR spectra. Because of the complexity of the fluorine-coupled spectra, variable angle spinning (VAS) was used to resolve the carbon-fluorine splittings. In order to study the orientational ordering over wide ranges of temperature, we have developed an empirical correlation between the order parameter and the value of a carbon-fluorine dipolar coupling constant. This enabled us to study the change in the order parameter with temperature. The results of applying this method to several structurally similar liquid crystals containing the same type of difluorinated phenyl ring are presented. A comparison is made to a similar mono-fluorinated liquid crystal.     

Testing assumptions about solute concentration dependence in liquid crystal NMR

The NMR spectra of four solutes, used as probes of liquid crystal orientational order, were analyzed. For each solute, samples were prepared at different solute concentrations, and the concentration dependence was used to extrapolate zero-concentration properties. The mean-field (Maier-Saupe) model when applied to solutes neglects solute-solute interactions and assumes all solutes in a mixed-solute sample see the same average environment. The first assumption is only valid as one approaches zero concentration, while experiments are typically carried out at concentrations between 0 and 10 mol %. The solute concentration dependence has in the past been "scaled out" using an internal solute reference as an orientational standard. We measured the concentration dependence of the orientational order parameter and calculate the corresponding interaction energies based on a mean-field interaction potential for a solute. We find agreement at the 3% level between experiments for different solutes while using (i) the zero-concentration values as solute-dependent orientational references and (ii) scaling to either order parameters or interaction energies; these two scalings gave equivalent but not identical results. We find, too, that errors inherent in the experiment and the calculations will limit attempts to refine the theory to push the comparisons beyond the 2% level.     

Orientational ordering in nematic liquid crystals 1CB-d11 dissolved in 5CB-d6

The ²H-N.M.R. spectra of mixtures of the non-mesogenic compound 4-cyano-4'-methylbiphenyl (1CB) and the nematic liquid crystal 4-cyano-4'-n-pentylbiphenyl (5CB) are measured as a function of concentration and temperature. Concentrations of up to 25 mol% 1CB have no effect on the N.M.R. spectrum and therefore on the orientational order of 5CB at a given reduced temperature. The order matrix of the 1CB is calculated from the measured quadrupole couplings. The results are analysed in terms of a model for orientational order that includes two anisotropic terms: (a) interaction between the molecular quadrupole moment and the mean electric field gradient of the medium, and (b) short range repulsive interactions. An estimate of the molecular quadrupole moment tensor of 1CB is obtained from the analysis.     

Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the SA to SC* (SC) transition

We investigated the correlation between orientational order and microscopic organization of the molecules in smectic A and chiral (racemic) smectic C phases by means of solid-state C-13 NMR, powder X-ray diffraction, and electrooptical measurements. The compounds under investigation are 4-((S)-2-methyloctanoyl)phenyl 4'-nonylbiphenyl-4-carboxylate ((S)-MONBIC) and its corresponding racemic compound ((S, R)-MONBIC). Static C-13 NMR indicates that: (1) the orientational angle of the tail with respect to the magnetic field decreases slightly both in the S_A and S*_C phases as decreasing temperature, and (2) the angle of the core with respect to the field decreases in the S_A phase but increases in the S*_C phase as decreasing temperature. Analysis of C-13 T₁ reveals that the dynamic molecular deformation for the core part can occur near the transition. We discuss the dynamic molecular deformation in comparison with the reorientation of the director at the S_A to S*_C transition. Based on the experimental results, we propose the structural model in which describes the microscopic organization of the molecules in the mesophases.     

Side group functionalized liquid crystalline polymers and blends VII. Phase behaviour and elastic constants K1 and K3 for hydrogen bonded blends of a functionalized LC copolymer with a low molecular mass non-mesogenic dopant

New hydrogen bonded blends of LC copolymers containing functional carboxyl groups with a low molecular mass pyridine-containing dopant were obtained and the orientational, optical and elastic properties of the blends were measured using the Fréedericksz method of threshold transitions in a magnetic field. The averaged order parameter S of the hydrogen bonded blends is found to be lower than that of the initial functionalized LC polymers. Furthermore, a considerable increase in the K₃/K₁ ratio is observed caused by an increment in the average 'effective' length of the hydrogen bonded mesogenic group. For the first time it is proven that LC blends with hydrogen bonded mesogenic groups obey the same main relationship of orientational elastic deformations as common nematic LC polymers with covalent bonding of mesogenic side groups.     

Structural ordering and molecular-optical properties of a discotic liquid crystal (Colho)

The infrared absorbance and the dispersion of the refractive indices in the region 0.49-0.63 µm of the discotic mesogen 2,3,6,7,10,11-hexaheptyloxytriphenylene were studied for the columnar and isotropic phases. The temperature dependence of the orientational order parameter S and the local field parameters of a light wave acting on the sample have been evaluated. It is shown that the mixing of molecular excitations modifies the behaviour of S as a function of temperature. Molecular-optical properties of the triphenylene derivative, such as the anisotropy of the polarizability, Δγ and the mean polarizability, &gammatilde; were investigated on the basis of the generalized Lorenz-Lorentz relation for uniaxial liquid crystals. The experimentally observed changes of these properties in the columnar phase seem to be affected by the induction mechanism of the change in the molecular polarizability with increasing S.     

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.     

Order parameters of the mesogenic core in two nematic liquid crystals containing a four-unit link

A series of compounds containing a four-unit link, 4'-n-alkylphenyl-4-n-hexyloxycinnamates, has been synthesized, and their mesomorphic behaviour has been determined. The orientational ordering of two compounds in this series has been studied in detail by 2D carbon-13 nuclear magnetic resonance with variable angle sample spinning. The carbon-proton dipolar coupling constants were obtained and the order parameters were calculated for different molecular segments (two phenyl rings, the central link containing an ethylenic motif, and the methylene carbons of the alkoxy chain). The temperature dependence of the local order parameters of each fragment were determined using these data and C-13 chemical shifts which were measured as a function of temperature. In these non-rigid molecules, the ratio between the order parameters of the two phenyl rings was found to be temperature dependent. The ring bearing the double bond is more ordered at low temperatures compared to the ring bearing the short alkyl chain, but this behaviour is reversed near the clearing point. The position of the major axis of the rigid core was calculated from the order parameter of one ring and the order parameter of the ethylenic link. It was found that this major axis forms a constant angle with respect to the para axis of the phenyl ring connected to the ethylenic group. On the other hand, the angle of this major axis with respect to the para axis of the phenyl ring connected to the carboxylic group is temperature dependent, varying from 11° at T/T_NI = 0·92 to 7° at T/T_NI = 0·98.     

Non-monotonic temperature dependence of the alignment of a nematic mixture on a ferroelectric substrate

Polarization optic techniques have been applied to study specific features of the anisotropic interaction between a dye-doped eutectic mixture of nematic liquid crystals p-methoxybenzylidene-p-n-butylaniline and p-ethoxybenzylidene-p-n-butylaniline and a polar surface of a ferroelectric triglycine sulphate crystal over the temperature range including the substrate Curie point T _c. It has been found that the temperature-induced structural changes in the nematic layer in the vicinity of T _c are related to the changes in the orientational part of the tensor order parameter Q_ik . The temperature dependence of the director angle θ¯, averaged over the nematic layer, has been obtained from the effective dichroism values of solute absorption. The experimental data were interpreted using the model, in which the anisotropic part of the surface energy has two terms with orthogonal easy axes.     

Structural and electro-optical investigations of the smectic phase of chlorine-substituted banana-shaped compounds

Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an XB2 phase behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by 13C NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the XB2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers. which exhibits antiferroelectric switching     

Deuterium NMR Studies on the Orientational Order Generated in Strained Rubbers

The present paper deals with the induced orientational order of the probe molecules dissolved in the uniaxially strained rubbers measured by using deuterium NMR. The distinctive dependence of the quadrupolar splitting on the swelling, elongation and crosslinking density was observed. The orientational order arising from the correlation between chain segments decreases with the increase of the numbers of both links between junctions and solvent molecules around segments.     

Alignment behaviour of the discotic nematic phase of 2,3,6,7,10,11-hexa(4-n-octyloxybenzoyloxy)triphenylene on polyimide and cetyltrimethylammonium bromide coated substrates

The orientational behaviour of the nematic discotic phase of 2,3,6,7,10,11-hexa(4-n-octyloxybenzoyloxy)triphenylene (C8OBT) on substrates coated with a polyimide or cetyltrimethylammonium bromide (CTAB) was investigated by polarizing optical microscopy. The averaged order parameters and directions of the triphenylene core and the carbonyl groups of C8OBT were evaluated by an infrared dichroic method. The discotic nematic (N_D) phase of C8OBT exhibits a homeotropic alignment on a polyimide film, a typical nematic schlieren texture on a glass substrate, and a tilted or planar homogeneous alignment on a CTAB-coated substrate. The order parameter of the triphenylene core is higher on a polyimide film (S = 0.6) than on a CTAB-coated substrate (S = 0.2), whereas that of the carbonyl groups remains roughly constant at 0.2 to 0.3 independent of the substrate for the N_D phase.     

Orientational ordering of ferroelectric thiobenzoate liquid crystals

The orientational ordering of a series of ferroelectric thiobenzoate liquid crystals was studied by natural abundance ¹³C NMR spectroscopy. The technique used was a combination of variable angle spinning (VAS) and separated local field spectroscopy (SLF). With rapid sample spinning about an axis forming an angle of c. 45° with respect to the magnetic field, the smectic director aligns parallel to the spinning axis, leading to narrow peaks in the ¹³C NMR spectrum. The two-dimensional NMR spectroscopic method SLF allows the observation of decoupled ¹³C signals in the ω2 dimension and first-order Csbnd;H splitting patterns in the ω1 dimension, from which the dipolar C-H coupling constants for individual bonds can be obtained. The order parameters for different molecular segments of eight different compounds, all containing two phenyl rings linked by a thioester group, were obtained this way. A considerable influence of length, branching and chirality of the aliphatic chain on the order parameters was observed.     

NMR determination of smectic ordering of probe molecules

The NMR spectra of the three solutes ortho-, meta-, and para-dichlorobenzene in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB are analyzed to yield two orientational order parameters for each solute. Extrapolation of the asymmetry in the energy parameters that describe the orientational ordering in the nematic phase are used to provide estimates of the strength of the nematic potential in the smectic A phase. The experimentally determined asymmetry of the orientational order parameters in the smectic A phase is then used in conjunction with Kobayashi-McMillan theory applied to solutes to give information about the smectic A layering and the nematic/smectic A coupling. In both smectic A solvents, the solute smectic coupling constant, tau, is negative (with the origin fixed at the center of the smectic layer) for all solutes. The signs and relative values of tau indicate that the ortho and para solutes favor the interlayer region while the meta solute is more evenly distributed throughout the layers.     

Elastic deformations in a magnetic field of a liquid-crystalline polymer with mesogenic groups in the main and side chains

Orientational elastic deformations in a magnetic field and phase transition temperatures of a thermotropically mesogenic “combined” aromatic polyester with mesogenic groups, both in the main and in the side chains, have been investigated in the range of 900 to 10,200 mol. wt. It was shown that in this molecular weight range the birefringence of a completely oriented nematic and correspondingly, the degree of its orientational order S are independent on molecular weight. When the relative temperature ΔT changes from −2 to −30°C the orientational order parameter increases from 0.35 to 0.55. The bend elasticity constants K₃ coincide in the order of magnitude with those for high and low molecular weight nematics investigated previously and their dependence on molecular weight has not been detected.