Unexpected high ordering of a [60]Fullerene nitroxide in the nematic phase of 4-4′-azoxyanisole

A nitroxide [60]fullerene adduct containing a pyrrolidine-1-oxyl group has been synthesized. Its orientational order in the nematic phase of the liquid crystal solvent 4,4′-azoxyanisole (PAA) has been measured from the variation of the EPR spectral parameters on passing from the isotropic to the nematic phase. Highly resolved EPR lines allow for precise evaluation of the shifts of the g, a _N and a _H values. Since the g and the hyperfine tensors are known, the order matrix could be obtained. This is compared with the one calculated with a theoretical model based on short range solute-solvent interactions, which predicts a considerable degree of orientation of the molecular axes, despite the almost spherical shape of the molecule. The agreement with experimental findings is quite good and it is further improved if a bent structure of the pyrrolidine ring is taken into account.     

Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Liquid Crystalline Organisation and Reaction Rate in the Formation of Liquid Crystal Thermoset Networks from Diepoxides and Dicyanates

Summary: The influence of the type of mesophase on the rate of reaction in amine curing or anionic polymerisation of mesogenic diepoxides, and in the cyclotrimerisation of mesogenic dicyanates has been investigated by isothermal DSC and IR-spectroscopy. Epoxide/amine systems were found to react faster in the nematic than in the isotropic phase. Anionic polymerisation of epoxides is an example where increase of the reaction rate occurs with the transition from a mesophase of higher order (smectic) to phases with lower order (nematic, isotropic). For triaromatic dicyanates with one or three methyl groups at the central ring cyclotrimerisation is faster in the isotropic than in the nematic phase showing an increase of activation energy in the nematic phase with the number of methyl groups. A pronounced rate increase has been observed in the smectic phase as compared to the nematic phase for the unsubstituted triaromatic dicyanate. In this case activation energies in the two phases are comparable with those of non liquid crystalline dicyanates.     

Dielectric study of the liquid crystalline dimer alpha, omega-bis(4'-cyanobiphenyl-4-yloxy)decane

The dielectric properties of a nematogenic dimer alpha,omega-bis(4-cyanobiphenyl-4-yloxy)decane in the nematic and isotropic phases have been investigated in the frequency range between 100 Hz and 13 MHz. It was found that the compound is characterized by a positive dielectric anisotropy. The dielectric constant in the nematic phase is lower than that in the isotropic phase, which suggests variation in the conformational distribution of the dimer after the phase transition. Only one relaxation process, both in the nematic and isotropic phases, has been observed in the frequency range used: the relaxation frequency has been found to take values between 2 and 4 MHz, depending on the temperature.     

Thermotropic homopolyesters. III. Preparation and properties of polymers based on 4′-hydroxyphenyl-4-hydroxycinnamate

Low-molecular-weight 4′-acetoxyphenyl-4-acetoxyoinnamate, as well as several polyesters synthesized from this monomer and aliphatic dibasic acids, exhibit thermotropic nematic phases. DSC heating curves for all of the polymers exhibit multiple transitions. The amount of crystallinity of these polymers at room temperature is small and the degree of order along the chain axis in the crystalline phase is poor. For the lower homologues the nematic phase exists over a broad temperature range of approximately 100°C. The polyester from chiral (+)-3-methyl adipate forms a thermotropic cholesteric phase. Both the diacetoxy monomer and azelate polymers of low molecular weight adopt the homeotropic texture on glass slides, but with increasing molecular weight the planar texture becomes preferred. Investigation of the effects of electric fields in the conduction regime upon the nematic phase of the diacetoxy monomer revealed that Williams domains are formed only with difficulty. In most cases, a stationary pattern appeared instead. At higher voltage the dynamic scattering mode (DSM) was obtained, and above this a field-induced transition to the isotropic phase. The azelate polyesters exhibited Williams domains and the DSM in the conduction regime. The formation time for Williams domains was fairly short for polymers having η_inh < 0.44 dL/g, but increased to 80 min when η_inh = 0.68 dL/g. The DSM was only observed for polymers having η_inh < 0.61 dL/g. For these polymers the critical frequency separating the conduction and dielectric regimes exhibits a stronger temperature dependence than that of low-molecular-weight nematogens. A new instability pattern is reported for the azelate polyesters in the dielectric regime.     

Effect of terminal polar substituents on the nature of smectic A liquid crystals of a series of 4-(4'-octyloxybenzoyloxy)benzylidene-4'-substituted anilines

The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4'-octyloxy-benzoyloxy)benzylidene-4'-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO₂ and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.     

Magnetic alignment of aqueous CTAB in nematic and hexagonal liquid crystalline phases investigated by spin-1 NMR

Spin-1 NMR has been used to characterize the magnetically aligned nematic and hexagonal liquid crystalline phases of aqueous cetyltrimethylammonium bromide (CTAB). A nematic/hexagonal biphasic region has been identified for the first time in this system. The nematic phase is characterized by an order parameter of smaller magnitude and greater temperature dependence. Magnetic alignment kinetic rates of the two phases differ greatly, with the nematic phase showing magnetic alignment much faster than the hexagonal phase. Equilibration has been monitored over time by measuring the change in quadrupole splitting as a function of temperature. As the sample equilibrates the temperature dependence of the splitting decreases logarithmically. This work also demonstrates how the phase and order of the liquid crystal can be manipulated during the early part of equilibration.     

Mesomorphic properties of orthopalladated complexes of laterally substituted azobenzene derivatives, 2-alkoxycarbonyl-4-(4-ethoxyphenylazo)phenyl 4-methoxybenzoates

Homologous series of 2-alkoxycarbonyl-4-(4-ethoxyphenylazo)phenyl 4-methoxybenzoates (L) and their orthopalladated μ-chloro dimers (Pd₂Cl₂L₂) have been synthesized and their thermotropic phase transition behaviour has been characterized. The ligands, L, are three-ring nematogens with their central rings substituted in the 2-position by n-alkyl chains of varied length. The thermal stability of the nematic phases decreases rapidly with increasing side chain length and then levels off as the series is ascended. In every case the mesomorphic phase appears only over a small temperature range (less than 20° C). The orthopalladated species also proved to be nematogenic. Melting and clearing points rise on the average by 87° C and 126° C, respectively, and thus the temperature ranges of the nematic phases are enlarged considerably upon metallation. In relation to the observed stabilization of the nematic phases, particularly of higher homologues, a lath-like molecular structure, in which the side chains of the ligands fill up the clefts of the core part, has been deduced for a Pd2Cl2L2 homologue from its solution ¹H-NMR spectrum.     

Rheological evidence for the existence of subphases in the liquid crystalline 4-n-alkoxybenzoic acids

The rheological behaviour of the homologous series, the alkyloxybenzoic acids, has been studied at steady flow and small amplitude oscillatory shear. The temperature dependencies of the viscosity were compared with differential scanning calorimetry data in order to estimate pretransitional phenomena in mesophases. The values of the viscosity and flow activation energy (E) can be indicative of the mesophase state. 4-n-pentyloxybenzoic acid yields a classic nematic (N) phase while the next homologues show a cybotactic nematic phase having a higher E value. The N phase is a Newtonian fluid of lower viscosity than that of the isotropic phase. The cybotactic nematic phase by its rheological properties takes an intermediate position between the N phase and a smectic C (SmC) phase. The SmC phase is a viscoelastic and viscoplastic medium. The SmC phase of 4-n-octyloxy-, 4-n-nonyloxy-, 4-n-decyloxy- and 4-n-dodecyloxybenzoic acid can exist in high- and low-ordered states. The more ordered smectic phase is characterised by higher stiffness, viscosity and yield stress than those of the less ordered. The SmC phase of 4-n-hexadecyloxybenzoic acid is characterised by the only state with unstable value of apparent viscosity. The phase state and the transition temperature can be dependent on the thermal history of the sample and a mechanical shear.     

Self‐Association of an Enantiopure β‐Pentapeptide in Nematic Liquid Crystals

Herein, we report for the first time that nematic liquid‐crystalline environments drive the reversible self‐aggregation of an enantiopure β‐pentapeptide into oligomers with a well‐defined structure. The peptide contains four (1S,2S)‐2‐aminocyclopentane carboxylic acid (ACPC) residues and the paramagnetic β‐amino acid (3R,4R)‐4‐amino‐1‐oxyl‐2,2,5,5‐tetramethylpyrrolidine‐3‐carboxylic acid (POAC). The structure of the oligomers was investigated by electron paramagnetic resonance (EPR) spectroscopy, which allowed us to obtain the intermonomer distance distribution in the aggregates as a function of peptide concentration in two nematic liquid crystals, E7 and ZLI‐4792. The aggregates were modeled on the basis of the EPR data, and their orientation and order in the nematic phase were studied by the surface tensor method.     

Surface ordering at the air–nematic interface. Part 2. A spectroscopic ellipsometry study of orientational order

Spectroscopic ellipsometry has been used to measure enhanced orientational ordering at the nematic–air interface of 8CB as the smectic A phase was approached by cooling from the isotropic phase. The depth profile of the orientational order has been estimated by calculating the ellipsometric parameters for a homeotropic uniaxial surface film on a uniaxial sub‐phase using the Abelès matrix method. This showed that the depth of the enhanced orientationally ordered region was ～10 nm at 0.5°C above the nematic–smectic A transition. This is substantially less than the thickness of the region with surface enhanced smectic order as determined by neutron reflection and a model of the surface structure consistent with both sets of results is proposed.     

Dielectric relaxation processes in the nematogen 4-cyanophenyl 4-n-heptylbenzoate

Experimental studies of the dielectric relaxation in the nematic and isotropic phases of 4-cyanophenyl 4-n-heptylbenzoate have been performed in the frequency range from 1 kHz to 1 GHz. In the nematic phase, measurements of the complex dielectric permittivity for two orientations of the director n , parallel and perpendicular to the measuring electric field E , have been carried out. It has been found that for E n , as well as in the isotropic phase, the dielectric spectra consist of two elementary domains, described by the Cole-Cole equation, whereas for E n three Cole-Cole-type domains can be distinguished. The possible molecular reorientational movements corresponding to these domains are discussed and comparison with the theoretical predictions is made.     

Monte Carlo simulation of the self-assembly and phase behavior of semiflexible equilibrium polymers

Grand canonical Monte Carlo simulations of a simple model semiflexible equilibrium polymer system, consisting of hard sphere monomers reversibly self-assembling into chains of arbitrary length, have been performed using a novel sampling method to add or remove multiple monomers during a single MC move. Systems with two different persistence lengths and a range of bond association constants have been studied. We find first-order lyotropic phase transitions between isotropic and nematic phases near the concentrations predicted by a statistical thermodynamic theory, but with significantly narrower coexistence regions. A possible contribution to the discrepancy between theory and simulation is that the length distribution of chains in the nematic phase is bi-exponential, differing from the simple exponential distribution found in the isotropic phase and predicted from a mean-field treatment of the nematic. The additional short length-scale characterizing the distribution appears to arise from the lower orientational order of short chains. The dependence of this length-scale on chemical potential, bond association constant, and total monomer concentration has been examined.     

Thermotropic homopolyesters—VI. A study of polymers based on 4′-hydroxyphenyl-4-hydroxybenzoate

Polyesters were synthesized from 4′-acetoxyphenyl-4-acetoxybenzoate and dibasic acids having 6–12 methylene units using the transesterification procedure described by van Luyen and Strzelecki. These polymers are designated S_n, where n is the number of methylene units in the diacid. The transition temperatures of our polymers stand in reasonable agreement with those reported by Strzelecki and Liébert. The nematic phase extends over a broad temperature range (70–100°), and the biphasic region spans an additional 50°. The texture of the nematic phase is normal for S7-1 having η_inh = 0.30 dl/g, but the other polymers show bright nematic droplets on a darker background. The density of droplets decreases with increasing η_inh in the S7 series, and as η is increased for the other polymer homologues. Only S7-1 exhibits Williams domains on application of an electric field. Quite different properties are found for the polyester having η_inh = 0.80 dl/g prepared from 4′-hydroxyphenyl-4-hydroxybenzoate and the diacid chloride. It melted sharply 24° higher, and its nematic phase showed a normal texture. Moreover, its nematic-isotropic transition appears in the DSC as a sharp peak, and the biphasic region spans only 9°. We believe the transesterification reaction occasionally involves the internal ester linkage in the monomer unit, producing a polymer with no definite repeating unit structure and a distribution of hard segment lengths.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Solution combustion derived nanocrystalline Zn(2)SiO(4):Mn phosphors: a spectroscopic view

Manganese doped nanocrystalline willemite powder phosphors Zn(2-x)Mn(x)SiO(4) (0.1(6)A(1) ground state. The mechanism involved in the generation of a green emission has been explained in detail. The effect of Mn content on luminescence has also been studied.     

The mesomorphic anomaly of 4-[2-(4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-pentylcyclohexyl-1-carboxylates

Four homologous series of 4-[2-[4-alkoxy-2,3,5,6-tetrafluorophenyl)ethynyl] phenyl trans-4-alkylcyclohexyl-1-carboxylates have been prepared. Their liquid crystalline behaviour was investigated by optical polarizing microscopy and DSC. Series A, B and C exhibit the nematic phase. The phase transition behaviour of series D is anomalous. The compounds of series D with a short alkoxy chain exhibit enantiotropic smectic A and nematic phases, while those with a long alkoxy chain exhibit only the nematic phase.     

Synthesis and properties of poly(1,2–8,9-trans-quinacridono)

The aromatic ladder polymer, poly(1,2–8,9-trans-quinacridono) (PQ) has been synthesized. The intermediate polymer was also characterized. Electron paramagnetic resonance (EPR) and conductivity measurements were made on PQ. PQ packs in nematic liquid crystal form. Iodine intercalates and destroys the order during doping, while raising its conductivity by five orders of magnitude.     

Corresponding states in the smectic-A phase

In the last few years, experimental data of many different sources has been gathered to show that order parameter curves of essentially all nematic liquid crystals (NLCs) compounds can be reduced to a single and universal curve of the corresponding states; experimental data of an impressive number of different nematic compounds has been linearly rescaled in such a way that all them coalesced in a single curve. The surprising novelty observed on NLCs is that this universal behaviour is not restricted to the neighbourhoods of a critical region, as is usually expected from critical phenomena, but it covers the entire domain of the nematic phase, from the nematic isotropic temperature to the nematic crystalline temperature. The aim of this paper is to study the statistical mechanics fundamentals of this behaviour and show that it is a consequence of the universality of the angular part of the potential of the interaction between the particles composing the liquid crystal (LC). We will illustrate this behaviour presenting evidences that it can be found on the smectic-A phase.     

Solute induced liquid crystalline behaviour of a quaternary ammonium salt and its application to structure determination

A new liquid crystalline phase, induced by the addition of small amounts of a non-mesogenic solute (such as dimethyl sulphoxide or methyl iodide) to a quaternary ammonium salt, N -methyl- N, N, N-trioctadecylammonium iodide (MTAI), has been detected by NMR and optical microscopic studies. In some cases, there is a coexistence of nematic and smectic phases. Information on the ordering of the phases in the magnetic field of the spectrometer has been derived from NMR spectra of a dissolved molecule, 13C-enriched methyl iodide. The low order parameter of the pure thermotropic nematic phase of the salt provides firstorder spectra of the dissolved oriented molecules. Analyses of spectra of cis , cis -mucononitrile exemplifies the utility of the MTAI nematic phase in the determination of structural parameters of the solute.