NMR relaxation study of molecular dynamics in columnar and smectic phases of a PAMAM liquid-crystalline co-dendrimer

We present the first results obtained by proton (¹H) nuclear magnetic relaxation studies of molecular dynamics in a supermolecular liquid-crystal dendrimer exhibiting columnar rectangular and smectic-A phases. The ¹H spin-lattice relaxation time (T₁) dispersions are interpreted using two relaxation mechanisms associated with collective motions and local molecular reorientations of the dendritic segments in the low- and high-frequency ranges, respectively. The T₁ values show a drop around 2.3 MHz that is attributed to a contribution coming from cross-relaxation between ¹H and nitrogen nuclear spins. In the high-frequency range the motions appear to be of similar nature in both mesophases and are ascribed to reorientations of dendritic segments (belonging to the core and/or to the mesogenic units) characterized by two correlation times. Notable differences in the dynamics between the columnar and layered phases are observed in the low-frequency range. Depending on the mesophase they are discussed in terms of elastic deformations of the columns and layer undulations. In this study we find that the dendritic core influences the dynamics of the mesogenic units both for local and collective motions. These results can be understood in terms of spatial constraints imposed by the dendritic architecture and by the supermolecular arrangement in the mesophases.     

Contribution of proton NMR relaxation to the investigation of molecular dynamics in columnar mesophases of discotic and polycatenar molecules

We present in this work a review concerning wide frequency rangeT ₁ proton NMR relaxation studies performed in compounds exhibiting columnar mesophases, namely the Col_ho mesophase in the case of a liquid crystal of discotic molecules and the ø_h mesophase in the case of a liquid crystal of biforked molecules. These NMR relaxation studies were performed combining conventional and fast field cycling NMR techniques in a frequency range between 100 Hz and 300 MHz. The possibility of probing such a large frequency range has provided a way to effectively distinguish the influence, on theT ₁ relaxation profiles, of the different molecular movements observed in this type of mesophases. In addition, we present a comparison between the molecular dynamics in columnar (ø_h) and lamellar (SmC) mesophases exhibited by the same biforked compound.     

热致液晶CPHOB的有序参数与核磁共振弛豫

从室温至380K测量了热致液晶4-己氧基苯甲酸4-氰基苯酯（CPHOB）的氢核磁共振谱．其固态至向列相的转变温度T_sn=347K，向列相至各向同性液体的转变温度T_ni=359K．得到了液晶态有序参数的温度关系．讨论了氢核磁共振的自旋晶格弛豫机制． 关键词：     

Study of dipole dynamics and pre-transitional effects at isotropic to nematic phase transition by low-frequency dielectric relaxation measurements

Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperatureT _IN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5–10?MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequencyf _R is found to follow an Arrhenius shift, with an activation energy of 1.7?eV. Temperature variation of the dielectric strength (Δε = ε _o ? ε_∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent α_eff = 0.072 indicates weak growth of transitional fluctuations across the IN transition.     

Features of the magnetic and dielectric behavior of mesophases of chromium (III) complexes with azacyclic ligands

Two paramagnetic (1, 2) chromium metallomesogens were studied by the EPR, magnetic-susceptibility, and dielectric-spectroscopy methods. These mesogens possess columnar liquid-crystal phase polymorphism. For mesogen 1 in the columnar Col_xd phase, an irreversible structural phase transition was detected by the EPR method. An anomalous nonlinear temperature dependence of the resonance fields of the EPR line positions and of the fine-structure parameter D in the mesophases of compound 1 was observed. It was shown that this anomaly is associated with the soft mode of crystal lattice. The dielectric spectroscopy data provide support for the existence of the soft mode, which is probably due to the transition of the system from a paraelectric to a dipole-ordered state. The structure of the hexagonal columnar plastic phase of mesogen 2 is formed by a single type of paramagnetic monomer chromium center and does not exhibit any specific features.     

Enhancement of Nematic Order and Global Phase Diagram of a Lattice Model for Coupled Nematic Systems

We use an infinite-range Maier–Saupe model, with two sets of local quadrupolar variables and restricted orientations, to investigate the global phase diagram of a coupled system of two nematic subsystems. The free energy and the equations of state are exactly calculated by standard techniques of statistical mechanics. The nematic–isotropic transition temperature of system A increases with both the interaction energy among mesogens of system B, and the two-subsystem coupling J. This enhancement of the nematic phase is manifested in a global phase diagram in terms of the interaction parameters and the temperature T. We make some comments on the connections of these results with experimental findings for a system of diluted ferroelectric nanoparticles embedded in a nematic liquid-crystalline environment.     

On the nature of spin-lattice relaxation in nematic MBBA

The measurements of the frequency dependence of the proton T₁ in nematic MBBA have been extended to 270 MHz, and the results have been compared with the predictions of the order fluctuation and self-diffusion relaxation theories. Both the frequency and the temperature dependences of T₁ are shown to be characteristic of a mechanism controlled by molecular self-diffusion.     

Dielectric and orientational properties of nematogenic 4-<Emphasis Type="Italic">n</Emphasis>-octyl(4′-cyanophenyl)benzoate

Experimental studies of the dielectric properties in the nematic and isotropic phases of 4-cyanophenyl-4′-n-octylbenzoate (8CPB) have been performed in the frequency range from 50 kHz to 100 MHz. The relaxation process related to the rotation around the short molecular axis has been analysed. The results obtained have revealed that in the vicinity of the I-N phase transition, the molecular subdiffusional rotation process occurs. It is connected with the pretransitional effects which are observed both in the static and dynamic dielectric measurements. On the basis of the temperature dependence of the relaxation time in the nematic and isotropic phases, the orientational order parameter 〈P₂〉 has been determined. The values of 〈P₂〉 obtained in this way have been compared with those evaluated from the measurements of the polarized electronic absorption of the dichroic dye, dissolved as guest probe in the mesogenic host.     

Synthesis,characterization and mesomorphic properties of new liquid-crystalline compounds involving ester–chalcone linkages

We report the synthesis and evaluation of thermal behaviour of two novel series of chalcone-based liquid-crystalline compounds. The flexibility in these systems is provided by attaching straight alkoxy chain at one end. All the mesogens have been characterized by spectroscopic methods such as ¹H-NMR, FTIR, mass spectra, UV/Vis, and also elemental analysis. Their thermal behaviour was evaluated mainly by polarizing optical microscopic observation and DSC studies. It has been observed that the majority of the compounds display nematic and/or smectic mesophases. In both the series, compounds having C₃ to C₇ tail display only enantiotropic nematic phase, whereas C₈ and C₁₀ homologues exhibit enantiotropic smectic C and nematic phases. The higher homologues with C₁₂, C₁₄ and C₁₆ tails show the smectic C phase only.     

Molecularengineering of discotic nematic liquid crystals

Connecting two columnar phase forming discotic mesogens via a short rigid spacer leads to the formation of a π-conjugated discotic dimer snowing discotic nematic (N _D) phase. Attaching branched-alkyl chains directly to the core in hexaalkynylbenzene resulted in the stabilisation of N_D phase at ambient temperature. Pentalkynylbenzene derivatives possessing a combination of normal- and branched-alkoxy chains display a very broad N_D, phase which is stable well below and above the room temperature.     

Phase separation in nematic and smectic a phases

Until recently no example was known with a miscibility gap separating two non-ordered mesophases (i.e.: nematic, smectic A or C) of the same symmetry. In most cases of binary solutions it was observed that either the phase diagram is of the simple eutectic type with quasi ideal behaviour of the mixtures or the liquid crystalline phases are destabilized at the benefit of the isotropic solution (Figure 1). These properties of easy miscibility among mesogens have been long and widely applied at different stages of the study and application of liquid crystals.     

Determination of the Maier-Saupe strength parameter from dielectric relaxation experiments: a molecular dynamics simulation study

Molecular dynamics simulations have been performed to investigate the rotational motion in the nematic and isotropic phases of a model mesogenic system in which the interactions between the molecules are represented by the Gay-Berne potential. First-rank end-over-end rotational relaxation times, analogous to those measured using dielectric relaxation spectroscopy for real mesogens with a longitudinal electric dipole, have been determined as a function of temperature and density. The relaxation times at temperatures throughout the nematic region are found to be larger than the values extrapolated from the isotropic phase to the same temperature. The simulation results are compared with the extended Debye theory for dielectric relaxation in the nematic phase. This relates the reduction in the relaxation rate to the retardation factor which depends on the Maier-Saupe strength parameter, and in turn is defined uniquely by the second-rank orientational order parameter. The simulations indicate that the retardation factor at constant strength parameter is density dependent, a feature neglected in the relaxation theory. We compare the simulation results where possible with experiment.     

Design, synthesis and characterization of a linear hydrogen bonded homologous series

A linear hydrogen bonded liquid crystalline homologous series has been synthesized and characterized. Hydrogen bond is formed between p-n-dodecyloxy benzoic acid and various p-n-alkyl benzoic acids whose alkyl chain vary from octyl to ethyl. Synthesized complexes are characterized by FTIR, ¹H NMR and ¹³C NMR studies for inferring the formation of hydrogen bonds. Polarizing Optical Microscopy (POM) and DSC studies reveal various mesophases and their corresponding transition temperatures along with respective enthalpy values. All the seven synthesized complexes exhibit rich liquid crystalline mesomorphism. A new phase namely smectic X has been observed in five of the complexes with a narrow thermal range. This phase has been characterized by optical textural, DSC, tilt angle and helicoidal pitch studies. Smectic X is sandwiched between traditional smectic C and re-entrant smectic C (designated as C_R) phases. Homeotropic transition in nematic phase is observed in all the mesogens and thus these materials can be used as thermally controlled optical shutters. Tilt angle in smectic C, smectic X and smectic C_R phases have been experimentally elucidated for all the mesogens.     

NMR investigations of molecular motions inp-n-hexyloxybenzylidene-p′-n-propylaniline

Proton spin-lattice relaxation times,T ₁, have been measured in the smectic phases, S _G ² , S _G ¹ and S_A, and in the nematic phase of HxBPA, in the temperature range, 220K<T<360 K. In the S _G ¹ and S _G ² phases,T ₁ has been measured at 15 and 40 MHz. The (S _G ¹ →S _G ² ) and (S _G ² →S _G ¹ ) transitions are clearly seen as discontinuities inT ₁. The former transition is seen to be due to possible freezing or change of hydrocarbon chain motions of the molecule. Our data in the S _G ¹ phase have been fitted to a model in which anisotropic rotational diffusion of the end hydrocarbon chains as also that of the rigid part of the molecule are considered. In the nematic phase, at 351 K, the observed behaviour ofT ₁, measured in the frequency range, 5 to 40 MHz, agrees well with the theoretical predictions of Uklejaet al and Freed, who take into account long range collective order fluctuations and local reorientations. Using these theories, the correlation time for short range reorientations has been calculated from our results to be 4.3 × 10⁻¹⁰ and 1.1 × 10⁻⁹ s respectively.     

Proton magnetic relaxation studies of molecular diffusion in the liquid crystal 40.5

Nuclear magnetic relaxation spectroscopy is used to study the molecular dynamics in a liquid crystal butoxy benzylidene pentylaniline (40.5) in the frequency range 4 to 30 MHz and the results are compared with two other members of the same homologous series (viz 40.8 and 40.6). Spin lattice relaxation time studies indicate that molecular self diffusion (SD) and reorientation processes (R) dominate the relaxation process and their relative contributions are quantified. This contrasts with the case where order director fluctuations (ODF) effectively mediate relaxation process and all the three processes are found to be important in 40.6 in a similar frequency range.T _1D in 40.5 in the nematic phase shows temperature dependence indicating that ODF that is present at low frequencies might be diffusion assisted. These relaxation data are analysed in theS _B phase of this compound also to obtain contributions to the relaxation process. These results are also analysed to obtain different parameters associated with the above dynamical processes.     

Phase transition behavior of hydrogen bonded liquid crystal (6BA)<Subscript>2</Subscript>-(BPy)<Subscript><Emphasis Type="Bold">x</Emphasis></Subscript> as studied by <Superscript>2</Superscript>H NMR

The thermal properties and the orientational order of hydrogen-bonded liquid crystals (6BA)₂-(BPy)_0.4 and (6BA)₂-(BPy)_0.3 (6BA: 4-n-hexylbenzoicacid, BPy: 4,4’-bipyridine) were investigated by DSC and ²H NMR. On cooling, isotropic liquid - liquid crystal phase transition temperatures were T _C= 409 and 405 K for (6BA)₂-(BPy)_0.4 and (6BA)₂-(BPy)_0.3, respectively. Thermal anomalies in the liquid crystal phase were observed at T _LC1= 402 and T _LC2= 375 K for (6BA)₂-(BPy)_0.4 and at T _LC1= 398 and T _LC2= 375 K for (6BA)₂-(BPy)_0.3. For (6BA)₂-(BPy)_0.4, only the smectic component was created above T _LC1. In addition, the nematic component was created below T _LC1. The nematic component gradually changed to the smectic component with decreasing temperature and only the smectic component was observed below T _LC2. For (6BA)₂-(BPy)_0.3, only the nematic component was created above T _LC2. The phase transition from the nematic phase to the smectic phase took place at around T _LC2.     

Influence of the nematic order on the rheology and conformation of stretched comb-like liquid crystalline polymers

We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose, two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase, the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t _r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine: is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t _r ^1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t _r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly, the relaxation is the same at all scales of the whole polymer chain. Received 5 May 1999 and Received in final form 18 October 1999     

Dielectric investigations in the nematic and hexatic smectic B phases of 4-hexyl-4′-[2-(4-isothiocyanatophenyl)ethyl]-1,1′-biphenyl

Temperature-dependent dielectric characterization of 4-hexyl-4′-[2-(4-isothiocyanatophenyl)ethyl]-1,1′-biphenyl (HIEB), which exhibits smectic B and nematic phases, has been carried out over the frequency range of 1 Hz to 10 MHz for homeotropic and planar alignment of sample molecules. This compound shows positive dielectric anisotropy (Δε = ε _∥ ? ε _⊥ > 0) in the nematic (N) phase. One mode of dielectric relaxation showing Arrhenius behaviour has been detected in the hexatic smectic B (SmB) phase. Various electrical parameters, namely, dielectric anisotropy, relaxation frequency and activation energy, have been determined in the N and SmB phases.     

Deuteron NMR study of dynamics and order in the columnar phase of a fluorinated discotic liquid crystal

Measurements have been made of deuteron quadrupolar splitting and spin—lattice relaxation times T _1Q and T _1z in the columnar phase of a ring deuterated 1-fluoro-2,3,6,7,10,11-hexahex-yloxytriphenylene (F-HAT6) as a function of temperature and at two different Larmor frequencies. The disc-like molecules are stacked, with only short range positional order, into columns which are arranged on a 2-dimensional hexagonal lattice. To describe small step reorientations of these molecules, a rotational diffusion model which uses a space-fixed frame to diagonalize the molecular diffusion tensor is adopted. The principal diffusion constants in this so called ‘anisotropic viscosity model’ Dα and Dβ are for rotations of a molecule around and perpendicular to the columnar axis, respectively. A global target analysis of the spectral densities J ₁(ω₀) and J ₂(2ω₀) at seven temperatures and two Larmor frequencies in a single minimization procedure was carried out. It was found that Dα Dβ, which is consistent with the picture that the motion towards or away from the local director (i.e., the columnar axis) tends to disrupt the packing of molecules within the column. Furthermore, the activation energies Eαβ and Eβ_a for these motions were found to be 67.7 kJmol^?1 and 56.3kJmol^?1. That these values are higher in comparison with those for HAT6 is interesting, and believed to be a result of a strong lateral dipole in F-HAT6.     

分子双轴性与向列型液晶在各向同性相的最低过冷温度

在分子场近似下,研究分子双轴性对向列型液晶在各向同性相最低过冷温度T^*的影响.理论结果表明,通过考虑分子双轴性,各向同性相最低过冷温度T^*与向列相—各向同性相相变温度T_c的比值向实验值靠近.当分子双轴性参数δ取到0.3时,T^*/T_c达到0.9817,已很接近0.99的实验值. 关键词：