Orientation order and dynamics of CDCl3 in ethylcellulose/chloroform liquid crystal phases

Abstract

Deuterium quadrupole echo spectroscopy has been used to probe the orientational behaviour and dynamics of CDCl₃ molecules in ethylcellulose/chloroform chiral nematic phases. The ²H spectra of both left-handed and right-handed mesophases, formed by ethylcellulose with degrees of substitution of 2·3 and 3·0 respectively, were examined as functions of temperature and concentration. The observed quadrupolar splittings are relatively large and the magnitudes of the order parameters of the solvent molecules show little correlation with the handedness of the chiral phases. However, the ²H line shapes of these two types of chiral phase exhibit rather different temperature dependences. Spectral simulations show that the observed line shapes and their temperature variations are mainly determined by diffusion of CDCl₃ molecules along the pitch axis. The effects of centrifugation on the ²H spectra are also described.     

Orientation control over bent-core smectic liquid crystal phases

Bent-core smectic liquid crystal (LC) phases (B phases) have been widely studied since their unusual polar and chiral properties were discovered in the 1990s. Relatively few studies have examined the mechanisms by which the orientation of the B phase may be controlled to produce the type of macroscopic domain that is essential for organic semiconductor, optical device and patterning applications. This review is intended to cover recent progress towards controlling the B phases upon layering, including anisotropic treatment methods and topographical confinement methods. Finally, this review closes with a discussion of B smectic phases that have been fabricated for use in certain applications.     

Subdiffusive dynamics of a liquid crystal in the isotropic phase

The isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the beta-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1 ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures Tc are 244+/-6 and 258+/-6 K, respectively, the latter is some 10 K below the corresponding experimental values. The different values of Tc we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these "cages," estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320 K to a few nanoseconds at 275 K.     

Molecular dynamics simulations of side chain liquid crystal polymer molecules in isotropic and liquid-crystalline melts

A detailed molecular dynamics simulation study is described for a polysiloxane side chain liquid crystal polymer (SCLCP). The simulations use a coarse-grained model composed of a combination of isotropic and anisotropic interaction sites. On cooling from a fully isotropic polymer melt, we see spontaneous microphase separation into polymer-rich and mesogen-rich regions. Upon application of a small aligning potential during cooling, the structures that form on microphase separation anneal to produce a smectic-A phase in which the polymer backbone is largely confined between the smectic layers. Several independent quenches from the melt are described that vary in the strength of the aligning potential and the degree of cooling. In each quench, defects were found where the backbone chains hop from one backbone-rich region to the next by tunneling through the mesogenic layers. As expected, the number of such defects is found to depend strongly on the rate of cooling. In the vicinity of such a defect, the smectic-A structure of the mesogen-rich layers is disrupted to give nematiclike ordering. Additionally, several extensive annealing runs of approximately 40 ns duration have been carried out at the point of microphase separation. During annealing the polymer backbone is seen to be slowly excluded from the mesogenic layers and lie perpendicular to the smectic-A director. These observations agree with previous assumptions about the structure of a SCLCP and with interpretations of x-ray diffraction and small angle neutron scattering data. The flexible alkyl spacers, which link the backbone to the mesogens, are found to form sublayers around the backbone layer.     

Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system

Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rhorho(c) (at the temperature T). The relaxation times follow a power-law scaling as the critical density is approached, suggesting that this density-driven glass transition concurs with mode coupling theory.     

Molecular dynamics and rheological properties of concentrated solutions of rodlike polymers in isotropic and liquid crystalline phases

A molecular theory is presented for the dynamics of rodlike polymers in concentrated solutions. The theory describes the rotational motion of rods in both the isotropic phase and the liquid crystalline phase. Combined with the molecular expression of the stress tensor, it also gives a unified rheological constitutive equation which predicts the nonlinear viscoelasticity in both phases. As an illustrative example, the steady-state viscosity η at zero-shear-rate is calculated. The predicted dependence of η on molecular weight and concentration agrees fairly well with experiments.     

Molecular dynamics simulations studies of nanoparticles in an isotropic liquid crystal matrix: Single particle behavior and pairwise interactions

We report results of molecular dynamics simulation studies of the behavior of spherical nanoparticles (NPs) in a dense isotropic nematogen matrix comprised of soft spherocylinders (SSCs). The SSCs exhibit a tendency for frustrated planar anchoring at the NP surface that results in a long-range (compared to the size of the NPs and SSCs) reduction in local orientational ordering and increased fluctuations in local orientational ordering compared to the pure isotropic phase of the SSCs. The potential of mean force between two nanoparticles exhibits a novel long-range repulsive tail separated from short-range molecular packing peaks by a shallow local minimum in free energy. The long-range repulsion is caused by NP-induced ordering fluctuations while the shallow minimum results from increased local ordering within the confinement region in between two NPs. The influence of the NPs on local orientational order in the nematogen matrix and the nematogen-induced interaction between NPs are found to depend strongly on the size of the NPs.     

Determination of solvatochromic solvent polarity parameters for the characterisation of some nematic liquid crystal in anisotropic and isotropic phases

Characterisation of liquid crystals (LCs) as solvents is needed, to obtain the polarity and solvatochromic polarity parameters of these media. Polarity parameters demonstrate the effects of LC media on the photo-physical behaviour of solute molecules in an anisotropic medium. The practical limitations in determining solvent polarity scale parameters for LCs can overcome the overlapping absorption band of LCs and solvent-sensitive standard compounds or their insolubility in LCs. In this work, we report Kamlet–Abboud–Taft polarity functions of some nematic LCs in different temperatures and phases, isotropic and anisotropic, with the solvatochromic method, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-phenolate standard probe. In addition, a new azo and coumarin dye were used as probes to obtain some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced.     

Molecular dynamics of oligofluorenes: a dielectric spectroscopy investigation

The molecular dynamics were investigated in a series of "defect-free" oligofluorenes up to the polymer by dielectric spectroscopy (DS). The method is very sensitive to the presence of keto "defects" that when incorporated on the backbone give rise to poor optical and electronic properties. Two dielectrically active processes were found (beta and alpha process). The latter process (alpha) displays strongly temperature dependent relaxation times and temperature- and molecular weight-dependent spectral broadening associated with intramolecular correlations. The glass temperature (Tg) obeys the Fox-Flory equation and the polymer Tg is obtained by DS at 332 K. The effective dipole moment associated with the alpha process is 0.27 +/- 0.03 D.     

Reentrant isotropic phase in a discotic liquid crystal mixture

We have used X-ray diffraction and optical microscopy to study binary mixtures of 2,3,7,8,12,13-hexa-n-tetradecanoyloxy- and 2,3,7,8,12,13-hexa(4-n-undecyloxybenzoyloxy) truxenes. Our measurements confirm the existence of a reentrant isotropic phase, which is however always preceded by a mixed phase region rather than by a pure columnar phase. The nematic N_D phase displays an anomalously negative thermal expansion coefficient. We speculate on mechanisms for reentrancy in discotic mesogens.     

Evolution equation for a disclination line located between the uniaxial and isotropic phases of a nematic liquid crystal

We derive a supplemental evolution equation for a disclination line located on an interface between the uniaxial and isotropic phases of a nematic liquid crystal. This equation provides an additional kinetic relation accounting for the motion of an interfacial disclination line. In our treatment of the problem we neglect fluid motion. Our approach is based on the notion of configurational forces. To illustrate the role of our additional evolution equation, we consider two simple examples. We also identify an expression for the configurational force exerted by the uniaxial phase at the defect located on the phase interface.     

Nuclear spin relaxation in biaxial liquid crystal phases

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J_mLm'L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the 'biaxial' spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Nuclear spin relaxation in biaxial liquid crystal phases

Abstract

The orientation-dependent spin-lattice relaxation rates for biaxial liquid crystal phases are given explicitly in terms of spectral densities J _mLm′L (ω) described by Berggren et al. (1993, J. chem. Phys., 99, 6180). It is recognized that the ‘biaxial’ spectral densities are not observed in biaxial phases unless the director is oriented away from the external magnetic field.     

Orientation of liquid crystal molecules in polydimethylsiloxane micro-channels

The orientation of liquid crystal (LC) molecules confined in polydimethylsiloxane micro-channels, which are produced using mould-replica technique, with a rectangular cross section, is studied. The rod-like molecules of the LC adopt the vertical anchoring at every surface of the channels. The macroscopic alignment of the confined LC can be changed by altering the aspect ratio of the channels. The change in the macroscopic alignment of the LC is attributed directly to the restructuring of the proportion of the number of molecules anchored to the respective surfaces due to the changes in specific area of the bounding surface of the channels. Solutions for the elastic energy function obtained using conformal mapping method were used to predict director profile of the LC in the confinement.     

Configurational temperature and local properties of the anisotropic Gay-Berne liquid crystal model: applications to the isotropic liquid/vapor interface and isotropic/nematic transition

Molecular simulations in the isothermal statistical ensembles require that the macroscopic thermal and mechanical equilibriums are respected and that the local values of these properties are constant at every point in the system. The thermal equilibrium in Monte Carlo simulations can be checked through the calculation of the configurational temperature, k(B)T(conf)=<|?(r)U(r(N))|(2)>/, where ?(r) is the nabla operator of position vector r. As far as we know, T(conf) was never calculated with the anisotropic Gay-Berne potential, whereas the calculation of T(conf) is much more widespread with more common potentials (Lennard Jones, electrostatic, ...). We establish here an operational expression of the macroscopic and local configurational temperatures, and we investigate locally the isotropic liquid phase, the liquid?/?vapor interface, and the isotropic-nematic transition by Monte Carlo simulations.     

Dynamics of a discotic liquid crystal in the isotropic phase

Optically heterodyne-detected optical Kerr effect (OHD-OKE) experiments are conducted to study the orientational dynamics of a discotic liquid crystal 2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (HPT) in the isotropic phase near the columnar-isotropic (C-I) phase transition. The OHD-OKE signal of HPT is characterized by an intermediate power law t(-0.76+/-0.02) at short times (a few picoseconds), a von Schweidler power law t(-0.26+/-0.01) at intermediate times (hundreds of picoseconds), and an exponential decay at long times (tens of nanoseconds). The exponential decay has Arrhenius temperature dependence. The functional form of the total time dependent decay is identical to the one observed previously for a large number of molecular supercooled liquids. The mode coupling theory schematic model based on the Sjogren [Phys. Rev. A 33, 1254 (1986)] model is able to reproduce the HPT data over a wide range of times from <1 ps to tens of nanoseconds. The studies indicate that the HPT C-I phase transition is a strong first order transition, and the dynamics in the isotropic phase display a complex time dependent profile that is common to other molecular liquids that lack mesoscopic structure.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

Abstract

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4′-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral–racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic–smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.     

Optical activity and light scattering in the isotropic and smectic A* phases of a highly chiral smectic liquid crystal

The results of optical activity measurements on the smectic A* phase of 1-methylheptyl 4'-[(4-n-tetradecyloxyphenyl)proprioloyloxy]biphenyl-4-carboxylate (14P1M7) and the chiral nematic phase of a chiral-racemic mixture of S-4-(2-methylbutyl)phenyl 4-decyloxybenzoate (CE6) are shown to be extremely similar. This is in full agreement with the proposed model of the A* phase as a twistgrain-boundary (TGB) phase. In addition, new light scattering measurements using circularly polarized light in a back-scattering geometry yield information on the fluctuations in the isotropic phase. Unlike in chiral nematics where only one structural mode is affected, the data show a strong deviation from the normal temperature dependence near the isotropic-smectic A* transition for two structural modes. Possible reasons for this behaviour in highly chiral smectic liquid crystals are discussed.