The temperature dependence of nematic liquid crystalline polymer melt diffusion

Abstract

Polymer chain diffusion in the nematic mesophase was studied using a model main chain liquid crystalline (LC) polyether based on 2,2′-dimethyl-4,4′-dihydroxyazoxybenzene and mixed alkane spacers. A side chain LC polymethacrylate containing an azobenzene mesogenic group was also investigated. Tracer diffusion coefficients were determined as a function of temperature by an ion-beam depth profiling technique, forward recoil spectrometry. The results confirm that main chain LC polymer chain dynamics are dramatically affected by phase transitions and sample geometry. This behaviour is in marked contrast to the side chain LC polymer which exhibited no phase dependence on the part of the tracer diffusion coefficient.     

Texture dependence of capillary instabilities in nematic liquid crystalline fibres

Static and dynamic linear analyses of axisymmetric capillary instabilities in textured nematic liquid crystalline fibres are performed using the equations of nemato-statics and inviscid nemato-dynamics. Three representative textures, viz. axial, onion, and radial, are analysed to show all possible effects of Frank gradient elasticity on the wavelength selection and growth rate of peristaltic modes driven by surface area reduction. It is found that Frank elasticity may tend to stabilize or destabilize the fibre, depending on the initial fibre texture. Axial textures tend to stabilize the fibre through the director splay-bend distortions driven by surface tilting. Onion textures are destabilized by decreasing azimuthal bend elastic energy caused by surface displacement. Radial textures exhibit a stabilizing tilt mechanism due to bend modes and a destabilizing displacement mechanism due to splay modes, but the former is predicted to be dominant. The static analysis provides good estimates of the instability thresholds while the transient energy balance provides information on the fastest growing modes. The static and dynamic results are compared and shown to be fully consistent. The couplings between splay and/or bend distortions, surface tilting, and surface displacement in nematic fibres are characterized and used to explain the deviations from the classical Rayleigh instability.     

Effects of flexibility on liquid crystalline polymer behavior: The nematic broken rod

A new theory for liquid crystalline polymers is developed, and its behavior in simple shear flow is analyzed. The theory accounts for molecular flexibility by employing a microstructure consisting of two rigid rods linked by a joint with a tunable stiffness. The probability distribution function equation for the orientation of the arms of the broken rod is derived. The adaptation of the smoothed particle hydrodynamics (SPH) technique for obtaining numerical solutions to this theory is detailed. The behavior of the theory at equilibrium is derived analytically and compared with numerical results; the SPH technique is then used to obtain results in flow. It is found that in the limit of a nearly stiff joint, the model gives behavior that is very similar to that of rigid rod polymers, the only difference being a lesser tendency to tumble due to greater variation in the order parameter. For nearly free joints, the shear flow induces interesting dynamics for the transition between states with the arms outstretched and those where they are folded up (so‐called “hairpins” of main‐chain LCPs). © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 281–300, 1999     

Photoluminescent nematic liquid crystalline elastomer actuators

Nematic liquid crystalline elastomer (LCE) actuators possessing both photoluminescent (PL) and stimuli-responsive functions were fabricated and studied. PL-dyes (1-pyrenemethyl acrylate and 4-bromo-2,6-bis-(1?-methyl-benzimidazolyl) pyridine loaded with Eu(III) ion) were synthesised and characterised, and then the dyes were mixed with an acrylate side-on liquid crystalline monomer, a cross-linker and a photo-initiator. Through magnetic field alignment, well-defined LCE micropillar PL actuators were fabricated from the mixed samples by a method combining soft lithography and photo-polymerisation/photo-cross-linking. Microscopic observations indicated that the LCE micropillars showed reversible thermomechanical deformation at the nematic-to-isotropic transition temperature. During the reversible contraction and extension process, the LCE actuator containing 1-pyrenemethyl moieties showed stable photoluminescence, while for the LCE actuator doped with 4-bromo-2,6-bis-(1-methyl-benzimidazolyl)pyridine/Eu(III) complex, the PL emission was quenched at about 100°C, which was before the pillars contraction occurring at a higher temperature. When cooled down to room temperature, the contracted LCE micropillars recovered their original shape and the initial PL emission state. The micron-sized LCE actuators can be used for thermomechanical devices and machines with different PL functions at the same time.     

Molecular dimensions and melt rheology of an all‐aromatic liquid crystalline polymer

The molecular dimensions and melt rheology of a thermotropic all‐aromatic liquid crystalline polyester (TLCP) composed of p‐hydroxy benzoic acid, hydroquinone, terephthalic acid, and 2,4‐naphthalenedicarboxylic acid is examined. The Mark–Houwink exponent (α) of 0.95 is estimated for the TLCP. The persistence length estimated from molecular weight (M) and intrinsic viscosity ([η]) data using the Bohdanecky–Bushin equation is about 95 Å, whereas that estimated from light scattering data is 117 Å. These persistence lengths and the observed α value, both higher than those for flexible polymers, suggest that the present TLCP is a semirigid polymer. The zero shear melt viscosity (η₀) varies with approximately M⁶ for molecular weight M > 3 × 10⁴ g/mol; below this molecular weight, η₀ varies almost linearly with M. Widely different entanglement molecular weights (M_e) are predicted, depending on the method used; the plateau modulus estimates M_e of about 8 × 10⁵ g/mol, whereas the ratio of mean square end‐to‐end distance and molecular weight (〈R²〉₀/M) predicts M_e's either too small (0.33 g/mol) or too large (2.5 × 10⁶ g/mol), depending on the theory used. Although the change in the molecular weight dependency of melt viscosity appears to be associated with the onset of entanglement coupling of the semirigid molecules, its origin needs further investigation. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2378–2389, 2001     

The kinetics of nematic phase size growth during the phase transition in an isotropic melt of liquid crystalline azomethine dimer

Polarizing microscopy was used to study the kinetics of formation and droplet size growth of the ordered (nematic) phase during the phase separation of an azomethine dimer melt at various rates of cooling. The statistical droplet size distribution of the nematic phase during phase separation was described by a model derived in terms of the thermodynamics of irreversible processes. Two kinetic phase separation stages were observed and described by the universal scaling function.     

Phase transitions in mixtures of a side-on-side chain liquid crystalline polymer and low molar mass nematic liquid crystals

Miscibility phase diagrams of mixtures of side-on side chain liquid crystalline polymers (s-SCLCP) and low molar mass liquid crystals (E48 and E44) have been established by means of polarized optical microscopy and light scattering. E48 and E44 are cyanobiphenyl-based eutectic nematic liquid crystal (LC) mixtures with nematic-isotropic transition temperatures of 93 and 105 C, respectively. The phase diagram of the s-SCLCP/E48 system reveals the coexistence of an isotropic nematic region and a single nematic phase in order of descending temperature. The single nematic phase suggests that the pair is miscible in the nematic region. On the other hand, the s-SCLCP/E44 mixture shows liquid liquid and nematic nematic coexistence phases, suggestive of the immiscibility character of the pair. These nematic phase diagrams of the s-SCLCP/E48 and s-SCLCP/E44 have been analysed in the context of the combined Flory-Huggins (FH) free energy for isotropic mixing and the Maier-Saupe (MS) free energy for nematic ordering of the mesogens. This combined FH/MS theory is capable of predicting the observed nematic phase diagrams consisting of liquid liquid, liquid nematic, nematic nematic, and the pure nematic regions. The change of colour accompanying the appearance and disappearance of the inversion walls may be attributed to the temperature dependence of birefringence.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4-n -hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0–29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

Visco-elastic properties of reentrant nematic liquid crystalline mixtures

The visco-elastic properties for binary mixtures of 4- n -hexyloxy-4'-cyanobiphenyl (6OCB) and 4- n -octyloxy-4'-cyanobiphenyl (8OCB) are investigated in detail by a light scattering technique. The mixtures exhibit a reentrant nematic (RN) phase between the smectic A (SmA) and crystal phases in the range 22.0-29.5 wt % of 6OCB. The viscosity and the elastic constant increase with cooling in both the nematic and reentrant nematic phases. It is also found that the mixtures exhibit anomalously large values of viscosity and elastic constant near the phase transition, i.e. a pretransitional phenomenon can be observed. The activation energy for viscosity is smaller in the RN phase than in the ordinary nematic phase appearing at higher temperatures, while the normalized elastic constant (defined as the ratio of the elastic constant to the square of the dielectric anisotropy) is larger. Moreover, the visco-elastic behaviour depends on the specific time during which the sample has been kept in the SmA phase. These results confirm that the SmA phase has a strong influence on the formation of molecular aggregates, which plays an important role in the reentrant phenomenon of these systems.     

Direct decoration of disclinations by solidification-induced band texture for a nematic side chain liquid crystalline polymer

For a nematic polymethacrylate side chain liquid crystalline polymer, g 154 N 298 I (°C), the solidification-induced band texture has been observed aligned along the disclination under a polarizing optical microscope, when the specimen was quenched from 280°C to room temperature. The decoration technique of solidification-induced band texture, which is usually reported for main chain liquid crystalline polymers, was then introduced to reveal the director field pattern along a disclination for this side chain liquid crystalline polymer. It was found by infra-red dichroism measurements that the director orientation is parallel with the direction of the band. On this basis, disclinations with strength s=±1/2 and s=±1 were mapped according to the corresponding pattern of solidification-induced band texture. In addition, two types of inversion wall, loop-like and splay-type walls, were also found to be decorated by the solidification-induced band texture.     

Effects of temperature on the alignment and electrooptical responses of a nematic nanoscale liquid crystalline film

The surface-induced alignment and electrooptical (EO) dynamics of a 50-nm-thick liquid crystalline (4-n-pentyl-4'-cyanobiphenyl; 5CB) film were studied at three temperatures: 25 and 33 degrees C (near the crystalline-nematic and nematic-isotropic transition temperatures, respectively) and 29 degrees C (a median temperature in the stability region of the nematic phase). The ZnSe surfaces that entrap the liquid crystal (LC) film have been polished unidirectionally to produce a grooved surface presenting nanometer-scale corrugations, a structure that induces a planar and homogeneous orientation in the nematic phase. The present work attempts to understand the influences of temperature on the surface-induced alignment and corresponding EO dynamics of the material. Step-scan time-resolved spectroscopy measurements were made to determine the rate constants for the electric-field-induced orientation and thermal relaxation of the 5CB film. The field-driven orientation rates vary sensitively with temperature across a range that spans the stability limits of the nematic phase; the relaxation rates, however, vary very little across this same temperature range. We propose that these differences in LC behavior arise as consequence of the interplay of the temperature dependence of the elastic constants, viscosity, and degree of orientational order of the LC medium. A simple theoretical model provides some understanding of these behaviors.     

The electro-optics of nematic liquid crystals stabilized by a polymer network

Samples of nematics stabilized by a polymer network, which are new composite materials, were prepared. A ZhK-1277 nematic composite and a bisphenyl-A-dimethacrylate monomer were used. Polymerization was conducted via UV radiation. The electro-optic properties, i.e., the dependence of transmittance and the turn-on and turn-off times on the electric voltage and layer thickness, of the resulting material and a pure nematic were studied. The experimental results are explained by the domain structure of the nematic in a polymer network, according to which the liquid crystal in an electro-optic cell is composed of oriented domains separated by thin partitions of the polymer. The size of the domain regions of the liquid crystal is 2 μm.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4'-butylaniline (MBBA) and 4-n-pentyl-4'-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

On the temperature dependence of the order parameter of liquid crystals over a wide nematic range

Abstract

Based on the ideas of Landau-de Gennes theory applied to nematic liquid crystals, several forms for the variation of the order parameter as a function of temperature are investigated over a wide nematic range. These functional forms are used to fit the experimental order parameters, determined through the use of C-13 NMR, for 4-methoxybenzylidene-4′-butylaniline (MBBA) and 4-n-pentyl-4′-cyanobiphenyl (5CB), and the physical significance of the parameters is discussed. A comparison of the results shows that the functional form which fits the experimental data best is similar to the Haller equation, a useful relation which is usually regarded as empirical. In this case, the coefficients resulting from a semi-empirical approach based on the Landau-de Gennes treatment may be thought of as quantifying the importance of the structure and rigidity of the liquid crystal in determining the temperature dependence of the order parameter for that liquid crystal. In the process, we have also examined the pretransitional behaviour in the C-13 NMR chemical shifts of liquid crystals observed within a few tenths of a degree above the nematic to isotropic transition temperature.     

Novel atropoisomeric binaphthyl-containing liquid crystalline copolymers forming chiral nematic phases

Novel liquid crystalline (LC) acrylate side group copolymers, which consist of nematogenic phenyl 4-methoxybenzoate acrylate monomer (A) and novel chiral binaphthyl (BN) methacrylate monomers (MB-n) have been synthesized. The copolymers prepared differ in the spacer lengths of MB-n (n 3,5,11) and in their compositions. The homopolymers of the three new chiral binaphthyl monomers MB-n were also prepared. Copolymers with a low concentration of binaphthyl monomer units (less than 16 mol%) display a cholesteric mesophase. The induced chirality in the polymers is due to atropoisomerism (C2-symmetry) of the molecules. The helical twisting powers (beta), caused by the atropoisomeric units in the synthesized copolymers, were determined, and their temperature dependencies studied. The unusually high negative temperature coefficient of beta observed above the glass transition temperature is explained in terms of conformational changes of the BN molecules in the copolymers.     

The kinetics of cholesteric phase growth from an isotropic melt of liquid crystalline polymer with mesogenic groups in the main chain

The kinetics of growth of cholesteric phase drops in the phase transition (during cooling) of an isotropic melt of a liquid crystalline polymer containing mesogenic groups in the main chain was studied by polarization optical microscopy. The statistical drop size distribution was described in terms of the equilibrium thermodynamics of irreversible processes. An analysis of the time dependences of the mean diameter of drops revealed the existence of two stationary stages of the kinetics of cholesteric phase growth and allowed these stages to be quantitatively described by the universal law of the growth of clusters.     

Antiferroelectric behavior in a liquid crystalline polymer

Electro-optic and polarization reversal measurements were performed on a side-chain polymer exhibiting a chiral smectic phase. Based on the observed electro-optic and current responses, we discuss the possibility of an antiferroelectric structure in the polymer. In order to establish a model for the observed behavior, a detailed comparison with the properties of the low molar mass antiferroelectric substance MHPOBC was made. The birefringence modulation in the chiral smectic polymer, originating from the field-induced antiferroelectric-to-ferroelectric transition, is analyzed for the case of a randomly oriented sample. The result shows that the coincidence of birefringence modulation and polarization current peaks is strong evidence for the existence of antiferroelectric order in the smectic layers. Such coincidence was observed both for MHPOBC and the chiral side-chain polyacrylate.