Thermotropic Liquid Crystalline Research Articles

Thermal Properties of Nylon/Nylon and Thermotropic Liquid Crystalline Polymer/Nylon Blends

Thermal Properties of Nylon/Nylon and Thermotropic Liquid Crystalline Polymer/Nylon Blends

Synthesis and thermotropic liquid-crystalline properties of N-alkylpyridinium bromides substituted with a terphenylene moiety

Molecules containing a terphenylene core, two alkyl chains and a pyridinium ring associated with its bromine counterion were synthesised and their liquid crystalline properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results were compared with those of chemical intermediates, which also develop a liquid crystalline behaviour. Both intermediates and pyridinium salts showed a rich polymorphism at temperatures ranging from around 100 to 200°C and 115 to 220°C, respectively. X-ray results indicate that both intermediates and pyridinium salts develop tilted smectic mesophases with molecules stacked in single and double layers, respectively. The tilt angle of some of these compounds decreases so markedly upon cooling that molecules attain almost an orthogonal position. The stacking of molecules in the smectic layers was explained in terms of the mutual repulsion interactions between the terphenylene core, the alkyl chains and the ionic species (the pyridinium ring associated with its counterion) and it was proposed that the π–π interactions between the long aromatic cores counterbalance the strong forces between the ionic species, leading to a full segregation of these molecular parts in periodic sublayers. A molecular arrangement model is proposed for these salts.

Synthesis and Characterization of Thermotropic Liquid Crystalline Polyesters/Glass Fiber by <i>In Situ</i> Process

Liquid crystalline polyesters containing central biphenylene moiety in the mesogenic unit with ester linkages to the different lengths of flexible spacers were synthesized using melt polycondensation reaction. Glass fiber, as a reinforce, was meltpolymerized with 4-acetoxybenzoic(ABA),4,4＇-diacetoxybiphenyl and terephthalic acid(TA) to make TLCPs with a good mechanical properties. All the obtained compounds were characterized by conventional spectroscopic methods. The thermal behavior of the polymers has been characterized using polarized optical microscopy and differential scanning calorimetry. The effect of monomer structure and mechanical properties has been investigated on the prepared polyesters and is also compared with that of the analogous polyester by other processing mode. Optical microscopy showed a highly threaded liquid crystalline texture to a high birefringent Schlierene texture characteristic of the nematic phase for all polyesters. DSC experiments were also found in accord with mesophase formation. The melting points of polymers having a central 4,4’-biphenyldicarboxylate residue have two broad endotherms. The glass transition temperature values of polyesters are not effect by the glass fiber. According to the SEM result showed that the composite had strong fiber-matrix adhesion. From the mechanical test result, it showed that TLCP are reinforced by rigid fiber further improvement of properties depending on fiber content, fiber fabric construction, fiber type. The mechanical properties of GF/TLCP had been improved greatly as the content of glass fiber was increased,but GF/TLCP mechanical properties do not improve remarkably as glass fiber content more than 30wt%.The effect of copolymerization with added glass fiber in synthesis process is compared with that of the anlogous homopolyesters by other processing mode. Our finding shows that the copolymers exhibit reduced technical process and maintained mechanical properties.

Diffusivity and Solubility of Organic Solutes in Supported Liquid Crystal Membranes

The electro-optical properties of thermotropic liquid crystalline (LC) materials have been the subject of significant research effort due to their well-established applications in display technology; however, relatively little work has been done concerning the transport of dissolved solutes in LC phases, limiting their potential use in applications including chemical separation and sensing. Supported liquid crystal membranes were synthesized by impregnating porous cellulose nitrate (CN) membranes with 4-cyano-4'-octylbiphenyl (8CB) from chloroform solutions under vacuum. The resulting membranes were stable under aqueous conditions. Measurements of the 8CB-CN membrane transport properties were performed at 38 degrees C (nematic) and 44 degrees C (isotropic) for 11 aromatic solutes, including positional isomers, in aqueous solution. Solute diffusivity and solubility was calculated using a time-lag technique based on the study of the transient (unsteady state) and steady-state permeation regimes. The solubility and diffusivity of aqueous aromatic solutes in 8CB LCs depended significantly on intra- and intermolecular hydrogen bonding, and more specifically the number of hydrogen-bonding sites on a solute that are available for interactions with the aqueous and LC phases. In the nematic phase of 8CB, shape specific affinity was observed for para isomers and other rodlike solutes. A decreased activation energy for diffusion was observed at the isotropic to nematic phase transition for o-hydroxybenzoic acid, as expected based on the increased order in the nematic phase. Permeation selectivities for the separation of positional isomers by the 8CB-CN membranes indicated high selectivity only for the hydroxybenzoic acid isomers, due to the propensity of o-hydroxybenzoic acid to form intramolecular hydrogen bonds.

Solvent‐Free Protein Liquids and Liquid Crystals

Molten proteins: Warming a freeze-dried powder of a stoichiometric ferritin–polymer ionic construct produces a solvent-free liquid protein that exhibits viscoelastic and thermotropic liquid-crystalline properties at 30 °C (see polarized-light micrograph), and transforms into a Newtonian fluid above 40 °C.

Synthesis of Thermotropic Liquid Crystalline Polyimides with Siloxane Linkages

Abstract Thermotropic liquid crystalline (LC) semi-aliphatic polyimides have been developed from 3,3′,4,4′-tetracarboxybiphenyl dianhydride (BPDA) and diamines containing siloxane spacer units. These polyimides exhibited high thermal stability and lower transition temperature compared to the corresponding polyimides containing alkyl or oxyethylene spacer units.

Poly(aryleneethynylene)s

Poly(aryleneethynylene)s (PAE) are easily synthesized, chemically stable, and color-responsive towards their surroundings. These superbly chromic polymers are attractive as active transducers in sensors and advanced organic electronic devices. PAEs are generally fluorescent with emission maxima ranging from 420-600 nm, and can be either water- or organo-soluble. PAEs from linear building blocks are rigid rod molecules and display a host of supramolecular arrangements in solution and in the solid state. The poly(para-phenyleneethynylene)s (PPE) are lyotropic or thermotropic smectic liquid crystalline. In the solid state, PPEs display lamellar supramolecular structures that lead to distinct stranded nanoscale morphologies. The spectroscopy and the optical properties of the PAEs and in particular of the PPEs are dominated by their conformation, which is influenced by solid-state packing, solvent, temperature and other factors. The presence of twisted and planar forms and their interconversion leads to attractive structure/property relationships; PAEs are of use as ingredients for advanced supramolecular materials.

Liquid crystalline matrix polymers for aramid ballistic composites

AbstractFive semi‐flexible thermotropic liquid crystalline (LC) polyesters and poly(ester‐amide)s were synthesized and used as matrix resins for Twaron™ aramid‐based ballistic fabrics. The ballistic performance was investigated as a function of the neat resin content. For the most successful liquid crystalline polyester system, the effect of blending with styrene‐ethylene‐butylene‐styrene (SEBS) and polyvinyl butyrate (PVB) rubber was also explored. The best neat resin V50 values were obtained for 20 wt% LC polyester (LCPE)‐Twaron™ composites, that is, 418 m.s−1, whereas SEBS/LCPE and PVB/LCPE modifications resulted in maximum V50 values of 460 and 466 m.s−1, respectively. It was found that the ballistic impact resistance is strongly influenced by the elastic modulus of the resin component and to a lesser extent to the level of adhesion between the resin and fabric. The effect of resin content, resin strength, elongation‐at‐break, and resin toughness on the ballistic impact resistance was found to be small. The best ballistic protection could be obtained when the Young's modulus of the LC resin was in the range of 0.01–1 GPa. This result seems to be in agreement with existing inter‐yarn friction models. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Synthesis and Characterization of Benzylidene and Azobenzene Based Photochromic Liquid-Crystalline Polymers

Two series of novel thermotropic main chain liquid crystalline polymers containing photocrosslinkable chalcone and photoisomerizable azobenzene moieties were synthesized from bis[4-( m-hydroxyalkoxy)benzylidene]cycloalkanones [ m = 6,8,10] with 4,4'-azobenzene dicarbonylchloride. The chemical structure of the monomers and polymers were confirmed by spectral techniques. The mesophase properties were investigated using hot stage optical polarized microscopy and differential scanning calorimetry. All the polymers exhibited liquid crystalline properties. The thermogravimetric analysis data revealed that the polymers were stable up to 220 °C and underwent two-stage decomposition. Photocrosslinking property of the polymers was investigated using UV spectroscopy. The cyclopentanone containing polymers showed faster photocrosslinkability than that of their cyclohexanone counterpart. In addition to the photocrosslinkability of cycloalkanone units, the azo moiety displayed E-Z (trans-cis) isomerization in the polymer backbone.

Supramolecular Ring Banded Prototype Liquid Crystalline Oligo(phenylenevinylene)

We report a new ring banded supramolecular structure in thermotropic liquid crystalline oligo(phenylenevinylene) (OPV) via a melt crystallization process. A series of structurally different OPV molecules were synthesized using tricyclodecanemethanol (TCD) as a bulky pendant unit to trace ring banded morphology. Among all, an OPV molecule with rigid bis-TCD units in the central core and flexible dodecyl chains at the outer phenyl rings (BTCD-BDD-OPV) was found to show ring banded morphologies, which is a first of its kind in pi-conjugated materials. BTCD-BDD-OPV experiences strong aromatic pi-pi interactions in both film and liquid crystalline (LC) frozen stage. The pi-induced aggregation leads to lamellar self-assembly of OPV-mesogens that subsequently undergo helical crystal growth, thereby producing dark and bright ring banded patterns. Variable temperature X-ray diffraction analysis revealed the existence of three peaks at 27.07, 13.97, and 8.90 A corresponding to 001, 002, and 003 fundamental layers, respectively, thus confirming the lamellar self-assembly of OPV-mesogens. Electron microscopic (SEM and TEM) analysis of the LC frozen sample showed images confirming helical microcrystalline assembly and providing direct evidence for the self-organization mechanism. Detailed photophysical experiments such as excitation, emission, and time-resolved fluorescence decay studies indicated that BTCD-BDD-OPV has very strong pi-pi interaction in both film and LC frozen stage, which was found to be main driving force for the formation of supra-ring structure. Upon illumination with light, the OPV chromophores in the LC phase were excited and the color of the samples turned into luminescent green ring bands.

Hierarchical structures of self-assembled helical conjugated polymers—Magnetically forced alignment of liquid crystalline poly( meta-phenylene) derivatives

We synthesized poly( meta-phenylene) derivatives with liquid crystalline (LC) groups consisting of phenylcyclohexyl (PCH) or cyanobiphenyl (CB) moiety in the side chains. Thermotropic LC phases and self-assembled structures of the polymers were investigated through polarized optical microscope (POM), differential scanning calorimeter (DSC), and X-ray diffraction (XRD) measurements. The polymers showed discotic liquid crystallinities based on helical structure of the main chains, where the helical chains are self-assembled to form columns along the helical axis. Macroscopic alignments of the polymers were carried out using a magnetic field of 12 T in liquid crystal phases. XRD measurements of the aligned polymers showed that the columns of the main chains are aligned perpendicular to the magnetic field.

Thermal decomposition behavior and durability evaluation of thermotropic liquid crystalline polymers

The thermal decomposition behavior and degradation characteristics of four different thermotropic liquid crystalline polymers (TLCPs) were studied. The thermal decomposition behavior was determined by means of thermogravimetric analysis (TGA) at different heating rates in nitrogen and air. The order of the thermal stability was as follows: multi-aromatic polyester > hydroxybenzoic acid (HBA)/hydroxynaphthoic acid (HNA) copolyester > HNA/hydroxyl acetaniline (HAA)/terephthalic acid (TA) copolyester > HBA/Poly(ethylene terephthalate) (PET) copolyester. The activation energies of the thermal degradation were calculated by four multiple heating rate methods: Flynn-Wall, Friedman, Kissinger, and Kim-Park. The Flynn-Wall and Kim-Park methods were the most suitable methods to calculate the activation energy. Samples were exposed to an accelerated degradation test (ADT), under fixed conditions of heat (63±3 °C), humidity (30±4%) and Xenon arc radiation (1.10 W/m2), and the changes in surface morphology and color difference with time were determined. The TLCPs decomposed, discolored and cracked upon exposure to ultraviolet radiation.

Structure–property relationship of thermotropic liquid-crystalline vinyl polymers containing no traditional mesogen

A series of non-mesogenic vinyl monomers, 2,5-bis(alkoxycarbonyl)styrene, were synthesized and polymerized via free radical polymerization. The alkoxy groups were systematically varied to investigate the effects of their size and architecture on the thermotropic liquid-crystalline properties of the resultant polymers. Although no traditional mesogen was present in the macromolecular structure, all the polymers revealed stable hexagonal columnar liquid-crystalline phase at the temperatures well above their glass transitions when the molecular weights were high enough, as evidenced by a combinatory analysis of differential scanning calorimetry, polarized light microscopy, and one- and two-dimensional wide-angle X-ray diffraction techniques. For the polymers containing five carbon atoms in the alkoxy terminals, the temperatures of glass transition and mesophase formation decreased and the d-spacing value of mesophase increased as the methyl substituent moved away from the connecting phenyl ring. Increasing the number of methyl group or the size of the substituent had the same effect. In the case of polymers with Y-shaped alkoxy terminals, the larger the side groups, the lower the glass-transition temperature and mesophase formation temperature and the larger the d-spacing value of mesomorphic structure.

Synthesis and characterization of side‐chain cholesteric liquid‐crystalline polysiloxanes containing different space groups

AbstractA series of liquid‐crystalline (LC) polysiloxanes were synthesized by two different cholesteric monomers, cholest‐5‐en‐3‐ol(3β)‐10‐undecenoate and cholesteryloxycarbonylmethyl 4‐allyloxybenzoate. The chemical structures and LC properties of the monomers and polymers were characterized by various experimental techniques, including Fourier transform infrared spectroscopy, 1H‐NMR, elemental analysis, differential scanning calorimetry, and polarized optical microscopy. The specific rotation absolute values increased with increasing rigid spacers between the main chain and the mesogens. All of the polymers exhibited thermotropic LC properties and revealed cholesteric phases with very wide mesophase temperature ranges. With a reduction in the soft‐space groups in the series of polymers, the glass‐transition temperature and the isotropic temperature increased slightly on heating cycles. Reflection spectra of the cholesteric mesophase of the series of polymers showed that the reflected wavelength shifted to short wavelengths with decreasing soft‐space groups in the polymers systems, which suggested that the helical pitch became shorter with increasing rigid‐space groups. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Preparation and characterization of new poly-pyrrole having side chain liquid crystalline moieties

We have synthesized a novel liquid-crystalline (LC)-conducting polymer by introducing LC group into pyrrole monomer and polymerizing it with FeCl 3. The N-substituted pyrrole with LC group (Py-RedII) was prepared by direct reaction of potassium pyrrole salt with 2-[ N-ethyl- N-[4-[(4-nitrophenyl)azo]-phenyl]amino]ethyl-3-chloropropionate (RedII). The polymerization of this monomer was successful, giving a conjugated polymer system with liquid crystalline moieties in moderate yield. The polymer obtained was soluble in organic solvents and had a thermotropic liquid crystallinity with mosaic texture characterized by polarizing optical microscopy. Phase transitions, thermal analysis and rheological studies were also evaluated by means of differential scanning calorimetry (DSC), thermogravimetry analysis (TGA) and scanning electron microscopy (SEM), respectively. Spectral properties of poly (2-[ N-ethyl- N-[4-[(nitrophenyl)azo]phenyl]amino]ethyl- N-pyrrolyl propionate) (PPy-RedII) were characterized by UV, IR, 1H NMR, and 13C NMR spectroscopies. The photoluminescence spectrum of the film showed that maximum photoluminescence peak emission is located at 437 nm, corresponding to the photon energy of 2.5 eV. Electrical conductivity of PPy-RedII was studied by the four-probe method and produced a conductivity of 7.5 × 10 − 4 S cm − 1 .

Polydomain model predictions of molecular orientation in isothermal channel flows of thermotropic liquid crystalline polymers

AbstractWe report process simulations of molecular orientation of liquid crystalline polymers for isothermal channel flows in extrusion. The simulations use a “polydomain” model due to Larson and Doi (Larson and Doi, J. Rheol., 35, 539 (1991)), which is implemented by exploiting a nearly exact analogy with a fiber orientation model that is widely used for analysis of composites processing. Simulation results are compared to experimental data previously obtained using a customized X‐ray capable extrusion die that allows a wide range of channel flow geometries to be explored. Competition between inhomogeneous shear and extension in these kinematically complex flows has a profound effect on the resulting molecular orientation distributions. We find that the Larson–Doi model successfully predicts most aspects of the experimental observations, demonstrating its utility for process modeling. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Rheological, Dynamic Mechanical and Thermal Properties of Thermotropic Liquid Crystalline Polymer/Unsaturated Polyester/Glass Fiber Hybrid Composites

Rheological, Dynamic Mechanical and Thermal Properties of Thermotropic Liquid Crystalline Polymer/Unsaturated Polyester/Glass Fiber Hybrid Composites

Effect of Polyphosphazene on the Properties of Blend of Nylon 6,6/Thermotropic Liquid Crystalline Polymer

This paper demonstrates the effectiveness of polyphosphazene elastomer as compatibilizer on the properties of Nylon 6,6/thermotropic liquid crystalline polymer blends. In situ fibrillation of thermotropic liquid crystalline polymers develops in the shear flow field in presence of thermoplastic matrix. The addition of polyphosphazene into the blend facilitates the structural development of thermotropic liquid crystalline polymer. Polyphosphazene elastomer reacts at the interface resulting in reduction of viscosity, which in turn reduces the droplet size enabling fine dispersion of the Liquid Crystalline Polymer (LCP) fibrils. Thermal stability does not change appreciably, but in case of ternary blend Tg value slightly shifted towards the higher side compared to pure matrix indicating the increase in miscibility in the ternary blends. Morphological study demonstrates the significance of compatibilization in immiscible polymer blends.

Landau–de Gennes theory of biaxial nematics reexamined

Recent experiments report that the long-looked-for thermotropic biaxial nematic phase has been finally detected in some thermotropic liquid crystalline systems. Inspired by these experimental observations, we concentrate on some elementary theoretical issues concerned with the classical sixth-order Landau-de Gennes free energy expansion in terms of the symmetric and traceless tensor order parameter Q alpha beta. In particular, we fully explore the stability of the biaxial nematic phase giving analytical solutions for all distinct classes of the phase diagrams that theory allows. This includes diagrams with triple, critical, and tricritical points and with multiple (reentrant) biaxial and uniaxial phase transitions. A brief comparison with predictions of existing molecular theories is also given.

A novel microstream injection molding method for thermotropic liquid crystalline polymers to promote mechanical isotropy: A matrixing microbeam X-ray study

The effects of flow-altering inserts and mold cavity geometry on the mechanical properties of an injection molded liquid crystalline polymer were studied to produce parts with properties approaching macroscopically isotropic state. By inserting fine metal mesh barriers to the gates of the mold cavities, a large number of highly oriented microstreams are produced. After their creation these highly oriented streams of differing flow vectors intertwine and this texture remains reasonably intact even after substantial shearing and extension history imparted on them during ensuing flow into the cavity. This method is effective in the interior away from the skin regions formed under the shearing flow during injection. The local molecular orientation was determined using a matrixing microbeam WAXS technique that allows precision movement of the sample in the microbeam X-ray. Samples produced with the 1.0mm2 mesh showed large variations in the local symmetry axis with respect to the machine as measured by microbeam X-ray diffraction incrementally from the edge to the core of the parts. In comparison, samples with no mesh insert showed only gradual changes in the tilt angle (angle between local symmetry axis and flow direction). The modulus and tensile strengths of all samples with the 1.0mm2 mesh inserts were found to approach virtual global mechanical isotropy.