Nematic Liquid Crystalline Phase Research Articles

Synthesis of Novel Glass-Forming Liquid Crystals Containing Acrylic Acid Trimer Core Unit and Mesogenic Moiety, and Their Use in Cholesteric Reflection Films

A novel medium molecular mass liquid-crystalline (LC) material to form glassy nematic phase has been developed to provide uniformly aligned LC films. The LC material consists of an acrylic acid trimer as a core structure and a mesogenic moiety. The resulting LC trimer showed a nematic LC phase and vitrified to form monodomain films at room temperature. This glassy LC trimer is miscible with a chiral photochromic dopant, providing monodomain, aligned, defect-free cholesteric reflection films, the color of which is tunable by UV irradiation.

Multiple short time power laws in the orientational relaxation of nematic liquid crystals

Relaxation in the nematic liquid crystalline phase is known to be sensitive to its proximity to both isotropic and smectic phases. Recent transient optical Kerr effect (OKE) studies have revealed, rather surprisingly, two temporal power laws at short to intermediate times and also an apparent absence of the expected exponential decay at longer times. In order to understand this unusual dynamics, we have carried out extensive molecular dynamics simulations of transient OKE and related orientational time correlation functions in a system of prolate ellipsoids (with aspect ratio equal to 3). The simulations find two distinct power laws, with a crossover region, in the decay of the orientational time correlation function at short to intermediate times (in the range of a few picoseconds to a few nanoseconds). In addition, the simulation results fail to recover any long time exponential decay component. The system size dependence of the exponents suggests that the first power law may originate from the local orientational density fluctuations (like in a glassy liquid). The origin of the second power law is less clear and may be related to the long range fluctuations (such as smecticlike density fluctuations)--these fluctuations are expected to involve small free energy barriers. In support of the latter, the evidence of pronounced coupling between orientational and spatial densities at intermediate wave numbers is presented. This coupling is usually small in normal isotropic liquids, but it is large in the present case. In addition to slow collective orientational relaxation, the single particle orientational relaxation is also found to exhibit slow dynamics in the nematic phase in the long time.

Synthesis of ABA Triblock Copolymers via Ring Opening Metathesis Polymerization Using a Bimetallic Initiator: Influence of a Flexible Spacer in the Side Chain Liquid Crystalline Block

A series of monomers with side chain liquid crystals (SCLCs) were synthesized for ring opening metathesis polymerization (ROMP). The liquid crystals (LCs) used were 4-hydroxybenzoic acid 4-methoxyphenyl ester (MPOB-H), which is known to exhibit a nematic liquid crystalline phase, and biphenyl-4-carboxylic acid 4-(1-butoxycarbonylethoxy)phenyl ester (BPP4-H), which is known to exhibit a smectic phase. The side chains differed in spacer length, spacer type, and the nature of the LC. Monomers were polymerized using a bimetallic ROMP initiator [Mo(NAr)(OCMe3)2CH]2C6H4 (where Ar = 2,6-diisopropyl phenyl); both homopolymers and ABA type triblock copolymers, where the B block is the LC functional monomer, and A is methyltetracyclodocene (MTD) were prepared. The polymers gave unimodal peak distributions with polydispersities <1.22. Incorporation of a polyoxyethylene spacer decreased the glass transition temperature of the polymer block to −25 from +20 °C, the Tg when an alkyl spacer was used. Although no distinct LC phase was observed with the polyoxyethylene spacer when MPOB was used, when BPP4 was attached via a polyoxyethylene spacer, a distinct liquid crystalline transition was observed. Polymers with an alkyl spacer exhibited liquid crystalline behavior and good phase segregation on the basis of differential scanning calorimetry and small-angle X-ray scattering studies.

Comparison of the Crystal and Molecular Structures of Three Similar 4-Heptyl-biphenyl Compounds: 4-Heptyl-4′-cyanobiphenyl, 4-Heptyl-3′-cyanobiphenyl, and 4-Heptyl-4′-nitrobiphenyl

The structure of 4-heptyl-4′-cyanobiphenyl (7CB4), 4-heptyl-3′-cyanobiphenyl (7CB3), and 4-heptyl-4′-nitrobiphenyl (7NB4) have been investigated at 22°C in the crystalline phase to gain insight into the influence of packing arrangements by intermolecular interactions and of conformation of similar compounds by varying the terminal part of the molecule. The compound 7CB4 exhibits a nematic liquid-crystalline phase that is not present for the other two compounds. The packing of the three compounds is quite different, although two of them belong to the same space group, P21/a.

Molecular Flexibility as a Factor Affecting the Surface Ordering of Organic Adsorbates on Metal Substrates

The effect of molecular flexibility on the surface ordering of complex organic adsorbates is explored, using alpha,omega-dihexylquaterthiophene (DH4T) and mixed DH4T|tetracene phases on Ag(111) as model systems. The structure of DH4T/Ag(111) interfaces is determined by the flexibility of the hexyl chains at either end of the quaterthiophene backbone: Above 273 K, DH4T forms a nematic liquid crystalline phase with a director close to the [112] direction of the silver substrate. At 273 K, a reversible phase transition to a long-range ordered, point-on-line coincident phase is observed. However, this ordered state is still affected substantially by the flexible nature of DH4T, which materializes in a large number of local structural defects. If traces of DH4T are coevaporated with tetracene, inclusions of a 1:1 stoichiometric DH4T|tetracene phase are found in a tetracene/Ag(111) matrix (alpha-phase). In this mixed phase, the two surface enantiomers of pro-chiral DH4T on one hand and tetracene on the other form a complex stripe structure. The mixed phase shows a higher degree of order than present at the pure DH4T/Ag(111) interface, which also lacks chiral organization. The addition of tetracene molecules as structural templates stabilizes certain conformations of DH4T and thus, by balancing its structural flexibility, allows the surface-induced chirality of DH4T to become a decisive factor in determining the structure of the mixed phase.

Isotropic-to-nematic transition in liquid-crystalline heteropolymers: I. Formalism andmain-chain liquid-crystalline polymers

We formulate a density functional approach for arbitrarily branched liquid-crystalline (LC)heteropolymers consisting of elongated rigid rods coupled through elastic joints. The theoryexactly accounts for the energetic and entropic single-chain effects, whereas the interchainexcluded volume effects are treated within the Onsager approximation. We apply thetheory to finite-length main-chain LC polymers composed of rigid mesogens coupled byflexible spacers of finite length, modelled initially as chains of thin rods. The theory thenallows an easy passage to the wormlike chain limit for the spacers. Employing a bifurcationanalysis we analytically obtain the stability boundaries of the isotropic phasetowards the nematic liquid crystalline phase, as a function of the relative size of themesogens with respect to the spacers and the spacer flexibility. From the sameanalysis we also obtain the distribution of the incipient nematic ordering at thespinodal density as a function of position along the chain, including the end-effects.

Induced Phase Separation in Low-Ionic-Strength Cellulose Nanocrystal Suspensions Containing High-Molecular-Weight Blue Dextrans

Spontaneous entropic phase separation phenomena occur in a wide range of systems containing highly anisotropic colloidal particles. Among these are aqueous suspensions of negatively charged cellulose I nanocrystals produced by sulfuric acid hydrolysis of native cellulose, which phase separate into isotropic and chiral nematic liquid-crystalline phases. Phase separation of an isotropic phase from a completely ordered nanocrystal suspension may be induced by the addition of salts or nonadsorbing macromolecules. In previous work (Edgar, C. D.; Gray, D. G. Macromolecules 2002, 35, 7400-7406), an isotropic phase was found to form over a period of several days when blue dextran (a sulfonated triazine dye, Cibacron blue 3G-A, covalently attached to high-molecular-weight dextran chains) was added to initially ordered suspensions. Here we report work showing that the observed phase separation was associated with the charged dye molecules attached to the dextran. The Cibacron blue 3G-A dye attached to blue dextran was found to induce greater phase separation than free (unbound) dye; at increasing ionic strength, depletion attractions due to the blue dextran increasingly contribute to the phase separation.

Isotropic to Nematic Liquid Crystalline Phase Transition ofF-Actin Varies from Continuous to First Order

We report that the properties of the isotropic to nematic liquid crystalline phase transition of F-actin depend critically on the average filament length. For average filament lengths longer than 2 microm, we confirm previous findings that the phase transition is continuous in both alignment and concentration. For average filament lengths shorter than 2 microm, we show for the first time a first order transition with a clear discontinuity in both alignment and concentration. Tactoidal droplets of coexisting isotropic and nematic phases, differing in concentration by approximately 30%, form over the course of hours and appear to settle into near equilibrium metastable states.

Synthesis and investigation of the effect of nematic phases on the glass‐transition behavior of novel cycloaliphatic liquid‐crystalline poly(ester amide)s

AbstractWe designed and developed novel cycloaliphatic liquid‐crystalline (LC) poly(ester amide)s to investigate the effects of nematic LC phases and hydrogen‐bonding interactions on the glass‐transition behavior. Three series of poly(ester amide)s based on commercially important poly(1,4‐cyclohexanedimethylene terephthalate) were synthesized with two new cycloaliphatic diamines {3,8‐bis(aminomethyl)‐tricyclo [5.2.1.0.(2,6)]decane (tricyclic) and 1,3‐cyclohexane bismethylene amine (monocyclic)} and a linear counterpart (1,6‐hexamethylene diamine). The compositions of the ester/amide units in the copolymers were varied up to 50% by the adjustment of the amounts of the diol and diamine in the feed. The structures of the polymers were confirmed with NMR and Fourier transform infrared, and their inherent viscosities were measured at 30 °C with an Ubbelohde viscometer. Thermal analysis revealed that the poly(ester amide)s having less than 25 mol % amide linkages were thermotropic and LC, and threadlike nematic phases were observed under a polarizing microscope. The introduction of nematic, LC phases drastically affected the glass‐transition temperatures of the copolymers, and a plot of the composition versus the glass‐transition temperature passed through a maximum for lower amide incorporation, regardless of the structural differences of the amide units (cyclic or linear). This nonlinear Flory–Fox trend was correlated to the cooperative effect of the strong alignment of polymer chains in the nematic phases and intermolecular packing induced by the hydrogen bonding in the poly(ester amide)s. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5557–5571, 2006

Structure of a liquid crystalline fluid around a macroparticle: Density functional theory

The structure of a molecular liquid, in both the nematic liquid crystalline and isotropic phases, around a cylindrical macroparticle, is studied using density functional theory. In the nematic phase the structure of the fluid is highly anisotropic with respect to the director, in agreement with results from simulation and phenomenological theories. On going into the isotropic phase the structure becomes rotationally invariant around the macroparticle with an oriented layer at the surface.

Dynamic interactions between nematic point defects in the spinning extrusion duct of spiders

Spider silk fibers have remarkable mechanical properties as a result of an ultraoptimized spinning process. Silk fibers are spun from a lyotropic nematic liquid crystalline anisotropic fluid phase which undergoes significant structural changes throughout the spinning pathway. In the silk extrusion duct, those structural changes are expected to be driven by elastic-mediated interactions between point defects. In this work, the interaction between two point defects of opposite topological charges located on the axis of a cylindrical cavity is studied using a tensor order parameter formalism. Distinct regimes leading to defect annihilation and structural transitions are described in detail. The driving force setting the defects into motion is also examined. The different results suggest that the tensorial approach is primordial in describing the complicated physics of the problem. The phenomenon described is important to the understanding of the process-induced structuring of silk fibers and to defect physics in a more general context.

Mesogenic 4-alkoxy-2-hydroxy-4’-formylazobenzenes

Homologous 4-alkoxy-2-hydroxy-4’-formylazobenzenes (Alk = C3H7, C6H11, C8H17) were synthesized and were shown to produce monotropic nematic liquid crystalline phase. The products were characterized by electron absorption and 1H and 13C NMR spectra. The effect of lateral hydroxy group on their mesomorphic properties was analyzed.

Thermal studies on polythiophene containing mesogenic side chains

Series of thiophenes containing mesogenic side chains at the 3 rd position are synthesized. The thermal transitions and thermal stability of the synthesized monomers and polymers are studied. The polarizing microscopic studies of synthesized monomers showed nematic liquid crystalline phase and these mesophases further confirmed by differential scanning calorimetric study. The obtained melting and the isotropic temperatures decrease linearly with the increase of aliphatic chain length. The transition temperatures determined from DSC analysis, overlap well with the temperatures obtained from optical microscopy studies. Thermal stability of the compounds is analysed using thermogravimetric studies. Thermal stability of monomers and polymers has been determined by calculating IPDT values. Structural influence on thermal degradation patterns of monomers and polymers are also discussed.

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.

Isotropic−Nematic Phase Transition of Single-Walled Carbon Nanotubes in Strong Acids

We present the first quantitative assessment of the maximum amount of nanotubes that can exist in the isotropic phase () of single-walled carbon nanotubes (SWNTs) in Brønsted-Lowry acids. We employ a centrifugation technique in conjunction with UV-vis-nIR spectroscopy to quantify , which is also the critical concentration of the isotropic-nematic transition of SWNTs in strong acids. Centrifugation of biphasic dispersions of SWNTs, that is, acid dispersions consisting of an isotropic phase in equilibrium with an ordered nematic liquid crystalline phase, results in a clear phase separation, where the isotropic phase is supernatant. Dilution of the isotropic phase with a known amount of acid followed by UV-vis-nIR absorbance measurements yields , that is, the maximum concentration of SWNTs that can exist in the isotropic phase in a given acid for a given SWNTs' length distribution. At low SWNT concentration (below 200 ppm) in superacids, light absorbance in the range from 400 to 1400 nm scales linearly with concentration. This Beer's law behavior yields calibration curves for measuring SWNTs' concentration in acids. We find that the critical concentration of the isotropic-nematic transition increases with acid strength in accordance with the previously proposed sidewall protonation mechanism for dispersing SWNTs in acids.

Supramolecular Thermotropic Liquid Crystalline Materials with Nematic Mesophase Based on Methylated Hyperbranched Polyethylenimine and Mesogenic Carboxylic Acid

AbstractSummary: Supramolecular interaction of fully methylated hyperbranched polyethylenimines (PEI) with a mesogen‐based carboxylic acid, 5‐(p‐cyanobiphenoxy)pentanoic acid, results in the formation of supramolecular complexes exhibiting thermotropic liquid crystalline (LC) mesophases. In contrast to the common smectic mesophases of most dendritic LC polymers, nematic LC phases were observed. The complexation of PEI and the mesogen units is due to electrostatic interaction between the carboxylate groups and the ammonium end groups of PEI. LC properties were investigated by a combination of differential scanning calorimetry, polarizing light optical microscopy, and X‐ray diffractometry.Schematic illustration of the supramolecular assembly of CBPA with PEIMe backbone.magnified imageSchematic illustration of the supramolecular assembly of CBPA with PEIMe backbone.

Synthesis and characterization of a series of thermotropic liquid crystalline copolyester nanocomposites

AbstractA series of aromatic thermotropic liquid crystalline copolyester (TLCP) nanocomposites were prepared by the in situ intercalation polymerization of p‐acetoxybenzoic acid (ABA), terephthalic acid (TPA), and diacetoxynaphthalene (DAN) isomers in the presence of the organoclay. The DAN isomers used in this study were 2,3‐ and 2,7‐naphthylene. We examined the variation of the liquid crystallinity, morphology, and thermal properties of the nanocomposites with organoclay content in the range 0–10 wt %. All the polymer nanocomposites were fabricated with a molar ratio of ABA:TPA:DAN = 2:1:1; they were shown to consist of a nematic liquid crystalline phase for low organoclay contents (≤5 wt %), whereas the hybrids with a higher concentration of organoclay (≥10 wt %) were found not to be mesomorphic. By using transmission electron microscopy, the clay layers in the 2,3‐DAN copolyester hybrids were found to be better dispersed in the matrix polymer than those in the 2,7‐DAN copolyester hybrids. The introduction of an organoclay into the matrix polymer was found to improve the thermal properties of the 2,3‐DAN copolyester hybrids. However, the thermal properties of the 2,7‐DAN copolyester hybrids were found to be worse than those of the pure matrix polymer for all organoclay compositions tested. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 387–397, 2006

Preorganization Effect of a Polar Supermolecule on Dielectric Anisotropy in a Nematic Liquid Crystalline Phase

We have prepared novel preorganized compounds in which 4-(4-fluorophenyl)phenyl and 4-(trifluoromethoxy)phenyl are connected via a rigid catechol unit or a flexible spacer and investigated their structural effects on the induced dielectric anisotropy in the mixture with a host material. The nonsymmetric bimesogenic compounds with the flexible spacer were found to be a dopant with negative dielectric anisotropy in host A composed of fluoro-substituted phenyl esters, whereas they were found to be a dopant with large positive dielectric anisotropy in host B (6OCB). The rigid U-shaped compound enhanced Δe in both hosts A and B. We discuss effects of host−guest interaction on conformational change of the guest molecule in the nematic liquid crystalline phase.

Extraordinary Doping Effects of Chiral Helical Linear Tetrapyrrole–Zn(II) Complexes on Chiral Nematic Induction of MBBA Liquid Crystal

Abstract The nematic liquid crystalline phase of N-(4-methoxybenzylidene)-4-n-butylaniline (MBBA) was effectively transformed into the chiral nematic phase upon addition of zinc(II) complexes of the linear tetrapyrrole bearing chiral auxiliaries at the helix terminal.

Temperature-Dependent Raman Spectroscopic Study of the Nematic Liquid Crystal 4-n-Pentyl-4′-Cyanobiphenyl

The spectral behaviors of 4-n-pentyl-4'-cyanobiphenyl (5 CB) have been studied by means of temperature-dependent Raman spectros-copy in the range between --70 and 70 degrees C. The nu(C identical with N) bands in the Raman spectra were found splitting below the transition temperature from the solid to the nematic liquid crystalline phase at approximately 24 degrees C, suggesting the existence of solid crystalline polymorphism. The interfacial structures of 5 CB on metal plate surfaces have been reexamined by surface-enhanced Raman scattering (SERS) at different temperatures. On Ag and Au, the asymmetric shapes of the nu(C identical with N) bands suggest that there should exist different binding schemes for 5 CB on metal surfaces. These asymmetric bands in the nu(C identical with N) stretching region were found to vary in changing temperature.