Nematic Liquid Crystalline Phase Research Articles

Single crystal XRD study of 4,4′-dipyridyl — p-ethoxybenzoic acid and N,N′-dipyridylpiperazine — p-ethoxybenzoic acid co-crystals: Direct evidence of H-bond interaction in the corresponding liquid-crystalline mesogenic phases

Supramolecular H-bonded complexes formed between p-ethoxybenzoic acid and 4,4′-dipyridyl or N,N′-dipyridylpiperazine were obtained as nematic liquid-crystalline phases. The corresponding crystals suitable for single crystal X-ray analysis were isolated and investigated. The stoichiomtery of complex 1 is (4,4′-dipyridyl)-2(p-ethoxybenzoic acid) and that of complex 2 is (N,N′-dipyridylpiperazine)-2(p-ethoxybenzoic acid). The molecular packing observed in the crystal structures investigated indicates that the mesogenic phases of the complexes are induced by H-bond interactions.

Synthesis and thermal properties of side‐chain liquid crystalline poly[1‐({(4‐methoxyazobenzene‐4′‐oxy)alkyl}thio)‐2.3‐epoxypropane

A novel series of side‐chain liquid crystalline copolymers, i.e. poly‐[1‐({(4‐methoxyazobenzene‐4′‐oxy)alkyl}thio)‐2.3‐epoxypropane]s, were synthesised in which the number of methylene units in the alkyl group of the side chain varies from 3 to 6. The structures of compounds obtained were characterised by elemental analysis and 1H NMR spectroscopy. Thermal properties were investigated using polarising optical microscopy, differential scanning calorimetry and X‐ray diffraction analysis. The synthesised thiols and corresponding polyepichlorohydrin‐based copolymers exhibited thermotropic liquid crystalline behaviour and nematic phases with schlieren textures. The thiols have lower transition temperatures than the corresponding copolymers. In addition, the degree of substitution of the copolymers synthesised increased with an increase in number of methylenes in the side chain. The phase transition temperatures increased and the mesomorphic temperature ranges widened with the increasing degree of substitution of the copolymers.

Liquid Crystalline Features in a Polyolefin of Poly(methylene-1,3-cyclopentane)

A liquid crystalline phase has been discovered in a polyolefin of poly(methylene-1,3-cyclopentane) (PMCP) having low molecular weight, which was obtained with cyclization polymerization of 1,5-hexadiene (HD) using a zirconocene catalyst in the presence of chain transfer reagents. PMCP clearly shows fluidity and birefringence above the glass transition temperature under the ambient conditions. Schlieren-like texture, which indicates nematic liquid crystalline phase, has been detected in places. Clear diffraction pattern is observed in wide-angle X-ray diffraction analysis below the isotropization temperature. The isotropization temperature decreases with decreasing of molecular weight of the PMCP. PMCP with the higher molecular weight takes the longer time for liquid crystallization from isotropic phase. Non-Newtonian viscosity region in the range of low share rate, which is a characteristic of liquid crystal, has been detected below the isotropization temperature.

Photoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene) cycloalkanone units

AbstractPhotoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene)cycloalkanone moieties were synthesized and characterized by structural, thermal, mesomorphic, and optical measurements. The bis(benzylidene) cycloalkanone chromophores in the main chain can constitute both as a mesogen and photoreactive center, whereas 1,3,4‐oxadiazole is a well‐known fluorophore. The thermal properties of polymers were found to be inversely proportional not only to the spacer length but also to ring‐size of cycloalkanones. Hot stage polarized optical microscopic investigations displayed enantiotropic nematic liquid crystalline phases and development of grainy to schlieren textures depends on the length of flexible spacer in the polymer backbone which was in accordance with DSC analysis. Both photoisomerization and photodimerization are observed from the absorption spectra and discussed. The fluorescence spectra in solution state at various concentrations showed that the polymers show blue‐emission maxima and the Stokes shifts being 48–49 nm. The energy transfer occurred when increasing the concentration of the solution. The band gap energies calculated from the absorption spectra are in the range of 3.17–3.41 eV. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5760–5775, 2008

Synthesis by ATRP and effects of molecular weight on photomechanical properties of liquid crystalline polymers containing side-chain azobenzene chromophores

The synthesis of a series of azobenzene containing liquid crystalline methacrylic homopolymers, poly(4-ω-methacryloyloxy-hexyloxy-4′-ethoxyazobenzene) [Poly( M6A)], with distinct average chain lengths and low polydispersity has been achieved by Atom Transfer Radical Polymerization (ATRP) in THF solution using allyl 2-bromoisobutyrate as initiator and Cu(I)Br as catalyst. Under the adopted conditions the living centers concentration is found to be constant throughout the polymerization process and well defined chain end-groups are obtained. All the obtained polymeric samples, having average molecular mass ( M ¯ n ) ranging from 3300 to 14000 g/mol, exhibit smectic and nematic liquid-crystalline phases on heating, with transition temperatures strongly dependent on polymerization degree, as characterized by differential scanning calorimetry and polarized optical microscopy. The photomechanical effects (i.e. the dependence of volume and density) exhibited upon trans-to- cis and cis-to- trans photoisomerization of the azobenzene mesogenic groups have been investigated by ellipsometry and related to molecular weight, with particular attention to important parameters for potential applications such as the relative variation of total volume, response time, stability and reproducibility.

On the Stability of the Nematic Order Observed During the Cold-crystallization of PET

The rotational isomeric state analysis of poly(ethylene terephthalate) (PET) was carried out by using parameters adjusted so as to reproduce the experimental results of 1H NMR and dipole moment measurements on ethylene glycol dibenzoate. The central OC-CO bond of the ethyleneglycol unit prefers to be in the gauche form at lower temperatures, suggesting that the PET chains should contain many kink conformations in the amorphous state. In the following part, the nematic conformation of the same C-C bond was studied by preparing a model compound which is capable of forming an enantiotropic nematic liquid crystalline (LC) phase. IR studies indicate that the trans fraction of the C-C bond increases from ∼15% in the isotropic liquid to ∼50% in the nematic LC state at the phase boundary. For this drastic change in the conformation at the transition, the C-C bond should be required to pay certain entropy penalty. These results are however compatible with the recent observations by SAXS and DPLS that the formation of nematic LC-like domains proceeds prior to the evolution of primary crystal nucleation during the cold-crystallization of PET. The PET chain is known to take the planar all-trans conformation in the crystalline state. Finally a possible mechanism leading to the nematic order in the amorphous PET is presented. Growth of the nematic LC-like domains in the amorphous phase may lead to a spinodal microphase separation, as suggested by Kaji et al.

Molecular Dynamics Study of Flexo-Electric Effect in the Banana-Shaped Gay-Berne System

To study flexo-electric effect of nematic liquid crystalline phase, molecular dynamics simulation of a system composed of banana-shaped molecules consisting of a couple of Gay-Berne particles is carried out. A fan-shaped simulation cell together with an external field is utilized to obtain equilibrium nematic phase with bend deformation. An order of the second axis of molecules is shown to be induced by bend deformation of the principal axis of the order. The flexo-electric coefficient is obtained for some bend angles of constituent molecule, which exhibits systematic temperature dependence.

Liquid crystalline phase of G-tetrad DNA for NMR study of detergent-solubilized proteins.

The liquid crystalline phase consisting of the potassium salt of the dinucleotide d(GpG) is compatible with detergents commonly used for solubilizing membrane proteins, including dodecylphosphocholine, the lysolipid 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine, and small bicelles consisting of dihexanoyl phosphatidylcholine and dimyristoyl phosphatidylcholine. The chiral nematic liquid crystalline phase of d(GpG) consists of long columns of stacked G-tetrad structures and carry a net negative charge. For water-soluble systems, the protein alignment induced by d(GpG) is very similar to that observed for liquid crystalline Pf1 bacteriophage, but of opposite sign. Alignment of the detergent-solubilized fusion domain of hemagglutinin is demonstrated to be homogeneous and stable, resulting in high quality NMR spectra suitable for the measurement of residual dipolar couplings.

Rheology of carbon nanofiber-reinforced polypropylene

The rheological properties of two different nanocomposite systems consisting in the dispersion of carbon nanofibers (CNFs) in polypropylene are investigated. The nanoreinforced systems were identically prepared with two CNFs that differ only in the length of the fibers being otherwise identical to analyze the effect of fiber aspect ratio. Linear dynamic viscoelasticity and the steady-state rheology of the two different nanocomposites are presented. The system reinforced with CNFs with larger aspect ratio shows several rheological features that resemble peculiarities of rodlike polymers in the nematic liquid crystalline phase.

Static dielectric pretransitional effects in thermotropic liquid crystals

AbstractThe paper presents a review of recent results on static dielectric behaviour in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices. Temperature behaviour of static permittivity and its derivative are interpreted in terms of a basic thermodynamic quantities increment (internal energy, entropy and Helmholtz free energy) induced by a probing electric field.

Cover Picture: Liquid Crystalline Peptide Nanowires (Adv. Mater. 22/2007)

AbstractLiquid‐crystalline peptide nanowires consisting of aromatic dipeptides are easily prepared by a sonication in a volatile organic solvent, report Hyotcherl Ihee, Sang Ouk Kim, and co‐workers on p. 3924. The colloidal nematic liquid crystalline phase of the rigid nanowires allows for a macroscopically ordered morphology of the nanowires under an external electric field. The cover image presents a broad field view of nanowires oriented by an electric field between two electrodes.

Liquid Crystalline Peptide Nanowires

The synthesis and liquid crystalline behavior of peptide nanowire is presented. An aromatic dipeptide rapidly assembles into robust one-dimensional nanowires having a large aspect ratio in a volatile organic solvent. The stable dispersion of rigid nanowires shows a colloidal nematic liquid crystalline phase over a broad concentration, which is particularly advantageous for fabricating well-aligned nanowire morphologies under an external field.

Preparation and characterization of side-chain liquid crystalline polymers containing chenodeoxycholic acid residue

A series of new side-chain liquid crystalline polymers containing chenodiol residue derived from 24-[4′-hydroxybiphenyl-4-yl-4-(allyloxy)benzoyloxy]-3α,7α-di{n-[4′-(4-ethoxybenzoyloxy)biphenyl-4-yloxy]-n-oxoalkanoyloxy}-5β-cholane was designed and prepared. The chemical structures of the monomer and polymer were confirmed by Fourier transform infrared and 1H NMR spectra. The mesomorphic properties of monomer and polymer were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction. The side-chain liquid crystalline polymers revealed wide mesophase temperature range and high thermal stability, and they showed nematic liquid crystalline phase. The influence of flexible space group length on thermal properties and specific rotation was examined.

Synthesis by ATRP and Properties of Liquid Crystalline Three-Armed Star Polymers Containing Azobenzene Chromophores

Atom Transfer Radical Polymerization (ATRP) has been applied to synthesize liquid crystalline methacrylic polymers having three-arms star structure, starting from a central core of C3 symmetry. By just changing the duration of the polymerization process, macromolecules having distinct average chain lengths with low polydispersity values and well defined end-groups have been obtained. All the obtained star-shaped polymers, characterized by differential scanning calorimetry and polarized optical microscopy, exhibit smectic and nematic liquid-crystalline phases on heating, with transition temperatures strongly dependent on the average polymerization degree, and high clearing points. The comparison of these systems with the analogous linear derivatives pointed out higher glass transition temperature values, although they are characterized by lower average polymerization degree. Such behavior can be attributed to the stiffness of the aromatic core, which limits the free movement of chain ends.

Stability of Nematic and Smectic Phases in Rod-Like Mesogens with Orientation−Dependent Attractive Interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.

Long-Lived Giant Number Fluctuations in a Swarming Granular Nematic

Coherently moving flocks of birds, beasts, or bacteria are examples of living matter with spontaneous orientational order. How do these systems differ from thermal equilibrium systems with such liquid crystalline order? Working with a fluidized monolayer of macroscopic rods in the nematic liquid crystalline phase, we find giant number fluctuations consistent with a standard deviation growing linearly with the mean, in contrast to any situation where the central limit theorem applies. These fluctuations are long-lived, decaying only as a logarithmic function of time. This shows that flocking, coherent motion, and large-scale inhomogeneity can appear in a system in which particles do not communicate except by contact.

Lyotropic liquid crystalline poly(aryl ether ketone) copolymer containing phthalazinone moiety and biphenyl mesogen

A novel lyotropic liquid crystalline material poly(aryl ether ketone) copolymer containing phthalazinone moiety and biphenyl mesogen named P-8515 was developed by a mild solution polycondensation method. The molecular weight ( M n) was 53,000 and the value of molecular weight distribution index (MDI) was 2.49 detected by GPC. The critical concentration ( C ∗) of P-8515 was 36 wt% and P-8515 exhibited characteristic nematic lyotropic liquid crystalline phases in NMP solution at different concentrations and the phase morphology changed to a typical threaded texture when shear forces were induced from PLM observations. The T g value was 238 °C and the value for 5% weight loss temperature was 515 °C in nitrogen from DSC and TGA determinations, respectively.

Synthesis and Physical Properties of Ferrocene Derivatives (XX): Crystal Structure of a Liquid-Crystalline Ferrocene Derivative, 1,1′-bis [9-[4-(4-methoxyphenoxycarbonyl)phenoxy] nonyloxycarbonyl]Ferrocene

Single-crystal X-ray structure analysis has been performed for symmetrical 1,1′-disubstituted ferrocene derivative. This compound exhibits liquid crystallinity as a function of temperature with nematic and smectic liquid-crystalline phases. The X-ray structure analysis revealed that two substituents existed in the same directions, and as such induced the cis conformation (“U” shape) of the molecule. The molecules in the unit cell are arranged in a bookshelf manner. Within each layer, CH-π intermolecular interaction was observed at the substituents between neighboring molecules. It is considered that this interaction plays a predominant role in stabilizing the crystal packing arrangement and the mesomorphism.

Interface states in polyfluorene-based metal–insulator–semiconductor devices

Hybrid metal–insulator–semiconductor structures based on ethyl-hexyl substituted polyfluorene (PF2/6) as the active polymer semiconductor were fabricated on a highly doped p-Si substrate with Al 2 O 3 as the insulating oxide layer. We present detailed frequency-dependent capacitance–voltage ( C – V ) and conductance–voltage characteristics of the semiconductor/insulator interface. PF2/6 undergoes a transition to an ordered crystalline phase upon thermal cycling from its nematic-liquid crystalline phase, confirmed by our atomic force microscope images. Thermal cycling of the PF2/6 films significantly improves the quality of the (PF2/6)/Al 2 O 3 interface, which is identified as a reduced hysteresis in the C – V curve and a decreased interface state density ( D it ) from ∼3.9 × 10 12 eV −1 cm −2 to ∼3.3 × 10 11 eV −1 cm −2 at the flat-band voltage. Interface states give rise to energy levels that are confined to the polymer/insulator interface. A conductance loss peak, observed due to the capture and emission of carriers by the interface states, fits very well with a single time constant model from which the D it values are inferred.

Liquid crystalline semirigid polyesters based on phenylstilbene analogues of 1,3,4‐thiadiazole

Semirigid polyesters composed of phenylstilbene analogues of 1,3,4‐thiadiazole, 2‐(2‐phenylethenyl)‐5‐phenyl‐1,3,4‐thiadiazole (PEPT), linking an octamethylene chain at different disubstituted (3,3′‐, 3,4′‐, 4,3′‐ and 4,4′‐) positions, were prepared from four diphenols of the PEPT and sebacoyl chloride by interfacial polycondensation. The effect of polymer structure on thermotropic liquid crystalline (LC) and optical properties is discussed. Differential scanning calorimetry (DSC) measurements, optical texture observations and powder X‐ray diffraction patterns showed that the polymer linking the octamethylene chain at the 4,4′‐position (4,4′‐PEPT) has a linear structure and forms an enantiotropic nematic LC phase. Polymers linking the octamethylene chains at the 4,3′‐ (4,3′‐PEPT), the 3,4′‐ (3,4′‐PEPT) and the 3,3′‐positions (3,3′‐PEPT) positions have a less linear structure and display monotropic smectic phase or no LC phase. Solution and solid‐state UV‐visible and emission spectra indicated that the polyesters exhibit absorption maxima due to the PEPT moieties and fluoresce blue light, but low or no quantum efficiencies were recognized. The polyesters emitted weak polarized fluorescent light at room temperature.