Nematic Liquid Crystalline Phase Research Articles

Mirror Symmetry Breaking by Chirality Synchronisation in Liquids and Liquid Crystals of Achiral Molecules.

Spontaneous mirror symmetry breaking is an efficient way to obtain homogeneously chiral agents, pharmaceutical ingredients and materials. It is also in the focus of the discussion around the emergence of uniform chirality in biological systems. Tremendous progress has been made by symmetry breaking during crystallisation from supercooled melts or supersaturates solutions and by self-assembly on solid surfaces and in other highly ordered structures. However, recent observations of spontaneous mirror symmetry breaking in liquids and liquid crystals indicate that it is not limited to the well-ordered solid state. Herein, progress in the understanding of a new dynamic mode of symmetry breaking, based on chirality synchronisation of transiently chiral molecules in isotropic liquids and in bicontinuous cubic, columnar, smectic and nematic liquid crystalline phases is discussed. This process leads to spontaneous deracemisation in the liquid state under thermodynamic control, giving rise to long-term stable symmetry-broken fluids, even at high temperatures. These fluids form conglomerates that are capable of extraordinary strong chirality amplification, eventually leading to homochirality and providing a new view on the discussion of emergence of uniform chirality in prebiotic systems.

Synthesis and thermotropic properties of polyazomethines–containing side chain azobenzene moieties

Aromatic polyazomethines–containing azobenzene side groups have been synthesized by solution polycondensation reaction of aromatic dialdehydes bearing flexible spacers between the aromatic rings with aromatic diamines having preformed azobenzene side groups. Thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, and wide angle X-ray diffraction have been used to evaluate their thermal and mesomorphic properties. The target polyazomethines showed nematic liquid crystalline phase, large mesophase range, and good thermal stability.

Synthesis and phase behavior of a polynorbornene‐based molecular brush with dual “jacketing” effects

ABSTRACTIn this study, a series of well‐defined liquid crystalline molecular brushes with dual “jacketing” effects, polynorbornene‐g‐poly{2,5‐bis[(4‐methoxyhenyl)oxycarbonyl] styrene} (PNb‐g‐PMPCS), were synthesized by the “grafting through” method from ring opening metathesis polymerization of α‐norbornenyl‐terminated PMPCS. The rigid PMPCS side chain was synthesized by Cu(I)‐catalyzed atom transfer radical polymerization initiated by N‐[(2‐bromo‐2‐methylpropanoyl)ethyl]‐cis‐5‐norbornene‐exo‐2,3‐dicarboximide. The chemical structures of the molecular brushes were confirmed by 1H NMR and gel permeation chromatography (GPC), and the thermal properties were studied by thermogravimetric analysis (TGA). GPC results reveal that the molecular brushes have relatively narrow polydispersities. TGA results show that the molecular brushes have excellent thermal stabilities. The PMPCS side chains in all the molecular brushes form the columnar nematic liquid crystalline phase, which is a little different from the behavior of linear PMPCS possibly due to the confinement or other effects of the brush architecture which leads to decreased order. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2116–2123

Synthesis of polymerizable liquid crystalline monomers and their side chain liquid crystalline polymers bearing azo-ester linked benzothiazole mesogen

A series of azo-ester linked benzothiazole mesogen containing new polymerizable liquid crystalline (LC) methacrylate monomers M1–M4 having different terminal substituents (H, CH3, OCH3, and OC2H5) on the benzothiazole moiety and their side chain liquid crystalline polymers (SCLCPs) P1–P4 were synthesized and characterized. The chemical structures, thermal stability, and LC phase behaviors of monomers and polymers were examined by the usage of different experimental techniques. All the synthesized monomers and polymers exhibited excellent thermal stability. Monomer M1 (without terminal substitution) displayed both nematic and smectic phases while monomers M2–M4 (CH3, OCH3, and OC2H5 substituted) showed only nematic phase. On the other hand, all the prepared polymers exhibited only nematic LC phase. Polymers P1–P4 showed two strong absorption bands in the range of 261–262 and 376–413 nm whereas polymers P1–P4 exhibited strong fluorescence emission in the range of 522–524 nm. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were found to be −4.87 to −4.80 eV and −2.36 to −2.14 eV, respectively. The synthesized polymers could be potential candidate as fluorescent materials in the polymer light-emitting diode applications.

POSS-Containing Jacketed Polymer: Hybrid Inclusion Complex with Hierarchically Ordered Structures at Sub-10 nm and Angstrom Length Scales

A series of organic–inorganic hybrid polymers, PnPOSS (n = 6, 10, which is the number of carbon atoms in the flexible alkyl spacers), containing a mesogen-jacketed liquid crystalline polymer (MJLCP) as the main chain and a crystalline nanobuilding block, polyhedral oligomeric silsesquioxane (POSS), in the side chains were designed and synthesized for constructing a hybrid inclusion complex. The phase structures and phase transitions of PnPOSS were investigated by differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, and high-resolution transmission electron microscopy. With the competitive self-assemblies of the two covalently connected building blocks, namely MJLCP and POSS moieties, PnPOSS shows various phase structures including a sub-10 nm hexagonal columnar (Colh) or columnar nematic liquid crystalline phase, an angstrom rhombohedral crystalline (KR) phase, and a hierarchically ordered phase with Colh and KR coexisting. This work provides a new approach for the design and synthesis of hybrid inclusion materials.

Cryogenic-temperature electron microscopy direct imaging of carbon nanotubes and graphene solutions in superacids.

Cryogenic electron microscopy (cryo-EM) is a powerful tool for imaging liquid and semiliquid systems. While cryogenic transmission electron microscopy (cryo-TEM) is a standard technique in many fields, cryogenic scanning electron microscopy (cryo-SEM) is still not that widely used and is far less developed. The vast majority of systems under investigation by cryo-EM involve either water or organic components. In this paper, we introduce the use of novel cryo-TEM and cryo-SEM specimen preparation and imaging methodologies, suitable for highly acidic and very reactive systems. Both preserve the native nanostructure in the system, while not harming the expensive equipment or the user. We present examples of direct imaging of single-walled, multiwalled carbon nanotubes and graphene, dissolved in chlorosulfonic acid and oleum. Moreover, we demonstrate the ability of these new cryo-TEM and cryo-SEM methodologies to follow phase transitions in carbon nanotube (CNT)/superacid systems, starting from dilute solutions up to the concentrated nematic liquid-crystalline CNT phases, used as the 'dope' for all-carbon-fibre spinning. Originally developed for direct imaging of CNTs and graphene dissolution and self-assembly in superacids, these methodologies can be implemented for a variety of highly acidic systems, paving a way for a new field of nonaqueous cryogenic electron microscopy.

Chiral power change upon photoisomerization in twisted nematic liquid crystals

In this work, we use the photoisomerization of azobenzenes, a phenanthrospirooxazine, and a fulgide in a twisted nematic liquid crystalline phase to change the chiral twisting power of the system. The changes are probed by the rotatory power of linearly polarized light. Time resolved and steady state experiments are carried out. The chiral change and the photoisomerization process have similar characteristic recovery times and activation energy, thus probing that the change is induced by the modification in the chemical composition of the photochromic dopant system. The amplitude of the light twisting power change correlates with the order change in the liquid crystal (LC) but not with the modification in the absorption characteristics of the system. This indicates that the driving force of the chiral change is the microscopic order modification in the LC phase that affects the helical pitch of the phase.

Spontaneous mirror symmetry breaking in a re-entrant isotropic liquid.

An achiral double-swallow tailed tetracatenar compound forms a re-entrant isotropic liquid between a bicontinuous cubic phase and a nematic liquid crystalline phase. This isotropic liquid was found to represent a conglomerate of two immiscible chiral liquids. The chirality is retained and eventually becomes uniform at the transition to the cubic I432 phase.

A topotactic transition in a liquid crystal compound

The title compound has two crystal phases related by an enantiotropic single-crystal-to-single-crystal transition and a nematic liquid crystalline phase before transition to the isotropic liquid phase.

Liquid crystalline textures and polymer morphologies resulting from electropolymerisation in liquid crystal phases

A small fraction of an acrylate liquid crystalline monomer (≤5%) is mixed into nematic and smectic liquid crystalline phases, and polymerised through the application of a voltage (electropolymerisation).

Synthesis and solid-state properties of thermotropic liquid crystalline polypeptide bearing imidazolium and p-tolyl groups

Polypeptides bearing imidazolium and p-tolyl groups have been synthesized by click chemistry between poly(γ-3-azidopropyl-l-glutamate) (PAPLG) and 3-butyl-1-propargyl imidazolium bromide (BPIB) and 1-methyl-4-prop-2-ynyloxybenzene (MPB). The FTIR results revealed α-helical conformation of all resulting polypeptides in the solid-state. POM and WAXS study revealed that polypeptides with high p-tolyl contents (molar percentage ⩾54%) possessed nematic liquid crystalline phases and hexagonal packing in the solid-state. DSC analysis revealed the solid to liquid crystalline transitions in the temperature range of 35–42°C.

Nematic and lamellar liquid-crystalline phases in suspensions of charged silica-coated gibbsite platelets

Computer simulations, theoretical investigations and experiments carried out over the last 20 years have demonstrated that suspensions of hard plate-like colloids display a rich liquid-crystal phase behaviour including nematic and columnar phases. Recently, it has become clear that charged colloidal platelets display an even richer phase behaviour including smectic A and smectic B phases. Here, we report on the formation of liquid crystals in suspensions of charged silica-coated gibbsite platelets in dimethylformamide in the presence of 1 mM salt and without added salt. A nematic phase is observed in the system with added salt, while in the system without added salt a lamellar (smectic A) liquid-crystalline phase is formed as demonstrated by optical observations and synchrotron small angle X-ray scattering measurements. The observed phases are in contrast with the columnar phase that would be expected if the platelets were interacting through hard-core repulsions. A bifurcation analysis suggests that, on decreasing ionic strength, first the columnar phase is destabilised in favour of nematic order, which in turn is destabilised in favour of lamellar order. Our experimental and theoretical results support the suggestion that the appearance of nematic and lamellar phases is related to the strength and range of the double-layer repulsion.

Liquid crystalline behavior of graphene oxide in the formation and deformation of tough nanocomposite hydrogels.

In this paper, we report the formation and transformation of graphene oxide (GO) liquid crystalline (LC) structures in the synthesis and deformation of tough GO nanocomposite hydrogels. GO aqueous dispersions form a nematic LC phase, while the addition of poly(N-vinylpyrrolidone) (PVP) and acrylamide (AAm), which are capable of forming hydrogen bonding with GO nanosheets, shifts the isotropic/nematic transition to a lower volume fraction of GO and enhances the formation of nematic droplets. During the gelation process, a phase separation of the polymers and GO nanosheets is accompanied by the directional assembly of GO nanosheets, forming large LC tactoids with a radial GO configuration. The shape of the large tactoids evolves from a sphere to a toroid as the tactoids increase in size. Interestingly, during cyclic uniaxial tensile deformation a reversible LC transition is observed in the very tough hydrogels. The isolated birefringent domains and the LC domains in the tactoids in the gels are highly oriented under a high tensile strain.

Nematic order drives macroscopic patterns of graphene oxide in drying drops.

We report on a series of experiments on large-area ordered patterns of graphene oxide on solid substrates deposited from aqueous dispersions by directed drop evaporation. The aqueous dispersion of graphene oxide exhibits phase transitions from isotropic to liquid crystalline nematic phases via a biphasic region with increasing concentration. In the single nematic phase, schlieren textures accompanied by oriented bands are frequent. Drying of drops in each phase results in deposition covering the whole drop base. The dynamic process of drop drying is analyzed based on the weight loss, radius change, and texture change over time. It is found that the radial bands develop in the nematic drops in the vicinity of the receding of the contact line and subsequently transform into birefringent stripes after drying. Study into the structure and morphology of the stripes reveals anisotropic wrinkling of graphene oxide sheets. The nature of stripe orientation is strongly dependent on the local nematic order at the dewetting water front. Various macroscopic patterns with different stripe orientations including radial spokes, spider webs, and parallel stripes have been generated by tuning the nematic order of drops.

Dispersive power and crossover temperature, TCO, of two polar nematic liquid crystals in the visible spectral region

Refractive index studies are carried out on two highly polar liquid crystals: 1. N-(p-n-methoxy benzylidene)-p-amino benzonitrile, PmBAB, 2. N-(p-n-ethoxy benzylidene)-p-amino benzonitrile, PeBAB. The experimental investigations are carried out in the visible region at four different wavelengths, namely, 633, 589, 546 and 436 nm. The two compounds exhibit only the nematic liquid crystalline phase in between the isotropic and crystalline solid. The dispersive power ω is estimated for two consecutive wavelengths for the case of <n>, ne and no for different wavelengths and found to be constant with temperature. Further the temperature gradients of ne and no are estimated, and the crossover temperature is obtained using dno/dT for all the wavelengths.

Liquid crystalline phase formation in suspensions of solid trimyristin nanoparticles.

The presence of liquid crystalline phases in suspensions of solid lipid nanoparticles can increase the risk of their gelling upon administration through fine needles. Here we study the formation of liquid crystalline phases in aqueous suspensions of platelet-like shaped solid lipid nanoparticles. A native lecithin-stabilized trimyristin (20 wt %) suspension was investigated at different dilution levels by small-angle X-ray scattering (SAXS) and visual inspection of their birefringence between two crossed polarizers. For trimyristin concentrations φMMM < 6 wt %, the dispersed platelets are well separated from each other whereas they start to self-assemble into stacked lamellae for 6 wt % ≤ φMMM < 12 wt %. For φMMM ≥ 12 wt %, the SAXS patterns become increasingly anisotropic, which is a signature of an evolving formation of a preferred orientation of the platelets on a microscopic scale. Simultaneously, the suspensions become birefringent, which proves the existence of an anisotropic liquid crystalline phase formed in the still low viscous liquid suspensions. Spatially resolved SAXS scans and polarization microscopy indicate rather small domains in the (sub)micrometer size range in the nematic liquid crystalline phase and the presence of birefringent droplets (tactoids). The observed critical concentrations for the formation of stacks and the liquid crystalline phase are significantly higher as for equivalent suspensions prepared from triglycerides with longer chains. This can be explained with the lower aspect ratio of trimyristin platelets. Special emphasis is put on the isotropic-liquid crystalline phase transition as a function of the ionic strength of the dispersion medium and φMMM. Higher salt concentrations allow shifting of the phase transition to higher trimyristin concentrations. This can be attributed to a partial screening of the repulsive forces between the platelets, which allows higher packing densities within the platelet stacks and of remaining isolated platelets.

Synthesis, characterization, and mechanical properties of a novel terphenyl liquid crystalline epoxy resin

ABSTRACTA novel terphenyl liquid crystalline (LC) epoxy resin was synthesized and characterized by 1H‐NMR, Fourier transform infrared spectroscopy, differential scanning calorimetry (DSC), and polarizing optical microscopy. Depending on the curing temperature, the synthesized resin formed both smectic and nematic LC phases. A time‐temperature‐transformation diagram was constructed to optimize the curing process, which helped in the preparation of LC and isotropic system. The terphenyl epoxy resin obtained exhibited higher acid resistance than a comparable Schiff‐base epoxy resin, and also displayed excellent fracture toughness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41296.

Coumarin-based emissive liquid crystals

Calamitic luminophores are presented based on an elongated aryl-substituted coumarin core. The substitution pattern at terminal and lateral positions were systematically modified. The new compounds exhibit smectic C and/or nematic liquid crystalline phases. The chromophores show fluorescence in the blue spectral region. The benzopyranones are miscible with acrylate-substituted reactive nematic mesogens. Uniaxial orientation of the mixed systems was achieved by surface-assisted alignment on polyimide orientation layers. The film anisotropy was stabilised by subsequent photocrosslinking. The crosslinked oriented films display linear polarised photoluminescence upon isotropic excitation.

Persistence of nematic liquid crystalline phase in a polyfluorene‐based organic semiconductor: A high pressure study

ABSTRACTThe role of high pressure on a low molecular weight nematic liquid crystalline organic semiconductor, ethyl‐hexyl substituted polyfluorene (PF2/6) is investigated using photoluminescence (PL), Raman scattering, and X‐ray scattering studies at pressures from 1 to 8 GPa. The PL and the Raman data under pressure are consistent with each other with no abrupt changes in the pressure coefficients of PL or Raman peaks. The PL energies redshift and broaden, consistent with both enhanced intra‐ and interchain interactions. The Raman peak positions yield pressure coefficients similar to other phenyl based π‐conjugated polymers. The broadening of a doublet peak in the 1135 cm−1 region indicates a more planar backbone conformation with increasing pressure. X‐ray scattering indicates that the torsion angle between adjacent repeats reduces with increasing pressure and reverts back with decompression. The intermolecular structure is weakly ordered (frozen nematic) and essentially maintained with increasing pressure, in contrast to a high molecular weight PF2/6. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1014–1023

Synthesis and characterization of noncentrosymmetric main chain hydrogen bonded macromolecules incorporating an azo-stilbazole acceptor

The syntheses and characterization of three noncentrosymmetric main chain hydrogen bonded macromolecules which incorporate diethyl-[4-(pyridin-4-ylazo)-phenyl]-amine hydrogen bond acceptor units are reported. These macromolecules participate in strong intermolecular hydrogen bonding as demonstrated using attenuated total reflectance (ATR) FTIR. The phase transitions of these macromolecules depend on the identity of the hydrogen bond donor. Mixtures of these azo-stilbazole macromolecules with noncentrosymmetric hydrogen bonded liquid crystalline macromolecules possess nematic liquid crystalline phases when the concentration of the azo-stilbazole macromolecule is ≤ 50 mol%.