Liquid Crystalline Oligomers Research Articles

Rheology of oligomer melts in the nematic and isotropic states.

Oligomers prepared by chain extension of liquid crystalline monomers are thermotropic. The alignment of liquid crystalline oligomers to shear flow via direct ink write printing is an increasingly popular approach to prepare aligned and 3-D printed liquid crystalline elastomers (LCEs). Here, we are concerned with the contribution of order and thermal history on the rheological properties of liquid crystalline. When the oligomers begin in a polydomain nematic state, the transition to an aligned nematic state occurs gradually over a wide range of shear rates. Conversely, when the oligomers begin in an isotropic state they behave as a Newtonian fluid until a critical shear rate is reached, at which point they align in a critical manner. It is shown that by either decreasing liquid crystalline content or increasing temperature, the viscosity of the oligomer melt decreases while this critical shear rate increases. In addition, the normal stress of oligomers is positive over all shear rates but decreases significantly in magnitude with increasing temperature. By combining the analysis of both temperature and liquid crystalline content, it is demonstrated that the temperature relative to the nematic-isotropic transition temperature is key to the oligomers' unique flow behaviors.

Confinement-Induced Fabrication of Liquid Crystalline Polymeric Fibers.

In aqueous media, liquid crystalline droplets typically form spherical shapes in order to minimize surface energy. Recently, non-spherical geometry has been reported using molecular self-assembly of surfactant-stabilized liquid crystalline oligomers, resulting in branched and randomly oriented filamentous networks. In this study, we report a polymerization of liquid crystalline polymeric fibers within a micro-mold. When liquid crystal oligomers are polymerized in freely suspended aqueous media, curvilinear and randomly networked filaments are obtained. When reactive liquid crystalline monomers are oligomerized in a micro-channel, however, highly aligned linear fibers are polymerized. Within a top-down microfabricated mold, a bottom-up molecular assembly was successfully achieved in a controlled manner by micro-confinement, suggesting a unique opportunity for the programming architecture of materials via a hybrid approach.

Liquid crystalline oligomers with a central core unit containing urethane groups

ABSTRACT Here, we show a generic approach to synthesis of oligomers which include modelling LC main chain polymers, LC side chain polymers or LC dendrimers. Four new families of LC oligomers derived from 1,6-hexamethylene diisocyanate and their di- or trimers (uretdione, biuret or isocyanurate) and ω-hydroxyalkylated cyanobiphenyls were prepared. Their mesomorphic and optical properties were investigated by differential scanning calorimetry and polarised optical microscopy. Mesomorphic properties were crucially impacted by the structure of the central core spacer (urethane, uretdione, biuret or isocyanurate). Stabilisation of the LC state of synthesised oligomers was attributed to the H-bonding interactions confirmed by FTIR spectra.

Rheology of liquid crystalline oligomers for 3-D printing of liquid crystalline elastomers.

Liquid crystalline monomers can be oligomerized and subsequently 3-D printed to prepare liquid crystalline elastomers (LCEs) with spatial variation of the nematic director to create soft materials that undergo complex shape change when subject to stimulus. Here, we detail the correlation of alignment in 3-D printed LCE on the shear history of the oligomeric ink. This coupling is evident both in the polymerization of sheared LCE samples as well as steady-state rheological experiments that quantify the time-dependent flow behaviors of these complex fluids. Under a steady shear flow, oligomeric LC inks transition from a nematic state with unaligned (polydomain) orientation to a uniaxially aligned (monodomain) nematic phase over a large range of applied strain. After cessation of shear flow, the oligomeric LC inks return the polydomain orientation over approximately 30 minutes. The alignment of liquid crystalline segments in the LCE (and the associated stimuli-response of the materials) is ultimately correlated to the degree of strain applied to the ink.

Mesogenic Properties of Aromatic Oligoesters Derived from Kink-Structured Monomers

Novel liquid crystalline oligomers were prepared using different compositions of kink-structured aromatic dicarboxylic acids, aromatic diols, and 4-hydroxybenzoic acid via high-temperature polycondensation. The reaction products were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, and polarized optical microscopy. As a result, the samples containing kinked units with phenyl or naphthalene moiety had a broad processing window limited by the melting process and the isotropization, whereas one based on diphenic acid was almost entirely in an amorphous state. The surface properties of the oligomers were evaluated by wetting measurements using a static contact angle analysis.

Fluorescent nematic liquid crystalline oligomers with reversible texture and photoluminescence response to temperature

Fluorescent nematic liquid crystalline oligomers demonstrating thermosensitive liquid crystalline properties and dynamic fluorescence properties.

Continuous spinning aligned liquid crystal elastomer fibers with a 3D printer setup.

Fibrous liquid crystalline elastomers (LCE) are an attractive variant of LCE-based actuators due to their small thickness, leading to faster response times to stimuli, as well as the increased mechanical strength. Fabrication of LCE fibers has been attempted by various research groups using electro-spinning or micro-fluidic techniques, without much success. Here we propose an alternative way to achieve single-step continuous spinning LCE fibers in a more scalable and robust way, based on a liquid-ink 3D printer. We demonstrate this technique in our home-made device by dynamically extruding/stretching liquid crystalline oligomer mixed with photo-reactive cross-linker, to fix the aligned network under UV light after extrusion. The report also describes a protocol for material synthesis and identifies optimal conditions for the stable fiber spinning process. Microns-thick LCE fibers with two different compositions have been successfully spun, and demonstrated enhanced mechanical properties with the inherited thermal-actuation capability. This technique also demonstrates the potential to fine-tune the mechanical properties of fibers to enable further development in fiber-based LCE applications.

Novel nematic and glassy liquid crystalline oligomers as electroluminescent organic semiconductors

ABSTRACT Several oligomers (MWs > 1000 g mol−1) with highly rigid and conjugated aromatic cores, incorporating 1,4-disubstituted phenylene, 2,7-disubstituted fluorene, 1,4-disubstituted thiophene and/or 2,7-disubstituted carbazole rings with lateral and terminal substituents were synthesised as novel organic semiconductors. These novel liquid crystalline oligomers exhibit glass transition temperatures above room temperature, high clearing points and a broad nematic temperature range as well as electroluminescent behaviour in solution-processed devices.

Self-assembly, phase behaviour and ion-conducting property of pyridinium-based ionic liquid-crystalline oligomers

ABSTRACT Series of oligomeric ionic liquid crystals (OILCs) based on various anions including bromide (Br−), tetrafluoroborate (BF4 −), dodecylbenzenesulphonate (A-SO3 −) and (+)-10-camphorsulphonate (D-SO3 −) are synthesised by use of pyridine-containing monomers, terminal bromo-substituted alkanes and anion replacement reagents. The effect of anions on the chemical structure, liquid-crystalline behaviour, and ionic conductivity is investigated. The OILCs show various mesophases including smectic, hexagonal columnar and bicontinuous cubic phase due to the change in anions. These mesophase types are determined by two incompatible segments derived from ionic mesogens and flexible alkyl chains in the microstructure. For the OILCs based on A-SO3 − and D-SO3 − anions, a curvature of the interfaces between the layers arises due to the increased size of one segment, resulting in columnar and cubic mesophases. The ionic conductivity is investigated in the way that a lithium ion solution passing through the oriented OILC films at room temperature. For the OILC films with various anions, ionic conductivities of the measuring system increase a little in the order A-SO3 − > D-SO3 − > BF4 −> Br−. The values based on A-SO3− anion films show nearly twofold increase compared with those based on Br- anions, which can be attributed to the effects of both ion migration and ionic channels in the microstructures.

Transition in nanoscale electrical conductivity in the Langmuir-Blodgett film of a novel liquid crystalline oligomer

We have studied the nanoscale electrical conductivity of a monolayer film of a novel star shaped liquid crystalline molecule, hexatriphenylene substituted anthraquinone (AQD6). The molecule has a central core of electron deficient anthraquinone moiety connected to six electron rich triphenylene moieties by flexible alkyl chains. The monolayer formed at air-water interface was transferred onto the solid substrates by Langmuir-Blodgett (LB) technique and its surface topography was imaged using an atomic force microscope (AFM). The limiting area obtained from the surface pressure-area per molecule isotherm and the topography of the AFM images suggest that the anthraquinone moiety of the AQD6 molecule is organized in face-on configuration on the substrate and the triphenylene moieties are in edge-on configuration extended away from the substrate. We have studied the electrical conductivity of the AQD6 monolayer deposited on gold coated silicon substrate using a current sensing AFM. Analysis of current (I) − voltage (V) characteristics of the metal-monolayer film-metal junction showed a transition from direct tunneling to an injection tunneling. Further, we have estimated the barrier height and the effective mass of electron in the metal-monolayer film-metal junction.

Luminescent fluorinated chiral liquid crystalline oligomers containing Eu(III) complexes

ABSTRACTLuminescent fluorinated chiral liquid crystalline oligomers containing Eu(III) complexes (Eu-LCOs) with good liquid crystalline properties and obvious luminescence properties were prepared using Poly(methylhydrogeno)siloxane (PMHS), chiral liquid crystalline monomer (M1), fluorinated liquid crystalline monomer (M2), fluorinated Eu(III) complex (M3). The chemical structures, liquid crystalline behaviours of Eu-LCOs were characterised by various experimental techniques. The introduction of small quantity of fluorinated Eu(III) complexes endowed the oligomers with excellent luminescence properties. The Eu(III) complexes did not change the liquid crystalline textures of the oligomers. Fourier transform infrared imaging showed that Eu(III) complexes were evenly distributed in oligomers. In order to express the mutual effect and distribution of the components, a structural representation of Eu-LCOs was established. The Eu-LCOs displayed wide mesophase temperature ranges and reversible mesomorphic phase transitions. The Eu-LCOs can emit soft red light when being excited. Luminescence intensities of Eu-LCOs gradually increased with an increase of Eu(III) complexes from 0 to 1.0 mol%. However, the luminescence intensities of Eu-LCOs decreased monotonically with the increase of temperature in liquid crystalline phase.

Perylene-Based Liquid Crystals as Materials for Organic Electronics Applications.

Columnar phases formed by the stacking of disclike molecules with an intimate π-π overlap forms a 1D pathway for the anisotropic charge migration along the columns. Columnar phases have great potential in organic electronic devices to be utilized as active semiconducting layers in comparison to organic single crystals or amorphous polymers in terms of processability, ease of handling, and high charge carrier mobility. Intelligent molecular engineering of perylene and its derivatives provided access to tune the physical properties and self-assembly behavior. The columnar phase formed by perylene derivatives has great potential in the fabrication of organic electronic devices. There are several positions on the perylene molecule, which can be functionalized to tune its self-assembly, as well as optoelectronic properties. Thus, many liquid-crystalline molecules stabilizing the columnar phase, which are based on perylene tetraesters, perylene diester imides, and perylene bisimides, have been synthesized over the years. Their longitudinal and laterally extended derivatives, bay-substituted derivatives exhibiting a columnar phase, are reported. In addition, several liquid-crystalline oligomers and polymers based on perylene derivatives were also reported. All such modifications provide an option to tune the energy levels of frontier molecular orbitals with respect to the work function of the electrodes in devices and also the processability of such materials. In this feature article, we attempt to provide an overview of the molecular design developed to tune the applicable properties and self-assembly of perylene derivatives as well as recent developments related to their application in the fabrication of organic solar cells, organic light-emitting diodes, and organic field-effect transistors.

Liquid crystalline non-linear S-shaped oligomers consisting of azobenzene and biphenylene units: synthesis, characterisation and influence of central spacer

ABSTRACTA series of non-linear S-shaped liquid crystal oligomers wherein the molecule consists of biphenylene moiety as a central core unit and two symmetrical side arms azobenzene moieties joined to catechol as a linkage group have been successfully synthesised and characterised. The members in this series possess different inner spacer with carbon number n ranging from 4 to 9 while the outer spacer length located in azobenzene moieties are remains unaltered. The members with even parity exhibit monotropic phase, whereas homologues with odd parity display enantiotropic phase. The appearance of nematic, smectic A and smectic B phases was validated with texture observation under polarised light and X-ray diffraction (XRD). The XRD study on the S-shaped oligomer indicates that the arrangement of smectic phase exists as an intercalated structure. The S-shaped oligomer shows photoisomerisation properties in solution whereby the trans to cis isomerisation for this molecule is accomplished 1140 s, whereas reverse process under thermal back reaction occurred in 4620 min.

Developments in liquid-crystalline dimers and oligomers

ABSTRACTLiquid-crystalline dimers and bimesogens have attracted much attention due to their propensity to exhibit the spontaneously chiral twist-bend mesophase (NTB), most often by dimers with methylene spacers. Despite their relative ease of synthesis, the number of ether-linked twist-bend materials significantly lags behind those of methylene-linked compounds. In this work, we have prepared and studied a range of ether-linked bimesogens homologous in structure to the FFO9OCB; as with methylene-linked systems, it appears that it is molecular topology and the gross molecular shape that are the primary drivers for the formation of this phase of matter. Dimers and bimesogens are well studied within the context of the twist-bend phase; however, present understanding of this mesophase in oligomeric systems lags far behind. We report our recent efforts to prepare further examples of oligomeric twist-bend nematogens, including further examples of our ‘n+1’ methodology, which may allow the synthesis of high-purity, monodisperse materials of any given length to be prepared. We have observed that there is a tendency for these materials to exhibit highly ordered soft-crystalline mesophases as opposed to the twist-bend phase.

Liquid crystalline oligomers derived from cholesterol: synthesis and columnar mesomorphism

ABSTRACTThe novel columnar liquid crystals based on symmetric hairpin-shaped cholesterol tetramers bridged by rigid or hydrogen-bonding Schiff-base spacers were prepared by reacting cholesterol dimer with phenylenediamines or dihydrazides in yields of 70–85%. Structural characterisation of these new compounds had been achieved by infrared, nuclear magnetic resonance and mass spectrometry. Their mesomorphic properties were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. They exhibited good hexagonal columnar mesophase with three molecules in a disc of the columnar hexagonal state. This study implied that the symmetric cholesterol tetramers with rigid cores or hydrogen-bonding cores prefer to columnar mesophase.

Reduced graphene oxide/liquid crystalline oligomer composites based on reversible covalent chemistry.

As the properties of materials can be determined by their structures, a novel liquid crystalline oligomer (LCO) was first designed and prepared. The LCO possessed reactivity with reduced graphene oxide (RGO) and could be used to modify RGO. Maleimide groups could be grafted onto the RGO surface via the Diels-Alder reaction, whereby the mesogenic units enable the LCO to have optical rotation and excellent liquid crystalline properties, while the carboxyl groups could improve the dispersibility of RGO in solvents and disperse more TiO2 on the surface of modified RGO than on pure RGO. Spectroscopic tools (Raman and XRD) were used to confirm the completion of the reaction. A Fourier transform infrared imaging was utilized to analyze the dispersibility of RGO in the RGO-LCO composites. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were used to determine the thermal properties of the composites. Due to the excellent thermal property of RGO and the interactions between RGO and LCO, the dispersed RGO can increase the decomposition activation energy and the glass transition of composites. RGO can enhance the photocatalytic degradation of TiO2 due to its high electron mobility and large specific surface area. By preventing the aggregation of TiO2 and RGO, thus improving the solubility and dispersion stability of the RGO and increasing the affinity of RGO to TiO2, the LCO-modified RGO was better than pure RGO to enhance the photocatalytic degradation efficiency of TiO2.

A Liquid Crystalline Oligomer Exhibiting Nematic and Twist-Bend Nematic Mesophases.

The twist-bend nematic phase (NTB ) has been described as the structural link between the untilted uniaxial nematic phase (N) and the helical chiral nematic phase (N*). The NTB phase exhibits phenomena of fundamental importance to science, that is, 1) the spontaneous formation of a helical pitch on the nanometer scale in a fluid and 2) the spontaneous breaking of mirror symmetry, leading to the emergence of chiral domains in an achiral system. In this Communication, we present a study on T49 [bis(4-(9-(4-((4-cyanobenzoyl)oxy)phenyl)nonyl)phenyl) 4,4'-(nonane-1,9-diyl)dibenzoate], a liquid-crystalline oligomer exhibiting the twist-bend nematic phase, which has a molecular length that is of comparable dimensions to the sub-10 nm pitch determined for CB9CB, and provide new insights into the differentiation between the nano- and macro-science for self-assembling supermolecular systems.

Covalent modification of reduced graphene oxide by chiral side-chain liquid crystalline oligomer via Diels–Alder reaction

RGO was dispersed in the CSLCO matrix via DA reaction, and the composites have excellent properties.

Star-shaped triphenylene discotic liquid crystalline oligomers and their hydrogen-bonded supramolecular complexes with simple acids

The synthesis and self-assembly properties of four novel star-shaped oligomers containing triphenylene discotic mesogens are reported.

Novel photoluminescent chiral liquid crystalline oligomers containing lanthanide ions

Chiral liquid crystalline oligomers with lanthanide ions distributed in helical matrixes can emit red or green light.