New ferroelectric and antiferroelectric liquid crystals studied by complementary methods

Four new synthesised liquid crystalline compounds belonging to the homologous series of fluorinated biphenyl benzoate esters have been studied to compare their dielectric and electrooptic properties. Three of the studied compounds exhibited ferro- and antiferroelectric phases while one of them exhibited only one liquid crystalline phase – ferroelectric SmC*. No paraelectric phase was detected and straight transition between isotropic liquid and ferroelectric phases was observed for all studied compounds. Tilt angle for all of the studied compounds was equal to ca. 45? in the liquid crystalline phases, except temperature range close to the isotropic liquid–ferroelectric smectic phase transition. Temperature dependences of helical pitch, spontaneous polarisation and switching time have been determined. Based on XRD results, temperature dependence of the layer thickness has also been found. Only one relaxation process has been revealed in the ferroelectric as well as antiferroelectric phases, even the bias field up to 8 V/µm was applied. The dielectric and electrooptic data are discussed based on the mean-field theory predictions.     

Electro-optic and dielectric properties of new binary ferroelectric and antiferroelectric liquid crystalline mixtures

Several new binary liquid crystalline mixtures have been designed and their properties were studied by complementary methods. It has been shown that even both pure components used for the mixture design possess the ferroelectric behaviour; the induced antiferroelectric smectic phase has been detected for one of the prepared mixtures. The phase diagram has been constructed and the existence of the antiferroelectric phase was confirmed by switching time and dielectric spectroscopy measurements. Some of the resulted mixtures possess very high values of the tilt angle that reaches close below 45° degrees at saturation. Values of spontaneous polarisation were found within 50–200 nC/cm² in dependence of the mixture’s composition. Due to specific properties, the obtained mixtures might be interesting for further design of multicomponent mixtures and formulation of the advanced nanocomposite systems.     

Effect of chain length and fluorination on the dielectric and electro-optic properties of three partially fluorinated biphenylyl benzoate rigid core based ferroelectric liquid crystals

Three partially fluorinated ferroelectric liquid crystals (4F4R, 4F5R and 7F3R) with biphenylyl benzoate rigid core have been investigated by frequency-dependent dielectric spectroscopy and electro-optic methods. Molecular structures of the compounds differ only by the length of the carbon chain and the number of oligomethylene spacers. 4F4R and 4F5R exhibit ferroelectric SmC* phase over a considerable temperature range and directly melts into isotropic phase, 4F4R also shows a ferroelectric type subphase. Only Goldstone mode (GM) relaxation is observed in these compounds, but no soft mode (SM). 7F3R exhibits both the SmC* and SmA* phases, has higher stability of SmC*. Both the GM and SM relaxations are observed in this case, signifying that SM relaxation is possible only when SmC* is formed on cooling from SmA*. Increase of dielectric strength and critical frequency with temperature, in all cases, has been explained in the light of generalised Landau model. Spontaneous polarisation is found to decrease with increasing flexibility, and optical tilt depends more on fluorination than on chain length.     

Impact of silica nanoparticles dispersion on the dielectric and electro-optical properties and absorption spectra of host ferroelectric liquid crystal

The present work concerns with the investigation of the effect of dispersion of Silica (SiO₂) nanoparticles (NPs) in host ferroelectric liquid crystal (FLC) KCFLC10S on the dielectric and electro-optical properties and ultraviolet-visible (UV-VIS) absorption spectra of the pristine and dispersed systems. We have found that the dispersion of SiO₂ NPs in the host FLC strongly influences the various properties of dispersed systems. No evidence of aggregates and clumps in the dispersed system has been observed. Due to SiO₂ NPs dispersion, a rapid decrease in dielectric permittivity ε’, increase in conductivity σ with frequency, increase in spontaneous polarisation P_s and decrease in switching time with bias voltage have been observed. Based on the absorption spectra, we have also made an attempt to link the electro-optical and dielectric response with the mechanism of FLC–NPs interactions.     

Critical behaviour of the order parameters at the SmC* to SmA phase transition in a ferroelectric liquid crystal mixture

Critical behaviour of the order parameters has been investigated in the ferroelectric liquid crystal mixture ZLI‐3654 in a 7.5 µm thick planar cell. The temperature dependence of the primary (tilt angle) and secondary (spontaneous polarisation) order parameters is considered. The critical exponent (β) has been evaluated from the fitting of the temperature dependence of the experimental data for both tilt angle and spontaneous polarisation. Experimental results are compared with the predictions of the de Gennes and Landau models.     

Dielectric and electro-optical response of a room temperature tri-component antiferroelectric mixture

Frequency- and temperature-dependent dielectric and switching parameters of a room temperature tri-component antiferroelectric liquid crystal mixture W-287 have been determined. Dielectric, optical texture and thermodynamic studies show wide room temperature range antiferroelectric SmC*_a (?91.1°C to <–25°C) phase in addition to high temperature paraelectric SmA* (?2.6°C) and ferroelectric SmC* (?4.4°C) phases. The dielectric studies carried out in the frequency range of 1–35 MHz under planar anchoring condition of the molecules show five different relaxation modes appearing in the SmA*, SmC* and SmC*_a phases. Using Curie–Weiss law fit, ferroelectric SmC* to paraelectric SmA* transition temperature has been found to be 91.8°C. The dielectric response of SmC*_a phase exhibits unusually three relaxation modes due to collective as well as individual molecular processes in addition to phason mode in the SmC* phase and amplitudon mode in the SmA* phase. Spontaneous polarisation, switching time and rotational viscosity have also been determined. The maximum value of P_S is ?300 nC/cm², whereas viscosity is moderate. Switching time is of the order of few milli seconds.     

Highly tilted antiferroelectric (R) enantiomers useful for the formulation of eutectic mixtures

ABSTRACT

Novel chiral three-ring (R) enantiomers were synthesised using optically active (R)-(?)-2-octanol. Properties, such as the sequence of phases, the transition temperatures and enthalpies, were tested by a polarising optical microscope and differential scanning calorimeter. An antiferroelectric smectic phase (SmC_A*) with a direct transition from the antiferroelectric to the isotropic phase (SmC_A*-Iso) was observed for three esters with an achiral C₃F₇CH₂O(CH₂)₃O– terminal chain. Bi- and multicomponent mixtures with a broad temperature range of the antiferroelectric phase and good electro-optical properties were formulated. Helical pitch of pure esters and mixtures was measured by spectrophotometry method.     

Dielectric properties of four room temperature ferroelectric and antiferroelectric multi-component liquid crystalline mixtures

Dielectric properties of four recently formulated room temperature multi-component liquid crystalline mixtures with paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC*_A) phases have been studied as a function of temperature and frequency. Under planer anchoring condition, dielectric spectroscopy revealed all the characteristic modes: low frequency P_L and high frequency P_H mode in SmC*_A phase, Goldstone mode (GM) in SmC* phase and soft mode (SM) in SmA* phase. Dielectric behaviour has also been studied under the application of DC bias electric field. With bias electric field, we have been able to study the soft mode dielectric behaviour in the SmC* phase. An unknown high frequency mode (X-mode) with and without bias is also observed in SmC* phase. Dielectric results are explained in the light of generalised Landau theory. The mixtures show very high soft mode electroclinic coefficient in the SmA* phase in addition to fast switching in SmC*_A and SmC* phases [30].     

X-ray diffraction and dielectric spectroscopy studies on a partially fluorinated ferroelectric liquid crystal from the family of terphenyl esters

The fluorinated compound, (S)-4′′-(6-perfluoropentanoyoxyhexyl-1-oxy)-2′,3′-difluoro-4-(1-methylheptyloxycarbonyl)-[1,1′:4′,1′′]-terphenyl, which exhibits antiferroelectric SmC_A*, ferroelectric SmC* and paraelectric SmA* phases, has been investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and frequency-dependent dielectric spectroscopy methods. X-ray studies have revealed that the layer thickness remains almost constant in the SmA* phase but within the SmC* and SmC_A* phases it decreases with decreasing temperature, a step jump being observed only at the SmA*–SmC* transition. The tilt angle in the SmC_A* phase decreases from 22.2° to 19.5°, and in the SmC* phase it decreases from 18.8° to 5.5°. Spontaneous polarisation is found to be quite high and varies between 74.1 and 118.7 nC cm^?2. The variation in ε′ and ε′′ with temperature shows a discontinuous change at the transition temperatures. Goldstone mode relaxation is only observed in the ferroelectric and antiferroelectric phases and is found to be of the Cole–Cole type. The soft mode is observed on application of a bias field near the SmC*–SmA* transition. Neither the soft mode nor the anti-phase azimuthal angle fluctuation mode is observed in SmC_A*. Rotational viscosity decreases quite rapidly with temperature but in a different manner in the ferroelectric and antiferroelectric phases. Activation energy for this process is found to be 48.14 kJ mol^?1 in the SmC* phase.     

Effect of lateral methoxy substitution on mesomorphic and structural properties of ferroelectric liquid crystals

A new series of materials with a chiral fragment derived from lactic acid and a methoxy group as lateral substituent in different positions of the molecular core was synthesised and investigated. Derivatives with ester or ether linkages of the non‐chiral chain were also studied. Depending on the molecular structure, cholesteric, twist grain boundary smectic A (TGBA*), chiral smectic A (SmA*) or chiral smectic C (SmC*) phases were detected. In derivatives with the ester linkage and a methoxy group at the nearest and the next nearest phenyl ring to the non‐chiral chain these phases completely disappear. On the other hand, a methoxy group on the phenyl ring close to the chiral chain provides a compound with low layer shrinkage at the SmA*–SmC* phase transition (“de Vries” behaviour). The temperature dependence of the spontaneous polarisation, the tilt angle, the layer spacing as well as the complex permittivity were studied and the results discussed in terms of molecular structure.     

Dielectric and electro-optic properties of two biphenylyl benzoate-based ferroelectric mesogens with tilted hexagonal phases

ABSTRACT

Two partially fluorinated ferroelectric liquid crystals (2F3R, 3F3R) with biphenylyl benzoate core have been investigated by frequency domain dielectric spectroscopy and electro-optic method. Both the compounds exhibit SmC* phase over a wide range of temperature along with tilted hexatic phase SmF* and soft crystal phases SmJ* (in 2F3R) and SmG* (in 3F3R) and one coexistence phase of (SmF*+SmC*). Both BOO phason and tilt phason relaxations are observed in SmF* phase and cooperative relaxation behaviour is also observed in crystal-like smectic phases. Goldstone mode relaxation is observed in SmC* phase and Maxwell Wagner mode is observed in all the phases. In 2F3R soft mode is detected in both SmC* and SmA* phases but not in 3F3R which melts directly to isotropic phase from SmC* phase. Compounds possess moderate values of spontaneous polarisation and exhibit quite sharp electrical response especially in 2F3R. Rotational viscosities, pitch and elastic constant are also measured. Effect of chain length and fluorination on various physical parameters has been discussed.     

PDLC films prepared by electron beam and ultraviolet curing: influence of curing conditions on the electro-optical properties

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation processes using ultraviolet (UV) and electron beam (EB) radiation. A mixture of the nematic LC material E7, an aromatic polyester acrylate, and additional monomeric acrylates was exposed to the EB radiation. A photoinitiator was included in the initial mixture in the case of UV exposure. The electro-optical behaviour of the PDLC films obtained has been investigated as a function of the chosen radiation. The transmission versus voltage curves strongly depend on the curing conditions, and are highly reproducible. Threshold and saturation voltages continuously increase with increasing dose values for UV-cured films, whereas plateau values were reached for EB-cured samples. A small memory effect has been observed for UV-cured systems.     

Effect of two different size chiral ligand-capped gold nanoparticle dopants on the electro-optic and dielectric dynamics of a ferroelectric liquid crystal mixture

Nanocolloids consisting of a ferroelectric liquid crystal (FLC) doped with different concentrations (0.10 and 0.50 wt.%) of surface treated gold nanoparticles (GNPs) differing in size (1.77, 5.5 nm) are prepared and characterised. The effects of doping on the clearing temperatures as well as electro-optic and dielectric parameters of a FLC mixture are presented. The clearing temperatures remain invariant with doping. A remarkable increase in the spontaneous polarisation is noticed due to the addition of the GNPs with chiral monolayer capping. Tilt angle and switching time, at least in their tendency, become slightly reduced and increased, respectively. Depending on the size of the nanoparticles, surface plasmon resonance is observed to be slightly increased by increasing the surface. In addition, a small change in localised electric field is found upon doping. The increase in the dielectric permittivity and the dielectric strength is observed and attributed to the parallel coupling between the dipoles of functionalised GNPs, induced by external electric field, and the vector of the spontaneous polarisation of the FLC matrix. A decrease in relaxation frequency is observed. A substantial increment of one order in the dc conductivity is also observed for the nanocolloids.     

Effect of dispersion of CdSe quantum dots on phase transition,electrical and electro-optical properties of 4PP4OB

We prepared composites of a liquid crystalline material, 4-pentylphenyl 4-octyloxybenzoate (4PP4OB) and cadmium selenide quantum dots (CdSe-QDs) and investigated their thermodynamic, electro-optical and dielectric properties. The effect of QDs on transition temperature from isotropic to nematic and nematic to smectic A phases was evaluated in this study. The effect of CdSe-QDs inclusion on various display parameters on host liquid crystals was studied in the nematic phase. The electrical parameters of the composites – relative permittivity, dielectric anisotropy, dielectric loss and dielectric relaxation – were investigated in the nematic and smectic phases. The changes in dielectric parameters of the composites are explained in terms of Maier–Meier theory.     

Effect of potassium permanganate on morphological,structural and electro-optical properties of graphene oxide thin films

This work investigated the effect of Potassium Permanganate (KMnO₄) on graphene oxide (GO) properties, especially on electrical properties. The GO thin films were deposited on a glass substrate using drop casting technique and were analysed by using various type of spectroscopy (e.g. Scanning Electron Microscopy (SEM), Ultra- Violet Visible (UV–VIS), Fourier Transform Infrared (FTIR), X-Ray Diffraction (XRD), optical band gap, Raman Spectroscopy). Furthermore, the electrical experiments were carried out by using current–voltage (I-V) characteristic. The GO thin film with 4.5 g of KMnO₄ resulted in higher conductivity which is 3.11 × 10^?4 S/cm while GO with 2.5 g and 3.5 g of KMnO₄ achieve 2.47 × 10^?9 S/cm and 1.07 × 10^?7 S/cm, respectively. This further affects the morphological (SEM), optical (band gap, UV–Vis, FTIR, and Raman), and crystalline structural (XRD) properties of the GO thin films. The morphological, elemental, optical, and structural data confirmed that the properties of GO is affected by different amount of KMnO₄ oxidizing agent, which revealed that GO can potentially be implemented for electrical and electronic devices.     

A novel approach to discuss the viscosity Arrhenius behaviour and to derive the partial molar properties in binary mixtures of N,N-dimethylacetamide with 2-methoxyethanol in the temperature interval (from 298.15 to 318.15) K

Calculation of excess properties in N,N-dimethylacetamide + 2-methoxyethanol binary mixtures at (298.15, 308.15 and 318.15) K from experimental density, viscosity and sound velocity values were presented in previous work. Applications of these experimental values to test different correlation equations as well as their corresponding relative functions were also reported. Considering the quasi-equality between the Arrhenius activation energy Ea and the enthalpy of activation of viscous flow ΔH*, here we can define partial molar activation energy Ea₁ and Ea₂ for N,N-dimethylacetamide and 2-methoxyethanol, respectively, along with their individual contribution separately. Correlation between Arrhenius parameters reveals interesting Arrhenius temperature with a comparison to the vaporisation temperature in the liquid vapour equilibrium, and the limiting corresponding partial molar properties that can permit us to estimate the boiling points of the pure components.     

Effect of the comonomer addition sequence in the synthesis of an acrylonitrile terpolymer on the solution rheology and fiber properties

Polyacrylonitrile terpolymers based on acrylonitrile, acrylic acid, and methyl acrylate were synthesized by reversible addition–fragmentation chain-transfer polymerization with varying the monomer addition sequence during the synthesis. The monomer introduction sequence in the synthesis of a ternary copolymer significantly affects both the rheological properties of solutions and the strength of obtained fibers.