Electro-optic properties of thiol-ene polymer stabilized ferroelectric liquid crystals

Electro-optic properties of polymer stabilized ferroelectric liquid crystal (PSFLC) systems are examined as a function of varying concentrations of either a linear or crosslinked thiol-ene polymer. The thiol-ene method of polymer stabilization is a drastic change from previous studies designed to avert the problem of polymer phase separation. FLC rise time and tilt angle measurements were used to determine the effects of the polymer network on the optical properties of the system. The addition of monomer impurities to both systems demonstrated a reduction in tilt angle, which translated into decreased switching speeds in both systems prior to polymerization. The crosslinked thiol-ene system showed increased switching times due to the creation of polymer in the interlayer spacing of the FLC, but exhibited minimal increase in the rotational viscosity of the system. In addition, the crosslinked polymer systems resulted in an increase in the liquid crystalline order, which produced an increase in the contrast ratio of the system. The linear polymer system showed drastically different results as compared with the crosslinked system. The rise time and tilt angle measurements decreased upon polymerization of the linear thiol-ene and the rotational viscosity and contrast ratio values also decreased. We suggest that the linear thiol-ene polymer phase separation from the interlayer spacing leads to a microscopic misalignment of the FLC molecules, causing a decrease in the optical properties of the LC.     

Formation of a host nanostructure for ferroelectric liquid crystals using thiol-ene polymers

Liquid crystal displays are a subject of intense research interest because of their application to high definition display devices. Recently, polymer stabilized ferroelectric liquid crystals (PSFLCs) have been investigated due to the enhanced electro-optic properties of FLCs. We have utilized thiol-ene photopolymerizations to form a PSFLC system. Thiol-ene photopolymerizations are radical reactions, which proceed via a step growth reaction mechanism. During the polymerization, the polymer network structure is trapped into place due to the rapid transition from low molecular mass monomers and oligomers to high molecular mass polymer. This aspect is evidenced by phase transition data for the FLC, which indicates that the monomer and polymer are not phase separated from the FLC. Infrared dichroism shows that both monomer and polymer are ordered in parallel with the smectic layers of the FLC. Small angle X-ray scattering (SAXS) data show that both monomer and polymer are swelling the smectic layers. Thus, a polymer nanostructure is produced that serves as an ordered, stabilizing host for the FLC.     

Evolution of the switching current during the preparation of polymer network-ferroelectric liquid crystal microcomposites

The polymer network-ferroelectric liquid crystal (FLC) microcomposites are prepared by UV crosslinking of a chiral prepolymer diluted in a FLC. A fast cross-linking process involving 20 mW cm^-2 UV intensity produces material with ferroelectric properties which have strongly deteriorated compared with the pure FLC. By a slow process using a UV intensity of 2 mW cm^-2 the ferroelectric properties are less affected. It is supposed that the very dense polymer network, which arises during the fast process, is responsible for hindering the switching process and decreasing the spontaneous polarization. The photopolymerization kinetics are determined by measuring the switching current during the UV irradiation. The characteristic time of cross-linking is mainly reflected in the evolution of the spontaneous polarization and rotational viscosity with time.     

Evolution of the switching current during the preparation of polymer network-ferroelectric liquid crystal microcomposites

The polymer network-ferroelectric liquid crystal (FLC) microcomposites are prepared by UV crosslinking of a chiral prepolymer diluted in a FLC. A fast cross-linking process involving 20 mW cm^-2 UV intensity produces material with ferroelectric properties which have strongly deteriorated compared with the pure FLC. By a slow process using a UV intensity of 2 mW cm^-2 the ferroelectric properties are less affected. It is supposed that the very dense polymer network, which arises during the fast process, is responsible for hindering the switching process and decreasing the spontaneous polarization. The photopolymerization kinetics are determined by measuring the switching current during the UV irradiation. The characteristic time of cross-linking is mainly reflected in the evolution of the spontaneous polarization and rotational viscosity with time.     

Polarization and viscosity measurements of ferroelectric liquid crystalline monomer polymer mixtures

Measurements of spontaneous polarization, tilt angle and response time on mixtures of a ferroelectric side chain polyacrylate with a low molecular mass liquid crystal of a very similar structure are reported. From the obtained values the rotational viscosity for the ferroelectric switching process is calculated. All mixtures exhibit an Arrhenius-like temperature dependence of the rotational viscosity, the values for the polymer being three orders of magnitude higher than for the low molecular mass compound.     

Effect of CdSe quantum dots doping on the switching time,localised electric field and dielectric parameters of ferroelectric liquid crystal

A systematic study highlighting the effect of cadmium selenide quantum dots (CdSe QDs) with varying concentrations of 0.05, 0.10 and 1.0 wt% doping on the electrooptical and dielectric parameters of ferroelectric liquid crystal (FLC) is presented. No considerable change is observed in phase transition temperature and tilt angle with CdSe QDs doping at lower and higher dopant level. Substantial enhancement of localised electric field at higher doping level (1.0 wt%) of CdSe QDs manifested the ≈48% reduction in the switching response of FLC nanocolloids at 30°C. Reduction in the spontaneous polarisation, dielectric constant and absorption strength could be attributed to the antiparallel correlation among dopant and matrix molecules, ion capturing in the capping additive layer and enhancement of the rotational viscosity of the nanocolloids, respectively. Goldstone mode relaxation frequency is found to be decreased with doping up to 0.10 wt% concentration and showed reverse effect at higher QDs concentration. QDs doping effect on the photoluminescence intensity is also discussed.     

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.     

Polymerization of polymer/ferroelectric liquid crystal composites formed with branched liquid crystalline bismethacrylates

Polymerization and phase behaviour of a branched liquid crystalline bismethacrylate in a ferroelectric liquid crystal (FLC) were characterized. Addition of the monomer increases the temperature range of the smectic A phase, and, at relatively low concentrations of monomer, the temperature range increases to more than 10 times that observed in the neat FLC. Other phases such as the smectic C* phase are no longer exhibited as the monomer interferes with the inherent tilt of the FLC molecules. After polymerization, the polymer network phase separates and the phase transition temperatures return to values close to those of the FLC. The monomer also shows a high degree of orientational order before polymerization, which is retained to a large extent after polymerization. The order in the polymer network results in considerable birefringence at temperatures well above the clearing point of the pure FLC. This behaviour is induced by the order of the polymer network and interactions of the FLC molecules with the network which prevent a fraction of the FLC molecules from exhibiting typical phase behaviour. In the formation of the branched liquid crystalline bismethacrylates/FLC composites, a steady increase is observed in the polymerization rate as the polymerization temperature increases and the order of the FLC phase decreases, a behaviour significantly different from that observed in other monomer/FLC mixtures for which the polymerization rate increases as the order of the FLC phase increases. Additionally, at appropriate polymerization temperatures around the clearing point, polymerization driven endothermic phase transitions may be observed.     

Phase behaviour and electro-optic characteristics of a polymer stabilized ferroelectric liquid crystal

The effects of adding a diacrylate monomer or its polymerized network to a ferroelectric liquid crystal have been characterized. The monomer lowers the temperatures of transition to the more ordered phases, whereas the polymer network phase separates into polymer rich and LC rich phases and has little effect on the LC phase behaviour. Ferroelectric polarization decreases comparably in both monomer and networked systems. As the network concentration increases, the size of LC domains decreases considerably. With low concentrations of polymer and, thus large LC domains, optical response and tilt angle remain fairly independent of polymer concentration, but as the polymer concentration increases, switching speed and tilt angle decrease dramatically. Polymerization rate maxima increase with monomer concentration until saturation of monomer in the liquid crystal is reached. The rate maxima then decrease as monomer must diffuse from monomer rich droplets. Double bond conversion during the polymerization is comparable for all monomer concentrations below 50 per cent.     

Mechanism of electrooptic switching time enhancement in ferroelectric liquid crystal/gold nanoparticles dispersion

It is well established that incorporation of nanoparticles (NPs) in the structure of ferroelectric liquid crystals (FLCs) leads to a decrease in their electrooptic response time. Several approaches have been suggested to explain this effect (decrease in rotational viscosity of FLCs, ions enhanced localised electric field, dipole–dipole interaction among NPs and FLC molecules, FLC ordering). In this article, we will report the role of the voltage divider formed by the structural elements of a FLC cell based on ferroelectric liquid crystal/gold nanospheres (FLC/GNSs) dispersion in enhancement of the switching time. Using the impedance spectroscopic measurements, it was demonstrated that the dispersing of GNSs leads to the increase in the voltage drop on FLC/GNSs layer in comparison with the pristine FLC one. Consequently, the electrooptic response time of the FLC/GNSs cell is faster than that of the pristine one. However, the rotational viscosity of the FLC does not depend on the presence of the GNSs.     

Ferroelectric liquid crystal gels Network stabilized ferroelectric display

Liquid crystal (LC) mixtures of a reactive diacrylate and a commercial ferroelectric liquid crystal (FLC) mixture were produced. The mixtures were brought into cells provided with orientation layers in which various orientations such as uniaxial and twisted orientations, could be induced. When the desired orientation had been obtained, the polymerization of the reactive molecules was induced creating a three-dimensional anisotropic network containing the FLC molecules which were not chemically attached to the network (ferroelectric gels). The presence of the anisotropic network was found to have a large effect on various properties of the FLC molecules. For example, complex orientations (for example, 180° twisted) obtained by polymerization (formation of the gel) in the nematic phase remained unchanged when the gel was cooled to the FLC phase. Likewise uniaxial orientation, which cannot be realised in thick cells containing the pure FLC, could be realised when ferroelectric gels were used. For this reason the ferroelectric gels will be referred to as an anisotropic network-stabilized FLC. Optical properties such as effective birefringence and tilt angle were also influenced by the network. When use was made of a network with a lower birefringence than the FLC, a reduced effective birefringence in gels was observed. This is important for technologies involving thicker cells. The switching covered a range of voltages in the case of the gels, instead of showing a well-defined threshold voltage as in the bulk. The possibility of using this effect to obtain grey scales in cells during passive addressing has also been demonstrated.     

The synthesis and the bulk rheological properties of the highly-branched block polyethers

Several kinds of highly-branched block polyethers were synthesized via anion ring-opening polymerization of propylene oxide (PO) and ethylene oxide (EO), using phenol-amine resin (PA) as the initiator. The rheological properties determined by rotational rheometer all followed the regular rules of polymer systems: under a certain conditions, the bulk polyethers were pseudoplastic and non-Newtonian fluid, and with the increasing of the shear rate and temperature, the apparent viscosity of the block copolymers were reduced. In addition, modulus determination showed that such polyether molecules presented preferable viscosity compared to the elasticity, meanwhile, storage modulus, loss modulus and compound viscosity all decreased with the increasing of temperature. Storage modulus and loss modulus increased along with the scanning frequency increasing. But compared with the same kind of linear polymers, the significant difference was the low melt viscosity, which attributed to the special three-dimension space structure hindering the entanglement of chains. Furthermore, the rheological properties among the several block polyethers showed differences obviously. In other words, the number of block and the content of EO all have a significant effect on the rheological properties, specifically, the modulus will increase with the increasing of the block number and the EO content.     

Large-scale molecular dynamics simulations of alkanethiol self-assembled monolayers

Large-scale molecular dynamics simulations of self-assembled alkanethiol monolayer systems have been carried out using an all-atom model involving a million atoms to investigate their structural properties as a function of temperature, lattice spacing, and molecular chain length. Our simulations show that the alkanethiol chains of 13-carbons tilt from the surface normal by a collective angle of 25 degrees along next-nearest-neighbor direction at 300 K. The tilt structure of 13-carbon alkanethiol system is found to depend strongly on temperature and exhibits hysteresis. At 350 K the 13-carbon alkanethiol system transforms to a disordered phase characterized by small collective tilt angle, flexible tilt direction, and random distribution of backbone planes. The tilt structure also depends on lattice spacing: With increasing lattice spacing a the tilt angle increases rapidly from a nearly zero value at a=4.7 A to as high as 34 degrees at a=5.3 A at 300 K for 13-carbon alkanethiol system. Finally, the effects of the molecular chain length on the tilt structure are significant at high temperatures.     

Monolayers of diphenyldiacetylene derivatives: Tuning molecular tilt angles and photopolymerization efficiency via electrodeposited Ag interlayer on Au

An electrodeposited Ag adlayer (upd, underpotential deposition) is utilized to improve monolayer photopolymerization of diphenyldiacetylene derivatives (DPDAs) that would otherwise exhibit no polymerization in solid state. Topochemical reaction of diacetylene derivatives via solid-state 1,4-addition yields polydiacetylenes which are of great importance due to properties associated with their ene-yne conjugated backbones. The polymerization efficiency heavily depends on the molecular arrangement in the crystals. For example, crystals of most DPDA derivatives show no activity for topochemical reaction because the bulky phenyl end groups space out the triple bonds and thus DPDAs require relatively large translation and rotation angles for polymerization. In principle, topochemical reaction is viable if molecules are in optimal arrangement. The upd interlayer can be applied to tune the adsorbate-substrate interactions, intermolecular spacing, and the molecular tilt angle by controlling the coverage of the Ag adlayer. It is thus possible to manipulate the molecular arrangement of DPDAs for the subsequent polymerization. Successful photopolymerization of DPDA monolayers is realized from the decrease in nu(C[triple bond]C) intensity by infrared reflectance absorbance spectroscopy, growth of ene-yne pi-pi* transition by UV-vis measurements, and enhanced electrochemical stability by the cathodic desorption protocol. The optimal efficiency of polymerization takes place on upd-modified substrates that can generate approximately 45 degrees tilt angle for DPDA derivatives.     

Preparation and Properties of Submicrometer Particle Methyl Methacrylate-Butyl Acrylate Polymers with Multilayer Morphology

Abstract

Multilayer submicrometer particles having a core and 'a shell consisting of poly(methyl methacrylate) and an interlayer of a crosslinked copolymer of poly(butyl acrylate) were prepared by seeded emulsion polymerization. The interlayer encapsulates the core and hinders the encapsulated polymer from being dissolved. On dispersion stirring of the melt of a multilayer polymer, the encapsulation is disrupted and the amount of insoluble polymer corresponds to the mass of the middle layer of poly-(butyl acrylate). The impact resistance of films prepared from slightly crosslinked polymer samples increases markedly. Provided that the density of the interlayer network is high, the stirring causes the impact resistance of the polymer to be gradually reduced so that for highly crosslinked polymer material it becomes lower than for untreated polymer.     

POLYMERIC SURFACTANTS BASED ON OLEIC ACID IV. Lamellar Liquid Crystal Polymerization of Sodium Oleate/Oleic Acid/Aliphatic Diene/Water System

The lamellar liquid crystalline phase in the system consisting of sodium oleate (NaOL), oleic acid (OLA), and water was determined. The interlayer spacing (d) of the lamellar liquid crystal was measured through small angle X-ray diffraction, which indicated that oleic acid molecules were solubilized between the end methyl groups at low concentrations, and then were located within the hydrocarbon chain layer with further increase of its concentration. Cross-linking agents were added to the system, which were found being located partly in between the end methyl groups and partly within the hydrocarbon chain layers. The liquid crystal phase of NaOL/OLA/H₂O system with the cross-linking agent was polymerized at 60 °C, which turned out to be a mixture of liquid crystals and solids. Interlayer spacing decreased by about 10 Å, indicating a disruption of the ordered structure by the polymerization. The polymerization took place not only within the hydrocarbon layer, but also in between the layers separated by the end methyl groups. The resulting polymer lowered the surface tension of water to below 30 mN/m, with a critical micellization concentration of about O.25g/L.     

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT

Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm², 240 mPa.s, 150 µs and 28^?, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.     

Fast switching response and dielectric behaviour of fullerene/ferroelectric liquid crystal nanocolloids

The electro-optical and dielectric responses of the fullerenes C₆₀-doped ferroelectric liquid crystal (FLC) nanocolloids are reported. Order parameter and phase transition temperature remain invariant as a function of varying dopant concentration (0.10 wt% to 0.50 wt%). Faster switching response of nanocolloids comparing to that of the non-doped FLC is manifested by increase in the localised electric field (around 76% increment for 0.50 wt%), while reduction in the spontaneous polarisation could be the result of anti-parallel correlation amid dopant and FLC dipoles. Decrease in dielectric constant, absorption strength, dielectric strength and rotational viscosity of FLC nanocolloids than that of non-doped FLC is the other consequence of C₆₀ doping. Goldstone-mode relaxation frequency is found to be increased with increasing doping concentration of C₆₀ in FLC.     

Synthesis of a liquid crystalline azobenzene monomer for photoalignment of ferroelectric liquid crystals

A liquid crystalline dimethacrylate-based monomer containing an azobenzene group was synthesized. The miscibility of the azo monomer and its crosslinked polymer network with ferroelectric liquid crystals (FLCs) was improved by the liquid crystallinity of the monomer. By performing thermal polymerization of the azo monomer dissolved in a FLC host under linearly polarized visible light irradiation, following a pre-irradiation with unpolarized UV light at room temperature, bulk alignment of the FLC could be induced in the absence of surface orientation layers, as a result of the photo-orientation of azobenzene moieties related to the reversible trans-cis photoisomerization of the chromophore. The optical and rubbing-free alignment of a FLC could be achieved with as few as 0.5 wt % of the azo polymer network. This represents a step toward the preparation of uniform samples of photoaligned FLC by reducing the amount of the azo polymer network that may be phase-separated from the FLC host.     

Experimental investigation of the compressible continuum theory of a homeotropically aligned ferroelectric liquid crystal

The optical tensor configuration in a homeotropically aligned ferroelectric liquid crystal (FLC), SCE13 cell, is investigated by means of optical excitation of half leaky guided modes. A thin slab of FLC is confined between a high index pyramid and a low index substrate whose indices bound those of the liquid crystal. In this geometry there exists a small angle range over which a series of sharp resonant modes may propagate in the liquid crystal. Detecting the angular dependent reflectivity for plane polarized radiation and subsequently fitting the data by iteratively modelling from multilayer Fresnel theory, a full characterization of the tilt and twist profile throughout the cell is achieved. The temperature dependence of the tilt of the principal director, which is related to the smectic cone angle, and of the optical permittivity, as well as the pitch have been obtained. The tilt director profile across the cell is interpreted using a compressible continuum theory for SmC* liquid crystals which includes the possibility of variable cone angle and layer spacing.