Effects of the structures of epoxy monomers on the electro-optical properties of heat-cured polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared from thermal polymerisation-induced phase separation in heat-curable monomers/nematic liquid crystal (LC) mixtures. For PDLCs with a certain amount of LCs, the microstructure and the refractive index of polymer networks could be influenced by the relative content of epoxy monomers, owing to their different chemical structures. The effect of these factors on the electro-optic properties of films was also investigated.     

Effects of chemical structure and composition of the polymer matrix on the morphology and electro-optical performance of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) systems on the basis of nematic liquid crystal E7 and amphiphilic binary copolymers of acrylic acid (AA) with such acrylates as 2-ethylhexyl acrylate (EHA), n-butyl acrylate (BA), and methyl acrylate (MA) are investigated. It is shown that the liquid crystal (LC) drops in the copolymer EHA–AA have submicrometre sizes, and their dependence on the composition of the photo-curable monomer mixture is described by a parabolic curve. The highly oriented domain structure in the same system is first revealed when electric field is applied. The threshold voltage for all systems begins to increase with some critical composition of a monomer mixture in which the longer the hydrocarbonic radical in an acrylate molecule, the higher the content of AA. The PDLC system based on the BA–AA copolymer with 30 wt% LC exhibits the least value of the driving voltage, 1 V μm^–1, and the lowest memory effect.     

Study on the electro-optical properties of polyimide-based polymer-dispersed liquid crystal films

The reflectivity control device, initially developed for attitude control, is utilised to control the solar sail orbit by switching the states between absorption and specular reflection. Actually, the major parts of the device are the polymer-dispersed liquid crystal (PDLC) films. Here, PDLC films based on polyimide (PI) as polymer matrix and a low molecular weight LC can be prepared by the thermally induced phase separation (TIPS) method. The influences of cooling rate and the content of LC on the size and uniformity of LC droplets dispersed in a polymer matrix by a TIPS process were investigated. It was found that a fast cooling rate gave smaller droplet sizes and hence a more uniform distribution as compared to the ones produced under a slow cooling rate. If the LC content was increased, the droplet size would be increased. Furthermore, the effect of LC droplet size on the electro-optical properties of the PI-based PDLC films was discussed, such as transmittance, threshold voltage, driving voltage and contrast ratio (CR).     

The physical properties of alkene-terminated liquid crystal molecules/E8 mixture and the electro-optical properties as they doped in polymer-dispersed liquid crystal systems

Effects of the content of fluorinated alkene-terminated liquid crystal (LC) molecules on the physical properties of the fluorinated alkene-terminated LC/E8 mixture were studied. The morphology and electro-optical properties as they doped in polymer-dispersed liquid crystal (PDLC) films were investigated. The detailed discussion of the obtained results is given. As a result, comparing with the physical properties of the series of LC mixtures with the same content of the analogous fully saturated compounds doped with E8, we find that the birefringence is significantly larger for the LC mixture with the alkene-terminated materials. Both fluorinated alkene-terminated LC molecules and the analogous fully saturated compounds doped with E8 reduce the driving voltage of PDLC films. Moreover, PDLC films with the fluorinated alkene-terminated LC molecules possessed higher contrast ratio and faster response time than that of the PDLC films prepared by adding the same mass fraction of the analogous fully saturated compounds. Thus, the ability to manipulate physical properties of LC mixture and electro-optical properties of PDLC films by changing the LC molecular structures may have future relevance for new LC structures design and applications of PDLC films.     

Effects of monomer structure on the morphology of polymer networks and the electro-optical properties of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared by photochemical polymerisation with a series of (meth)acrylate monomers. The effects of monomer structure on the morphology of polymer networks in the PDLC films were studied. The acrylate monomers without sidegroup chain formed uniform polymer networks. The methacrylate monomers with methyl as their sidegroup chains formed lace-like networks. The size of the LC droplets increased with increasing the length of the flexible chain of both methacrylate and acrylate monomers. Meanwhile, the effects of the morphology of the polymer network on the electro-optical properties of PDLC films were also investigated.     

Effects of functionality of thiol monomer on electro-optical properties of polymer-dispersed liquid crystal films

In this paper, polymer-dispersed liquid crystal (PDLC) films which based on the acrylate and the thiol monomers were first prepared by ultraviolet-initiated polymerisation. The electro-optical properties and morphologies of the PDLC films were systematically investigated. The functionality of thiol monomers and their feed ratio showed great influence on the properties of the fabricated PDLC films because of the existence of competition between thiol–acrylate reaction and acrylate monomer polymerisation reaction. This made the polymer network and electro-optical properties of the PDLC films easily tunable by the introduction of the thiol monomers. When added four-functional thiol monomer PETMP with appropriate concentration into the PDLC system, lower driven voltage and higher contrast ratio were achieved.     

Polyimide thin films as compensators for liquid crystal displays

The disadvantages of NW-TN-LCD (normally white twisted nematic liquid crystal display) are discussed. The reason that the negative birefringent polyimide thin films are used to compensate NW-TN-LCD to decrease off-axis leakage, improve contrast ratios and enlarge viewing angles is explained. A certain polyimide thin film is taken as an example to show the compensation effect on NW-TN-LCD. © 1997 John Wiley & Sons, Ltd.     

Effects of the fluorinated liquid crystal molecules on the electro-optical properties of polymer dispersed liquid crystal films

The different fluorinated liquid crystal (LC) molecules doped to E8 were used as LC component to prepare polymer dispersed liquid crystal (PDLC) films. The mass fraction of the LC mixture is fixed 50.0 wt%. Results indicate that doping 8.0 wt% fluorinated LC molecule ME3CP to E8 significantly reduced the driving voltage of the PDLC films, and the driving voltage reduced with the rise of mass fraction of ME3CP. Besides, the terminal flexible chain length of the fluorinated LC molecule influenced the LC mixture properties based on E8, such as the dielectric anisotropy, birefringence and viscosity of the LC mixture, and the morphology and the electro-optical properties of PDLC films were controlled not only by the physical properties of the LC mixture, but also by the terminal flexible chain length of the fluorinated LC molecule .     

Preparation and electro‐optical properties of polymer dispersed liquid crystal films with relatively low liquid crystal content

In this paper, polymer dispersed liquid crystals (PDLC) films with LC content as low as 40 wt% were prepared, and the electro‐optical properties were carefully investigated. To accomplish this, different (meth)acrylate copolymerizaiton monomers have been used. The electro‐optical properties and morphologies of the PDLC films were strongly influenced by the chemical structure of copolymerization monomers (hydroxypropyl methacrylate (HPMA), glycidyl methacrylate, hydroxypropyl acrylate) and their feed ratio. Lower driven voltage and higher contrast ratio were achieved when the PDLC films showed a morphology with suitably LC domain size. At high HPMA content, a thin polymer film was formed on the surface of PDLC samples, which is beneficial to decrease the total LC content in PDLC devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Advanced wide viewing angle technology in liquid crystal display modes

This paper reviews the advanced optical configurations for in-plane switching (IPS) and vertical alignment (VA) liquid crystal (LC) cells with wide viewing angle in a visible wavelength range. Optical compensation and optimisation to eliminate off-axis light leakage in the dark state is performed on a Poincaré sphere using the trigonometric and the Muller matrix method. By optimising the wavelength dispersion of used optical retardation films, we could achieve wide-view characteristics for both the IPS and VA LC cells. In addition, we show the advanced wide-view technology for a reflective LC mode.     

Digital optical memory devices based on polymer-dispersed liquid crystals films: appropriate polymer matrix morphology

ABSTRACT

Permanent memory effect (PME) in polymer dispersed liquid crystal (PDLC) allows a greater applicability range than traditional PDLCs. One of the most interesting application could be the possible storing of optical information, the so-called Digital Optical Memory (DOM) devices. To test this application it would be required a display structure having an array of pixels addresses. Each pixel was filled with PDLC film with PME and electric field can be independently applied to different PDLC elements to define on/off pixel states (transparent or scattering states).PDLC films were obtained from a mixture of E7 nematic liquid crystal and poly(ethylene glycol) dimethacrylate with 875 g mol^-1oligomer as precursor of the polymeric matrix. The effect of the curing temperature and the UV light intensity as well time exposure during the polymerisations on the electro-optical performance of PDLC films were investigated. In this way, a high transparency state (T_OFF=55%) for a long period of time at room temperature even after the applied voltage has been switched off were obtained, started from an opaque state (T₀=0%) and after reaching a transparent state (T_MAX=75%), which causes 73% PME. The application to an 8x8 passive matrix using PDLC with PME is also demonstrated as proof-of-principle.     

Liquid crystal alignment on ion-beam irradiated bismuth-doped tin oxide films and their application to liquid crystal displays

Uniform and defect-free homogeneous alignment of liquid crystal (LC) molecules on solution-derived bismuth-doped tin oxide (TBO) films has been achieved using ion-beam (IB) irradiation. We performed measurements and physicochemical analysis to verify and establish the cause of the successful LC alignment. In addition, we measured the electro-optical characteristics of twisted-nematic cells with IB-irradiated TBO films to explore the suitability of this material for liquid crystal displays (LCDs). The results indicate that this approach will allow the fabrication of high-performance enhanced LCD devices.     

Effects of crosslinking agent/diluents/thiol on morphology of the polymer matrix and electro-optical properties of polymer-dispersed liquid crystal

To study effects of the crosslinking agent/diluents/thiol on morphology of the polymer matrix and the electro-optical properties of polymer-dispersed liquid crystal (PDLC) films, samples were prepared by ultraviolet (UV)-initiated polymerisation. Due to the interaction between thiol–acrylate reaction and acrylate monomers polymerisation, the sample compositions were the foremost determinant to the microstructures which in turn played an essential role on the electro-optical properties of the PDLC films. With the increasing content of the crosslinking agent, the LC droplet size decreased, while the thiol had a contrary effect on the LC droplet size. It was demonstrated that the superior properties of the low-driven voltage (37.2 V), the high contrast ratio (148.2), the short response time (14.9 ms) and the high saturation transmittance (86.6%) could benefit from a novel microstructure which had a dense surface and meshes with microspheres attached. It was of great significance for the optimisation and the potential applications of the PDLC films.     

Polymer-dispersed liquid crystal composites for bio-applications: thermotropic,surface and optical properties

Polymer-dispersed liquid crystal (PDLC) systems based on polysulfone as carrying matrix and 4-cyano-4?-pentylbiphenyl (5CB) liquid crystal (LC) were obtained as thin transparent films. The PDLC films were prepared by solvent- and thermally induced phase separation methods, with various compositions in the two components. Information on the phase separation was obtained by polarised light optical microscopy, differential scanning calorimetry and scanning electron microscopy. The PDLC composites show well-defined droplets of submicrometric size, around 650 nm for a medium content of LC and around 250 nm for a low one. The droplets show a radial configuration and a homeotropic alignment of the LC molecules within. By contact angle measurement and surface free energy calculations, it was established that self-assembling of aliphatic units of the two composite components, at droplet interface, is the driving force of the homeotropic alignment. Moreover, these data indicated the potential biocompatibility of the studied composites. The photophysical behaviour shows a better light emission of the PDLCs containing bigger droplets.     

Concept of power consumption saving in transflective liquid crystal displays

This study proposes the concept of power consumption saving in transflective liquid-crystal displays (TR-LCDs). This concept is based on the design of a TR-LCD with three different display modes, namely, transmissive (T), reflective (R) and transflective (TR). Among the three, the TR mode is most often used to operate the designed LCD. To conserve power and extend the battery life, the designed LCD can be switched to operate in T/R mode in an environment with dim/stable ambient light. The highest operation voltages of both T and R modes are less than half of that of the TR mode. Moreover, the strong and weak surface anchoring energies that affect TR-LCD performance and the various pretilt angles of LCs are discussed. Moreover, the operating voltage (3.7 V) to obtain the good dark state of the three modes is independent of the errors of cell gap fabrication and surface anchoring energy, and temperature effect. The concept can be applied to portable electronic gadgets in which power consumption is an important issue.     

Enhanced wavelength-sensitivity of volume holographic grating with dual-photoinitiation system in polymer-dispersed liquid crystal

We propose a method to improve the wavelength-sensitivity of a volume holographic grating by mixing dual-photoinitiation dopants, namely, rose bengal (RB)/N-phenylglycine (NPG) system and methylene blue (MB)/p-toluenesulfonic acid (PTSA) system, which enable the fabrication of gratings upon both green and red light illumination. The RB/NPG green light photoinitiation system is chosen in combination with the red-active MB/PTSA system because RB and MB absorb light near 567 and 665 nm independently. In this case, the holographic gratings, recorded in polymer-dispersed liquid crystal, can be fabricated simultaneously by two different visible laser lights with an output wavelength of 532 and 632.8 nm. The electro-optical performances of the gratings exposed upon 532 and 632.8 nm have been implemented: the diffraction efficiencies (DE) are 75% and 57% respectively when the exposure time is optimised to 2 min and 17 min; the threshold voltage is approximately 2.9 V/μm. The enhanced wavelength-sensitivity of holographic gratings allows for the storage of three-dimensional (3D) images on the same hologram plate, and these 3D images are easily reconstructed by both red and green light.     

Langmuir-Blodgett films as aligning layers for homeotropic alignment of liquid crystal molecules

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

Transitions of bipolar to radial orientation of liquid crystal droplets in amphiphilic system of PDLC film

The orientation order of nanoscale droplets of thermotropic liquid crystals (LCs) suspended in polymer dispersed liquid crystal (PDLC) solutions prepared with different medias (e.g., polymers, surfactants, nonpolar materials like dyes) respond sensitively and differently via molecular interactions. Such a valuable knowledge provides basis for understanding the properties of PDLC devices. Previously, many studies have explored the droplets size, electro-optical property variations in PDLC films by varying the materials types and its compositions. However, the variations in droplet orientation order with respect to material type and composition provide a new class of study in this particular field. The current study explored the transition in droplet orientation from bipolar to radial on varying the amphiphilic block copolymer concentrations. Further, the variations in surface energies of LCs in different series of block copolymer medias were investigated by contact angle measurements.     

Influence of the remanent polarisation on the liquid crystal alignment in composite films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and a cyanobiphenyl-based liquid crystal

Polymer-dispersed liquid crystals (PDLCs) of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and nematic 4-cyano-4?-n-hexylbiphenyl (6CB) or 4-cyano-4?-n-pentylbiphenyl (5CB) were prepared to study the effect of the remanent polarisation of the polymer on the liquid crystal alignment. We measured the macroscopic alignment of the liquid crystal molecules in the thickness direction by means of Infrared Transition-Moment Orientational Analysis. Electrical poling at 100 V/µm caused an increased order parameter up to 0.15. After subsequent annealing above the nematic-to-isotropic phase-transition temperature, the order parameter was reduced to 0.02. Nevertheless, the order parameter was still higher than for non-poled film indicating a slight orientation in thickness direction. Both values are lower than those expected from model calculations. In agreement with dielectric measurements, we attribute this result to the shielding effect of mobile charge carriers within the liquid crystal inclusions.     

Thermotropic and optical behaviour of new PDLC systems based on a polysulfone matrix and a cyanoazomethine liquid crystal

The thermotropic and optical characterisations of a novel polymer dispersed liquid crystal (PDLC) system based on polysulfone UDEL P-1700 as polymer matrix and a low molecular weight liquid crystal compound containing a mesogenic azomethine core and a cyano-substituent were investigated. The PDLC samples were prepared by solvent-induced phase separation (SIPS) and thermally induced phase separation (TIPS) methods using various compositions in the two components. Information on the morphology and phase structure was obtained by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. PDLC systems with well-defined droplets were obtained for the composite with medium content of liquid crystal compound. The optical characterisation of these materials was performed by analysing their UV–visible absorption and photoluminescence emission as a function of the liquid crystal aggregation degree.