Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

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.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

The fabrication of polymer dispersed liquid crystal based on SiO2 photonic crystal template

The conductance of polymer matrix is an important factor for the property of the polymer dispersed liquid crystal (PDLC). The nanographites are dispersed into the polymer matrix for optimising the dielectric conductive property. The synthesised nanoparticles SiO₂ was used as photonic crystal (PC) to work as a template for fabricating PDLC films. A mixture of pre-polymer and liquid crystals (LCs) was infiltrated into the void of the PC and polymerised under ultraviolet light. The void of the PC made uniform the dispersion of the liquid crystals in the films. The optical property of the PDLC films was optimised by doped nanographites and negative charge SiO₂ template. The effect of negative charge SiO₂ and nanographites on the threshold voltage and driving voltage was researched. The morphology of the PDLC films was studied by the FTIR image. The dispersed LCs droplets were uniformly affected by the addition of the nanographites. The LCs droplets dispersed in the polymer were located in the void of the SiO₂ photonic crystal.     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

The fabrication of novel optical diffusers based on UV-cured polymer dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) with different sizes of the LC droplets are prepared based on the ultraviolet (UV) light curable acrylate monomers/LCs composites to fabricate the optical diffuser films. To acquire light diffusers with high optical performance, the effects of the monomer structure and the UV light intensity on the micro-structure of the PDLC films are studied. Results show that the PDLC films could exhibit a strong light scattering at the premise of maintaining high transmittance in the visible region. As the LC droplets are spherically dispersed in the polymer networks, when the size of LC droplets is about 3.0 μm, the haze can reach 88.5% and the transmittance is nearly 90.0%, which can be used as a bottom diffuser film. While when the size of LC droplets is about 10.0 μm, the haze and transmittance are 39.2% and 90.2%, respectively; hence, it can be a good choice for a top diffuser film. With the advantages of simple preparation, roll-to-roll industrial production and tunable optical properties, it is supported that the films based on UV-cured PDLC films can be applied as outstanding optical diffuser films in the liquid crystal display industry.     

Orientational control of liquid crystal molecules via carbon nanotubes and dichroic dye in polymer dispersed liquid crystal

Polymer dispersed liquid crystals (PDLCs) using nematic liquid crystal and photo-curable polymer (NOA 65) were prepared by polymerisation-induced phase separation technique, in equal ratio (1:1) of polymer and liquid crystal (LC). We demonstrate that doping of small amount (0.125%, wt./wt.) of multiwall carbon nanotubes (CNTs) and orange azo dichroic dye in PDLC generously controlled the molecular orientation, dynamics of LC in droplet and size of droplets. The effects of multiwall CNTs and dye on PDLCs were studied in terms of transition temperature, droplet morphology, transmittance characteristic, contrast ratio and response time. The results exhibited that the values of the threshold electric fields were reduced from 8 V/µm (pure PDLC) to 1.18 and 1.72 V/µm, doped with multiwall CNTs and dye, respectively. The CNTs-doped PDLC shows faster switching response as compared with pure PDLC and dye-doped PDLC. However, dye-doped PDLC shows much higher contrast among all PDLC samples. Further, the results also illustrate that the birefringence value of LC in PDLCs was changed with doping of CNTs and dye.     

Memory effects in chiral nematic liquid crystals doped with functionalised single-walled carbon nanotubes

Octadecylamine-functionalised single-walled carbon nanotubes (SWCNTs) were dispersed into nematic liquid crystals (LCs) doped with chiral molecules. The collective orientation of nematic LC molecules in helical layers was manipulated by varying dopant concentration. Highly anisotropic nature of SWCNTs enhanced the anisotropy of the LC as confirmed by polarised fluorescence spectroscopy. The π–π interaction of SWCNTs present in the planar alignment layers and twisted nematic LC molecules affects the molecular relaxation process. An irreversible electro-optic memory in the material has been observed.     

Liquid-crystalline epoxies dispersed liquid crystals

Dispersing liquid crystal droplets in a rigid polymer matrix results in an electrically controllable light scattering medium. A polymer with high glass transition temperature phase-separated with a low molecular weight liquid-crystalline material is a good binder for polymer dispersed liquid crystals (PDLC). Main-chain liquid-crystalline epoxies were synthesized for the phase separation with low molecular weight nematic liquid crystals E7 (mixture of 4-alkyl-4′-cyanobiphenyls) and studied for their mesomorphic properties and response to an applied electrical field.     

Cyano terminated tolane compounds for polymer dispersed liquid crystal application: relationship between cyano terminated tolane based molecular structures and electro-optical properties

ABSTRACT

The structures of the liquid crystal (LC) molecules have a key role in impacting the electro-optical performance of a polymer dispersed liquid crystal (PDLC) film. In this paper, the relationship between the LC molecular structures and the electro-optical properties of PDLC films is investigated based on an unexplored cyano-terminated tolane compounds (CTTCs) doped E8 LCs/UV polymers system. Due to the high polarity of CTTCs, LCs doped with the cyano-terminated tolane (CTT) molecules exhibit high birefringence and large positive dielectric anisotropy. On the whole, PDLC films doped with the CTT molecules exhibit a lower driving voltage than that doped with the pure E8. More excitingly, PDLC films based on CTT molecules with larger length-to-width ratio and longer conjugated system show higher contrast ratio (CR) and faster response time. Eventually, the mechanism of the effects of CTT-based molecular structures and the relationship between the electro-optical performance of PDLC films and CTT molecules are illustrated. This work paves a new way for optimising the electro-optical properties of PDLC films.     

Reverse-mode polymer dispersed liquid crystal films prepared by patterned polymer walls

This article proposes a methodology to prepare polymer dispersed liquid crystal (PDLC) films working in the reverse-mode operation, where the ion-doped nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δε) were locked by polymer walls. On-state and off-state of films were controlled by an electric field. In the absence of an electric field, it appears to be transparent. In the field, the homogeneous alignment NLCs form dynamic scattering, giving rise to opaque. The effect of the cylindrical holes with different diameters of photo masks and liquid crystal Δε on the electro-optical properties and transmittance wavelength range of 400–3000 nm light of samples were investigated. It was found that it exhibited very good electro-optical characteristics, high contrast ratio and excellent infrared energy-efficient of films used as switchable windows.     

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.     

Effects of off-state alignment in polymer dispersed liquid crystals

Partial off-state alignment of the liquid crystal in polymer dispersed liquid crystal (PDLC) droplets was obtained by the application of electric or magnetic fields during their formation. Photopolymerization was used to induce phase separation of the liquid droplets from monomer/liquid crystal solutions. Substantial director directionality was retained in these PDLC films after removal of the fields used during their formation. This alignment affected both the off-state and the on-state electro-optic properties of the films. Transverse electrical fields (5 to 60 V across a 15 μm thickness) applied during PDLC formation from a solution of E7 (BDH Ltd) in a monomer resulted in PDLC films with progressively lower off-state scattering and lower threshold voltage. Strong longitudinal magnetic fields (9 to 14 T) applied during PDLC formation with these materials resulted in strong polarization effects in the light scattering off-state. In the infrared region, where there is less light scattering than in the visible region, the longitudinally aligned films shows tunable birefringent electro-optic effects while retaining the fast time response characteristics of PDLC films with small droplet sizes.     

Electro-Optical Properties of Inorganic Oxide/Liquid Crystal Composite Film by the Sol-Gel Process

The Electro-optical properties of a composite ceramic/liquid crystal membrane, with the dispersed liquid crystals (LCs) in the high dielectric constant inorganic oxides, were investigated. The composite membranes having bicontinuous micron structure of matrices and LCs were made by the sol-gel process via the syntheses of multicomponent sols using tetra-n-butyltitanate, barium diethoxyethoxide, methyltriethoxsilane and polyethylene glycol (PEG) for phase separation. The switching behaviors of the composite films have been observed by applying AC voltages at 1 kHz. It was also confirmed that the driving voltage of the composite film depended on the composition of the inorganic oxide matrix. The composite films of high dielectric constant were driven at an electric field under 10 kV/cm, much less than that of existing PDLC.     

Optical properties of stretched polymer dispersed liquid crystal films

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The 'anomalous' nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.     

Effects of off-state alignment in polymer dispersed liquid crystals

Partial off-state alignment of the liquid crystal in polymer dispersed liquid crystal (PDLC) droplets was obtained by the application of electric or magnetic fields during their formation. Photopolymerization was used to induce phase separation of the liquid droplets from monomer/liquid crystal solutions. Substantial director directionality was retained in these PDLC films after removal of the fields used during their formation. This alignment affected both the off-state and the on-state electro-optic properties of the films. Transverse electrical fields (5 to 60 V across a 15 μm thickness) applied during PDLC formation from a solution of E7 (BDH Ltd) in a monomer resulted in PDLC films with progressively lower off-state scattering and lower threshold voltage. Strong longitudinal magnetic fields (9 to 14 T) applied during PDLC formation with these materials resulted in strong polarization effects in the light scattering off-state. In the infrared region, where there is less light scattering than in the visible region, the longitudinally aligned films shows tunable birefringent electro-optic effects while retaining the fast time response characteristics of PDLC films with small droplet sizes.     

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.     

Reduced OFF-axis haze in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) are composite materials formed by micron-sized droplets of liquid crystals (LCs) dispersed in a polymer matrix, which can be turned from an opaque state to a transparent one by application of a suitable electric field. PDLCs have been proposed in applications related to the control of light transmittance on large surfaces (light shutters, displays, rear mirrors). Despite several advantages, PDLCs’ main drawback is haze, i.e. the fast decay of transmission at large viewing angles. In this paper, a method for achieving highly transparent PDLC devices over a wide range of viewing angles is proposed. The method is based on the use of PDLCs with tilted elongated LC droplets and driven by opportune electric fields, which are experimentally calculated and able to ensure an almost constant value for OFF-axis transmittance.     

Dielectric relaxation in polymer dispersed nematic liquid crystal films

Dielectric absorption studies for polymer dispersed liquid crystal (PDLC) samples of different nematic (4-hexyl-4'-cyanobiphenyl, 6CB) contents (20, 30, and 40 wt%) have been carried out in the frequency range from 10kHz to 10MHz. A method for evaluation of the fraction of the molecules which are not affected by the polymer surface anchoring forces in the nematic droplets is proposed.