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).     

Interfacial Interaction of Polymer/Liquid Crystal Molecules and Electrooptical Properties of their Composite Systems

Polymer/liquid crystal composite films were prepared from a solution of polymer and nematic liquid crystal (LC) by a solvent casting method. The phase-separated structure of the composite film was controlled by the solvent evaporation rate. The light-scattering profile of a poly(diisopropyl fumarate)/LC: 40/60 w/w solution during solvent evaporation exhibited a periodic structure, indicating that the phase-separated structure was formed by spinodal decomposition. The aggregation structure of the composite film was investigated with a scanning electron microscope (SEM). SEM observation of the composite film suggested the presence of periodicity and dual connectivity of polymer and LC phases. The faster the solvent was evaporated, the smaller the LC channel (domain) size in the composite film. The composite film, composed of poly(methyl methacrylate) (PMMA) and a nematic LC (E44) with a positive dielectric anisotropy, exhibited remarkable and reversible light-scattering-light-transmission switching, under the modulation of an ac electric field. The light-scattering state was dependent on such optical heterogeneities as spatial distribution of the nematic directors and/or mismatching in the refractive indices of the components. The electrooptical behavior of the composite film was strongly dependent on the LC channel (domain) size in the composite film. The transmittance increased and the rise and decay response times (τ_R and τ_D), decreased and increased, respectively, with an increase in the size of the LC channel (domain).

The electrooptical switching properties for the polymer/LC composite film should be influenced by miscibility between the polymer and the LC phases. The miscibility between both phases was evaluated from a distribution of relaxation time for interfacial polarization. The anchoring effect was also investigated by measuring the nonlinearity of the dielectric constant for the composite system.     

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 .     

An electrically light-transmittance-switchable film with a low driving voltage based on liquid crystal/polymer composites

ABSTRACT

As a typical class of electrically light-transmittance-switchable (ELTS) composites materials, polymer dispersed liquid crystal (PDLC) films have been widely used in displays, smart windows, light shutters, etc. However, the commercialised PDLC film still requires a comparatively high voltage to maintain its transparent state, leading to huge power consumption and even a potential safety risk. In this regard, we proposed a ‘heat followed UV’ stepwise polymerisation strategy for preparing a kind of ELTS film with a low driving voltage (~20.7 V) through constructing a coexistent system of polymer dispersed and polymer stabilised liquid crystal (PD&SLC). In this new PD&SLC system, vertically orientated polymer networks were formed within LC domains to induce the vertical alignment of LC, thereby reducing the driving voltage. Also, the as-made PD&SLC film exhibited good flexibility due to the high content of polymer. Moreover, the effects of the liquid crystalline polymerisable monomers content on the polymer morphologies as well as the electro-optical properties of the as-made PD&SLC films were elaborately investigated.     

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.     

Liquid crystal-polymer composites. Anomalous electro-optical curve

Liquid crystal-polymer composites containing a low proportion of polymer are of interest because of their low driving voltage, but they can show anomalous electro-optical curves, such as a minimum in the electro-optical curve. We explain the existence of this minimum, by a partial orientation of liquid crystal domains, together with the presence of mobile polymer fragments. This partial orientation is visible using a polarizing microscope, and the presence of polymer that is not linked to the main network is proved by size exclusion chromatography of the liquid crystal extracted from the composite.     

The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules

Highly mono-sized poly(methyl methacrylate) (PMMA)/liquid crystal (LC) microcapsules having a mono-sized single LC domain were prepared by the solute codiffusion method and solvent evaporation. The size of the LC domain in the microcapsules could be controlled by the amount of LC introduced during the swelling stage. The electro-optical properties of the polymer dispersed liquid crystal (PDLC) prepared by using the microcapsules was highly improved. In particular, the threshold voltage was lowered and the switching behaviour with an applied electric field was sharpened drastically compared with PDLC prepared simply by solvent evaporation-induced phase separation.     

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.     

A new reverse mode light shutter from silica-dispersed liquid crystals

Nanoparticle dispersions in liquid crystalline materials at low concentrations allow both investigating the formation of defects in liquid crystal (LC) and enhancing the light-scattering properties of LC optical devices. Reverse mode LC dispersions are LC devices, which look like transparent in their OFF state, when no electric field is applied, and opaque in their ON state. In this paper, a new reverse mode device, formed by a dispersion of a LC mixture in a silica nanoparticle crosslinked network, is presented. The morphology and the electro-optical properties of these silica nanoparticle/LC composites were investigated for two different LC mixtures with a negative dielectric anisotropy. The observed transmittances and relaxation times were found to depend strongly on the silica amount and chemical–physical properties of LC used in the sample preparation.     

Liquid crystal alignment properties of 2-naphthoxymethyl-substituted polystyrenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of 2-naphthoxymethyl-substituted polystyrenes with different contents of naphthoxymethyl side groups were investigated. The polymer films exhibited good optical transparency in the visible light region (400–700 nm). The LC cells made from the unrubbed films of polymers having more than 57 mol%?of 2-naphthoxymethyl containing monomeric units showed homeotropic LC alignment with a high pretilt angle of about 90^o. Good electro-optical characteristics, such as the threshold voltage, response time, voltage holding ratio and residual DC voltage were observed for the LC cells fabricated with the polymer having 100 mol%?of 2-naphthoxymethyl containing monomeric units as an LC alignment layer.     

Studies on the electro-optical and the light-scattering properties of PDLC films with the size gradient of the LC droplets

Polymer dispersed liquid crystal (PDLC) films with the size gradient of the LC droplets were prepared based on the epoxy/acrylate hybrid polymer matrix. The ultraviolet (UV) intensity gradient was induced by the UV-absorbing dye over the thickness of the samples. Taking advantage of the difference between the epoxy monomers and acrylate monomers in polymerisation rates and the UV intensity gradient, the gradient distribution of the LC droplet size was formed in PDLC films. The effect of the size gradient of the LC droplets on the electro-optical and the light-scattering properties of PDLC films was investigated. The results showed that due to the size gradient distribution of the LC droplets, PDLC films could exhibit the strong light scattering in the UV-visible-near infrared (VIS-NIR) region. Consequently, it provides a potential approach for modulating NIR light transmittance.     

Electro-optical properties of holographically patterned polymer-stabilized cholesteric liquid crystals

Electro-optical properties of cholesteric liquid crystals (LCs) with holographically patterned polymer stabilization were examined. It is hypothesized that increasing the LC domain size in a single dimension, relative to a random three-dimensional network of LC pockets separated by polymer strands, will allow enhanced electro-optical properties of the final device. Prior to holographic patterning, polymer stabilization with large elastic memory was generated by way of high irradiation intensities and optimized material choices. High irradiation intensities are required for the holographic patterning process to maintain polymer layer formation. At optimized conditions, polymer patterning of the stabilization allowed for an approximate 20% increase in the clear state transmission of the device, and allowed for an approximate 3 V µm^-1 reduction in the overall switching voltage as compared to an analogous floodlit irradiated sample. Switching times were increased at most threefold with holographic patterning, but all relaxation times were below 20 ms. The enhanced electro-optical properties appear to stem from a single dimension domain size increase, which allows for a reduction in the LC/polymer interaction.     

Effects of silver nanoparticle doping on the electro-optical properties of polymer stabilized liquid crystal devices

ABSTRACT

Polymer stabilised liquid crystal (PSLC) devices are widely used in various smart light modulation occasions. Their electro-optical properties can still be improved to address future challenges. It is well known that doping liquid crystal (LC) materials with nanoparticles can change the material’s electro-optical performance. In this study, silver nanoparticles (AgNPs) with size about 15–20 nm and surfactant were doped into PSLC devices. The effects of AgNPs doping on the PSLC electro-optical performances were studied. The result shows that AgNPs and surfactant doping can increase the dielectric anisotropy of LC mixture. PSLC devices with AgNPs doping have lower driving voltage and response time than un-doped PSLC devices. Therefore, doping of a specific concentration of AgNPs resulted in PSLC devices with improved electro-optical performance.     

不同光滑度立方体氧化铁对液晶电光性能的改善

向列相液晶被广泛应用于液晶显示中,但是由于杂质的存在,会导致液晶的驱动电压变大,增加能耗。 为了降低阈值电压和饱和电压,通常向液晶中添加纳米颗粒来提高电光性能。 本文利用水热法制备了表面粗糙和光滑的两种立方体Fe₂O₃纳米颗粒,其形貌均匀,尺寸约550 nm。 将二者分别掺杂到向列相液晶E7中,结果表明,粗糙立方体Fe₂O₃/E7复合体系具有比光滑立方体Fe₂O₃/E7复合体系和向列相液晶E7更优的电光性能,且在掺杂质量分数为0.4%时,其电光性能达到最优,阈值电压和饱和电压分别降低9.9%和11.6%,对比度增大80%,响应时间降低至6.0 ms。 这归因于粗糙立方体Fe₂O₃具有足够的表面积和表面所带电荷更多,所以会更易吸附体系中的杂质离子和减弱杂质离子的屏蔽作用,从而提高了电光性能。     

Vertical alignment of liquid crystal on tocopherol-substituted polystyrene films

We synthesised a series of vitamin-based and renewable tocopherol-substituted polystyrene (PTOC#, # = 20, 40, 60, 80 and 100), where # is the molar content of tocopherol moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a higher molar content of tocopherol side group showed vertical LC alignment behaviour. The vertical alignment (VA) behaviour was well correlated with the surface energy value of these polymer films. For example, VA was observed when the surface energy values of the polymer were smaller than about 35.22 mJ/m² generated by the nonpolar tocopherol moiety having long and bulky carbon groups. Good electro-optical characteristics, such as voltage holding ratio and residual DC voltage, and aligning stabilities at 200°C and ultraviolet irradiation of 10 J/cm² were observed for the LC cells fabricated using PTOC100 as a LC alignment layer. Therefore, it was first found that the renewable tocopherol-based materials can produce an eco-friendly vertical LC alignment system.     

Influence of low concentration silver nanoparticles on the electrical and electro-optical parameters of nematic liquid crystals

Thermodynamical, optical, dielectric and electro-optical characterisation of nematic liquid crystals (LCs) and silver nanoparticle (NP) composites have been carried out. Transition temperatures of pure and composites systems have been measured. Thermodynamical studies suggest increase of clearing temperature of the composite material as compared to the pure material. Threshold voltage for switching from bright to dark state and splay elastic constant of the pure and composite materials have been determined. From frequency dependence of dielectric measurements, permittivity, loss, relaxation frequency and dielectric strength of flip-flop mechanism of LC molecules in the nematic phase have been calculated. Dielectric properties of composites have been explained in reference of Maier and Meier theory. The effects of doping of NPs on dielectric and electro-optic properties of LC-NP composites have been discussed.     

Influence of SMA content on the electro-optical properties of polymer-dispersed liquid crystal prepared by monodisperse poly(MMA-co-SMA)/LC microcapsules

Monodisperse poly(methyl methacrylate) (PMMA) particles containing various concentrations of stearyl methacrylate (SMA) were prepared, and a liquid crystal (LC) was swollen into the particles using a solute co-diffusion method (SCM). Phase separation behaviors between the polymer and LC were monitored by utilizing an optical and a polarized microscope (OM/POM). The monodisperse LC microcapsules were then applied to a polymer-dispersed liquid crystal (PDLC), and the electro-optical properties were investigated. As a result, the threshold and driving voltages were improved when the SMA content increased. The long alkyl chains of SMA in the capsules should exist at the interface of the LC and polymer resulting in an enhancement of phase separation between the polymer and LC, which largely influences the electro-optical properties of PDLC.     

Liquid crystal alignment behaviours on poly(methyl methacrylate) having polyhedral oligomeric silsesquioxane groups

A series of poly(methyl methacrylate) derivatives containing polyhedral oligomeric silsesquioxane (POSS) groups (MCP#) were synthesised via free radical polymerisation (FRP) using methacryl isobutyl POSS (MA-POSS) and methyl methacrylate as monomers to investigate liquid crystal (LC) alignment property of these polymer films. The LC cells made from the films of the polymers having 100 mol% of MA-POSS units (MCP100) showed vertical LC alignment having a pretilt angle of about 90°. The vertical LC alignment behaviour on the MCP100 film was ascribed to the very hydrophobic MCP100 surface having the surface energy value smaller than about 23 mJ/m² generated by the nonpolar bulky POSS group. Good electro-optical characteristics, such as voltage holding ratio (VHR) and residual DC voltage (R-DC), were observed for the LC cells fabricated using MCP100 as a LC alignment layer.     

Superior electro-optic properties of liquid crystal system using cobalt oxide nanoparticle dispersion

In this article, we study the electro-optical (EO) properties of the homogeneous aligned nematic liquid crystal (N-LC) doped with cobalt oxide (Co₃O₄) nanoparticles (NPs). The EO characteristics of Co₃O₄ doped N-LC are higher performance, indicating lower threshold voltage (1.33 V), faster rising time (1.479 ms), and faster falling time (9.343 ms) than pure liquid crystal (LC) cells. We have demonstrated these results by investigating the relationship between dielectric constants and LC device properties. Furthermore, we proved NPs doped N-LC cells drive low power operation without capacitance hysteresis. Our experimental results were verified by software simulation based on general physical properties.     

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.