Effect of graft polymer prepared by living radical polymerisation on electro-optical properties of polymer dispersed liquid crystal

In this study, a graft polymer matrix prepared by living radical polymerisation had been incorporated into polymer dispersed liquid crystals (PDLCs). The electro-optical properties of the PDLCs were investigated. The results showed that the length and density of graft chain had a great influence on the memory effect of the PDLCs. Low-driving-voltage and weak-memory-effect PDLCs could easily be obtained with a graft polymer matrix.     

Facile fabrication of polymer-dispersed liquid crystal films via nucleophile-initiated thiol-ene click reaction

Nucleophile-initiated thiol-ene click reaction was employed to prepare polymer-dispersed liquid crystal (PDLC) films initiated by 4-dimethylaminopyridine (DMAP). The PDLC films were prepared in different temperature. It is found that the reaction temperature has a big influence on the electro-optical properties of PDLC films and 60°C is the optimal temperature to fabricate PDLC film which has high ON state transmittance (T_on) and low OFF state transmittance (T_off). UV-Visible spectrophotometer, scanning electron microscopy and polarised optical microscope were employed to explore the obtained PDLC films. Nucleophile-initiated thiol-ene click reaction, without UV-light, probably provides a new pathway to prepare PDLC films.     

Effects of thiol monomers on the electro-optical properties of polymer-dispersed liquid crystal films prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACT

Nucleophile-initiated thiol-ene click reaction is a highly novel and efficient method of preparing polymer-dispersed liquid crystal (PDLC) films. The effects of thiol monomers on the electro-optical properties of PDLC films prepared by nucleophile-initiated thiol-ene click reaction were investigated in this work. The thiol monomers were dithiol, trithiol, tetrathiol or their mixture. It was found that the increase of functionality could lead to the increase of threshold voltage and saturation voltage and the decrease of off-state transmittance. The influence of reaction temperature was also investigated. The results indicated that functionality and reaction temperature had combined effects on the electro-optical properties of PDLC films.     

Effects of the chain length of crosslinking agent and dye-doped amount on the electro-optical properties of polymer-dispersed liquid crystal films prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACT

A series of crosslinking agents with different chain lengths were used to explore their effect on electro-optical properties of dye-doped polymer dispersed liquid crystals (PDLCs) film prepared by nucleophile-initiated thiol-ene click reaction. The influences of dye contents and temperatures on electro-optical properties were also investigated. It was found that the increase in chain length of crosslinking agent leads to the increase in driving voltage and the decrease in memory effect. The decrease in reaction temperature causes a decrease in driving voltage and an increase in transmittance. Particularly, the dye content could be optimised to obtain promising materials with minimum driving voltage and high contrast ratio for display applications.     

Effects of alkyl chain length of monomer and dye-doped type on the electro-optical properties of polymer-dispersed liquid crystal films prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACT

Polymer-dispersed liquid crystal (PDLC) films containing a series of monomers with different alkyl chain lengths were prepared by nucleophile-initiated thiol-ene click reaction. The effect of alkyl chain length of monomers, dye and temperature on electro-optical properties of PDLC films was investigated. It was found that the alkyl chain length and polymerisation rate of monomers together determine the size of liquid crystal (LC) droplets, thus affecting the electro-optical properties of PDLC. In addition, the type and content of dyes could be optimised to obtain PDLC materials with better comprehensive properties for display.     

Surfactant-doped reverse-mode polymer-dispersed liquid crystal display with enhanced properties

In this article, a nonionic surfactant Brij30 has been used to vertical align the negative anisotropic liquid crystal. The surfactant is further used in reverse-mode polymer-dispersed liquid crystal display to enhance the electro-optical properties. This work shows an enhancement of contrast ratio simply with the addition of surfactant. In addition, contact angle and surface free energies have been determined to judge the role of surfactant in control of surface orientation of liquid crystal molecules.     

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).     

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.     

A novel polymer dispersed liquid crystal film prepared by reversible addition fragmentation chain transfer polymerization

Polymer as an important component of polymer dispersed liquid crystal (PDLC) has a great influence on electro-optical properties. In this letter, the effect of molecular weight of polymer matrix on the electro-optical properties of PDLC films was investigated with reversible addition fragmentation transfer (RAFT) polymerization. It was found that the saturation voltage and memory effect were apparently influenced by molecular weight of polymer which can be regulated efficiently by irradiation time, while the morphology of liquid crystal droplets kept unaltered. It was estimated that the increase of molecular weight of polymer enhanced entanglement between polymer and liquid crystal, which induced the different surface interaction and electro-optical properties.     

The complete morphological,electro-optical and dielectric study of dichroic dye-doped polymer-dispersed liquid crystal

Polymerisation-induced phase separation method was used to prepare dye-doped polymer-dispersed liquid crystal (DPDLC) films consisting of nematic liquid crystal, photo-curable pre-polymer and a dichroic dye. The change occurring on droplets morphology with the change in dye concentration and voltage was investigated by means of polarising optical microscope and scanning electron microscope. Electro-optic behaviour of these composite films in the presence of an externally applied AC electric field (0–100 V, 200 Hz) and temperature range 25–50°C was evaluated using He–Ne laser. The maximum absorbance of dye and effect of electric field on the absorbance of dye was studied using UV-Vis spectrophotometer. Dielectric relaxation spectroscopy was carried out in the frequency range 20 Hz–20 MHz and over the temperature range 25°C–90°C. Dielectric parameters such as distribution parameter, relaxation frequency, relaxation strength and relaxation time of these DPDLC films were calculated, and the modelling of experimental data was done using Debye and Cole–Cole method.     

Photopolymerisation kinetics and electro-optical properties in mixtures of dichroic dye-doped nematic liquid crystal and photocurable polymer

This work reports the effect of dye on the photopolymerisation and electro-optical properties of polymer dispersed liquid crystal (PDLC) composite films. Dichroic PDLC (DPDLC) films based on a photocurable polymer and nematic liquid crystal (LC) with an azo dichroic dye were prepared by photopolymerisation-induced phase separation method. Polarising optical microscopy has been used for monitoring the phase separation kinetics and two-phase morphology evolution in the DPDLC system. LC domains with radial structures during initial period of phase separation adopted a resultant morphology of bipolar configuration over the course of polymerisation. The phase separation and morphology of LC domains was found to be dependent on the amount of dye used. Moreover, the addition of small amount of dye reduced the switching voltage, and enhanced the contrast ratio with improved switching time in the PDLC films. It was shown that, under the application of an electric field, the molecular orientation and absorbance of dichroic dye can be controlled in DPDLC to induce non-linearity and colour contrast without the use of polarisers.     

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.     

The effect of molecular weight of polymer matrix on properties of polymer-dispersed liquid crystals

The electro-optical properties and memory effects are important characters of polymer-dispersed liquid crystal (PDLC) devices. Molecular weight of polymer matrix influences the morphology of liquid crystal droplets in PDLC film and the performance of PDLC devices. In this letter, PDLC films were doped with a small amount of chain transfer agent (CTA), in order to control the molecular weight of polymer matrix. It is observed that the addition of CTA induces a reduction of threshold voltage. In addition, the effect of molecular weight on memory effects of PDLC devices was also discussed. It is found that the entanglement between polymer and liquid crystal molecule increased with the molecular weight, which reduces the memory effect.     

Optical properties of electron beam- and UV-cured polypropyleneglycoldiacrylate/liquid crystal E7 systems

This contribution focuses on a detailed investigation of the relationship between the method of polymerisation/cross-linking, such as slow and rapid UV radiation, and high voltage accelerated electron beam (EB), and the obtained physical properties including phase diagrams, polymerisation and phase separation kinetics, morphologies and electro-optic responses of polypropyleneglycoldiacrylate (PPGDA) monomers, in the presence of the nematic liquid crystal E7. The longer the spacing between double bonds, the more rapid was the photopolymerisation under both UV systems; however, the reverse was proved under EB. More homogenous and regular morphologies were obtained under EB curing.

The electro-optical responses of various polymer dispersed liquid crystals (PDLCs) systems exhibited remarkable differences between the UV-cured samples and those cured by the EB technique. It was found that the threshold and saturation voltages considerably increased in the case of the UV-cured systems. Other results involving the contrast ratio, which is higher for EB-cured systems, confirm their higher quality, although the rapid photopolymerisation UV source was employed, which slightly improved the electro-optical responses. Moreover, EB curing leads to high enough conversions without a photoinitiator, which may act as an impurity that might have a strong impact on the electro-optical performance of the obtained PDLCs.     

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.     

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.     

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.