PDLC films prepared by electron beam and ultraviolet curing: influence of curing conditions on the electro-optical properties

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation processes using ultraviolet (UV) and electron beam (EB) radiation. A mixture of the nematic LC material E7, an aromatic polyester acrylate, and additional monomeric acrylates was exposed to the EB radiation. A photoinitiator was included in the initial mixture in the case of UV exposure. The electro-optical behaviour of the PDLC films obtained has been investigated as a function of the chosen radiation. The transmission versus voltage curves strongly depend on the curing conditions, and are highly reproducible. Threshold and saturation voltages continuously increase with increasing dose values for UV-cured films, whereas plateau values were reached for EB-cured samples. A small memory effect has been observed for UV-cured systems.     

Effects of a chemically modified multiwall carbon nanotubes on electro-optical properties of PDLC films

Polymer dispersed liquid crystal (PDLC) films are fabricated by well-known polymerization-induced phase separation method. In this paper, the dispersion of multi-walled carbon nanotubes (MWCNT) in liquid crystals has been enhanced by chemical modification and we have investigated their effects on the morphology, electro-optical properties and conductivity of the PDLC films. Results indicated that the threshold voltage and the saturation voltage of PDLC films decreased with the increase of the doping concentration of MWCNT or chemically modified MWCNT, because carbon nanotubes can enhance the electric field by reducing the resistivity of the medium and increasing the capacitance of the cells. It can be viewed obviously that the contrast ratio of the PDLC films doped with the chemically modified MWCNT is higher than that of the MWCNT.     

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.     

CO-sensing properties of undoped and doped tin oxide thin films prepared by electron beam evaporation

Undoped thin films of tin oxide and those doped with indium oxide and nickel oxides were deposited by electron beam evaporation. The effects of the film thickness and preparation conditions (films prepared with or without the presence of oxygen environment during deposition) on the optical and carbon monoxide sensing properties of the films were studied. The films were characterized using X-ray diffraction and X-ray photoelectron spectroscopy and optical spectroscopy techniques. All the films were found to be amorphous. It was found that the sensitivity of the films to CO increased with the thickness and the porosity of the films. It was found that their selectivity to CO gas relative to CO₂ and SO₂ gases could be improved upon doping the films with indium (or nickel) oxide.     

固化条件对聚合物分散液晶膜电光性能的影响

选用聚乙二醇二缩水甘油醚(EGDE)/季戊四醇缩水甘油醚(PERTGE)/1,8-二氨基-3,6-二氧杂辛烷(EDBEA)/向列相液晶(SLC1717)复合体系,在不同的固化条件下,通过热聚合诱导相分离方法制备了一系列电光性能不同的聚合物分散液晶(polymer dispersed liquid crystal,简称PDLC)膜.研究了固化温度和固化时间对制备的PDLC膜中聚合物网络的微观形貌和电光性能的影响.结果表明,随着固化温度的升高以及固化时间的缩短,PDLC膜的对比度、驱动电压和开态响应时间逐渐增大,而关态响应时间逐渐减小.在固化温度为363.2 K,固化时间为7 h时,所制备的PDLC膜具有较佳的电光性能.     

An investigation on a novel PDLC film’s fabrication and its electro-optical properties

A polymer dispersed liquid crystal (PDLC) film that has good electro-optical properties is produced by the method of polymerized-induced phase separation. Based on the application foreground, its capability parameters, such as contrast ratio, work voltage, and visual angle, are characterized for the first time by a white light but not a fixed wavelength light. The results show the PDLC film has a low work-voltage of 20 V, more than 150° visual angle, high stability, and long lifetime. The differences between plastic and glass ITO-coated substrates of PDLC films are also studied in this paper. The plastic substrate has better property and will have a wider perspective especially in the portable, tender and folded display devices. Due to adjustable properties of film by electric field, PDLC has the potential application for display device, sensor, switch, grating, and new generation analytical apparatus.     

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.     

Effect of droplet size on the dielectric properties of PDLC films

Polymer dispersed liquid crystal (PDLC) films (5CB/PMMA, 60/40) of different droplet size were prepared by a solvent-induced phase separation method under different N₂ flow speeds. The effects of droplet size on the thermal transitions of the LC and various dielectric properties such as dielectric constant, conductance, dielectric loss, and the electric field induced in a droplet were examined. The configuration of the LC in the film with smaller droplets can be identified by comparing the dielectric constant of the film with the one predicted by Boettcher's mixture formula. In addition, the effect of droplet size on the electro-optical response of the PDLC film was investigated. Variations of the conductance and the dielectric constant of the film were analyzed under various AC frequencies, with the purpose of elucidating the polarization mechanism of the LC molecules in the droplet. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1373–1381, 1997     

Structure,microstructure and optical properties of cerium oxide thin films prepared by electron beam evaporation assisted with ion beams

Cerium oxide thin films were prepared by combined electron beam evaporation and ion beam assisted deposition techniques (EBE–IBAD). Their crystallographic structures, microstructures, and optical properties were studied as a function of the substrate temperature (200 °C and 500 °C) and the dose of Ar⁺ or O₂⁺ ion assistance during growth. X-ray diffraction was used to estimate the crystallographic texture, grain size, microstrain and lattice constant. Sample microstructure was studied by scanning electron microscopy. Transmission UV–vis spectroscopy was employed to obtain optical information (band gap, density, and refractive index). All films showed a cubic CeO₂ structure with different preferential growth depending on the preparation conditions. The bombardment with Ar⁺ ions during film deposition proved to be very effective for changing the film structure, hindering columnar growth and producing smaller grain sizes and higher values of microstrain and lattice constant. Films grown at 200 °C and Ar⁺ ion assistance showed the highest density, the smallest grain size (～10 nm) and a high expansion of the lattice constant (up to ～1%). This expansion is related to the presence of Ce³⁺ at the grain boundaries. Ion assistance during the growth leads to films with higher values of refractive index and lower values of band gap.     

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 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.     

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.     

Thermo-mechanical and electro-optical properties of polymer dispersed liquid crystal films

We have observed a strong dependence of the electro-optical properties of polymer dispersed liquid crystal (PDLC) films on temperature. One plausible explanation for this dependence is the thermal expansion of the films. We investigated the thermal expansion of various PDLC films using a thermal mechanical analyser (TMA). As the temperature increased, the films expanded and their droplet size decreased, and their contrast, response time, and hysteresis changed simultaneously. We observed obvious changes of thermal expansion coefficient of the films. We studied the qualitative relationship between thermo-mechanical properties and electrooptical properties. The thermal expansion of PDLC films could be adjusted by changing the degree of cross-linking of the polymer.     

Investigation of carbonaceous insulating films by electron beam X-ray microanalysis

Quantitative investigations of insulators by electron beam X-ray microanalysis are normally only possible using special preparation techniques for the samples. Samples were coated by evaporation with thin carbon films, but the analysis will fail if the samples themselves contain carbon. Two ways are proposed for solving this problem. Samples and reference samples with similar composition were initially coated by evaporation with carbon. The k-ratios necessary for the determination of element concentrations were calculated either experimentally by relation of the intensities to those of carbon standards or by computer calculation of the film thicknesses using the k-ratios of reference samples. By means of this calculation it was possible to correct the usually measured k-ratios or intensities of samples via modified depth distribution functions.     

Structural and catalytic properties of ultra-dispersed silver powders prepared by metal evaporation with high-power electron beam. 1. Structural properties

Summary Structural properties of ultra-dispersed silver powders synthesized via high-temperature electron-beam evaporation of silver in different gases (Ar, N₂, He) were studied with XRD, TEM, EDX and XRF. The formation of specific oxide-like species on the particle surface of an Ag(Ar) powder was found. It was shown that these particles can contain argon atoms.     

Modification of electro-optical properties of polymer dispersed liquid crystal films by iniferter polymerization

In this letter, iniferter polymerization was employed to prepare polymer dispersed liquid crystal (PDLC) films. Polystyrene (PS) was prepared as a macro-iniferter (MI). With the addition of MI in PDLC films, poly(methyl acrylate)-b-polystyrene was prepared in situ and used as polymer matrix in photopolymerization induced phase separation (PIPS). A reduction in driving voltages and an improvement in the ON state transmittance were observed for the sample prepared with a small amount of MI; while a poor electro-optical performance was obtained for that without any MI. Moreover, molecular weight and refractive index of the polymer matrix could be easily adjusted by the concentration of MI, and the matrix seems to be a prospective material for the PDLC devices.     

Quantification of the electron beam damage of thin films

Owing to diverse possible mechanisms of the e-beam damage, some changes either in the crystalline structure, chemical composition or film thickness occur as the proper damage of the virgin state. Changes in the chemical bonds are often connected with some change in thickness or structure so that the other two are the most important items to be detected, preferably in real time of the electron-microscopical or electron-spectroscopical examination. We propose to measure the film thickness on the basis of the most probable energy loss due to the electron pass through the film and back, i.e. on the basis of the position of the broad background maximum in the electron spectra of stratified specimens. A change in crystallinity can be sensed through the elastic peak intensity. Placing a measurement window between these features in the energy scale and measuring the energy filtered background during the irradiation, we get a pronounced dependence of the signal on the dose which unambiguously reveals the damage limit in the form of a curve knee. In some cases, two stages of damage are detectable.     

Thermoelectric properties of PEDOT films prepared by electrochemical polymerization

Thermoelectric (TE) properties of flexible and free‐standing poly(3,4‐ethylenedioxythiophene) (PEDOT) films synthesized via galvanostatic polymerization of 3,4‐ethylenedioxythiophene in propylene carbonate containing sulfated poly(β‐hydroxyethers) (S‐PHE) as polymer electrolyte were elaborately studied. Both electrical conductivities (σ ) and Seebeck coefficients (S ) of the PEDOT:S‐PHE films were increased by decreasing the temperature (T ) or by increasing the current density (J ) during electrosynthesis. Possible reasons for the lack of a trade‐off relation commonly observed between σ and S are discussed on the basis of SEM and oxidation‐level measurements. Preparation of the PEDOT:S‐PHE films was optimized with respect to T and J . In addition, the oxidation level of the PEDOT:S‐PHE films was controlled by potential and the change of their TE performances was discussed in conjunction with the change of chemical species involved. The power factor (PF = σS ²) of the PEDOT:S‐PHE films reached 7.9 μW m^?1 K^?2, leading to a dimensionless TE figure‐of‐merit (ZT ) of 0.013. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 524–531     

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.