Electro-optical Properties Of Polymer-dispersed Liquid Crystal Research Articles

Polymer-Dispersed Liquid Crystal Films on Flexible Substrates with Excellent Bending Resistance and Spacing Stability

Polymer-dispersed liquid crystals (PDLCs) are very attractive due to their electrically switchable properties. However, current PDLC films still have problems such as high driving voltages, low contrast ratio (CR), and poor bending resistance and spacing stability. To solve these problems, a PDLC film with a system of coexisting polymer spacer columns and polymer network was proposed. First, based on the adhesive systems of IBMA and UV6301, the effects of IBMA concentration and LC content on the morphology of the polymer network and the electro-optical properties of PDLC were investigated, respectively. Then, the effects of the process conditions of mask polymerization such as temperature, time, and UV light intensity on the morphology and electro-optical properties of the polymer spacer columns were systematically investigated. It was found that PDLC films with the coexistence system exhibit both excellent electro-optical properties and outstanding bending resistance and spacing stability. Thus, it provides new practical possibilities for the preparation of high-performance PDLC films used in flexible devices.

Improving PDLC electro-optical performance using a photomask multi-step polymerisation method

ABSTRACT Polymer-dispersed liquid crystals (PDLCs) have been intensively studied for their light scattering properties and are of great use in electro-optical display devices. In this paper, high performance PDLCs were prepared by a multi-step polymerisation method using a photomask through a polymerisation-induced phase separation (PIPS) process. The effect of the photomask on the polymerisation of polymerisable monomers was investigated and it was confirmed that the photomask could effectively polymerise the polymerisable monomers in the light-transmitting fraction preferentially to form polymer-dominated spacer columns. In the subsequent complementary polymerisation process, the liquid crystal content of the previously unpolymerised fraction increases, which leads to a decrease in threshold and drive voltage and an increase in contrast of the PDLC film. Compared to conventional polymerisation methods, the multi-step polymerisation method can reduce the threshold voltage to 38.5 V, the drive voltage to 55.8 V and the contrast to 42, which greatly improves the overall electro-optical properties of PDLC, and the formation of spacer columns improves the bonding of the film, offering new possibilities for achieving energy efficient PDLC films and flexible films.

Effects of formulation composition and CeO2 nanoparticles doping on the morphologies of polymer spacer columns and electro-optical properties of PDLC

Polymer dispersed liquid crystal (PDLC) with polymer spacer columns were prepared by ultraviolet (UV) stepwise polymerization induced phase separation (PIPS) process. The influences of the formulation composition, such as the internal proportion of monomers and the liquid crystal (LC) content, on the morphologies of polymer spacer columns and electro-optical properties of PDLC films, were researched. Due to the difference in cross-linking density and polymerization rate, PDLC films exhibit different morphologies of polymer spacer columns and electro-optic properties. Based on the above, the CeO2 nanoparticles were doped into PDLC films. The influences of the doping content of CeO2 nanoparticles on the polymer network and electro-optical properties of PDLC were studied, and the EDS spectra were used to characterize the extent of dispersion of CeO2 nanoparticles in the polymer matrix. The results of the study showed that the contrast ratio (CR) of PDLC film increases from 10.9 to 27.3 at 0.8% CeO2 nanoparticles doping, while the saturation voltage (Vsat) decreases from 71.3 V to 47.9 V. Therefore, the work offers a new approach to the preparation of PDLC films with excellent comprehensive performance.

Faster switching polymer dispersed liquid crystal devices incorporated with functionalized SWCNTs

Polymer dispersed liquid crystals (PDLCs) were fabricated using acrylate-based UV curable monomer and a low birefringent nematic liquid crystal, 5PCH, by polymerization induced phase separation method. The effect of ODA-SWCNT dispersion on electro-optical properties of PDLC was investigated and compared with the undoped PDLC. The total response time reduces by 70%, and threshold voltage decreases by 42% in PDLC prepared with 0.015 wt% ODA-SWCNT nanocomposites of 5PCH, compared to the PDLC prepared with pure 5PCH. The optical transparency and sharpness of the image in ON-state was found to be remarkably better in the ODA-SWCNT incorporated PDLC as compared to the PDLC prepared with pure 5PCH.

The effect of the LC mixtures with the different clearing point on temperature dependence of the electro-optical properties of polymer dispersed liquid crystals

Two groups of polymer dispersed liquid crystals (PDLCs) have been studied, results indicated ,when the liquid crystals (LCs) and the UV monomers before curing in the preparation had poor miscibility, the PDLCs showed poor electro-optical properties. Then, we obtained a series of LC mixtures based on E8 and B001, the clearing point of which changed from 71.6 °C to 142.0 °C. The further study on the effect of temperature on the electro-optical properties of PDLCs indicated that PDLCs with high temperature resistance may be obtained by increasing the clearing point of LCs coupled with manipulating other LC parameters.

Influence of ZnO NPs on morphological and electro-optical properties of polymer-dispersed liquid crystals

ABSTRACT Polymer dispersed liquid crystal (PDLC) is a striking material and has an enormous role in electro-optic display devices. In this paper, nanoparticles (NPs) doped PDLCs were prepared from a mixture of UV curable liquid polymer, nematic liquid crystal (LC), and ZnO NPs by the polymerisation induced phase separation (PIPS) process. The morphological and electro-optical properties of PDLC with various diluent contents, liquid crystal contents, and nanomaterial doping were investigated comprehensively, and a morphological study shows that multiple factors jointly affected the droplet size, thereby altering a variety of properties including driving voltage and contrast. Compared with pure PDLC, the NPs doped PDLC reduces the threshold voltage from 39.5 V to 21.1 V and the driving voltage from 71.0 V to 45.0 V, which greatly improves the electro-optical performance of PDLC with a few sacrifices in contrast. This opens new possibilities for achieving a wide range of applications for energy-efficient PDLC.

A study on electro-optical properties of polymer dispersed liquid crystal films with thiol-isocyanate-ene ternary network prepared by nucleophile-initiated thiol-ene click reaction and thiol-isocyanate coupling reaction

ABSTRACT A series of polymer dispersed liquid crystal (PDLC) films with thiol-isocyanate-ene ternary network were successfully prepared by nucleophile-initiated thiol-ene click reaction and thiol-isocyanate coupling reaction. The effects of isocyanate content, reaction temperature and liquid crystal (LC) content on the electro-optical properties of PDLC were investigated. It is found that with the decrease of isocyanate content or reaction temperature, there are lower voltages as well as higher transmittance. When the LC content increases, the polymer structure changes and the phase inversion occurs. The results indicate that PDLC films with good electro-optical properties could be prepared by nucleophile-initiated thiol-ene click reaction and thiol-isocyanate coupling reaction, providing a new idea for PDLC preparation.

A study on electro-optical properties of polymer dispersed liquid crystal films doped with barium titanate nanoparticles prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACT Polymer-dispersed liquid crystal (PDLC) films doped with barium titanate (BTO) nanoparticles were prepared by nucleophile-initiated thiol-ene click reaction. The effects of BTO content and reaction temperature on the electro-optical properties of PDLC were explored. It is found that the size of liquid crystal droplets is bigger when the amount of BTO increases. As the BTO content increases, the transmittance first rises and then falls, while the voltage that drives the liquid crystal orientation first drops and then rises. The electro-optical properties of PDLC doped with BTO show a similar changing trend with the change of reaction temperature. However, when the reaction temperature becomes higher, the influence of BTO on the electro-optical properties of PDLC becomes greater. Doping BTO helps to obtain PDLC films with good electro-optical properties.

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

ABSTRACTThe nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.

Fabrication of dye-doped polymer-dispersed liquid crystals with low driving voltage based on nucleophile-initiated thiol-ene click reaction

ABSTRACTDye-doped polymer-dispersed liquid crystal (PDLC) is an attractive material for application in smart windows. We report a novel dye-doped PDLC system with low driving voltage (Vdr) fabricated by nucleophile-initiated thiol-ene click reaction for the first time. The Vdr decreased significantly with the addition of monofunctional monomer. The influences of reaction temperature on the electro-optical properties of PDLC are also investigated in this work.

Analysis of morphological behaviour and electro-optical properties of silica nanoparticles doped polymer dispersed liquid crystal composites

We have prepared silica nanoparticles (SNPs) doped polymer dispersed liquid crystal (PDLC) composites by using varying concentrations of SNPs. The varying concentration of SNPs has a profound impact on the morphology and electro-optical properties of PDLC. The polarizing optical microscopy (POM) studies revealed a substantial non- homogeneity/dual sized droplet morphology in the LC droplet size induced by the SNPs. The relative transmission was enhanced in the SNP-PDLC composites as compared to the undoped PDLC. The threshold voltage (Vth) of the doped composites decreased as compared to the undoped PDLC but increasing SNP concentration had a slight impact on the threshold voltage (Vth) as well as transmission. The effect of SNPs on nematic-isotropic transition temperature (TN-I) and ionic conductivity was also clarified.

Preparation and Performance Testing of Flexible PDLC Films

A new kind of polymer dispersed liquid crystal (PDLC) films based on nematic liquid crystal E7 and pre-polymeric composition epoxy resin E51 were prepared by polymerization induced phase separation via heating curing in this paper. The electro-optical properties of PDLC films were affected by many processing factors, for example, the weight ratio of the pre-polymer and liquid crystal, the curing temperature, curing time and film thickness. The electro-optical properties of PDLCs fabricated under different conditions were tested. The performance parameters such as contrast ratio, threshold voltage and the saturation voltage were calculated. In our case, the optimal preparation condition was 65% liquid crystal content with a curing time of 6 hours at 65oC. The PDLC film prepared in this paper showed a threshold voltage of 14.11V and a saturation voltage of 49.47V. The effect of film thickness on the electro-optical properties of PDLCs had been analyzed as well. The threshold voltage and saturation voltage of PDLC increase as the film thickness increases as so as the contrast ratio.

A review of the electro-optical properties and their modification by radiation in polymer-dispersed liquid crystals and thin films containing CdSe/ZnS quantum dots

This review summarizes our results on the electro-optical properties of switchable periodic structures holographically synthesized in polymer-dispersed liquid crystals (PDLCs) and of thin polymer films containing CdSe/ZnS quantum dots. In particular, we discuss the (1) effects of the microstructure and gamma-radiation on the electro-optical properties of PDLCs, (2) switching characteristics of PDLC-based periodic structures, as well as of complementary “reverse-mode” structures, and (3) effects of simultaneous irradiations by the 532-nm laser and gamma-rays on the photoluminescence characteristics of thin films containing CdSe/ZnS quantum dots.

Polymer-dispersed liquid-crystal voltage sensor

We present a novel electric-field and voltage sensor based on the electro-optical properties of polymer-dispersed liquid-crystals (PDLCs). In principle, the transmittance of PDLCs is a nonlinear function of the applied electrical field. To measure an AC field we superposed to it a known DC field. This allowed us to achieve linearization of the PDLC response and to measure transmittance changes independently of the light-intensity level variations. Validation experiments are presented.

TSC study and electro-optical properties of epoxy/acrylic polymer-dispersed liquid-crystal film in DICY thermal cure

We report on the structures and electro-optical properties of epoxy/acrylic polymer-dispersed liquid-crystal (PDLC) films. A thermal stimulated current (TSC) analysis was used to investigate the physical structures of PDLC. In the TSC spectrum of PDLC, three relaxation peaks were observed: the glass transition of the liquid crystal, the glass transition of the polymer matrix, and the ρ transition. The ρ transition represents the discharge behavior of space charges, and its intensity increased as the curing time and content of the curing agent dicyandiamide (DICY) increased. The pre-UV-cured films with different DICY contents were thermally cured at 130 °C for various periods. The electro-optical properties of PDLC, such as the contrast ratio and switching voltage, increased as the curing time of DICY, the content of DICY, or both increased. As the ambient temperature increased from 10 to 40 °C, the contrast ratio and switching voltage of PDLC gradually decreased. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 507–514, 2001

Stability Studies on Polymer Dispersed Liquid Crystal Films

Polymer Dispersed Liquid Crystal(PDLC) is a class of electroscattering materials exhibiting very promising properties for display device applications. For the last few years, the electrooptical properties of PDLC have improved drastically to use projection device with high brightness. We have studied stability of PDLC films which formed by ultraviolet initiated photopolymerization and liquid crystal phase separation. We evaluated several stability items in PDLC films such as running and environment test, accelerated life time at low and elevated temperature, voltage holding ratio(VHR) variations according to UV exposing time and temperature.