Electro-optical Properties Of Polymer Dispersed Liquid Crystal Films Research Articles

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.

Dual-field responsive polymer-dispersed liquid crystal films with polymer spacer columns and fluorescent properties

ABSTRACT Polymer dispersed liquid crystal (PDLC) films with polymer spacer columns were prepared by photomask stepwise polymerisation method. The effects of crosslinker content and liquid crystal (LC) content on the morphologies of polymer spacer columns and electro-optical properties of PDLC films were investigated. Due to the difference of crosslink density and system viscosity, the polymer spacer columns exhibit different morphologies and the electro-optical properties of PDLC films are also altered. Based on the above, the PVA coatings loaded with rare earth phosphor were introduced into PDLC films. The influences of rare earth phosphor concentration on the fluorescent and electro-optical properties of PDLC films were studied. Also, it was found that the UVaging resistance of PDLC films was enhanced because of the loading of rare earth phosphor absorbing UV light. Therefore, the successful preparation of PDLC films with polymer spacer columns and reversible fluorescence properties opens new possibilities for expanding the flexible display applications of PDLC.

Polymer dispersed liquid crystals doped with CeO2 nanoparticles for the smart window

ABSTRACT In this paper, rare earth oxide CeO2 nanoparticle-doped polymer dispersed liquid crystal (PDLC) films were prepared by the polymerisation induced phase separation (PIPS) process. The effects of the crosslinking agent TMPTA content, liquid crystal (LC) content and the CeO2 content on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It has been found that by changing the TMPTA content, LC content and doping a small amount of CeO2 nanoparticles, the electro-optical properties of PDLC films can be optimised. After adding a low concentration of CeO2 nanoparticles, the electro-optical properties of the PDLC film have been well optimised: after adding 0.8% CeO2, Vth decreased from 26.6 V to 16.8 V, and CR increased from 89.5 to 137.6. Consequently, this work offers a meaningful approach to optimise the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.

Enhancement of electro-optical properties of polymer dispersed liquid crystals by doping with nano-sized electrospun fibers

In this paper, in the mixed system of five kinds of acrylate polymeric monomers, nematic liquid crystals and reticular nanofiber films, the polymer dispersed liquid crystal (PDLC) films were prepared by the polymerization-induced phase separation method, and their properties were analyzed by combining the electro-optical curves and microstructure. The experimental results have proved that the cavity size of the polymer network can be controlled by adjusting the ratio of photo-polymeric monomers and the content of liquid crystal, thus directly affecting the electro-optical properties of PDLC films. This research will improve the potential technical applications of PDLC devices inflexible displays.

Effects of oxygen heterocyclic acrylate monomers on the morphologies and electro-optical properties of polymer dispersed liquid crystal composite films

The current work aimed to fabricate polymer dispersed liquid crystal (PDLC) films using heterocyclic acrylate monomers containing oxygen atoms. The incorporation of oxygen heterocycles into the host polymer matrix significantly improved the electro-optical properties of PDLC films. Particularly, the high contrast ratio 330.0 was realized for the monomer Trimethylolpropane formal acrylate (TFA). Further, the excellent electro-optical properties, including low driving voltage (10.0 V), moderately high contrast ratio (64.0) and short response time (<8.0 ms) were obtained when the liquid crystals/polymer composite film thickness decreased from 20 μm to 8 μm. This work has provided a novel materials system to promote the application of PDLC films in the thin-layer liquid crystal display.

Influence of SiO2 nanoparticles on electro-optical and thermophysical properties of polyacrylate/liquid crystal composites

ABSTRACTIn this paper, SiO2 nanoparticle-doped polymer dispersed liquid crystal (PDLC) lenses are made from a mixture of prepolymer, 5CB liquid crystal and SiO2 nanoparticles by the polymerisation induced phase separation (PIPS) process. The effect of SiO2 nanoparticles on the electro-optical properties of PDLC films are studied. It is established that SiO2 nanoparticles affect the microstructure of PDLC films significantly because of the formed agglomerates of SiO2 nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the response time for doped systems with small amount of SiO2 nanoparticles. We also observe a shift of nematic–isotropic transition temperature as a function of SiO2 nanoparticle contents. A good agreement between the electro-optical study and thermophysical properties is reached.

A study on the polymer structures and electro-optical properties of epoxy-mercaptan-based polymer dispersed liquid crystal films

ABSTRACTIn this paper, polymer dispersed liquid crystal (PDLC) films based on epoxy-mercaptan system were prepared by thermal-initiated polymerization. The effects of the liquid crystal (LC) content, the proportion and the functionality of epoxy monomers on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It was found that the morphologies of the polymer matrix can be altered from polymer meshes to polymer balls by increasing the LC content as well as the functionality of epoxy monomers. Accordingly, the electro-optical properties could be regulated by the morphologies of polymer networks. Especially, the as-made PDLC films with homogeneous porous structures exhibited the optimal electro-optical properties. Consequently, this work offers a meaningful approach to control the microstructures and optimize the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.

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

ABSTRACTThe 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.

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

ABSTRACTEffects 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.

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.

Effects of Mesogenic Compound with High Birefringence on Electro-Optical Properties of Heat-Cured Polymer-Dispersed Liquid Crystal Films

Addition of mesogenic compound with high birefringence in nematic liquid crystal resulted change in the electro-optical properties of polymer-dispersed liquid crystal (PDLC) films considerably. The PDLC films were prepared from thermally polymerization-induced phase separation in heat curable monomers/nematic liquid crystal mixtures, and the effect of mesogenic compound with high birefringence on the electro-optical properties of PDLC films was studied. Therefore, when the mesogenic compound was doped in different concentrations, their quality fraction played considerable role in deciding the electro-optical parameters of the PDLC films.

Influence of the multi-functional epoxy monomers structure on the electro-optical properties and morphology of polymer-dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation method with nematic LC content as low as 40 wt%, and the electro-optical properties were carefully investigated. To accomplish this, the structure of multi-functional curable epoxy monomers with different composition feed ratios and the weight percentages of the two groups were examined in this study. The combined effects of heat-curable monomers’ structure on the conspicuous morphology of polymer network of PDLC films formed small holes and suitably distributed coin-like networks in both groups A and B, respectively. The detailed characteristics and morphology of polymer network of PDLC films were analyzed by employing liquid crystal device parameter tester, UV-Vis-NIR spectrophotometer and scanning electron microscope. Meanwhile, the enhanced curing temperature effects on the alkyl chain length, short flexible chain length, and rigid chain segment containing epoxy monomers structure on the increasing morphology of polymer network as well as electro-optical properties of PDLC films were also studied. It was found that the LC domain size of the polymer network could be regulated by adjusting the structure and composition ratio of curable epoxy monomers, and then the electro-optics of the PDLC films could be optimized, which is beneficial for decreasing the total LC content in PDLC devices.

Effects of a triethylamine catalyst on curing time and electro-optical properties of PDLC films

Polymer dispersed liquid crystal (PDLC) films can be prepared by the thermal polymerization-induced phase separation method, but the curing time is more than 7 hours, which leads to high energy consumption. In this study, the effects of triethylamine as a catalyst on the curing process and the electro-optical properties of PDLC films have been studied. Furthermore, PDLC films are prepared with different amounts of triethylamine to investigate the best ratio of the catalyst. It was found that using triethylamine as the catalyst could shorten the curing time from 7 hours to 1 hour, and the film with 5 wt% of triethylamine has the best electro-optical properties, and is as good as one prepared by the traditional method. All in all, employing triethylamine as a catalyst has been proved to be an energy efficient way of making PDLC films.

大分子引发剂的主链柔性对PDLC电光性能的影响

Polymer dispersed liquid crystal(PDLC) films were prepared with living graft macro-initiator(MI) with different main-chain flexibility,synthesized by reversible addition-fragmentation chain transfer(RAFT) and iniferter polymerization with four kinds of monomers with different glass transition temperature.The effect of main-chain flexibility of MIs on morphologies,transmittance,switching voltage and hysteresis of PDLC films based on graft polymer had been investigated.It was observed that the OFF-state transmittance(TOFF) and the size of LC droplet decreased at first and then increased with the increase in main-chain flexibility of MIs,and both showed the lowest value when MI was RAFT-P(BMA-co-CMSI).However,Vth and Vsat showed an opposite trend.It was found that the Vth value first increased from 8.0 V to 17.7 V and then decreased to 7.3 V,while the Vsat value first increased from 25.6 V to 32.9 V and then decreased to 20.2 V.In addition,the hyste-resis showed complex changes,due to a variety of factors.Therefore,the change of main-chain flexibility of MIs successfully improved the electro-optical properties of PDLC films based on graft polymer.

The UV polymerisation temperature dependence of polymer-dispersed liquid crystals based on epoxies/acrylates hybrid polymer matrix components

Hybrid polymers, which form a combination of monomers with different reactive groups and curing mechanisms, synergistically combine the properties of the two constituent homopolymers. In this paper, polymer-dispersed liquid crystal (PDLC) films using epoxies / acrylates as hybrid polymer matrix components were prepared. The UV polymerisation temperature dependence of the microstructure and the electro-optical properties of PDLC films based on epoxies / acrylates hybrid polymer matrix had been studied. The electro-optical properties of PDLC films depend strongly on the liquid crystal (LC) domain size and morphology of PDLC films, which could be controlled by the polymerisation rate of the curable monomers and the diffusion rate of the LC molecules during the film preparation process. The polymerisation rate of the curable monomers and the diffusion rate of the LC molecules depend exponentially on the UV polymerisation temperature. Therefore, this research reports an example of methods to prepare PDLC films based on a hybrid polymer matrix and obtain PDLC films which possess good electro-optical properties.

The influence of the structure of curable epoxy monomers on the electro-optical properties of polymer dispersed liquid crystal devices prepared by UV-initiated cationic polymerisation

Electro-optical properties are important characteristics of polymer dispersed liquid crystal (PDLC) devices. In this paper, PDLC films based on liquid crystal (LC)/curable epoxy monomers/cationic photoinitiator composites were prepared by ultraviolet-initiated (UV-initiated) cationic polymerisation using triphenylsulfonium hexafluoroantimonate (Ar3S+SbF6 −) as the photoinitiator. The effects of the structures of curable epoxy monomers on PDLC films were investigated. The microstructures of the PDLC films were strongly influenced by the curable epoxy monomers, which in turn influenced the electro-optical properties of PDLC films. It was found that the LC domain size of the polymer network could be regulated by adjusting the structure of curable epoxy monomers, and then the electro-optical properties of PDLC films could be optimised.

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.

Studies on electro-optical properties of polymer dispersed liquid crystal films based on epoxy resins prepared by UV-initiated cationic polymerisation

Epoxy polymer dispersed liquid crystal (PDLC) films based on ethyleneglycol diglycidyl ether (EGDE)/nematic liquid crystal (SLC1717) composites were prepared by ultraviolet-initiated cationic polymerisation using triphenylsulfonium hexafluoroantimonate (Ar3S+SbF6 −) as the photoinitiator. The effect of the composition of the mixture and curing temperature on the microstructure of the polymer network and the electro-optical properties was investigated. It was found that the liquid crystal (LC) domain size of the polymer network could be regulated by adjusting the LC content and the curing temperature. The electro-optical properties of PDLC films could then be optimised.

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.

Temperature dependence of electro-optical characteristics of polymer dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films consist of microdroplets of a liquid crystal dispersed in a polymer matrix. Their applications are based on the electrically controllable light scattering properties of the liquid crystal droplets. The effects of temperature on the electro-optical properties of PDLC films have been rarely investigated. In this work, we studied the light transmission on varying the temperature and frequency. It was observed that the transmission at a fixed voltage decreased with increasing temperature above 43°C, independent of frequency. We examined possible origins of this unusual dependence of the transmission on the temperature. It was concluded that conductivity effects due to free ions newly created at high temperatures could be responsible for the unusual behaviour observed.