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

ABSTRACT

In 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.     

The fabrication of polymer dispersed liquid crystal based on SiO2 photonic crystal template

The conductance of polymer matrix is an important factor for the property of the polymer dispersed liquid crystal (PDLC). The nanographites are dispersed into the polymer matrix for optimising the dielectric conductive property. The synthesised nanoparticles SiO₂ was used as photonic crystal (PC) to work as a template for fabricating PDLC films. A mixture of pre-polymer and liquid crystals (LCs) was infiltrated into the void of the PC and polymerised under ultraviolet light. The void of the PC made uniform the dispersion of the liquid crystals in the films. The optical property of the PDLC films was optimised by doped nanographites and negative charge SiO₂ template. The effect of negative charge SiO₂ and nanographites on the threshold voltage and driving voltage was researched. The morphology of the PDLC films was studied by the FTIR image. The dispersed LCs droplets were uniformly affected by the addition of the nanographites. The LCs droplets dispersed in the polymer were located in the void of the SiO₂ photonic crystal.     

大分子引发剂分子量对接枝聚合物基体的PDLC电光性能的影响

以可逆加成-断裂链转移(RAFT)、引发转移终止(iniferter)活性自由基聚合相结合的方法,用一步法制备了不同分子量的大分子引发剂RAFT-PS-co-PCMSI(MI),并通过紫外光聚合诱导相分离法制备了以接枝聚合物为基体的聚合物分散液晶(PDLC)膜.研究了不同分子量的MI对PDLC的微观形貌,关闭状态透光率,阈值电压,饱和电压以及记忆效应等方面的影响.研究表明,降低PDLC中MI的分子量,会使得液晶微滴粒径增大,阈值电压(Vth)、饱和电压(Vsat)减小,记忆效应、关闭状态透光率升高.     

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).     

Liquid Crystal-Polymer Composite Materials

Polymer dispersed liquid crystal (PDLC) films can be switched electrically from a light-scattering off-state to a highly transparent on-state. Thin films were prepared via a polymerization-induced phase separation process, using electron beam radiation. The liquid crystal (LC)/polymer materials were obtained from blends of an eutectic nematie mixture E7 and a polyester acrylate-based polymer precursor. The optical and electro-optical properties of the PDLC films obtained depend strongly on the LC concentration. The LC solubility limit in the polymer matrix and the fractional amount of LC contained in the droplets were determined by means of calorimetrie measurements.     

Propriétés optiques et thermophysiques de composites polyméres-cristaux liquides (PDLC)

Polymer dispersed liquid crystal (PDLC) films can be switched electrically from a light scattering off-state to a highly transparent on-state. Thin films were prepared via a polymerization-induced phase separation process using electron beam radiation. The liquid crystal (LC)/polymer materials were obtained from blends of a eutectic nematic mixture E7 and a polyester acrylate based polymer precursor. The optical and electrooptical properties of the obtained PDLC films strongly depend on the LC concentration. The LC solubility limit in the polymer matrix and the fractional amount of LC contained in the droplets were determined by calorimetric measurements.     

Effects of hyperbranched prepolymers prepared from butyl acrylate and butyl methacrylate on the electro‐optical properties of polymer dispersed liquid crystal

Two hyperbranched prepolymers were synthesized via the reversible addition‐fragmentation chain transfer copolymerization of butyl acrylate or butyl methacrylate with divinyl benzene, respectively. These prepolymers were used in the photopolymerization‐induced phase separation process of preparing polymer dispersed liquid crystal (PDLC) films with no risk of gelation. The morphologies of PDLC samples were investigated. Of particular interest was that the driving voltage (V₉₀) of PDLC with high glass transition temperature (T_g) matrix was as low as 5 V, whereas the V₉₀ of PDLC with low T_g matrix was still high. An explanation for this phenomenon is the hypothesis that different interaction modes function at the interface of liquid crystal and polymer. Meanwhile, the hysteresis of the two systems was minor, which correlated with the hyperbranched structure of polymer. The on‐state transmittance (T_ON) of PDLC films enhanced because of the well matching between the refractive index of polymer and that of liquid crystal. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Digital optical memory devices based on polymer-dispersed liquid crystals films: appropriate polymer matrix morphology

ABSTRACT

Permanent memory effect (PME) in polymer dispersed liquid crystal (PDLC) allows a greater applicability range than traditional PDLCs. One of the most interesting application could be the possible storing of optical information, the so-called Digital Optical Memory (DOM) devices. To test this application it would be required a display structure having an array of pixels addresses. Each pixel was filled with PDLC film with PME and electric field can be independently applied to different PDLC elements to define on/off pixel states (transparent or scattering states).PDLC films were obtained from a mixture of E7 nematic liquid crystal and poly(ethylene glycol) dimethacrylate with 875 g mol^-1oligomer as precursor of the polymeric matrix. The effect of the curing temperature and the UV light intensity as well time exposure during the polymerisations on the electro-optical performance of PDLC films were investigated. In this way, a high transparency state (T_OFF=55%) for a long period of time at room temperature even after the applied voltage has been switched off were obtained, started from an opaque state (T₀=0%) and after reaching a transparent state (T_MAX=75%), which causes 73% PME. The application to an 8x8 passive matrix using PDLC with PME is also demonstrated as proof-of-principle.     

Preparation and electro‐optical properties of polymer dispersed liquid crystal films with relatively low liquid crystal content

In this paper, polymer dispersed liquid crystals (PDLC) films with LC content as low as 40 wt% were prepared, and the electro‐optical properties were carefully investigated. To accomplish this, different (meth)acrylate copolymerizaiton monomers have been used. The electro‐optical properties and morphologies of the PDLC films were strongly influenced by the chemical structure of copolymerization monomers (hydroxypropyl methacrylate (HPMA), glycidyl methacrylate, hydroxypropyl acrylate) and their feed ratio. Lower driven voltage and higher contrast ratio were achieved when the PDLC films showed a morphology with suitably LC domain size. At high HPMA content, a thin polymer film was formed on the surface of PDLC samples, which is beneficial to decrease the total LC content in PDLC devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation of polymer-dispersed liquid crystal doped with indium tin oxide nanoparticles

To study polymer-dispersed liquid crystal (PDLC) films doped with indium tin oxide (ITO) nanoparticles (NPs), samples were prepared by ultraviolet-initiated polymerisation based on the thiol–acrylate system. Owing to the interaction between PDLC system and ITO NPs, the content and the size of ITO NPs are the main determinants to the microstructure which plays an essential role on the electro-optical and anti-infrared properties of the PDLC films. In the polymer matrix, a novel microstructure consisting of a dense surface, micron-sized meshes and submicron meshes is found to benefit the better performances of the low driven voltage (20.7 V), the relatively high contrast ratio (8.3) and the lowest transmittance(500–2500 nm) on average at about 3.55% with maximum of merely 7.6%. Thus, it lays a solid foundation for the further investigations on the microstructure and the performance of the PDLC films. Meanwhile, it is proved that the PDLC film, improved performance through doping ITO NPs, is promising to be a superior choice in the field of energy-saving.     

Influence of chain length of prepolymers in permanent memory effect of PDLC assessed by solid-state NMR

ABSTRACT

The relationship between linear chain (ethylene oxide units) length of polymerisable monomers with morphology, electro-optical properties and ¹³C nuclear magnetic resonance (NMR) spectroscopy of the corresponding polymer-dispersed liquid crystal (PDLC) films was investigated. The preferred liquid crystal molecule alignment and permanent memory effect of PDLC were greatly influenced by the length of the molecular chain of prepolymers to be incorporated as a polymer matrix. By increasing the number of ethylene oxide in prepolymer chain and maintaining the number of functionalities (polymerisable groups in each monomer molecule), the permanent memory effect of PDLC increased, as proved by solid-state ¹³C NMR spectroscopy.     

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.     

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.     

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.     

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.     

Effect of electric field on phase separation of polymer dispersed liquid crystal

The kinetics of the polymerization induced phase separation of liquid crystal (LC)/monomer mixture has been investigated by means of depolarized light intensity technique and polarized light microscope (PLM). To examine the effect of the electric field, a DC electric field was applied across the mixtures during the phase separation process. The kinetic study indicates that the phase separation process is accelerated when the electric field is applied. The morphologies of the formed polymer dispersed liquid crystal (PDLC) films were observed by PLM. The electric field applied during the phase separation process yields the PDLC with small LC domains and fine morphologies. The clearing temperature (T_NI) of the formed PDLC films was measured by the PLM and it is found that the T_NI increases with the applied electric field intensity.     

High on-state clarity polymer dispersed liquid crystal films

Abstract

Results are presented which show that the on-state clarity of a UV cured polymer dispersed liquid crystal (PDLC) film depends on the refractive index of the final polymer in the PDLC film, the ordinary refractive index of the liquid crystal, the solubility of the liquid crystal in the prepolymer and the rate at which the film is cured. Liquid crystal mixtures for use in PDLC films are chosen such that the ordinary refractive index of the liquid crystal is equal to the refractive index of the polymer matrix. It has been shown previously that a large quantity of liquid crystal remains dissolved in the polymer matrix, thus increasing the mismatch between the refractive index of the polymer and the ordinary refractive index of the liquid crystal and therefore reducing the on-state clarity. For liquid crystal mixtures which have high solubility in the prepolymer (>60 per cent) the mismatch in the refractive indices can be very large and the on-state clarity of the resulting film can be very poor (T _on<70 per cent). Results are presented which show that it is possible to increase the on-state clarity of such films by increasing the rate at which these films cure. If the liquid crystal is less soluble in the prepolymer (<45 per cent), a PDLC film formed from such a liquid crystal/ prepolymer system often has very good on-state clarity (T _on>75 per cent) be it cured slowly or quickly. Results are also presented which show that in order to achieve a true measure of on-state clarity it is necessary to use a small collection angle (<3°) in the detecting optics. If larger collection angles are used, the photodetector collects light which is scattered out of the specular beam, thus leading to a false measure of on-state clarity.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.     

Orientation and anchoring effects in stretched polymer dispersed nematic liquid crystals

The induction of liquid crystal orientation through mechanical stretching was investigated for polymer dispersed liquid crystals (PDLCs) by means of infrared dichroism. Using a nematic liquid crystal BL006 and polyacrylic acid as the polymer matrix, it was possible to stretch the PDLC films with BL006 in either the isotropic or the nematic phase. After cooling the films under strain to room temperature, the molecular orientation of BL006 was found to be much higher for films that contained isotropic liquid droplets of BL006 at the time of stretching than for films that had nematic droplets. Stretching PDLC films with isotropic droplets results in no molecular orientation, but the orientation is induced during the subsequent cooling when BL006 goes through the isotropic-to-nematic phase transition. Interestingly for PAA/BL006, the nematic director orients along the long axes of the elongated droplets despite liquid crystal anchoring perpendicular to the polymer interface.