Polymer Dispersed Liquid Crystal System Research Articles

Effect of a Polymercaptan Material on the Electro-Optical Properties of Polymer-Dispersed Liquid Crystal Films.

Polymer-dispersed liquid crystal (PDLC) films were prepared by the ultraviolet-light-induced polymerization of photopolymerizable monomers in nematic liquid crystal/chiral dopant/thiol-acrylate reaction monomer composites. The effects of the chiral dopant and crosslinking agents on the electro-optical properties of the PDLC films were systematically investigate. While added the chiral dopant S811 into the PDLC films, the initial off-state transmittance of the films was decreased. It was found that the weight ratio among acrylate monomers, thiol monomer PETMP and the polymercaptan Capcure 3-800 showed great influence on the properties of the fabricated PDLC films because of the existence of competition between thiol-acrylate reaction and acrylate monomer polymerization reaction. While adding polymercaptans curing agent Capcure 3-800 with appropriate concentration into the PDLC system, lower driven voltage and higher contrast ratio were achieved. This made the polymer network and electro-optical properties of the PDLC films easily tunable by the introduction of the thiol monomers.

Effects of functionality of thiol monomer on electro-optical properties of polymer-dispersed liquid crystal films

ABSTRACTIn 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.

Effect of an additive on the permanent memory effect of polymer dispersed liquid crystal films

BACKGROUND Polymer dispersed liquid crystal (PDLC) films have a range of applications varying from switchable windows to new optical memory devices. They switch from an opaque state, when liquid crystal molecules are randomly dispersed, to a transparent state, when liquid crystal molecules are aligned, simply by applying a small voltage. On some PDLC films, the transparent state is retained even when the voltage is switched off; this is called permanent memory effect (PME). This process is totally reversible, returning to the initial opaque state either through heating or applying a higher frequency electric field when using a two frequency liquid crystal. RESULTS In this work, the impact on the electro-optical response of PDLC films as a result of adding an additive was studied. This performance is affected by several factors, the most important being the anchorage force. This anchorage force is modified with the addition of an additive such as TX-100. As a result, it was verified that the addition of TX-100, increased the PME up to 98.7% and decreased the voltage required to achieve 90% of maximum transmittance to 2 V µm−1. CONCLUSIONS The addition of a surfactant, as TX-100 in this case, was shown to be a valuable method to control the PME in PDLC systems in particular those prepared by PIPS from the oligomer polyethylene glycol dimethacrylate and the monomer triethylene glycol dimethacrylate in mixture with the nematic LC E7. This new advance on liquid crystal field also allows development of a new memory device based on a write–read–erase cycle. © 2015 Society of Chemical Industry

Photocure kinetics and control of LC microdroplets for acrylate-based polymer dispersed liquid crystal with different methacrylate contents

This paper reports on the relationship between the photocure kinetics and the morphological properties of UV-curable acrylate-based polymer dispersed liquid crystal (PDLC) systems with the various 2-ethylhexylacrylate (2-EHA) and 2-ethylhexylmethacrylate (2-EHMA) contents using photo-differential scanning calorimetry (photo-DSC), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). Photo-DSC analysis using an autocatalytic kinetics model demonstrated that the cure extent and cure rate gradually decreased and then leveled off as the concentration of 2-EHMA increased up to 25 wt%, which was well confirmed by using FTIR spectroscopy. The morphological observations revealed that increasing the 2-EHMA content increased the size of the LC droplets within the acrylate-based polymer matrix. Notably, there was a marked increase in the number of LC droplets larger than 10 μm in PDLC composites containing more than 20 wt% 2-EHMA, suggesting that the addition of more than 20 wt% 2-EHMA to the acrylate-based PDLC compounds is not desirable. The presence of 2-EHMA clearly affected the photocure behaviors of acrylate-based PDLC compounds, which interestingly means that the slow reactivity of methacrylate monomer could be simply used to control the phase separation rate and the microstructures of LC droplets of acrylate-based PDLC systems. Open image in new window

Selective Amplitude-frequency Electro-optical Modulation by Polymer-dispersed Liquid Crystal Films Aligned by Teflon Nanolayers

We report the amplitude-frequency electro-optical (EO) modulation by a specific type of polymer-dispersed liquid crystal (PDLC) composites – planar single layers of large nematic microdroplets aligned by teflon nanolayers. Such a surface modifying PDLC system exhibits a selective modulated 2nd harmonic EO response by the dielectric oscillations of the nematic director. The bandlike behavioural characteristic of the amplitude-frequency modulation of light, achieved by the single-layered PDLC films, can be tuned by AC voltage applied on the PDLC cell. This property can be applied to tunable EO modulators operating in the infrasound frequency range.

Effect of Host Polymer Matrices on Electro Optical and Dielectric Behaviour of Polymer Dispersed Liquid Crystal System

The effect on the electrical characteristics of poly (methyl methacrylate) (PMMA) and poly (methyl methacrylate – co – butyl acrylate) (PMMABA) containing liquid crystal (E8) as dispersed material, is investigated in terms of electro optical and dielectric relaxation spectroscopy (DRS) technique. This polymer dispersed liquid crystal (PDLC) composite film is sandwiched between two glass slides with a transparent conductive substrate such as indium tin oxide (ITO). The resulting assembly forms an electro-sensitive material that can be switched from a OFF state to a ON state by the application of an external electric field or thermal ramp. In the present investigation PMMA / E8, PMMABA / E8 and PMMA+PMMABA / E8 composite films of 30/70 wt / wt % were prepared by Solution Induced Phase Separation (SIPS) method. Morphological study showed that homogeneous LC phase is embedded in a spongy – like polymer matrix. Electro optical behavior was determined under the condition of an externally applied AC electric field (0-200Vp-p, 50-1000Hz). DRS has been carried out in the frequency range from 20 Hz to 20 MHz and over the temperature range from 24°C to 75°C.The interfacial charge layer effect and qualitative evaluation of distribution of relaxation time is characterized by Cole-Cole model.

Study of polymer-dispersed liquid crystal systems using epoxies / acrylates as 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. It will be of interest to use these monomers together, with the aim of forming polymer-dispersed liquid crystal (PDLC) systems based on a hybrid polymer network. In this paper, PDLC systems using epoxies / acrylates as hybrid polymer matrix components were prepared. The effects of the ratio of epoxy monomers and acrylate monomers on morphology and the electro-optical properties of PDLC systems were investigated. It was found that the liquid crystal domain size of the polymer network could be regulated by adjusting the ratio of epoxy monomers and acrylate monomers, and that the electro-optical properties of PDLC systems could then be optimised.

Effects of chemical structure and composition of the polymer matrix on the morphology and electro-optical performance of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) systems on the basis of nematic liquid crystal E7 and amphiphilic binary copolymers of acrylic acid (AA) with such acrylates as 2-ethylhexyl acrylate (EHA), n-butyl acrylate (BA), and methyl acrylate (MA) are investigated. It is shown that the liquid crystal (LC) drops in the copolymer EHA–AA have submicrometre sizes, and their dependence on the composition of the photo-curable monomer mixture is described by a parabolic curve. The highly oriented domain structure in the same system is first revealed when electric field is applied. The threshold voltage for all systems begins to increase with some critical composition of a monomer mixture in which the longer the hydrocarbonic radical in an acrylate molecule, the higher the content of AA. The PDLC system based on the BA–AA copolymer with 30 wt% LC exhibits the least value of the driving voltage, 1 V μm–1, and the lowest memory effect.

Thermotropic and optical behaviour of new PDLC systems based on a polysulfone matrix and a cyanoazomethine liquid crystal

The thermotropic and optical characterisations of a novel polymer dispersed liquid crystal (PDLC) system based on polysulfone UDEL P-1700 as polymer matrix and a low molecular weight liquid crystal compound containing a mesogenic azomethine core and a cyano-substituent were investigated. The PDLC samples were prepared by solvent-induced phase separation (SIPS) and thermally induced phase separation (TIPS) methods using various compositions in the two components. Information on the morphology and phase structure was obtained by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. PDLC systems with well-defined droplets were obtained for the composite with medium content of liquid crystal compound. The optical characterisation of these materials was performed by analysing their UV–visible absorption and photoluminescence emission as a function of the liquid crystal aggregation degree.

Isotropic-nematic transition at the surface of a liquid crystal embedded in an aerosil network

We reexamined the isotropic-nematic (Iso-N) phase transitions of 4-n-heptyl cyanobiphenyl (7CB) embedded in aerosils of silica nanoparticle dispersions using highly sensitive differential scanning calorimetry (HS-DSC), polarizing optical microscopy (POM), and retardation measurements. We found a simple and very profound relationship between the calorimetric and optical measurements; in addition to double DSC peaks, which have been observed previously, a two-step change of the retardation was clearly observed by varying temperature. From our analysis, there is no doubt that the Iso-N liquid-crystal phase transition certainly occurs in two steps, i.e., the bulk transition takes place at first and then the surface transition takes place upon cooling. We should note that the surface transition takes place below the bulk transition temperature, the opposite of what was recently observed in a polymer-dispersed liquid-crystal system.

Electro-optical characteristics of dye-doped polymer dispersed liquid crystals

The electro-optical properties of dye-doped Polymer Dispersed Liquid Crystal (PDLC) films were examined as a function of the polymer UV curing temperature, injection temperature and applied voltage. In addition, the effects of the morphology on the electro-optical properties were examined. The contrast ratio and driving voltage of the PDLC films were affected considerably by changes in the UV curing temperature but the injection temperature had a much lesser effect. The color difference and electro-optic characteristics of the dye doped PDLC film were investigated, where the PDLC films were doped with RGB dyes for flexible display applications. The red-dye-doped PDLC film with small-sized particles showed good color differences. In this dye-doped PDLC system, the film driving voltage ( V on) of the PDLC increased with increasing liquid crystal droplet size, except for the TL205–prepolymer–red dye. In particular, the red-dye doped PDLC composition showed the lowest V on in the electro-optic characteristics of the dye-doped PDLC film.

The influence of crosslinking agents on the morphology and electro‐optical performances of PDLC films

AbstractThe morphology of polymer‐dispersed liquid crystal (PDLC) has a major impact on its electro‐optic properties. The PDLC was prepared by photochemical polymerization from UV‐curable monomers/crosslinking agent/initiator/LC mixture. The influences of crosslinking agents on the PDLC system had been studied. The microstructures of the PDLC were strongly influenced by the insertion of polyethylene glycol (600) diacrylate (PEGDA 600) crosslinker, relatively long and flexible branch, and diethylene glycol diacrylate (DEGDA) crosslinker along the network bone. The relative weight ratio of crosslinking agents and polymerization temperature had a significant effect on the LC domain size, and which resulted in influencing the electro‐optic properties of PDLC. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Effects of the chain length of crosslinking agents on the electro-optical properties of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared by photochemical polymerisation from an ultraviolet-curable monomer/crosslinking agent/initiator/liquid crystal (LC) mixture. The influences of the chain length of crosslinking agents on the PDLC system have been studied. The microstructures of the PDLC films were strongly influenced by the chain length of the crosslinker, which in turn influences the electro-optic properties of PDLC films. It is proved that adjusting the chain length of the crosslinking agents and the LC content can produce PDLC films that have good electro-optic properties with a lower LC content.

Polysulfone as polymer matrix for a novel polymer-dispersed liquid crystals system

We report the preparation and thermotropic characterization of a novel polymer-dispersed liquid crystal (PDLC) system based on the polysulfone (PSU) UDEL-1700 as a polymer matrix and on 4-[4-(n-butyloxy)phenylene-imino-methylidene]benzonitrile as a liquid crystal. PDLC films have been prepared using the thermal-induced phase separation (TIPS) technique, with different compositions in the two components. A detailed optical polarized microscopy and differential scanning calorimetry study has been carried out to evaluate a correlation between the composite composition and the PDLC formation. We found that PSU could be a good matrix for the formation of PDLC systems.

Polymer-encapsulated liquid crystals comprising montmorillonite clay

The morphologies of polymer-dispersed liquid crystals (PDLCs) and holographic PDLC (HPDLC) composites consisting of nematic liquid crystals filled with inorganic nanoparticles of either pristine montmorillonite (MMT) clay or organically modified MMT (OMMT) clay in polymer matrices are described. Different curing processes with ultraviolet irradiation and an Ar+ laser were applied to make the PDLCs and HPDLCs, respectively. In order to gain some prior knowledge of the dispersion of the inorganic nanoparticles in the polymer-encapsulated systems, wide-angle x-ray diffraction patterns from an MMT clay suspension in liquid crystal were observed at variable temperatures. Differential scanning photocalorimetry was carried out to observe the monomer conversion. The morphology of liquid-crystal droplets of the PDLCs was investigated by optical polarizing microscopy. The holographic nature of the HPDLCs was confirmed by both scanning electron microscopy and atomic force microscopy. It was found that the diffraction efficiency is markedly enhanced by doping a small amount of pristine clay platelets in the HPDLC. This preliminary study demonstrates the potential of a new optical device, which is made of MMT clay hybridized with liquid crystal in a PDLC system.

Polymer dispersed liquid crystals based on poly(styrene-b-ethylene oxide), poly(bisphenol a carbonate) or poly(methylphenylsiloxane), and 4′-(hexyloxy)-4-biphenyl-carbonitrile: Analysis of phase diagrams and morphologies generated

Thermodynamic phase behavior was studied for polymer dispersed liquid crystal (PDLC) films consisting of blends of different thermoplastics such as poly (bisphenol A carbonate) (PC), poly(methylphenylsiloxane) (PMPLSi), poly(styrene-b-ethylene oxide) (PSEO) block copolymer, and low-molecular weight 4′-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) liquid crystals. The phase diagrams of these PDLC systems were built up using thermal transitions determined by differential scanning calorimetry (DSC), transmission optical microscopy (TOM), and dynamic mechanical analysis (DMA). Final morphology of these PDLC films generated by temperature induced phase separation was monitored by TOM. The final morphology of HOBC blends with PC, PSEO, or PMPLSi could be modified by controlling the relative compositions resulting in PDLC systems with small and narrow size-distribution of nematic droplets, suitable for use in thermo-optical devices. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Humidity sensing based on nanoporous polymeric photonic crystals

In this work, an optical humidity sensor based on a nanoporous polymeric photonic crystal (PC) is demonstrated. The PC sensing structure is created by combining a holographic interference patterning technique with a modified holographic polymer-dispersed liquid crystal system. Changes in relative humidity (RH) induce the modification of the refractive index contrast between the nanoporous and nonporous regions, and thus the transmittance and bandgap position, of the PC structure. For a PC structure with 30% porosity and a grating spacing of 220 nm, a change in the RH from 40% to 95% at 34 °C results in a redshift of 43 nm in the central wavelength at the PC bandgap and an increase from 12% to 87% in the relative transmittance at λ = 600 nm. Other performance analyses have shown that the nanoporous polymeric PC-based humidity sensor is highly stable and reproducible, exhibits minimal hysteresis, and responds relatively fast.

Phase equilibria of polymer dispersed liquid crystal systems in the presence of an external electrical field

AbstractThe effect of an external electrical field on phase behaviors is reported for polymer dispersed liquid crystal films of 4′‐pentyl‐4‐biphenylcarbonitrile/poly(methyl methacrylate) binary mixtures with various polymer molecular weights. The experimental results show that increasing the molecular weight of the polymer or the electrical field intensity can give rise to an increase in the phase‐transition temperature and a widening of the binary phase region. The lattice theory, regarding a binary system consisting of a rigid nematic liquid crystal and a random polymer, has been extended to the case in which an external electrical field is present. A comparison of the theoretical predictions with the experimental results has been carried out, and satisfactory agreement has been found. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1898–1906, 2007

A novel process for holographic polymer dispersed liquid crystal system via simultaneous photo-polymerization and siloxane network formation

Formation of transmission holographic polymer dispersed liquid crystal gratings was studied for matrix components of trimethylolpropane triacrylate:trimethoxysilylmethyl methacrylate:1-vinyl-2-pyrrolidone (reactive diluent) in the range from 80:10:10,wt% to 10:80:10,wt% and E7 as nematic liquid crystal under the irradiation with Nd-YAG laser (532,nm). The optimum concentration of E7 in the recording solution was 35,wt% (65,wt% of matrix components) in presence of small amounts of radical photo-initiator system (Rose Bengal 0.05,wt%, NPG 0.1,wt%). When the concentration of methacrylate in matrix components was low ( 50,wt%) of the methacrylates. Contrary to that gratings with reasonably high and stable diffraction efficiency were successfully fabricated with trimethoxysilylmethyl methacrylate, gratings with only low diffraction efficiency were obtained for non-cross-linkable trimethylsilylmethyl methacrylate. With higher concentration of photo-sensitizer and photo-initiator (0.2,wt%; 1.0,wt%), and shorter irradiation time, gratings with high diffraction efficiency could be fabricated only for the photo-initiator system of 3,3′-carbonylbis(7-diethylaminocoumarin) and diphenyliodonium hexafluorophosphate with shorter induction period (∼174,s). Grating with diffraction efficiency of 72% was obtained with trimethoxysilylpropyl acrylate (80,wt% in the matrix component) and 35,wt% E7. By increasing the concentration of diphenyliodonium hexafluorophosphate to 2,wt%, diffraction efficiency increased to 85% and induction period was shortened to 129,s, and low volume shrinkage of 8% was attained via simultaneous radical cross-linking of trimethylolpropane triacrylate and siloxane network formation of trimethoxysilyl groups of trimethoxysilylmethyl acrylate by atmospheric moisture catalyzed by cationic species produced from the initiator system. In SEM morphology, although gratings formed with high concentration of trimethoxysilylpropyl acrylate had some cracks in polymer matrix, the largest grating spacing indicating the lowest volume shrinkage and very regular and well-defined gratings were observed.

Siloxane‐containing Bifunctional Epoxide Together with Multi‐functional Acrylates to Improve the Performance of Holographic Gratings

AbstractWell‐defined transmission holographic gratings with high diffraction efficiency were fabricated by irradiating a mixture of dipentaerythritol penta‐/hexa‐functional acrylates, 1‐vinyl‐2‐pyrrolidone and 1,3‐bis(3‐glycidoxypropyl)‐1,1,3,3‐tetramethyldisiloxane or 1,3‐bis[2‐(1,2‐epoxycyclohex‐4‐yl)ethyl]‐1,1,3,3‐tetramethyldisiloxane in the absence (photo‐polymer systems, 30 : 10 : 60 in weight ratio; 80 or 55% efficiency) or presence (holographic polymer dispersed liquid crystal systems: 45 : 9 : 36 : 10 in weight ratio; 83 or 67% efficiency) of commercial liquid crystalline compound E7 with diphenyliodonium hexafluorophosphate and 3,3′‐carbonylbis(7‐diethylaminocoumarin) photo‐initiating system by Nd:YAG laser (λ = 532 nm). When the ratio of dipentaerythritol penta‐/hexaacrylate and 1,3‐bis(3‐glycidoxypropyl)‐1,1,3,3‐tetramethyldisiloxane was changed to 50 : 40 in photo‐polymer system, the diffraction efficiency increased to 84%. They had smooth surface morphologies with regulated spacing. In polymer dispersed liquid crystal system, the same ratio of dipentaerythritol penta‐/hexaacrylate and siloxane‐containing epoxides gave the best results. 1,5‐Bis(3‐glycidoxypropyl)‐1,1,3,3,5,5‐hexamethyltrisiloxane and 1,5‐bis[2‐(1,2‐epoxycyclohex‐4‐yl)ethyl]‐1,1,3,3,5,5‐hexamethyltrisiloxane gave high diffraction efficiency with 10% E7 (97% and 75%). The trisiloxane derivatives gave gratings with considerably or moderately reduced angular deviation (0.83, 0.66 degree for trisiloxane from 1.2, 0.7 degree for disiloxane derivatives, respectively for signal beam, and 0.76, 0.70 degree from 1.1, 1.0 degree for the reference beam at 32 degree of external incident angle) from Bragg profile, namely 5.2 and 4.5% volume shrinkage for trisiloxane, and 7.5, 5.6% for disiloxane derivatives, respectively. Gratings with diffraction efficiency over 95% with the narrowest angular selectivity of 4.0 degree was obtained when trimethylolpropane triacrylate was used together with 1,5‐bis[2‐(1,2‐epoxycyclohex‐4‐yl)ethyl]‐1,1,3,3,5,5‐hexamethyltri‐siloxane (50 : 40) and 5% E7. These gratings were actually used to store real images.