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.     

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

Dye-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 (V_dr) fabricated by nucleophile-initiated thiol-ene click reaction for the first time. The V_dr 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.     

Surfactant-doped reverse-mode polymer-dispersed liquid crystal display with enhanced properties

In this article, a nonionic surfactant Brij30 has been used to vertical align the negative anisotropic liquid crystal. The surfactant is further used in reverse-mode polymer-dispersed liquid crystal display to enhance the electro-optical properties. This work shows an enhancement of contrast ratio simply with the addition of surfactant. In addition, contact angle and surface free energies have been determined to judge the role of surfactant in control of surface orientation of liquid crystal molecules.     

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.     

Effect of rGO on polymer-dispersed liquid crystal fabricated by RAFT polymerisation

In this study, we demonstrated that doping polymer matrix with a small amount of reduced graphene oxide (rGO) component (0.05–0.2%) had significant influence on the polymerisation kinetics and electro-optical performances of polymer-dispersed liquid crystal films (PDLCs) fabricated with macro reversible addition-fragmentation chain transfer agents. The effects of rGO content were studied in terms of morphology, compound viscosity, polymer conductivity, polymerisation kinetics and driving voltage of PDLCs. The results exhibited that higher rGO content increased the compound viscosity and the entire process proceeded slowly. Furthermore, the addition of rGO increased the polymer conductivity and local electric field, and reduced the saturation voltage as well as the threshold voltage from 27.3 to 19.5 V and 13.2 to 6.41 V, respectively.     

Chromaticity of polymer-dispersed cholesteric liquid crystals

The colour producing properties of the polymer-dispersed cholesteric liquid crystal colour display have been measured. We show how the central wavelength of the reflection spectrum depends on the sample voltage, chiral concentration, temperature, and viewing angle. From the reflection spectra we calculate the chromaticity coordinates for two situations and display them on chromaticity diagrams. The results indicate that these new systems may be suitable for the use in multicolour displays.     

Helical unwinding in polymer-dispersed ferroelectric liquid crystals

We demonstrate that ferroelectric liquid crystals dispersed in a polymer matrix can form uniaxially aligned elliptical droplets. The alignment is controlled by mechanical shear during the polymerization of a UV-curable adhesive. The possibility of prealignment makes polymer-dispersed ferroelectric liquid crystals suitable for application in flexible electro-optical displays. In contrast to surface stabilized ferroelectric liquid crystal displays, the electro-optical effect in our system is due to the deformed-helix ferroelectric (DHF) effect.     

Light transmission by polymer-dispersed liquid crystals

Light transmission of nematic liquid crystals dispersed in a polymeric matrix is investigated. The paper gives a theoretical analysis of light transmission by the liquid crystal-polymer and liquid crystaldye-polymer systems based on determination of the total cross section of extinction. The mathematical model is considered in the anomalous diffraction approximation. The theoretical calculations are in good agreement with experimental data obtained for films based on cyanodiphenyl dispersed into a polyvinyl acetate matrix.Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 6, pp. 118–122, November–December, 1993.Translated by L. Chernomorskaya     

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.     

“Guest-Host” effect in polymer-dispersed nematic liquid crystals

In this paper we report on a new optoelectronic material representing a polymer-dispersed liquid crystal (PDLC) film. This material exhibits optical response to the applied electrical field and is useful for displays and light gates. The effect of azo and anthraquinone dyes on the optical density of PDLC films has been studied. The proportion of the dye dissolved in an LC cell and the molar absorption coefficients were determined, and the influence of the dyes on the scattering efficiency of PDLC films was calculated.Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 6, pp. 114–117, November–December, 1993.Translated by L. Smolina     

Phase separation and gelation of polymer-dispersed liquid crystals

Polymerization-induced phase separation in polymer-dispersed liquid crystal is studied by computer simulations in two dimensions. The domain morphology resulting from phase separation is investigated by solving the coupled set of equations for the local volume fraction and the nematic order parameter, taking into account the viscoelastic effects and gelation due to polymerization. Comparing the morphology of phase separation by temperature quench, it is shown that the viscoelastic effects and gelation enable the polymer-rich phase to form a stable interconnected domain even when the polymer component is minority. The experimental evidence consistent with this characteristic feature is also given.     

Copolymer composition-dependent light transmission of polymer/liquid crystals composite films

Electrooptic responses (voltage and angular-dependent transmittance) of polymer/liquid crystal composite films with H, V, and unpolarized lights have been studied based on a nematic liquid crystal (Ro-5921) and four types of homopolymers and copolymers from ethyl methacrylate and styrene with different compositions. In this way, the index ratio of the polymer (n_p) to the ordinary refractive index of liquid crystal (n_o)(n_p/n_o) has been varied systematically, and the effect of the index ratio on viewing angle, applied voltage, response times, and transient response have been investigated. With increasing styrene content in the copolymer, droplet size increased, threshold (V_th) and saturation (V_sat) voltage, and rise time decreased. With n_p ≲ n_o, maximum transmittance occurred at normal incidence, regardless of the type of polarization. On the contrary with n_p > n_o, V-polarization gave a peak in the transmittance-voltage curve, and transmittance overshot upon removal of the field, and these were interpreted in terms of effective refractive index and two-step relaxations. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 55–64, 1998     

Photoswitching of holographic polymer-dispersed liquid crystals doped with chiral dopant

Chiral dopants were added to the formulation of holographic polymer-dispersed liquid crystals and the effects studied in terms of grating formation dynamics, morphology, diffraction efficiency, contrast ratio and electro-optical properties of the films. A gradual increase of real-time diffraction efficiency, decrease of droplet size and increase of diffraction efficiency of the composite film were obtained with the addition and increasing content of chiral dopant, due to the increased viscosity of the liquid crystal (LC) doped with the chiral dopant leading to decreased droplet coalescence. The contrast ratio decreased with increasing content of chiral dopant due to the difficult orientation of LC molecules caused by the formation of a helical structure. Addition of a small amount of the chiral dopant increased the driving voltage slightly, whereas the decay time is decreased significantly as a result of the high twisting of the helical structure.     

Effect of macro-RAFT agent on the morphology of polymer dispersed liquid crystals

In this study, macro-(RAFT) reversible additional fragmental chain transfer agent prepared by reversible additional fragmental chain transfer polymerisation has been incorporated into polymer dispersed liquid crystals (PDLCs). The effects of concentration, molecular weight and glass transition temperature of macro-RAFT agent were studied in terms of morphology, polymerisation kinetics, molecular weight of polymer matrix and electro-optical properties of the films. It was found that the key factor influencing morphology was the mobility of macro-RAFT agent chain rather than polymerisation rate and molecular weight of polymer matrix. Furthermore, the decrease in the mobility of macro-RAFT agent chain caused less liquid crystal nematic fraction, smaller liquid crystal domain size and greater driving voltage.     

UV tuning of the electro-optical and morphology properties in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro-optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro-optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro-optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro-optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro-optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.     

Phase separation of polymer-dispersed liquid crystals on a chemically patterned substrate

The surface-induced structure formation of polymer-dispersed liquid crystals (PDLCs) on a chemically patterned substrate was studied for the first time. The patterns on the substrate were successfully transferred to the PDLC film, resulting in alternating LC-rich and polymer-rich phases. This simple approach offers a new means of organizing micrometer-sized LC domains into well-ordered structures in a polymer matrix of PDLCs.     

Water-in-water emulsions stabilized by non-amphiphilic interactions: polymer-dispersed lyotropic liquid crystals

Emulsion systems involving surfactants are mainly driven by the separation of the hydrophobic interactions of the aliphatic chains from the hydrophilic interactions of amphiphilic molecules in water. In this study, we report an emulsion system that does not include amphiphilic molecules but molecules with functional groups that are completely solvated in water. These functional groups give rise to molecular interactions including hydrogen bonding, pi stacking, and salt bridging and are segregated into a dispersion of droplets forming a water-in-water emulsion. This water-in-water emulsion consists of dispersing droplets of a water-solvated biocompatible liquid crystal--disodium cromoglycate (DSCG)--in a continuous aqueous solution containing specific classes of water-soluble polymers. Whereas aqueous solutions of polyols support the formation of emulsions of spherical droplets consisting of lyotropic liquid crystal DSCG with long-term stability (for at least 30 days), aqueous solutions of polyamides afford droplets of DSCG in the shape of prolate ellipsoids that are stable for only 2 days. The DSCG liquid crystal in spherical droplets assumes a radial configuration in which the optical axis of the liquid crystal aligns perpendicular to the surface of the droplets but assumes a tangential configuration in prolate ellipsoids in which the optical axis of the liquid crystal aligns parallel to the surface of the droplet. Other classes of water-soluble polymers including polyethers, polycations, and polyanions do not afford a stable emulsion of DSCG droplets. Both the occurrence and the stability of this unique emulsion system can be rationalized on the basis of the functional groups of the polymer. The different configurations of the liquid crystal (DSCG) droplets were also found to correlate with the strength of the hydrogen bonding that can be formed by the functional groups on the polymer.     

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.     

Phase diagrams and morphology of polymer-dispersed liquid crystals: An analysis

Thermal and morphological studies of polymer-dispersed liquid crystals (PDLCs) for various compositions of liquid crystalline material 4-undecyloxybenzoic acid (UDBA) in two polymer matrices, polyvinylidenefluoride-co-hexafluoropropylene P(VdF–HFP) or polyethylene oxide (PEO), have been carried out using differential scanning calorimetry (DSC) and polarising optical microscopy (POM). Phase diagrams for different series of PDLCs have been analysed using the Flory–Huggins theory of isotropic mixing and the Maier–Saupe–McMillan theory, to include the anisotropic contributions. Mesogenic transitions of UDBA are observed to be greatly influenced when dispersed in these polymers. The morphologies and miscibility studies of these PDLCs suggest that UDBA is highly miscible in PEO, only partially miscible in poly(methyl methacrylate) (PMMA), but almost immiscible in P(VdF–HFP).