Photoinduced Phase Separation Research Articles

Photoinduced Phase Separation

A novel approach of photoinduced phase separation has been demonstrated with a photolabile anionic surfactant, mixed with an inert nonionic surfactant in the presence of salting-out electrolyte. Breakdown of the photolyzable surfactant results in hydrophobic photoproducts, which are emulsified by the remaining inert surfactant; added electrolyte resolves the emulsion into macroscopic oily and aqueous phases. The initial micellar systems can disperse an insoluble additive marker dye (shown), which may be spatially segregated from the aqueous environment by the action of UV light.

56.2: Invited Paper: Pixel-Isolated Liquid Crystal Mode for Plastic Liquid Crystal Displays

Abstract We proposed a pixel-isolated liquid crystal (PILC) mode for enhancing mechanical stability of flexible display applications. Since the LC molecules in this mode are isolated in pixels by patterned or phase-separated microstructures, the LC alignment is stable against external pressure. Moreover, the cell gap of our structure is uniformly preserved against bending deformation of the plastic substrates since two substrates are tightly attached each other by the solidified polymer layer produced by photo-induced anisotropic phase separation. For flexible display applications, we have tested the mechanical stability of electro-optic properties in PILC structures with plastic substrates. 1. Introduction In recent years, roll-up displays have drawn considerable attention for next-generation information displays because of their excellent portability such as light weight, thin packaging, and flexibility. Among several available technologies, it is expected that a liquid crystal display (LCD) using plastic film substrates is the most promising device because of its superior visibility with low power consumption over other displays such as organic light-emitting device or electrophoretic displays. However, there are still critical problems regarding the fabrication of commercially available plastic LCDs with current technologies based on the glass substrates. One of those problems is the instability of LC structures due to hydrodynamic properties of LCs at bending and another is the separation of two plastic substrates due to the flexibility of film substrates. Such problems do not exist in conventional glass-substrate-based LCDs since glass substrates can sustain a stable LC alignment condition against external bending or pressure. To solve these problems, several types of polymer wall and/or network as supporting structures have been proposed and demonstrated [1]-[7]. These structures were fabricated using an anisotropic phase separation method from polymer and LC composite systems by applying a patterned electric field or spatially modulated UV intensity. However, these methods require high electric field to initiate the anisotropic phase separation or remain residual polymers in unexposed regions that reduce optical properties and increase the operating voltage of the devices.

Electro-optics and structural peculiarities of liquid crystal–nanoparticle-polymer composites

The structural peculiarities and electro-optic performance of liquid crystal (LC)-colloidal nanoparticle (NP)-polymer (P) composites formed by photoinduced phase separation are considered. We classify these materials under two groups according to two limiting cases of polymer morphology. The first group corresponding to small polymer concentration comprises LCs filled with NPs that are stabilized with a polymer network. It is found that, in addition to the light scattering caused by the LC orientational defects, the refractive index mismatch between LC and NP aggregates may significantly affect the electro-optic contrast and its angular characteristics. The second group is represented by polymer dispersed liquid crystals (PDLCs) filled with NPs. It is established that, in the process of photoinduced phase separation of the LC-NP-prepolymer mixture, the nanoparticles are mainly involved with the polymer, serving as building blocks for the polymer matrix. When the aggregation rate of the NPs is high or their size is large, the NPs enhance light scattering in the polymer. For low aggregation rate, NPs modify the effective refractive index and/or the absorption coefficient of the polymer phase without producing any noticeable optical inhomogeneity. Additionally, we found that TiO2 NPs may cause a photochromic effect, which manifests itself in color changes in the course of the photoinduced phase separation. For PDLCs with optically transparent polymer matrices modified by NPs, it is shown that doping with NPs can be used to control the refractive index ratio of the LC and polymer. In this way one can modify the contrast and substantially reduce the off-axis haze of the PDLC. The observed effects show LC-NP-P composites as materials of considerable promise for LCD and other electro-optic applications.

Submillisecond switching of nematic liquid crystal in cells fabricated by anisotropic phase-separation of liquid crystal and polymer mixture

Liquid crystal (LC) cells consisting of a very thin (submicrometer) nematic LC layer switch with time response smaller than 1ms. The total response time, i.e., sum of the turn-on and turn-off times, can be made as small as 1.3ms. Such devices were prepared by a photoinduced anisotropic phase separation of mixtures of the LC and prepolymer. The formation of the LC–polymer bilayer in these devices was confirmed by polarized light microscopy and scanning electron microscopy. The phase separation method permits one to fine-tune the LC film thickness by varying the LC concentration in the mixture. This technique can be used to fabricate fast LC devices for TV and video applications.

Photo-induced phase separation effect in cuprates

Parent insulating cuprates appear to be unstable with regard to a self-trapping of the low-energy one-and two-center CT excitons with a nucleation of electron-hole (EH) droplets being actually the system of coupled electron CuO47- and hole CuO45- centers having been glued in lattice due to strong electron-lattice polarization effects. Such a system can be regarded as an EH Bose liquid described by the generalized Bose-Hubbard Hamiltonian. The scenario can readily explain photo-induced effects in underdoped cuprates. Indeed, illumination of a material with light leads to the generation of EH pairs that will proliferate and grow up to be a novel nonequilibrium EH droplet or simply to be trapped in either EH droplet with the rise in its volume fraction. During the illumination of underdoped cuprate near the insulator-metal transition, the material may even become superconducting.

Photoinduced phase separation and miscibility in the condensed phase of a mixed Langmuir monolayer.

We report our studies on the mixed Langmuir monolayer of mesogenic molecules, p-(ethoxy)-p-phenylazo phenyl hexanoate (EPPH) and octyl cyano biphenyl (8CB), employing the techniques of surface manometry and Brewster angle microscopy. Our studies show that the mixed monolayer exhibits higher collapse pressures for certain mole fractions of EPPH in 8CB as compared to individual monolayers. Also, a considerable reduction in the area per molecule is seen in the mixed monolayer, indicating a condensed phase. We have also studied the photostability of the mixed monolayer at different initial surface pressures. The mixed monolayer, under alternate cycles of UV and visible illumination, exhibits changes in surface pressures. This is due to the photoinduced transformation of EPPH isomers in the mixed monolayer. Our in-situ Brewster angle microscope studies for 0.5 mole fraction of EPPH in 8CB show a phase separation in the UV and a miscible phase in the visible, at low surface pressures ( approximately 5 mN/m). At higher surface pressures ( approximately 10 mN/m), under UV illumination, we find a phase separation which does not revert to a miscible phase under visible illumination.

Electro-optic effect, propagation loss, and switching speed in polymers containing nano-sized droplets of liquid crystal

Polymers containing droplets of liquid crystal smaller than 100 nm, which have good transparency and easily form films, were prepared under various conditions to evaluate their potential as electro-optic materials for waveguide-type devices. By varying the liquid crystal concentration and the strength of the UV irradiation causing photo-induced phase separation of the droplets, we were able to control the droplet size and density. We have clarified how the birefringence generated in an applied electric field, switching speed, and optical loss of light propagating in the film depend on droplet size and density. Polymer materials having a large electro-optic effect (δn = 0.001 at 8 V μm-1), low propagation loss (~2.5 dB cm-1), and fast response time (~10 μs) have been developed.

Effects of UV Irradiation in Memory-Type PDLC Films Formed by a Photo-Induced Phase Separation

Abstract We have studied the relationship between the UV irradiation time and electrooptical properties in the UV curable polymer dispersed liquid crystal (PDLC) film. Memory-type PDLC films are prepared with various terms of UV irradiation and the changes of transmittance, capacitance and resistance of the PDLC film are measured while the phase separation is advanced. The driving voltage increases in the PDLC film which is made with an insufficient or excessive UV irradiation. The memory transmittance also depends on the UV irradiation time.

Photo-oxidative behaviour of crude oils relative to sea pollution: Part II. Photo-induced phase separation

A phase separation appears during the photo-oxidation of a light crude oil (Zarzaitine from Algeria), an aromatic-rich petroleum distillate, and a model mixture. Analysis by steric exclusion chromatography shows that this phenomenon is due to the formation of high molecular weight species which are insoluble in the hydrocarbon mixture. The phenomenon is linked to the radical propagation of the oxidation reaction and mainly involves the aromatic fraction. Its mechanism is discussed on the basis of experimental results.