Nematic liquid crystals polymer dispersed

To produce novel LC phase behavior and properties, a variety of polymer/LC composites have been developed. These include systems which employ liquid crystal polymers (5), phase separation of LC droplets in polymer dispersed liquid crystals (PDLCs) (4), incorporating both nematic (5,6) and ferroelectric liquid crystals (6-10). Polymer/LC gels have also been studied which are formed by the polymerization of small amounts of monomer solutes in a liquid crystalline solvent (11). The polymer/LC gel systems are of particular interest, rendering bistable chiral nematic devices (12) and polymer stabilized ferroelectric liquid crystals (PSFLCs) (1,13), which combine fast electro-optic response (14) with the increased mechanical stabilization imparted by the polymer (75). 

Polymer-dispersed liquid crystals (PDLCs) are made up of nematic liquid crystals dispersed in a solid continuous polymer matrix. These are prepared by mixing a reactive monomer into a non-polymerisable LC medium and then polymerising the reactive monomer to create a polymer matrix, at the same time capturing the LCs as dispersed droplets, greater than 1 pm in diameter, i.e. the wavelength of visible light.3 -33... 

Nematic Ordering in Polymer Dispersed Liquid Crystals... 

Dispersions of micron-sized droplets of nematic liquid crystal in a polymer matrix form the basis of a potentially... 

R. Ondris-Crawford, E. P. Boyko, B. G. Wagner, etal.. Microscope textures of nematic droplets in polymer dispersed liquid crystals, J. Appl. Phys., 69, 6380 (1991). 

P. S. Drzaic, Polymer dispersed nematic liquid crystal for large area displays and light valves, J. Appl Phys. 60, 2142 (1986). 

Y. K. Fung, A. Borsmik, S. Zumer, et al., Pretransitional nematic ordering in liquid crystals with dispersed polymer networks, Phys. Rev. E, 55, 1637 (1997). 

Abstract Monte Carlo simulations of lattice spin models are a powerful method for the investigation of confined nematic liquid crystals and allow for a study of the molecular organization and thermod3mamics of these systems. Investigations of models of polymer-dispersed liquid cr3rstals are reviewed devoting particular attention to the calculation of deuterium NMR spectra from the simulation data. 

Based on the director distribution we can derive the electrooptical response of a nematic liquid crystal cell (such as birefringence), rotation of the polarization plane of the incident light, total internal reflection, absorption, or some other important characteristics of the cell. In this chapter we will consider in detail these particular features of the electrooptical phenomena in uniform structures. Special attention will be paid to their possible applications. Electrooptics of the isotropic phase and polymer nematics, including Polymer Dispersed Liquid Crystals (PDLC), are also discussed. 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

Liquid crystals and colloids have different structures, i.e., there exist colloids dispersed in liquid-crystalline media as either matrices or LC-droplets in an isotropic matrix, prepared in polymer-dispersed liquid crystals. Literature has described the method for the preparation of liquid-crystalline colloidal particles (i.e., photo-crosslinking of dispersion), which leads to large colloidal particles with a broad size distribution [153]. In the latter case, the matrix is hard (crosslinked polymer) and the LC-droplet is mobile. The perfectly inverted system for colloids in a LC-matrix is represented by the liquid-crystalline colloidal particles in a liquid-like isotropic matrix. These studies describe a system prepared by photopolymerization of a nematic liquid-crystalline monomer dispersion in a viscous solvent. The liquid crystalline colloidal particles are manipulated in electrical fields, due to their anisotropic properties. Generally, the anisotropic... 

Systems consisting of a low molar mass liquid crystal and a polymer are currently of great interest with respect to applications and due to the intriguing finite size effects. This chapter describes some aspects of liquid crystals embedded in dense polymer binders and low concentration polymer networks that modify the bulk liquid crystal phase, with added emphasis on chirality. The introduction to the phenomenological description is followed by the modeling of field effected chiral nematic droplets in polymer-dispersed liquid crystal systems. Next the orientational ordering induced by polymer networks is described, and finally the usefulness of these materials for direct-view reflection displays, bistable displays, and light valves is reviewed. 

To conclude this section on 2D NMR of liquid crystals, some studies of more exotic liquid crystalline systems are pointed out. Polymer dispersed nematic liquid crystals have attracted much attention because of their applications as optical display panels. Deuteron 2D quadrupole echo experiments have been reported [9.28] in the isotropic and nematic phases of / -deuterated 5CB dispersed in polymers. A similar technique was used [9.29] to study two model bilayer membranes. Both studies allow determination of the lineshape F(u ) due to quadrupolar interactions and the homogeneous linewidth L(u ) of the individual lines [9.28]. The 2D quadrupole echo experiment has also been used [9.30] to separate chemical shift and quadrupolar splitting information of a perdeuterated solute dissolved in a lyotropic liquid crystal. The method was compared with the multiple-quantum spectroscopy that is based on the observation of double-quantum coherence whose evolution depends on the chemical shift but not on the quadrupolar splitting. The multiple-quantum method was found to give a substantial chemical shift resolution. The pulse sequences for these methods and their treatment using density matrix formalism were summarized [9.30] for a spin 1=1 system with non-zero chemical shift. Finally, 2D deuteron exchange NMR was used [9.31] to study ring inversion of solutes in liquid crystalline solvents. 

The technique has also been used to study the dynamic behaviour of a polymer-dispersed liquid crystal subjected to an electric field [18]. The liquid polymer used was the commercially available nematic liquid crystal mixture E7, which contains four nitrile and ethyl substituted bi- and tri-phenyls. It was blended with a polymer precursor consisting of a mixture of an acrylate monomer, an acrylate oligomer and a UV curing agent. The 2D correlation analysis showed that the rigid core of the liquid crystal molecules re-orients as a unit, and suggests that the polymer side chains existing in the interface between the polymer and the liquid crystals may re-orient in phase with the liquid crystal re-orientation by interaction with the liquid cryst molecules. 

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