Polymer host, dielectric constant

The term photochromism can be defined as a light-driven reversible transformation between two isomers possessing different absorption spectra.111,21 The two isomers differ from one another not only in their absorption spectra, but also in their geometrical structures, oxidation/reduction potentials, refractive indices, and dielectric constants. When such photochromic chromophores are incorporated into functional molecules, such as polymers, host molecules, conductive molecular wires, or liquid crystals, the functions can be switched by photoirradiation.[3 61 Photostimulated reversible changes in refractive index can also be applied to optical waveguide switching.171 This chapter reviews applications of photochromic chromophores, especially diar-ylethene derivatives, in various photo switching molecular systems. 

A detailed consideration of more advanced theoretical treatments clarifies the role played by the polymer dielectric constant. In the absence of intermo-lecular electrostatic interactions, one would desire the lowest possible dielectric constant, e.g., PMMA would be a better host matrix than polycarbonate. This is because the dielectric constant of the polymer host would act to attenuate the poling field felt by the chromophores. On the other hand, in the presence of intermolecular electrostatic interactions, optimum electro-optic activity will be achieved for polymer hosts of intermediate dielectric constant. The dielectric constant of the host acts not only to attenuate the externally applied poling field, but also fields associated with intermolecular electrostatic interactions. 

Detailed theoretical calculations establish that phase diagrams separating homogeneous from phase-separated regions will depend on intermolecular electrostatic interactions, the strength of the applied poling field, and on the dielectric constant of the host polymer. In Fig. 16, we show phase diagrams calculated as a function of N, 8, and E. 

Some degree of solvency of the plasticizer for the host polymer is essential for plasticization. Not surprisingly, a match of solubility parameters of the plasticizer and polymer is often a necessary but not a sufficient condition for compatibility. In the case of PVC, the dielectric constants of the plasticizer should also be near that of the polymer. 

Templated mesoporous silica material MCM-41 [1] consists of hexagonal arrays of cylindrical pores with diameters between 1.5 and 20 nm, narrow pore size distributions and negligible pore networking. These properties make these materials ideal for fundamental studies aimed at determining the effect of surface forces, confinement and reduced dimensionality on the phase behavior of host molecules. The features of MCM-41 materials [2] make them suitable for a number of applications in catalysis, adsorption, optics, as low dielectric constant materials to insulate integrated circuits, and as host materials for polymers, nanoparticles and enzymes [2], The gas-liquid transition of adsorbates in templated mesoporous silica materials has been extoisively studied by experiment, theory and molecular simulation [3], From a molecular simulation viewpoint, a number of silica pore models have... 

Maximum ON-state transmittance occurs when the refractive index of polymer ( p) matches with the ordinary refractive index ( ) of LC. During the film formation, it is possible that some fraction of LC dissolved in polymer matrix can have a profound effect mi the PDLC film properties. Therefore, determination of partition of LC between the polymer and LC phases is an important factor in evaluating the performance of PDLC films. The reorientation of LC portion in a PDLC composite film is responsible for the optical non-linearity and electro-optical properties of the device. The absorption of LC into an isotropic polymer results in the LC becoming a part of the polymer phase. In this state, reorientation of LC does not happen with an applied electric field, leaving less amount of LC behind for scattering of light. Therefore, selection of suitable concentration of LC in PDLC films is crucial in optimizing film properties. LC dissolved in polymer matrix alters refractive index, dielectric constant, viscosity etc. of the host polymer. As explained earlier, for best electro-optical responses, a polymer and LC material are chosen on the basis of... 

The excitation of the surface plasmon is found to be an extinction maximum or transmission minimum. The spectral position v half-width (full width at half-maximum) T and relative intensity f depend on various physical parameters. First, the dielectric functions of the metal and of the polymer Cpo(v) are involved. Second, the particle size and shape distribution play an important role. Third, the interfaces between particles and the surrounding medium, the particle-particle interactions, and the distribution of the particles inside the insulating material have to be considered. For a description of the optical plasmon resonance of an insulating material with embedded particles, a detailed knowledge of the material constants of insulating host and of the nanoparticles... 

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