Electro-optical behavior

Optical and electro-optical behavior of side-chain liquid crystalline polymers are described 350-351>. The effect of flexible siloxane spacers on the phase properties and electric field effects were determined. Rheological properties of siloxane containing liquid crystalline side-chain polymers were studied as a function of shear rate and temperature 352). The effect of cooling rate on the alignment of a siloxane based side-chain liquid crystalline copolymer was investigated 353). It was shown that the dielectric relaxation behavior of the polymers varied in a systematic manner with the rate at which the material was cooled from its isotropic phase. 

Phase Behavior and Electro-optic Properties. With the great potential of PSFLCs, it is important to understand the changes induced by the polymer network on the FLC properties. In order to do so, two different non-mesogenic monomers, HDDA and PPDA, have been used. The structure of these monomers is quite similar (See Figure 1) with the only difference being that the phenyl group in PPDA is replaced with a six carbon alkyl chain for HDDA. These monomers, despite their structural similarity, have much different physical properties and consequently the polymers formed from HDDA and PPDA may also influence the FLC phase and electro-optic behavior differently. 

The introduction of a polymer network into an FLC dramatically changes phase and electro-optic behavior. Upon addition of monomer to the FLC, the phase transitions decrease and after polymerization return to values close to that observed in the neat FLC. The phase behavior is similar for the amorphous monomers, HDD A and PPDA. The electro-optic properties, on the other hand, are highly dependent on the monomer used to form the polymer/FLC composite. The ferroelectric polarization decreases for both HDDA and PPDA/FLC systems, but the values for each show extremely different temperature dependence. Further evidence illustrating the different effects of each of the two polymers is found upon examining the polarization as both the temperature and LC phase of polymerization are changed. In PPDA systems the polarization remains fairly independent of the polymerization temperature. On the other hand, the polarization increases steadily as the polymerization temperature of HDDA systems is increased in the ordered LC phases. 

Many other interesting examples of spontaneous reflection symmetry breaking in macroscopic domains, driven by boundary conditions, have been described in LC systems. For example, it is well known that in polymer disperse LCs, where the LC sample is confined in small spherical droplets, chiral director structures are often observed, driven by minimization of surface and bulk elastic free energies.24 We have reported chiral domain structures, and indeed chiral electro-optic behavior, in cylindrical nematic domains surrounded by isotropic liquid (the molecules were achiral).25... 

Note 3 A polymer that exhibits a nonlinear optical effect due to anisotropic electric susceptibilities when subjected to electric field together with light irradiation is called an electro-optical polymer. A polymer that exhibits electro-optical behavior combined with photoconductivity is called a photorefractive polymer. 

The supporting medium (aqueous or organic solvents membrane-mimetic compartments) also has a profound influence on the optical and electro-optical properties of nanosized semiconductor particles. This dielectric confinement (or local field effect) originates, primarily, in the difference between the refractive indices of semiconductor particles and the surrounding medium [573, 604], In general, the refractive index of the medium is lower than that of the semiconductor particle, which enhances the local electric field adjacent to the semiconductor particle surface as compared with the incident field intensity. Dielectric confinement of semiconductor particles also manifests in altered optical and electro-optical behavior. 

The effect of cationic polyacrylamide adsorption on the stability of aqueous cellulose suspension has been electro-optically studied by Khlebtsov et al. [19,20], The variations in transmitted light modulation in low-frequency (3-45 Hz) square-pulsed a.c. fields have been found to correlate well with the adsorption isotherm. The influence of sodium carboxymethylcellulose adsorption on the electro-optical behavior of negatively charged Si02 (ani-sometric aggregates, consisting of monodisperse spherical particles) has also been studied, and an increase of the particle induced dipole moment and its time of relaxation has been found [21,22],... 

III. ELECTRO-OPTICAL BEHAVIOR OF COLLOID PARTICLES IN POLYELECTROLYTE SOLUTION... 

In Figure 2 are represented the electro-optical effect a, the electrophoretic mobility Ue, and the relaxation time r of the particle disorientation after the switching off of the electric field as a function of the initial polyelectrolyte concentration. One observes that the a and r variations correspond to the variation of f/e, i.e., the electrostatic attraction of the polyelectrolyte to the oppositely charged surface, which is the main driving force for the adsorption, governs the electro-optical behavior and stability of the suspension containing this polyelectrolyte. 

Several examples can be given in order to demonstrate the similarity existing between the electro-optical behavior of a suspension stabilized by polyelectrolyte adsorption and the behavior of polyelectrolyte in solution. 

The origin of the additional LF effect, which appears in stabilized suspensions in the frequency range 102-104 Hz, will be commented on here bearing in mind that the electric properties of the adsorbed macromolecules govern the electro-optical behavior of these suspensions. 

Petkanchin IB, Buleva M. Influence of polyelectrolyte adsorption on the electro-optical behavior of aerosil particles. Comm Dept Chem BAS 1991 24 570-575. 

Electro-Optic Behavior of Twisted-Nematic Liquid Crystals... 

Conventional AMLCDs use nematic liquid crystals in a twisted configuration between two sheets of glass or plastic, one of which is the backplane containing the TFTs. The space between the two sheets is called the cell gap. Display operation relies on some subtle features of the electro-optic behavior of nematic liquid crystals... 

Electro-Optic Behavior of Organic Light-Emitting Diodes... 

A new class of liquid crystal/polymer network composite with very small amounts of polymer network (3 Wt%) is described. These composites are formed by photopolymerization of the monomers in-situ from a solution of monomer dissolved in low-molar-mass liquid crystals. Several techniques have proven useful to characterize these polymer networks. This review describes polymer network structure and its influence on electro-optic behavior of liquid crystals. Structural formation in these composites begins with the phase separation of polymer micronetworks, which aggregate initially by reaction-limited, and then by diffusion-limited modes. The morphology can be manipulated advantageously by controlling the crossover condition between such modes, the order of the monomer solution prior to photopolymerization, and the molecular structure of monomers or comonomers. 

D. Kang, J.E. Maclennan, N.A. Clark, A.A. Zakhidov, R.H. Baughman, Electro-optic behavior of liquid-crystal-filled silica opal photonic crystals Effect of liquid-crystal alignment Phys. Rev. Lett. 86, 4052-4055 (2001)... 

Bent-core LC molecules are one fascinating LC class featured by a banana-shaped or bent-core molecular shape which have attracted a great deal of interest for then-exciting electro-optical behavior [65,66]. When the bent-core molecule 13 was used (Scheme 4.5), a columnar phase of hybrid GNPs were obtained (Fig. 4.10) [50]. 

K. Suda, K. Akagi, Electro-optical behavior of ferroelectric liquid crystalline polyphenylene derivatives. J. Polym. Sci. Part A Polym. Chem. 46, 3591-3610 (2008)... 

H.P. Hinov and Y. Marinov, Theoretical considerations and experimental illustration of the electro-optic behavior of longitudinal flexoelectric domains under the joint action of DC and AC voltages the case of strong anchoring. Mol. Cryst. Liq. Cryst. 503(1), 45-68, (2009). 

A physical model describing the electro-optic behavior of switchable optical elements based on electro wetting. J Appl Phys 96 6267-6271... 

Figure 5 shows typical responses for this particular sample (0.5% sphere volume fraction). A basic difference in the electro-optical behavior of spherical compared to anisometric particles is the linear response at low field intensity. The modulated signal follows the frequency of the applied field with a phase difference of —90°. With an increase in frequency the ac component relaxes, changing in both amplitude and phase and passing to double frequency, while the dc component changes... 

---