Photoalignment mechanism

A photodecomposition-type material does not include optically sensitive structures, and the determination of the photochemical reaction for such materials is rather more difficult than for others. Therefore, determination of the photoalignment mechanism for a photodecomposition-type material is difficult to compare with that for the other two mechanisms. 

Besides a chemical reaction, the UV light can induce an asymmetric potential field under which the stable configuration is characterized by the dye absorption oscillator perpendicular to the induced light polarization. One of the possible photoaligning mechanisms in azo-dye films is a pure reorientation of azo-dye molecules [10, 18, 42, 43]. Diffusion mechanisms of the azo-dye reorientation can be used to explain this phenomenon. 

Several mechanisms of photoalignment reactions have been reported in the literature and hence a variety of photoalignment materials are available for director patterning. For the manufacturing of a patterned retarder applied in a transflective display (Sect. 9.4.2), we have tested two commercially avail-... 

Photoalignment by using a photodecomposition mechanism has been applied to a wide variety of materials compared with the other two methods, photoisomeriza-... 

Yadavalli NS, Santer S (2013) In-situ atomic force microscopy study of the mechanism of surface relief grating formation in photosensitive polymer films. J Phys Chem B 115 1363-1367 Yeh P, Gu G (1999) Optics of liquid crystal displays. Wiley, New York Yu CJ, Kim DW, Kim J, Lee SD (2005) Polarization-invariant grating based on a photoaligned liquid crystal in an oppositely twisted binary configuration. Opt Lett 30 1995-1997... 

To describe the physical mechanisms of LC photoalignment with a special emphasis on the most useful photosensitive materials and preparation procedures suitable for the purpose. 

Progress in the application of LC photoaligning for LCDs has stimulated many fundamental studies of its mechanism. However, an adequate explanation of the photoalignment phenomenon is still absent. Several publications have appeared recently and provide only a qualitative explanation of the phenomenon of photoalignment. A more adequate explanation of the process is still in progress, taking into account both photochemical and non-photochemical transformations in photosensitive layers under the action of polarized or non-polarized, but directed, light. 

Many publications concerning the mechanism of the photoalignment in various cinnamate side-chain polymers, which provide a homogeneous LC alignment, were reviewed in [8]. According to some of them, the LC alignment is parallel to the polarization direction of the LPUVL. The effect may take place due to both the photochemical cis-trans isomerization and dimerization processes. 

V. G. Nazarenko, O. P. Boiko, A. B. Nych, Yu. A. Nastishin, V. M. Pergamen-shchik, and P. Bos, Selective hght-indnced desorption the mechanism of photoalignment of liquid crystals at adsorbing sohd surfaces. Europhysics Letters 75, 448 (2006). 

Comparison of the Characteristics of Photoalignment Layers for Different Mechanisms of LC Photoalignment... 

As follows from Table 3.1, pure reorientation (diffusion) is the most suitable mechanism for LC photoalignment. The commercialization of the materials produced by the Die Corporation (Japan) is expected soon [45, 46]. The first target should be the photoaligning of monomers and LC polymers for phase retarder applications (Chapter 5). Photoaligning with azo-dye layers will be considered below. 

Table 3.1 Characteristics of photoalignment layers for various mechanisms of LC photoalignment (Chapter 2)...

Polarized electroluminescence (EL) spectra can also be used to characterize the quality of the photoaligned layer. The brightness and polarization ratio of EL is shown to depend on the composition and processing conditions of the alignment layer. Photoalignment provides a non-contact method to achieve macroscopic orientation of the chromophore without mechanical damage. EL with a polarization ratio of 11 1 (Figure 3.20) has been obtained from a fluorene-based nematic network formed by the selective polymerization of diene photoactive... 

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