Polarizers/Polarization direct photoalignment

A vast amount of investigations on photoalignment of LCs has been reported during the past one and a half decades. Irradiation with LPL induces photo-induced anisotropy, in which molecules are generally oriented to an unexcitable direction, namely, perpendicular to the electric vector of the polarization direction. This effect is called photoinduced optical anisotropy (Seki, 2004 Natansohn and Rochon, 2002 Ichimura, 2000). 

FIGURE 7.2 Schematic illustration of photoalignment of azobenzene-contaiiiing PLCPs. After the trans-cis-trans cycles, the azobenzene mesogens are aligned perpendicularly to the LPL polarization direction. The photoalignment state can be destroyed by CPL irradiation or heating. 

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. 

Figure 5.12 Analyzer angle, (p versus sample rotation angle. 9 curves (experimental points) measured by transmission null eUipsometry for the optical film shown in Figure 5.11 [13]. The film is optically equal to the positive A crystal plate with a slow axis perpendicular to the polarization direction of the actinic light used in photoalignment processing. The in-plane retardation is 185.9 nm...

It was also reported that photoreactivity of side-chain liquid-crystalline polymers can align liquid crystals both in a parallel mode or perpendicularly, depending on the degree of the photoreaction of the polymers. Presumably this particular polymer can multi-photoalign the liquid crystal pattern without a change of the direction of the linearly polarized UV light. The chemical structure of such an aligning polymer is depicted as follows ... 

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. 

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