PHOTOALIGNMENT

Anchoring cumulative effect in liquid crystals

We predict and experimentally confirm that two consecutive photoalignment treatments with beams of perpendicular polarizations can compensate each other and restore the initial anchoring.

In our approach, each treatment of the surface is an additive contribution to the anchoring. Initial spatial-angular distributions of the easy axis and of the anchoring energy are intrinsic characteristics of the anchoring surface. This random orientation may be masked but not eliminated by photo-exposure as demonstrated experimentally below.

The cell (Figure a, microscope view between crossed polarizers) was subjected to two consecutive UV-light treatments. The first treatment (Figure b) produced the easy axis perpendicular to i.e. we induced a 90 degree twist structure in the cell. The second exposure restored the initial isotropic angular distribution in perfect details.

We proposed a tensor description of an aligning surface that considers both homogeneous and inhomogeneous parts of LC anchoring and the affect of different exposures on the alignment of liquid crystal. We showed that in the first approximation, each treatment of the aligning surface gives an additive contribution to the anchoring but does not eliminate the initial anchoring pattern.

Non-monotonic exposure dependence of the pretilt angle and surface polarity of the photo-orientant f-pvcn

We observed and studied the non-monotonic UV-exposure dependence of the pretilt angle of LC on a photoaligning surface. We found this non-monotonic dependence on fluorinated polyvinyl-cinnamate (F-PVCN) surface while non-fluorinated PVCN revealed monotonic decrease of the pretilt angle with exposure. These dependencies correlate with light-induced changes of the polarity of F-PVCN and PVCN surfaces. The results points to an important role of the surface polarity and its light-induced changes in the mechanism of generation of the pretilt on photoaligning materials.

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Figure 1. Dependence of the pretilt angle vs
irradiation time for PVCN and PVCN-F.

a) PVCN

b) PVCN-F

Photo 1. The toner powder on the PVCN film (a) and
PVCN-F film (b) after different exposure time