Chemistry Reference
In-Depth Information
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Figure 6.48.
The para-trans transition of azobenzene molecule.
6.6.4.
Photo-isomeric change
Eich
et al.
(1987) developed an effect to utilize an optical information stor-
age effect which is totally different from the above thermo-optical effect.
Under illumination by a laser beam the azobenzene undergoes the para-
trans transition, shown in Figure 6.48. If azobenzene units are attached
to the backbone of side chain liquid crystalline polymers the azobenzene
units in the presence of an electric field are aligned uniformly. The trans-
configuration is not stable and will relax back to the para configuration
under light irradiation or thermal effect. The azobenzene units don't relax
back to exactly the original position but a deformation occurs. Accordingly
a birefringence change occurs. The effect can be enhanced by applying a
polarized light because the optical axis of the azobenzene units prefers to
align themselves to be perpendicular to the polarization direction.
Eich
et al.
(1987) applied 70 mol % mesogenic group — cyano-phenyl-
benzoate and 30 mol % azobenzene as the side group to form a polyacrylate
copolymer. The sample cell is homeotropically aligned. Under the illumina-
tion of a laser beam the azobenzene accordingly undergoes the para-trans
isomeric change, and thus the local orientation is changed. In such way
the holographic recording is then utilized by this liquid crystalline polymer
film. The thickness of the liquid crystalline polymer is 10 microns, the glass
transition temperature is
T
g
=30
◦
C and the nematic to isotropic transition
is
T
c
= 103
◦
C. The definition of the holograph strips reached 300 lines/mm,
the recording content is about 1 Gbit/cm
2
. The holography can be kept for
a few weeks. Heating the sample erases the storage image.
The side chain liquid crystalline polymer as storage display has such
advantages as easy erasure, long working life, high contrast and reliable
storage,
etc
. The molecular features can be tailored. The future of its
application is expected to be bright.
