Page 193 - Photoreactive Organic Thin Films

I 7 2 MIKHAILV. KOZLOVSKY, LEV M. BLINOV,AND WOLFGANG HAASE

5,7 CONCLUSIONS

The chiral side chain polymers derived from asymmetric esters of terephthalic
acid and hydroquinone can form (in a broad temperature range, including
ambient temperature) an unusual mesophase (the "isotropic smectic" phase,
IsoSm*) characterized by high transparency and optical isotropy within the
visible wavelength range, combined with a hidden layered smectic ordering
and some elements of helical superstructure at shorter dimensions of 10 to
250 nm. The short-pitch TGB A* model seems to be the most adequate for
the mesophase structure.
If functionalized with an azobenzene chromophore, the resulting, copoly-
mers also form the IsoSm* phase in a broad range of compositions. They
show photochromism and photoinduced birefringence typical of azo dye LC
copolymers, but they differ from the latter by their spontaneous formation of
transparent, optically isotropic films, thus allowing for easy production of
thick (up to 30 um) photoactive layers. On the other hand, as compared with
amorphous azobenzene-containing polymers, our copolymers possess some
hidden liquid-crystalline ordering, which allows "imprinting" of photo-
induced orientation of chromophores into the mesogenic matrix and/or main
chain conformation during phase formation. Such imprinting can occur either
on cooling or isothermally (in the case of kinetically governed transitions) and
results in larger values of photoinduced birefringence and its higher stability.
Birefringence can be induced in optically isotropic photochromic IsoSm*
copolymers by UV illumination and/or by visible light within the blue-green
wavelength range of 400 to 500 nm. The latter is rather stable and shows no
saturation up to several hours of illumination, whereas the birefringence
induced by polarized UV light (300 to 400 nm) within the first few seconds
disappears with further illumination. On the other hand, pretreatment with
UV light increases substantially the sensitivity of the copolymer to the next
photorecording within the blue-green range. Moreover, hidden UV images
can be written by UV irradiation through a mask, and the visible images can
be further developed by polarized uniform visible light.
Generally, photochromic IsoSm* films are considerably more sensitive to
phase grating recording (created, e.g., by two laser beams of opposite circular
polarization) than to intensity grating recording (with linearly polarized
light). The gratings can be stable (for several months) or unstable (disappear-
ing in few minutes) depending on the laser beam intensity, film pretreatment,
and type of grating. Stable gratings can be erased with UV or circular light
irradiation, or by heating the film above the phase transition point.
The most remarkable feature of the reported chiral azo dye copolymers,
however, is the possibility of controlling their phase transitions by light illu-
mination. This capability is observed for the copolymer compositions within
the range of metastable IsoSm* phase formation, and it has been applied for
the novel photorecording technique, LCPT (light-controlled phase transition)
recording. The recording is carried out with unpolarized visible light of low
intensity, and the images are recorded as Sm A scattering patterns on the
IsoSm* background. They can be easily read out with nonpolarized light,
both in the scattering mode and in the transmission (dia) mode.