Modern Polymer Spect..

induced alignment of the investigated polyester and the following scenario is suggested
for optical storage: the film obtained by casting from solution consists of'
macroscopically isotropic niicro-domains. The irradiation with linearly polarized
laser light leads to a melting of these domains, caused by direct heating due to
absorption or a deposition of heat through traris-cis-trms isonierization cycles of
the azobenzeiie moieties [95]. The origin of the orientation effect is attributed to
tivm,r-cis photoisomerization of the azo-group, followed by a c-is-trans thermal or
photoisomerization. In the cis-form, this functionality is able to undergo some
reorientation with respect to its original alignment. The absorption of the polarized
'writing' beam will continue as long as a component of the electric dipole vector of
the azo-group lies in the direction of the laser polarization. The trurzs-cis reorientation
process will therefore continue until all the azo-groups have been oriented
perpendicular to the polarization vector of the laser beam [86. 1011. In the polyesters
investigated here, this orientation remains after the 'writing' beam is turned
off and a long-tenn stable, macroscopic orientation is established.
Apart from this long-term stability, the possibility to erase and reinduce this
anisotropy in the polymer is a prerequisite for its application as a reversible optical
data-storage medium. To demonstrate the loss of mesogen-alignment during heating
and the perfect reversibility of a 'write-erase-write' cycle, Figure 2-37a shows a
stack plot of the i(C-N)-band region in the polarization spectra of a P10a12 film
during heating from 32 "C to 100 "C (isotropization temperature, 68 "C) and in
Figure 2-37b the dichroism of the v(C-N)-band of the same polymer is shown
(from left to right) after irradiation, upon heating the irradiated sample to 100°C
and after reirradiation. The aliiiost perfect coincidence of repeated writing cycles
is demonstrated in more detail in Figure 2-38a, which displays the simultaneous
photo-induced alignment of the different segments during laser irradiation in an
order parameter/tinie-plot. Figure 2-38b exhibits the intermediate thermal erasure
of the first orientation as a function of temperature. The loss of orientation for the
different polymeric segments is also coupled and takes place in a narrow teniperature
interval. A comparison of Figure 2-38b with the transition temperature ranges
of the individual phases in Table 2-2 shows that, during erasure, the anisotropy
is mainly retained in the less ordered mesophases 3 and 4. Beyond 54 "C, only a
small, residual anisotropy is observed due to recrystallization effects [96], which
completely disappears at the clearing point of phase 2.
2.7 Orientation of Liquid Crystals Under
Mechanical Forces
Apart from the orientation mechanisms discussed in the previous sections, a preferential
alignment of liquid crystals or liquid-crystalline polymers may also bc
induced by the application of mechanical forces. Depending on the investigated
system, this can be either done by shearing between plates or in stress-strain niea-