Page 168 - Photoreactive Organic Thin Films

5. CHIRAL POLYMERS WITH PHOTOAFFECTED PHASE BEHAVIOR FOR OPTICAL DATA STORAGE J 47

because the orientational order in the LC state restricts the reorientation of
the mesogenic side chains. That disadvantage can be easily overridden, how-
ever, by preliminary misalignment of the LC order using irradiation with non-
polarized or circularly polarized UV light. On the other hand, higher ordering
of the mesogenic and dye groups in the mesophase leads to higher glass
temperatures and, potentially, to enhanced stability of the recorded data or
images. Moreover, the competition between the thermodynamic equilibrium
LC structure and the photo-optically induced one might result in a number of
advanced and sophisticated photorecording techniques, as we will show later
in this chapter.
There have also been reports on the preparation of polar materials by a
photo-electro-poling technique that combines the optically induced
quadrupolar depletion of chrornophores in the direction of the light electric
vector with an additional field-induced orientation of dipolar chro-
43 45
rnophores. " The latter allows the preparation of "cold electrets," which
are interesting for nonlinear optical applications, such as optical harmonic
generation, wave mixing, etc. 3
Among the known LC phases used for photoaddressed polymer applica-
tions, chiral systems attract particular interest due to such features as
selective reflection of light, circular polarization of the propagating/reflected
light, and ferroelectricity. Thus, for example, Bobrovsky et al, have reported
recently on the controlled change of a helical pitch and, hence, of a color of
46
cholesteric photochromic copolymers, and dual photoaddressing using light
of different wavelengths in terpolymers containing both azobenzene and
47
benzylidene-jp-menthanone photochromic moieties. Moreover, the photo-
induced chromophore reorientation was shown to unwind totally the helical
structure of cholesteric oligomers and polymers. 48
We should note here that most of the publications consider the photo-
orientation process and the birefringence it introduces to be the only effect of
the light illumination onto dichroic LC polymers. Nevertheless, the elongated
E-form and the bent Z-form of the same azo dye molecule generally should
reveal quite different phase behaviors: The former favors formation of LC
phases, but the latter can hardly accommodate to the mesophase. Hence, the
illumination should affect substantially mesophase properties and phase tran-
sitions of dichroic LC polymers. Eich, Wendorff, Reck and Ringsdoffs early
29
paper discusses a large shift—up to 10 K—of the clearing point in a nematic
azo dye copolymer under illumination. It is no surprise, therefore, that the
UV illumination within the temperature gap between the clearing points of
completeiy-trans and completely-ds isomers, i.e., isotropization temperatures
in darkness and under illumination, causes the {isothermal) clearing transi-
49
tion, as reported by Hayashi et al. for a high-ordered smectic phase of an
azo dye polyacrylate. In that case, the temperature variation of the photo-
SQ
induced phase transition does not exceed 6 K. However, Ikeda et aL report
an isothermal photoinduced N-Iso phase transition of photochromic side
chain poly(meth)acrylates within a much broader temperature range (from
130°C down to the room temperature), including the region below T . The
backward transition to the nematic state occurs thermally in darkness, taking
some seconds close to the clearing point but much longer at lower tempera-