main chain, combined main and side chain, hydrogen-bonded or ionic aggregation of
photochromic LC polymer systems, as well as photoresponsive LC dendrimers,
polymer-dispersed liquid crystal (PDLC), and more. The interplay of photochromic
moiety/LC phase with light may vary with composition (Prasad et al. 2005).
The azo-only side-chain liquid crystalline polymer has the ability to self-organize
to form LCP as well. This type of polymer generates stable photoinduced birefringence due to the cooperative motion of light-absorbing azobenzene chromophores,
and the derived hologram shows minimum degradation. This offers ideal material for
optical data storage applications. Cholesteric polymer films having planar orientation exhibit a selective reflection of light. The action of light leads to the isomerization of both chiral and photochromic group (or the photochromic group alone),
which induce change in the helical twisting power of the chiral fragments. This
process is accompanied by sharp changes in the supramolecular helical structure and
the optical properties of the polymer.
Another series of LC architecture originates from bisbenzylidenes. They have
several isomeric structures. Their molecular design is such that it has dual characteristic of LC and chromophore, respectively. The photoactivity toward higher
wavelength UV radiation is exhibited by cross-linking via 2π + 2π coupling reaction
resulting in cyclobutane ring. This type of cross-link is usually not stable to lower
wavelength UV radiation. Bisbenzylidene polymers are found to be thermally stable,
and the photo cross-linking ability makes them more stable. Thus, having large
electro-optic coefficient, these polymers can be applied in the area of nonlinear
optics. The conjugated architecture has added another feature of electronically
conducting polymer. A polymer designed with cinnamic acid moiety also possesses
identical photoresponse ability and forms similar cross-link under suitable irradiation.
Under photoactive LCP, holographic polymer-dispersed liquid crystals (PDLC)
can be discussed as well. Holographic PDLCs (H-PDLCs) are usually formed by
illumination of interference patterns on a mixture of photoactive monomers and
liquid crystals (Kubytskyi and Reshetnyak 2006). Local differences in photopolymerization rates and reduced miscibility induce the LC domains to be periodically formed with the period of the interference pattern, in low-light intensity areas.
An electric field may be used to manipulate the orientation of the LC making the
hologram to remain “hidden.”
Liquid crystalline (LC) azobenzene polymers have several features which differentiate it from their amorphous counterparts (Tong et al. 2004):
• The alignment of azobenzene groups induced by irradiation with linearly polarized light is thermally stable. For amorphous polymers, photoinduced orientation,
obtained at room temperature, will diminish during increase of temperature and
disappear at T > T g due to the thermal relaxation. For LC polymers, the opposite
may happen as the initial photoinduced orientation may enhance at T > Tg
because of natural tendency for self-organization of the azobenzene mesogenic
groups in a LC phase.
• The photochemically induced LC-isotropic phase transition takes place when the
rodlike trans isomer converts into the bent cis isomer as the latter disturbs the LC
14 Photoactive Liquid Crystalline Polymers
395
photochromic LC polymer systems, as well as photoresponsive LC dendrimers,
polymer-dispersed liquid crystal (PDLC), and more. The interplay of photochromic
moiety/LC phase with light may vary with composition (Prasad et al. 2005).
The azo-only side-chain liquid crystalline polymer has the ability to self-organize
to form LCP as well. This type of polymer generates stable photoinduced birefringence due to the cooperative motion of light-absorbing azobenzene chromophores,
and the derived hologram shows minimum degradation. This offers ideal material for
optical data storage applications. Cholesteric polymer films having planar orientation exhibit a selective reflection of light. The action of light leads to the isomerization of both chiral and photochromic group (or the photochromic group alone),
which induce change in the helical twisting power of the chiral fragments. This
process is accompanied by sharp changes in the supramolecular helical structure and
the optical properties of the polymer.
Another series of LC architecture originates from bisbenzylidenes. They have
several isomeric structures. Their molecular design is such that it has dual characteristic of LC and chromophore, respectively. The photoactivity toward higher
wavelength UV radiation is exhibited by cross-linking via 2π + 2π coupling reaction
resulting in cyclobutane ring. This type of cross-link is usually not stable to lower
wavelength UV radiation. Bisbenzylidene polymers are found to be thermally stable,
and the photo cross-linking ability makes them more stable. Thus, having large
electro-optic coefficient, these polymers can be applied in the area of nonlinear
optics. The conjugated architecture has added another feature of electronically
conducting polymer. A polymer designed with cinnamic acid moiety also possesses
identical photoresponse ability and forms similar cross-link under suitable irradiation.
Under photoactive LCP, holographic polymer-dispersed liquid crystals (PDLC)
can be discussed as well. Holographic PDLCs (H-PDLCs) are usually formed by
illumination of interference patterns on a mixture of photoactive monomers and
liquid crystals (Kubytskyi and Reshetnyak 2006). Local differences in photopolymerization rates and reduced miscibility induce the LC domains to be periodically formed with the period of the interference pattern, in low-light intensity areas.
An electric field may be used to manipulate the orientation of the LC making the
hologram to remain “hidden.”
Liquid crystalline (LC) azobenzene polymers have several features which differentiate it from their amorphous counterparts (Tong et al. 2004):
• The alignment of azobenzene groups induced by irradiation with linearly polarized light is thermally stable. For amorphous polymers, photoinduced orientation,
obtained at room temperature, will diminish during increase of temperature and
disappear at T > T g due to the thermal relaxation. For LC polymers, the opposite
may happen as the initial photoinduced orientation may enhance at T > Tg
because of natural tendency for self-organization of the azobenzene mesogenic
groups in a LC phase.
• The photochemically induced LC-isotropic phase transition takes place when the
rodlike trans isomer converts into the bent cis isomer as the latter disturbs the LC
14 Photoactive Liquid Crystalline Polymers
395