incorporated as side units of the polymeric chain. As mentioned before, the type of
chromophore and the length of the spacer have a major influence dictating whether
the material will exhibit an amorphous or a liquid crystalline phase. Longer flexible
spacers usually lead to liquid crystalline materials that have demonstrated to generally lead to higher and more stable values of the photoinduced anisotropy than
amorphous polymers. The methylenic chain provides an efficient decoupling of
the azobenzene and main chain dynamics favoring the photoinduced molecular
reorientation, therefore leading to large optical anisotropy. Moreover, the mesogen
to mesogen interactions help to stabilize the induced orientation and can promote
further amplification of the light-induced order even after the illumination has
ceased.
Side chain liquid crystalline azobenzene homopolyesters as the ones shown in
Fig. 7 show mesomorphic behavior at room temperature for certain methylenic
spacers. Despite their low T g , sometimes below room temperature, they are able to
register large and stable values of optical anisotropy with high sensitivity (low
energy), a behavior that is related to the mesomorphic character of this polymer
series.
Poly(meth)acrylic azobenzene containing homopolymers with a suitable
methylenic spacer present also liquid crystalline behavior. As in the case of the
polyesters, these materials are able to register large and stable values of the photoinduced birefringence. This can be optimized by the use of cleverly designed azo
chromophores as done by Ikeda and coworkers. These authors prepared a side chain
liquid crystalline photoresponsive polymers with azotolane side units (Fig. 5b). The
azotolane consists on an azobenzene group directly connected to a tolane one. The
highly conjugated resultant chromophore has an increased anisotropy of the molecular polarizability. The material is able to register birefringence values as high as 0.7
(measured at 633 nm), exceeding the values obtained with conventional azobenzene
polymers (Okano et al. 2006). Other macromolecular architectures such as dendritic
functionalized at the periphery with azobenzene chromophores with liquid crystal
behavior have also been explored in terms of their photoinduced optical response
leading to birefringence values similar to those obtained in side chain linear ones.
Although covalent functionalization has been predominant as the preparation
method in this type of application, the azo functionalization of polymeric structures
through noncovalent interaction (hydrogen, ionic, or halogen bonding) has also led
to large and stable values of photoinduced optical anisotropy.
Despite the large and stable photoinduced anisotropy obtained in liquid crystalline homopolymers, their use as volume holographic materials is restricted due to the
large extinction coefficient of the chomophore at the recording wavelengths, which
limits the sensitized thickness to few microns. Also, the presence of liquid crystal
domains lowers the optical quality of the films. To fully exploit the benefits of
volume holography, transparent films in the orders of several hundreds of microns
need to be prepared. Trying to retain the benefits of liquid crystalline polymers for
this application, several approaches have been undertaken, such as the statistical
copolymerization with non-absorbing monomers, the use of block copolymers with
azo liquid crystalline block and a second non-absorbent amorphous one, or the direct
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
463
chromophore and the length of the spacer have a major influence dictating whether
the material will exhibit an amorphous or a liquid crystalline phase. Longer flexible
spacers usually lead to liquid crystalline materials that have demonstrated to generally lead to higher and more stable values of the photoinduced anisotropy than
amorphous polymers. The methylenic chain provides an efficient decoupling of
the azobenzene and main chain dynamics favoring the photoinduced molecular
reorientation, therefore leading to large optical anisotropy. Moreover, the mesogen
to mesogen interactions help to stabilize the induced orientation and can promote
further amplification of the light-induced order even after the illumination has
ceased.
Side chain liquid crystalline azobenzene homopolyesters as the ones shown in
Fig. 7 show mesomorphic behavior at room temperature for certain methylenic
spacers. Despite their low T g , sometimes below room temperature, they are able to
register large and stable values of optical anisotropy with high sensitivity (low
energy), a behavior that is related to the mesomorphic character of this polymer
series.
Poly(meth)acrylic azobenzene containing homopolymers with a suitable
methylenic spacer present also liquid crystalline behavior. As in the case of the
polyesters, these materials are able to register large and stable values of the photoinduced birefringence. This can be optimized by the use of cleverly designed azo
chromophores as done by Ikeda and coworkers. These authors prepared a side chain
liquid crystalline photoresponsive polymers with azotolane side units (Fig. 5b). The
azotolane consists on an azobenzene group directly connected to a tolane one. The
highly conjugated resultant chromophore has an increased anisotropy of the molecular polarizability. The material is able to register birefringence values as high as 0.7
(measured at 633 nm), exceeding the values obtained with conventional azobenzene
polymers (Okano et al. 2006). Other macromolecular architectures such as dendritic
functionalized at the periphery with azobenzene chromophores with liquid crystal
behavior have also been explored in terms of their photoinduced optical response
leading to birefringence values similar to those obtained in side chain linear ones.
Although covalent functionalization has been predominant as the preparation
method in this type of application, the azo functionalization of polymeric structures
through noncovalent interaction (hydrogen, ionic, or halogen bonding) has also led
to large and stable values of photoinduced optical anisotropy.
Despite the large and stable photoinduced anisotropy obtained in liquid crystalline homopolymers, their use as volume holographic materials is restricted due to the
large extinction coefficient of the chomophore at the recording wavelengths, which
limits the sensitized thickness to few microns. Also, the presence of liquid crystal
domains lowers the optical quality of the films. To fully exploit the benefits of
volume holography, transparent films in the orders of several hundreds of microns
need to be prepared. Trying to retain the benefits of liquid crystalline polymers for
this application, several approaches have been undertaken, such as the statistical
copolymerization with non-absorbing monomers, the use of block copolymers with
azo liquid crystalline block and a second non-absorbent amorphous one, or the direct
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
463