the film thickness (usually few micrometers). Since contraction along the director
increases with decreasing order, the contraction is larger at the entry face of the film
and decreases as going deeper in the film. To accommodate these differences in
deformation, the system bends. Even not having a phase change as in liquid crystalline elastomers, light-induced disorder in glassy systems has a large impact in
mechanical properties giving a clearly visual bending. The film can be reversed to
the original flat state by illuminating it with visible light transforming cis isomers to
the trans form and homogenizing the film to the initial all-trans ordered state.
Bending directionality can also be changed by modifying the alignment of the
director of these monolithically oriented cantilevers. If the director of the film is
homeotropically aligned, the sample bends away from the light as opposed to the
planarly oriented samples. An increase of disorder in homeotropic samples leads to
isotropic expansion of the film in the plane. The absorption gradient in the depth is
responsible of the gradient in expansion that leads to film bending away from the UV
light source.
Precise control over the bending direction was obtained in polydomain samples
prepared from similar materials and film thickness by using linearly polarized light
as exciting agent. Bending was again induced toward the source; however, bending
direction was always perpendicular to linearly polarized light direction what was
ascribed to selective excitation of liquid crystal domains oriented parallel to light
polarization direction (Yu et al. 2003).
The previous examples show how intensity gradients in the film thickness,
intrinsically occurring in highly absorbing azobenzene samples, can be used for
the bending of photoresponsive liquid crystalline polymeric films. Spatially inhomogeneous excitation in the plane of the film can also induce morphing of liquid
crystalline elastomeric films having azobenzene dispersed as shown by PalffyMuhoray and coworkers (Camacho-López et al. 2004). Illumination in the absorption band of the azo chromophore with a Gaussian beam slightly smaller than the
sample results in rapid deformation of the film around the illuminated region. When
these films are illuminated while they are floating in water, they swim away the high
intensity region of the spot.
Besides of making use of intensity gradients, engineering of the director configuration results in further control and optimization of the photomechanical response.
Broer and coworkers explored the effect of splay-bend configurations. The authors
took advantage of the fact the contraction along the director associated to the
increase of disorder is accompanied by an expansion perpendicular to the director.
A change of the director over the thickness of the film results in a gradient of
mechanical properties even with no gradient in stimulus. This allows to substantially
reduce the azobenzene content since no absorption gradients are needed in this case.
Bending in these director engineered films comes from contraction at the entry face
of the film and the expansion at the opposite face (or vice versa depending on the
relative director orientation of the faces with respect to the lamp). This configuration
optimizes also photoresponse benefiting from the opposite mechanical behavior of
the two sides of the film, expansion and contraction, as compared to previously
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
471
increases with decreasing order, the contraction is larger at the entry face of the film
and decreases as going deeper in the film. To accommodate these differences in
deformation, the system bends. Even not having a phase change as in liquid crystalline elastomers, light-induced disorder in glassy systems has a large impact in
mechanical properties giving a clearly visual bending. The film can be reversed to
the original flat state by illuminating it with visible light transforming cis isomers to
the trans form and homogenizing the film to the initial all-trans ordered state.
Bending directionality can also be changed by modifying the alignment of the
director of these monolithically oriented cantilevers. If the director of the film is
homeotropically aligned, the sample bends away from the light as opposed to the
planarly oriented samples. An increase of disorder in homeotropic samples leads to
isotropic expansion of the film in the plane. The absorption gradient in the depth is
responsible of the gradient in expansion that leads to film bending away from the UV
light source.
Precise control over the bending direction was obtained in polydomain samples
prepared from similar materials and film thickness by using linearly polarized light
as exciting agent. Bending was again induced toward the source; however, bending
direction was always perpendicular to linearly polarized light direction what was
ascribed to selective excitation of liquid crystal domains oriented parallel to light
polarization direction (Yu et al. 2003).
The previous examples show how intensity gradients in the film thickness,
intrinsically occurring in highly absorbing azobenzene samples, can be used for
the bending of photoresponsive liquid crystalline polymeric films. Spatially inhomogeneous excitation in the plane of the film can also induce morphing of liquid
crystalline elastomeric films having azobenzene dispersed as shown by PalffyMuhoray and coworkers (Camacho-López et al. 2004). Illumination in the absorption band of the azo chromophore with a Gaussian beam slightly smaller than the
sample results in rapid deformation of the film around the illuminated region. When
these films are illuminated while they are floating in water, they swim away the high
intensity region of the spot.
Besides of making use of intensity gradients, engineering of the director configuration results in further control and optimization of the photomechanical response.
Broer and coworkers explored the effect of splay-bend configurations. The authors
took advantage of the fact the contraction along the director associated to the
increase of disorder is accompanied by an expansion perpendicular to the director.
A change of the director over the thickness of the film results in a gradient of
mechanical properties even with no gradient in stimulus. This allows to substantially
reduce the azobenzene content since no absorption gradients are needed in this case.
Bending in these director engineered films comes from contraction at the entry face
of the film and the expansion at the opposite face (or vice versa depending on the
relative director orientation of the faces with respect to the lamp). This configuration
optimizes also photoresponse benefiting from the opposite mechanical behavior of
the two sides of the film, expansion and contraction, as compared to previously
16 Light-Sensitive Azobenzene-Containing Liquid Crystalline Polymers
471