Finkelmann in monodomain samples of liquid crystalline polysiloxanes cross-linked
with azobenzene chromophores (Finkelmann et al. 2001) and later by Terentjev
(Cviklinski et al. 2002) and Keller in acrylate-based materials (Li et al. 2003). After
the photoinduced contraction, the samples expanded back recovering the initial shape
upon thermal relaxation. The large differences found between amorphous and liquid
crystalline systems has its origin in the interplay, in these last ones, between the elastic
properties of the polymeric network and the ordering of the mesogenic units. As
mentioned before, the photoinduced isomerization of azobenzene chromophores into
the bulky cis form produces an isothermal decrease of the molecular order, leading
eventually to a transition to the isotropic state. Due to the coupling of the mesogens and
the polymer chain, the orientational order reduction leads to a strong change of the
polymer chain configuration accordingly, increasing the strain of the material. Because
of this strain amplification and the anisotropic mechanical properties of these materials,
focus in this field of photoresponsive soft actuators has been put in cross-linked liquid
crystalline polymeric systems being in situ photopolymerization of reactive mesogens,
the dominant sample processing methodology.
The photoinduced linear contraction of these systems can be used to pull objects
but also be transformed into rotational motion as demonstrated by Ikeda by using a
photoactive LCP sheet around two pulleys. By irradiating with UV and Vis at two
different specific locations extension and contractions of the film are transformed in
a rotational motion (Yamada et al. 2008).
Beyond the contraction of uniaxially oriented samples, many other types of film
deformations have been pursued towards more applied configurations that can turn
into functional elements in devices. Material design, engineering of the director
profile, and control over light excitation are key elements to have a well-defined
target actuation. For example, generation of gradients across the film thickness leads
to internal stresses that result in film bending, bringing these polymeric systems to
the third dimension. The group of Ikeda reported in 2003 the bending under the action
of light of cantilever type azobenzene rectangular samples with an area in the order of
tens of square millimeters and a thickness of few micrometers (Ikeda et al. 2003).
These authors shifted the focus from previously studied liquid crystal elastomers
cross-linked with azobenzene to glassy liquid crystalline polymeric networks prepared by only azobenzene monomers (mono and difunctionalized), aiming at a faster
mechanical photoresponse. Despite the photogenerated strain is much smaller, usually less than 2%, than in elastomers, cantilevers made of these glassy liquid crystalline polymeric systems show large bending. Irradiation with unpolarized UV light of
monodomain samples above the T g led to bending of the film towards the light source
and in the direction of the liquid crystal director. Despite the authors heated the
sample above the T g to observe the photoinduced deformation, later was demonstrated that bending could also take place in the glassy state, below T g . The origin of
this film bending is at the cis population gradient induced by UV light. Due to the
large extinction coefficients of azobenzene chromophores in the UV, light of this
range of wavelengths is largely absorbed in the first micron of the film, therefore
appearing a steep profile of UV intensity and, as a result, a gradient of cis isomers over
470
C. Sánchez-Somolinos
with azobenzene chromophores (Finkelmann et al. 2001) and later by Terentjev
(Cviklinski et al. 2002) and Keller in acrylate-based materials (Li et al. 2003). After
the photoinduced contraction, the samples expanded back recovering the initial shape
upon thermal relaxation. The large differences found between amorphous and liquid
crystalline systems has its origin in the interplay, in these last ones, between the elastic
properties of the polymeric network and the ordering of the mesogenic units. As
mentioned before, the photoinduced isomerization of azobenzene chromophores into
the bulky cis form produces an isothermal decrease of the molecular order, leading
eventually to a transition to the isotropic state. Due to the coupling of the mesogens and
the polymer chain, the orientational order reduction leads to a strong change of the
polymer chain configuration accordingly, increasing the strain of the material. Because
of this strain amplification and the anisotropic mechanical properties of these materials,
focus in this field of photoresponsive soft actuators has been put in cross-linked liquid
crystalline polymeric systems being in situ photopolymerization of reactive mesogens,
the dominant sample processing methodology.
The photoinduced linear contraction of these systems can be used to pull objects
but also be transformed into rotational motion as demonstrated by Ikeda by using a
photoactive LCP sheet around two pulleys. By irradiating with UV and Vis at two
different specific locations extension and contractions of the film are transformed in
a rotational motion (Yamada et al. 2008).
Beyond the contraction of uniaxially oriented samples, many other types of film
deformations have been pursued towards more applied configurations that can turn
into functional elements in devices. Material design, engineering of the director
profile, and control over light excitation are key elements to have a well-defined
target actuation. For example, generation of gradients across the film thickness leads
to internal stresses that result in film bending, bringing these polymeric systems to
the third dimension. The group of Ikeda reported in 2003 the bending under the action
of light of cantilever type azobenzene rectangular samples with an area in the order of
tens of square millimeters and a thickness of few micrometers (Ikeda et al. 2003).
These authors shifted the focus from previously studied liquid crystal elastomers
cross-linked with azobenzene to glassy liquid crystalline polymeric networks prepared by only azobenzene monomers (mono and difunctionalized), aiming at a faster
mechanical photoresponse. Despite the photogenerated strain is much smaller, usually less than 2%, than in elastomers, cantilevers made of these glassy liquid crystalline polymeric systems show large bending. Irradiation with unpolarized UV light of
monodomain samples above the T g led to bending of the film towards the light source
and in the direction of the liquid crystal director. Despite the authors heated the
sample above the T g to observe the photoinduced deformation, later was demonstrated that bending could also take place in the glassy state, below T g . The origin of
this film bending is at the cis population gradient induced by UV light. Due to the
large extinction coefficients of azobenzene chromophores in the UV, light of this
range of wavelengths is largely absorbed in the first micron of the film, therefore
appearing a steep profile of UV intensity and, as a result, a gradient of cis isomers over
470
C. Sánchez-Somolinos