Stimuli-Responsive Actuators
The unique characteristics of LCE is the strong correlation of the macroscopic
deformation and the molecular alignment. The LCE systems undergo fast and
large reversible change in dimension when the state of order is changed. A transition
to a low ordered state will bring about a dimensional contraction along the director in
a uniaxially aligned state. In the absence of a volumetric change, the contraction
along the director is always compensated by the expansion in other directions. When
a volumetric expansion occurs, on the other hand, both the components are shifted
toward the same sign. Depending on the alignment director in LC networks, various
deformations such as bending, folding, and twisting occur. The LCE actuators are
susceptible to heat (thermo-actuators), solvent (chemo-actuators), and light (photoactuators). Majority of previous works focus on thermo-actuators. Upon increasing
temperature from the aligned state (LC phase) to the randomly oriented state
(I phase), the polymeric LC networks expand in a direction perpendicular to the
LC director while contracting along the director. Magnitude of the response is
closely correlated to the cross-linking density and thermal expansion coefficient.
Solvent-induced actuators are derived on the basis of the selective swelling in a
certain solvent. The programmed 2D structures transform to the 3D objects
depending on the polarity of solvents. Various shapes such as cube, pyramid, and
Phlat Ball have been reported (Jeong et al. 2011). Humidity-responsive LC actuators
are developed using self-assembly of supramolecular networks via chemical crosslinking and hydrogen bonding (Dai et al. 2013). After the breakage of hydrogen
bonding, the film responds to the humidity change in the atmosphere, causing a
bending deformation (Fig. 9).
Recently, the light-driven photo-actuators receive a lot of intensive attentions
because the mechanical deformation can be switched rapidly by changing the
wavelength of light (Kim et al. 2013). When chromophores are parts of the networks
or bound to them, light energy may affect the conformation of the networks due to
isomerization. The LCE containing azobenzene moieties, for instance, shows a
reversible expansion and contraction by conformational transition from trans- to
cis-isomeric states. The photo-actuators in the form of fiber are also obtained from
melt spinning process. Upon irradiating UV light (365 nm), the fiber bends toward
the irradiation direction and transforms back to the original state when exposed to the
visible light (450 nm). The reversible bending performance in a desired direction is
beneficial for the fabrication of wireless 3D actuators (Fig. 10).
Advanced Composites
With the addition of inorganic fillers, physical properties of LC mixtures are
improved significantly (Wang et al. 2012). The aligned LCP/carbon nanotube
(CNT) composites display the anisotropic mechanical properties. As shown in
Fig. 11, the mesogenic LC molecules are preferentially oriented parallel to the highly
aligned CNT. The composite film exhibits a higher tensile strength along the parallel
direction of an array of CNT compared to the perpendicular direction. Furthermore,
the incorporation of aligned CNTs provides the higher electrical conductivity along
the CNT-aligned direction.
4 Anisotropic Liquid Crystal Networks from Reactive Mesogens
107
The unique characteristics of LCE is the strong correlation of the macroscopic
deformation and the molecular alignment. The LCE systems undergo fast and
large reversible change in dimension when the state of order is changed. A transition
to a low ordered state will bring about a dimensional contraction along the director in
a uniaxially aligned state. In the absence of a volumetric change, the contraction
along the director is always compensated by the expansion in other directions. When
a volumetric expansion occurs, on the other hand, both the components are shifted
toward the same sign. Depending on the alignment director in LC networks, various
deformations such as bending, folding, and twisting occur. The LCE actuators are
susceptible to heat (thermo-actuators), solvent (chemo-actuators), and light (photoactuators). Majority of previous works focus on thermo-actuators. Upon increasing
temperature from the aligned state (LC phase) to the randomly oriented state
(I phase), the polymeric LC networks expand in a direction perpendicular to the
LC director while contracting along the director. Magnitude of the response is
closely correlated to the cross-linking density and thermal expansion coefficient.
Solvent-induced actuators are derived on the basis of the selective swelling in a
certain solvent. The programmed 2D structures transform to the 3D objects
depending on the polarity of solvents. Various shapes such as cube, pyramid, and
Phlat Ball have been reported (Jeong et al. 2011). Humidity-responsive LC actuators
are developed using self-assembly of supramolecular networks via chemical crosslinking and hydrogen bonding (Dai et al. 2013). After the breakage of hydrogen
bonding, the film responds to the humidity change in the atmosphere, causing a
bending deformation (Fig. 9).
Recently, the light-driven photo-actuators receive a lot of intensive attentions
because the mechanical deformation can be switched rapidly by changing the
wavelength of light (Kim et al. 2013). When chromophores are parts of the networks
or bound to them, light energy may affect the conformation of the networks due to
isomerization. The LCE containing azobenzene moieties, for instance, shows a
reversible expansion and contraction by conformational transition from trans- to
cis-isomeric states. The photo-actuators in the form of fiber are also obtained from
melt spinning process. Upon irradiating UV light (365 nm), the fiber bends toward
the irradiation direction and transforms back to the original state when exposed to the
visible light (450 nm). The reversible bending performance in a desired direction is
beneficial for the fabrication of wireless 3D actuators (Fig. 10).
Advanced Composites
With the addition of inorganic fillers, physical properties of LC mixtures are
improved significantly (Wang et al. 2012). The aligned LCP/carbon nanotube
(CNT) composites display the anisotropic mechanical properties. As shown in
Fig. 11, the mesogenic LC molecules are preferentially oriented parallel to the highly
aligned CNT. The composite film exhibits a higher tensile strength along the parallel
direction of an array of CNT compared to the perpendicular direction. Furthermore,
the incorporation of aligned CNTs provides the higher electrical conductivity along
the CNT-aligned direction.
4 Anisotropic Liquid Crystal Networks from Reactive Mesogens
107