Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 125

where l i ( i ¼ x , y , z ) is the dimension in the i -direction and the subscript 0 denotes
the undeformed state. As can be seen in Fig. 5 , the contractive strain in the
x -direction increases with V 0 above a threshold voltage amplitude ( V 0c ), and levels
off at sufficiently high V 0 . In contrast, g y remains almost zero even at high V 0 .
The two-dimensional deformation mode is independent of V 0 and c x . A reduction in
c x increases the magnitude of the strain, and the maximum |g x | reaches ca. 15% for
c x ¼ 3 mol%. A decrease in c x also reduces V 0c . A detailed discussion of V 0c will be

given in Sect. 3.5.
Nematic gels with random polydomain textures under electric fields exhibit
simple (three-dimensional) uniaxial deformation [ 13 ]. Figure 6 shows the electrical
deformation of a cylindrical-shaped polydomain nematic gel in an unconstrai-
ned geometry. The chemical structures of the mesogen, the cross-linker, and the
swelling solvent are identical to those of the monodomain samples in Figs. 4 and 5 .
The cross-linking reaction without taking mesogen alignment into account results
in a nematic elastomer with a polydomain texture that possesses randomly oriented
local directors without a global director. As shown in Fig. 6 , the field-induced defor-
mation is uniaxial elongation along the field axis: The dimension in the electric field
increases, whereas the dimensions in the other two directions decrease equivalently.
The directors have a random configuration in the initial state, and the electric field
induces a rotation of the local directors toward the field direction.
The results shown in Figs. 4 - 6 were obtained for nematic gels with dielectrically
positive mesogens whose molecular long axes are parallel to the dipole moment.
The director of dielectrically positive nematogens aligns in the field direction. The
electrical stretching of dielectrically positive nematic gels is parallel to the field
direction, independently of the type of initial director configuration (monodomain
or polydomain). Figure 7 shows the electrical deformation of a cylindrical-shaped
polydomain nematic gel with negative dielectric anisotropy [ 13 ] . Both the mesogen
and the swelling solvent in this gel, whose chemical structures are shown in Fig. 7a ,
are dielectrically negative (i.e., the long axes are normal to the dipole moment).

Fig. 6 Electro-mechanical effect of a cylindrical-shaped polydomain nematic gel with positive
dielectric anisotropy. (a) Observation geometry. (b) Electrical deformation under a DC field of
E ¼ 0.6 MV/m. The specimen is elongated in the field direction and contracted in the other two
directions. Observation under natural light. (c) Strains as a function of field strength. From [ 13 ]