Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 126

Fig. 7 Electro-mechanical effect of a cylindrical-shaped polydomain nematic gel with negative
dielectric anisotropy. (a) Molecular structures of the constituent mesogen and swelling solvent in
the gel. (b) Strains as a function of field strength. From [ 13 ]. (c) Schematics of electrical
deformation of dielectrically positive ( D e > 0) or negative ( D e < 0) polydomain nematic gels.
In the nematic gels in the present study (Fig. 2 ), the stretching direction of network backbone is
parallel to the mesogen alignment [ 33 , 34 ]

The dielectrically negative nematic gels are elongated in the direction normal to the
field axis in accordance with the realignment direction of the local directors. These
results indicate that the stretching direction in the electrical deformation of nematic
gels is governed by the sign of the dielectric anisotropy, which is schematically
shown in Fig. 7c . In general, the stretching direction of polymer backbone becomes
either parallel or normal to the alignment direction of dangling mesogens, depend-
ing on the spacer length and the chemical structures of side-chain type LCEs [ 32 ] .
The LCEs in this study correspond to the former case (i.e., parallel), which was
confirmed by FTIR spectroscopy [ 33 , 34 ] .

3.2 Electro-Optical Effect

The characteristic two-dimensional deformation of monodomain nematic gels
under electric fields originates from the rotation of the director. The electrically
driven director rotation is evident from the large change in optical birefringence as
shown in Fig. 4b . The appearance of the gel under cross-polarized conditions
changes from bright to dark, which corresponds to a change in the director orienta-
tion from the x -direction to the z -direction.
Figure 8a shows the effective birefringence ( Dn eff : Dn eff ¼ n x n y where n x
and n y are refractive indices in the x - and y -directions, respectively) in the x - y plane
as a function of V 0 [ 19 ]. Note that the birefringence data in Fig. 8 was obtained
using a cell filled with optically transparent silicone oil in order to observe the
optical effect purely originating from the gel, although 5CB was employed as the
surrounding fluid in Fig. 4 . The gels exhibit a finite Dn eff at V 0 ¼ 0( Dn eff 0 ) due to

the original global uniaxial orientation in the x -direction (i.e., n x 0 > n y 0 ¼ n z 0