Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 133

showed that the deformation is dominated by that driven by director rotation while
the contribution of the Maxwell effect to strain is minor, until the director rotation is
completed [ 41 ] .

3.5 Threshold for the Onset of EOM Effect

A finite threshold for the onset of EOM effects is observed in Figs. 5 and 8 . In the
case of nematic liquids, the determinant of the threshold for the director rotation is
voltage, which is a main feature of the Fredericks transition [ 6 , 42 ] . Does electric
field or voltage determine the threshold for LCEs? This issue was first discussed in
studies of the EO response of nematic gels in the constrained geometry of g z ¼ 0
[ 15 , 16 ]. Figure 12 shows Dn eff in the low V 0 region for the two unconstrained
samples with the same c x (14 mol%) but different film thicknesses ( d g 0

¼ 23 and

47 m m) and cell gaps ( d t ¼ 40 and 60 m m) [ 19 ] . In cells composed of two layers

(film and silicone oil) differing in thickness and dielectric constant, the voltage ( V c,g )
and field strength ( E c,g ) acting on the film are given by:

e s d g V c
e s d g þ e g d t d g

V c ; g ¼ (12a)

V c ; g
d g

E c ; g ¼ (12b)

where e g and e s are the dielectric constants for the gels along the field axis and
silicone oil, respectively. For simplicity, we assume no pre-tilting of nematogens in
the initial state, and approximate e g by the dielectric constant normal to the long

0.03

0.025

d g 0 ( m) d t ( μ m)

μ

23 40

47 60

0.02

0 50 100 150 200

V 0 (V)

Fig. 12 Effective birefringence ( Dn eff ) at low-voltage amplitudes for nematic gels with c x ¼ 14
mol% and different values of d g 0 and d t . The arrows indicate the threshold voltage amplitudes
( V 0c ) for the onset of electro-optical effects. From [ 19 ]