Applications of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 80

Accordingly, the piezoelectric coefficients measured for FLCEs and for other
ferroelectric materials have a similar magnitude because for similar forces the
elastomers are deformed more strongly.
Concerning the shape variations resulting from the application of an external
electric field, the consequences of an induced ferroelectric switching will be
discussed at first. As evident from Fig. 14a , both polar states are very similar and
the deformation resulting from the rotation of the director on the cone can be
considered to be rather small. In fact some “vibrations” have been observed during
rapid switching of low-molar-mass ferroelectrics; thus the shape variations must be
rather small [ 178 , 179 ] . The electroclinic effect, on the other hand, promises much
larger deformations (see Fig. 14b ) and most experiments have focused in this
direction. In free-standing films, the smectic layers are parallel to the film surface
and the polar axis lies in the plane of the film. This arrangement is ideal for
observation of the electroclinic effect around the smectic-A* to chiral smectic-C*
phase transition (see Fig. 19a ). If an electric field is applied parallel to the smectic
layers, the chiral mesogens will tilt such that the induced polarity is opposite to the
electric field. The tilt of the molecules is accompanied by a decrease in the smectic
layer thickness. Due to the stacking of the smectic layers in the film, the thickness
variation of the various layers is expected to add up and a reasonable change of the
film thickness should be observed.
These considerations have led to several attempts to measure the thickness
change of films in dependence on an applied electric field. In a first experiment,
a Michelson interferometer was used for the determination of the thickness change
in a 75-nm thick free-standing smectic film of FLCE2.1 (Scheme 1 ). With this
method, a strain of 4% at a field of 1.5 V/ m m was measured [ 170 ] , which would
correspond to a ¼ 10 15 m 2 /V 2 . In a second experiment (with the similar

material FLCE2.2) reflection microscopy was used. This allowed simultaneous
measurement of the decrease in film thickness and the associated increase in
width of the sample (Fig. 20 ) . In this experiment, a strain of only 1% was detected
at a field of 3 V/ m m[ 171 ] , corresponding to a ¼

55

10 15 m 2 /V 2 . Most probably
the smaller value is correct [ 171 ], because its magnitude is consistent with tilt angle
susceptibilities of similar materials [ 23 , 25 ] , for which induced tilt angles of the

1

Fig. 19 (a) FLCE film crosslinked in the smectic-C* phase. (b) FLCE film crosslinked in the
smectic-A* phase