Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 122

Table 1 Sample characteristics of monodomain nematic gels
Nematic gel Crosslinker

concentration ( c x )

Gel thickness
( d g 0 )

Spacer
thickness ( d p )

Cell gap
( d t )

Solvent
content

( m m) ( m m) ( m m) (mol%) (vol%)

SNE-3 3 26 20 40 82

SNE-7 7 34 25 40 74

SNE-7D 7 43 40 50 78

SNE-10D 10 40 40 50 73

SNE-14 14 23 25 40 50

irradiating the cell using a wavelength of 526 nm. The cell gap was controlled using a
spacer with a thickness of d p ranging from 20 to 40 m m (Table 1 ). The cross-linker
concentration ( c x ) was varied from 3 to 14 mol%. The resulting gel films were
carefully separated from the cell and immersed in dichloromethane to wash out the
unreacted materials and nematic solvent. The swollen films were gradually deswollen
by increasing the methanol content in the swelling solvent. The fully deswollen films
were dried and thereafter allowed to swell in a cyanobiphenyl type LMM-LC,
4- n -pentyl-4 0 -cyanobiphenyl (5CB; Fig. 2 ) at25 C until the swelling was equili-
brated. The 5CB content in the swollen films increases with a decrease in c x and
ranged from 82 to 50 vol% (Table 1 ). In the fully swollen films, the LCE and 5CB are
completely miscible and form a single nematic phase, which is confirmed by a single
nematic-isotropic transition temperature [ 30 ] . It should be noted that both of the
constituent nematogens, A-6OCB and 5CB, in the swollen elastomers are dielectri-
cally positive, i.e., the molecular long axis of each nematogen tends to align parallel to
the direction of electric fields.

2.2 Observation Geometry

As demonstrated later, unconstrained geometry is crucial for the observation of
pronounced EOM effects. The swollen elastomer films were placed between two
glass plates with optically transparent indium tin oxide (ITO) electrodes so that the
films could have no mechanical constraints from the rigid electrodes, i.e., the cell
gap was larger than the film thickness, as shown in Fig. 3 . The cell was filled with a
silicone oil, which was optically transparent and immiscible with the swollen
elastomers, in order to observe the EO effect purely originating from the gels.
The gel thickness ( d g 0 ) and cell gap ( d t ) for each sample are listed in Table 1 .
A square AC electric field was imposed normally on the initial director of the
swollen elastomers. The frequency was 1 kHz unless specified otherwise. In the
measurement of optical birefringence, the specimen was placed such that the initial
director was at an angle of 45 relative to the crossed polarizers. The intensity of the
transmitted light ( I ) was measured by a photodiode detector as a function of voltage
amplitude ( V 0 ). The effective birefringence in the x - y plane ( Dn eff ) was evaluated