Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 43

a

b

z z

n

n

j

90- j

F

d

f

l

y y

d

f

l

x x

Fig. 17 Smectic-C elastomer with uniformly aligned director but conical layer distribution
subjected to shear strain perpendicular to the director [ 117 ] (a) and corresponding reorientation
process that produces an S C monodomain (b). Reprinted with permission from [ 87 ]. Copyright
(2008) American Chemical Society

leads to an anisotropic distribution of the layer normals on the cone causing a non-
centrosymmetric phase structure but cannot induce a completely uniform orienta-
tion of both the layer normal and the director. Nevertheless, the partially oriented
phase structure can be fixed permanently by chemical crosslinking, yielding highly
transparent elastomers which show piezoelectric properties and second harmonic
generation [ 80 - 83 , 115 ] . The same orientation procedure has also been deployed to
align main chain S C elastomers [ 116 ].
A mechanical deformation that is consistent with the biaxial S C phase symmetry
and that is able to induce a macroscopic orientation of the layer normal and the
director is a shear mechanical field. Hiraoka et al . prepared side chain S C * LSCEs
by applying a shear deformation perpendicular to the director followed by
chemical crosslinking [ 117 , 118 ]. For this, the uniaxially aligned elastomer
with uniformly oriented director but conical layer distribution is mounted into a
mechanical shear setup (Fig. 17a ) . The elastomer is sheared stepwise at room
temperature until the shear angle is consistent with the tilt angle of the S C phase
structure. Between each shear step a sufficiently high relaxation time should
be maintained to avoid damaging the sample. The mechanical field induces
a continuous reorientation of the S C phase structure where the director rotates
towards the shear diagonal, while the layer normal takes up the position perpen-
dicular to the shear mechanical field (Fig. 17b ) . This multi-step orientation
process can take up to several hours. It must be avoided that the chemical
crosslinking reaction is completed before a macroscopic orientation is induced.
If a hydrosilylation reaction is used, the reaction speed is rather slow at ambient
temperature. However, it might be advisable to cool the sample during the
orientation process to slow down the crosslinking process and to provide enough
time for a successful alignment. The sheared state which corresponds to an S C
monodomain structure can be fixed permanently by completing the chemical
crosslinking reaction at elevated temperatures - completely analogous to nematic
elastomers. Following this procedure, main chain LSCEs showing S C and
chiral S C * phase structures have also been realized by Sanchez et al . and Heinze
et al . , respectively [ 86 , 87 , 119 ] .