Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 35

director. Subsequently, transition to the smectic state occurs and uniformly aligned
layers are built up. By deploying a second crosslinking step this macroscopic
orientation can be fixed permanently. In contrast to the anisotropic deswelling
technique, this subsequent orientation of the director and the layer normal induces
globally a prolate chain conformation, which is inconsistent with the equilibrium
conformation of the smectic polymer melt. By variation of the composition in
coelastomers of smectogenic and nematogenic side-chains, it is possible to induce
systematically this specific phase behavior. With increasing fraction of
nematogenic side chains, the stability of the smectic phase decreases and for a
certain composition range the phase sequence smectic-A, nematic, isotropic is
observed. Inducing a macroscopic orientation then becomes easy by applying
uniaxial mechanical stretching. Of course, the mesogenic side chains must possess
a flexible spacer with an odd number of spacer atoms or a long spacer (x > 7) to give
locally a prolate chain conformation in the nematic state. This synthetic concept
is very robust and works for a wide range of chemical constitutions, including
smectogenic side chains carrying perfluorinated tails [ 75 , 77 ].
In contrast to classical side chain elastomers, smectic-A main chain elastomers
exhibit prolate chain conformations. Consequently, macroscopically oriented
samples can be prepared according to the method of K upfer et al . utilizing a second

crosslinking step under uniaxial deformation [ 31 ]. Analogous, S A LSCEs based on
side chain elastomers with side-on attached mesogenic units can be prepared [ 97 ] .
In general the practical aspects presented in detail for nematic elastomers can be
used for the synthesis of S A LSCEs as well. However, a few additional remarks will
be given in the following box.

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While deswelling under uniaxial mechanical load is performed at room temper-
ature for nematic elastomers, this procedure is usually not applicable for smectic
side-chain elastomers. Especially for elastomers carrying partially fluorinated
mesogenic side-chains, the deswelling occurs too fast to allow for a good
orientation of the layer normal and the director. As soon as the smectic state is
reached the layer planes can couple to the mechanical field and a uniaxial
mechanical deformation becomes insufficient to produce a monodomain elas-
tomer (see Sect. 3.2 ). It is therefore advisable to perform the deswelling at
elevated temperature. The elastomer is heated to the isotropic state under
uniaxial load until most of the solvent is evaporated. Afterwards it is slowly
cooled to a temperature slightly below T n,i , detectable by a spontaneous length
change. The load is successively increased until a perfectly transparent sample
is obtained. After 2 h of crosslinking in the nematic state, the elastomer is cooled
to the smectic state and the crosslinking reaction is completed over several days.
In the smectic phase the load can be further increased if necessary, as the
elastomers can now stand much higher mechanical stress.
Alternatively, the orientation can be carried out at room temperature in a
chamber saturated with toluene vapor. This is achieved by placing a few
drops of toluene in a chamber of suitable size. After the chamber is filled with
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