Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 24

Fig. 7 Principal route to liquid single crystal elastomers (LSCEs)
procedures have to be applied. This is the case for the mechanical field induced
orientation of S C elastomers where two separate steps are necessary to orient both
the layer normal and the director. For biaxial nematic and biaxial S A phases the
mesogens exhibit orientational long range order in all three spatial directions, and
the minor director m is used in addition to the major director n to denote the
orientation of the minor molecular axes. Here, a second orientation step is necessary
to produce monodomains with a macroscopic 3D orientation of the LC phase
structure. These two cases will be discussed in Sect. 4.2 .
4.1 Permanent Orientation of the Main Axis
For the preparation of monodomains with respect to the main axis, two principal
strategies can be deployed (Fig. 8 ). The first approach takes advantage of the anisot-
ropy of the polymer chain conformation of the network strands and is based on
considerations discussed in Sect. 3.2 . Mechanical deformation of polydomain LCEs
leads to a globally anisotropic chain conformation and can induce a macroscopic
orientation of the LC phase structure above a certain threshold stress. By introducing a
globally anisotropic chain conformation a priori during synthesis, LSCEs are accessi-
ble. This can be realized by chemical crosslinking after or during orientation by means
of mechanical or viscous flow fields and will be discussed in detail in Sect. 4.1.1 .
Following the second route, the anisotropic physical properties of the LC phase
structure, such as the anisotropy of the diamagnetic or dielectric susceptibility, can
be utilized to orient the mesogenic units in a magnetic or electric field, respectively.
Macroscopic alignment can also be achieved by surface effects, a technique well
known from low molar weight LCs. As the mesogenic units are covalently linked to
the polymer chains, the orientation of the LC phase structure leads directly to the
formation of a globally anisotropic chain conformation. Both are fixed permanently
by chemical crosslinking in the aligned state, which will be discussed in Sect. 4.1.2 .
If the first route is chosen, large-area elastomers can be produced with a wide
variety of film thicknesses. As precursors, only polymeric materials, for example,
photo-crosslinkable polymers or lightly crosslinked polymer gels, can be used.
The second route is usually limited by the available field strength to thin film
geometries. Typically photo-crosslinkable LC polymers or monomer/crosslinker
mixtures (usually containing additional photo-initiators and/or inert LCs as
solvents) can serve as starting materials. The latter offer the advantage that molding
techniques can be applied and elastomeric materials of more complex geometries
can be realized.