Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 20

along the stretching direction, locally a prolate chain exists. In the opposite case the
mesogenic units and the stretching direction are aligned perpendicular to each
other, indicating locally an oblate chain conformation. In order to obtain conclusive
information about the local orientation of mesogenic units with respect to the
stretching direction, equilibrium conditions have to be ensured. Sometimes this
can be problematic, as for high drawing speeds and high cooling rates non-
equilibrium orientations or non-uniform orientations within the fiber cross-section
may be frozen-in.
3.2 Orientation of Polydomain Networks
As a direct consequence of the interaction between LC order and polymer chain
conformation, the global state of order in polydomain LCEs can be manipulated by
mechanical stretching. It is well known from conventional rubbers that mechanical
deformation induces changes in the macroscopic chain conformation of the network
strands. Uniaxial elongation causes the formation of prolate chain conformations
while biaxial stretching, which is equivalent to uniaxial compression, establishes
global oblate chain conformations. This has important consequences for LC
elastomers. By changing the macroscopic chain conformation so that it is consistent
with the phase symmetry of the LC state, macroscopic alignment can be induced.
Uniaxial stretching a polydomain nematic elastomer with locally prolate chain
conformation, e.g., a main chain polymer, a side chain side-on polymer or a side
chain end-on polymer with an odd number of spacer atoms, produces a transparent
polymer network with uniform orientation of the nematic director along the stress
axis above a characteristic strain. We shall refer to this alignment of the director in
the film plane as homogeneous orientation. The formation of a monodomain is
shown for a nematic side chain elastomer in Fig. 2 . Above the characteristic
threshold strain, the directors of the individual domains rotate towards the mechan-
ical stress axis. The resulting macroscopic orientation of the director can be easily
proven by X-ray scattering or IR dichroism. This process is completely reversible.
When the mechanical stress is released, the sample relaxes back into the poly-
domain equilibrium state.
The concept of mechanical field induced orientation can easily be transferred to
nematic elastomers with oblate chain conformation, i.e., side chain end-on
elastomers with an even number of spacer atoms. In order to achieve a monodomain
structure, a globally oblate chain conformation has to be established. This can be
achieved by uniaxial compression or biaxial stretching of the polydomain elastomer
which induces a uniform homeotropic alignment of the nematic director perpendic-
ular to the film plane. Up to now, this orientation technique has only been realized
experimentally for chiral nematic elastomers [ 72 ].
Mechanical fields can also be deployed to achieve a macroscopic orientation of
the helicoidal z -axis of cholesteric elastomers. As discussed above, nematic side
chain polymers with odd spacer length exhibit a locally prolate chain conformation