Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 21

Fig. 5 Local polymer chain conformation of nematic side chain elastomers (with respect to the
nematic director n ) and resulting global chain conformation in the cholesteric phase structure (with
respect to the cholesteric helix axis h )
with respect to the local director. In the helicoidal phase structure of chiral nematic
elastomers, however, this orientation corresponds to an overall oblate chain confor-
mation with respect to the helix axis as the local director rotates continuously along
this axis (Fig. 5 ). Therefore a globally oblate chain conformation has to be
established in order to achieve a cholesteric monodomain. Using biaxial stretching
or uniaxial compression, elastomers with homeotropically aligned helix axes can be
obtained [ 72 ]. In the case of a chiral nematic polymer with even spacer length and
thus locally oblate chain conformation with respect to the local director, a prolate
chain conformation exists with respect to the helix axis. Uniaxial mechanical
stretching induces a macroscopic orientation of the helix axis along the stretching
direction in the film plane [ 73 ] .
Smectic side-chain polymers prefer locally oblate chain conformations, indepen-
dent of the spacer length or attachment geometry. Analogous to oblate nematic
polydomain elastomers, biaxial mechanical stretching or uniaxial compression can
be used to orient S A polydomain elastomers. This achieves a simultaneous orientation
of the director and the smectic layer normal in a uniform homeotropic fashion [ 74 ] .
Upon uniaxial stretching a polydomain elastomer in the smectic-A phase,
the layer planes usually couple to the mechanical field. This process does not