Order and Disorder in Liquid-Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 198

observed in these materials. Obviously this requires oriented monodomain samples,
which will now be discussed in some detail.

3.3 Monodomain “Single Crystal” Nematic Elastomers

Without special precautions, nematic elastomers form nonuniform polydomain
textures during fabrication. As a result, such a sample is opaque due to strong
light scattering by disoriented domains. Over the years many attempts, stimulated
by the analogy with the conventional liquid crystals, have been made to align
polydomain nematic elastomers with magnetic or electric fields. These attempts
proved to be unsuccessful, leading to the conclusion that the fields are too weak to
cause any significant re-alignment. Under these conditions, mechanical stretching
is the only remaining appropriate external field. Alignment of a polydomain
elastomer by stretching is readily observed with the naked eye: after a certain
degree of extension the initially opaque sample becomes clear and fully transparent
[ 52 ]. The threshold stress, s c , is small, of the order of 10 4 N/m 2 . The optical
transparency of monodomain elastomer samples is rather perfect, in contrast to
aligned samples of low-molecular-mass nematics that are still turbid due to thermal
director fluctuations. However, for elastomers, n is anchored to the rubbery matrix
and the director fluctuations are suppressed. This observation gives a hint as to why
application of electric or magnetic fields is insufficient to orient nematic elastomers.
The field acts on the highly polarizable mesogenic cores and its influence is
amplified by the cooperative nature of the long-range orientational order. However,
in elastomers the nematic cooperative factor is limited by the net size (4-5 nm) and,
compared to low-molecular-mass nematics, much larger electric (magnetic) fields
are needed to align the director. In a typical rubber, the average distance between
crosslinks is small. Using the characteristic value of the rubber modulus m ¼ n s k B T

10 5 N/m 2 at room temperature ( k B T 10 21 J), the average separation is 4

n s 1/3

4 nm. Mechanical fields act directly on the polymer network as a whole,
and thus the reorientation of the mesogenic cores linked to the backbones is much
easier than by electric or magnetic fields.
A small mechanical strain, e ¼ l 10%, acting directly on the polymer

backbones, is enough to align the mesogenic cores. These then can be crosslinked to
create a highly ordered elastomer monodomain. A very successful procedure along
these lines is the two-step crosslinking process by Kupfer and Finkelmann [ 71 , 72 ] ,
who developed an important yet simple technique for making so-called liquid
single crystal elastomers (LSCE). Chains are first lightly crosslinked in the isotropic
swollen state. These are then stretched in a uniaxial fashion and the solvent
is slowly removed while a second crosslinking proceeds in the aligned nematic
state. After this reaction is complete, the stress is removed and the system becomes
a clear monodomain. Its stability is remarkable, even after heating to the isotropic
phase and cooling back down to the nematic state. Hence, the overall director
orientation is “imprinted” by the second crosslinking step, which provides the

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