Order and Disorder in Liquid-Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 212

external field and then crosslinked, we can expect the crosslinks to be in registry
with the smectic layers and to stabilize the lamellar structure against layer displace-
ment fluctuations. This situation will facilitate the theoretical prediction [ 128 , 129 ]
that translational order can be enhanced and even become a truly long-range order.
If the crosslinking is first made in nematic or isotropic phase, then uniaxially
alignment is accomplished to form a monodomain nematic elastomer, and only
after that cooled down to the smectic phase, the result will be opposite. Though the
sample will preserve uniaxial alignment, the layer positions will be frustrated due to
random crosslink positioning. In this case, crosslinks provide a random network of
defects that could destroy the smectic order [ 130 - 132 ] . The final thermodynamic
state of the sample will depend on the relative impact of crosslinking at the first
stage and at the final stage when the network is fixed.
Earlier experiments by Wong et al. [ 138 ] used a polyacrylate-based side-chain
smectic elastomer samples with about 5 mol% crosslinks. In this case, the elastomer
sample was prepared via reaction with a crosslinking agent in toluene. Alignment
was achieved in situ by stretching by 25% the freely suspended sample in the
nematic phase and subsequently cooling into an aligned smectic phase. This situa-
tion differs strongly from the method described in the previous paragraph.
There are several other possible ways to prepare well-aligned smectic elastomer
samples crosslinked directly in the smectic phase. Low-molecular-weight meso-
gens are easily aligned by surface forces. Driven by the tendency to minimize the
surface energy, smectic membranes (freely suspended smectic films) with a perfect
homeotropic alignment are easily formed [ 134 ]. Smectic polymer materials are
much more viscous than their low-molecular-weight counterparts. Still uniform
smectic membranes can be made close to the clearing temperature to the isotropic
phase or even above it. After cooling down into the smectic phase, the films can be
crosslinked by UV irradiation. Such methods have been used to produce planar
films [ 139 - 144 ] and even curved elastomer films in the shape of inflated balloons
[ 145 , 146 ] . However, no high-resolution X-ray work has been performed with these
types of sample.
The elastic properties of monodomain smectic elastomers are different from
those of nematic elastomers [ 147 ] . The stress-strain diagram shows a considerable
anisotropy of the elastic moduli. Stretching along the layer normal is associated
with a large modulus of ~10 7 N/m 2 , comparable to the smectic compressional
modulus B in low-molecular-mass and polymer smectics. This value is about two
orders larger than the modulus in the plane of the layers, which is comparable to the
shear modulus m ~10 5 N/m 2 characteristic of the isotropic state. These observa-
tions indicate that the crosslinks are strongly pinned by smectic layers. As a result,
when stretching along the layer normal the crosslinks cannot glide through the
layers. The associated modulus is therefore associated with deformation energy of
the smectic layers and is not rubbery. The physical reason for the large anisotropy in
smectic networks is clear: stretching along the layer normal attempts to change the
layer spacing, which is resisted by the smectic ordering. The mechanical field acts
on the mesogenic in the smectic layers rather than on the crosslinks responsible
for rubber elasticity. Such high elastic anisotropy is unprecedented even in strongly