Order and Disorder in Liquid-Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 187

by an (often flexible) spacer. Main-chain LC polymers are built up by combining
rod-like mesogenic fragments and flexible moieties in alternating succession. In a
somewhat more modern terminology, one can divide each case into “end-on” and
“side-on” LC polymers, which differ in the way the rod-like mesogenic fragment
is attached to the spacer. The properties of the nematic phase formed by these two
types of polymer appear to be very different. In Sect. 3, these results are extended
to the properties and anisotropic shapes of nematic elastomers.
Monomer and polymer smectic LC phases are discussed in Sects. 4 and 5 .
Smectic systems consist of stacks of liquid layers in which thermally excited
fluctuations cause the mean-squared layer displacements to diverge logarithmically
with the system size (Landau-Peierls instability). As a result, the positional corre-
lations decay algebraically as r ( being small and positive) and the discrete
Bragg peaks change into singular diffuse scattering with an asymptotic power-law
form (see Sect. 4.1 ). In Sect. 4.2, some background information is summarized
about random fields, the presence of which can lead to disorder. For smectic
elastomers, the layers cannot move easily across the crosslinking points where
the polymer backbone is attached. Consequently, layer displacement fluctuations
are suppressed, which under certain assumptions has been predicted to effectively
stabilize the one-dimensional (1D) periodic layer structure. On the other hand,
crosslinks provide a random network of defects that could destroy the smectic
order. Thus, in smectic elastomers two opposing tendencies exist: suppression of
layer displacement fluctuations that enhances the translational order, and random
quenched disorder that leads to a highly frustrated equilibrium state. These two
aspects are discussed in Sect. 4.3 . The signature of (dis)order is found in the
lineshape of the X-ray peaks corresponding to the smectic layer structure. For
experimental aspects of the high-resolution X-ray methods involved we refer to
Obraztsov et al. [ 7 ]. The experimental situation regarding order/disorder due to
crosslinking smectic elastomers is reviewed in Sect. 5.1 for end-on side-chain
smectic polymers and includes a discussion of the nematic-smectic transition. In
Sect. 5.2, the discussion is extended to main-chain smectic elastomers and to a par-
ticular side-on side-chain system in Sect. 5.4 . Finally, in Sect. 6 some conclusions
and an outlook are given.
2 Liquid-Crystalline Polymers
2.1 Conformation and Structure
When LC fragments are covalently linked to a polymer chain, the material acquires
the properties of a mesogenic polymer. Such polymeric liquid crystals have an
intrinsic conflict between the drive of the backbone to adopt a random coil confor-
mation and the tendency to LC order associated with the mesogenic units. Flexibil-
ity of the backbone chain as well as of the connecting spacer is essential to give the