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Order and Disorder in Liquid-Crystalline
Elastomers
Wim H. de Jeu and Boris I. Ostrovskii
Abstract Order and frustration play an important role in liquid-crystalline polymer
networks (elastomers). The first part of this review is concerned with elastomers in
the nematic state and starts with a discussion of nematic polymers, the properties of
which are strongly determined by the anisotropy of the polymer backbone. Neutron
scattering and X-ray measurements provide the basis for a description of their
conformation and chain anisotropy. In nematic elastomers, the macroscopic shape
is determined by the anisotropy of the polymer backbone in combination with the
elastic response of elastomer network. The second part of the review concentrates
on smectic liquid-crystalline systems that show quasi-long-range order of the
smectic layers (positional correlations that decay algebraically). In smectic elasto-
mers, the smectic layers cannot move easily across the crosslinking points where
the polymer backbone is attached. Consequently, layer displacement fluctuations
are suppressed, which effectively stabilizes the one-dimensional periodic layer
structure and under certain circumstances can reinstate true long-range order. On
the other hand, the crosslinks provide a random network of defects that could
destroy the smectic order. Thus, in smectic elastomers there exist two opposing
tendencies: the suppression of layer displacement fluctuations that enhances trans-
lational order, and the effect of random disorder that leads to a highly frustrated
equilibrium state. These effects can be investigated with high-resolution X-ray
diffraction and are discussed in some detail for smectic elastomers of different
topology.
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