Chemistry Reference
In-Depth Information
5 Conclusions and Outlook
The preparation of LCEs comprises sophisticated chemistry that brings together the
broad areas of research on macromolecular networks and on liquid crystals. For a
successful synthesis it is not sufficient to combine only conventional principles and
methods of both fields. In addition specific new features emerge that have to be
considered.
Conventional principles and methods concern the synthetic routes for macromo-
lecular networks and the realization of the liquid crystalline state by mesogenic
monomer units. Network chemistry has to consider the reactivity and functionality
of the monomer units. In most cases, this excludes ionic polymerization techniques
and reduces utilizable methods to radical polymerization and polymer analog
reactions for side chain networks, and to polycondensation or polyaddition
reactions for main chain elastomers. The chemistry of the crosslinking process
and the chemical constitution of the crosslinker have to be adapted to the polymeri-
zation process. Applying photo-chemistry of suitable functional monomer units
opens an additional, versatile pathway to build up the network structure.
The liquid crystalline state can be systematically introduced by rigid anisometric
building blocks or by amphiphilic moieties. According to their chemical constitu-
tion and basically following the same systematic as known from low-molar-mass
liquid crystals, all types of LC phase structure are accessible. Additional aspects to
be considered arise from modification of the state of order due to the decreased
specific volume of the macromolecules (compared to that of the monomers) and the
reduced translational and rotational mobility of the mesogenic units due to their
linkage within the macromolecule.
New features to be taken into account for the synthesis of LCEs are (1) the
interaction between the LC order and the conformation of the polymer backbone,
and (2) the effect of the chemical constitution of the crosslinker on both the local
topology of the network and the LC order. The interaction between the LC order
and the conformation of the polymer backbone causes a deviation from a conven-
tional statistical spherical coil to an oblate or prolate chain conformation. Without
any precautions during the synthesis, LCEs are obtained in a polydomain structure
due to maximization of the entropy of the overall chain conformation. The crucial
interplay of LC order and conformation of the polymer backbone, however, offers
the chance to induce macroscopically an overall anisotropic chain conformation by
an external mechanical field. If the symmetry of the mechanical field is consistent
with the symmetry of the LC phase structure, a uniform orientation is produced.
Introducing an additional chemical crosslinking process, the macroscopic uniform
alignment becomes locked-in permanently and a liquid single-crystal elastomer
(LSCE) is realized. Alternatively, uniform alignment of LC monomers or linear
pre-polymers can be induced by electric or magnetic fields or surface effects.
Subsequently, a chemical crosslinking reaction performed in the aligned state
yields LSCEs. However, these conventional orientation techniques are limited to
very small sample thicknesses.
Search WWH ::
Custom Search