Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 6

network and may be considered as a local defect within the LC phase structure of
the network. This topology is not only determined by the chemical constitution of
the multifunctional molecule, e.g., rod-like or flexible, but it is also affected by the
local order imprinted during the crosslinking procedure. A network synthesized in
the liquid crystalline state differs from a network with the same chemistry that was
synthesized within the isotropic state. To minimize the effect of crosslinking
molecules, their concentration has to be minimized. The influence of

F

can be
shown using an elementary molecular theory of amorphous polymeric networks
introduced by Flory in 1953. This ideal network theory does not consider dangling
chain ends (i.e., a network chain that is connected to a junction of the network at
only one end) or loops (i.e., a network chain, both ends of which are attached to the
same polymer chain) and assumes that all junctions of the network have a function-
ality of

2. Such an ideal network can of course not be achieved in reality. In the
ideal network the number of network chains is denoted as v and the number of
junctions as

F >

. The formation of a perfect network can then be imagined as a process of
end-linking of network chains with a crosslink. Herby, the number of chain-ends
2

m

. Fewer
junctions are needed when the functionality of the crosslink is higher [ 11 , 12 ]. A
higher functionality thus leads to fewer local defects in LC elastomers.
In the following sections some selected examples of the chemistry of LC
networks will be summarized. While the synthesis of LC side chain elastomers
mainly follows the radical polymerization technique and the polymer analogous
addition reaction, LC main chain elastomers are exclusively synthesized by
polycondensation or polyaddition reactions.

n has to be equal to the number of functional groups Fm ,sothat m ¼ 2 n=f

2.1 Side Chain Elastomers

In side chain elastomers, the mesogenic moieties, which can be rods, discs, or
amphiphiles, are attached as side-groups to a polymer main chain via a flexible,
aliphatic spacer. The existence of this spacer is crucial for the formation of LC
phases as it lowers the tendency for crystallization and allows for a sufficient
decoupling of the mesogenic units from the polymer backbone. This partial
decoupling is necessary to allow the polymer chains to gain some entropy while
the mesogenic side chains can exhibit orientational (and positional) long-range
order of the LC phase. Concepts to change the LC phase behavior by modifying the
chemical constitution are well known from linear polymers and crosslinking usu-
ally does not change the phase behavior dramatically. Rod-like mesogenic units are
typically composed of two aromatic rings which are linearly connected via ester or
ether bonds. For short aliphatic spacers and short tails of the mesogenic units,
nematic phases are observed. However, with increasing spacer or tail length the
stability of smectic phases increases. Mesogens based on three aromatic rings show
preferably nematic phases but suffer from high transition temperatures. For the
preparation of cholesteric elastomers part of the nematogenic side chains is