Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 5

with a polymerization process of a new mesogenic monomer. Here all the factors
mentioned above have to be considered and the knowledge of the liquid crystalline
phase behavior of the corresponding linear polymer has to be elucidated. The
consolidated findings of intensive research on linear LC polymers will help to
evaluate a chemical constitution of a new monomer that successfully yields an
LCE. The second strategy avoids all these problems by already starting with a linear
LC polymer with known LC phase behavior. A crosslinking process - either via
suitable functionalization of the linear polymer or by g -irradiation - hardly modifies
the LC phase behavior.
For the synthesis as well as for the usability of the networks the transition
temperatures are most important. They determine the regime of use and hence the
functionality of the LC structure. These are the liquid crystalline to isotropic phase
transformation temperature T LC,i and the glass transition temperature T g , where the
material transfers from the glassy state into the LC state. A transformation from the
LC into the crystalline phase at T c,LC is mainly suppressed due to the complex
structure of LCEs and should be avoided as it might destroy the network.
T LC,i should be above the temperature-regime of interest in cases where the
physical properties of the elastomers within the LC phase, such as ferroelectricity or
optical properties, are to be used. If changes in the physical properties are to be
exploited, such as length changes at the phase transition, T LC,i must be adjusted to
the desired transition temperature. For polymers with discotic or calamitic
mesogenic groups, the phase transition temperatures are determined by the struc-
ture of the mesogenic monomer and of the main chain. The principles of their
manipulation are well known from work on linear LC polymers. Additionally T LC,i
can be systematically modified by copolymerization with different mesogenic or
non-mesogenic co-monomers.
T g of LC elastomers is determined by the flexibility of the main chain, by
interactions between the mesogens and the main chain, as well as by interactions
between the mesogens. In the case of elastomers, the crosslinking density also plays
an important role for the glass transition. This is rather different depending on the
flexibility of the crosslinker used. If a flexible crosslinker is used, T g decreases with
respect to the linear polymer for low crosslinking densities. Here the crosslinker
acts as softening agent. If the crosslinking density is further increased, T g rises
due to increasing immobilization of the network strands. For a rigid, mesogenic
crosslinker, T g increases continuously with the concentration of the crosslinker.
A high crosslinking density offers the chance to freeze-in the LC structures into
anisotropic glasses, yielding duromers with highly interesting properties. Those
materials have been investigated, for example, in the work of Broer (for a review,
see [ 10 ] ). For systems where dynamic processes such as electromechanical behav-
ior or photo-mechanics are of interest, a T g below 20 C is favorable. Strategies to
obtain such low T g will be described below.
Another very important aspect for the synthesis of LC elastomers is the func-
tionality determines the number of chains that meet at a

junction. The local topology of the crosslink determines the properties of the

F of the crosslink. F