Side Chain 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. 

In the following we will outUne two basic methods to synthesize LC side chain elastomers. As a starting point for the synthesis of LC elastomers a mixture of mesogenic monomers and bi- or multi-functional crossUnker molecules may serve. This will be discussed in the first part of the section. Alternatively, polymer analogous reactions, where the mesogenic moieties are attached to a polymer backbone, can be employed, which will be discussed in the second part of the section. 

If LC monomers are used as starting materials, it is very important to crmsider that monomer and polymer networks can differ in their phase behavior as previously mentioned. This is a particular issue for nematic elastomers. Only very few examples are known in which the temperature regime of a nematic phase of a monomer overlaps with the nematic temperature regime of the polymer. The systematic that was hereby obtained for the LC phase behavior of linear polymers also holds for LC polymer networks because for elastomers the cmicentration of the mesogens is much higher than that of the crosslink. The chemistry that can be used for the crosslink is determined by the chemistry of the polymerization technique. 

Another method is based on the synthesis of functional linear prepolymers. This can be achieved by copolymerization of the mesogenic monomer with a functional co-monomer, e.g., co-monomers containing a reactive hydroxy group. In a seccnid reaction step, these functional linear polymers can be crosslinked, e.g., with a diisocyanate crosslinker [15]. 

Functionalized linear polymers were S3mthesized by Schuring et al. [18]. With linear poly(siloxanes) obtained by a hydrosilylation reaction of a 

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The application of the previously discussed techniques to induce monodomain structures in side-chain liquid-crystalline polymers by the application of electric or electromagnetic fields, by shearing or on anisotropic surfaces, frequently leads to comparatively low, macroscopically uniform orientation. Additionally, the methods are limited to a sample thickness of about 100 pm. Liquid-crystalline side-chain elastomers do not have this restriction, because a high macroscopic orientation can be induced in polymeric networks by mechanical deformation up to a sample thickness of about a centimeter [103, 109]. The synthesis of such systems can be performed by crosslinking linear, side-chain liquid-crystalline polymers to networks [llOj. The inherent combination of rubber elasticity and liquid-crystalline phase behavior, may then be exploited for the induction of a macroscopic mesogen orientation by mechanical deformation. 

To obtain the liquid crystalline state in a polymer network, several strategies are conceivable. They are all based on well known principles evaluated during the last few decades for linear liquid crystalline polymers. The monomer units of the network have to consist of mesogenic moieties, which are either rigid rods or discs in the case of thermotropic polymorphism or amphiphiles in the case of lyotropic polymorphism. The mesogenic units can be attached either as side chains to the monomer units yielding side chain elastomers (Fig. la, b) or directly linked... 

Fig. 1 Different attachmcait geometries for the synthesis of LCEs side chain elastomers with end-on (a) or side-on (b) attached mesogenic side chains and main chain elastomers with mesogenic units incorporated end-on (c) or side-on (d) into the polymer main chain...

An example for the synthesis of a side chain elastomer using radical polymerization of acrylates is presented in Scheme 1 and was demonstrated by Thomsen and co-workers [14]. Here, the mesogenic groups are attached side-on to the polymer-backbone. This attachment geometry is useful for a number of applications, because... 

Compared to side chain elastomers the preparation of main chain elastomers with suitable transition temperatures is rather challenging. What is more, due to the rigid rod-like mesogenic moieties in the polymer main chain, most linear main chain liquid crystalline polymers exhibit high clearing temperatures and tend to crystallize [23], making them unfavorable for many LC elastomer applications. 

Scheme 5 Combined main chain side chain elastomer synthesized by Wennter et al. [29]...

It is also possible to use the vinyl-terminated pre-polymer to crosslink a side chain LC polymer. This was demonstrated by Wermter et al. [29] who crosslinked a side chain end-on polymer similar to that shown in Scheme 3 with a main chain polymer carrying vinyl end-groups (Scheme 5). The side chain component can be understood as a multifunctiOTial crosslinker for the main chain elastomer and also acts as a plasticizer, further decreasing the transition temperature to about 90 C. Such combined main chain/side chain elastomers show, if oriented to a permanent monodomain (Sect. 4) extremely high length changes at the phase transformation to... 

Fig. 5 Local polymer chain conformation of nematic side chain elastomers (with respect to the nematic director n) and resulting global chain conformation in the cholesteric phase structure (with respect to the cholesteric helix axis h)...

In some cases it is also possible to induce a global chain conformation of the LSCE opposite to the local chain conformation of the polymeric starting material which will be discussed for cholesteric and smectic-A side chain elastomers. 

Once the reaction is successful, the elastomer strip is loosened from the centrifuge wall with the non-cutting side of a thin lancet. The elastomer film is carefully removed from the cell together with the Teflon support and cut into smaller (usually two or three) pieces. For cutting the swollen polymer gel the best method is to use a carpet knife and a hammer. That way, a clean cut of both the gel and the Teflon band is achieved. It is useful, especially if working with sticky side chain elastomers, to keep some toluene at the preparation table to clean the tools before reuse. If possible, both long rims of the film should also be cut with a carpet knife, because small defects at the rim can... 

In contrast to classical side chain elastomers, smectic-A main chain elastomers exhibit prolate chain conformations. Consequently, macroscopically oriented samples can be prepared according to the method of Kiipfer et al. utilizing a second crosslinking step under uniaxial deformation [31]. Analogous, Sa LSCEs based on side chain elastomers with side-on attached mesogenic units can be prepared [97]. 

Fig. 20 (a) Molecular structure of the smectic side-chain elastomer with 96% side-on group Rl and 4% crosslinker R2. (b) 2D X-ray picture of the elastomer with the smectic layer peaks visible axially [135]... 

The basic chemistry of liquid crystal elastomers is very similar to the chemistry of linear liquid crystal polymers. One can distinguish between main- and side-chain elastomers where the mesogenic units are either segments or side groups of the monomer imits of the macromolecular chain. The elastomers only differ from linear liquid crystal polymers by conventional crosslinking reactions. New aspects for chemistry appear when liquid crystal networks are required that spontaneously, and without any external fields, exhibit a macroscopically uniform orientation of the director. For these conditions, networks have to be sythesized where the chain conformation is consistent a priori with the uniform liquid crystal order [5]. 

In this short review we have summarized our current knowledge of two key issues concerning the elasticity of uniaxial nematic side chain elastomers (NEs). The measurements taken on the dry and swollen NEs, show that there are two types of NEs. 

Stein P, ABfalg N, Einkelmann H, Martinoty P (2001) Shear modulus of polydomain, monodomain and non-mesomorphic side-chain elastomers influence of the nematic order. Eur Phys J E 4 255... 

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