Main Chain Elastomers

The chemistry of main chain elastomers is limited to step-growth reactions, i.e., polycondensation and polyaddition reactions, which demand the highest purity of the starting materials and experimental conditions which exclude side reactions. 

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. 

Despite their more complex chemistry, main chain LC elastomers have gained much interest in recent years due to the direct coupling of the liquid crystalline order and the polymer backbone conformation. The ground breaking predications of de Gennes were also based on main chain elastomers [3]. 

Crosslinking of terminally functionalized LC polymers with a suitable multifunctional crosslinker was introduced by Zentel et al. [26]. They crosslinked allyl side-groups of a liquid crystalhne polyester with an oligo-siloxane crosslinker, yielding elastomers with Sa and Sb phase behavior and rather high transition temperatures. 

One of the difhculties of this method is that both the content of end-groups of the polymer and the amount of volatile crosslinker is hard to measure and weigh, respectively, so that it is extremely difficult to keep to the exact stoichiometry. This often results in high soluble contents of the elastomers and imperfect comparability of the samples. 

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Bergmann, G. H. E. Einkelmann, H. Percec, V. Zhao, M., Liquid-Crystalline Main-Chain Elastomers. Makromol. Rapid Commun. 1997,18, 353-360. 

The X-ray pattern taken at room temperature from a stretched sample is reported in Figure 11. A broadened wide angle reflection at the equator is evident. The smectic layer reflections can be observed at the meridian, indicating a perpendicular orientation of the smectic layers to the stress direction. The orientation is therefore dominated by the polymer chains, as it was reported also in the case of highly stretched fibers of a smectic main chain elastomer (35). 

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...

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. 

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... 

Scheme 8 Components for the synthesis of main chain elastomers (MCLCEs) in a solvent-free one-pot reaction employing a photo-initialized thio-ene polycondensation [39]...

Ishige and co-workers [36-38] produced Sca main chain elastomers with transition temperatures of about 180 °C also utilizing a one-pot method, by a melt trans-... 

This robust synthetic approach has frequently been used to produce LSCEs in the last few years as it works well for side chain as well as main chain elastomers. It is also applicable for polymer networks of different LC phase structures (smectic, cholesteric, lyotropic hexagonal) as long as they exhibit prolate chain conformations. We will therefore give a more detailed description in the following paragraphs. 

For main chain elastomers, a proper preparatirai of the reaction mixture is cmcial as volatile reactants are often involved and the exact stoichiometry has to... 

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]. 

Conventional principles and methods concern the synthetic routes for macromo-lecular networks and the realization of the liquid crystalline state by mesogenic monomer tmits. Network chemistry has to consider the reactivity and functionality of the monomer tuiits. In most cases, this excludes ionic polymerization techniques and reduces utihzable methods to radical polymerization and polymer analog reactions for side chain networks, and to polycondensation or polyadditirai reactions for main chain elastomers. The chemistry of the crosslinking process and the chemical constitution of the crosslinker have to be adapted to the polymerization process. Applying photo-chemistry of suitable functional mmiomer units opens an additional, versatile pathway to build up the network structure. 

Beyer P, Terentjev EM, Zentel R (2007) Monodomain liquid crystal main chain elastomers by photocrosslinking. Macromol Rapid Commun 28(14) 1485-1490. doi 10.1002/ marc.200700210... 

Heinze P, Finkehnann H (2010) Shear deformation and ferroelectricity in chiral SmC main-chain elastomers. Macromolecules 43(16) 6655-6665. doi 10.1021/mal002084... 

Krause S, Zander F, Bergmann G, Brandt H, Wertmer H, Finkelmann H (2009) Nematic main-chain elastomers coupling and orientational behavior. C R Chim 12 85-104... 

Fig. 7 Orientational order parameter of a nematic main-chain elastomer from D-NMR around the N-I transition for different crosslink concentrations x [90]...

As mentioned earlier, for end-on main-chain smectic polymers the polymer chains connect the smectic layers. As a result, polymer defects are expected to be directly translated into layer distortions (Fig. 4b-d). This probably offsets any possible influence of damping of the layer fluctuations (potentially leading to increased order) because of the connectivity of the layer structure via the chains. As mentioned already in Sect. 2.3, main-chain polymers and elastomers have little tendency to form a smectic phase. They have been less thoroughly investigated than their side-chain counterparts. X-ray structural information of several main-chain elastomers with about 10% of approximately the same cyclic multifunctional crosslink have been compared with their homopolymer counterparts by De Jeu et al. [155]. As no results are available for other crosslink concentrations, little can be said about the specific contribution of the crosslinks to disorder. 

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