Lyotropic elastomers

For the macroscopic orientation of lyotropic elastomers in mechanical fields the same principles as for thermotropic elastomers can be applied. In some ways the situation is even simpler as the rod-like micelles of the hexagonal phase are compatible with an overall prolate network chain conformation. In addition, the lamellar phase built up from disk-like micelles requires an overall oblate chain conformation to form a liquid single-crystal hydrogel (LSCH) [98]. 

Locally oblate lyotropic elastomers with lamellar phase structure (L -phase) can be oriented by uniaxial compression, as outlined above for thermotropic smectic-A elastomers. Fischer et al. synthesized crosslinked polysiloxane elastomers carrying non-ionic amphiphilic side-groups attached with their hydrophobic end to the polymer backbone. They were able to compress elastomer samples between Teflon half-cylinders to about half of their original thickness. The orientation of the phase structure - except for some unoriented domains - was demonstrated by means of H-NMR spectroscopy on the directly deuterated samples as well as by X-ray scattering. The preferred orientation of the director, and hence the amphiphilic side chains, was found to be parallel to the axis of compression with the amphiphilic bilayers aligned perpendicularly [98, 99]. 

In the phrase liquid-crystalline, the crystalline adjective refers to the faa that these materials are sufSdentiy ordered to diffract an X-ray beam in a way analogous to that of normal crystalline materials. On the other hand, the liquid part specifies that there is frequently sufSdent disorder for the material to flow like a liquid. liquid crystals can be divided into thermotropic, that exhibit a phase transition with change of temperature, and lyotropic, that exhibit phase transition as a function of both temperature and concentration of the LC molecules in a solvent. Both low molecular wdght materials and polymers " can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. LC networks of flexible chains have the following three properties (1) they can be extensively deformed (as described for elastomers throughout this book), (2) the deformation produces alignment of the chains, and (3) alignment of the chains is central to the formation of LC phases. Elastomers of this type have been the subject of numerous studies, as described in several detailed reviews. ... 

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... 

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. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

Fleischmann EK, Forst FR, Zentel R (2014) Liquid-crystalline elastomer fibers prepared in a microfluidic device. Macromol Chem Phys 215 1004-1011 Flory PJ (1956) Phase equilibria in solutions of rod-like particles. Proc R Soc Lond A 234 73-89 Forney BS, Guymon CA (2011) Fast deswelling kinetics of nanostructured poly (N-isopropylacrylamide) photopolymerized in a lyotropic liquid crystal template. Macromol Rapid Commun 32 765-769... 

---