Rigid liquid-crystalline networks

Rusakov 107 108) recently proposed a simple model of a nematic network in which the chains between crosslinks are approximated by persistent threads. Orientional intermolecular interactions are taken into account using the mean field approximation and the deformation behaviour of the network is described in terms of the Gaussian statistical theory of rubber elasticity. Making use of the methods of statistical physics, the stress-strain equations of the network with its macroscopic orientation are obtained. The theory predicts a number of effects which should accompany deformation of nematic networks such as the temperature-induced orientational phase transitions. The transition is affected by the intermolecular interaction, the rigidity of macromolecules and the degree of crosslinking of the network. The transition into the liquid crystalline state is accompanied by appearence of internal stresses at constant strain or spontaneous elongation at constant force. 

T. Ikeda, J. Mamiya and Y. Yu, Angew. Chem. Int. Ed, 2007, 46, 506. Cross-linked Liquid Crystalline Systems From Rigid Polymer Networks to Elastomers, ed. D. Broer, G. Crawford and S. Zumer, CRC Press, Boca Raton, FL, 2011. 

Liquid crystalline thermosets (LCT) with a rigid rod-like mesogenic group capped firom both ends by reactive fimctional groups can be used as matrices for production of advanced composites, coatings or adhesives (1, 2). hi this case ordering of the chains of the final network polymer is the only way to improve its physical and mechanical properties. 

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... 

Storz R, Komp A, Hofbnann A, Finkebnann H (2009) Macromol Rapid Commun 30 615 Crawford GP, Broer D, Zumer S (2011) Cross-Linked Liquid Crystalline Systems From Rigid Polymer Networks to Elastomers. CRC Press... 

There have been several mechanical studies performed on gelled networks of stiff LCPs, some where the liquid crystallinity of the precursors was retained in the gels [278,780] and others where stiff mesogenic segments were connected by rigid branchpoints, leading to loss of mesomorphicity [277, 311, 312, 393, 400, 409]. Thus far, the results are limited to reported stress-strain curves [278, 393] and to values of static shear modulus obtained by linear compression ex-... 

Broer D, Crawford GP, Zumer S (2011) Cross-linked liquid crystalline systems liom rigid polymer networks to elastomers (liquid crystals book series). CRC Press, Boca Raton Camacho Lopez M, Finkelmann H, Palffy-Muhoray P, Shelley M (2004) Fast liqnid-ciystal elastomer swims into the dark. Nat Mater 3 307-310 Cviklinski J, Tajbakhsh AR, Terentjev EM (2002) UV isomerisation in nematic elastomers as a route to photo-mechanical transducer. Eur Phys J E 9 427 34 de Gennes PG (1969) Possibilites offertes par la reticulation de polymeres en presence d nn eristal liquide. Phys Lett 28A 725-726... 

Carfagna C, Amendola E, Giamberini M (1994c) Rigid-rod networks—liquid-crystalline epoxyresins. Compos Stmct 27 37 3... 

It is significant that the reinforcement degree corresponds to a class of polymer forming a nanocomposites matrix. The largest values of / are obtained for polymers whose chains are able to stretch on the silicate platelet surface (rigid-chain polyimide, crystallising polypropylene and thermotropic liquid crystalline polyester), intermediate values for polymers whose chains are able to stretch only partly (polycarbonate, poly (butylenes terephthalate) and amorphous polyamide-6) and the smallest values for nanocomposites on the basis of epoxy polymer, the capability of chains stretching of which decreases sharply because of the availability of transverse covalent bonds network [30]. 

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