Liquid crystalline elastomeric systems

Incorporation of flexible siloxane spacers into side chain or main chain liquid crystalline polymers have been shown to drastically reduce the transition temperatures 255,267,271,272,277) anc[ aiso increase the response time of the resultant systems to the applied thermal, optical or electrical fields 350-353>. In addition, siloxanes also provided elastomeric properties and improved the processibility (solution or melt) of the resulting liquid crystalline copolymers. 

This mechanical differential behaviour can be easily explained by the fact that, in main-chain LSCEs, all the azobenzene molecules are part of the main polymer backbone. In this way, the generation of the bent cis isomer of the azo chromophore in the elastomeric network uptm suitable illumination not only decreases the liquid-crystalline order at the molecular level but also affects dramatically the conformation of the main polymer backbone. On the other hand, in side-chain LSCEs, the different monomers are connected to the main polymer backbone through flexible spacers, which decreases significantly the coupling between the geometrical change suffered by the azo dye upmi isomerisation and the main polymer matrix. Therefore, in side-chain LSCEs, the photo-mechanical effect is mainly due to the alteration of the ordering of the system. As a result, the photo-mechanical effect becomes less notable in side-chain LSCEs. 

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