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Statistical models of elasticity in main chain and smectic liquid ...

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Chapter FourThe <strong>elasticity</strong> <strong>of</strong> <strong>smectic</strong>-AelastomersAmodel <strong>of</strong> <strong>smectic</strong> elastomers is developed <strong>in</strong> this chapter, start<strong>in</strong>gfrom a microscopic model that is used calculate the properties <strong>of</strong> an averagecha<strong>in</strong>, <strong>and</strong> then us<strong>in</strong>g the aff<strong>in</strong>e approximation to build up the properties<strong>of</strong> the whole elastomer.4.1 IntroductionSmectic <strong>liquid</strong> s<strong>in</strong>gle crystal elastomers share properties with both <strong>smectic</strong><strong>liquid</strong> crystals <strong>and</strong> nematic elastomers. In this <strong>in</strong>troduction some <strong>of</strong> the properties<strong>of</strong> <strong>smectic</strong> <strong>liquid</strong> crystals are discussed, aswell as methods used to describethem.InparticulartheHelfrich-Huraulteffect, observedexperimentally<strong>in</strong> <strong>smectic</strong> A (SmA) <strong>liquid</strong> crystals by Clark <strong>and</strong> Meyer [55], is exam<strong>in</strong>ed.Some <strong>of</strong> the experimentally measured properties <strong>of</strong> SmA elastomers are thenreviewed, <strong>and</strong> the terms used <strong>in</strong> the cont<strong>in</strong>uum model to describe <strong>smectic</strong>elastomers summarised.4.1.1 Smectic <strong>liquid</strong> crystal order parameterSmectic <strong>and</strong> nematic <strong>liquid</strong> crystals are both composed <strong>of</strong> rod like molecules.However <strong>in</strong> the case <strong>of</strong> <strong>smectic</strong> <strong>liquid</strong> crystals, a short range anisotropic attractive<strong>in</strong>teraction causes a modulation <strong>in</strong> the density <strong>of</strong> the rods <strong>in</strong> additionto the alignment <strong>of</strong> the rods along a particular direction. This modulation <strong>in</strong>the density dist<strong>in</strong>guishes the <strong>smectic</strong> phase from the nematic phase <strong>and</strong> canbe used to def<strong>in</strong>e an order parameter for the <strong>smectic</strong> phase. The density can81

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