Orientability of the minor director of homeotropically aligned smectic-a elastomers in external mechanical fields.

We present studies on bulk smectic-A copolymer networks with end-on attached homeotropically oriented mesogens that show spontaneous optical biaxiality at room temperature. Orthoscopic and conoscopic investigations under uniaxial extension in the layer planes give first evidence of the orientability of the minor director in mechanical fields yielding biaxial monodomains with 3-d orientational long-range order of all three principle axes. This is an important step towards the synthesis of permanently oriented biaxial monodomain elastomers for which highly interesting mechanical and optical properties are expected.

Mechanics of Homeotropically Ordered Smectic-A Elastomers with Global Oblate Chain Conformation.

Smectic-A elastomers combine one-dimensional translational order of rod-like segments with the rubber elasticity of a polymer network. In recent years, detailed investigations were carried out on elastomers showing a global prolate chain conformation. In this communication, the first experiments on fluorinated SA elastomers exhibiting a global oblate chain conformation are presented, where the polymer chains are on average compressed along the layer normal of the lamellar phase structure. The mechanical anisotropy is studied by means of thermoelastic experiments and stress-strain measurements. For the first time, the layer compression modulus B of smectic elastomers is directly measured. B is significantly larger as compared to conventional low molar mass liquid crystals and decreases significantly with increasing local disorder introduced by the isotropic crosslinker.

Random disorder and the smectic-nematic transition in liquid-crystalline elastomers.

We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition. At an increased cross-link concentration of 10% the smectic order disappears and gives way to extended short-range layer correlations. In this situation neither a smectic-nematic nor a nematic-isotropic transition is observed anymore. The occurrence of disorder at a relatively large cross-link concentration only, indicates that smectic elastomers are rather resistant to a random field. The temperature dependence of the correlation lengths and thermoelastic behavior suggest a shift to a "parasmectic" regime of a first-order smectic-isotropic transition.