A New Type of Macroscopically Oriented Smectic‐A Liquid Crystal Elastomer

Abstract

Smectic‐A liquid single crystal elastomers (SA‐LSCE) are materials where the rubber elasticity of the polymer network is combined with the one‐dimensional positional long‐range order of mesogenic groups which are covalently attached to the network. In the systems investigated so far, a mechanical deformation of the network causes significant reorientation processes of the layered structure. We present a new type of SA‐LSCE in which this structure remains unaffected on mechanical deformation both parallel and perpendicular to the director. The thermoelastic behavior, macroscopic dimensions, and stress–strain measurements parallel as well as perpendicular to the director are investigated. X‐ray studies confirm that a deformation parallel and perpendicular to the layer normal does not alter the macroscopically ordered lamellar structure with respect to the order parameter and the layer correlation length. We propose a simple picture where defects within the lamellar structure might account for these findings.magnified image

Monodomain Liquid Crystal Main Chain Elastomers by Photocrosslinking

“…[7] Recently Komp and Finkelmann reported a new type of smectic-A LCE, in which mesogens with perfluorinated alkyl chains at both ends were attached side-on to a poly(methylsiloxane) backbone. [34] In their systems, they found an initial longitudinal modulus of E ¼ 14 MPa and a significant softening at a threshold strain of e ¼ 5%. For this elastomer the smectic structure (layer spacing, director orientation) also remained unaffected by longitudinal deformation, again in contrast to Nishikawa's networks.…”

Numerical study of stretched smectic-Aelastomer sheets

“…Other work on Sm-A elastomers has used very different sample aspect ratios, such as those used by Komp and Finkelmann [7] (2 cm × 4 mm × 100 μm). The finite element results shown in Fig.…”

“…More detailed x-ray studies of similar side-chain systems reveal that the layers behave as if they are embedded in the rubber matrix [6]. Later experiments on side-chain systems with different chemistry have shown the same threshold behavior, but the samples remain transparent throughout the deformation [7]. The behavior at the threshold has been shown to depend on the type of smectic ordering present [8] and on the degree of cross-linking in the elastomer [9].…”

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

“…[3] Analogous to this procedure, macroscopically oriented S A main-chain elastomers and side-chain elastomers with side-on attached mesogens were synthesised. [4,5] In contrast, classical smectic side-chain polymers with end-on attached mesogenic units prefer a locally oblate chain conformation, where the polymer chains are mainly located between the smectic layers. [6,7] A uniaxial mechan- Smectic-A elastomers combine one-dimensional translational order of rod-like segments with the rubber elasticity of a polymer network.…”

“…Analogous observations were reported for S A LSCEs with planar orientation and global prolate chain conformation. [4,5,8,21] To determine the elastic moduli of elastomer E1, the stress was measured in dependence of the applied strain at T ¼ 40 8C far away from the glass transition and the smecticisotropic phase transformation (Figure 2b). Perpendicular to the layer normal, a linear stress-strain behaviour is found with a modulus of E x ¼ 5.2 Â 10 5 N Á m À2 which is significantly larger compared to the modulus in the isotropic state…”