Free-standing Smectic LC Elastomer Films
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Free-standing Smectic LC Elastomer Films C. Tolksdorf1, R. Zentel1, R. Köhler2, U. Dietrich2, M. Lösche2, R. Stannarius2 1 Universität Mainz, Institut für Organische Chemie, Duesbergweg 10-14, D-55099 Mainz, 2 Universität Leipzig, Institut für Experimentalphysik I, Linnestrasse 5, D-04103 Leipzig ABSTRACT We probe the mesophase transitions and layer structures in thin ordered smectic liquid crystalline elastomer films by means of x-ray diffraction and optical microscopy. Oriented elastomer films of submicrometer thickness are produced by crosslinking freely suspended smectic polymer films. After crosslinking, the mesomorphism is similar to that of the precursor polymer. Smectic layers align parallel to the film plane. The layer spacing increases with temperature in the SmC* phase while it decays above the SmC*-SmA transition. Keywords: Smectic elastomers, x-ray, optical microscopy
INTRODUCTION Liquid crystal elastomers (LCE) combine the rubber elastic properties of polymer networks with the anisotropy of liquid crystalline materials [1]. In smectic elastomers, orientational as well as positional order is present. Particularly the ferroelectric smectic modifications open promising perspectives for application as sensors or actuators [2]. The elastic network is soft enough to allow switching of the mesogenes by application of external electric fields [2,3]. In addition to potential technical applications, smectic LCE are interesting from a scientific point of view [4]. Many of the complex structural couplings of electrical, optical and mechanical properties have not been analyzed on a quantitative level so far, some fundamental features are still only poorly understood. Structure, dynamics and mechanical properties of ferroelectric LCE have been investigated with various experimental methods (e. g. [5-14]). It has been shown, e.g. that the elastic response of smectic LCE may be strongly anisotropic and that the modulus for deformations normal to the smectic layers may exceed the entropy elasticity of the network by orders of magnitude [9]. The goal of this paper is a structural assessment of ordered ('single crystal') smectic LCE films. Small angle x-ray scattering of substrate-supported and freely suspended films provides information about the layering, phase structures and mesophase transitions in the films. Optical microscopic investigations supplement these x-ray data.
EXPERIMENTAL The precursor material is a random side-chain copolymer (Fig.1 ). It consists of a siloxane backbone with statistically distributed mesogenic and crosslinker side chains. The ratio of nonsubstituted, mesogenic and crosslinking moieties is 2.7:0.95:0.05. The crosslinker unit is similar CC2.6.1
in structure and length to the mesogenic side chain, a photoreactive group is attached at its end. In the smectic phases, the mesogenic side chains form aligned layers, with the siloxane backbone essentially located between them in separate layers. The particular choice of the long reactive units guarantees that the crosslinking of the sample o
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