Rheological properties of chiral liquid crystals possessing a cholesteric–smectic A transition

Abstract

We have measured the rheological properties of two cholesterol derivatives (cholesteryl myristate and cholesteryl nonanoate) in the vicinity of their cholesteric–smecticA transitions. The results for the two compounds differ qualitatively, and are in agreement with results based on optical observations of new defects in cholesteryl nonanoate showing that this material, traditionally considered as a typical cholesteric, in fact exhibits a TGBA phase between the cholesteric and smectic A phases.

Notes

^†Such power laws are reported for other liquid crystal phases and can be understood as defect-mediated regimes, adapting to liquid crystals the theory developed long ago by Orowan for the motion of line defects in solids [13].

Shear rate versus stress at different temperatures for cholesteryl tetradecanoate (CT). Open symbols: cholesteric phase, T=82, 79, 78.7, 78.4, 78.2°C (γ·∼σ^m , m=1). Filled symbols: smectic phase, T=77.9 (m=1.35), 77.6 (m=1.55), 77.1 (m=1.8), 76.8°C (m=2). To aid reading, bold lines with exact slopes corresponding to m=1 and m=2 are represented

Shear rate versus stress at different temperatures for cholesteryl nonanoate (CN). Open symbols: triangles, for T=75, 72, 71.1°C (T > T _N*–SmA+1°C), m=1; squares for T=70.9, 70.7, 70.5, 70.3°C (T _N*–SmA<T < T _N*–SmA+1°C), from m=1 (low stresses) to m=2 (higher stresses). Filled symbols: smectic phase, 70°C, m=2. To aid reading, bold lines with exact slopes corresponding to m=1 and m=2 are represented

^‡The exponent m=2 has also been reported for nematic polymers in solution and was recently explained as a defect-mediated regime [16], following Kléman and co-workers [13]. Nevertheless m=2 has never been reported for a thermotropic nematic to our knowledge.