J. Phys. II France
Volume 4, Numéro 12, December 1994
Page(s) 2149 - 2159
DOI: 10.1051/jp2:1994253
J. Phys. II France 4 (1994) 2149-2159

Comparison between the tilted SmC $_{\alpha}^*$ and SmC $_{\gamma}^*$ phases of MHPOBC studied by optical techniques

John Philip, Jean-René Lalanne, Jean-Paul Marcerou and Gilles Sigaud

Centre de Recherches Paul Pascal, CNRS, Avenue du docteur Albert Schweitzer, 33600 Pessac, France

(Received 1 July 1994, received in final form 7 September 1994, accepted 12 September 1994)

We present optical investigations in the SmC $_{\alpha}^*$ and SmC $_{\gamma}^*$ phases of a low molar mass chiral liquid crystal, 4-(1-methylheptyloxycarbonyl) phenyl 4 '-octyloxy biphenyl-4-carboxylate [MHPOBC(R)]. The polarizing microscope observations (PMO), optical rotatory power (ORP) and anisotropic Rayleigh scattering (ARS) investigations show that the smectic C $_{\alpha}^*$ structure strongly differs from the SmC $_{\gamma}^*$ phase. The ORP in the smectic C $_{\alpha}^*$ phase has been found to be very weak (less than 0.2 deg/mm), compared to ordinary liquids with strongly chiral molecules and to other smectic C * phases. Although pretransitional ORP effects in the vicinity of the SmA $\rightleftharpoons$ SmC * phase transition have been reported earlier in chiral liquid crystals, no such behaviour is noticed in this compound at the SmA $\rightleftharpoons$ SmC $_{\alpha}^*$ and SmC $_{\alpha}^*$ $\rightleftharpoons$ SmC * phase transitions. The absence of critical behaviour in the ARS at the SmA $\rightleftharpoons$ SmC $_{\alpha}^*$ phase transition is a clear indication of the nonexistence of "local biaxial" character of the SmC $_{\alpha}^*$ phase. Moreover, the PMO studies also confirm the "macroscopically uniaxial nature" of all these phases except SmC $_{\gamma}^*$ phase, which was found to be birefringent. The possibility of a divergence in the "biaxial fluctuations" at the SmC $_{\gamma}^*$ $\rightleftharpoons$ Sm $_{\rm A}^*$ phase transition is also discussed.

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