The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls |
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Authors: | H J Coles C Strazielle |
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Institution: | 1. C.N.R.S. Centre de Recherches sur les Macromolécules 6 , rue Boussingault, 67083, Strasbourg, Cedex, France;2. Physics Department , Schuster Laboratory, University of Manchester , Manchester, MI39 PL, U.K.;3. C.N.R.S. Centre de Recherches sur les Macromolécules 6 , rue Boussingault, 67083, Strasbourg, Cedex, France |
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Abstract: | The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC n H 2n + 1) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ~3°C above the clearing temperature Tc where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at Tc . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to Tc where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase. |
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Keywords: | benzalazines mesogenic compounds analytical |
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