Pressure effect on the nematic-isotropic transition in dilaterally substituted nematogens

• Published in: Liquid crystals 2001-02, Vol.28 (2), p.291-297
• Main Authors: Daoudi, A., Longuemart, S., Roussel, F., Kolinsky, C., Bayle, J. P.
• Format: Article
• Language: English
• ISSN: 0267-8292; 1366-5855
• DOI: 10.1080/02678290010007549

The effect of pressure on the liquid crystal properties of two new dilaterally substituted nematogens has been studied. The method employed involves measurement of the thermal pressure variation of a sample under isochoric conditions. The pressure-temperature phase diagrams were determined. As for unsubstituted compounds, the nematic phase is stabilized upon application of pressure. The values of the (∂T/∂P) NI slopes of the clearing lines for these nematogens are, however, significantly higher than those generally characterizing rod-shaped nematics. The entropy separation method was used to estimate the constant volume and the volume-dependent terms of the entropy changes at the nematic-isotropic transition. The values of these contributions were determined on the basis of thermobarometric data showing that the transition entropy is strongly volume-dependent. This suggests that the higher values of the (∂T/∂P) NI slopes observed for these compounds can be related to the rearrangement of the lateral flexible chains in the nematic phase when the pressure increases, leading to a decrease of the excluded free volume caused by the bulky core of the molecules.