High pressure studies on 4, 4'-di-n-alkoxyazoxybenzenes

Abstract

The effect of pressure on the phase transitions in the first six homologues of 4,4'-di-n-alkoxyazoxybenzenes is investigated using differential thermal analysis as the probe. The slope dT/dP for the nematic-isotropic transition alteranates as the number of the carbon atoms in the alkoxy end chain is successively increased. No such behaviour is observed for the solid-nematic transition. The experimental values of dT/dP for the nematic-isotorpic transition for all the compounds agreereasonably well with those evaluated from the Clausius-Clapeyron equation. In the case of the pentyl compound which has recently been found to exhibit a highly ordered smectic phase (Sx, as yet unidentified) in addition to the nematic phase at atmospheric pressure, it is seen that an additional lower temperature mesophase appears at a pressure of 2.55 kbar which grows at the expense of the Sx phase. Beyond 6.7 kbar, the Sx phase disappears and only the new pressure induced phase exists. The other interesting result observed is that the monotropic smectic C phase of the hexyl derivative becomes enantiotropic at higher pressures, with a resulting triple point at 1.25 kbar, 98Oc.