Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) under hydrostatic pressure

Abstract

The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 500MPa using high pressure optical and DTA measurements. The known enantiotropic phase transitions of HHTT, i.e. crystal (Cr)–helical phase (H), H–hexagonal columnar phase (Colh ) and Colh–isotropic liquid (I ) were observed up to 32MPa. Application of hydrostatic pressures above 32MPa results in the H and Colh phases becoming monotropic, depending upon the applied pressure. The H phase was observed as a monotropic phase in the pressure region between 32 and about 180MPa. Thus, the I Colh H Cr transition sequence appeared only on cooling under these pressures, while the Cr Colh I transition occurred on heating. Further increases in pressure above a second limiting value leads to the Colh phase becoming monotropic. Thus the I Colh Cr transition sequence appeared on cooling, while the Cr I transition was observed on heating. The T vs. P phase diagram based on the data obtained in the heating mode contains two triple points; one is estimated as 40MPa, 77.2ß C for the Cr–H–Colh triple point and the other is extrapolated as 285MPa, 118.3ß C for the Cr–Colh–I triple point. These triple points de ne the upper limits for the appearance of the stable H and Colh phases, respectively.