Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) under hydrostatic pressure
Titel:
Phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) under hydrostatic pressure
Auteur:
Verschenen in:
Liquid crystals
Paginering:
Jaargang 28 (2001) nr. 11 pagina's 1679-1690
Jaar:
2001-01-01
Inhoud:
The phase behaviour of the discotic mesogen 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT) was investigated under hydrostatic pressures up to 500 MPa 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 32 MPa. Application of hydrostatic pressures above 32 MPa 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 180 MPa. Thus, the I MColh MH MCr transition sequence appeared only on cooling under these pressures, while the Cr MColh MI transition occurred on heating. Further increases in pressure above a second limiting value leads to the Colh phase becoming monotropic. Thus the I MColh MCr transition sequence appeared on cooling, while the Cr MI 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 40 MPa, 77.2°C for the Cr-H-Colh triple point and the other is extrapolated as 285 MPa, 118.3°C for the Cr-Colh-I triple point. These triple points define the upper limits for the appearance of the stable H and Colh phases, respectively.
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