Phase behavior of four homologous compounds of 4-n-alkyl-4′-isothiocyanatobiphenyl (nBT) under pressure
The phase behavior of four homologous compounds of 4-n-nonyl-, 4-n-decyl-, 4-n-undecyl-, and 4-n-dodecyl-4′-isothiocyanatobiphenyl (9BT, 10BT, 11BT, and 12BT) was re-investigated to characterize their high-pressure mesophases under pressures up to 150 MPa using a polarizing optical microscope equipped with a high-pressure optical cell and a wide-angle X-ray diffractometer equipped with a high-pressure sample vessel. The pressure-induced mesophases of 9BT and 10BT appearing under pressures above about 60 and 100 MPa, respectively, were identified as nematic (N) and SmA phases, which indicate the reversible Cr-CrE-N-I and Cr-CrE-SmA-I phase transitions under elevated pressure. 11BT and 12BT exhibited the reversible transition of Cr-CrE-SmA-I in the low-pressure regions below about 5 and 23 MPa, respectively. Both the stable CrE phases changed into the monotropic (and metastable) one under higher pressures, in which the Cr-SmA-I and I-SmA-CrE-Cr phase transitions on heating and cooling processes, respectively, were recognized.