THE DISCONTINUITY OF THE ISOTROPIC - MESOPHASE TRANSITION IN N-CYANOBIPHENYLS HOMOLOGOUS SERIES FROM 4CB TO 14CB. NONLINEAR DIELECTRIC EFFECT (NDE) STUDIES

摘要

Results of studies of the static nonlinear dielectric effect (NDE) in trie homologous scries of 4-cyano-4-n-alkylbiphenyls (nCB, from n = 3 to n = 14), exhibiting both isotropic - nematic (I-N) and isotropic -smectic A (I-SmA) phase transitions, are presented. Tests were conducted in the isotropic phase where the reciprocal of NDE shows a linear dependence vs. temperature or pressure, also in the immediate vicinity of the clearing point. It was found that the discontinuity (ΔT) of the isotropic - mesophase transition increases with the length of the alkyl-chain in the nCB molecule, from 0.7 K (4CB) to ca. 11 K (14CB). Regarding the influence of pressure, it continuously increases the value of ΔT(P) for the case of the I-N transitions, as shown for 5CB up to P = 215MPa . For the the I-SmA transition discontinuity first drops down to ΔT(p)_(min) ≈ (1/2)[ΔT(0.1MPa)] and next strongly increases with pressure rising up to P = 375MPa.