CALCULATION OF THE CONFORMATION ENTROPIES OF DIMER LIQUID-CRYSTALS AND COMPARISON WITH THE OBSERVED TRANSITION ENTROPIES AT CONSTANT VOLUME

RIS analysis of the deuterium quadrupolar splitting data was performed for alpha,omega-bis[(4,4'-cyanobiphenylyl)oxy]alkane dimer liquid crystals having -O(CH2)(n)O- flexible spacers (n = 9 (CBA-9) and n = 10 (CBA-10)) between the 4,4'-cyanobiphenylyl ends according to the scheme previously established. The analysis indicates that most of the conformers involved in the range 0 < psi(1),psi(2) < 45 degrees adopt spatial configurations reasonably consistent with the nematic arrangement of mesogenic cores in both dimer LC systems, where psi(1), and psi(2) denote the inclination angles of the terminal mesogenic cores with respect to the molecular axis. The conformational entropy changes at the crystal-nematic (CN) and nematic-isotropic (NI) interphases estimated on this basis are as follows: CBA-9, Delta S-en(conf) = 59.6, Delta S-ni(conf) = 13.3; CBA-10, Delta S-ni(conf) = 64.2, Delta S-ni(conf) = 15.6 (J mol(-1) K-1 units). The values of the entropies Delta S-tr(conf) thus derived were compared with the constant-volume transition entropies (Delta S-tr)(v) determined by the PVT measurements reported in the accompanying paper: CBA-9, (Delta S-cn)(v) = 53.9, (Delta S-ni)(v) = 7.9; CBA-10, (Delta S-cn)(v) = 62.4, (Delta S-ni)(v)) = 13.3 (J mol(-1) K-1 units). In view of the uncertainties involved in the estimation of the entropies both in theory and in experiments, the correspondence is quite favorable. The conformation of the spacer undoubtedly plays an important role in determining the phase behaviors of these main chain liquid crystals. It is pointed out that the discrepancy between the calculation and experiment may be further improved by considering other contributions such as (1) the entropy changes due to the orientation of the anisotropic molecules in the liquid crystalline state and (2) the possibility of the entropy loss during the compression to achieve constant-volume transitions. It is concluded that the observed increase in the quadrupolar and dipolar splittings with decreasing temperature arises mainly from the variation of the order parameter of the molecular axis.