Pressure-volume-temperature (PVT) measurements were performed on
Mainchain liquid crystals having a regularly alternating rigid-flexible repeating structure often exhibit distinct odd-even oscillation in their thermodynamic quantities with respect to the number of methylene units in the spacer. The effect has been widely observed from dimer (DLC) to polymer liquid crystals (PLC), but not in the monomer liquid crystals (MLC) which comprise a mesogenic unit carrying one or two tails. The degree of oscillation varies depending on the chemical structure of the linking group used to join the spacer with mesogenic units. In our previous work, we have suggested that such characteristics are mainly due to the geometrical arrangement of the linkage. In this work, we have prepared a series of carbonate-type MLCs NC$$OCOO(CH,),.,CH,, where q5 is a phenyl group, as well as DLCs NCqh$OOCO(CH,),O-COO$qCN. The thermodynamic behaviors at the nematic-isotropic NI transition have been compared with those previously reported for the ether-type DLCs. For the dimer series, the molecular orientation and the spacer conformation were investigated by using *H NMR technique.The origin of the difference in the odd-even behaviors has been traced back to the geometrical characteristics of the linking groups, which affect the relative orientation of the terminal mesogenic cores, thus leading to modification of the conformational distribution in the nematic state. The analysis has yielded the nematic conformation which satisfactorily reproduces the observed values of the dipolar and quadrupolar couplings. The bond conformation vs. bond order plots are shown for the n odd and n even systems together with the corresponding results derived previously for the ether-type DLCs. Although the numbers of bonds of the flexible spacer are not exactly comparable with each other, the conformational ordering along the spacer tends to be pronounced in both DLC systems.
RIS analysis of the deuterium quadrupolar splitting data was performed for a,a»-bis [(4,4'cyanobiphenylyl)oxy]alkane dimer liquid crystals having -0(CH2)"0flexible spacers (n = 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 < if i,if 2 < 45°a dopt spatial configurations reasonably consistent with the nematic arrangement of mesogenic cores in both dimer LC systems, where ifi and if 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, AS"nf = 59.6, AS"nf = 13.3; CBA-10, AS"nf = 64.2, AS"nf = 15.6 (J mol-1 K-1 units). The values of the entropies AS™111 thus derived were compared with the constant-volume transition entropies (AStr)" determined by the PVT measurements reported in the accompanying paper: CBA-9, (AScn)i> = 53.9, (ASn;)" = 7.9; CBA-10, (AScn)i) = 62.4, (ASni)u = 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.
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