Mean field analysis of four liquid crystalline odd-even ester dimers

A mean field analysis is presented for four liquid crystalline ester dimers, Dn, containing the dimethylbenzalazine mesogen, alkanedioyloxy flexible spacers from 7 to 10 carbon atoms and acetate terminal groups. The conformations of the dimers, in the RIS approximation, were generated from the known crystallographic coordinates of D8 and D9. The energy of each conformer is split into an internal ( conformation dependent) part and an external ( orientation dependent) part. After proper averaging over all orientations and conformations, the orientation-conformation partition function is evaluated and, from that, the Helmholtz free energy. A qualitative agreement between calculated and observed thermodynamic properties is obtained. In fact, the theoretical analysis correctly predicts strong odd-even fluctuations for the mesogenic group order parameter, S, as well as for transition entropy, Delta S-NI, and transition temperature, T-NI. The distribution of conformers is similar for dimers having the same parity of the spacer. For even dimers, the calculated fraction of linear extended conformers in the nematic phase at the N-I transition is around 47%, whereas it is less than 3.6% for odd dimers.