EVEN-ODD EFFECTS IN LIQUID-CRYSTAL DIMERS WITH FLEXIBLE SPACERS - A TEST OF THE ROTATIONAL ISOMERIC STATE APPROXIMATION

In this work we analyse the consequences of relaxing the rotational is omeric state (RIS) approximation in predicting the transitional proper ties of nematogenic systems formed by two mesogenic moieties linked by a flexible spacer, as in the homologous series of alpha,omega-bis(4'- cyanobiphenyl-4-yl) alkanes and alpha,omega-bis(4'-cyanobiphenyl-4-ylo xy) alkanes, generically referred to as CB dimers. Comparison is made therefore between predictions of the nematic-isotropic transition temp eratures, entropy changes, and order parameters obtained from the RIS model, and those by taking into account the full distribution of torsi onal angles. The continuous model predicts a damping of the even-odd e ffects for long chain systems, with entropy changes and order paramete rs at the nematic-isotropic transition tending to those of the corresp onding monomers.