Splay and bend elastic constants in the nematic phase of some disulfide bridged dimeric compounds

We report measurements of the temperature dependences of the splay ( K-11 ) and bend (K-33) elastic constants of several homologues and a binary mixutre of disulfide bridged symmetric dimers made of alkoxy cyanobiphenyl monomeric units. All of them have an even number m of carbon and sulfur atoms in the linking chain and expected to have linear conformations. Both K-11 and K-33 alternate depending on the parity of x which is the number of carbon atoms in the alkyl chain of a monomer. We will refer to this as 'odd-even' effect. This effect, occurs near the nematic-isotropic transition temperature. T-NI, but monotonically decrease with m at lower temperatures. We argue that these results imply large rotational fluctuations of the two halves of the dimers about the S-S bond giving rise to average bent conformations near T-NI. At lower temperatures those conformations which can adapt better to the given curvature deformation in the medium are favored.