MOLECULAR ASSOCIATION IN NEMATIC PHASES OF CYCLIC LIQUID-CRYSTAL OLIGOMERS

We analyzed molecular ordering in the nematic phase of cyclic LC oligo mers with mesogenic groups attached to a central siloxane ring. We foc us on relationships between the local spatial arrangement of these mol ecules and low-angle X-ray scattering phenomena as revealed by compute r molecular modeling. Computer simulation shows that the presence of s trong low-angle X-ray reflections in the nematic phase is caused by lo cal association of the molecules. The cyclic molecules form double-row associations with up to six molecules packed in a single string. The presence of the siloxane central rings with a high local concentration of Si-O atomic groups with strong scattering power is critical for th e appearance of these low-angle X-ray reflections with significant int ensity. Cycling of siloxane groups into short rings hinders rotation a nd restricts mobility of these fragments, resulting in a high localiza tion of strong scattering groups. Computer simulation of X-ray propert ies of partially ordered systems within the ''lattice approximation'' works well for crystal lattices and highly ordered mesophases like sme ctics. However, this approach underestimates local distortions of posi tional ordering and does not adequately describe short-range molecular ordering in the nematic phase. Application of straightforward calcula tions of the form-factor of molecular clusters reproduces major featur es of these unusual X-ray scattering patterns but falls short in descr ibing fine details such as diffuse background scattering, peak profile s, and exact d-spacings.