title: Phase structure and molecular dynamics of liquid-crystalline side-on organosiloxane tetrapodes.
abstract: X-ray diffraction and proton NMR relaxation measurements were carried out on two liquid-crystalline organosiloxane tetrapodes with side-on mesogenic groups, exhibiting nematic and smectic- C phases, and on a monomeric analog. Packing models for the mesophases exhibited by these systems are proposed on the basis of x-ray diffraction data. As a consequence of microsegregation, the aromatic cores are packed in between two sublayers formed by a mixture of interdigitated aliphatic and siloxane chains. The mixed sublayers are characteristic for the tetrapodes with side-on mesogenic groups presented in this work and have not been observed in tetrapodes with terminally attached mesogens. The tilt angle in the smectic- C phase is found very large, i.e., approximately 61 degrees -62 degrees . Notably, smectic- C clusters are present also in the whole temperature range of the nematic phase. NMR relaxometry yields T(1)-1 dispersions clearly different from those of conventional calamitics. The influence of molecular tendency to form interdigitated structures is evidenced by frequency-dependent relaxation rate in the isotropic phase-indicating the presence of ordered clusters far above the phase transition-and by the diminished role of molecular self-diffusion in ordered phases. Nematiclike director fluctuations are the dominating relaxation mechanism whereas the translational displacements are strongly hindered by the interdigitation of dendrimer arms.
authors.affiliation: Centro de Física da Matéria Condensada, Universidade de Lisboa, Lisboa, Portugal.
authors.forename: D
authors.initials: D
authors.lastname: Filip
authors.forename: C
authors.initials: C
authors.lastname: Cruz
authors.forename: P J
authors.initials: PJ
authors.lastname: Sebastião
authors.forename: M
authors.initials: M
authors.lastname: Cardoso
authors.forename: A C
authors.initials: AC
authors.lastname: Ribeiro
authors.forename: M
authors.initials: M
authors.lastname: Vilfan
authors.forename: T
authors.initials: T
authors.lastname: Meyer
authors.forename: P H J
authors.initials: PH
authors.lastname: Kouwer
authors.forename: G H
authors.initials: GH
authors.lastname: Mehl