title: Molecular ordering in the high-temperature nematic phase of an all-aromatic liquid crystal.
abstract: We report the structural characterization of the nematic phase of 2,6-biphenyl naphthalene (PPNPP). This lath-like all-aromatic mesogen provides a valuable benchmark for classical theories of nematic order. PPNPP exhibits a very high temperature nematic phase (417-489 °C) above an enantiotropic smectic A phase. X-ray diffraction reveals a surprisingly strong tendency towards molecular layering in the nematic phase, indicative of "normal cybotaxis" (i.e. SmA-like stratification within clusters of mesogens). Although stronger at low temperatures, the layering is evident well above the smectic A-nematic transition. The nematic order parameter is evaluated as a function of temperature from the broadening of the wide-angle diffuse diffraction feature. Measured values of the orientational order parameter are slightly larger than those predicted by the Maier-Saupe theory over the entire nematic range except for a narrow region just below the clearing point where they significantly drop below the theoretical prediction.
authors.affiliation: Dipartimento di Scienze e Ingegneria della Materia, dell'Ambiente ed Urbanistica and CNISM, Università Politecnica della Marche, Via Brecce Bianche, 60131 Ancona, Italy. o.francescangeli@univpm.it.
authors.forename: Francesco
authors.initials: F
authors.lastname: Vita
authors.affiliation: Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands. t.j.dingemans@tudelft.nl.
authors.forename: Maruti
authors.initials: M
authors.lastname: Hegde
authors.affiliation: Netherlands Organization for Scientific Research (NWO), DUBBLE CRG at the ESRF, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Portale
authors.affiliation: Netherlands Organization for Scientific Research (NWO), DUBBLE CRG at the ESRF, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France.
authors.forename: Wim
authors.initials: W
authors.lastname: Bras
authors.affiliation: European Synchrotron Radiation Facility, Boîte Postale 220, 38043 Grenoble Cedex, France.
authors.forename: Claudio
authors.initials: C
authors.lastname: Ferrero
authors.affiliation: Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290, USA.
authors.forename: Edward T
authors.initials: ET
authors.lastname: Samulski
authors.affiliation: Dipartimento di Scienze e Ingegneria della Materia, dell'Ambiente ed Urbanistica and CNISM, Università Politecnica della Marche, Via Brecce Bianche, 60131 Ancona, Italy. o.francescangeli@univpm.it.
authors.forename: Oriano
authors.initials: O
authors.lastname: Francescangeli
authors.affiliation: Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands. t.j.dingemans@tudelft.nl.
authors.forename: Theo
authors.initials: T
authors.lastname: Dingemans