Molecular ordering in the high-temperature nematic phase of an all-aromatic liquid crystal.

Francesco Vita,Edward T Samulski,Theo Dingemans,Giuseppe Portale,Claudio Ferrero,Oriano Francescangeli,Wim Bras,Maruti Hegde

doi:10.1039/c5sm02738a

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.

Highlights

Zannoni[4] that this qualitative agreement with experiment could be explained by generic aspects of a N phase comprised of locally ordered clusters of molecules persisting above the clearing point into the isotropic (I) phase
We found good agreement between the transition temperature values measured by X-ray diffraction (XRD) and differential scanning calorimetry (DSC)
The onset of the N phase was observed at 470 1C when the pair of broad isotropic rings typical of the I phase (Fig. 3a) changed into the anisotropic diffraction pattern of the aligned nematic phase (Fig. 3b–f)

Summary

Introduction

Zannoni[4] that this qualitative agreement with experiment could be explained by generic aspects of a N phase comprised of locally ordered clusters of molecules persisting above the clearing point into the isotropic (I) phase. In spite of this drastic simplification, the prediction of the temperature dependence of orientational order hP2i by the M–S theory fits surprisingly well the N phases of many low molecular weight liquid crystals.

Results

Conclusion