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Issue 25, 2014
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Thermotropic orientational order of discotic liquid crystals in nanochannels: an optical polarimetry study and a Landau–de Gennes analysis

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Abstract

Optical polarimetry measurements of the orientational order of a discotic liquid crystal based on a pyrene derivative confined in parallelly aligned nanochannels of monolithic, mesoporous alumina, silica, and silicon as a function of temperature, channel radius (3–22 nm) and surface chemistry reveal a competition of radial and axial columnar orders. The evolution of the orientational order parameter of the confined systems is continuous, in contrast to the discontinuous transition in the bulk. For channel radii larger than 10 nm we suggest several, alternative defect structures, which are compatible both with the optical experiments on the collective molecular orientation presented here and with a translational, radial columnar order reported in previous diffraction studies. For smaller channel radii our observations can semi-quantitatively be described by a Landau–de Gennes model with a nematic shell of radially ordered columns (affected by elastic splay deformations) that coexists with an orientationally disordered, isotropic core. For these structures, the cylindrical phase boundaries are predicted to move from the channel walls to the channel centres upon cooling, and vice-versa upon heating, in accord with the pronounced cooling/heating hystereses observed and the scaling behavior of the transition temperatures with the channel diameter. The absence of experimental hints of a paranematic state is consistent with a biquadratic coupling of the splay deformations to the order parameter.

Graphical abstract: Thermotropic orientational order of discotic liquid crystals in nanochannels: an optical polarimetry study and a Landau–de Gennes analysis

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Publication details

The article was received on 27 Jan 2014, accepted on 27 Mar 2014 and first published on 28 Mar 2014


Article type: Paper
DOI: 10.1039/C4SM00211C
Citation: Soft Matter, 2014,10, 4522-4534

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    Thermotropic orientational order of discotic liquid crystals in nanochannels: an optical polarimetry study and a Landau–de Gennes analysis

    A. V. Kityk, M. Busch, D. Rau, S. Calus, C. V. Cerclier, R. Lefort, D. Morineau, E. Grelet, C. Krause, A. Schönhals, B. Frick and P. Huber, Soft Matter, 2014, 10, 4522
    DOI: 10.1039/C4SM00211C

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