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Collective Orientational Order and Phase Behavior of a Discotic Liquid Crystal Under Nanoscale Confinement

Abstract

The phase behavior and molecular ordering of hexakishexyloxy triphenylene (HAT6) DLC under cylindrical nanoconfinement is studied utilising differential scanning calorimetry (DSC) and dielectric spectroscopy (DS), where cylindrical nanoconfinement is established through embedding HAT6 into the nanopores of anodic aluminum oxide membranes (AAO), and a silica membrane with pore diameters ranging from 161 nm down to 12 nm. Both unmodified and modified pore walls were considered, and in the latter case the pore walls of AAO membranes were chemical treated with n octadecylphosphonic acid (ODPA) resulting in the formation of a 2.2 nm thick layer of grafted alkyl chains. Phase transition enthalpies decrease with decreasing pore size, indicating that a large proportion of the HAT6 molecules within the pores has a disordered structure, which increases with decreasing pore size for both pore walls. In the case of the ODPA modification the amount of ordered HAT6 is increased compared to the unmodified case. The pore size dependencies of the phase transition temperatures were approximated using the Gibbs Thomson equation, where the estimated surface tension is dependent on the molecular ordering of HAT6 molecules within the pores and upon their surface. DS was employed to investigate the molecular ordering of HAT6 within the nanopores. These investigations revealed that with a pore size of around 38 nm, for the samples with the unmodified pore walls, the molecular ordering changes from planar axial to homeotropic radial. However, the planar axial configuration, which is suitable for electronic applications, can be successfully preserved through ODPA modification for most of the pore sizes.

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Publication details

The article was received on 26 Oct 2018, accepted on 03 Dec 2018 and first published on 03 Dec 2018


Article type: Paper
DOI: 10.1039/C8NA00308D
Citation: Nanoscale Adv., 2019, Accepted Manuscript
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    Collective Orientational Order and Phase Behavior of a Discotic Liquid Crystal Under Nanoscale Confinement

    A. Yildirim, K. Sentker, G. J. Smales, B. R. Pauw, P. Huber and A. Schoenhals, Nanoscale Adv., 2019, Accepted Manuscript , DOI: 10.1039/C8NA00308D

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