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Propriétés physiques des cristaux liquides discotiques nanoconfinés

Abstract : The aim of this work is to conduct fundamental and experimental studies of the physical properties of columnar discotic liquid crystal (CDLCs) confined in highly ordered porous templates at the nanoscale. CDLC molecule of planar shape, consist in rigid polyaromatic nuclei surrounded by functionalizable flexible aliphatic chains, and are capable of self-assembly in columns, thereby promoting overlap of their π electron orbitals. This makes these materials real candidates for applications in molecular electronics and photovoltaics due to the possibility of migration of the charge carriers along their columns. However, these applications require a good control of the parameters affecting the alignment mechanisms in the columnar phases of large single domains, preferably at room temperature. A very promising approach to optimize the diffusion lengths of charge carriers has been recently proposed, based on the formation of oriented CDLC nanowires by self-assembly in so-called "templates". However, structural and dynamical proprieties and confinement effects are still scarce, and could be a real scientific lock to their implementation. Our study is focused on commercial CDLCs (HPT) and Py4CEH which are confined in porous alumina and porous silicon membranes with pore diameters of c.a. tens of nm. The phase diagram was first studied by DSC and more deeply characterized by neutron scattering. In confined geometries, we observe a depression of the phase transition temperatures, a broadening of the columnar phase stability domain and an opening of hysteresis loops amplified by smaller pore size. A high orientational order was found in the bulk columnar phases by solid-state NMR, and the structure of confined columnar systems, dominated by a radial distribution with homeotropic anchoring was observed. The molecular dynamics was studied by quasielastic neutron scattering. It is affected by confinement: large lengthscale motions are massively slowed down, whereas the rapid and local dynamics becomes submitted to large distributions of correlation times.
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  • HAL Id : tel-00979588, version 1

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Makha Ndao. Propriétés physiques des cristaux liquides discotiques nanoconfinés. Autre [cond-mat.other]. Université Rennes 1, 2013. Français. ⟨NNT : 2013REN1S131⟩. ⟨tel-00979588⟩

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