7 research outputs found
Matériaux cristaux liquides magnétiques
Les Ă©lastomĂšres cristaux liquides (ECL) offrent la possibilitĂ© d'obtenir des actionneurs, des muscles artificiels et autres senseurs. En effet, ces matĂ©riaux combinent les propriĂ©tĂ©s Ă©lastiques des Ă©lastomĂšres aux propriĂ©tĂ©s d'organisation liĂ©es au cristal liquide et peuvent ainsi changer de forme sous l'effet de la tempĂ©rature. En ajoutant des nanoparticules magnĂ©tiques, la matrice ECL offre l'intĂ©rĂȘt de rĂ©pondre Ă un champ magnĂ©tique.
Dans cette thÚse, le polymÚre sélectionné est un polysiloxane. Il a une température de transition vitreuse trÚs basse et il est donc trÚs flexible à température ambiante. De plus, il est facile à réticuler. Les nanoparticules choisies sont des nanobùtonnets de cobalt, car leur forme anisotrope permet d'avoir une susceptibilité magnétique élevée. Pour comprendre les mécanismes qui sont en jeu, nous nous intéressons d'abord à l'étude des systÚmes non réticulés, c'est-à -dire aux polymÚres cristaux liquides dopés avec des nanoparticules magnétiques (PCLM), et nous les comparons aux polymÚres cristaux liquides seuls (PCL). Sur ces polymÚres une étude structurale poussée a été menée afin d'avoir une analyse des matériaux à diffÚrentes échelles. On constate qu'en presence d'une interaction de la matrice cristal liquide avec les nanobùtonnets de cobalt, les mésogÚnes du PCLM s'orientent mieux sous champ magnétique. Dans le cadre des études magnétiques, on observe que les PCLM présentent des champs coercitifs plus élevés que ceux prevus par la théorie. Ceci est dû au fait que le PCLM est un milieu dilué qui réduit les interactions dipolaires.
Ensuite nous avons Ă©tudiĂ© les systĂšmes rĂ©ticulĂ©s, et pour ce faire, nous avons mis au point les conditions de synthĂšse. Les matĂ©riaux obtenus sont cristal liquides et ferromagnĂ©tiques. Nous souhaitions appliquer le champ magnĂ©tique sur le composite et observer sa dĂ©formation mĂ©canique. Cependant pour voir un changement de forme du matĂ©riau, il faut qu'il soit monodomaine. Ainsi nous avons rĂ©alisĂ© des synthĂšses du composite sous champ magnĂ©tique, Ă tempĂ©rature ambiante, pour orienter les mĂ©sogĂšnes, ainsi que les nanobĂątonnets de cobalt, dans une mĂȘme direction. AprĂšs avoir observĂ© les Ă©chantillons, en diffraction des rayons X et en magnĂ©tomĂ©trie, il apparait que les nanabĂątonnets sont bien alignĂ©s. Par contre, pour orienter les mesogĂšnes, il apparait qu'il serait prĂ©fĂ©rable de se placer Ă la tempĂ©rature isotrope en appliquant le champ magnĂ©tique.
Cette étude a validé la faisabilité de réaliser des réseaux élastomÚres mésomorphes magnétiques et a montré l'apport potentiel du dopage de la matrice cristal liquide par des nanobùtonnets de cobalt.Liquid-Crystalline Elastomers (LCE) allow to obtain actuators and other sensors. These materials can change shape under temperature. In addition to magnetic nanoparticles, LCE offer the interest of responding to a magnetic field.
In order to understand the mechanisms at stake, we start by the study of no cross-linkage systems that is to say by liquid-crystalline polymers doped with magnetic nanoparticles (PCLM). Then, we compare it at only liquid-crystalline polymers (PCL). The strutural study of these polymers shows that in presence of an interaction between the liquid-crystalline matrix and the cobalt nanorods, mesogens of PCLM show a better alignment under magnetic field.
Then, we have studied the cross-linkage systems. The Materials obtained are liquid-crystal and ferromagnetic. To see a shape modification of the material it must be monodomain. Thus, we have realized syntheses of composite under magnetic field at room temperature, to aligne the mesogens and the cobalt nanorods in the same direction. The observation of these samples enables to see that the cobalt nanorods are well aligned. However, to aligne the mesogens it appears that it would be better to use an isotropic temperature and to apply the magnetic field.
This study has approved the feasibility to realize magnetic mesomorphous elastomers and has shown the contribution of doping of the liquid-crystalline matrix by cobalt nanorods
In Situ and Ex Situ Syntheses of Magnetic Liquid Crystalline Materials: A Comparison
International audienceMagnetic hybrid liquid crystalline composites have been obtained either by thermal decomposition of a cobalt precursor in a solution containing a liquid crystal polymer or by dispersing preformed cobalt nanorods in a liquid crystal polymer matrix. The final materials are all mesomorphous and ferromagnetic. Their magnetic characteristics are compared as a function of the synthesis method
Room-Temperature, Strain-Tunable Orientation of Magnetization in a Hybrid Ferromagnetic Co Nanorod-Liquid Crystalline Elastomer Nanocomposite
International audienceHybrid nanocomposites based on magnetic nanoparticles dispersed in liquid crystalline elastomers are fascinating emerging materials. Their expected strong magnetoâelastic coupling may open new applications as actuators, magnetic switches, and for reversible storage of magnetic information. We report here the synthesis of a novel hybrid ferromagnetic liquid crystalline elastomer. In this material, highly anisotropic Co nanorods are aligned through a crossâlinking process performed in the presence of an external magnetic field. We obtain a highly anisotropic magnetic material which exhibits remarkable magnetoâelastic coupling. The nanorod alignment can be switched at will at room temperature by weak mechanical stress, leading to a change of more than 50â% of the remnant magnetization ratio and of the coercive field
Room-Temperature, Strain-Tunable Orientation of Magnetization in a Hybrid Ferromagnetic Co Nanorod-Liquid Crystalline Elastomer Nanocomposite
International audienceHybrid nanocomposites based on magnetic nanoparticles dispersed in liquid crystalline elastomers are fascinating emerging materials. Their expected strong magnetoâelastic coupling may open new applications as actuators, magnetic switches, and for reversible storage of magnetic information. We report here the synthesis of a novel hybrid ferromagnetic liquid crystalline elastomer. In this material, highly anisotropic Co nanorods are aligned through a crossâlinking process performed in the presence of an external magnetic field. We obtain a highly anisotropic magnetic material which exhibits remarkable magnetoâelastic coupling. The nanorod alignment can be switched at will at room temperature by weak mechanical stress, leading to a change of more than 50â% of the remnant magnetization ratio and of the coercive field
Liquid Crystalline PolymerâCo Nanorod Hybrids: Structural Analysis and Response to a Magnetic Field
International audienceThis work deals with the structural analysis of side-chain liquid crystalline polysiloxanes, doped with magnetic cobalt nanorods, and their orientational properties under a magnetic field. These new materials exhibit the original combination of orientational behavior and ferromagnetic properties at room temperature. Here we show that, within the liquid crystal polymer matrix, the cobalt nanorods self-assemble in bundles made of nanorod rows packed in a 2-dimensional hexagonal lattice. This structure accounts for the magnetic properties of the composites. The magnetic and orientational properties are discussed with respect to the nature of the polymer matrix
Liquid Crystalline PolymerâCo Nanorod Hybrids: Structural Analysis and Response to a Magnetic Field
International audienceThis work deals with the structural analysis of side-chain liquid crystalline polysiloxanes, doped with magnetic cobalt nanorods, and their orientational properties under a magnetic field. These new materials exhibit the original combination of orientational behavior and ferromagnetic properties at room temperature. Here we show that, within the liquid crystal polymer matrix, the cobalt nanorods self-assemble in bundles made of nanorod rows packed in a 2-dimensional hexagonal lattice. This structure accounts for the magnetic properties of the composites. The magnetic and orientational properties are discussed with respect to the nature of the polymer matrix