Electrostriction and giant permittivity of polymer nanocomposites

Par Philippe Poulin, CRPP, CNRS-Université de Bordeaux

Mardi 30 Mai, 14h, Salle des séminaires (215), 2ème étage, Bâtiment A4N

Abstract :

The inclusion of conductive particles into insulating polymer matrices allows the synthesis of nanocomposites with tunable dielectric properties. In particular, giant permittivity is achieved when the conductive inclusions form near percolated networks. The permittivity of such nanocomposites strongly varies when the soft polymer matrix is deformed, giving rise to large electrostriction coefficients as needed in variable capacitors of energy harvesting devices.
Near percolated networks can easily be obtained with particles of anisotropic shape, such as carbon nanotubes, metal nanowires or graphene platelets. Because of their large aspect ratio, the particles exhibit a large excluded volume and a resultant low percolation threshold compared to spherical or quasi-spherical particles. The control of the spatial organization of the particles in the matrix is critical for the control of the material dielectric properties. We will present recent approaches to control the ordering of carbon nanotubes using emulsion templates to obtain enhanced dielectric permittivity and electrostriction coefficients [1]. We will also discuss differences or rod like particles from graphene platelets with giant anisotropy. The percolation behavior of graphene platelets has been recently predicted to be far more complicated than generally anticipated by excluded volume concepts [2]. Here, by characterizing the percolation transition in a liquid crystalline graphene based elastomer composite, we confirm experimentally that graphene flakes self-assemble into nematic liquid crystals (LCs) at concentrations below the percolation threshold [3]. We find that the competition of percolation and LC transition provides a new route towards high-permittivity materials. Near-percolated liquid crystalline graphene based composites display a giant permittivity along with a low loss tangent. The near percolated nanocomposites exhibit large permittivity variations in response to small strain deformations, giving rise to a giant electrostriction coefficients of about M= -5×10-14 m2/V2 at 100 Hz. The present materials are promising for uses in variable capacitors of energy harvesters. Their implementation in actual electronic devices is currently investigated.

[1] Luna, A., Yuan, J., Neri, W., Zakri, C., Poulin, P. and Colin, A., (2015) Giant Permittivity Polymer Nanocomposites Obtained by Curing a Direct Emulsion. Langmuir 31: 12231-12239.
[2] Mathew, M.; Schilling, T.; Oettel, M., Phys. Rev. E 2012, 85 (6), 061407.
[3] Graphene Liquid Crystal Retarded Percolation for High Permittivity Materials , J. Yuan, A. Luna, W. Neri, C. Zakri, T. Schilling,  A. Colin, P. Poulin , Nat. Comm. (2015) 6, 8700.