Dielectric investigations on carbon nanotubes doped polymer dispersed liquid crystal films
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Dielectric investigations on carbon nanotubes doped polymer dispersed liquid crystal films Constantin Paul Ganea1, Doina Manaila-Maximean 2,a , Viorel Cîrcu3,b 1 National Institute of Materials Physics, POBox MG 07, 077125 Magurele, Romania 2 Department of Physics, University Politehnica of Bucharest, 313 Spl. Independentei, 060042 Bucharest,
Romania
3 Department of Inorganic Chemistry, University of Bucharest, 23 Dumbrava Rosie st, Sector 2,
Bucharest 020464, Romania Received: 29 May 2020 / Accepted: 22 September 2020 © Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We obtained carbon nanotubes (CNTs) doped polymer dispersed liquid crystal (PDLC) films using the nematic E7 and polymethyl methacrylate, a composite that combines the benefic characteristic of the liquid crystals (LC) and carbon nanoparticles. The clearing temperatures recorded by differential scanning calorimetry for the PDLC blends were found to be lower than the value recorded for pure E7 LC mixture with no significant impact of the CNTs’ concentration. Broadband dielectric spectroscopy (DS) measurements were performed in the (10−1 ÷ 107 ) Hz frequency range, in the temperature domain (280–350) K. From the DS study, a two order magnitude variation of the conductivity over the entire temperature range was observed. The presence of CNTs results in an increase of electrical conductivity, with increasing concentration. Because the loss tangent spectra have complex shapes, they were fitted using the generalized Havriliak–Negami functions, and the characteristic relaxation times were extracted. The dependency of the characteristic relaxation time on temperature was modeled using the Vogel–Fulcher–Tammann function, and it showed a temperature variation according to the Arrhenius law. The increase of the CNT concentration increases the activation energy of the molecular electric dipoles of the LC. The interface LC-polymer interactions influence the nematic to isotropic phase transition of the LC.
1 Introduction The composites known as Polymer Dispersed Liquid crystals (PDLCs) are soft films consisting of liquid crystal (LC) domains embedded in a continuous polymer matrix [1]. Usually, in these composites the dimensions of the confined LC droplets vary from hundreds of nanometers till a few microns. For LCs with positive dielectric anisotropy, ε > 0, when the device is in the field “OFF” state, the LC molecules are randomly oriented and the PDLC film scatters the light. When applying an alternative electric field, the LC dipoles orient in the direction of the electric field, thus changing the refractive index of the LC droplet, which becomes (ideally) equal to that of the polymer, and the film turns transparent [1]. Besides the
a emails: [email protected]; [email protected] (corresponding author) b e-mail: [email protected] (corresponding author)
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applications of these devices as light shutt
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