Characterization of thin transparent polymeric films obtained by plasma polymerization technique and their application to liquid crystal cells

Abstract

The aim of this work Although liquid crystal displays (LCD) are quite ubiquitous in the modern world, there is still a great run for better and cheaper LCD. The main physical phenomenon that makes LCD as valuable consists in the particular way polarized light propagates through anisotropic media in general and liquid crystals in particular. Different from solid anisotropic media, the actual anisotropy of liquid crystals is imposed by surface interactions. The concepts of aligning layer and anchoring have been coined. Normally, a thin lightly dielectric polyimide film separates the liquid crystal from conducting transparent electrodes. An applied electric field between these electrodes can reorient the liquid crystal inside (the bulk) and change the transmittance of the cell. An electric field can rather easily do so such that the response time to the applied film, τon, is normally less than 1 ms. Switching off the field, liquid crystal relaxes to the initial state, the only “driving force” now, in the absence of the electric field, remains the anchoring of the liquid crystal to the surface. If this anchoring is not very strong the relaxation time, τoff, can be as large as seconds, a unsuitable value for practical purposes. Much stronger anchoring overwhelms this shortcut by the expense of using thin film transistors, difficult to insert and quite costly. Not long ago, a “fast switching response” has been observed using conducting polymers. [1] as aligning films. Conducting electro active polymers such as polypyrrole (PPyr), Polyaniline (PAn), polythiophene (PTh), or poly-o-anisidine (PoA) are complex dynamic structures that captivate the imagination of those involved in intelligent materials research [2]. Although promising response times, τoff 1. The chemical nature of the substance used, for instance polyaniline, or polypyrrole; , of 1-2 ms, even 0.5 ms, have been observed, the rate of defected samples is unacceptable high. Therefore, there is a strong incentive to carry on investigation in the field. There are many parameters that should be considered; among them we quote: 2. The way of inducing the polymerization process, either chemically, electrelectro-chemically, by DC or Rf plasma reactor; 3. the nature and number of doping or included ions; 4. Their mobility 5. Possible red-ox reactions at ITO/polymer and/or polymer/liquid crystal interfaces; 6. Thickness of the aligning films 7. Roughness or porosity of the film In this thesis we will present all the results obtained with these films about “the fast switching response”, a characterization study made with various instruments like SEM, AFM ecc. on these films and other measurements like current curves on the LC cells made with these films, all realized for better understand the properties of these films deposited via DC plasma polymerization