Dielectric Properties of Electrostatic Self-Assembled (ESA) Films
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Dielectric Properties of Electrostatic Self-Assembled (ESA) Films Ramazan Asmatulu, Brian Geist, William B. Spillman, Jr., and Richard O. Claus Virginia Tech Applied Biosciences Center, 106 Plantation Road, Virginia Polytechnic Institute and State University 0356, Blacksburg VA 24061 ABSTRACT Capacitance measurements were conducted to determine the dielectric constant of electrostatic self assembly (ESA) films over various temperature and frequency ranges at 1 mV and 15% relative humidity. Measurement electrodes were fabricated on the ESA films using silver grease and a brass bar. In these tests, polymeric nanofilms were fabricated on gold-coated glass slides using the ESA process. Thicknesses of the films were between 100 nm and 600 nm obtained by depositing different number bilayers of negatively charged Poly s-119 (PS-119) or heparin and positively charged poly(diallyldimethylammonium chloride) (PDDA). The test results showed that dielectric constant values were around 2. Based on the test results, we concluded that this is a technique that might prove useful to estimate the capacitance and dielectric constant values of nanostructured ESA films, which can be largely used in the near future. 1.INTRODUCTION Thin film coatings are widely used in many applications for mostly conductivity / isolation, corrosion protection, impact/wear resistance and biocompatibility/biodegradability of material surfaces. Since the 1990s, a growing need has arisen for electrostatic self-assembled (ESA) films that can be utilized in many specific applications (e.g., new tools and machine parts), as well [1,2]. These films can be fabricated on glasses, metals and alloys, ceramics, polymers and their compounds using a dipping technique to modify the performance (optical, electrical, tribological, thermal, magnetic and electrostatic) of the materials, and also protect the surface from the environmental effects (e.g., humidity, ultraviolet light, temperature, static and dynamic forces, etc.) [1-6]. There is a basic concept behind the ESA process, shown in Figure 1. On the left of the Figure 1, the substrate surface has been thoroughly cleaned and electrically charged through chemical processing. Typically, cleaning of the substrate exposes functional groups at the surface, and thus effectively produces a net electrically charged molecular layer. The charged substrate is then dipped into a solution containing cationic (e.g., poly(diallyldimethylammonium chloride) - PDDA) that are attracted to the anionic surface (e.g., gold, glass, etc.) and selfassemble into a single layer of molecules [1-4]. Physical, chemical and physicochemical results show that the molecular order of such individual monolayers is nearly perfect, in much the same way that electrical charge on a free conducting surface distributes itself uniformly over the surface in order to minimize total system energy. Subsequent anionic (e.g., PS-119, heparin, etc.) and cationic monolayers are added in bilayer pairs, by alternately dipping the substrate into those positively a
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