Thin Conducting Polymer Films: Effect of Thickness on Wetting Behavior of Polyaniline Ultra Thin Films
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ABSTRACT Wetting properties and surface structure of polyaniline (PANI) thin films were studied as a function of the polymer thickness, using AFM and neutron reflectivity. Increasing the oxidation state as well as polymer thickness was found to increase the tendency of the polymer to form polycrystalline films. Three main regions with characteristic behavior were observed: ultra thin films of the order of several molecular layers, an intermediate thickness film of several molecular layers to -150nm, and films thicker than 150nm. In the ultra thin region the polymer formed nano-scale spherical domains. In the intermediate one, the films tend to be continuous and amorphous. In the thick films, crystallinity was observed. INTRODUCTION The stability and structure of ultra-thin polymer films on a solid substrate are of significance both technologically, for applications ranging from coating, electrical layers to lubrication, and fundamentally, for understanding the interfacial behavior of polymers. A number of factors govern the stability of thin films including properties of the film itself and the interactions of the polymer with the surrounding. Film thickness, viscosity, surface tension and interaction with the solid surface are among the stability controlling parameters [1]. The present study focuses on the effects of film thickness of polyaniline (PANI), a rigid conducting polymer, on the stability and structure of the film. A break-up of a thin liquid film, as well as thin flexible polymer films can be described in terms of capillary instability mechanism [2,3] in which the film is subject to thermal fluctuations [3-7]. When the fluctuations exceed a minimum threshold, the film breaks up. The dewetting behavior of flexible and semi-flexible polymers has been the topic of several studies [5-8]. In the linearized capillary wave instability model, thermally induced thickness fluctuations are exponentially amplified for wave vectors less than a critical wave vector qc. The fastest fluctuations have a time independent wave vector qm=(3/2) /2(a/h)2 and a rate of growth, Rm= 3ya4/(4rlh5 ), where a2=IAV(6iry), A is the Hamaker constant, y is the surface tension, ri is the viscosity, and h is the layer thickness [2,3]. The current work introduces the interfacial behavior of thin films of a rigid polymer, polyaniline. In addition to being a model for rigid polymers, polyaniline is one of the most studied conducting polymers. One of the major goals in research of conducting polymers is to search for materials with electrical conductivity comparable to
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Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
or higher than that of good conductors. Some theoretical results suggest that the intrinsic conductivity of conducting polymers might exceed that of Cu. However, in real systems, this conductivity is limited by disorder and other defects. Understanding the basic features of the film/fiber and its formation, would be an immense step towards a methodical way of making polymer films/fibers exhibiting wel
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