Surface characteristics and wetting behavior of carbon nanotubes
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Surface characteristics and wetting behavior of carbon nanotubes Asa H. Barber, Luqi Liu, Sidney R. Cohen1 & H. Daniel Wagner Department of Materials and Interfaces & 1Chemical Research Support Weizmann Institute of Science Rehovot 76100 ISRAEL ABSTRACT The wetting properties and surface characteristics of individual carbon nanotubes are elucidated by immersing the nanotube into various organic liquid. The resultant force acting on the nanotube can be used to evaluate a liquid contact angle at the nanotube surface from classical methods. This technique was shown to be accurate enough to discern differences in wetting behavior due to both structural and chemical changes in the nanotube structure. INTRODUCTION Carbon nanotubes show promise as a reinforcing phase in composite materials due to their excellent mechanical properties [1-6]. For effective composite reinforcement, the adhesion between the nanotube and surrounding matrix, which is usually polymeric, must be good. The properties of an interface between the polymer and nanotube will be strongly dependent on those of the nanotube surface. Therefore, as with typical engineering fibers, it is important to evaluate the surface properties of nanotube materials. The characterization of fibers surfaces in general has used a wide variety of methods [7]. However, the most important technique has been to probe the wetting behavior of fiber surfaces. Typically this is achieved by placing a droplet of the matrix polymer onto the fiber surface and observing the contact angle. The classical Young's equation can then be used to describe the solid-liquid interaction from surface energy considerations thus:
γ SV = γ SL + γ LV cosθ
(1)
where γSV, γSL and γLV are the surface energies at the solid-vapor, solid-liquid and liquid-vapor interfaces, and θ is the well known liquid contact angle with the solid surface. Previous studies have sought to relate the polymer liquid contact angle on a fiber to overall composite mechanical properties, with good correlations found [8-11] due to improved interfacial adhesion. Carbon nanotubes present problems when attempting to evaluate their wetting characteristics as the common method of observing liquid wetting is more difficult due to the extremely small nanotube dimensions. Therefore, any observations require high resolution imaging, typically electron microscopy [12, 13], which can further complicate experiments because of the relatively high vacuum required. A more promising method for evaluating wetting of fibers is the Wilhelmy plate technique [14]. The basis of this technique involves the measurement of forces acting on a fiber when it is partially submerged within a liquid. For favorable wetting, where the
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contact angle of the liquid with the fiber is less than 90°, the force acting on the fiber and pulling it into the liquid (Fr) is related to the surface tension of the liquid (γLV) by:
Fr = γ LV πdcosθ
(2)
Where d is the external diameter of the fiber. This technique has been successfully applied to closed
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