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The contact angle between multiwall carbon nanotubes, carbon nanofiber and carbon fibers, and polypropylene and polyethylene glycol was measured using electron microscopy. Good wetting was observed using both polymers. The Owens–Wendt plotting procedure was used to obtain the polar, dispersive and total surface energy parameters for the three types of carbon materials. Minor
differences only were found between these.

Carbon nanotube-based composites have generated considerable interest over the past few years in the materials research community because of their potential for large increases in strength and stiffness relative to conventional carbon fiber-reinforced polymer composites [1,2]. Key aspects of the performance of nanotube- based composites include the extent to which a
tube can be wetted by a given polymer. The interface between matrix and reinforcement plays a crucial role in the physical performance of the composite. In particular, the nature of the interface is dominant in fracture toughness, corrosion and moisture resistance. In the case of non-functionalized carbon nanotubes the contribution of wetting to adhesion is considered to
be significant. Wetting of nanotubes by the surrounding media is necessary in order to couple the inherent strength of the nanotubes to the matrix, unless direct chemical bonding is induced. If the interface is weak the composite has low strength and stiffness, but high
resistance to fracture. In the case of strong interfaces the composite has high strength and stiffness, but is brittle. As the wetting properties of a fiber may differ significantly from those of a plane solid surface [3], some theoretical studies predict that this effect becomes
more prominent as the fiber diameter decreases and a transition from partial wetting to non-wetting will occur at the nanometric scale [4,5]. Qualitative studies show excellent wettability of the graphitic inner tube walls by water [6]. Dujardin et al. [7,8] used
transmission electron microscopy to measure the contact angle of metals on carbon nanotubes and estimate the energy surface parameters both for single wall and multiwall carbon nanotubes and found that the transition from non-wetting to at least partial wetting occurs
with liquids that have surface tension between  130–190 mJ/m2. Barber et al. [9] utilized atomic force microscopy and the Wilhelmy balance method to quantify the contact angle between nanotubes and a variety of organic liquids and showed that most organic liquids at least partially wet a nanotube surface, although this becomes less favorable as the liquid becomes
more polar. Such work is technically demanding and time consuming to execute, hence, a more
direct and straightforward method is desirable. In the present Letter, the contact angle between molten polymers subsequently cooled on graphite fibers, nanofibrils and multi-walled carbon nanotubes ismeasured using electron microscopy. Thus, the wetting
behavior of polymer melts on these carbon surfaces
may be evaluated.