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This work attempts to clarify the influence of surface roughness on the thermodynamic interactions between carbon particles and polymer melts. The surface energy of carbon black (CB) and short carbon fibers (VGCF) having different surface roughnesses was estimated by inverse gas chromatography (IGC) and highly sensitive isothermal calorimeter (HS-ITC) measurements using lowmolecular-weight analogues of polymers as probes. We confirmed that the carbon surfaces possess energetic heterogeneity with the most active sites at the graphite crystalline edges, and the interactions in play are van der Waals in nature. Competitive adsorption of two chemically different polymers by incorporation of the carbon particles into the polymer blends was investigated based on SEM and TEM observations. We found that the selective location of CB in the polymer blends does not always depend on the surface tension of polymers but seems to be governed largely by the flexibility of the polymer chains. In VGCFfilled HDPE/PMMA composites, a self-assembled VGCF/HDPE network throughout the PMMA matrix was observed where the flexible HDPE chains are preferentially absorbed at the rough ends of the VGCF filaments. These experimental results lead to the conclusion that surface roughness strongly affects the carbon-polymer interactions, and the entropy penalty may play a main role in competitive adsorption of polymers on the rough carbon surfaces.

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Morphology and electrical properties of short carbon fiber-filled high-density polyethylene (HDPE)/poly(methyl methacrylate)(PMMA) polymer blends have been studied. The percolation threshold of HDPE50/PMMA50 blends filled with vaporgrown carbon fiber (VGCF), 1.25 phr VGCF content, is much lower than those of the individual polymers. The SEM micrographs verified that the enhancement of conductivity could be attributed to the selective location of VGCF in the HDPE phase. A double percolation is the basic requirement for the conductivity of the composites, i.e., the percolation of carbon fibers in the HDPE phase and the continuity of this phase in the blends, which hereby are defined as the first percolation and the second percolation, respectively. The SEM micrographs also showed that the short carbon fibers could affect the morphology of the blends. With the increase of VGCF content, the HDPE domains are elongated from spherical into strip shape, finally develop to a continuous structure. As a result, the second percolation threshold of the blends filled with 2.5 phr VGCF, 20 wt % HDPE, is lower than that of the blends filled with 1.5 phr VGCF, 30 wt % HDPE. The influence of molding temperature and time on the second percolation threshold has also been investigated. For the composites molded at a lower temperature, the second percolation threshold is shifted to a higher VGCF content, but there is little influence of molding time on the second percolation threshold.