Ultrathin polymer film is deposited on the surfaces of vapor-grown carbon nanofibers by a
plasma polymerization using acrylic acid as a monomer. Small angle light scattering is used to investigate the dispersion behavior of the carbon nanofibers suspended in water and provides information on the mechanism by which plasma treatment assists dispersion. Both plasma-treated and untreated nanofibers exhibit a hierarchical morphology consisting of small-scale aggregates that agglomerate to form fractal clusters that eventually precipitate. The time evolution of small-scale aggregation and large-scale agglomeration is studied by fitting the scattering data to a unified model. The morphology of the small-scale aggregates is also studied by extracting the size distribution from the angle-dependence of the scattered intensity, using the maximum entropy method in conjunction with a simplified tube form factor. The aggregates are side-by-side bundles of individual nanofibers or more complex structures. Plasma treatment not only contributes to breaking up of the small-scale aggregates into smaller sizes but also inhibits their agglomeration. For untreated fibers, large agglomerates appear immediately after sonication and their size remains almost unchanged during the precipitation process. For treated fibers, precipitation dominates during the first 8 h, leaving small entities in suspension which form agglomerates after a few days.
