Small angle light scattering study of improved dispersion of carbon nanofibers in water by plasma treatment
- a The Key Laboratory of Rubber-Plastics (Qingdao University of Science and Technology), Ministry of Education, Qingdao 266042, China
- b Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA
- c Shandong University, Jinan 250061, China
- d The Institute for Advanced Nanomaterials, Tongji University, Shanghai, China
- e The Research Institute of Micro/Nano Science & Technology, Shanghai Jiaotong University, Shanghai 200240, China
- f Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA
- Received 2 January 2009, Accepted 13 April 2009, Available online 22 April 2009
Abstract
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.
1. Introduction
Carbon nanotubes are intrinsically one-dimensional structures that are chemically inert, and possess extraordinary mechanical and electronic properties [1] and [2]. Unfortunately, the advantages of carbon nanotubes have not been fully realized because the dispersion of carbon nanotubes is particularly intractable [3], [4], [5] and [6]. Although it is well known that plasma treatment modifies the surfaces of carbon nanotubes and thus improves the dispersion of carbon nanotubes, the measurement of the degree of dispersion remains challenging and the mechanism by which plasma treatment aids in the dispersion of carbon nanotubes is unknown. Here we use light scattering to quantify the dispersion of plasma-treated carbon nanofibers and elucidate their dispersion mechanism.
Deposition of a coating on carbon nanotubes could alter the surface properties of the nanotubes, offering a potential technological opportunity for improved dispersion. Plasma polymerization has been used in surface and interface engineering for improving adhesion, hydrophobicity and hydrophilicity, corrosion resistance, and surface etching [7]. Low-temperature plasma polymerization treatment, a room temperature and environmentally benign process, can be used for surface modification and thin film deposition on almost all substances. Deposition of ultrathin films of highly crosslinked polymers on the surface of carbon nanofibers (or nanotubes) by a plasma polymerization has been achieved [8] and [9]. However, the nature of the dispersed entities in various media and the evolution of the dispersed state under quiescent conditions following sonication are unknown.
Many types of carbon nanotubes exist and the terminology is not universal. Carbon nanofibers are similar to multiwalled carbon nanotubes, but the carbon nanofibers are larger in diameter and much lower in cost, thus making them more suitable to practical applications.
Scattering methods have recently been employed to provide structural information about nanotube morphology [10], [11], [12], [13] and [14]. In this work, we use small angle light scattering to infer the morphology of the plasma-treated vapor-grown carbon nanofibers and quantify the state dispersion of as-received and plasma-treated carbon nanofibers in water as a function of time. In order to better understand the state of aggregation of the nanofibers, we also estimate the size distribution from the light scattering data using the maximum entropy method [15] and [16]. We use the Irena code developed by Ilavsky and Jemian to get the maximum entropy solution [17] and [18].
The time evolution of the scattering data show that plasma treatment not only inhibits large-scale agglomeration, but also contributes to the morphological change of the short-scale bundling of fibers.
These observations have significant implications regarding the use of plasma-treated carbon nanofibers as a reinforcing filler to enhance the mechanical properties of polymer composites. The dispersion of untreated carbon nanofibers in matrix polymers has proven difficult, and the resulting composites do not show the anticipated properties. After plasma treatment, the dispersion of nanofibers in polymer matrices is greatly enhanced, thus leading to improved mechanical properties [9]. Light scattering study of plasma-treated carbon nanofibers suspended in media provides a morphological basis for improved performance of plasma-treated nanofiber reinforced polymer composites. Our observation is in good agreement with experimental investigation for enhanced dispersion of plasma-treated nanofibers in polymer composites [9].
2. Experimental
The carbon nanotubes we used are vapor-grown carbon nanofibers or simply carbon nanofibers. Vapor-grown carbon fibers have larger diameters than carbon nanotubes. Details regarding the nanofibers are given by Koerner et al. [19]. The vapor-grown nanofibers were provided by Applied Sciences Inc. Cedarville, OH. Pyrograf®-III PR19LHT nanofibers are vapor grown and subsequently heated to temperatures up to 3000 °C. The Pyrograf®-III carbon nanofibers normally contain a few concentric cylinders but may also be nested truncated cones. Typically the cores are open.
The carbon nanofibers (PR19HT) are plasma-treated using acrylic acid as a monomer. The plasma-coating facility is a homemade system. The plasma reactor for thin film deposition of nanoparticles has been introduced previously. The plasma reactor for plasma treatment of carbon nanofibers consists mainly of a radio frequency source, glass vacuum chamber and press gage [7], [20] and [21]. The vacuum chamber of the plasma reactor has a long Pyrex glass column about 80 cm in height and 6 cm in internal diameter. The powder was vigorously stirred at the bottom of the tube and thus the surface of particles can be continuously renewed and exposed to the plasma during the plasma polymerization processing. A magnetic bar was used to stir the powders. The gases and monomers were introduced from the gas inlet during the plasma cleaning treatment or plasma polymerization. Before the plasma treatment, the basic pressure was pumped down to less than 50 mtorr and then the carrier gas (such as argon) or monomer vapors were introduced into the reactor chamber. The operating pressure was adjusted by the gas/monomer mass flow rate. During the plasma polymerization processing, the input power was 20 W and the system pressure was 300 mtorr. The plasma treatment time was 10 min.
TEM samples were prepared by allowing a drop of nanofiber suspension to dry onto Cu grids with holy-carbon film. The high-resolution transmission electron microscopy (HRTEM) experiments were performed using a JEOL JEM 2010F electron microscope with a field emission source. The accelerating voltage was 200 kV.
The dispersion efficiency was determined using a small angle light scattering photometer–a Micromeritics Saturn Digitizer (www.micromeritics.com). The data are reported in reciprocal space as intensity vs. the magnitude of the scattering vector, q. Light scattering covers the regime 10−6 Å−1 < q < 10−3 Å–1, where q = (4π sin θ)/λ, θ being half the scattering angle, and λ being the wavelength of the radiation in the medium. This q range corresponds to length-scales (∼q−1) from 100 μm at low-q to 1000 Å (0.1 μm) at high q. Deionized water was used as background.
3. Results and discussion
The as-received PR19HT powder consists of loosely aggregated nanofibers. Some nanofibers are curved with open ends. A representative HRTEM image of the original Pyrograf®-III PR19HT (Fig. 1) shows the graphite structure with the interlayer spacing d = 0.34 nm. Their diameters range from 20 to 100 nm in TEM. No iron catalyst particles are found by TEM.
