FTIR Characterization of the Reactive Interface of Cobalt Oxide Nanoparticles Embedded in Polymeric Matrices Rina Tannenbaum,* ,² Melissa Zubris, ² Kasi David, ² Dan Ciprari, ² Karl Jacob, ‡,§ Iwona Jasiuk, ⊥ and Nily Dan # School of Materials Science and Engineering, School of Polymer, Textile and Fiber Engineering, School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, Department of Mechanical Engineering, Concordia UniVersity, Montreal, Quebec, Canada, and Department of Chemical Engineering, Drexel UniVersity, Philadelphia, PennsylVania ReceiVed: August 10, 2005; In Final Form: December 6, 2005 Fourier transform infrared spectroscopy (FTIR) was used as a novel characterization method to determine the properties of the interface that developed when cobalt oxide nanoparticles were self-assembled in a poly- (methyl methacrylate) (PMMA) matrix. The method employed the distinct changes that were observed in the infrared spectra of the polymer upon adsorption onto the cobalt oxide nanoparticles, allowing a quantitative determination of the average number of contact points that the average polymer chain formed with the surface of a cobalt oxide nanoparticle of average size. The results obtained with this method compared favorably to those ob- tained by the coupling of transmission electron microscopy (TEM) experiments with thermogravimetric analysis (TGA). On the basis of both methods, we concluded that the interfacial region created between the cobalt oxide nanoparticles and PMMA is extremely sensitive to the chain length, i.e., the number of anchor points and the density of the polymer layer increase with chain molecular weight. At molecular weights of ∼250 000, the density of the polymer layer saturates at a value that correspond to that of very thin PMMA films. 1. Introduction Nanostructured materials consist of phases with dimensions in the nanometer size range (1 to 100 nm). This is the range where atomic and molecular phenomena strongly influence the macroscopic material properties. It has been shown that the mechanical, 1-3 electronic, 4-6 magnetic, 6-8 and optical properties of a material vary as a function of the nanoscale domain size, while they are less sensitive to the size of micron-scale domains. As a result, composite materials in which nanoparticles, 6,9-16 nanofibers, or nanotubes 17-20 are dispersed in a matrix of another material, frequently display different material properties than composites based on larger particles. 21-26 The definition of nanocomposite materials encompasses a large variety of systems made of distinctly dissimilar compo- nents, where at least one component of the composite has nanometer size dimensions. 17-26 The general class of organic/ inorganic nanocomposite materials is a fast growing area of research. Significant effort has been focused on the ability to obtain control of the structure of nanoscale materials in the composite via innovative synthetic approaches. 28-34 The properties of nanocomposite materials depend not only on the properties of their individual components but also on their organization in the composite as well as their interfacial characteristics. The latter is especially important in these systems where the ratio between the surface area and the volume is high. Indeed, several studies indicate that nanocomposites can display new properties which are not present in the constituent parent materials. 1-8 One of the key factors influencing the macroscopic material behavior of composites, e.g., the load transfer between matrix and filler, 35-38 is the interfacial region between these two components. Reducing the size of filler while preserving the filler volume fraction leads to a dramatic increase of the volume fraction of the interfacial region (interphase). The interfacial region in a polymer nanocomposite, depicted schematically in Figure 1a, has distinct structural features, depending on the strength of the interactions between the filler particles and the polymer matrix. A weak attractive interaction will result in the extension of the polymer chains into the bulk polymer matrix, creating a diffuse interface, as shown in Figure 1b. Conversely, a strong attractive interaction involves the effective adsorption of polymer chains onto the surface of the fillers via bonding of functional groups of the polymer with reactive sites on the surface of the nanofillers. 39 Such an interaction will result in a flat (i.e., dense) configuration of the polymer on the surface of the nanofillers and a compact interface, as shown in Figure 1c. In addition, the strength of the interaction of a polymer molecule with the surface of the filler controls also the polymer molecular conformations and resulting interactions (e.g., entanglement distribution) in a larger region surrounding the filler (as shown in Figure 1a). Thus, control of the particle/polymer interface will enable control of the mechanical and structural properties of the nanocomposite materials. As shown in Figure 1, the particle/polymer interface may be characterized through several parameters such as the number of “adsorbed” chains (namely, chains that have at least one segment in contact with the particle), the thickness of the * Address correspondence to this author. E-mail: rina.tannenbaum@ mse.gatech.edu. ² School of Materials Science and Engineering, Georgia Institute of Technology. ‡ School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology. § School of Mechnaical Engineering, Georgia Institute of Technology. ⊥ Department of Mechanical Engineering, Concordia University # Department of Chemical Engineering, Drexel University. 2227 J. Phys. Chem. B 2006, 110, 2227-2232 10.1021/jp054469y CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006