Designed Interfaces in Polymer Nanocomposites: A Fundamental Viewpoint
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Linda S. Schadler, Sanat K. Kumar, Brian C. Benicewicz, Sarah L. Lewis, and Shane E. Harton Abstract Using nanocomposites in design-critical applications requires an understanding of their structure–property–function relationships. Despite many reports of highly favorable properties, the behavior of polymer nanocomposites is not generally predictable. The ability to tailor the filler/matrix interaction and an understanding of the impact of the interface on macroscopic properties are key to designing their properties. Tailoring can be achieved by grafting short molecules or polymer chains from the surface with precise control over their chain length (1–1000 mers), graft density (0.01–1 chains/nm2), and chemical architecture. The challenge is understanding the impact of the modified surfaces on the properties of the interfacial polymer, which can be more than 50% of the volume of the polymer matrix and, hence, can exert significant control over the macroscopic behavior of the nanocomposite. This article highlights the fundamental technical challenges that need to be overcome before spherical nanoparticle or nanotube composites can be designed. In particular, we discuss results from the recent literature that have significantly advanced our ability to predict and control nanocomposite properties through the use of designed interfaces.
Introduction Nanoscale fillers blended with polymers (“nanocomposites”) offer the real possibility of creating materials with properties that are not realizable with traditional, micron-scale fillers. These unusual properties arise because of three attributes of nanofillers. First, they can have properties distinct from micron-scale fillers. For example, carbon nanotubes are as stiff as graphite fibers, but are almost an order of magnitude stronger.1 Second, nanoscale fillers play the role of small mechanical, optical, and electrical defects. These provide an opportunity for multifunctionality (e.g., scratch-resistant, transparent polymers2). Third, they create a
large volume of interfacial polymer with properties different from the bulk, providing an opportunity for tailoring properties. It is imperative to have a fundamental understanding of each of these aspects to understand structure–property–function relationships in these materials and to design composites with specific properties. In this review, focus is placed on the third aspect, and the role of the interface in modifying the thermomechanical properties of the composite is critically examined. Because this area of research is extremely broad, the discussion is further focused on the case of curved nanofillers (spherical and cylindrical); these systems are of particular
MRS BULLETIN • VOLUME 32 • APRIL 2007 • www.mrs.org/bulletin
interest because they provide a contrast to the case of composites with micron-scale spherical or cylindrical fillers, which have been studied for nearly half a century. In traditional composites, the interface is critically important for controlling properties and has been the focus of significant
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