Polymer nanocomposites (PNCs)–that is, nanopar ticles (spheres, rods, plates) dispersed in a polymer matrix–have garnered substantial academic and industrial in terest since their inception, circa 1990. This is due in large part to the incredible promise demonstrated by these early efforts: PNCs will not only expand the per form ance space of traditional filled polymers, but introduce completely new combinations of properties and thus enable new applications for plastics. Low volume additions (1–5%) of nanopar -ticles, such as layered silicates or carbon nanotubes, provide property enhancements with respect to the neat resin that are comparable to those achieved by conventional loadings (15–40%) of traditional fillers. The lower loadings facilitate proc essing and re duce component weight. Most important, though, is the unique value - added properties not normally possible with traditional fillers, such as reduced permeability, optical clarity, self - passivation, and increased re sis tance to oxidation and ablation. These characteristics have been transformed into numerous commercial suc cesses, including automotive parts, coatings, and flame retardants. This issue of the MRS Bulletin provides a snapshot of these exemplary successes, future opportunities, and the critical scientific challenges still to be addressed for these nanoscale multiphase systems. In addition, these ar ticles provide a perspective on the current status and future directions of polymer nanocomposite science and technology and their potential to move beyond additive concepts to designed ma te rials and devices with prescribed nanoscale composition and morphology.

Layered smectite nanoclays are being developed for incorporation into a variety of host polymer systems. Nanoscopic phase distribution can impart enhanced stiffness at low addition levels and improve barrier and flame-retardant properties. When combined with other inorganic and organic modifiers, nanoclays can provide synergies to generate the desired formulation properties and cost/per form ance characteristics. Developments with existing nanoclay products using conventional amine chemistries are described for thermoplastic, thermoset, and rubber formulations. Nanoclays are demonstrating unique, multidimensional per form ance and proc essing capabilities. Commercial applications are emerging in a variety of diverse markets ranging from automotive to packaging.

One important application of nanocomposites is their use in engineered structural composites. Among the wide variety of structural applications, fiber-reinforced composites for aerospace structures have some of the most demanding physical, chemical, electrical, thermal, and mechanical property requirements. Nanocomposites offer tremendous po tential to improve the properties of advanced engineered composites with modest additional weight and easy integration into current proc essing schemes. Sig nificant progress has been made in fulfilling this vision. In particular, nanocomposites have been applied at numerous locations within hierarchical composites to improve specific properties and optimize the multifunctional properties of the overall structure. Within this ar ticle, we review the status of nanocomposite incorporation into aerospace composite structures and the need for continued development.

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 inter face on macroscopic properties are key to designing their properties. Tailoring can be achieved by grafting short mole cules 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 ar ticle highlights the fundamental technical challenges that need to be overcome before spherical nanopar ticle 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.

Controlling the dispersion of nanoparticles in polymeric matrices is the most significant impediment in the development of high-perform ance polymer nanocomposite ma te rials and results primarily from the strong interpar ticle interactions between the nanopar ticles. This review examines the theoretical and experimental strategies employed in developing appropriate chemical and physical methods to achieve controlled dispersion of nanopar ticles. Methods to characterize the state of dispersion, including force and electron micros copy, and scattering, electrical, and mechanical spectroscopy, are considered with special emphasis on achieving quantitative meas ures of the dispersion. Some of the outstanding issues, such as long-term aging and the implication for the dispersion of nanopar ticles, development of high-throughput methods for rapid screening, and methods for in-line monitoring, are also discussed.

The remarkable electrical and thermal conductivities of isolated carbon nanotubes have spurred worldwide interest in using nanotubes to enhance polymer properties. Electrical conductivity in nanotube/polymer composites is well described by percolation, where the presence of an interconnected nanotube network corresponds to a dramatic increase in electrical conductivity ranging from 10−5 S/cm to 1 S/cm. Given the high aspect ratios and small diameters of carbon nanotubes, percolation thresholds are often reported below 1 wt% although nanotube dispersion and alignment strongly influence this value. Increases in thermal conductivity are modest (∼3 times) because the inter facial thermal re sis tance between nanotubes is considerable and the thermal conductivity of nanotubes is only 104 greater than the polymer, which forces the matrix to contribute more toward the composite thermal conductivity, as compared to the contrast in electrical conductivity, >1014. The nanotube network is also valuable for improving flame-retardant efficiency by producing a protective nanotube residue. In this ar ticle, we highlight published research results that elucidate fundamental structure–property relationships pertaining to electrical, thermal, and/or flammability properties in numerous nanotube-containing polymer composites, so that specific applications can be targeted for future commercial success.

Bionanocomposites have established themselves as a promising class of hybrid materials derived from natural and synthetic biodegradable polymers and organic/inorganic fillers. Different chemistries and compositions can lead to applications from tissue engineering to load-bearing composites for bone reconstruction. A critical factor underlying biomedical nanocomposite properties is interaction between the chosen matrix and the filler. This ar ticle discusses current efforts and key research challenges in the development of these ma te rials for use in potential biomedical applications.

A single printed circuit board includes thousands, sometimes even hundreds of thousands, of solder joints. The failure of even a single solder joint is usually enough to compromise the functionality of an electronic device or system. PbSn solder had been the standard ma te rial for these joints until various regulations around the world began to limit Pb use. SnAgCu and related alloys are quickly replacing PbSn, but much still needs to be understood and controlled. None of the paradigms for understanding the mechanical response of PbSn alloys is applicable to lead-free alloys. Much of the surprising behavior of SnAgCu solder arises from the complex and fascinating nature of its solidification behavior. In this ar ticle, the impact of solidification on the microstruc ture and therefore the mechanical properties of these solder joints will be addressed in the context of microelectronics proc essing. The need for better simulations of SnAgCu solder behavior will also be examined. Notably, modelers will have to account for a variety of new parameter dependencies not previously considered.