Polymer nanocomposites with prescribed morphology: Going beyond nanoparticle-filled polymers

Polymer nanocomposites (PNCs), i.e. nanoparticles (spheres, rods, and plates) dispersed in a polymer matrix, have garnered substantial academic and industrial interest since their inception, ca. 1990. With respect to the neat matrix, nanoparticle dispersion has been shown to enhance physical (e.g., barrier, erosion resistance, and reduced flammability), thermomechanical (e.g., heat distortion temperature, thermal expansion coefficient, and stiffness), and processing (e.g., surface finish and melt strength) characteristics. Beyond maximization of the nanoparticle dispersion, however, the morphology of these materials is many times uncontrolled, yielding isotropic nanofilled systems, not necessarily spatially engineered, designed and tailored materials. To impact high-technology applications requiring unique electrical, thermal, and optical properties, manufacturing techniques enabling control of the nanoparticle arrangement and distribution must be developed. This paper will examine the status of approaches for directing the hierarchical morphology of nanoparticle dispersions in three dimensions, and beyond uniaxial alignment, using examples from the literature to highlight the potential and issues. Ultimately, two general approaches to this challenge are emerging, namely, external-in (directed patterning of nanoparticle dispersions) and internal-out (mesophase assembly of nanoparticles).