We examine the structure of densely grafted polymer layers grown from the surfaces of spherical nanoparticles over a broad range of graft densities and chain lengths. Dynamic light scattering (DLS) experiments show that the hydrodynamic thickness of the polymer layer, h, scales as h µ N 4/5 for short chains and high grafting densities, that is, in the concentrated polymer brush (CPB) regime, whereas h µ N 3/5 for long enough chains (semidilute polymer brush, SDPB). The mean field theory of Wijmans and Zhulina is able to collapse approximately all of our data and those in the existing literature (even on other polymers) into an apparently universal form. From these findings, we conclude that the result h µ N 4/5 for the CPB is an intermediate crossover scaling, relevant to particles of finite curvature, analogous to the h µ N observed for concentrated flat brushes. Second, the scaling h µ N 3/5 uniquely reflects the increased space available to the chain segments as one proceeds away from a curved grafting surface. Under these situations, the chains experience less packing frustration as compared to a planar brush, and the semidilute polymer brush shows scaling behavior analogous to chains in good solvent, even though the chains are much more extended.

We critically explore the role of particle dispersion on the melt state mechanical properties of nanocomposites formed by mixing polystyrene homopolymers with polystyrene grafted silica nanoparticles. We selected this system since we previously showed that nanoparticle spatial distribution can be controlled through judicious choices of the brush and matrix parameters. Here we focus on the temporal evolution of the nanoparticle self-assembly dispersion state and its effect on mechanical reinforcement using rheology, electron microscopy, and the measurement of nanoscale particle dynamics using X-ray photon correlation spectroscopy. Nanoscale and macroscopic experiments show that a composite with percolating sheets of particles displays "gel-like" or solid-like mechanical behavior at lower particle loadings than one with uniform particle dispersion. This conclusion allows us to conjecture that mechanical reinforcement is primarily controlled by interparticle interactions (including those facilitated by the grafted chains) and that the matrix plays a relatively minor role. This statement has far-reaching consequences on the design of polymer nanocomposites with desired properties.

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.

A small amount of alumina nanoparticles in polymethylmethacrylate causes a sharp depression of the glass transition temperature (Tg) accompanied by a toughening of the composite. We investigated this phenomenon using multispeckle x-ray photon correlation spectroscopy. Measurements reveal a dynamic structure factor that has the form exp[-(t/taua)beta], with beta greater than 1. We show for the first time that beta(T) tracks the internal stress at the polymer-particle interface. The internal stress, which we propose arises due to the entropic penalty that the polymer faces in the presence of the nanoparticles, engenders temporally heterogeneous dynamics. In the jammed glassy state, we show that the dominant fast relaxation mode--taumax--aided by a weak dewetting interface relieves the stress and follows the variations in Tg.

The use of polymer derived ceramics (PDCs) has been slowly increasing in the aerospace and friction markets as matrix materials for ceramic composites. They have demonstrated exceptional properties as brake rotors and wear parts but their cost has severely limited large scale insertions into the market. A new family of lower cost ceramic forming, siloxane based polymers has been developed that yields ceramic materials similar in behaviour to higher cost carbosilane derived ceramics. This paper discusses the new PDC class and provides a comparison demonstrating the similarity in properties. Both polymers have been utilised as the matrix for brake rotors and the lower cost PDCs have shown slightly better friction performance in an automotive brake rotor application.