Structure and Properties of Nanocomposites Containing Anisotropic Nanoparticles

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This dissertation deals with polymeric materials containing dispersed anisotropic nanoparticles such as nanotubes, nanofibers, and nanoplatelets. Such polymer nanocomposites have attracted much attention since they exhibit a host of superior properties over the parent polymer. For example, nanoparticles can impart flame-retardancy, high electrical conductivities, and high mechanical stiffness to the polymer. Despite the growing interest in these materials, many aspects remain poorly characterized, and the connection between properties and microstructure is still not fully understood. This provides the motivation for the present study.

In the first part of this study, we focus on the flammability behavior of polymer nanocomposites containing multi-walled carbon nanotubes (MWNTs). It has been shown that MWNTs impart flame-retardancy to the polymer at low loadings, and moreover, the flame-retardancy correlates with the rheological properties of the nanocomposite. Here, we show that the aspect ratio of MWNTs is a key parameter in controlling both the rheology and flammability. Particles with a larger aspect ratio impart much higher storage moduli and complex viscosities to the nanocomposites compared to equivalent mass loadings of particles with a smaller aspect ratio. Additionally, in flammability experiments, the larger-aspect-ratio particles lead to a greater reduction in mass loss rate, i.e., they are more effective at reducing flammability. 

In the second part of this study, we focus on the conductivity of nanocomposites containing particles such as MWNTs or carbon nanofibers (CNFs). When these materials are processed by compression molding or melt extrusion, the conductivities of the resulting composites are often found to be disappointingly low. Here, we show that the conductivities can be increased, sometimes by orders of magnitude, simply by subjecting the sample to quiescent annealing at temperatures above the polymer's glass transition temperature (Tg). We demonstrate these results for both MWNT and CNF-based composites in polystyrene (PS). The mechanism behind the conductivity increase is shown to involve an increase in the connectivity of the particle network, which is reflected in dynamic rheological measurements as an increase in the plateau modulus at low frequencies. 

In the final part of this study, we present a simple method to improve the rheology and flammability properties of nanocomposites formed from polymers and clay platelets. These materials are usually made by combining a polymer with a commercial organoclay 

powder. We show that by fractionating the clay to exclude low-aspect ratio particles and aggregates, we can improve their dispersion (exfoliation) in the polymer. The resulting composites have higher optical transparency and better rheological properties for a given mass loading of clay. When these composites are subjected to a flame, we find a more uniform residue when compared to samples with the commercial organoclay. The fractionated clay also shows an interesting behavior when dispersed in water, where it forms birefringent gels at high particle loadings.