Understanding the Thermodynamic Behavior of Nanoparticles to Tailor Polymer Nancomposite Structure

The work presented in this dissertation is an attempt to understand the entropic and enthalpic forces that govern the dispersion and dissolution of nanoparticles in solutions and in thin polymer films with the end-goal of producing highly tailored products.

In the first part, neutron reflectivity was used to study the impact of nanoparticle presence on the surface segregation of deuterated polystyrene (dPS) in a polystyrene matrix. The impact of the presence of cylinders (carbon nanotubes), sheets (graphene), and spheres (polystyrene soft nanoparticles) on the surface segregation process and ultimate structure were examined. Experimental data indicate that the presence of the nanoparticles slows the dPS diffusion in all cases, and the soft nanoparticles, which contained branching and more chain ends than the dPS linear polymer matrix, are entropically driven to the air surface, resulting in a decreases of excess dPS at the surface and a decrease in free energy of the system. Graphene had the opposite effect, segregating to the silicon surface due to a higher surface energy and enhancing the dPS segregation to the air surface.

The next part focuses on developing a protocol using static light scattering and refractometry to quantitatively determine the solubility behavior of boron containing nanoparticles. With scattering, the second virial coefficient is obtained and used to calculate the solute-solvent interaction parameter, [chi], which quantifies the mixing behavior. UV-Vis spectroscopy and physical observations were also used to describe the systems. The solubility behavior of carboranes, boron nitride nanotubes and sheets, and single walled carbon nanotubes (SWNTs) were quantified. In all cases there is good agreement between the measured data and [chi]. Suitable solvents were also predicted based on the calculation of the Hildebrand solubility parameter, [delta]. Use of [delta] to predict solubility shows good agreement for the smaller particles, but is more suspect for the nanotubes and sheets due to additional entropic factors.

Finally, two purification techniques for SWNTs, acid purification and purification via centrifugation in surfactant, were examined. Experimental evidence indicates that centrifugation leads to the isolation of more pristine tubes, appropriate for applications that require increased electrical conductivity.

The solubility of nanoparticles in solution is quantified using static light scattering and refractometry (to obtain the solute-solvent interaction parameter and refractive index increment), and purification techniques are examined for single walled carbon nanotubes.