Elucidating the Importance of Structure, Surfaces, and Interfaces in Polymer Nanoparticles and Nanocomposites

This dissertation details research conducted to elucidate the importance of structure, surfaces, and interfaces in both polymeric nanoparticles and polymer nanocomposites. The fundamental understanding that is garnered in these studies provides a foundation to rationally develop nanocomposites tailored for unique functionalities, performance and applications.

Soft polymeric nanoparticles, have shown to imbue non-traditional diffusive properties, the strength of which decreases with crosslinking density of the nanoparticle. The crosslinking dependent morphology of these nanoparticles is first characterized in a dilute solution of good solvent (Chapter 2). The scattering results revealed that the structure ranges from a swollen polymer in good solvent (0% XL), to a collapsed polymer in theta solvent (0.4%XL), to unequivocally particle-like ( ≥ 0.8%XL). The transition to a particle-like morphology hinges on the clear presence of a measurable surface. To better understand the mechanisms behind their non-traditional diffusive properties, the internal dynamics of these nanoparticles were probed (Chapter 3). While globally exhibiting Zimm-like dynamics, the nanoparticles showed a heterogeneity of local internal dynamics, exhibiting significantly slowed dynamics on length scales that contained crosslinks and linear-like dynamics over length scales where crosslinks were mostly absent. The clear separation of internal dynamics magnifies the importance of the nanoparticle’s core-shell structure.

The consequences of surfaces and interfaces within polymer nanocomposites is also explored. The permanency of a bound polymer layer is characterized by monitoring the time evolution of the volume fraction profile of an adsorbed polymer layer (Chapter 4). While the total thickness of the layer remained unchanged, the composition varied indicating that individual chains are not necessarily “irreversibly” adsorbed. Additionally, the molecular weight dependence on the kinetics of chain desorption are studied, finding that desorption in the melt transitions from diffusion limited to a combination of diffusion and surface detachment limited with increasing molecular weight. Finally interfaces between a fire-retardant small molecule and polymer is compatibilized using a polymeric dispersant (Chapter 5). The average and homogeneity of particle size is improved in melt mix blends of the three components as the polymer dispersant can disrupt the intramolecular hydrogen bonding within the flame-retardant with intermolecular interactions.