Nanoparticle Composite Materials: Controlling Nanoparticle Organization and Nanoparticle Effects on Polymer-Composite Dynamics

<p>One of the main challenges of the materials science is synthesizing materials with custom designed properties. One approach to address this challenge is creating composite materials. Combining nanoparticles (NP) and polymers is a common way to realize such composites. Due to the interesting properties of NP (eg. quantum confinement and high surface-to-volume ratio), nanoparticles have many desirable features. Polymers are one of the most ubiquitous materials are also highly controllable. In this thesis use molecular dynamics simulations as our primary tool to study polymer-nanoparticle composite materials.</p> <p>In the first part of our research, we study the dynamics and stability of DNA-functionalized NP superlattices. Our findings can possibly lead to a general design rule for DNA-functionalized NP that consequently can lead to synthesize of NP lattices by design. Ordered structures of NP can show many interesting properties. For example, a cubic diamond (CD) lattice of NP can exhibit a complete optical bandgap. Due to its low volume fraction and the need for highly directional bonds, the CD lattice is one of the most challenging crystallographic symmetries to synthesize. Using DNA-functionalized NP and combined with the DNA origami self-assembly approach, we introduce a method to create NP that will form bonds with tetrahedral symmetry and synthesize the cubic diamond lattice. We show the importance of longer-ranged interactions between NPs for the relative stability of the two polymorphs of the diamond lattice: CD and hexagonal diamond (HD). We also study the different factors affecting the relative stability of CD lattice as a function of NP size and DNA stiffness values. We show that the rotational degrees of freedom dominate the change in free energy of lattices as a function of NP size. Furthermore, we study the equilibrium crystallite shape of the CD lattice. We find that, by changing the stiffness of DNA linkers, it is possible to tune the crystallite shape of CD lattice from a complete octahedron to a cube-truncated octahedron.</p> <p>In the second part of our research, we study the effects of NP size on the dynamical and structural properties of interfacial polymers. Our findings can possibly leads to synthesize of materials with fundamentally different behavior using different relative size of the NP and the polymer chain. NPs can also be used to improve the properties of polymeric materials (eg. mechanical, electrical and optical); many of these properties can be tuned by NP size. We study the effects of the NP size and the attractive interaction strength between the NP and polymers on the dynamics and structure of interfacial polymers. The dynamics changes results in changes of the glass transition temperature (Tg) of the composite. We find that the relative size of the NP compared to the polymer chain plays an important role in the dynamics and structural behavior of interfacial polymers; the relative size can fundamentally and dramatically change the composite behavior. The glass transition temperature for composite with smaller NP size relative to polymer chain increases linearly by increasing the interaction strength; in contrast, NPs with a size comparable or larger than that of the polymer chains from alayer with very slow dynamics ("bound layer") around the NP. The bound layer "cloaks" the effects of NP on the rest of polymer, resulting in a saturation of Tg as the interaction strength increases. The NP also changes the structure and alignment of interfacial polymer chains; these changes diminish as the NP size decreases.</p>