Controlling Particle Structures in Polymers through Nanocomposite Processing

The use of polymer nanocomposites (PNCs) for their unique properties has been around for over a century, used in anything from airplanes to raincoats. In the past 30 years, the fabrication of advanced composite materials has expanded this field into a vast array of applications, tailoring the optical, mechanical, and electrical properties of the material for any specific use. As with any composite, the goal is to take advantage of the desirable properties in each individual component and form an overall superior material. In this body of work, we focus on mixing inorganic nanoparticles (NPs), which are strong and dense, with various polymers, typically used for being tough, light, and easy to work with. The reason for using nano-sized fillers is to maximize the inorganic surface area with which the polymer can interact with, allowing for a minimum amount of filler to be used with maximum benefit, though this is not always practically the case. The interaction between the NP and polymer is only optimized if the NP structure can be controlled. Each of the chapters in this thesis work toward finding new, and practical, methods for understanding and controlling NP dispersions in polymers. In each of the chapters, we focus primarily on the use of silica NPs, ranging from 10-100 nm in diameter, studying methods for controlling their dispersion in polymers like polystyrene (PS), polyethylene (PE), polyisoprene (PI), poly(2-vinyl pyridine) (P2VP), poly(ethylene oxide) (PEO), poly(methyl acrylate) (PMA), and others. First, we take a closer look at how to control and quantify “well-dispersed” NPs in a polymer matrix, taking advantage of various techniques to stabilize the NPs in solution before casting them into the polymer. Once we understand how to reliably disperse the NPs, we can begin to find ways to reorganize them into structures that could provide further improvements in the mechanical properties of the composites, again focusing on methods that would be practically relevant in any polymer system. These techniques take advantage of thermodynamic and kinetic drivers to reorder the NPs in amorphous and semi-crystalline polymers. Forming bound layers of a polymer on a favorably interacting NP surface can stabilize the NPs in a variety of polymer systems, providing initially well-dispersed systems for further study. Alternatively, the grafting of chains onto the NP surface leads to various self-assemblies of the NPs in different matrices, depending on the interaction of the grafted and matrix chains. Starting with well-dispersed NPs in a semi-crystalline polymer allows us to take advantage of the crystallization process to kinetically force NPs into hierarchical structures throughout the composite. This concept alone encompasses a bulk of this thesis – a technique that simply requires the isothermal crystallization of the polymer at different temperatures to achieve vastly different NP structures. Understanding the interaction of the NPs and the crystal is studying using extensive calorimetry and microscopy experiments, specifically determining how to define the confinement of the system due to the presence of NPs and their effect on growth and nucleation. The resulting alignment of NPs into the interlamellar region of the crystal is then analyzed in detail using a correlation function, commonly applied to neat semi-crystalline structures, but applied here for the first time to a PNC. This analysis provides new insights into the alignment process and ways for quantifying the degree of NP alignment. The alignment technique is then applied to several other systems for the specific focus of improving the mechanical properties of unique and industrially relevant PNCs, specifically using polymer grafted NPs. Finally, we briefly discuss the effect of annealing time and temperature on NP dispersion, dynamics, and resulting in unprecedented changes in the macroscopic properties of the material, uncovering new insights in the aging of PNCs. Each of these techniques provides details around controlling the organization and structure of NPs in polymers for the purpose of improving their mechanical properties, all while simply changing the way in which the material is processed