2012 Theses Doctoral

Designer Polymer Superstructures from Solid Phase "Click" Chemistry

Dach, Benjamin Isaac

This thesis describes the preparation of crosslinked organic polymer-silica nanocomposites and surface modification by dynamic covalent chemistry. Chapter 1 provides a review of solid phase chemistry and the various techniques and methods that are used to perform the surface experiments. The philosophy of `click' chemistry is discussed and its various applications in the realms of polymer brushes and surfaces are explained. In Chapter 2, a new method to form cross-linked polymer-silica nanocomposites is described. Mixtures of nanoparticles (NPs) bearing terminal alkyne groups with NP bearing terminal azide groups are cured by copper catalyzed alkyne-azide click chemistry to produce crosslinked "Matrix-Free" Nanocomposites (MFNs). MFNs hold promise to provide exceptionally high strength and toughness by virtue of the continuous network of covalent polymer bridges that forms between nanoparticles upon curing.

A method of cleaving polystyrene and polyacrylate polymers on silica NPs by HF etching of the silica cores is described in Chapter 3. It is demonstrated that the polymers can be successfully cleaved under the acidic conditions, but the ester linkages on the polymer backbone are also cleaved, thereby degrading the polymer.

In Chapter 4, a simple method is demonstrated to covalently link polymers to silicon substrates with quantitative control of polymer areal density. The approach is based upon solid phase synthesis coined by Bruce Merrifield. In spite of recent developments in polymerization techniques, difficulties remain in synthesizing and separating "designer" polymers with controlled architecture (i.e., branch structure, block structure and shape) and well defined molecular weight. Additionally, cleavage of molecules from a surface is usually performed under harsh acidic conditions, which can damage the polymers. By chemically grafting heterobifunctional polymers to a surface and utilizing a photocleavable nitrobenzyloxycarbonyl (NBOC) surface linker, well-defined, tailor-made polymer structures that can be cleaved under relatively mild conditions are constructed.

Chapter 5 describes `clicking' polymers to silica NPs to produce both linear and dendritic architectures. Linear polymer growth has traditionally been used to covalently attach polymers to a NP surface for applications in drug delivery, flexible electronics, and protein purification. However, dendritic polymers on NPs have been shown to provide additional functionality, and our results show that steric hindrance of the large polymer brushes primarily affects the loading potential of the NP. To overcome this, loading potentials from growing dendritic polymer brushes without surface spacer ligands are compared to growing dendritic polymer brushes with surface spacer ligands to increase polymer loading.