Date of Award

8-9-2014

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Brian C. Benicewicz

Abstract

This research focuses on creating and investigating polymer/organic bound interfaces on nanoparticles with advanced architectures to tailor the properties of polymer nanocomposites for various applications. Reversible addition-fragmentation chain transfer (RAFT) polymerization and a toolbox of surface functionalization from the simple to the advanced were developed to prepare the polymer nanomaterials.

In the first part (Chapter 2), a variety of RAFT agents (xanthate, dithiocarbamate and trithiocarbonate) were used to mediate the polymerizations of several classes of free radical polymerizable monomers. These monomers consist of styrene, methyl acrylate, methyl methacrylate, vinyl acetate and isoprene, which have different activities and require different classes of RAFT agents to control the polymerizations. RAFT agents containing the tertiary and α-EWG R groups demonstrated excellent control over the molecular weights with little effect on the PDIs. MADIX agents and trithiocarbonate RAFT agents containing the similar R groups generated polystyrene with a high PDI (~2.0) and a low PDI (~ 1.1) respectively with control over the polymer molecular weights.

These differences can result in either sharp or fuzzy interfaces on nanoparticles with similar chain lengths and chain densities. A dithiocarbamate RAFT agent exhibited excellent control over the polymerization of vinyl acetate. In addition, trithiocarbonates were used as thermally stable RAFT agents that could mediate the polymerization of isoprene with predictable molecular weights. Polyisoprene grafted silica particles are expected to improve the dispersion of particles in rubber matrices, which is critical for mechanical reinforcement.

In the second part, two classes of water soluble polymers were grafted on nanoparticles via surface-initiated RAFT polymerizations. A new RAFT agent, 4-cyanopentanoic acid N-pyrroledithiocarboxylate (CPDC) was invented for mediating the polymerization of N-vinylpyrrolidone. The synthesis of poly(vinylpyrrolidone) (PVP) grafted nanoparticles was confirmed by FTIR, TGA, 1H NMR and TEM. The synthesis of dye-labeled poly(methacrylic acid) (PMAA) grafted silica nanoparticles was studied with two methods. In the first method, “one-pot” click reactions between azide attached silica nanoparticles and alkyne functionalized molecules (alkyne based dye and 4-pentynoic acid) were used. In the second method, surface-initiated RAFT polymerization of tertbutylmethacrylate (tBuMA) was conducted on dye-labeled CPDB coated silica nanoparticles followed by sequential removal of the thiocarbonylthio end groups and the tert-butyl moieties. Additionally, as a more straightforward strategy, direct polymerization of methacrylic acid on silica nanoparticles with a diameter size as small as 15 nm was conducted via the RAFT polymerization technique. A variety of PMAA brushes with different lengths and densities were prepared on nanoparticle surfaces with excellent control.

In the third part (Chapter 5), a direct-coprecipitation of iron salts strategy was used to generate superparamagnetic nanoparticles with a saturation magnetization of 59.5 emu/g. A silica coating was applied and used to stabilize the magnetic nanoparticles and create a convenient platform for further functionalization. PMAA brushes were prepared on the magnetic nanoparticles with an average diameter size as small as 10 nm via surface-initiated RAFT polymerization of methacrylic acid while maintaining good dispersibility in solutions. The synthesis was confirmed by FTIR, TGA, VSM, TEM and AFM. The polymer grafted magnetic nanoparticles were removed from water solutions after antimicrobial testing using a magnet, thereby avoiding nano-based pollution of the environment.

In the last part (Chapter 6), the surface of silica nanoparticles was modified with a variety of functionalities, from the simple to the advanced. A series of luminescent particles with different sizes were prepared. Dye-labeled monolayer carboxylic acid coated silica nanoparticles with a range of graft densities were synthesized and the particles have strong fluorescence. It was shown that when the commonly-used antibiotics, such as penicillin-G, were linked to carboxylic acid grafted silica nanoparticles, their bacteriocidal efficiencies were increased significantly, even to antibiotic-resistant MRSA. We hypothesize that the increased antimicrobial activity is ascribed to locally high concentrations of antibiotics bound to nanoparticles, which overwhelms the resistance of bacterial strains. β-Cyclodextrin grafted nanoparticles were prepared to capture acyl-homoserine lactone molecules in the bacterial quorum sensing

(QS) process. Advanced bimodal PEG and PMAA grafted nanoparticles, and poly(MAAb-NIPAM) grafted particles were designed and prepared via the “grafting to” and“grafting from” techniques. A new strategy of aqueous-based surface functionalization of particles with water soluble polymers was developed.

Rights

© 2014, Lei Wang

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