Date of Award

2017

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

College of Arts and Sciences

First Advisor

Brian Benicewicz

Abstract

This work focused on the synthesis and characterization of polymer-grafted nanoparticles for various applications including drug-delivery, directed self-assembly and mechanical reinforcement applications. The surfaces of inorganic particles were modified with polymers of different composition, chain length, graft density and polymer architecture depending on the specific needs of each project. The surface modifications were mainly achieved by surface-initiated reversible addition fragmentation chain transfer (RAFT) polymerization, which is a very versatile technique to prepare nanocomposites with desired properties.

The first part of this work (Chapters 2 & 3) focused on novel self-assembly techniques. Chapter 2 described the design and characterization of a new self-assembly technique called surface-initiated polymerization-induced self-assembly (SI-PISA). We rationally designed this one-pot synthesis method based on mixed brush grafted silica nanoparticles. Nano-assemblies with different shapes including 1D strings, 2D disks, 3D vesicles and solid spheres were obtained at high solid content in various solvent systems. The scheme is based on sequential SI-RAFT polymerization strategy. A solvent-miscible brush was first grafted onto 15 nm silica NPs, and self-assembly was subsequently induced by the polymerization of a second brush that was solvent-immiscible. Self-assembly occurred in situ with the SI-polymerization of the second brush. The shape of the nano-objects was found to be controlled by the chemical structure of grafted polymers, chain length of grafted polymers, and reaction media.

In Chapter 3, we utilized the low blocking efficiency of reversed monomer addition order in combination with surface initiated RAFT polymerization to establish a facile procedure towards mixed polymer brush grafted nanoparticles SiO2-g-(PS1, PS1-b-PMAA). The SiO2-g-(PS, PS-b-PMAA) nanoparticles were analyzed by GPC deconvolution, and the fraction of each polymer component was calculated. Additionally, the SiO2-g-(PS, PS-b-PMAA) were amphiphilic in nature, and showed unique self-assembly behavior in water.

The second part of this work (Chapter 4) is based on a drug delivery application. A pH and thermal dual-responsive nanocarrier with silica as the core and block copolymer composed of poly(methacrylic acid) (PMAA) and poly(N-isopropylacrylamide) (PNIPAM) as the shell was prepared by surface initiated RAFT polymerization. These dual-responsive nanoparticles were used as carriers to deliver the model drug doxorubicin (DOX) with unusually high entrapment efficiency and loading content. The release rate was controlled by both pH and temperature of the surrounding medium. Moreover, these particles selectively precipitated at acidic conditions with increased temperature, which may enhance their ability to accumulate at tumor sites. Cytotoxicity studies demonstrated that DOX-loaded nanoparticles are highly active against Hela cells, and more effective than free DOX of equivalent dose. A cellular uptake study revealed that SiO2-PMAA-b-PNIPAM nanoparticles could successfully deliver DOX molecules into the nuclei of Hela cells. All these features indicated that SiO2-PMAA-b-PNIPAM nanoparticles are a promising candidate for therapeutic applications.

The third part of this work (Chapter 5 & 6) is surface functionalization of silica nanoparticles with low Tg rubbery polymers, and the use of these polymer-grafted nanoparticles as mechanical reinforcement fillers. Chapter 5 documented the collaborative work between our research group and Michelin North America Inc. to study the influence of grafting density and molecular weight of grafted polymer on mechanical properties of polyisoprene tire rubber. The grafting of polyisoprene (PIP) to different types of silica has been studied and developed by RAFT polymerization processes. This has been shown to be applicable for preparing grafted nanoparticles that are useful for exploring new surface interactions between silica fillers and rubber materials. Scale up approaches have been successful and detailed mechanical property studies were documented to assess the potential of these new graft architectures on improving rubbery composite properties. Chapter 6 is focused on another rubber material, polychloroprene. Compared with other rubbers, polychloroprene exhibits excellent resistant to oil, grease, wax, ozone and harsh weather conditions. RAFT polymerization and surface-initiated RAFT polymerization (SI-RAFT) of polychloroprene was studied. The SI-RAFT polymerization rate of chloroprene was found to be slower than free solution RAFT polymerization, and further regulated by graft density of the grafted polymers. The resulting polychloroprene-grafted silica nanoparticles were directly crosslinked to get matrix-free polychloroprene nanocomposites that showed good nanoparticle dispersion and superior mechanical properties compared with unfilled polychloroprene rubber

Rights

© 2017, Yang Zheng

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