Document Type

Dissertation

Date of Award

2016

Keywords

Polymer chemistry

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Alistair J. Lees, Chair

Second Advisor

Wayne E. Jones, Jr. Faculty Advisor

Third Advisor

Chuan-Jian Zhong Member

Subject Heading(s)

Polymer chemistry

Abstract

Our society has made significant advancements in technology as it continues to grow in size which in turn has led to an accumulating amount of toxic threats. Some types of harmful pollution our society is currently facing include industrial waste such as organic dyes, pharmaceutical pollution and chemical warfare agents (CWAs). To date the nerve agent, O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate), also known as VX, is the world’s most lethal chemical substance. Some of these deadly nerve agents have been employed in various conflict and terrorist attacks. Currently available CWA degradation techniques include incineration and water hydrolysis followed by biotreatment with enzymes. Drawbacks to these techniques include the selectivity of the analyte, degradation of the enzyme over extended treatment time, and lack of robustness for practical applications. A more effective approach may be achieved by heterogeneous catalysis employing nanostructured materials. Solid catalysts including titania have demonstrated a means to effectively destroy CWAs.

TiO2 is regarded as an efficient photocatalyst for degradation of organic toxins due to its strong oxidative power, high stability, low cost, and environmental friendliness. TiO2 nanofibers represent an alternative materials approach to conventional nanoparticle composites for use in photocatalytic degradation. Nanofibers were fabricated using a sol gel synthesis and electrospinning; a non-mechanical, electrostatic process using electrically driven jets producing fiberous matt that is followed by a thermal treatment resulting in TiO2. Further modification of TiO2 with metal nanoparticles introduces the area of study known as plasmonics. Materials possess unique optical characteristics that can further enhance the destruction of toxic threats.

The synthesized fibers were used directly in photocatalytic degradations of 2- chloro ethyl ethylsulfide (CEES) and dimethyl methylphosphonate (DMMP) and were found to exhibit enhanced rates of degradation. It was seen that saturation of the TiO2 nanofibers with water prior to exposure with CEES showed an overall increased the degradation under UV irradiation. Photocatalytic degradations of DMMP were designed to demonstrate the role of surface area in the degradation process. The comparison of nanofibers vs. nanoparticles supports a conclusion that surface area is not a critical factor in the degradation of target species.

Electrospun nanofibers of polymethyl methacrylate (PMMA) and titanium triisopropoxide (TTiP) were found to possess catalytic properties when introduced to methyl paraoxon, a simulated chemical warfare agent (SCWA). In addition to the photocatalytic advantages of these fibers, increased flexibility and durability were observed compared to electrospun TiO2 nanofibers. The resulting fibers would also be better compatible with low temperature processing of multifunctional materials including metal-organic frameworks (MOFs) and sensors. It is shown that the presence of TTiP within the polymer/MOF composite increased percent conversion and lowered the half-life of the reaction. Results acquired are the best to date according to literature. It has been hypothesized there are multiple competing degradation mechanisms that are dependent on the source of irradiation used to drive the degradation reaction.

Plasmonics have inspired a significant amount of interest in various research communities for applications in nanophotonics, optics, catalysis, and energy conversion. Materials possessing surface plasmon resonances (SPR), such as silver nanoparticles, have been studied and are known to exhibit appealing optical characteristics. Ag nanoparticles were deposited on the surface of the TiO2 fiber through a polyol synthesis with silver nitrate. The hypothesis is the emission lifetime of Ag-TiO2 will have a smaller intensity then that of TiO2, this in turn would mean the recombination rate of electron hole pair in Ag-TiO2 is slower than that of TiO2. Degradations of methyl paraoxon with TiO2 and Ag-TiO2 did show that the metal deposited TiO2 had an enhancement in the percent conversion to the nitrophenoxide product of methyl paraoxon.

In this work, it is shown there are many factors involved in optimizing the photocatalytic performance of TiO2/ polymer composite nanofibers. The combination of novel nanotechnology with advancements in photocatalysis will provide new benefits and improvements with filtration, and self-decontaminating textiles and paints. The diversity of applications these materials can be incorporated in has the ability to be life changing for civilians and warfighters who are in constant threat of toxic agents. As research in this field continues to progress, degradation rates will only continue to increase in attempts to achieve airborne decontamination on a time-scale of milliseconds and liquid decontamination in seconds.

Comments

Copyright by Danielle L. McCarthy 2016, All Rights Reserved

Additional Advisors: Gerard S. McGrady Member; Bruce White Outside Examiner

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