Date of Degree

2-2015

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor(s)

Nan-loh Yang

Alan M. lyons

Subject Categories

Chemistry | Mechanics of Materials | Polymer Chemistry

Keywords

photocatalysis, polyacetal, singlet oxygen, superhydrophobic, surface, TiO2

Abstract

Polyacetal polymers are thermoplastic resins that play an important role in industry because of numerous industrial applications including automobile; household appliance; etc. The first part of this thesis (Chapter 2) is about the synthesis of a new acetal copolymer that exhibits superior thermal stability. The second part of this thesis (Chapter 3) is about the preparation and applications of TiO2-based polymer nanocomposite films, where the reactive oxygen species (ROS) are generated on the solid surface. Catalytic nanocomposite films are an active area of research because of their potential uses for environmental remediation and chemical synthesis. Furthermore, to enhance surface functionality, superhydrophobic surfaces are prepared using catalyst particles, where the ROS could be generated at the solid-liquid-gas interphase. These works are presented in the third part of this thesis (Chapters 4 and 5).

Acetal copolymers represent a family of well-established engineering thermoplastics serving a broad range of important industrial applications including replacement for metals. Their structure consists of oxymethylene units with a low concentration of co-monomer units. By interrupting the facile hemiacetal hydrolysis reaction that can propagate along the macromolecular chain, these co-units function as a "stopper" against degradation of the main block, -(CH2O)n-. The copolymer can also be blended with additives such as stabilizers and reinforcements more easily than the homopolymer due to more flexible polymer chains. Previous approaches have incorporated the "stopper" through cationic copolymerization of cyclic acetals such as ethylene oxide, dioxolane and dioxepane. The first part of this thesis describes the first synthesis of an eight-member ring acetal, 6-methyl-1, 3-dioxocane (MDOC), and its cationic copolymerization with trioxane initiated by boron trifluoride dibutyl etherate. The copolymerization process was monitored in situ using proton NMR. Incorporation of MDOC led to the insertion of the "stopper" unit, "-[CH2CH2CH(CH3)CH2CH2)O]-", thus synthesizing the new acetal copolymer. A superior copolymer thermal stability with a ~ 20°C increase in degradation onset temperature compared with end-capped polyoxmethylene was observed. Both TGA and DSC data indicated the random placement of the "stopper" in the copolymer likely due to efficient transacetalization because of the higher basicity and flexibility of the stopper unit compared with co-units comprising 2 to 4 carbons in length. DSC thermo-grams showed a melting curve of a polymer with melting point lower, as expected, than that of oxymethylene homopolymer. No homopolymer in the copolymer samples was in indicated by TGA. The new acetal copolymer, poly(6-methyl-1,3-dioxocane-co-trioxane), which has a "stopper" co-unit with five carbon atoms along the backbone, contains the longest reported stopper co-unit, potentially leading to improved elongation, and toughness and better compatibility with a range of additives compared to acetal homopolymers.

The second part of this thesis is focused on the design and preparation of photocatalytic surfaces. The use of TiO2 as a semiconducting heterogeneous photocatalyst for the photodegradation of organic pollutants has been extensively investigated as the material is non-toxic, inexpensive, and chemically stable over a wide pH range. Chapter 3 presents a novel lamination fabrication method that enables pre-formed TiO2 nanoparticles to become partially embedded in the surface of a thermoplastic polymer film. In this way, the particles are strongly adhered to the surface while remaining accessible to the aqueous solution. By modifying the fabrication conditions (e.g. temperature, pressure, polymer melt viscosity, etc.), the morphology of the hierarchical TiO2-polymer surface can be controlled and thus the rate of photocatalytic reactions can be increased. In addition, the fraction of TiO2 particles that become fully embedded in the polymer surface, and so inaccessible to photocatalysis reactions, can be reduced through lamination process control, thereby reducing costs.

Nanocomposite films were characterized (XPS, SEM, AFM, TGA) and tested by photooxidizing a Rhodamine B solution under either a UV lamp or natural sunlight. The morphology of the surface was correlated with both fabrication conditions and photocatalysis rate. This environmentally friendly technique is compatible with any type of TiO2 catalyst particle and so the wavelength response of the photocatalysis can be improved as particles that retain photocatalytic activity at longer wavelengths become commercially available. The wide variety of thermoplastic polymers that are compatible with the process will facilitate their introduction into a wide range of applications including waste water treatment and water purification.

In Chapter 4 and Chapter 5, a general approach is presented to incorporating particles into a superhydrophobic surface that catalyze the formation of reactive oxygen species. Superhydrophobic photocatalytic surfaces are prepared using hydrophilic TiO2 nanoparticles and hydrophobic Silicon-Phthalocyanine photosensitizer particles. A stable Cassie state was maintained, even on surfaces fabricated with hydrophilic TiO2 particles, due to significant hierarchical roughness. A triple phase photogenerator is designed and fabricated. By printing the surface on a porous support, oxygen could be flowed through the plastron resulting in significantly higher photooxidation rates relative to a static ambient. Photooxidation of Rhodamine B and BSA were studied on TiO2-containing surfaces and singlet oxygen was trapped on surfaces incorporating Silicon-Phthalocyanine photosensitizer particles. Catalyst particles could be isolated in the plastron to avoid contamination by the solution. This approach may prove useful for water purification and medical devices where isolation of the catalyst particle from the solution is necessary and so Cassie stability is required.

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