Chemistry & Biochemistry Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2752

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UNVEILING THE SELF-ASSEMBLY OF POLYMER-GRAFTED NANOPARTICLES IN SELECTIVE SOLVENTS
(2023) Lamar, Chelsey; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The self-assembly of inorganic nanoparticles (NPs) has garnered considerable attention due to the potential for fabricating functional structures with unique collective properties. In recent years, polymers have emerged as valuable candidates in assisting the organization of NPs into complex architectures with multiple capabilities. Researchers have shown that polymer-grafted nanoparticles (PGNPs) facilitate the use of advanced nanostructures with tailored properties in biomedical applications. Although, continued exploration of the rational design and tailoring of PGNP assemblies is needed to expand our understanding before we can fully realize the potential of these structures in desired applications. My dissertation aims to investigate the fundamental aspects and elucidate the underlying mechanisms in the self-assembly of PGNPs for modern biomedical applications. A facile and versatile solution-based strategy was utilized to explore the individual self-assembly of PGNPs with anisotropic NPs and the co-assembly of binary PGNPs with distinct sizes. We focused on designing, characterizing, and exploring the optical properties of hierarchical assembly structures produced from inorganic NPs tethered with amphiphilic block copolymers (BCPs). Individual PNGPs with anisotropic NPs and binary mixtures of small and large PGNPs produce vesicle structures with well-defined packing arrangements. My work shows how key parameters, including polymer chain length, nanoparticle size, and concentration, influence the self-assembly behavior and the formation of vesicles in each system. Through a combination of experimental observations and theoretical considerations, I highlight the significance of polymer shell shape in dictating the self-assembly behavior of individual anisotropic PGNPs. Moreover, I demonstrate that elevated temperatures impacted the stability and optical responses of the vesicle structures. In co-assembly studies, my work describes the macroscopic segregation of PGNPs with different sizes in the vesicular membrane, which is attributed to the conformation entropy gain of the grafted copolymer ligands. This research will provide valuable insights into the self-assembly behavior and fundamental design of PGNP structures relevant to biomedical applications.
Versatile Strategies for Multifunctional Polyolefins
(2023) Fischbach, Danyon Miles; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Polyolefins have quickly become one of the world’s most utilized products since their discovery in the 1950s. With 350 million tons produced each year, it is clear that the use of polyolefins is not subsiding in the near future. Instead, it is imperative to develop novel materials that are more efficient than their current counterparts. As the function of a plastic is derived from its properties, creating polyolefins with designable and targetable attributes is a major priority. The Sita group has played a huge role in the development of ‘precision’ polyolefins. The techniques employed allow for the scalable synthesis of a plethora of polyolefins. To do this, input variables such as the monomer, tacticity, molar mass, and molar mass distribution are controlled in an organized manner to affect output properties such as crystallinity, elasticity, and tensile strength. The ability to create diverse plastics is necessary for the functions asked of them, however, the missing element in almost all polyolefin synthesis is chemical functionality. The inert nature of polyolefins leads to limited reactivity, therefore, reducing possible chemical reactions, such as recycling. The goal of this work is to increase the scope of functional polyolefins so that new materials with improved properties can be produced. The first step in adding functionality is choosing the proper functional group. A drawback to many polyolefin functionalities currently under study is that they have a very limited scope. Functional groups are designed and used individually, requiring different compounds for each target functionality. To overcome this obstacle, aryl functional groups were targeted in this report. Phenyl functionalities are known for undergoing a range of chemical transformations leading to a wide variety of possible materials. Described in this report, aryl-functionalized polyolefins were synthesized using three different techniques. Each method has been shown to later undergo post-synthetic transformations to yield new functional groups that can either be used as contact points for macromolecular building blocks or as chromophores for optical observation. The single use or combination of these techniques has led to polyolefin-based materials that may in fact lower the barrier for the next-generation of functional polyolefins.
DESIGN AND SYNTHESIS OF POLYOLEFIN MATERIALS FOR NANOSTRUCTURED SELF-ASSEMBLY: BUILDING BLOCKS, COPOLYMERS, AND POLYMER CONJUGATES
(2022) Wentz, Charlotte Maria; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Polyolefin based materials are essential to today’s society in both simplistic commodity plastics to complex nanostructured materials and optoelectronic devices. In order to better understand these materials and make new impactful innovations, there is a barrier of fabrication, scalability, versatility, and programmability. The answer to the world’s plastic waste problem lies not in removing our use of polymers but relies in better understanding their properties, utilizing them as building blocks in advanced materials, and creating a long-lasting advanced material. Towards the goal of overcoming limitations in fabrication and scalability the work herein presents on utilizing a toolbox of living polymerization techniques such as living chain transfer polymerization (LCCTP) where new functionalities, stereochemical microstructures, optical properties, and physical properties of the polyolefin can be designed and systematically controlled. The polyolefins made through these techniques are scalable and versatile with end-group functionalization creating a seemingly endless choice of polymer building blocks and polymer materials. In line with creating new technologies that are programable the polyolefin building blocks made herein are utilized in multiple conjugates to create and understand methods and mechanisms of solid-state nanostructured self-assembly and access rare nonclassical phases that are highly desirable for their properties and uses in a plethora of applications. The conjugates investigated involve either a sugar-based head group covalently bond to a polymer tail to access rare and misunderstood Frank Kasper phase order-order transitions, or a perylene chromophore core covalently bond on both sides of the core in a linear fashion to polymer domains to create highly florescent or optically active materials that are useful in organic technologies such as solar cells, light emitting diodes, or nanotechnology. These perylene based conjugates can self-assemble into unique columnar phases and single gyroid phase. These results with conjugates provide methods for reliable and programmable access to rich phase behavior through the design of the polyolefin domains.
INVESTIGATION OF SOLID-STATE SELF-ASSEMBLY OF ONE- AND MULTI- COMPONENT SUGAR POLYOLEFIN CONJUGATES AND MECHANISMS FOR FRANK-KASPER MESOPHASE TRANSITIONS
(2020) Lachmayr, Katchen K.; Sita., Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The self-assembly of molecules provides the basis of life and has become ubiquitous for the development of nanostructured materials. Nanostructured materials have long reaching impacts for the furtherment of science, as the next revolution in technology requires the ability to fabricate nanostructured materials with sub-10-nm feature sizes. To this end, the work herein, presents on methods and mechanisms of solid-state nanostructured mesophase formation, through the isolation of complex phases and understanding of thermotropic order-order transition pathways. This extends to both solid-state classical and nonclassical phases, including the highly desirable, bicontinuous double gyroid, and uncommon Frank-Kasper (FK) phases resulting from self-assembly of the sugar polyolefin conjugates. The sugar polyolefin conjugates are produced through practical and scalable bulk quantities that demonstrate the dynamic self-assembly on the nanometer-scale, which results from rapid thermally induced order-order phase transitions. Production of different derivatives of the sugar polyolefin conjugates and blended systems, can significantly lower the barriers to access uncommon and complex mesophases through the elucidation of transition mechanisms for FK mesophases. To better understand the basic principles and mechanisms of formation that result in the complex packing motifs of FK phases, single- and multi-component systems were devised. These mechanisms avoid the need for large structural reconfigurations of spheres, and dynamic mass transfer, as previous solid-state thermotropic mechanisms have required. Additionally, a general strategy for design, modulation, and utilization of functionally competent soft matter solid-state FK phases is provided from developments with a two-component system utilizing a small molecule additive. These results demonstrate the sugar polyolefin conjugates as an exceptional class of self-assembling amphiphilic materials, which provide methods for reliably producing Frank-Kasper phases from single- and multi-component systems, in addition to remarkable classical phase behavior.
RESOLUTION IMPROVEMENT OF PHOTOLITHOGRAPHIC TECHNIQUES BASED ON VISIBLE LIGHT
(2017) Tomova, Zuleykhan; Fourkas, John T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The semiconductor industry is planning to use Extreme Ultraviolet lithography as its next-generation patterning technique. However, this technique has run into many roadblocks due to its cost and complexity. An alternative approach employs light in the near-UV. A 2-color photolithographic technique based on combination of two colors on the near-UV or visible light has shown promising results in creating structures with sizes at a fraction of the excitation light wavelength. One color of light excites photoinitiator molecules to a chemically active state that leads initiation of polymerization. A second color of light deactivates photoinitiator molecules before they form radicals, inhibiting polymerization. In this thesis we show how extending 2-color lithography to include a third color (3CL) can achieve super-resolution for applications requiring fabrication of closely packed structures. The advantage of the 3CL process is in its separation of polymerization initiation and deactivation steps by involving different chemical states that allow for more efficient deactivation and for increased resolution. Some of the crucial elements needed to achieve an optimized scheme for 3CL are the determination of the intramolecular transitions that participate in the process, the lifetimes of the photoinitiators, and the exposure parameters. Several photoinitiators were studied to determine the optimal exposure conditions. Polymerization action spectra and deactivation action spectra were used to determine the combinations of excitation and deactivation parameters resulting in the most efficient deactivation. The 2-beam initiation threshold (2-BIT) method was introduced for in situ measurement of the order of eective nonlinearity of photoresists. The order of the effective nonlinearity was determined for a series of photoinitiators under various excitation wavelengths and fabrication velocities. Additionally, a photoinitiator with a proportional velocity (PROVE) dependence, in which feature size increases with the velocity, was found to undergo efficient self-deactivation at increased temperatures. This dependence was demonstrated by gradually heating the sample and analyzing the fabricated feature sizes. Spot heating with a laser beam was also used to locally prevent polymerization. The correlation between polymerization rate and temperature opens opportunities for high speed fabrication that uses temperature gradients to create finer structures.
End-Group Functionalized Poly(α-olefinates) as Modular Building Blocks
(2016) Thomas, Tessy S.; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The development of living coordination chain transfer polymerization (LCCTP) has provided a viable path to practical and scalable bulk quantities of structurally well-defined polyolefins that are further characterized by having tunable molecular weight, narrow polydispersity and end-group moieties through the functionalization of the Zn(polymeryl)2 intermediate. These low molecular weight, atactic poly(α-olefinates) are attractive non-polar building blocks for new types of self-assembling polyolefin materials with tunable occupied volumes and lengths. In the present work, the investigation of self-assembled morphology manipulation through tunable occupied volume required a model α-olefin. Living coordinative group-transfer polymerization techniques were employed to determine the viability of low molecular weight, atactic poly(α-olefinates) (X-PAOs) as building blocks with bulky pendent groups. Application of these X-PAOs for the synthesis and self-assembly of high χ, low N amphiphilic diblock copolymers demonstrated the ability to manipulate the morphology of the thin film nanostructures through variation in occupied volume of the X-PAO domain. The resulting materials proved the combination of X-PAOs and polyester blocks provided a high enough χ in order to demonstrate self-assembly with an N as low as 50 monomer units while maintaining sub-20 nm domain spacings. It was of significant interest to develop a higher χ, lower N system to achieve sub-10 nm domain spacings. As a result a novel system using sugar-hybrid-PAO conjugates was developed. This system also demonstrated that variation in occupied volume of the X-PAO domain could influence thin-film morphology. The sugar-hybrid-PAO conjugates also demonstrated the ability to self-assemble in solution and encapsulate hydrophobic molecules. The sugar-hybrid-PAO conjugates proved to be a highly versatile system that has simplified the polysaccharide/synthetic block copolymer designs that have been previously used in the literature to obtain sub-10 nm domain spacings. The ability to generate hydrophobic building blocks using LCCTP has opened up the possibility of further investigations of new, advanced self-assembling materials and applications thanks to these readily-available X-PAOs modular building blocks.
Design and Synthesis of New Group (IV) Cyclopentadienyl Amidinate and Guanidinate Initiators for Controlling the Microstructure of Poly(α-olefins) During Living Coordinative Chain Transfer Polymerizations
(2014) Blakley, Cathryn Gail; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
C1-symmetric, cationic group 4 metal (Zr and Hf) mono-methyl complexes, {(η5-C5Me5)M[N(tBu)C(Me)N(Et)](Me)}[B(C6F5)4], are highly active initiators for the living and stereo-selective (isotactic) coordinative polymerization of propene and longer-chain α-olefins. Utilizing technology previously discovered but not yet fully utilized, it is possible to demonstrate the remarkable ability to stereo-engineer poly(α-olefins) with the use of a single initiator. A two-state living coordination polymerization process can be engaged by controlling the relative populations of the active and dormant species as a function of time to incorporate stereo-errors in a programmed fashion. Secondly, in the presence of excess equivalents of a main group metal alkyl such as diethylzinc (DEZ), rapid and reversible chain transfer between the active propagating species and the `surrogate' main group metal alkyl, which occurs at a rate that is significantly greater than propagation, serves as a work-around solution to the `one-chain-per-metal-site' limitation of a living polymerization. Successful adaptation of this reversible group transfer technology can include the rapid and reversible transfer of a polymeric group between `tight' and `loose' propagating ion pairs that mediated by excess DEZ to precisely control co-monomer incorporation. While this process of living coordinative chain transfer polymerization (LCCTP) can provide practical quantities of precision polyolefins, the exchange process results in loss of stereo-regularity in the final polymer microstructure. Strategies for achieving a high degree of stereo-regularity during LCCTP include the synthesis of new classes of configurationally stable and optically pure cyclopentadienyl, amidinate and guanidinate initiators that incorporate a distal, chiral substituent. A second strategy to create enantiomerically pure propagating species involves the adaptation of hydrozirconation to create a new class of terpene substituted cyclopentadienyl-amidinate complexes via insertion of an olefin into a Zr-H bond. The last attempt to impart stereocontrol under LCCTP conditions involves the addition of an enantiomerically substituent to the N-amidinate to ensure the same enantiofacial insertion.
Microfluidic Production of Polymeric Functional Microparticles
(2012) Jiang, Kunqiang; Raghavan, Srinivasa R; DeVoe, Don L; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation focuses on applying droplet-based microfluidics to fabricate new classes of polymeric microparticles with customized properties for various applications. The integration of microfluidic techniques with microparticle engineering allows for unprecedented control over particle size, shape, and functional properties. Specifically, three types of microparticles are discussed here: (1) Magnetic and fluorescent chitosan hydrogel microparticles and their in-situ assembly into higher-order microstructures; (2) Polydimethylsiloxane (PDMS) microbeads with phosphorescent properties for oxygen sensing; (3) Macroporous microparticles as biological immunosensors. First, we describe a microfluidic approach to generate monodisperse chitosan hydrogel microparticles that can be further connected in-situ into higher-order microstructures. Microparticles of the biopolymer chitosan are created continuously by contacting an aqueous solution of chitosan at a microfluidic T-junction with a stream of hexadecane containing a nonionic detergent, followed by downstream crosslinking of the generated droplets by a ternary flow of glutaraldehyde. Functional properties of the microparticles can be easily varied by introducing payloads such as magnetic nanoparticles and/or fluorescent dyes into the chitosan solution. We then use these prepared microparticles as "building blocks" and assemble them into high ordered microstructures, i.e. microchains with controlled geometry and flexibility. Next, we describe a new approach to produce monodisperse microbeads of PDMS using microfluidics. Using a flow-focusing configuration, a PDMS precursor solution is dispersed into microdroplets within an aqueous continuous phase. These droplets are collected and thermally cured off-chip into soft, solid microbeads. In addition, our technique allows for direct integration of payloads, such as an oxygen-sensitive porphyrin dye, into the PDMS microbeads. We then show that the resulting dye-bearing beads can function as non-invasive and real-time oxygen micro-sensors. Finally, we report a co-flow microfluidic method to prepare uniform polymer microparticles with macroporous texture, and investigate their application as discrete immunological biosensors for the detection of biological species. The matrix of such microparticles is based on macroporous polymethacrylate polymers configured with tailored pores ranging from hundreds of nanometers to a few microns. Subsequently, we immobilize bioactive antibodies on the particle surface, and demonstrate the immunological performance of these functionalized porous microbeads over a range of antigen concentrations.
PRINCIPLES OF COMPLEX POLYUBIQUITIN SIGNALING, THE STRUCTURAL BASIS FOR UBIQUITIN-UBISTATIN INTERACTIONS, AND NOVEL ASSAYS FOR THE CHARACTERIZATION OF DEUBIQUITINASES
(2013) Nakasone, Mark Alphonse; Fushman, David; Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Ubiquitination is the most versatile and is certainly one of the most difficult post-translation modifications to understand in eukaryotic life. In the process of ubiquitination the C-terminus of ubiquitin (Ub), a small 8.65 kDa protein is covalently attached to εNH2 groups of lysine side chains on target proteins. Once attached, additional Ubs can be added to the original Ub at eight unique linkage sites (M1, K6, K11, K27, K29, K33, K48, or K63) to form polyUb chains. This internal Ub-Ub linkage dictates the structural conformation of the polyUb chain, which in turn governs the receptors that can recognize a given chain. PolyUb chains were thought to be homogeneously linked until very recently when mixed linkage polyUb chains were detected on several cellular pathways. This observation implied that instead of having just eight distinct polyUb signals, there were now potentially quadrillions of unique chains. The results presented within represent the first in depth studies of mixed linkage polyUb chains, focusing on the structural impact of linkage mixing. For mixed K48 and K63 linked chains the findings support that their individual linkage properties are preserved regardless of linkage mixing. However, simulations for mixed linkage chains containing different linkages imply that many novel polyUb signals are possible. The ubiquitin-proteasome pathway is the primary mechanism to degrade short lived proteins in the cell and has also emerged as a top therapeutic target. Ubistatins, a class of small molecules bring about the same effects as existing proteasome inhibitor drugs by directly binding the polyUb chain. However, virtually nothing is known about the structural properties for any ubistatin/Ub complex. Here is provided the first structure of a ubistatin/Ub complex along with data that overwhelmingly validates the structure. Other important factors regarding the ubistatin/Ub interaction including the stoichiometry and dual hydrophobic / electrostatic binding mechanism are also uncovered. Proteomic analysis of polyUb conjugates has been limited to determining which linkage types are present. A novel method for K63 linked polyUb conjugates, which can measure consecutive K63 linkages is described here. This method allows the proteomics community to gain unprecedented information on cellular pathways utilizing K63 linkages.
Expanding the Range of Polyolefins through Living Coordinative Chain Transfer Polymerization
(2012) Wei, Jia; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The strategy, termed living coordinative chain-transfer polymerization (LCCTP), has been explored to boost the efficiency and versatility of polyolefin synthesis by coupling a reversible chain-transfer process with living coordination polymerization. LCCTP strategy not only overcomes the "one-chain-per-metal" limit on polymerization yield, but also provides opportunities to flourish the architectural, compositional and functional flexibility of polyolefin-based materials. A new strategy, named ternary living coordinative chain-transfer polymerization (t-LCCTP), extends the LCCTP methodology through employing the rapid and reversible chain-transfer process under living conditions between an active transition-metal propagating species, a primary surrogate trialkyaluminum, and a catalytic amount of diethylzinc as a secondary surrogate and chain-transfer mediator. This strategy provides a cost-effective, scalable process for the production of precision hydrocarbons, such as the low-molecular-weight oligomers from propene and alpha-olefins under near-ambient conditions. Having the advantage of using trialkyaluminum and diethylzinc as surrogate chain-growth sites, block and end-group functionalized polyolefin-based materials have been synthesized directly through chemical reactions of the Al-C/Zn-C bonds. Rapid and reversible chain-transfer between "tight" and "loose" ion pairs has been used to modulate the relative reactivities of ethene and 1-hexene or cyclopentene in a programmed fashion for LCCTP. Thus, different grades of a monodisperse polyolefin copolymer, such as the poly(ethene-co-1-hexene), have been obtained with a single cationic transition-metal catalyst. Through employing long chain alpha-olefins as co-monomers, a novel class of polyethene-based waxes has been synthesized with precisely tunable side-chain crystalline sizes. The discovery of a fundamentally novel Group 4 transition-metal binuclear catalyst has achieved the highly challenging goal of making ethene/propene (E/P) multi-block copolymers through steric-control over the "regional" and "local" hindrance around the binuclear catalyst molecule. Structural, thermal, surface morphological and mechanical characterizations of these E/P blocky materials unambiguously reveal their blocky nature and unique physical properties regarding to the traditional E/P random copolymers. Finally, LCCTP has been successfully coupled with this binuclear catalyst to provide a variety of polyethene-based blocky copolymers under chain-transfer conditions.

Chemistry & Biochemistry Theses and Dissertations

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