Theses and Dissertations from UMD
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Item Micelle and Aggregate Formation in Amphiphilic Block Copolymer Solutions(2010) Clover, Bryna Christine; Greer, Sandra C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The amphiphilic nature of many block copolymers causes self-aggregation and micelle formation in solvents that are miscible with only one of the block polymers (selective solvents). Micelle and aggregate formation of amphiphilic block copolymers in selective solvents is a function of temperature and concentration. Such self-aggregation has been examined here in a variety of block copolymer systems. In dilute solutions of Pluronic P85 (PEO26PPO40PEO26) (where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)) in D2O, transitions between clustered unimers, spherical micelles, cylindrical micelles, and finally lamellar micelles were observed with increasing temperature. The effect of pressure on this system was examined through small angle neutron scattering (SANS) techniques. At temperatures above 95 oC, a new phase of “demixed lamellae” was observed. Pressure effects on the transition temperatures between the phases of this system were investigated. The self-aggregation of Reverse Pluronic 17R4 (PPO14PEO24PPO14) in D2O has also been examined. The phase diagram of this system was determined through visual cloud-point techniques. Three distinct regions have been observed in solutions of this system, as a function of temperature and concentration: a cloudy, one-phase region; a clear, one-phase system; and a region of phase separation. Copolymer structures were examined in the clear and cloudy one-phase regions through SANS and dynamic light scattering (DLS) techniques. A network, or clustering, of unimers was observed in the cloudy phase. Aggregates in the clear, one-phase region could not be identified definitively as micelles. Finally, micellization of PEO132-PB89 (where PB is polybutadiene) has been studied in solutions of deuterated methanol and deuterated cyclohexane. Spherical micelles were observed in solutions of deuterated methanol. These micelles change little in size or shape over a 50 oC temperature span. The difference in aggregates in protonated and deuterated solvents was also examined. In deuterated cyclohexane, the copolymer formed flexible, cylindrical micelles below 40 oC. These micelles became spherical in shape at higher temperatures.Item NANOSTRUCTURE INVESTIGATION OF POLYMER SOLUTIONS, POLYMER GELS, AND POLYMER THIN FILMS(2009) Lee, Wonjoo; Bribeer, Robert M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis discusses two systems. One is structured hydrogels which are hydrogel systems based on crosslinked poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) containing micelles which form nanoscale pores within the PDMAEMA hydrogel. The other is nanoporous block copolymer thin films where solvent selectivity is exploited to create nanopores in PS-b-P4VP thin films. Both of these are multicomponent polymer systems which have nanoscale porous structures. 1. Small angle neutron scattering of micellization of anionic surfactants in water, polymer solutions and hydrogels Nanoporous materials have been broadly investigated due to the potential for a wide range of applications, including nano-reactors, low-K materials, and membranes. Among those, molecularly imprinted polymers (MIP) have attracted a large amount of interest because these materials resemble the "lock and key" paradigm of enzymes. MIPs are created by crosslinking either polymers or monomers in the presence of template molecules, usually in water. Initially, functional groups on the polymer or the monomer are bound either covalently or noncovalently to the template, and crosslinking results in a highly crosslinked hydrogel. The MIPs containing templates are immersed in a solvent (usually water), and the large difference in the osmotic pressure between the hydrogel and solvent removes the template molecules from the MIP, leaving pores in the polymer network containing functionalized groups. A broad range of different templates have been used ranging from molecules to nanoscale structures inclucing stereoisomers, virus, and micelles. When micelles are used as templates, the size and shape before and after crosslinking is an important variable as micelles are thermodynamic objects whose structure depends on the surfactant concentration of the solution, temperature, electrolyte concentration and polymer concentration. In our research, the first goal is to understand the micellization of anionic surfactants in polymer solutions and the corresponding hydrogels using small angle neutron scattering (SANS). SANS has been widely used to investigate structures ranging from sub-nanometer to sub-micrometer. Since the scattering lengths of H and D atoms are quite different, the scattering contrast can be enhanced (and varied) through isotopic labeling. It is possible to investigate the structure of micelles in polymer solutions and hydrogels using H/D contrast matching methods with SANS. For this aim, water-soluble and chemically crosslinkgable poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) was synthesized using group transfer polymerization. In order to control the size and shape of micelles, the degree of quaternization of the polymer was also controlled through the reaction of PDMAEMA with methyl iodide. The micellization of deuterated sodium dodecylsulfate (d-SDS) in (quaternized) PDMAEMA solutions and the corresponding hydrogels was then observed using SANS and the size and shape of d-SDS micelles was obtained by modeling. 2. Nanopatterning using block copolymer/homopolymer blends Block copolymers are well-known to self-assemble into meso- and nano-scale structures. The use of block copolymers for nanostructured patterns has attracted increasing attention due to their potential use as templates and scaffolds for the fabrication of functional nanostructures. In order to realize the potential of these materials, it is necessary to be able to control the orientation of the nanoscale pattern in a precise manner. Numerous methods such as manipulation of the interfacial surface energies, use of electric fields, and controlling the rate of solvent evaporation have developed to control orientation. In addition, it has been shown that nanopores within cylindrical domains oriented normal to the substrate can be generated by several methods. For example, one component can be degraded by UV exposure, or the homopolymer in a block copolymer/homopolymer blend can be extracted in a selective solvent. In our work, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP)/poly(4-vinylpyridine) (P4VP) films on silicon substrates were prepared using spincoating. The homopolymer was then extracted in ethanol generating pores perpendicular to the substrate. It is noted that the pore size and density were readily controlled by the amount of P4VP homopolymer in the PS-b-P4VP/P4VP solutions, giving simple control of the film structure. It was also possible to make pores more uniform and ordered by annealing in solvent vapor before extracting the homopolymer.Item Self-Assembled Photoresponsive and Thermoresponsive Fluids with Tunable Rheology(2009) Kumar, Rakesh; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids whose rheological properties can be tuned by light or heat (termed as photorheological (PR) or thermorheological (TR) fluids, respectively) have attracted a lot of attention as they can be useful in numerous applications such as drug delivery, coatings, sensors, and valves for microfluidic devices. However, current formulations of these fluids suffer from several limitations: in particular, they often require synthesis of complex organic molecules by elaborate procedures, and this limits the widespread use of these fluids. In this dissertation, we seek to develop and investigate new classes of PR and TR fluids based on organic molecules that are readily available and quite inexpensive. Since no new synthesis is required, these systems could prove to be more attractive for a variety of applications. In the first part of this study, we describe a new aqueous photorheological (PR) fluid based on the zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB) and the photosensitive molecule, ortho-methoxy cinnamic acid (OMCA). EDAB/OMCA fluids exhibit photogelling, i.e., a large (~ 10,000 fold) increase in viscosity upon exposure to UV radiation. We show that this photogelling is caused by the growth of long wormlike micelles in the sample. This structural change, in turn, is induced by the UV-induced isomerization of OMCA molecules from their trans to cis form. Evidence from zeta-potential studies, small-angle neutron scattering (SANS), and rheology are used to systematically reveal the molecular and microstructural mechanism for our results. In the second part of this study, we turn our attention to non-aqueous solvents and demonstrate a new class of PR fluids using such solvents. The PR effect here relies on transformations of "reverse" micellar structures formed by a well-known lipid (lecithin) in conjunction with para-coumaric acid (PCA). Lecithin/PCA fluids exhibit a substantial decrease in viscosity upon exposure to UV light (i.e., photothinning). Initially, the molecules self-assemble into long wormlike micelles, leading to highly viscoelastic fluids. Upon UV irradiation, PCA is photo-isomerized from trans to cis. This change in geometry induces a transition from long to short micelles. In turn, the solution viscosity is decreased by more than three orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. In the last study, we report a class of aqueous fluids whose viscosity increases upon heating (i.e., thermo-thickening). These fluids are mixtures of telechelic associating polymers (HEURs) and a type of supramolecules called cyclodextrins (CDs) in water. Interestingly, we observe this behavior only with a particular type of CDs, called alpha-CDs, and not with the other common CD types, i.e., beta- and gamma-CDs. These results are explained in terms of a competition between the hydrophobic end-caps and the hydrophilic backbone of the polymer for complexation with alpha-CD molecules. We have also investigated the effect of amphiphiles (single-tailed surfactants and double-tailed lipids) on the thermo-thickening. The addition of lipids substantially enhances the thermo-thickening behavior, which is explained to be due to an enhancement of the connectivity of hydrophobic junctions by lipid vesicles.Item Living Coordinative Chain Transfer Polymerization of 1-Alkenes(2008-12-05) Zhang, Wei; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A novel polymerization method, living coordinative chain transfer polymerization (CCTP), was recently developed with monocyclopentadienyl monoamidinate (CpAm) Group 4 metal complexes, which were previously applied for the traditional living coordination polymerization (TLCP) and stereomodulated degenerative transfer living (SDTL) coordination polymerization. In addition to a CpAm precatalyst and a cocatalyst, a chain transfer agent (CTA) was also added to the CCTP system. The CTA undergoes a rapid and reversible chain transfer with the Group 4 metal catalyst, which results in chain growth on an inexpensive main group metal alkyl. This new CCTP technique provides a practical solution towards the intrinsic problem, one chain per catalytic center, for a TLCP polymerization process. The first example of living CCTP was provided with ZnEt2 via Cp*HfMe2[N(Et)C(Me)N(Et)] (35) activated by [PhNHMe2][B(C6F5)4]. It was very efficient for the polymerization of ethene, propene, higher α-olefins and α,ω-nonconjugated dienes, and copolymerization of these monomers. The (co)polymers obtained possess very narrow polydispersity (PDI 1.03-1.10) and tunable molecular weights by several factors including a wide range of equivalents of ZnEt2. The living property of this CCTP system was further confirmed by kinetic studies and end group functionalization. The quantitative chain extension on zinc was clearly shown by in situ NMR spectroscopy. The coordinative chain shuttling polymerization (CCSP) was also studied while binary precatalysts, cocatalysts, or chain transfer agents were applied. The TLCP, SDTL and CCTP of propene via some new CpAm complexes other than 35 were also studied, including the zirconium analogue of 35, Cp*ZrMe2[N(Et)C(Me)N(Et)], and a series of binuclear complexes which have the common structure of [Cp*ZrMe2]2[N(tBu)C(Me)N(CH2)xNC(Me)N(tBu)] (26, x = 8; 27, x = 6; 28, x = 4). The formamidinate precatalyst Cp*ZrMe2[N(tBu)C(H)N(Et)] (12) was also covered in this study. Under both SDTL and CCTP conditions, the binuclear catalysts showed a tether-length dependent chain transfer process as observed by the polymerization results especially by the tacticity of resulting polypropene. Using CCSP process, multi-stereoblock polypropene was successfully prepared via 12 and 27. The structures and properties of these new complexes and (co)polymers were fully characterized by X-ray crystallography, elemental analysis, GPC, DSC, GC and high field NMR spectroscopy.Item SEQUENCE MODELING OF RAFT POLYMERIZATIONS WITH THE METHOD OF MOMENTS(2008-10-13) Zargar, Amin; Schork, Joseph; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Attempts to model the sequence structure of copolymers consisted of probabilistic functions that were incomplete and inaccurate. A novel technique to track sequence parameters is developed that determines not only copolymer composition, but sequence distribution as well. RAFT polymerizations are simulated with two independent and concurrent models to track MWD, conversion, copolymer composition, and sequence characteristics. Batch polymerizations are simulated with varying reactor conditions as a proof-of-concept to illustrate the power of the sequence model to track the composition of the polymer. Series of CSTR and PFR reactors with varying reactor conditions are then presented as applications to iteratively fine-tune copolymers with predetermined sequence and compositional structure.Item Micelles of Polybutadiene-b-Poly(Ethylene Oxide) in a Binary Solvent System(2008-05-01) Ploetz, Christopher D; Greer, Sandra C; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We studied the assembly behavior of a polybutadiene-b-poly(ethylene oxide) diblock copolymer in methanol, cyclohexane, and the corresponding partially miscible binary solvent system. Dynamic light scattering indicates that the copolymer forms coexisting spherical and cylindrical micelles in both of the pure solvents. In the binary solvent system, spherical micelles form in the methanol-rich phase for a wide range of temperatures. Conversely, micelles are present in the cyclohexane-rich phase only near the critical temperature. At the critical solvent composition, micelles form in the single phase region above the critical temperature. Size exclusion chromatography results for the binary solvent system show that the copolymer generally prefers the methanol-rich phase. The preference becomes more pronounced as temperature decreases.Item Ionizing Radiation-Induced Copolymerization of 2-Ethylhexyl Acrylate and Acrylic Acid and Ionomer Formation(2007-12-11) Weaver, Alia; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ionizing radiation-induced polymerization of acrylate esters is a technique employed for the curing of such materials for a variety of adhesive, coating, ink, and lithographic applications. The work presented in this dissertation involves the synthesis of a copolymer composed of 2-ethylhexyl acrylate (2-EHA) and acrylic acid (AA) using pulsed electron beam and gamma irradiation. The structure and synthesis kinetics of this copolymer were investigated by 1H nuclear magnetic resonance (NMR), electron pulse radiolysis with kinetic spectroscopic detection (PR-KSD), and Fourier transform infrared spectroscopy (FTIR). The effects of total dose, dose rate, and acrylic acid content on the polymerization reaction were studied. The conversion of 2-EHA monomer into polymer at a given total dose was found to be enhanced at lower dose rates and higher concentrations of acrylic acid. The pulse radiolysis investigation of the polymerization of 2-EHA and AA was performed through studies of four different types of systems: (i) neat 2-EHA, (ii) 2-EHA/methanol (MeOH) solutions, (iii) mixtures of 2-EHA and AA, and (iv) 2-EHA/AA/MeOH solutions. The build-up of carbon-centered neutral 2-EHA free radicals in neat 2-EHA was found to obey a second order rate law with a rate coefficient of ((7 ± 3) ´ 108)eEHA, whereas in 2-EHA/AA mixtures it was found to obey a pseudo-first order rate law with a rate coefficient of (1.5 ± 0.3) ´ 1010 mol-1 dm3 s-1. This phenomenon is suggested to originate in the increased H+ ion concentration in the presence of acrylic acid, which leads to a faster neutralization step of 2-EHA radical anions as they are transformed into neutral free radicals during the initiation step of the reaction. An investigation of the formation of ion-containing copolymers (known as ionomers) was performed using the radiation-synthesized poly(2-EHA-co-AA) and iron cations. Verification of successful incorporation of iron into the copolymer was identified by an asymmetric carboxylate stretch at 1600 cm-1 of the FTIR spectrum. TEM analysis of poly(2-EHA-co-AA)/Fe2+ ionomers formed from formulated compositions involving a 2:1 mole ratio of ferrous acetate to acrylic acid exhibited ionic clusters of approximately 100 nm in diameter, which may include up to 350 ferrous cations.Item SYNTHESIS, CHARACTERIZATION, AND KINETIC STUDIES OF IONIZING RADIATION-INDUCED INTRA- AND INTER-CROSSLINKED POLY(VINYL PYRROLIDONE) NANOHYDROGELS(2007-11-26) An, Jung-Chul; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A polymer nanohydrogel can be defined as a three-dimensional polymer network composed of hydrophilic crosslinked macromolecular chains filled with liquid and possessing a diameter of 1-102 nanometers. Nanohydrogels have drawn huge interest due to their potential applications, such as target-specific drug delivery carriers, absorbents, chemical/biological sensors, and bio-mimetic materials. However, the conventional methods of nanohydrogel synthesis require toxic chemicals (e.g., initiators, crosslinking agents) to form the gel structure. The additional steps required to remove unreacted or residual (undesired) substances cause nanohydrogel fabrication to be complicated, environmentally unfriendly, and unsuitable for biomedical use. This study aims to develop simple and efficient methods of producing nanohydrogels from polymeric, aqueous solutions using ionizing radiation. Poly(vinyl pyrrolidone) (PVP) nanohydrogels of various sizes and molecular weights were prepared by pulsed electron beam and steady-state gamma irradiation at different doses (5 and 10 kGy; 1Gy = 1 J kg-1) and temperatures (20 to 77 °C). The pervaded volume of the PVP chains becomes smaller at high temperatures (above 50 °C) due to the disruption of hydrogen bonds between water and PVP molecules which reduces the size and the molecular weight of the synthesized PVP nanohydrogels. The synthesis parameters (e.g., irradiation temperature, pulse repetition rate, dose rate, and solution concentration) were varied in order to control the size and the average molecular weight of the irradiated sample. In the absence of oxygen, the radiolytically produced free radicals of the thermally collapsed PVP molecules primarily underwent intra-crosslinking reactions, along with a minor contribution from inter-crosslinking reactions. The predominance of the intra-crosslinking mechanism was exhibited at high irradiation temperature (77 °C) in dilute solutions (c = 0.9 x 10-2 mol L-1). The formation of carbon-centered free radicals along the backbone of the PVP chain at higher pulse repetition rate (300 pulses per second) was found to enhance the intra-crosslinking reaction, thereby leading to the formation of smaller nanohydrogel molecules containing an average hydrodynamic radius (Rh) of 9.9 ± 0.1 nm.Item Toward Optimization of Photomodulation of Azobenzene-modified PPV Derivatives(2007-10-01) Grimes, Amy Frances; English, Douglas S; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Photophysical characterization of a family of photoswitchable conjugated polymers is presented in this work. Additionally the instrumentation constructed for these studies, a time-correlated single photon counting spectrometer, is discussed along with the methods used to characterize the instrument. Sample data and fitting procedures are presented. Overall instrument capabilities are also presented, specifically the use of the spectrometer to measure time-resolved fluorescence anisotropy. An example study probing the interactions between charged fluorophores and surfactant vesicles is included as a demonstration of a time-resolved fluorescence anisotropy application. Understanding the effect of side chain modifications on the emission of light from conjugated polymers is useful in the design of new polymers for applications in sensing and photovoltaics. This thesis focuses on determining the photophysical interactions between a photochromic side chain, azobenzene, covalently bound to a poly(p-phenylenevinylene) (PPV) derivative. Time-resolved and steady-state fluorescence measurements were employed in these studies. The photochromic azobenzene quenches emission from the PPV backbone differentially in its' two isomeric states. Both static quenching and non-radiative energy transfer were found to play important roles in the differential quenching of PPV emission by azobenzene. These studies led to the definition of a parameter to judge degree of difference in quenching between the two isomeric forms. This parameter, modulation efficiency (Emod), serves throughout the studies as an important figure of merit for the depth of modulation observed for structurally modified derivatives of the original azobenzene-modified PPV derivative. Maximizing the modulation efficiency was the aim of the PPV studies. The results presented here elucidated the complex photophysical processes that influence the emission properties of this family of azobenzene-modified PPV derivatives. Important guidelines to maximize modulation efficiency were determined based on these results which will aid researchers in the design of photomodulated conjugated polymers.Item Examination of Dynamic Processes in Living Ziegler-Natta Polymerization and New Polypropylene Architectures through Bimolecular Control(2006-11-24) Harney, Matthew Brian; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The cationic cyclopentadienyl zirconium acetamidinate (CPZA) complex {Cp*ZrMe[N(Et)C(Me)N(tBu)]}[B(Csub6 Fsub5)sub4], (Cp* = ηsup5-Csub5Mesub5) generated through activation of Cp*ZrMesub2[N(Et)C(Me)N(tBu)] via protonolysis of a methyl group with one equivalent of [PhNMesub2H][B(Csub6Fsub5)sub4], has been shown to be a highly active initiator for living alpha-olefin polymerization. Discrete cationic CPZA alkyl complexes of the general structure {Cp*ZrR[N(Et)C(Me)N(tBu)]}[B(Csub6Fsub5)sub4] were studied as model complexes for living polymers derived from this system. Detailed analysis of models in which R = nPr, iPr, iBu, and 2-Et-Bu, as well as living isotactic poly(1-butene) and living isotactic poly(1-sup13C-decene) revealed significant differences with respect to isomerization and decomposition. Studies carried out with various isotopically labeled iso-butyl derivatives revealed isomerization concurrent with decomposition, while the only case which did not provide evidence for isomerization was an isotopically labeled n-propyl derivative. Products of decomposition in cases of R = iBu and 2-Et-Bu included not only the expected alkenes from isomerization/beta-hydride elimination, but significant quantities of alkane. This is proposed to arise from competitive intramolecular abstraction of a hydrogen from the Cp* ligand. During decomposition, all species mentioned above disappeared in a first order manner, and all products of decomposition/isomerization appeared in a zero order manner. This is consistent with slow beta-hydride elimination followed by rapid isomerization, decomposition or reinsertion. Possible mechanisms of isomerization are discussed. The {Cp*ZrMe[N(Et)C(Me)N(tBu)]}[B(Csub6Fsub5)sub4] initiator was found to be an active initiator for the living isotactic polymerization of propylene ([mmmm] = 0.71). When substoichiometrically activated with only 0.5 equivalents of [PhNMesub2H][B(Csub6Fsub5)sub4], atactic polypropylene was produced due to the rapid degenerative transfer of methyl groups between active and dormant centers, which results in the racemization of active sites by virtue of the configurational instability of the dormant state. Successive additions of [PhNMesub2H][B(Csub6Fsub5)sub4] and a methylating agent were used to modulate the conditions of the polymerization system between degenerative transfer (at 50% activation) and fully activated conditions to produce the first discrete, homogeneous isotactic-atactic stereoblock polypropylene (sb-PP) materials. Preliminary tensile testing of three unique sb-PP materials displayed elastomeric properties that were heavily dependant on the stereoblock architecture. The synthesis of a wide range of polypropylene stereochemical microstructures between isotactic and atactic through bimolecular control by substoichiometric activation was also demonstrated, leading to the development of a fundamentally new architecture: stereogradient polypropylene.