Materials Science & Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2792
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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 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.