Materials Science & Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2792
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Item THE PHOTOCHEMISTRY OF POLYENYL RADICALS AND ITS APPLICATION TO UHMWPE FOR USE IN ARTIFICIAL CARTILAGE(2009) Kasser, Michael Jacob; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of UV light as an alternative to thermal treatments above the melting point (150 °C) to remove free radicals in irradiated UHMWPE was explored. It was found that, in contrast to the allyl free radical which is converted by 258 nm light to alkyl free radicals, polyenyl radicals are not converted to alkyl radicals by UV light. None-the-less, by sandwiching UV light treatments between low temperature thermal anneals (100 °C), it was possible to reduce free radical concentrations by 30%. This reduction was achievable for depths up to one millimeter. However, this reduction did not have a significant effect on oxidation due to an increase in oxidation susceptibility because of the concurrent increase in concentration of easily abstracted allylic hydrogens. By photoirradiating for the optimal amount of time, it was possible, for the first time, to synthesize a polyethylene sample whose residual free radicals consisted of almost entirely dienyl free radicals. This allowed unambiguous identification and simulation of dienyl free radical's EPR spectra to be a singlet containing nine peaks separated by 9 G hyperfine separation. Detailed studies of photoirradiation of UHMWPE containing free radicals revealed that photoirradiation with a continuous spectrum above 200 nm causes the decay of diene unsaturations and allyl free radicals, a reduction in the overall amount of free radicals, and an increase in the degree of unsaturation of polyenyl free radicals. Upon longer photoirradiation times, polyenyl radicals were converted from lower to higher degrees of unsaturation. This effect was identical in the presence and absence of oxygen, but was suppressed by hydrogen gas. These results showed that the conversion does not occur by a linear alkyl radical addition mechanism wherein alkyl radicals migrate to stable polyene unsaturations and polyenyl radicals thereby increasing their order, as previously suggested. The valid mechanism appears to be the direct photoconversion of diene unsaturations to dienyl radicals and lower order polyenyl radicals to higher order polyenyl radicals.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.