Browsing by Author "Wnuk, Andrew J."

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    Engineering properties of multiphase block copolymers
    Wnuk, Andrew J. (Virginia Polytechnic Institute and State University, 1979)
    Multiblock [-A-B-]n copolymers of bisphenol-A polycarbonate (I) and several poly(arylether sulfones) (II) have been investigated. The copolymers [see document for a diagram of copolymers (I) and (II)] were prepared from hydroxyl terminated oligomers (4,000 < M̅n < 30,000) by an interfacial technique which utilized phosgene as the coupling agent. Characterization of the oligomers and copolymers included end group analysis, membrane osmometry, and gel permeation chromatography. One of the most interesting aspects of block copolymers is their ability to undergo microphase separation above a critical block length. Either one or two phase block copolymers can be prepared by controlling the molecular weights of the parent oligomers. In the bisphenol-A polycarbonate/bisphenol-A polysulfone system, for example, strictly one phase materials, with only one intermediate glass transition temperature, were obtained at block lengths of less than 10,000 g/mole. Two-phase copolymers resulted when blocks exceeding 20,000 g/mole were coupled. Copolymers comprised of intermediately sized blocks (M̅n ≃16,000) could be obtained as either single or multiphase systems depending upon their previous thermal history. Homogeneous films, with a single intermediate Tg, were obtained via solution casting, whereas compression molding provided films exhibiting two Tg's. Subsequent DSC studies pointed out that microphase separation could be thermally, and irreversibly, induced by annealing above the Tg of the polysulfone blocks (190°C). Since polycarbonate and polysulfone are leading examples of tough, amorphous thermoplastics, the effects of microphase separation on the tensile, impact, and melt flow properties of the copolymers were investigated. A novel falling weight impact tester was designed and constructed to meet the needs of this study. The device was fully instrumented to provide a deceleration-time plot of the impact process by means of an accelerometer mounted in the projectile. Fracture energies for commercial homopolymers and graphite reinforced composites, in addition to polysulfone-polycarbonate block copolymers, were calculated from the impact curves. Both the tensile and impact properties of the copolymers improved with increasing polycarbonate content. Both single and multiphase materials were ductile and transparent as opposed to physical blends of the two. oligomers which were opaque and possessed poor mechanical properties. No differences due to microphase separation were observed in either the tensile or impact studies. The homogeneous copolymers displayed melt viscosities and activation energies nearly equal to those of the homopolymers. Much greater viscosities and activation energies were exhibited by the phase separated materials indicating that the heterogeneity was retained in the melt.
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    Structure and properties of copper oxide thin films
    Wnuk, Andrew J. (Virginia Polytechnic Institute and State University, 1977)
    Copper oxide thin films were prepared on glass substrates by radio frequency sputtering of a sintered CuO target in various argon-oxygen atmospheres. Films sputtered in pure argon consisted of both CuO and Cu₂O due to partial dissociation of CuO during deposition. Sputtering in 10% and 50% oxygen afforded films composed entirely of CuO₁₊ₓ. All the sputtered films exhibited extrinsic p-type semiconduction. Both resistivity and thermal activation energy decreased with increasing oxygen content of the sputtering atmosphere. The resistivity of the CuO/Cu₂O films decreased with increasing Cu₂O content. Annealing of the CuO₁₊ₓfilms at 300℃ in air increased their resistivity, activation energy, and visible transmittance, while decreasing the lattice parameters slightly. These changes were attributed to the loss of excess oxygen trapped within the film lattice during sputtering. Annealing of the CuO/Cu₂O films under the same conditions oxidized the Cu₂O to CuO and increased film resistivity to values greater than 10³Ω-cm. A high degree of preferred orientation was observed in the films sputtered in the presence of oxygen. These films were oriented with the (111) planes parallel to the substrate surface and remained so even after annealing in air at 300℃. The (111) planes in CuO are composed of equal concentrations of copper and oxygen atoms and were identified as the most densely packed planes in the lattice. Films consisting of Cu₂O only were prepared by reducing previously sputtered films in a mixture of CO and CO₂ at 300℃. The technique proved to be a convenient means for obtaining Cu₂O films with reproducible properties which were useful in explaining the behavior of the CuO/Cu₂O films.