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The thermal plasma production of fumed silica in a transferred arc consists of the decomposition of quartz to SiO (g) and oxygen followed by an oxidizing quench back to SiO2. The particles formed have diameters of the order of 10 to 20 nm and are linked in a three dimensional branched chain aggregate. Previous work by Addona and Munz (1999) demonstrated the technical feasibility of producing fumed silica using this method, but was unable to demonstrate the special rheological properties of the powder. The most important characteristic of fumed silica is the presence of hydroxyls on the surface of the particles, in the form of isolated hydroxyl groups, hydrogen-bonded hydroxyl groups and siloxane groups. / In the present work, we studied the changes in the powder quality by varying the quench conditions used for the production of the powder and by agglomerating the obtained particles. The fumed silica was agglomerated by conveying in a length of tubing with sharp bends. The powder was characterized using BET, Viscosity tests, FT-IR, TEM, SEM and XRD. The product was compared to both a commercial product (Aerosil 200) and the material previously produced by Addona. Tests were done before and after the agglomeration experiments. / The experimental results showed that the agglomeration had no effect on the powder's rheological properties. We concluded that the smaller viscosity values obtained for the plasma produced fumed silica were due to the lack of the free hydroxyl groups from the surface of the particles.

The nonlinear visoelastic behavior that accompanies large transient deformations in high molecular weight polymer melts and concentrated solutions is not at all well-understood, because equipment suitable for the investigation of such phenomena have not been available. This lack of understanding has limited the advancement of plastics processing technology, because the deformations that occur in polymer processing often take the melt well into the nonlinear regime. Furthermore, it has not been possible to test constitutive equations formulated to describe nonlinear phenomena. / A new rheometer, equipped with a specially designed shear stress transducer, has been developed to examine the nonlinear viscoelastic behavior of polymeric liquids. Deformations that can be generated include interrupted shear and large amplitude oscillatory shear. Using the new instrument, a thorough study was made of the rheological behavior of polyisobutylene at 22(DEGREES)C. The results obtained from these experiments were used to evaluate the predictive ability of a constitutive equation recently proposed by Mewis and De Cleyn.

Thesis (Ph D.)--Virginia Polytechnic Institute and State University, 1990. / Vita. Abstract. Includes bibliographical references (leaves 225-231). Also available via the Internet.

Thesis (Ph. D.)--Lehigh University, 2004. / Includes vita. Includes bibliographical references (leaves 228-233).

Valu Fa and Eastern Lau `s (two regions along Lau's back-arc spreading center) observed axial morphology suggest that Valu Fa is more magmatically robust than Eastern Lau despite Eastern Lau's spreading rate nearly doubling Valu Fa's. Early geochemical [Pearce et al., 1994] and geophysical [Martinez and Taylor, 2002] studies predict a gradational decrease in melting moving north from Valu Fa to Eastern Lau, but more recent geochemical and seismic observations ([Escrig, .et al 2009]; [Dunn and Martinez, 2011]; [Dunn et al., 2011]) show a sharper stepwise decrease in melting as the spreading center's ridge axis sweeps away from the Tofua Volcanic-Arc. As the ridge sweeps away from the volcanic-arc, the influence of the slab hydrated mantle in the melting structure of the ridge decreases. Furthermore, Eastern Lau produces a thinner crust than expected for a robust spreading center. 2-D numerical studies [Harmon and Blackmon, 2010] show a gradational decrease in melting from Valu Fa to Eastern Lau but with no corresponding thinning of Eastern Lau's crust. To understand the melting dynamics underneath Lau's back-arc spreading center and the Tofua Volcanic-Arc implementing the effects of 3-D mantle flow and slab hydration appears to be required. To explain the observed geochemical and seismic observations, three 3-D numerical were performed, using a community developed mantle convection solver (CitcomS). The first model shows that observed geometric and surface kinematic boundary conditions cause a steep gradational increase in relative melting area (anhydrous) moving northward with increasing spreading rate along the ridge axis from Valu Fa to Eastern Lau caused by a northwestern along axis mantle flow. A peak in the relative melting area appears particularly close to Eastern Lau where crust is thinnest. These predictions run in opposition to the observations. The second model shows including a viscosity reduction in the mantle wedge due to slab hydration causes a more subdued relative melting increase with spreading rate and "saddle" shaped decrease in relative melting area north of 20.9°S. This saddle shaped melting structure is caused by a reversal in along axis flow towards the southeast, which takes hot mantle from Eastern Lau and transports it underneath Valu Fa accounting for the anomalously thin crust observed at Eastern Lau. Finally, introducing a hydrated solidus increases the melt production under Valu Fau and causes a stepwise decrease in melt production at Eastern Lau due to its decreased proximity to the slab-hydrated region, consistent with the observed geochemical and seismic studies.

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The applicability of the conventional Hoffman-Weeks (HW) linear extrapolation for the determination of the equilibrium melting temperatures of semicrystalline polymers is critically reviewed. It is shown that the linear extrapolation of observed melting temperatures cannot, in general, provide a reliable estimate of the equilibrium melting temperature. A more rigorous nonlinear HW analysis is proposed in this dissertation, which yields more accurate estimates of the equilibrium melting temperatures for semicrystalline polymers. The proposed nonlinear HW analysis is successfully applied to the cases of isotactic polypropylene and poly(ethylene oxide). The predicted initial lamellar thickness as a function of the crystallization temperature matches well with experimental results and/or literature values. Results based on the nonlinear HW analysis are consistent with those obtained from the analysis of the temperature dependence of the crystal growth rates. The general applicability of the Lauritzen Hoffman (LH) secondary nucleation theory is also addressed for isotactic polypropylene and poly(ethylene oxide). While the LH theory provides an excellent account of the temperature dependence of spherulitic growth rates and ratio of nucleation constants for different regimes, it appears not to yield a meaningful value for the substrate length, L, for poly(ethylene oxide). In a second project, the effects of structural and topological constraints on the morphology, melting and crystallization behavior of ethylene/styrene copolymers are investigated. During cooling from the melt, the longest ethylene sequences crystallize into lamellae in the primary crystallization process, while the shorter ethylene sequences are suggested to form fringed micelles in the secondary crystallization process. Kinetic studies indicate that secondary crystallization is characterized by an Avrami exponent of ½ which is consistent with a one dimensional, diffusion controlled growth. The increase in the melting temperature of secondary crystals with crystallization time is tentatively explained by a decrease in the molar conformational entropy of the remaining amorphous fraction as a result of secondary crystallization, although the possible role of an increase of crystal lateral dimensions with time cannot be rigorously ruled out. / Ph. D.