Raked circular-cone aerobraking orbital transfer vehicle

An aerobraking orbital transfer vehicle (AOTV) (80) has aerobrake (82) with a blunted raked-off circular-cone configuration. The other components of the AOTV, including command/control module (95), fuel tanks (86, 88, 89 and 91), rocket engines (94) and afterbody (84), are positioned substantially along resultant force axis (104) of the AOTV (80). The axis (104) coincides with the resultant (sum of lift and drag) force vector. Afterbody (84) is mounted behind the aerobrake (82) with its length extending rearwardly from the aerobrake. The base flow clearance angle .phi. of the aerobrake (80) is 25.degree., thus allowing the afterbody (84) to extend rearwardly from the aerobrake (82) to a much greater extent than possible with a raked-off elliptic-cone aerobraking shield configuration. Afterbody size limitation and other problems associated with the raked-off elliptic-cone aerobraking shield configuration are alleviated by the combination of the aerobrake shape and positioning of the fuel tanks (86, 88, 89 and 91), rocket engines (94) and afterbody (84)

Surface Tension of High Density Polyethylene (HDPE) in Supercritical Nitrogen: Effect of Polymer Crystallization

This document is the accepted manuscript version of a published article. Published by Elsevier in the journal "Colloids and Surfaces A: Physicochemical and Engineering Aspects" volume 354, issues 1-3, page 347-352. doi:10.1016/j.colsurfa.2009.06.005.Surface tension of a polymer melt in a supercritical fluid is a principal factor in determining cell nucleation and growth in microcellular foaming. This work focuses on the surface tension of a crystalline polymer, high density polyethylene (HDPE), in supercritical nitrogen under various temperatures and pressures. The surface tension was determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The dependence of the surface tension on temperature and pressure, at temperatures above the HDPE melting point, ~125°C, was found to be similar to that of the amorphous polymer polystyrene (PS) in supercritical CO2, previously reported; i.e., the surface tension decreased with increasing temperature and pressure. Below 125°C and above 100°C, HDPE underwent the process of crystallization, where the surface tension dependence on temperature was different from that above the melting point, and decreased with decreasing temperature. Differential Scanning Calorimetry (DSC) characterization of the polymer was carried out to reveal the process of HDPE crystallization and relate to this surface tension behavior. It was found that the amount of the decrease in surface tension was related to the rate of temperature change and hence the extent of polymer crystallization.NSERC CR

Towards Maximal Cell Density Predictions for Polymeric Foams

Published by Elsevier in the journal "Polymer" volume 52, issue 24. doi:10.1016/j.polymer.2011.09.046.Self-consistent field theory is used to make direct predictions for the maximum possible cell densities for model polymer foam systems without recourse to classical nucleation theory or activation barrier kinetic arguments. Maximum possible cell density predictions are also made subject to constraining the systems to have maximal possible internal interface and to have well formed bubbles (no deviation from bulk conditions on the interior of the bubble). This last condition is found to be the most restrictive on possible cell densities. Comparison is made with classical nucleation theory and it is found that the surface tension is not an important independent consideration for predicting conditions consistent with high cell density polymeric foams or achieving the smallest possible bubble sizes. Instead, the volume free energy density, often labelled as a pressure difference, is the dominant factor for both cell densities and cell sizes.NSERC Canad

Origins of the failure of classical nucleation theory for nanocellular polymer foams

DOI: 10.1039/C1SM05575E (Paper) Soft Matter, 2011, 7, 7351-7358 This journal is © The Royal Society of Chemistry 2011Relative nucleation rates for fluid bubbles of nanometre dimensions in polymer matrices are calculated using both classical nucleation theory and self-consistent field theory. An identical model is used for both calculations showing that classical nucleation theory predictions are off by many orders of magnitude. The main cause of the failure of classical nucleation theory can be traced to its representation of a bubble surface as a flat interface. For nanoscopic bubbles, the curvature of the bubble surface is comparable to the size of the polymer molecules. Polymers on the outside of a curved bubble surface can explore more conformations than can polymers next to a flat interface. This reduces the free energy of the curved interface which leads to a significantly smaller barrier energy to nucleation and thus a much higher nucleation rate. Also, there is a reduction of unfavorable energetic contacts between polymer and fluid molecules in the vicinity of a curved interface. Polymers on the outside of a curved interface are less likely to find a portion of themselves in the interior of the unfavorable fluid bubble. A secondary cause of the failure of classical nucleation theory is due to the collapse of the bulk region inside the bubble. As the radius of a bubble is reduced, eventually the diffuse walls collide causing increased mixing of polymer and fluid molecules everywhere. This causes a reduction of internal energy associated with the interface, leading to smaller nucleation barrier energies and, again, a reduced barrier energy to nucleation.NSERC Canada, Strategic Projects Grant and Discovery Grant

Maximal cell density predictions for compressible polymer foams

Published by Elsevier in the journal "Polymer" volume 54, issue 2. doi:10.1016/j.polymer.2012.11.067.Thermodynamic upper bounds for polymer foam cell densities are predicted using compressible selfconsistent field theory. It is found that the incompressible limit always gives the highest, and therefore ultimate, upper bound. Qualitative comparisons between the compressible and incompressible cases agree, indicating that low temperatures and high blowing agent content should be used to achieve high cell densities. The inhomogeneous bubble structure reveals deviations from the expected homogeneous SanchezeLacombe equation of state, consistent with some experimental results. A generalized Sanchez eLacombe equation of state is discussed in the context of its suitability as a simple alternative to the SimhaeSomcynsky equation of state

Effect of Temperature and Pressure on Surface Tension of Polystyrene in Supercritical Carbon Dioxide

This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry B., 111, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/jp065851tThe surface tension of polymers in a supercritical fluid is one of the most important physicochemical parameters in many engineering processes, such as microcellular foaming where the surface tension between a polymer melt and a fluid is a principal factor in determining cell nucleation and growth. This paper presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendant drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and good qualitative agreement with experiment is obtained. The physical mechanisms for three main experimental trends are explained using SCFT, and none of the explanations quantitatively depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the slope of surface tension change with temperature is dependent on pressure.Natural Sciences and Engineering Research Council of Canada (NSERC) Canadian Foundation for Innovation (CFI) Canada Research Chairs (CRC) Progra

Effect of pressure and temperature on interfacial tension of poly lactic acid melt in supercritical carbon dioxide

© 2015. This manuscript version of Effect of Pressure and Temperature on Interfacial Tension of Poly lactic acid melt in supercritical carbon dioxide is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ This document is the accepted manuscript version of a published article. Published by Elsevier in the journal "Thermochimica Acta" volume 609, http://dx.doi.org/10.1016/j.tca.2015.04.005The interfacial tension of poly lactic acid (PLA) melt is measured in supercritical carbon dioxide (CO2) at the temperature range of 143 °C to 168 °C and CO2 pressures up to 2000 psi, using Axisymmetric Drop Shape Analysis Profile (ADSA-P). The results show a decrease in interfacial tension with increasing temperature and pressure. However, the interfacial tension dependency on temperature at high pressures decreases because of a reduction in CO2 solubility at high temperatures. The relationship between the interfacial tension and the density-difference of polymer-supercritical CO2 mixtures is also examined by the generalized Macleod equation. Moreover, the range of stability for the melted drop, in interfacial tension measurements, is obtained by dimensionless Bond number. The results indicate the validity of the measurements for Bond number between 0.36 and 0.48.Natural Sciences and Engineering Research Council (NSERC) Network for Innovative Plastic Materials and Manufacturing Processes (NIPMMP) Canada Research Chairs (CRC

Reduction of polymer surface tension by crystallized polymer nanoparticles

Copyright (2010) AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Chemical Physics 133 and may be found at http://dx.doi.org.proxy.lib.uwaterloo.ca/10.1063/1.3493334Self-consistent field theory is applied to investigate the effects of crystallized polymer nanoparticles on polymer surface tension. It is predicted that the nanoparticles locate preferentially at the polymer surface and significantly reduce the surface tension, in agreement with experiment. In addition to the reduction of surface tension, the width of the polymer surface is found to narrow. The reduced width and surface tension are due to the smaller spatial extent of the nanoparticles compared to the polymer. This allows the interface to become less diffuse and so reduces the energies of interaction at the surface, which lowers the surface tension. The solubility of the surrounding solvent phase into the polymer melt is mostly unchanged, a very slight decrease being detectable. The solubility is constant because away from the interface, the system is homogeneous and the replacement of polymer with nanoparticles has little effect.Natural Sciences and Engineering Research Council of Canad