Kumar, VipinGuo, Huimin2016-03-112016-03-112016-03-112015-12Guo_washington_0250E_15333.pdfhttp://hdl.handle.net/1773/35253Thesis (Ph.D.)--University of Washington, 2015-12This work explores the fabrication and properties of solid-state polymer nanofoams. Polymer nanofoams, which are expected to be the next generation cellular materials, have attracted much attention in the past decade. Nanoscale dimension of the bubbles could potentially render the nanofoams to have improved properties or some novel properties never observed before. There existed substantial challenges in creating polymer nanofoams and their properties were very poorly understood. This dissertation was set out to develop a versatile fabrication technique for creating polymer nanofoams, and to gain a better understanding their properties. Two approaches are initially investigated for enhancing cell nucleation and reducing cell size: varying intrinsic viscosity (or equivalently molecular weight) and varying glass transition temperature. Molecular weight and glass transition temperature are two of the most important parameters of polymers; however, their effect on cell nucleation was not reported previously in literature. In this study, it is found that the polymer with a higher molecular weight or higher glass transition temperature has a higher cell nucleation density and smaller cell size. These two approaches, however, show no success in creating nanofoams. A novel solid-state foaming process based on low-temperature carbon dioxide saturation is discovered and studied in this work. Using this novel process, nanofoams with cell size less than 100 nm are created for the first time in four polymers – polycarbonate (PC), polymethylmethacrylate (PMMA), polysulfone (PSU), and polyphenylsulfone (PPSU). Since the low-temperature saturation regime was never studied, fundamental aspects regarding to the solubility and diffusivity of carbon dioxide in polymers, as well as the glass transition temperature depression are first discussed. Lowering the saturation temperature (down to -30 C) is found to increase the carbon dioxide solubility substantially. The increased solubility depresses the glass transition temperature of the polymers significantly, even to below 0 C. Processing space has been established for each of the four polymers that relate process parameter (e.g. saturation temperature and foaming temperature) to cellular structures. Nanofoams are created with a smallest cell size around 20 nm and maximum cell nucleation density on the order of 1e15 cells per cubic centimeter. Also, a critical carbon dioxide concentration is observed, above which the cell nucleation density increases much more rapidly and the cell size drops drastically. In addition, it is discovered that above certain carbon dioxide concentration, as the foaming temperature increases the closed-cellular structure transitions to a porous, interconnected nanoporous structure. Several properties of PC nanofoams are investigated in this work. It is found that both tensile and impact properties are a strong function of foam density. Cell size doesn't seem to play a significant role. Reducing cell size results in much lower thermal conductivity, which enables the nanofoams to be potentially used as better insulation materials. Also, nanofoams are found to have a higher glass transition temperature and much improved light transmittance. This suggests the possibly of creating transparent nanofoams.application/pdfen-USmicrocellular; nanofoam; nanoporous; polymer; solid-state; solubilityMechanical engineeringmechanical engineeringThesis