Browsing by Author "Dr. Saad A. Khan, Committee Co-Chair"

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    Blend- and Surface-Assisted Foaming of Polymers with Supercritical Carbon Dioxide
    (2003-07-21) Siripurapu, Srinivas; Dr. Richard J. Spontak, Committee Co-Chair; Dr. Saad A. Khan, Committee Co-Chair; Dr. Joseph M. DeSimone, Committee Member; Dr. John H. van Zanten, Committee Member
    This thesis involves development of novel micro- and nanocellular foamed polymers using judicious polymer processing strategies with supercritical carbon dioxide (scCO₂). Apart from serving as a processing aid in the form of a transient plasticizer, scCO₂ is a powerful blowing agent to manufacture foamed plastics. It is also a viable replacement for the harmful chemical blowing agents such as chlorofluorocarbons, hydrofluorochlorocarbons and perfluorocarbons that are still prevalent in the foaming industry. This thesis focuses on improving our fundamental understanding of polymer foaming with scCO₂ to create novel materials that cannot be synthesized otherwise using traditional foaming technologies. In particular, we aim at creating new polymer foaming paradigms via either judicious polymer blending or introduction of surfaces to the polymer matrix. Novel experimental apparatus, utilizing both a continuous extrusion process and a batch process, have been designed and constructed to study various foaming applications with scCO₂. We show that microcellular foams (foams with pores on the order of 10 μm) containing semicrystalline polymers can be generated continuously by blending with a compatible amorphous polymer. Blends of miscible poly(vinylidene fluoride) (PVDF) – poly(methyl methacrylate) (PMMA) blends yield vastly improved microcellular morphologies compared to PVDF alone. We find that blend miscibility, viscosity reduction facilitated by scCO₂ and reduction or elimination of crystalline melting point of the polymer blend are key factors in producing these materials. The latter part of this dissertation investigates the feasibility of a scCO₂-based foaming procedure to generate micro and nanoporous thin polymer films. Our experimental findings reveal that controlling scCO₂ diffusion from film surfaces is the critical factor towards realizing uniform porosity in polymer films. We use a combination of physical constrains on film surfaces and introduction of interfaces via addition of a nanoscale filler or a tailored non-ionic surfactant to generate controlled foamed structures. Foaming experiments are conducted on 100 μm thick PMMA films with a variety of additives including colloidal silica (particle diameter of 10-12 nm), Zonyl fluorosurfactants, block and graft copolymer of PMMA with a CO₂-philic group such as a fluoropolymer (1,1-dihydroperfluorooctyl methacrylate) and a siloxane (poly (dimethyl siloxane)) in the presence of CO₂. The addition of a low molecular weight block copolymer (PMMA-b-PFOMA) with a CO₂-soluble block (PFOMA) and a polymer-miscible block (PMMA) are found to provide the highest increase in cell nucleation densities and smallest cell sizes.
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    Transport Properties of Hectorite Based Nanocomposite Single-Ion Conductors.
    (2003-04-14) Singhal, Ruchi Gupta; Dr. Saad A. Khan, Committee Co-Chair; Dr. Peter S. Fedkiw, Committee Chair; Dr. Jan Genzer, Committee Member
    Lithium-ion batteries are an important power source for small electronic technologies because of desirable characteristics including high-energy density, low weight, and excellent cycle performance. We investigated the electrochemical and rheological effects of clay nanocomposite fillers in lithium-ion battery electrolytes. Nanocomposite hectorite, a non-reactive smectite clay filler, was used in this study. Hectorite and other 2:1 layered clays (smectites) are unique in that they are characterized by a large negatively charged plate-like structure (˜250-nm diameter) with exchangeable counter cations sandwiched between thin platelets (˜1 nm). For lithium battery application, the native sodium cations on hectorite are exchanged for lithium ions and the plate-like particles are dispersed in high-dielectric solvents (e.g., ethylene carbonate (EC) and propylene carbonate (PC)) to create a physically gelled structure. The cation mobility is considerable relative to the mobility of the large anion clay platelets. Lithium-ion transference numbers of Li-hectorite in carbonate solvent have shown near unity values indicating efficient Li+ movement in a cell. Conductivity in our electrolytes is not as high as LiPF6 liquid electrolytes used in today's market, however, we hypothesized that the addition of low-molecular weight polymer compounds would improve conductivity. Our objective was to show improved conductivity in Li-hectorite/ethylene carbonate electrolytes with the addition of polyethylene glycol di-methyl ether (PEG-dm, 250 MW). Several combinations of clay and polymer loading are studied in an attempt to find an electrolyte with the highest conductivity. Finally, a preliminary comparison between hydroxyl terminated PEO (PEG) and PEG-dm as a polymer co-solvent with EC, is made with regards to rheological properties. We find all samples to exhibit gel-like behavior with room temperature conductivities of order 0.1 mS/cm. A maximum in conductivity is observed with increasing clay concentration. A maximum in clay basal spacing is also observed in the same concentration range, suggesting a direct correlation between conductivity and basal spacing. G' and yield stress increased by two orders of magnitude with increasing clay concentration and conductivity increased by one order of magnitude (from 5 to 25% clay), indicating clearly clay concentration to be the primary factor in determining the characteristics of these single ion conductors. Addition of PEG-dm to the base EC electrolyte shows moderate improvement in conductivity; the elastic modulus and yield stress also increase by a factor of three. Clay concentration had a dominating effect on all results including rheology results, when compared to solvent composition. PEG-dm electrolytes yielded a stronger gel sample when compared to PEG electrolytes. In addition, we found an interesting correlation between clay basal spacing and conductivity as a function of clay concentration.