Advanced Methods to Produce and Characterize Transport Properties of Novel Nanoporous Polymers
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In this work, experimental techniques, equipment, apparatuses, models, algorithms, and software codes were developed to explore the nanoporous structure of polyetherimide (PEI) created using the solid-state foaming process. First reported in 2001, researchers have wondered about the transport properties of this skin-covered nanoporous structure but no one had successfully developed the means to reliably produce, access, or characterize it. In this work, I developed techniques and equipment to produce flat, uniform, and dimensionally stable specimens with low variance. Then, I developed two unique methods to selectively and robustly remove the solid skin without damaging the underlying porous nanostructure. The resulting geometry brought about a fluid problem with peculiar boundary conditions, which has nontrivial solutions. Therefore, I solved the partial differential equations for these geometries in order to compute transport properties from experimental data. The full solution to this problem made use of analytic and numerical methods, software algorithms, and image processing. After developing the complete experimental and theoretical tool-sets, I obtained data on a wide range of nanoporous PEI structures, including permeability, diffusivity, tortuosity, and thermal conductivity. Permeability was found to be in the range of 10^-16 to 10^-15 m^2, comparable to that of limestone, volcanic rock, and sandstone. Diffusivities were in the range 0.01 - 0.025, comparable to very dense soil. Tortuosity, a property often wondered about by researchers beholding scanning electron micrographs (SEM) of this nanostructure, was computed from diffusivity and porosity data to be in the range 20 - 40. These are the first results on transport properties, some 15 years after the discovery of the nanostructure. During this doctoral work, several major discoveries were made. First, I found that PEI's nanostructure is impermeable to liquid water, however it allows gasses and wetting liquids (such as acetone and isopropyl alcohol) to freely flow through the structure. This phenomenon is similar to that exploited in the commercial polytetrafluoroethylene membrane GoreTEX. This waterproof-but-breathable property has many practical applications, especially given that PEI's nanostructure was shown to be waterproof at pressures of over 50 atmospheres, whereas GoreTEX can withstand less than 3 atmospheres. Additionally, I developed a new method to achieve a completely novel nanostructure in PEI by using a two-state foaming technique. These new nanostructures have a very unique appearance and resemble a hybrid of closed-cell and open-porous morphologies. Perhaps most notably, I also discovered a processing condition which produces PEI foams that are transparent. This is the first known occurrence of polymer foams with cells so small that light scattering is diminished to the point of optical clarity. The cells are so small that our SEM techniques were inadequate in imaging the structure. Only a few voids on the order of 10 nanometers were visible. However, the structure was found to be open-porous as well as optically transparent, similar to Aerogel, but a polymer. Lastly, I performed a feasibility study on 3D printing polymer bubbles. The idea was to create a nozzle that jets a stream of individual liquid polymer bubbles at high frequency, and deposit them in layers to create a 3D object. Although further work is necessary, I demonstrated the vision using liquid latex, butter, and paraffin wax as working materials, having successfully developed nozzles and techniques to create and release bubbles encapsulated by these substances. The most significant aspect of my work isn't in the fact that I was first in 15 years to obtain transport property data on nanoporous PEI, but in creating the experimental and theoretical tools as a platform for much broader future research in this field.
- Mechanical engineering