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Nano, Micro and Macro Scale Control of Porous Aerogel Morphology

Teo, Nicholas J
http://rave.ohiolink.edu/etdc/view?acc_num=akron154989595598542

Abstract Details

2019, Doctor of Philosophy, University of Akron, Polymer Engineering.
This research centers on combining different disparate technologies with the aerogel fabrication process to the control of nano, micro and macrostructure of aerogel morphology. It is envisioned that this control over structure at different length scales will enable aerogels to be used for various applications such as drug delivery, nanoparticle filtration or oil/water separation. Aerogels are a class of highly porous structures (>90% porosity) with inherently small pores with size typically in the range of 2-200 nm. These small pores are achieved through a combination of both the gel formation and supercritical drying steps, differentiating aerogels from other porous material counterparts (e.g. foams). The control of morphology is accomplished through manipulation of phase growth, introduction of dispersed phase liquids and templating of structures via 3D printing in conjunction with the aerogel fabrication process to produce a variety of aerogel structural forms such as foams, microparticles and mechanical metamaterials. In this work, two different polymeric material systems were studied, namely syndiotactic polystyrene and polyimide. Syndiotactic polystyrene was selected as it forms physically crosslinked, thermo-reversible gels which allow for on-demand gelation, as well as compatibility in water-in-oil emulsion systems. Polyimide was selected as a condensation sol-gel system provides increased flexibility in aerogel mechanical and chemical properties through different monomer selection. It was identified that chemical reaction kinetics, solvent effects, interfacial conditions and kinetics of phase separation all impact and control the structure-property relationships of aerogel materials. The body of work presented in this dissertation covers a wide variety of topics such as new synthesis of aerogel monoliths, microparticles and foams. The fabrication of these aerogel structures required thorough understanding of water-in-oil and oil-in-oil emulsion systems, solvent-monomer/polymer interactions, microfluidics and 3D printing. It was important for this work to adapt and incorporate these different topics within the restrictions and confines of the aerogel fabrication process. The effects of solvent properties (electron accepting capability and viscosity) on the tuning of aerogel pore structures were first evaluated. Both changes in solvent properties affected the gel times of the polyimide sol, thus enabling an increase in the time gap between thermodynamic phase transitions, leading to coarsening of polyimide strands. The gel time and the reaction rate were varied through two methods. First, the solvent acidity/basicity was selected to influence the forward reaction rate and conversion of the crosslinking reaction. Second, the solvent viscosity was varied through the addition of a viscosity modifier; to delay reactant diffusion rates to the reaction sites. Both these methods resulted in a shift in pore size distribution from predominantly mesopores to macropores. In the second part of the work, a microparticle formation process was developed that utilized a cheap, easily-assembled microfluidic device. This allowed the successful synthesis of monodisperse aerogel particles (95 % porosity) using a surfactant-free, oil-in-oil emulsion process. Extension of this microfluidic device with the addition of another flow enabled the creation of core-shell hollow microspheres with diameter of 500 microns and wall thickness of 5 microns. To our knowledge, these hollow aerogel microspheres are the first of its kind reported in literature. The third part of the study was focused on development of a set of hierarchical porous structures known as an aerogel foam, combining the properties of polymer foams (with micrometer size pores) and aerogels (mesoporous structure). These hierarchical structures were produced via emulsion-templating, followed by gelation of the continuous phase. Both water-in-oil emulsion and oil-in-oil emulsion systems were considered to account for the polymer systems that undergo respectively thermo-reversible gelation and sol-gel transition via chemical crosslinking. The resultant aerogel foams exhibited increased porosity due to the inclusion of the macrovoids. Finally, a new technique was developed to create complex aerogel shapes through the incorporation of 3D printing tools in the aerogel synthesis process. In this study, 3D printing was used to create sacrificial hollow molds that allowed the injection of the sol prior to gelation. The sol was allowed to cure in the hollow mold and removed from the mold through selective dissolution of the molds using a specific solvent pairing. This allowed for the creation of aerogels with curved surfaces, recessed spaces and intricate geometries. This method also allowed for the inclusion of an additional level of porosity to an inherently porous structure, resulting in a set of hierarchical porous structures.
Sadhan Jana (Advisor)
Younjin Min (Committee Chair)
Bryan Vogt (Committee Member)
Coleen Pugh (Committee Member)
George Chase (Committee Member)
220 p.
Polymer Chemistry; Polymers
aerogel; microfluidics; polyimide; 3D printing; emulsion-templating

Recommended Citations

Citations

  • Teo, N. J. (2019). Nano, Micro and Macro Scale Control of Porous Aerogel Morphology [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron154989595598542

    APA Style (7th edition)

  • Teo, Nicholas. Nano, Micro and Macro Scale Control of Porous Aerogel Morphology. 2019. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron154989595598542.

    MLA Style (8th edition)

  • Teo, Nicholas. "Nano, Micro and Macro Scale Control of Porous Aerogel Morphology." Doctoral dissertation, University of Akron, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=akron154989595598542

    Chicago Manual of Style (17th edition)

akron154989595598542
296
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This open access ETD is published by University of Akron and OhioLINK.