{"id":{"repo_id":"vt","oai_identifier":"oai:vtechworks.lib.vt.edu:10919/139861"},"canonical_url":"https://search.dev.ndltd.org/etd/vt/oai:vtechworks.lib.vt.edu:10919/139861","repository":{"repo_id":"vt","name":"Virginia Tech","base_url":"https://vtechworks.lib.vt.edu/oai/request"},"display":{"title":"Advancing Vat Photopolymerization of Photosensitive Polyimides: Process-Structure-Property Relationships","abstract":"Vat photopolymerization (VPP) is a robust additive manufacturing (AM) technology which can produce products with high resolution features, smooth service finish, and at relatively high throughput. These advantages are leveraged in applications like jewelry manufacturing as well as dental and medical device production. The primary challenge that limits VPP's adoption in broader applications is its limited material selection. Typical photocurable resins amenable to VPP are brittle, have limited thermomechanical performance, and are highly flammable. Aromatic polyimides have found broad adoption in aerospace, automotive, and electronics applications due to their exceptional thermal stability, mechanical strength, and chemical resistance. Their excellent dielectric properties are particularly advantageous for use in flexible circuits, insulating films, and semiconductor manufacturing. However, their excellent properties also make them challenging to process. For this reason, they are typically only found in films, tapes, rods, and plaques. Combining the geometric freedom of VPP with the material properties of aromatic polyimides would open a wide array of new applications. Existing efforts to add photosensitivity to polyimide precursor resins have been limited by challenging synthetic steps, limited solubility, high viscosity, and significant part shrinkage. Prior to this work, the state of the art approach was a method termed the supramolecular salt or \"polysalt\" approach. This strategy utilized electrostatic interactions between small molecule precursors to aromatic polyimides that had been modified to photopolymerize. Following templating the small molecule precursors with VPP, thermal post-processing was used to convert the small molecules first into a higher molecular weight poly(amic acid), then a cyclized polyimide, and finally to degrade the photopolymer scaffold. The aim of this work is to build on the existing understanding of the polysalt approach to evaluate how the VPP process, resin structure, and final part properties interact. The morphology of polysalt parts is explored extensively throughout the thermal post-process . PMDA/ODA and PMDA/DDS polysalt parts are manufactured and thermally post-processed to temperatures ranging from 100 to 400 °C in vacuum to capture morphology snapshots before and after imidization as well as acrylate scaffold degradation. It is found that PMDA/ODA yields highly ordered crystalline morphologies and strong interchain interactions via the polysalt approach while PMDA/DDS does not. This increased order also yields greater thermal stability and reduced porosity in PMDA/ODA polysalt parts. This work also provides a potential framework for the design of new photosensitive polyimide resins with tailored properties. It is also found that dimethyl esters of dianhydrides can be used in aromatic polyimide resin systems (specifically PMDA/ODA and PMDA/DDS) to adjust the acrylate content in what is termed the \"mixed polysalt\" approach. While reducing acrylate content leads to reduced gel stiffness making printing more challenging, it also reduces the buildup of stress during shrinkage in post-processing, leading to reduced part cracking. PMDA/DDS with 50% reduced acrylate content and PMDA/ODA with 20% reduced acrylate content are shown to be printable with gel moduli of 45 kPa and 62 kPa respectively. Further, the decrease in acrylate content leads to less evolution of pore forming gases during acrylate degradation leading to a significant improvement in final part density. Characterization of these factors enables balancing of these tradeoffs to successfully manufacture PMDA/ODA lattices with 0.50% porosity and an HDT of 438.0 °C. Finally, this work extends the resin characterization efforts introduced in earlier chapters by establishing a statistical framework for analyzing working curves. F-tests are established as a means to test whether Jacob's equation describes the relationship between cure depth and exposure for a given resin. In cases where lack-of-fit is determined, it is shown that the prediction of values such as Ec and Dp from the working curve is unreliable. Error expressions for derived values of the working curve such as Dp and Ec are also established. The impact of experimental design considerations such as exposure range, exposure average value, and unexplained variations from experimental setup can impact the error expressions for these derived values. This statistical framework not only advances the rigor of resin evaluation but also provides practical diagnostic tools that complement the polysalt and mixed polysalt strategies, ensuring that new photosensitive polyimide resins can be both effectively designed and reliably characterized for advanced VPP applications.","abstract_html":"Vat photopolymerization (VPP) is a robust additive manufacturing (AM) technology which can produce products with high resolution features, smooth service finish, and at relatively high throughput. These advantages are leveraged in applications like jewelry manufacturing as well as dental and medical device production. The primary challenge that limits VPP's adoption in broader applications is its limited material selection. Typical photocurable resins amenable to VPP are brittle, have limited thermomechanical performance, and are highly flammable. Aromatic polyimides have found broad adoption in aerospace, automotive, and electronics applications due to their exceptional thermal stability, mechanical strength, and chemical resistance. Their excellent dielectric properties are particularly advantageous for use in flexible circuits, insulating films, and semiconductor manufacturing. However, their excellent properties also make them challenging to process. For this reason, they are typically only found in films, tapes, rods, and plaques. Combining the geometric freedom of VPP with the material properties of aromatic polyimides would open a wide array of new applications. Existing efforts to add photosensitivity to polyimide precursor resins have been limited by challenging synthetic steps, limited solubility, high viscosity, and significant part shrinkage. Prior to this work, the state of the art approach was a method termed the supramolecular salt or "polysalt" approach. This strategy utilized electrostatic interactions between small molecule precursors to aromatic polyimides that had been modified to photopolymerize. Following templating the small molecule precursors with VPP, thermal post-processing was used to convert the small molecules first into a higher molecular weight poly(amic acid), then a cyclized polyimide, and finally to degrade the photopolymer scaffold. The aim of this work is to build on the existing understanding of the polysalt approach to evaluate how the VPP process, resin structure, and final part properties interact. The morphology of polysalt parts is explored extensively throughout the thermal post-process . PMDA/ODA and PMDA/DDS polysalt parts are manufactured and thermally post-processed to temperatures ranging from 100 to 400 °C in vacuum to capture morphology snapshots before and after imidization as well as acrylate scaffold degradation. It is found that PMDA/ODA yields highly ordered crystalline morphologies and strong interchain interactions via the polysalt approach while PMDA/DDS does not. This increased order also yields greater thermal stability and reduced porosity in PMDA/ODA polysalt parts. This work also provides a potential framework for the design of new photosensitive polyimide resins with tailored properties. It is also found that dimethyl esters of dianhydrides can be used in aromatic polyimide resin systems (specifically PMDA/ODA and PMDA/DDS) to adjust the acrylate content in what is termed the "mixed polysalt" approach. While reducing acrylate content leads to reduced gel stiffness making printing more challenging, it also reduces the buildup of stress during shrinkage in post-processing, leading to reduced part cracking. PMDA/DDS with 50% reduced acrylate content and PMDA/ODA with 20% reduced acrylate content are shown to be printable with gel moduli of 45 kPa and 62 kPa respectively. Further, the decrease in acrylate content leads to less evolution of pore forming gases during acrylate degradation leading to a significant improvement in final part density. Characterization of these factors enables balancing of these tradeoffs to successfully manufacture PMDA/ODA lattices with 0.50% porosity and an HDT of 438.0 °C. Finally, this work extends the resin characterization efforts introduced in earlier chapters by establishing a statistical framework for analyzing working curves. F-tests are established as a means to test whether Jacob's equation describes the relationship between cure depth and exposure for a given resin. In cases where lack-of-fit is determined, it is shown that the prediction of values such as Ec and Dp from the working curve is unreliable. Error expressions for derived values of the working curve such as Dp and Ec are also established. The impact of experimental design considerations such as exposure range, exposure average value, and unexplained variations from experimental setup can impact the error expressions for these derived values. This statistical framework not only advances the rigor of resin evaluation but also provides practical diagnostic tools that complement the polysalt and mixed polysalt strategies, ensuring that new photosensitive polyimide resins can be both effectively designed and reliably characterized for advanced VPP applications.","abstract_has_math":false,"creators":["Will, John William III"],"institution":"Virginia Tech","degree_name":"Doctor of Philosophy","degree_level":"doctoral","degree_discipline":"Materials Science and Engineering","degree_department":"Materials Science and Engineering","school":null,"contributors":[],"advisors":[],"committee_chairs":["Williams, Christopher Bryant","Long, Timothy E."],"committee_members":["Moore, Robert Bowen","Bortner, Michael J."],"year":2025,"date_issued":"2025-12-09","date_published":"2025-12-09","updated_at":"2026-07-22T22:19:15Z","subjects":["additive manufacturing","vat photopolymerization","aromatic polyimide","polymer processing","polymer characterization"],"languages":["en"],"rights":["In Copyright"],"rights_urls":["http://rightsstatements.org/vocab/InC/1.0/"],"identifier_entries":[{"key":"dc:identifier.other","label":"Dc Identifier Other","values":["vt_gsexam:44807"],"render_values":[{"text":"vt_gsexam:44807","href":null,"code":true}]}]},"links":{"outbound_url":"https://hdl.handle.net/10919/139861","outbound_label":"Handle","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.committeechair","label":"Committee Chair","values":["Williams, Christopher Bryant","Long, Timothy E."]},{"key":"dc:contributor.committeemember","label":"Committee Member","values":["Moore, Robert Bowen","Bortner, Michael J."]},{"key":"dc:contributor.department","label":"Department","values":["Materials Science and Engineering"]},{"key":"dc:creator","label":"Author","values":["Will, John William III"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2025-12-10T09:01:11Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2025-12-10T09:01:11Z"]},{"key":"dc:date.issued","label":"Date","values":["2025-12-09"]},{"key":"dc:publisher","label":"Institution","values":["Virginia Tech"]},{"key":"dc:type","label":"Dc Type","values":["Dissertation"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Materials Science and Engineering"]},{"key":"thesis:degree_level","label":"Degree Level","values":["doctoral"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Doctor of Philosophy"]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["Virginia Polytechnic Institute and State University"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["additive manufacturing","vat photopolymerization","aromatic polyimide","polymer processing","polymer characterization"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["In Copyright"]},{"key":"dc:rights.uri","label":"Rights URI","values":["http://rightsstatements.org/vocab/InC/1.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.other","label":"Dc Identifier Other","values":["vt_gsexam:44807"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://hdl.handle.net/10919/139861"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Vat photopolymerization (VPP) is a robust additive manufacturing (AM) technology which can produce products with high resolution features, smooth service finish, and at relatively high throughput. These advantages are leveraged in applications like jewelry manufacturing as well as dental and medical device production. The primary challenge that limits VPP's adoption in broader applications is its limited material selection. Typical photocurable resins amenable to VPP are brittle, have limited thermomechanical performance, and are highly flammable. Aromatic polyimides have found broad adoption in aerospace, automotive, and electronics applications due to their exceptional thermal stability, mechanical strength, and chemical resistance. Their excellent dielectric properties are particularly advantageous for use in flexible circuits, insulating films, and semiconductor manufacturing. However, their excellent properties also make them challenging to process. For this reason, they are typically only found in films, tapes, rods, and plaques. Combining the geometric freedom of VPP with the material properties of aromatic polyimides would open a wide array of new applications. Existing efforts to add photosensitivity to polyimide precursor resins have been limited by challenging synthetic steps, limited solubility, high viscosity, and significant part shrinkage. Prior to this work, the state of the art approach was a method termed the supramolecular salt or \"polysalt\" approach. This strategy utilized electrostatic interactions between small molecule precursors to aromatic polyimides that had been modified to photopolymerize. Following templating the small molecule precursors with VPP, thermal post-processing was used to convert the small molecules first into a higher molecular weight poly(amic acid), then a cyclized polyimide, and finally to degrade the photopolymer scaffold. The aim of this work is to build on the existing understanding of the polysalt approach to evaluate how the VPP process, resin structure, and final part properties interact. The morphology of polysalt parts is explored extensively throughout the thermal post-process . PMDA/ODA and PMDA/DDS polysalt parts are manufactured and thermally post-processed to temperatures ranging from 100 to 400 °C in vacuum to capture morphology snapshots before and after imidization as well as acrylate scaffold degradation. It is found that PMDA/ODA yields highly ordered crystalline morphologies and strong interchain interactions via the polysalt approach while PMDA/DDS does not. This increased order also yields greater thermal stability and reduced porosity in PMDA/ODA polysalt parts. This work also provides a potential framework for the design of new photosensitive polyimide resins with tailored properties. It is also found that dimethyl esters of dianhydrides can be used in aromatic polyimide resin systems (specifically PMDA/ODA and PMDA/DDS) to adjust the acrylate content in what is termed the \"mixed polysalt\" approach. While reducing acrylate content leads to reduced gel stiffness making printing more challenging, it also reduces the buildup of stress during shrinkage in post-processing, leading to reduced part cracking. PMDA/DDS with 50% reduced acrylate content and PMDA/ODA with 20% reduced acrylate content are shown to be printable with gel moduli of 45 kPa and 62 kPa respectively. Further, the decrease in acrylate content leads to less evolution of pore forming gases during acrylate degradation leading to a significant improvement in final part density. Characterization of these factors enables balancing of these tradeoffs to successfully manufacture PMDA/ODA lattices with 0.50% porosity and an HDT of 438.0 °C. Finally, this work extends the resin characterization efforts introduced in earlier chapters by establishing a statistical framework for analyzing working curves. F-tests are established as a means to test whether Jacob's equation describes the relationship between cure depth and exposure for a given resin. In cases where lack-of-fit is determined, it is shown that the prediction of values such as Ec and Dp from the working curve is unreliable. Error expressions for derived values of the working curve such as Dp and Ec are also established. The impact of experimental design considerations such as exposure range, exposure average value, and unexplained variations from experimental setup can impact the error expressions for these derived values. This statistical framework not only advances the rigor of resin evaluation but also provides practical diagnostic tools that complement the polysalt and mixed polysalt strategies, ensuring that new photosensitive polyimide resins can be both effectively designed and reliably characterized for advanced VPP applications."]},{"key":"dc:description.abstractgeneral","label":"General Abstract","values":["Vat photopolymerization (VPP) is a form of additive manufacturing (AM), or 3D printing, where light is used to cure liquid resin iteratively in thin layers to build a 3D object. It is a technology that can produce parts with very fine detail, smooth surfaces, and at relatively high speed. It is already used in industries like jewelry, dentistry, and medical devices. However, VPP is limited by its material selection. VPP materials are typically brittle, weak at high temperatures, and flammable. In contrast, aromatic polyimides are a class of polymers known for their excellent thermal stability, mechanical strength, chemical resistance, radiation resistance, and electrical properties. These qualities make aromatic polyimides essential in aerospace, automotive, and electronics applications for insulation films, flexible circuits, radiation protection, etc. Unfortunately, their excellent properties cause them to be difficult to process, so they are usually only made as thin films or solid blocks. Combining the precision of VPP with the performance of aromatic polyimides could open the door to a new generation of high-performance printed materials. The aim of this work is to evaluate how the VPP process, resin structure, and final part properties interact. The polysalt approach, a state-of-the-art method to print aromatic polyimides with VPP, is explored and expanded. Two different polyimide systems, known as PMDA/ODA and PMDA/DDS, are printed and heat-treated up to 400°C. It is found that PMDA/ODA forms a more ordered and stable structure than PMDA/DDS, leading to better heat resistance and fewer pores in the final parts. Additionally, this study introduces the mixed polysalt approach, which builds on the polysalt approach by allowing control over the amount of light-sensitive sacrificial template included in the resin. Reducing this light-sensitive template reduces cracking and porosity of the final parts at the cost of increasing the difficulty of printing. Through studying this tradeoff and the material properties throughout processing as a function of template content and part thickness, highly performant PMDA/ODA parts are produced with very low porosity (only 0.5%) and exceptional heat resistance, surviving temperatures above 430°C without deforming. Printing new resins (e.g. the polysalt and mixed polysalt resins) with VPP requires an understanding of how much light is needed to cure the material. Working curves are typically used in VPP to relate light with resin curing behavior. This research develops statistical methods for these working curves, helping to identify when the working curve fails to describe resin behavior accurately, and providing a measure of confidence for key material parameters. Together, these advances in material chemistry, printing strategy, and data analysis offer a foundation for creating next-generation high-performance VPP materials suitable for demanding engineering applications."]},{"key":"dc:description.degree","label":"Dc Description Degree","values":["Doctor of Philosophy"]},{"key":"dc:format.medium","label":"Dc Format Medium","values":["ETD"]},{"key":"dc:title","label":"Title","values":["Advancing Vat Photopolymerization of Photosensitive Polyimides: Process-Structure-Property Relationships"]}]}],"canonical_facts":{"dc:contributor.committeechair":["Williams, Christopher Bryant","Long, Timothy E."],"dc:contributor.committeemember":["Moore, Robert Bowen","Bortner, Michael J."],"dc:contributor.department":["Materials Science and Engineering"],"dc:creator":["Will, John William III"],"dc:date.accessioned":["2025-12-10T09:01:11Z"],"dc:date.available":["2025-12-10T09:01:11Z"],"dc:date.issued":["2025-12-09"],"dc:description.abstract":["Vat photopolymerization (VPP) is a robust additive manufacturing (AM) technology which can produce products with high resolution features, smooth service finish, and at relatively high throughput. These advantages are leveraged in applications like jewelry manufacturing as well as dental and medical device production. The primary challenge that limits VPP's adoption in broader applications is its limited material selection. Typical photocurable resins amenable to VPP are brittle, have limited thermomechanical performance, and are highly flammable. Aromatic polyimides have found broad adoption in aerospace, automotive, and electronics applications due to their exceptional thermal stability, mechanical strength, and chemical resistance. Their excellent dielectric properties are particularly advantageous for use in flexible circuits, insulating films, and semiconductor manufacturing. However, their excellent properties also make them challenging to process. For this reason, they are typically only found in films, tapes, rods, and plaques. Combining the geometric freedom of VPP with the material properties of aromatic polyimides would open a wide array of new applications. Existing efforts to add photosensitivity to polyimide precursor resins have been limited by challenging synthetic steps, limited solubility, high viscosity, and significant part shrinkage. Prior to this work, the state of the art approach was a method termed the supramolecular salt or \"polysalt\" approach. This strategy utilized electrostatic interactions between small molecule precursors to aromatic polyimides that had been modified to photopolymerize. Following templating the small molecule precursors with VPP, thermal post-processing was used to convert the small molecules first into a higher molecular weight poly(amic acid), then a cyclized polyimide, and finally to degrade the photopolymer scaffold. The aim of this work is to build on the existing understanding of the polysalt approach to evaluate how the VPP process, resin structure, and final part properties interact. The morphology of polysalt parts is explored extensively throughout the thermal post-process . PMDA/ODA and PMDA/DDS polysalt parts are manufactured and thermally post-processed to temperatures ranging from 100 to 400 °C in vacuum to capture morphology snapshots before and after imidization as well as acrylate scaffold degradation. It is found that PMDA/ODA yields highly ordered crystalline morphologies and strong interchain interactions via the polysalt approach while PMDA/DDS does not. This increased order also yields greater thermal stability and reduced porosity in PMDA/ODA polysalt parts. This work also provides a potential framework for the design of new photosensitive polyimide resins with tailored properties. It is also found that dimethyl esters of dianhydrides can be used in aromatic polyimide resin systems (specifically PMDA/ODA and PMDA/DDS) to adjust the acrylate content in what is termed the \"mixed polysalt\" approach. While reducing acrylate content leads to reduced gel stiffness making printing more challenging, it also reduces the buildup of stress during shrinkage in post-processing, leading to reduced part cracking. PMDA/DDS with 50% reduced acrylate content and PMDA/ODA with 20% reduced acrylate content are shown to be printable with gel moduli of 45 kPa and 62 kPa respectively. Further, the decrease in acrylate content leads to less evolution of pore forming gases during acrylate degradation leading to a significant improvement in final part density. Characterization of these factors enables balancing of these tradeoffs to successfully manufacture PMDA/ODA lattices with 0.50% porosity and an HDT of 438.0 °C. Finally, this work extends the resin characterization efforts introduced in earlier chapters by establishing a statistical framework for analyzing working curves. F-tests are established as a means to test whether Jacob's equation describes the relationship between cure depth and exposure for a given resin. In cases where lack-of-fit is determined, it is shown that the prediction of values such as Ec and Dp from the working curve is unreliable. Error expressions for derived values of the working curve such as Dp and Ec are also established. The impact of experimental design considerations such as exposure range, exposure average value, and unexplained variations from experimental setup can impact the error expressions for these derived values. This statistical framework not only advances the rigor of resin evaluation but also provides practical diagnostic tools that complement the polysalt and mixed polysalt strategies, ensuring that new photosensitive polyimide resins can be both effectively designed and reliably characterized for advanced VPP applications."],"dc:description.abstractgeneral":["Vat photopolymerization (VPP) is a form of additive manufacturing (AM), or 3D printing, where light is used to cure liquid resin iteratively in thin layers to build a 3D object. It is a technology that can produce parts with very fine detail, smooth surfaces, and at relatively high speed. It is already used in industries like jewelry, dentistry, and medical devices. However, VPP is limited by its material selection. VPP materials are typically brittle, weak at high temperatures, and flammable. In contrast, aromatic polyimides are a class of polymers known for their excellent thermal stability, mechanical strength, chemical resistance, radiation resistance, and electrical properties. These qualities make aromatic polyimides essential in aerospace, automotive, and electronics applications for insulation films, flexible circuits, radiation protection, etc. Unfortunately, their excellent properties cause them to be difficult to process, so they are usually only made as thin films or solid blocks. Combining the precision of VPP with the performance of aromatic polyimides could open the door to a new generation of high-performance printed materials. The aim of this work is to evaluate how the VPP process, resin structure, and final part properties interact. The polysalt approach, a state-of-the-art method to print aromatic polyimides with VPP, is explored and expanded. Two different polyimide systems, known as PMDA/ODA and PMDA/DDS, are printed and heat-treated up to 400°C. It is found that PMDA/ODA forms a more ordered and stable structure than PMDA/DDS, leading to better heat resistance and fewer pores in the final parts. Additionally, this study introduces the mixed polysalt approach, which builds on the polysalt approach by allowing control over the amount of light-sensitive sacrificial template included in the resin. Reducing this light-sensitive template reduces cracking and porosity of the final parts at the cost of increasing the difficulty of printing. Through studying this tradeoff and the material properties throughout processing as a function of template content and part thickness, highly performant PMDA/ODA parts are produced with very low porosity (only 0.5%) and exceptional heat resistance, surviving temperatures above 430°C without deforming. Printing new resins (e.g. the polysalt and mixed polysalt resins) with VPP requires an understanding of how much light is needed to cure the material. Working curves are typically used in VPP to relate light with resin curing behavior. This research develops statistical methods for these working curves, helping to identify when the working curve fails to describe resin behavior accurately, and providing a measure of confidence for key material parameters. Together, these advances in material chemistry, printing strategy, and data analysis offer a foundation for creating next-generation high-performance VPP materials suitable for demanding engineering applications."],"dc:description.degree":["Doctor of Philosophy"],"dc:format.medium":["ETD"],"dc:identifier.other":["vt_gsexam:44807"],"dc:identifier.uri":["https://hdl.handle.net/10919/139861"],"dc:language.iso":["en"],"dc:publisher":["Virginia Tech"],"dc:rights":["In Copyright"],"dc:rights.uri":["http://rightsstatements.org/vocab/InC/1.0/"],"dc:subject":["additive manufacturing","vat photopolymerization","aromatic polyimide","polymer processing","polymer characterization"],"dc:title":["Advancing Vat Photopolymerization of Photosensitive Polyimides: Process-Structure-Property Relationships"],"dc:type":["Dissertation"],"thesis:degree_discipline":["Materials Science and Engineering"],"thesis:degree_level":["doctoral"],"thesis:degree_name":["Doctor of Philosophy"],"thesis:institution_name":["Virginia Polytechnic Institute and State University"]},"updated_at":"2026-07-22T22:19:15Z"}