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School of Chemical and Biomolecular Engineering

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College of Engineering

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TRANSIENT AND MECHANICAL PROPERTIES OF POLY(PHTHALALDEHYDE) AND THE VARIABLE FREQUENCY MICROWAVE CURING OF HIGH-PERFORMANCE THERMOSETS

(Georgia Institute of Technology, 2022-08-08) Warner, Matthew J.

Research presented in this thesis is split into two parts. The first section involves tuning the transient and mechanical properties of poly(phthaldehyde) to form a flexible, liquefiable transient material that can depolymerize upon the flick of a metaphorical switch. Such materials can be useful for devices designed to vanish into their surroundings once used. This application-based research required further understanding of how plasticizer additives work to not only make flexible films in an efficient manner, but how they can also serve to decrease the freezing point of o-phthalaldehyde, poly(phthaldehyde)’s monomer unit, upon degradation such that said devices can effectively disappear. Chapter 1 section 1.1 introduces how poly(phthalaldehyde) works as a transient material, and Chapter 1 section 1.2 describes some important fundamental concepts pertaining to how plasticizers can efficiently provide flexibility to polymers and how they work to reduce poly(phthaldehyde)’s freezing point upon depolymerization. Chapter 2 describes an initial approach used to successfully make flexible poly(phthaldehyde) films, and Chapter 3 describes an improved approach utilizing fundamental principles discussed in Chapter 1 section 1.2. Lastly, challenges regarding flexible poly(phthaldehyde)’s low strength are discussed. The second section involves studying the variable frequency microwave curing of epoxy and cyanate ester resins. Such resins are used for a broad range of applications, including microelectronic packaging, circuit board substrates, lightweighting, high- temperature performance parts, etc. Regardless of the application some thermosets, particularly those that possess a high glass transition temperature, require elevated temperatures above 100°C and cure times above 2 hours for complete cure. Variable frequency microwave heating as an alternative to conventional, thermal heating has been proposed as a method for reducing cure times and temperatures. However, proposed and sometimes conflicting microwave heating phenomena described by scientists and engineers are still not very well understood. Thus, the overarching goal of this section is to better understand and use microwave-heating mechanisms that can be useful in reducing thermoset cure times. This involves using a microwave field’s ability selectively heat reactive species (i.e. a catalyst) at the microscopic level, which can occur when two different materials of dissimilar dielectric parameters are mixed. Chapter 4 briefly summarizes important fundamentals of matter-interactions with microwave electromagnetic fields, and how it pertains to selective heating phenomena. Chapter 5 and 6 describe the microwave curing of high glass transition temperature, homogeneous epoxy and cyanate esters respectively. Chapter 7 describes microwave enhanced curing of cyanate ester resin upon the addition of graphene and reduced graphene oxide, two microwave-absorbing, catalytic fillers. Finally, the problems regarding quantifying selective heating phenomena and dielectric property characterization are described.
Electrode Modifications for Improved Anion Exchange Membrane Water Electrolyzer Durability

(Georgia Institute of Technology, 2021-12-14) Dobbs, Alexandra

Within this dissertation electrode modifications for improved anion exchange membrane (AEM) electrolyzer durability are investigated. Ionic and inert catalyst binders were analyzed during electrolysis and the relationships between material properties and electrode stabilization or degradation are discussed. Systematic analysis of electrode formulations illuminates different catalyst deactivation mechanisms and potential solutions.
Channel engineering of hydroxide ion exchange polymers for electrochemical devices

(Georgia Institute of Technology, 2020-12-06) Huang, Garrett

Anion exchange membrane (AEM) fuel cells and electrolyzers are of interest because they have potential advantages over their acidic counterparts for the production and storage of renewable energy. AEM devices operate under high pH conditions where electrokinetics become more facile and allow for the use of non-noble catalysts. Furthermore, the membranes and ionomers can be made from inexpensive precursor materials which significantly drive down costs. A solid hydroxide ion conducting block copolymer AEM based on the vinyl addition polymerization of norbornene has been synthesized with efficient, phase segregated ion conduction channels, a chemically and thermally stable all-hydrocarbon backbone, and a mechanically robust supporting matrix. Light cross-linking was introduced to enable the use of polymers with high ion exchange capacity while maintaining reasonable water uptake and swelling. In this work, this newly developed class of polymers was extensively characterized and tested in fuel cells and electrolyzers to understand the relationship between polymer properties and device performance.
Synthesis and Applications of Depolymerizable, Phthalaldehyde-Based Polymers

(Georgia Institute of Technology, 2020-11-24) Engler, Anthony Christian

The field of depolymerizable polymers is rapidly expanding as scientists and engineers discover new materials and applications for their use. The ability of a polymer to revert back to its intrinsic monomers with minimal activation energy and without the need for solvents is highly attractive from a chemical recycling perspective. Polymers with these attributes have also found critical roles for niche applications in semiconductor manufacturing and so-called “transient technology”, where materials and devices must be able to degrade and vanish into their local environment after its pre-determined lifetime. Polyaldehydes are one class of materials that have the ability to depolymerize from solid state polymers in response to certain stimuli. The vast majority of polyaldehydes are only metastable at ambient temperatures, and breaking a covalent bond along the backbone of the polymer initiates spontaneous depolymerization because the monomer state possesses a lower thermodynamic energy. Although polyaldehydes were thoroughly studied in the 1970’s and 1980’s, their instability at ambient conditions prevented commercialization, and interest in the materials waned. The past decade has seen a resurgence in the study of polyaldehydes, largely with applications focused on their ability to depolymerize under mild conditions. In particular, poly(phthalaldehyde) has been extensively studied in both fundamental and applied investigations. The overarching goals of the research presented in this dissertation is to better understand the thermodynamics and mechanisms governing o-phthalaldehyde polymerizations so that advanced materials can be better designed for engineering applications. The first two projects described herein explore synthesis of materials. First, a microflow reactor is used to investigate the kinetics and polymer chain growth mechanism of cationically polymerized poly(phthalaldehyde) to determine the rate limiting step in creating large polymers. The second explores fundamental thermodynamics for the copolymerization of o-phthalaldehyde with aliphatic aldehydes that can help change the physical and chemical functionalities of the resulting polymers. The last three studies described focus on semiconductor manufacturing applications using these materials. First, polyaldehydes are used as dry-develop resists for optical lithography to print micron-sized features and delineates various development processes. These polymers are then investigated to their use as sacrificial barrier materials that can be removed via thermolysis to leave behind pristine substrates. Finally, the synthesis of silicon-containing polyaldehydes are described and characterized as to their use as a resist for thermal scanning probe lithography with improved resistance to oxygen-based plasma etching.
Application of sacrificial polymers in electronic and transient devices

(Georgia Institute of Technology, 2019-04-15) Jiang, Jisu

Sacrificial polymers are useful polymers to use in variety of applications. They can be used in microelectronic fabrication process to act as temporary placeholder and create air cavities for microelectronic fabrication and electronic packaging. More recently, there is interest in using sacrificial polymers as structural materials for transient electronic and other transient devices. Transient devices can be triggered to decompose or distintegrate by forming small monomer unit or dissolving in aqueous solution. The resulting devices can vanish and leave behind little or no traces. This provides great opportunity for disposal of electronic devices, as well as for military application where device discovery after usage is unfavorable. However, challenges are remaining to use the existing materials to achieve the application purpose to be met. In order to make them feasible for actual application, modification of polymer chemical structure and tuning of formulations are needed. Sacrificial polymers with suitable thermal stability and chemical miscibility with epoxy resin, such as high ceiling temperature epoxide functionalized poly(propylene carbonate), can act as a temporary placeholder to create nanoporous, low dielectric constant and loss printed circuit boards for faster device switching speed while still maintaining the original mechanical properties. Photosensitive, transient low ceiling temperature poly(phthalaldehyde) are of interest because they can make use of sunlight to trigger the transient properties. The transient and mechanical properties of poly(phthalaldehyde) transient polymer were tuned using different types of traditional and ionic liquid plasticizer for possible applications in a variety of conditions. To extend the application horizon of transient polymers, weakly basic amide and lactam were included in the photosensitive transient films and shown to delay the photo-induced degradation by forming weak conjugate acid in-situ. To make the fabrication process of light sensitive transient device easier, proof-of-concept bilayer photosensitive poly(phthalaldehyde) using photoacid diffusion was explored to make device fabrication more friendly. Prototypes of transient aircraft, transient parachute, and transient triboelectric nanogenerator were successfully demonstrated using the developed technologies. The advances in developing transient materials show a pathway forward to apply these technologies in defense and recyclable devices in the future.
Modifying the depolymerization of sacrifical polymers for electronic devices

(Georgia Institute of Technology, 2018-08-23) Phillips, Oluwadamilola S.

Decomposable polymers can be used in fabrication of integrated circuits and packages. Polymers that can be triggered to depolymerize into volatile, monomeric units are particularly useful as template materials for the creation of embedded-air cavities in the fabrication of traditional integrated circuits, MEMS packaging solutions, and transient (disposable) electronics. The understanding and controlling of the mechanism of thermal depolymerization of these polymers is important. The thermal decomposition of polypropylene carbonate has been changed by end-capping the polymer. This has expanded their use by allowing higher temperature thermal processes. The stimulus for the thermal decomposition has been controlled via photochemical reaction that can be targeted to specific wavelengths of light. The temperature range of the liquid-state after depolymerization of polyaldehdyes has been extended to lower temperatures by depressing the freezing point of the monomer. Liquid products are desirable because the monomer can absorb into the surrounding environment. Dual-layer structures have been demonstrated to limit the photo-chemical trigger to one region, where the acid-catalyst for degradation can propagate to unexposed regions. The multi-layer structure allows for fabrication of a wide-range of devices where the photosensitive trigger can be added at the last fabrication step. The results in this dissertation have created a pathway for the development and application of a new family of transient, photodegradable materials that can be tuned from wavelengths in the ultraviolet light region into the near-infrared regions at ambient or subzero temperatures.
Advances in low-k and transient polymers

(Georgia Institute of Technology, 2017-05-02) Schwartz, Jared Michael

This work focuses on directly photodefinable polymeric materials with lower dielectric constants and improved lithographic properties. Two materials were investigated. One material uses diazonaphthoquinone as the photo active compound with a polynobornene backbone polymer matrix. Epoxy was used to mitigate base uptake and allow cross-linking through both the multi-functional epoxy and the bis(diazonaphthoquinone). The second material uses low ceiling temperature polyaldehydes for packaging applications that have catalytic responses to small triggers. An in-situ NMR method for determining thermodynamic variables of low ceiling temperature polymers was established. The choice of polyaldehyde monomers was also considered for use in dry-developing applications. The dry-developing phenomenon will be utilized for the triggered disappearance of a polymer in a printed wiring board.
Improvements for chip-chip interconnects and MEMS packaging through MEMS materials and processing research

(Georgia Institute of Technology, 2015-01-09) Uzunlar, Erdal

Improvements for Chip-Chip Interconnects and MEMS Packaging Through Materials and Processing Research Erdal Uzunlar 129 Pages Directed by Dr. Paul A. Kohl The work presented in this dissertation focuses on improvements for ever-evolving modern microelectronic technology. Specifically, three topics were investigated in this work: electroless copper deposition on printed wiring boards (PWBs), polymer-based air-gap microelectromechanical systems (MEMS) packaging technology, and thermal stability enhancement in sacrificial polymers, such as poly(propylene carbonate) (PPC). In the electroless copper deposition study, Ag-based catalysts were identified as a low-cost and equally active alternative to expensive Pd-based catalysts. Hot H2SO4 treatment of PWBs was found as a non-roughening surface treatment method to minimize electrical losses. In MEMS packaging study, a sacrificial polymer-based air-gap packaging technique was improved in terms of identification and simplification of air-gap formation process options, optimization of thermal treatment steps, assessing air-gap formation performance, and analyzing the chemical composition of residue. It was found that non-photosensitive PPC leaves less residue, and creates more reliable air-gaps. The mechanical strength of air-gaps was found to come from residual stress in benzocyclobutene (BCB) caps. In thermal stability of PPC study, the mechanism of thermal stability increase on copper (Cu) surfaces was found as the complex formation between Cu(I) and iodonium of the photoacid generator (PAG), leading to hindrance of acid formation by PAG and restriction of acid-catalyzed decomposition of PPC.
Photo-definable dielectrics with improved lithographic, mechanical, and electrical properties

(Georgia Institute of Technology, 2015-01-05) Mueller, Brennen

Permanent dielectric materials are integral to the fabrication of microelectronic devices and packaging. Dielectrics are used throughout devices to electrically and mechanically isolate conductive components. As such, they are required to have low electrical permittivity and robust mechanical properties. For packaging applications, dielectrics can be directly photo-definable. Dielectrics need to have excellent lithographic properties. These properties are pivotal for enabling high yield and low cost fabrication of reliable, energy efficient devices. The aim of this work was to develop new positive tone dielectrics which have improved or application-specific lithographic, mechanical, and electrical properties. To this end, several new dielectric polymers and chemistries were evaluated and characterized. Initially, it was desired to develop a positive tone, polynorbornene (PNB) dielectric that utilizes diazonaphthoquinone (DNQ) photochemistry. Cross-linking was achieved with epoxy cross-linkers during a thermal cure. Several DNQ-containing compounds were evaluated, but only one had good miscibility with PNB. The dissolution characteristics of PNB were measured with respect to polymer composition, DNQ loading, and cross-linker loading. PNB films exhibited unique dissolution properties, and these measurements allowed for an optimum formulation to be developed. A formulation with 20 pphr DNQ and 10 pphr epoxy cross-linker had sufficient inhibition in unexposed regions and fast dissolution in exposed regions. The resulting dielectric was the first positive tone, DNQ-based PNB dielectric. After achieving photo-definability, the cross-linking of the cured dielectric was evaluated by characterizing the mechanical properties. It was discovered that DNQ acted as a cross-linker in these films, and this insight was key to achieving good curing of the dielectric. Several experiments were performed to support this conclusions, and the reaction kinetics of this cross-linking reaction were evaluated. This effort produced a functional, positive tone dielectric with a sensitivity of 408 mJ cm-2 and contrast of 2.3. The modulus was 2.0 to 2.6 GPa and the dielectric constant of 3.7 to 3.9, depending on the curing conditions. The DNQ cross-linking results led to the investigation of other cross-linking chemistries for positive tone dielectrics. A chemically amplified (CA) photochemistry was utilized along with a Fischer esterification cross-linking reaction. Patterning and cross-linking were demonstrated with a methacrylate polymer. Successful positive tone lithography was demonstrated at a high sensitivity of 32.4 mJ cm-2 and contrast of 5.2. Cross-linking was achieved at 120°C as shown by residual stress and solubility measurements. The CA photochemistry and Fischer esterification cross-linking were also demonstrated using a PNB dielectric, which was shown to have improved lithographic properties: a sensitivity of 8.09 mJ cm-2 and contrast of ≥ 14.2. Work was performed to evaluate the effect of the polymer composition on the mechanical and electrical properties. A polymer with 60 mol% hexafluoroisopropanol norbornene and 40 mol% tert-butyl ester norbornene exhibited a dielectric constant of 2.78, which is lower than existing positive tone dielectrics. It also outperformed existing dielectrics in several other categories, including dark erosion, volume change, cure temperature, and in-plane coefficient of thermal expansion. However, a limitation of this dielectric was cracking in thick films. The final study was to improve the mechanical properties of this CA PNB dielectric specifically to enable 5 µm thick films. First, a terpolymer was tested that included a non-functional third monomer. The dielectric constant increased to 3.48 with 24 mol% of the third monomer. Second, low molecular weight additives were used to lower the modulus. Only one of the five tested additives enabled high quality, thick films. This additive did not significantly affect the dielectric constant at low loadings. An optimized formulation was made, and processing parameters were studied. The additive decreased the lithographic properties, lowering the sensitivity to 175 mJ cm-2 and lowering the contrast to 4.36. In all, this work produced three functional dielectrics with positive tone photo-definability and good lithographic properties. Each dielectric can serve a variety of purposes in microelectronics packaging.
The morphology and coulombic efficiency of lithium metal anodes

(Georgia Institute of Technology, 2014-01-07) Goodman, Johanna Karolina Stark

Since their commercialization in 1990, the electrodes of the lithium-ion battery have remained fundamentally the same. While energy density improvements have come from reducing the cell packaging, higher capacity electrodes are needed to continue this trend. A lithium metal anode, where the negative electrode half reaction is the plating and stripping of metallic lithium, is explored as an alternative to current graphite anodes. The specific capacity of the lithium metal anode is over ten times that of the graphite anode, making it a serious candidate to further improve the energy density of lithium batteries. Electrodeposited lithium metal forms dendrites, sharp needles that can grow across the separator and short circuit the battery. Thus, a chief goal is to alter lithium’s plating morphology. This was achieved in two separate ionic liquid electrolytes by co-depositing lithium with sodium. The co-deposited sodium is thought to block dendritic sites, leading to a granular deposit. A nucleation study confirmed that metal deposits from the ionic liquid electrolyte containing sodium, prevented dendritic growth from nucleation on, and not after dendrites had already grown. A model based on the geometry of the nuclei was used to gain insight into the effect of the solid electrolyte interface (SEI) that forms on freshly deposited lithium metal. In addition to sodium, the effect of alkaline earth metals on the lithium deposit morphology was also explored. While these metals did not deposit from the ionic liquid electrolyte, their addition also resulted in granular, dendrite free, deposits. The alkaline earth additives generally increased the overpotential for nucleating on the substrate and lowered the current density achievable. Strontium and barium showed the least of these negative effects while still providing a dendrite free deposit. A second hurdle for lithium metal anodes is the instability between the electrolyte and lithium metal. A protective SEI layer that prevents undesired side reactions is difficult to form because of the large volume change associated with cycling. Formation of a better SEI on lithium metal was attempted through the addition vinylene carbonate, which greatly improved the coulombic efficiency of lithium metal plating and stripping. The effect of gases, such as oxygen, nitrogen and carbon dioxide, on the SEI layer was also investigated. It was found that the presence of nitrogen and oxygen improved the coulombic efficiency by facilitating a thinner SEI layer. This work presents attempts at improving the lithium metal anode both by increasing the coulombic efficiency of the redox process and by eliminating dendrite growth. The coulombic efficiency was improved through the bubbling of gases and addition of organic additives but work remains to increase this value further. Dendritic growth, which poses a safety hazard, was completely eliminated by two methods: 1) co-deposition and 2) adsorption of a foreign metal. Both methods could potentially be applied to different electrolytes, making them promising methods for preventing dendritic growth in future lithium metal anodes.