Doctor of Philosophy / Department of Chemistry / Christer B. Aakeröy / A family of six β-diketone based ligands capable of simultaneously acting as halogen-bond (XB) donors (each of para and meta substituted chloro, bromo and iodo functionalities) and chelating ligands was synthesized. Four ligands were characterized by X-ray diffraction to identify the structural behavior of the ligand itself. The free ligands bearing bromine and iodine show XB interactions (C-X···O) whereas the ligand containing chlorine did not show XB interactions. The corresponding Cu(II) complexes for ligands were also synthesized in different solvents such as acetonitrile, ethyl acetate and nitromethane. Both acetonitrile and ethyl acetate participate in XB interactions with XB donors (Br or I) although nitromethane does not participate in such interaction. Metal-ligand complexes with iodine as XB donor in the para position engage in XB interactions to make extended supramolecular architecture when the solvent is nitromethane. When the XB donor attached in the meta position of the ligand, formation of extended supramolecular architecture was seen even in the presence of a strongly coordinating solvent such as acetonitrile. Two tetra functionalized molecules bearing hydrogen-bond (HB) donors (-OH) and XB donors (-C≡C-I) and one tetra functionalized molecule which has only HB donors (-OH and -C≡C-H) were synthesized. The donor molecules themselves show potential for making HB and XB interactions with the available acceptor sites present in the system. The competition between intermolecular HB and XB was explored by co-crystallizing with suitable nitrogen based acceptors. HB and XB donors showed equal competitiveness toward common acceptors when making HB/ XB interactions. Furthermore, the geometry and relative positioning of the donor sites can, in certain cases, change the balance between the competing interactions by favoring HB interactions. A series of cavitands functionalized with XB donors, HB/XB donors and β-diketone have been synthesized. Binding preferences of XB and HB/XB cavitands towards a series of suitable HB/XB acceptors were studied in solid state and they have confirmed the presence of interactions between donor and acceptors. Cavitands with β-diketone functionality were subjected to binding studies with metal ions in solution as well as in the solid state. Successful metal-ligand complexation in solid state as well as in solution state based on UV/Vis titrations have been confirmed. In order to stabilize chemically unstable energetic compound, pentaerythritol tetranitrocarbamate (PETNC), a co-crystallization approach targeting the acidic protons was employed. A co-crystal, a salt and a solvate were obtained and the acceptors were identified as supramolecular protecting groups leading to reduced chemical reactivity and improved stability of PETNC with minimal reduction of desirable energetic properties. Several potential tetrazole based explosives which are thermal and impact sensitive and solid propellants which are impact sensitive were subjected to co-crystallization experiment to stabilize and enhance their properties. Co-crystals and salts of the explosives were obtained with suitable nitrogen based and oxygen based acceptors. The impact sensitivity and thermal instability of the explosives were improved with the introduction of co-formers. Oxygen based acceptors have shown more favorable explosive property improvements compared to nitrogen based acceptors with significant retention of explosive nature of the parent explosives.

<div>An improved understanding of energy localization ("hot spots'') is needed to improve the safety and performance of explosives. In this work I establish a variety of experimental and computational methods to aid in the investigation of hot spots. In particular, focus is centered on the implicit relationship between hot spots and energetic material sensitivity. To begin, I propose a technique to visualize and quantify the properties of a dynamic hot spot from within an energetic composite subjected to ultrasonic mechanical excitation. The composite is composed of an optically transparent binder and a countable number of HMX crystals. The evolving temperature field is measured by observing the luminescence from embedded phosphor particles and subsequent application of the intensity ratio method. The spatial temperature precision is less than 2% of the measured absolute temperature in the temperature regime of interest (23-220 C). The temperature field is mapped from within an HMX-binder composite under periodic mechanical excitation.</div><div> </div><div> Following this experimental effort I examine the statistics behind the most prevalent and widely used sensitivity test (at least within the energetic materials community) and suggest adaptions to generalize the approach to bimodal latent distributions. Bimodal latent distributions may occur when manufacturing processes are inconsistent or when competing initiation mechanisms are present.</div><div> </div><div> Moving to simulation work, I investigate how the internal void structure of a solid explosive influences initiation behavior -- specifically the criticality of isolated hot spots -- in response to a shock insult. In the last decade, there has been a significant modeling and simulation effort to investigate the thermodynamic response of a shock induced pore collapse process in energetic materials. However, the majority of these studies largely ignore the geometry of the pore and assume simplistic shapes, typically a sphere. In this work, the influence of pore geometry on the sensitivity of shocked HMX is explored. A collection of pore geometries are retrieved from micrographs of pressed HMX samples via scanning electron microscopy. The shock induced collapse of these geometries are simulated using CTH and the response is reduced to a binary "critical'’ / "sub-critical'' result. The simulation results are used to assign a minimum threshold velocity required to exhibit a critical response to each pore geometry. The pore geometries are subsequently encoded to numerical representations and a functional mapping from pore shape to a threshold velocity is developed using supervised machine-learned models. The resulting models demonstrate good predictive capability and their relative performance is explored. The established models are exposed via a web application to further investigate which shape features most heavily influence sensitivity.</div><div> </div><div> Finally, I develop a convolutional neural network capable of directly parsing the 3D electronic structure of a molecule described by spatial point data for charge density and electrostatic potential represented as a 4D tensor. This method effectively bypasses the need to construct complex representations, or descriptors, of a molecule. This is beneficial because the accuracy of a machine learned model depends on the input representation. Ideally, input descriptors encode the essential physics and chemistry that influence the target property. Thousands of molecular descriptors have been proposed and proper selection of features requires considerable domain expertise or exhaustive and careful statistical downselection. In contrast, deep learning networks are capable of learning rich data representations. This provides a compelling motivation to use deep learning networks to learn molecular structure-property relations from "raw'' data. The convolutional neural network model is jointly trained on over 20,000 molecules that are potentially energetic materials (explosives) to predict dipole moment, total electronic energy, Chapman-Jouguet (C-J) detonation velocity, C-J pressure, C-J temperature, crystal density, HOMO-LUMO gap, and solid phase heat of formation. To my knowledge, this demonstrates the first use of the complete 3D electronic structure for machine learning of molecular properties. </div>

Homolytic dissociation of the N-NO2 bond represents primary fission process of energetic materials under the influence of heat, impact, vibration and electric spark. The fission of nitramide bonds is characterized by homolytic bond dissociation energy BDE(RCON-NO2) or disproportionation bond energy DISP(RCON-NO2), which is expressed by an isodesmic reaction RCON-NO2 + SCON-H › RCON-H + SCON NO2, where SCON NO2 is a standard nitramide (1-nitropiperidin-2-on, NPO). This kind of virtual chemical calculation cancels the effect of electron correlation, accompanying the theoretical calculations of free radicals. In this thesis, the homolytic dissociation bond energy BDE(RCON-NO2) and disproportionation bond energy DISP(RCON-NO2) were evaluated for 13 cyclic nitramides using the DFT B3LYP/6-311+G(d,p) method and at the same time the total charges of corresponding nitro groups Q(NO2) were calculated by DFT B3LYP/6-31G(d,p) method. The evaluated BDE and DISP energies were correlated with detonation parameters as squares of detonation velocities and detonation heats. The resulting relationships allow a more detailed description of dependence between the molecular structure of evaluated nitramides and their explosive properties.

<div>From its advent in the 1980s until the 2000s, many of the advances in additive manufacturing (AM) technology were primarily focused on the development of various 3D printing techniques. During the 2000s, AM came to a juncture where these processes were well developed and could be used effectively for rapid prototyping purposes; however, the ability to produce functional components that could reliably perform in a given system had not been fully achieved. The primary focus of AM research since this juncture has been to transition AM from a rapid prototyping technique to a legitimate means of mass manufacturing end-use products. In order to make this happen, two significant areas of research needed to be advanced. The first area focused on advancing the limited selection and functionality of the materials being used for AM. The second area focused on the characterization of the end-use products comprised of these new materials.</div><div><br></div><div>The primary goals of the work described in this document are to substantially further the field of the additive manufacturing by developing new functional materials and subsequently characterizing the resultant printed components. The primary focus of the first two chapters (Chapters 2 and 3) is to further characterize an energetic material system comprising of aluminum (Al) particles embedded in a polyvinylidene fluoride (PVDF) binder, which has been shown to be compatible with AM. This material system has the ability to be implemented as a lightweight multifunctional energetic structural material (MESM); however, significant characterization of its structural energetic properties is needed to ensure reliable MESM performance. First, variations of a previously demonstrated Al/PVDF filament were investigated in order to determine the effect of material constituents on the structural energetic properties of the material. Seven different Al/PVDF formulations, with various particle loadings and particle sizes, were considered. The modulus of elasticity and ultimate strength for the seven formulations were obtained via quasi-static tensile testing of 3D printed dogbones. The energetic performance was quantified via burning rate measurements and differential scanning calorimetry (DSC) of 3D printed samples. Next, variations in the AM process were made and the effect of print direction on the same properties was determined. Once viable MESM performance was quantified, representative structural elements were printed in order to demonstrate the ability to create structural energetic elements. During quasi-static tensile testing, it was observed that aligning the load direction perpendicular to the print direction of the component resulted in inferior mechanical properties. This reduction in mechanical properties can be attributed to the lack of continuity at material interfaces, a well studied phenomena in AM.</div><div><br></div><div>This phenomena is the primary focus of the next two chapters (Chapters 4 and 5), which investigate the polymer healing process as it pertains to fusion-based material extrusion additive manufacturing, also known as fused filament fabrication (FFF). In the context of the FFF process, the extent of the polymer healing, or lack thereof, at the layer interface is known to be thermally driven. Chapter 4 quantifies the relationship between the reduction in mechanical properties and the temperature of the previously deposited layer at the time the subsequent layer is deposited. This relationship gives insight into which parameters should be closely monitored during the FFF process. The following chapter investigates incorporating plasma surface treatment as a means to improve the reduced mechanical properties seen in Chapter 3 and 4. As plasma surface modification can affect various stages of the polymer healing process, a variety of experiments were completed to determine which mechanisms of the plasma treatment were significantly affecting the mechanical properties of the FFF components. The thermal history was analyzed and it was hypothesized that enhanced diffusion at the layer interface was not a significant contributor to, but a rather a detractor from, the improved mechanical properties in this system. A variety of tests investigating how the plasma treatment was affecting the wettability of the surface were performed and all of the tests indicated that the wettability was increased during treatment and was likely the driving mechanism causing the improvement seen in the mechanical properties. These tests give some initial insight into how to successfully pair plasma treatment capabilities with FFF systems and give insights into how that plasma treatment can affect the polymer healing process in FFF applications.</div>

<p>The transformation of mechanical energy into thermal energy within composite energetic materials through various thermomechanical mechanisms is thought to lead to the creation of localized areas of intense heating. The growth of these “hot spots” is responsible for the bulk reaction or decomposition of the energetic material. Understanding the formation and growth of these hot spots has been an active area of research particularly for high-speed impact and shock conditions, but further work remains to be done in particular with respect to hot spot formation due to periodic mechanical excitation. Previous literature has established that many potential thermomechanical mechanisms may act at the interface between the constituent components of a composite energetic material. In order to provide further insight and guidance into the design of safer and more resilient energetic materials, the role of adhesion on hot spot formation for polymer bonded explosives (PBXs), a subset of composite energetic materials, was explored. Single HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane) crystals in polymer blocks were subjected to ultrasonic excitation and subsequent heating was captured via infrared thermography. Subsequent testing of HMX PBXs using a drop weight tower captured changes in the sensitivity of the energetic material. Variation of the polymer binder allowed for a range of adhesive and mechanical properties to be examined. These experiments on the role of adhesion under these kinds of excitations provided insight into how mechanical energy is being transformed into localized heating.</p>

This work presents findings in the combined experimental and computational study of the effects of anisotropy and microstructure on the behavior of HMX-based energetic materials. Large single crystal samples of β-HMX were meticulously created by solvent evaporation for experimental purposes, and respective orientations were identified via x-ray diffraction. Indentation modulus and hardness values were obtained for different orientations of β-HMX via nanoindentation experiments. Small-scale dynamic impact experiments were performed, and a viscoplastic power law model fit, to describe the anisotropic viscoplastic properties of the crystal. The anisotropic fracture toughness and surface energy of β-HMX were calculated by studying indentation-nucleated crack system formations and fitting the corresponding data to two different models, developed by Lawn and Laugier. It was found that the {011} and {110} planes had the highest and lowest fracture toughnesses, respectively. Drop hammer impact tests were performed to investigate effects of morphology on the impact-induced thermal response of HMX. Finally, the anisotropic properties obtained in this work were applied in a cohesive finite element simulation involving the impact of a sample of PBX containing HMX crystals with varying orientations. Cohesive finite element models were generated of separate microstructure containing either anisotropic (locally isotropic) or global isotropic properties of HMX particle. In comparison, the isotropic model appeared to be more deformation resistant.

<p>Explosives and rocket fuel are just two prime examples of energetic materials, compounds that contain a combustible fuel and oxidizer within the same substance. Recent advances have enabled the construction of energetic materials through multiple variations of additive manufacturing, principally inkjet, direct-write, fused filament fabrication, electrospray deposition, and stereolithography. Many of the methods used for creating multiple layered objects (three-dimensional) from energetic materials involve the use of highly viscid materials.</p> <p>The focus of this work was to design a process capable of additively manufacturing three-dimensional objects from melt-castable energetic materials, which are known for their low viscosity. An in-depth printer design and fabrication procedure details the process requirements discovered through previous works, and the adaptations available and used to construct an additive manufacturing device capable of printing both energetic and non-energetic (also referred to as inert) melt-castable materials. Initial characterization of three proposed inert materials confirmed their relative similarity in rheological properties to melt-castable energetic materials and were used to test the printer’s performance.</p> <p>Preliminary tests show the constructed device is capable of additively manufacturing melt-castable materials reproducibly in individual layers, with some initial successful prints in three-dimensions, up to three layers. An initial characterization of the printer’s deposition characteristics additionally matches literature predictions. With the ability to print three-dimensional objects from melt-castable materials confirmed, future work will focus on the reproducibility of multi-layered objects and the refined formulation of melt-castable energetic materials.</p>

<p>High explosives represent a class of materials known as energetic materials, in which providing an external stimulus of impact, heat, and electric shock can result in rapid exothermic reactions. Hence, there has always been a considerable research focus into the development, production, optimization, and control of these materials, aiming to increase explosive capabilities while also decreasing overall sensitivity to ignition.</p><p>The study of impact induced chemical initiation of explosives is an inherent multiscale problem that requires time and length scales not accessible by a single experiment or calculation. The works presented here provide a theoretical effort to contribute to bottom-up modeling of the physics and chemistry phenomena in reacting high explosives using molecular dynamics simulations. Focus will be placed how energy localizes in the molecular crystal TATB, an insensitive high explosive.</p><p>The first energy localization topic covered is an intra-molecular localization and distribution of the kinetic energy. Molecular dynamics is inherently classical, which partition energy equally between all modes. However, most molecular explosives should follow a quantum description, where energy is partitioned between modes following the Bose-Einstein distributions. A semi-classical approximation called the ‘quantum thermal bath’ is applied here to study classical vs. quantum effects for both shock and thermal initiation of chemistry. These results show, not only the importance of the changes to specific heat, which is expected, but the influence of the zero-point energy on reactivity.</p><p>The idea of energy localization is then expanded to the microstructural level, focusing on hotspots, which are areas of extreme temperature following interactions between a shockwave and the microstructure. To date, hotspots have been characterized and described by the localization of their temperature fields only. This work develops a description of the potential energy field in the hotspot, which is markedly different from the temperature field and cannot be predicted from it, as has been previously assumed. This latent potential energy, that is non-thermal, manifests from intra-molecular strain in which individual molecules in the hotspot become highly distorted. This strain energy is shown to be driven by plastic flow during the formation of the hotspot.</p><p>Lastly, the influence of the latent PE in hotspots on chemical reactivity is assessed. Reactive molecular dynamics calculations of shock induced pore collapse creates a hotspot in which deformed molecules can be separately assessed from undeformed ones. Deformed molecules are shown to react faster, follow different ensemble statistics, and undergo different first step reaction pathways. To better study these deformation under equilibrium, the Many-Bodied Steered MD method is developed in which multiple deformation modes are explored. It is shown that different deformation paths in the same molecule leads to different mechanochemical accelerations of kinetics and a different alteration of first step reaction pathways.</p>

<p dir="ltr">Smart energetics are energetic materials (propellants, explosives, and pyrotechnics) with on/off capabilities or in real time modification of combustion behavior. Solid propellants are known for many positive qualities such as their simplicity and low cost but also their glaring lack of active burning rate control. Previous proposed methods of active control of solid propellants include pintle valve actuation and electronically controlled solid propellants, however there is a need for improved methods. Surface area modification is one proposed method and can be employed in real time to affect the burning behavior of solid propellants. To this end, derivations were conducted regarding a slot adjacent to a solid propellant strand and the pressure and slot width threshold conditions that allow for burning to occur inside of the adjacent slot. The derivations considered different modes of combustion (convective and conductive) and combustion threshold conditions. The derivations resulted in five equations that were curve fit to existing literature for validation resulting in high R squared values. A demonstration of the creation of an adjacent slot with a piezoelectric actuator, a mini case study of the adjacent slot proposal, and a discussion of methods to create an adjacent slot as well as the effect of propellant selection on convective burning in slots were all done to follow up on the promising results of the theoretical work. </p><p dir="ltr">Furthermore, flexoelectricity is the coupling between strain gradient and charge generation and has been considered to modify the combustion characteristics of energetic materials. This work measured the flexoelectric properties of polymers and their associated energetic composites including polyvinylidene fluoride (PVDF), micron aluminum (μAl)/PVDF, nano aluminum (nAl)/PVDF, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), nAl/P(VDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)), μAl/P(VDF-HFP), hydroxylterminated polybutadiene (HTPB), ammonium perchlorate (AP)/HTPB, μAl/AP/HTPB, polytetrafluoroethylene (PTFE), and polydimethylsiloxane (PDMS). The measurements made on PVDF, μAl/PVDF, P(VDF-TrFE), P(VDF-HFP), PTFE, and PDMS were all within or near to the range of measurements from the literature. Novel measurements were made on nAl/PVDF, nAl/P(VDF-TrFE), μAl/P(VDF-HFP), HTPB, AP/HTPB, and μAl/AP/HTPB. Additionally, the effect of porosity, particle additions (μAl, nAl, or AP), and manufacturing method (3D printing, casting, different 3D printers, etc.) on the flexoelectric performance of these samples was investigated. It was found that large pores (millimeter scale) added via the infill pattern of 3D printed PVDF and Al/PVDF samples decreased the effective flexoelectric effect relative to the near full density control samples. This contrasts with previous work showing that adding small (micron scale) pores increases the flexoelectric performance of various polymers and energetic materials. Mixed results were found with respect to the effect of particle additions (μAl, nAl, or AP) on the flexoelectricity of a variety of materials. This may be explained by the competing effect of particle additions adding extra local strain gradients which amplify flexoelectricity but also replace some polymer binder material (PVDF, P(VDF-TrFE), P(VDF-HFP), and HTPB) with the particle additions (μAl, nAl, and AP) which are typically less flexoelectric. Our work demonstrates that manufacturing method does affect the flexoelectric properties of polymers and energetic composites. Lastly, our flexoelectric measurements of P(VDF-HFP) and PTFE may help explain accidents related to Magnesium-Teflon®-Viton® (MTV) flare systems that have, in many cases, been attributed to electrostatic discharge.</p>

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