The use of nanoreinforcements in automotive structural composites has provided promising improvements in their mechanical properties. For the first time, a robust statistical design of experiments approach was undertaken to demonstrate how key formulation and processing factors (nanofiber type, use of dispersing agent, mixing method, nanofiber weight fraction, and temperature) affected the dynamic mechanical properties of vapor-grown carbon nanofiber (VGCNF)/vinyl ester (VE) nanocomposites. Statistical response surface models were developed to predict nanocomposite storage and loss moduli as functions of significant factors. Only ~0.50 parts of nanofiber per hundred parts resin produced a roughly 20% increase in the storage modulus versus that of the neat VE at room temperature. Optimized nanocomposite properties were predicted as a function of design factors employing this methodology. For example, the use of highshear mixing (one of the mixing methods in the design) with the oxidized VGCNFs in the absence of dispersing agent or arbitrarily with pristine VGCNFs in the presence of dispersing agent was found to maximize the predicted storage modulus over the entire temperature range (30-120 °C). To study the key concept of interphase in thermoset nanocomposites, molecular dynamics simulations were performed to investigate liquid VE resin monomer interactions with the surface of a pristine VGCNF. A liquid resin having a mole ratio of styrene to bisphenol A-diglycidyl dimethacrylate monomers consistent with a 33 wt% styrene VE resin was placed in contact with both sides of pristine graphene sheets, overlapped like shingles, to represent the outer surface of a pristine VGCNF. The relative monomer concentrations were calculated in a direction progressively away from the surface of the graphene sheets. At equilibrium, the styrene/VE monomer ratio was higher in a 5 Å thick region adjacent to the nanofiber surface than in the remaining liquid volume. The elevated styrene concentration near the nanofiber surface suggests that a styrene-rich interphase region, with a lower crosslink density than the bulk matrix, could be formed upon curing. Furthermore, styrene accumulation in the immediate vicinity of the nanofiber surface might, after curing, improve the nanofiber-matrix interfacial adhesion compared to the case where the monomers were uniformly distributed throughout the matrix.

Molecular dynamics (MD) simulations have been performed to investigate the system equilibrium through the atomic/molecular interactions of a liquid vinyl ester (VE) thermoset resin with the idealized surfaces of both pristine vapor-grown carbon nanofibers (VGCNFs) and oxidized VGCNFs. The VE resin has a mole ratio of styrene to bisphenol-A-diglycidyl dimethacrylate VE monomers consistent with a commercially available 33 wt% styrene VE resin (Derakane 441-400). The VGCNF-VE resin interactions may influence the distribution of the liquid VE monomers in the system and the formation of an interphase region. Such an interphase may possess a different mole ratio of VE resin monomers at the vicinity of the VGCNF surfaces compared to the rest of the system after resin curing. Bulk nano-reinforced material properties are highly dependent on the interphase features because of the high surface area to volume ratio of nano-reinforcements. For example, higher length scale micromechanical calculations suggest that the volume fraction and properties of the interphase can have a profound effect on bulk material properties. Interphase formation, microstructure, geometries, and properties in VGCNF-reinforced polymeric composites have not been well characterized experimentally, largely due to the small size of typical nano-reinforcements and interphases. Therefore, MD simulations offer an alternative means to probe the nano-sized formation of the interphase and to determine its properties, without having to perform fine-scale experiments. A robust crosslinking algorithm for VE resin was then developed as a key element of this research. VE resins are crosslinked via free radical copolymerization account for regioselectivity and monomer reactivity ratios. After the VE crosslinked network was created, the constitutive properties of the resin were calculated. This algorithm will be used to crosslink equilibrated VE resin systems containing both pristine and oxidized VGCNFs. An understanding of formation and kinematics of a crosslinked network obtained via MD simulations can facilitate nanomaterials design and can reduce the amount of nanocomposite experiments required. VGCNF pull-out simulations will then be performed to determine the interfacial shear strength between VGCNFs and the matrix. Interphase formation, thickness and interfacial shear strength can directly feed into higher length scale micromechanical models within a global multiscale analysis framework.

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Analytic methods of elastic wave scattering in fiber-reinforced composite materials are investigated in this study to calculate the effective static stiffness (axial shear modulus, m) and wave properties (axially shear wave speed, B and attenuation, Y) in composites. For simplicity only out-of-plane shear waves are modeled propagating in a plane transverse to the fiber axis. Statistical averaging of a spatially random distribution of fibers is performed and a simultaneous system of linear equations are obtained from which the effective global wave numbers are numerically calculated. The wave numbers, K=Re(K)+iIm(K), are complex numbers where the real parts are used to compute the effective axial shear static stiffness and wave speed; the imaginary parts are used to compute the effective axial shear wave attenuation in composites. Three major parts of this study are presented. The first part is the discussion of multiple scattering phenomena in a successive-events scattering approach. The successive-events scattering approach is proven to be mathematically exact by comparing the results obtained by the many-bodies-single-event approach. Scattering cross-section is computed and comparison of the first five scattering orders is made. Furthermore, the ubiquitous quasi-crystalline approximation theorem is given a justifiable foundation in the fiber-matrix composite context. The second part is to calculate m, B and Y for fiber-reinforced composites with interfacial layers between fibers and matrix. The material properties of the layers are assumed to be either linearly or exponentially distributed between the fibers and matrix. A concise formula is obtained where parameters can be computed using a computationally easy-to-program determinant of a square matrix. The numerical computations show, among other things, that the smoother (more divisional layers), or thinner, the interfacial region the less damped are the composite materials. Additionally composites with exponential order distribution of the interfacial region are more damped than the linear distribution ones. The third part is to calculate m, B and Y for fiber-reinforced composites with interfacial cracks. The procedures and computational techniques are similar to those in the second part except that the singularity near the crack tip needs the Chebychev function as a series expansion to be adopted in the computation. Both the interfacial layers and interfacial crack cases are analyzed in the low frequency range. The analytic results show that waves in both cases are attenuated and non-dispersive in the low frequency range. The composites with interfacial layers are transversely isotropic, while composites with interfacial cracks are generally transversely anisotropic. / Ph. D.

This dissertation research addresses the interrelationships between chemistry and network structure in epoxy networks as well as how mechanical properties of the resulting networks are affected by these relationships. The effects of chemistry and network structure on interphase morphology and performance in vinyl ester/carbon fiber composites have also been investigated on both a macro and nanoscale. Thermosets were prepared with blends of bisphenol-A and novel phosphine oxide based diepoxide oligomers using a siloxane or a novolac crosslinking agent. In the siloxane cured networks the incorporation of the phosphine oxide group yielded networks with increased glass transition temperatures, from 71°C to 92°C, and water absorption, from 1 wt % to 5.5 wt %, due to the polar nature of the phosphine oxide bond. Higher char yields were also observed with the addition of the phosphorus, 27 wt % compared to 11 wt % for bisphenol-A epoxy networks. The bisphenol-A based epoxy/siloxane network was exceptionally ductile with a fracture toughness (K1c) of 2 MPa-m1/2. In networks prepared with the novolac crosslinking agent hydrogen bonding, observed using FTIR, was evident even at temperatures above the network Tg and resulted in increased rubbery moduli with phosphine oxide incorporation. Adhesive strengths to steel increased from ~9.7 MPa with bisphenol-A epoxy to ~13.8 MPa when the phosphine oxide containing epoxy was incorporated into the network. Within carbon fiber/vinyl ester composites, a series of tough ductile thermoplastics and a series of one-phase polyurethanes were investigated as carbon fiber sizings. The three poly(hydroxyether)s resulted in different interphase morphologies due to their respective interdiffusion into the vinyl ester resin. The unmodified poly(hydroxyether) was miscible with the vinyl ester resin at the elevated cure temperatures and adhesion between the fiber and bulk matrix was increased from 28 MPa with unsized fibers to 45 MPa with sized fibers. The carboxylate modified poly(hydroxyether) was also miscible at elevated temperatures, however the interdiffused region was narrower, ~5 mm. This system showed an increase in the fiber/matrix adhesion similar to that found for the unmodified poly(hydroxyether)/vinyl ester system and composite cyclic fatigue durability was improved by ~50 %. Using a poly(hydroxyether ethanolamine) interphase material, which was not miscible with the resin, resulted in a sharp interface. While the adhesion was not improved through the use of this sizing, the composite fatigue durability was still increased by a moderate amount, ~ 25%. The one-phase polyurethanes were dispersible in water with incorporation of a minimum of 0.08 equivalents of N-methyldiethanolamine per mole of diisocyanate. Fatigue durability in composite panels was not improved by the addition of the polyurethane sizings due to the miscibility of the sizing and the matrix. / Ph. D.

This work addresses the morphology of the wood/ Phenol-Formaldehyde (PF) adhesive interphase using yellow-poplar. In this case, morphology refers to the scale or dimension of adhesive penetration into wood. The objective is to develop methods for revealing ever smaller levels of wood/resin morphology. Dynamic techniques that are commonly utilized in polymer blend studies are investigated as potential methods for probing the wood/ adhesive interphase morphology. These are Dynamic Mechanical Analysis (DMA) and solid state NMR using CP/MAS. PF resin molecular weight is manipulated to promote or inhibit resin penetration in wood, using a very low or a very high molecular weight PF resin. With DMA, the influence of PF resin on wood softening is investigated. It is first demonstrated that the cooperativity analysis according to the Ngai coupling model of relaxation successfully applies to the in-situ lignin glass transition of yellow-poplar and spruce woods. No significant difference in intermolecular coupling is detected between the two woods. It is then demonstrated that combining simple DMA measurements with the cooperativity analysis yields ample sensitivity to the interphase morphology. From simple DMA temperature scans, a low molecular weight PF (PF-Low) does not influence lignin glass transition temperature. However, the Ngai coupling model of relaxation indicates that intermolecular coupling is enhanced with the low molecular weight PF. This behavior is ascribed to the low molecular weight PF penetrating lignin on a nanometer scale and polymerizing in-situ. On the other hand, a high molecular weight resin with a broad distribution of olecular weights (PF-High) lowers lignin glass transition temperature dramatically. This plasticizing effect is ascribed to a small fraction of the PF resin being low enough in molecular weight to penetrate lignin on a nanoscale, but being too dispersed for forming a crosslinked network. With CP/MAS NMR, intermolecular cross-polarization experiments are found unsuitable to probe the angstrom scale morphology of the wood adhesive interphase. However, observing the influence of the PF resins on the spin lattice relaxation time in the rotating frame, HT1r, and the cross-polarization time (TCH) is useful for probing the interphase morphology. None of the resins significantly affects the cross-polarization time, suggesting that angstrom scale penetration does not occur with a low nor a high molecular weight PF resin. However, the low molecular weight PF substantially modifies wood polymer HT1r, indicating that the nanometer scale environment of wood polymers is altered. On the other hand, the high molecular weight PF resin has no effect on wood HT1r. On average, the high molecular weight PF does not penetrate wood on a nanometer scale. Interestingly, the low molecular weight PF resin disrupts the spin coupling that is typical among wood components. Spin coupling between wood components is insensitive to the high molecular weight PF. Finally, it is noteworthy that the two PF resins have significantly different T1r 's in-situ. The low molecular weight resin T1r lies within the range of wood relaxations, suggesting some degree of spin coupling. On the other hand, the T1r of the high molecular weight PF appears outside the range of wood relaxations. Spin coupling between the high molecular weight resin and wood components is therefore inefficient. The CP/MAS NMR and DMA studies converge to identify nanometer scale penetration of the low molecular weight PF in wood. On the other hand, the high molecular weight PF resin forms separate domains from wood, although a very small fraction of the PF-High is able to penetrate wood polymers on a nanoscale. / Ph. D.

The development and commercialization of alkali ion secondary batteries has played a critical role in the development of personal electronics and electric vehicles. The recent increase in demand for electric vehicles has pushed for lighter batteries with a higher energy density to reduce the weight of the vehicle while with an emphasis on improving the mile range. A resurgence has occurred in lithium, and sodium, metal anode research due to their high theoretical capacities, low densities, and low redox potentials. However, Li and Na metal anodes suffer from major safety issues and long-term cycling stability. This dissertation focuses on the investigation of the interfacial chemistries between alkali metal-carbon host interactions and the electrode-electrolyte interactions of the cathode and anode with boron-based electrolytes to establish design rules for "lean" alkali metal composite anodes and improve long-term stability to enable alkali metal batteries for practical electrochemical applications. Chapter 2 of this thesis focuses on the design and preliminary investigation of "lean" lithium-carbon nanofiber (<5 mAh cm-2) composite anodes in full cell testing using a LiNi0.6Mn0.2Co0.2O2 (NMC 622) cathode. We used the electrodeposition method to synthesize the Li-CNF composite anodes with a range of electrodeposition capacities and current densities and electrolyte formulations. Increasing the electrodeposition capacity improved the cycle life with 3 mAh cm-2 areal capacity and 2% vinylene carbonate (VC) electrolyte additive gave the best cycle life before reaching a state of "rapid cell failure". Increasing the electrodeposition rate reduced cycling stability and had a faster fade in capacity. The electrodeposition of lithium metal into a 2D graphite anode significantly improved cycle life, implying the increased crystallinity of the carbon substrate promotes improved anode stability and cycling capabilities. As the increased crystallinity of the carbon anode was shown to improve the "lean" composite anode's performance, Chapter 3 focuses on utilizing a CNF electrode designed with a higher degree of graphitization and probing the interacting mechanism of Li and Na with the CNF host. Characterization of the CNF properties found the material to be more reminiscent of hard carbon materials. Electrochemical analysis showed better long-term performance for Na-CNF symmetric cells. Kinetic analysis, using cyclic voltammetry (CV), revealed that Na ions successfully (de)intercalated within the CNF crystalline interlayers, while Li ions were limited to surface adsorption. A change in mechanism was quickly observed in the Na-CNF symmetric cycling from metal stripping/plating to ion intercalation/deintercalation, enabling the superior cycling stability of the composite anode. Improving the Na metal stability is necessary for enabling Na-CNF improved long-term performance. Sodium batteries have begun to garner more attention for grid storage applications due to their overall lower cost and less volumetric constraint required. However, sodium cathodes have poor electrode-electrolyte stability, leading to nanocracks in the cathode particles and transition metal dissolution. Chapter 4 focuses on electrolyte engineering with the boron salts sodium difluoro(oxolato)borate (NaDFOB) and sodium tetrafluoroborate (NaBF4) mixed together with sodium hexafluorophosphate (NaPF6) to improve the electrode-electrolyte compatibility and cathode particle stability. The electrolytes containing NaDFOB showed improved electrochemical stability at various temperatures, the formation of a more robust electrode-electrolyte interphase, and suppression in transition metal (TM) reduction and dissolution of the cathode particles measured after cycling. In Chapter 5, we focus on the electrochemical properties and the anode-electrolyte interfacial chemistry properties of the sodium borate salt electrolytes. Similar to Chapter 4, the NaDFOB containing electrolytes have improved electrochemical performance and stability. Following the same electrodeposition parameters as Chapter 2, we find the NaDFOB electrolytes improves the stability of electrodeposited Na metal and the "lean" composite anode's cyclability. This study suggests the great potential for the NaDFOB electrolytes for Na ion battery applications. / Doctor of Philosophy / The ever-increasing demand for high energy storage in personal electronics, electric vehicles, and grid energy storage has driven for research to safely enable alkali metal (Li and Na) anodes for practical energy storage applications. Key research efforts have focused on developing alkali metal composite anodes, as well as improving the electrode-electrolyte interfacial chemistries. A fundamental understanding of the electrode interactions with the electrolyte or host materials is necessary to progress towards safer batteries and better battery material design for long-term applications. Improving the interfacial interactions between the host-guest or electrode-electrolyte interfaces allows for more efficient charge transfer processes to occur, reduces interfacial resistance, and improves overall stability within the battery. As a result, there is great potential in understanding the host-guest and electrode-electrolyte interactions for the design of longer-lasting and safer batteries. This dissertation focuses on probing the interfacial chemistries of the battery materials to enable "lean" alkali metal composite anodes and improve electrode stability through electrolyte interactions. The anode-host interactions are first explored through preliminary design development for "lean" alkali composite anodes using carbon nanofiber (CNF) electrodes. The effect on increasing the crystallinity of the CNF host on the Li- and Na-CNF interactions for enhanced electrochemical performance and stability is then investigated. In an effort to improve the capabilities of Na batteries, the electrode-electrolyte interactions of the cathode- and anode-electrolyte interfacial chemistries using sodium borate salts are probed using electrochemical and X-ray analysis. Overall, this dissertation explores how the interfacial interactions affect, and improve, battery performance and stability. This work provides insights for understanding alkali metal-host and electrode-electrolyte properties and guidance for potential future research of the stabilization for Li- and Na-metal batteries.

Applying sizing material (poly n-vinylpyrrolidone or PVP) around graphite fibers enhances the mechanical properties of carbon fiber reinforced epoxy composites. Understanding the influence of the interphase region between the carbon fiber and epoxy matrix is crucial in enhancing the performance of carbon fiber reinforced epoxy composite materials. In this work, simulated interphase regions, in the form of pure and modified epoxies were synthesized in the laboratory. Several characterization techniques were used to identify the properties of these modified epoxies. They were: 1) Tensile Tests: 2) Fracture Toughness Tests; 3) Thermogravimetric Analysis (TGA): 4) Differential Scanning Calorimetry (nSC); 5) Fourier Transform Infrared (FTIR) Spectrometry: and ()) Water Absorption. Young's modulus, yield stress, yield strain. ultimate tensile stress. ultimate tensile strain, tensile toughness, fracture toughness, and strain energy release rate were obtained from tensile and fracture tests. DSC and FTIR experiments were employed in this study to show the miscibility of PVP and epoxy resin. The pure and modified epoxy samples were immersed in water for about a month to determine their water absorptivity. Almost all epoxies remained unchanged in stiffness, with the exception of the sample 40 wt. % PVP. Only the pure epoxy and light PVP loading epoxies exhibited yield points. The ultimate properties worsened significantly with the increase of PVP loading. A decreasing trend was found in fracture toughness as PVP loading increased. All pure and modified epoxies exhibited sharp glass transition temperatures and the T_g's followed the Fox prediction. Downward frequencies shifting of carhonyl and hydroxyl groups were obtained from the PVP/epoxy blends by infrared study. This was believed to show evidence of hydrogen bond formation. All epoxy and modified epoxies were swollen in water absorption experiments. The samples reached equilibrium after about one month and water absorptivity was found to be a function of PVP content. These experiments sought to demonstrate the characteristics of the interphase region of the composites. / Master of Science

Thesis advisor: Dunwei Wang / Thesis advisor: Matthias Waegele / Lithium metal anode (LMA) holds great promise as alternative anode material for next-generation high energy density batteries. Efficiency and safety are two most critical concerns that impede practical application of LMA due to unstable interface between the electrode and the electrolyte. Solid electrolyte interphase (SEI), a passivation layer formed from electrolyte decompositions on the LMA surface, dictates the chemical and mechanical evolution of the electrode/electrolyte interface, and therefore directly affect the cycle life of lithium metal batteries. Although significant progress has been achieved to improve battery performance, a thorough understanding of SEI functions and properties is still inadequate. Both compositional and structural complexity severely hinder the efforts to uncover the SEI formation and evolution mechanism. To achieve stable lithium plating and stripping over cycling, it is necessary to lay a foundation of composition-structure-property relationships that can guide rational design of ideal SEI.First, to solve the safety and efficiency issues simultaneously, a facile and effective way to enable LMA in nonflammable electrolyte was identified by simply introducing oxygen into the battery. Reversible lithium plating and stripping was realized in a flame retardant triethyl phosphate solvent otherwise incompatible to LMA. A unique electrochemically induced electrolyte decomposition pathway was proposed and studied computationally and experimentally. The SEI formation mechanism enriches the knowledge of on the complex reactions toward an ideal SEI. The operation of Li-O2 batteries and Li-ion batteries were also demonstrated in a nonflammable phosphate electrolyte system. To understand the unique role of different SEI compositions, in the second part of this thesis, we designed and synthesized two-component artificial SEI model structures for comparison study. Our central hypothesis is that tailoring LiF and Li3PO4 compositions in the SEI layer can achieve a balanced and improved electrode/electrolyte stability. A magnetron sputtering method was developed to prepare LiF and Li3PO4 mixture films on Cu substrate. Preliminary results from battery cycling tests shows that mixture SEI structure is correlated to improved Coulumbic efficiency. Next, to understand detailed Li+ ion transport properties of the SEI. We presented an outline the current understanding of Li+ ion transport mechanisms and their dependence on the SEI. We also built on this fundamental knowledge to discuss practical effects in experimental systems. Lastly, we shared our perspectives on critical remaining questions in this field. In parallel to study on electrochemical energy system, developing electrochemical methods for integrated catalysis constitutes another part of thesis. We demonstrated that reactivity of an immobilized iron catalyst could be altered by application of an electrochemical potential to a surface to enable polymerization of different classes of monomers. A method was developed to pattern functional surfaces by using electrochemical potential to activate and deactivate polymerization reactions. The orthogonal reactivity of switchable polymerization catalysts was utilized to create patterned surfaces functionalized with two different polymers initiated from mixtures of monomers. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.