Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2005. / Includes bibliographical references (p. 195-207). / Molecular simulation has become a very powerful tool for understanding the process of polymer crystallization. By using carefully constructed simulations, one can independently observe the two phenomena responsible for melt crystallization: nucleation and growth. This research focuses on modeling the growth process using potentials that are well-parameterized for alkanes and polyethylene. In experiment and modeling of the kinetics of alkane crystallization, focus has been concentrated on growth rates very near the melting temperature, where the growth of these systems is optically observable. In this temperature range near .., diffusion is not a limiting factor, which has led to theory that models the thermodynamic driving force and its effect on kinetics. Phenomenologically, one observes a maximum growth rate at a temperature intermediate between the glass transition temperature ... and the melt temperature ... This arises as a competition between a thermodynamic driving force towards crystal growth, associated with locking chains into crystallographic registry and the ability of chains to diffuse to the new layer and rearrange themselves conformationally to satisfy the restrictions of crystal symmetry. The thermodynamic driving force is rate limiting at high temperatures, while melt mobility is rate limiting at low temperatures. Growth rates are of interest to the polymer processing community, who require accurate crystallization kinetic data over the entire temperature range, in order to predict solidification under process conditions and thus final fiber properties. / (cont.) A model which retains its connection to molecular structure would certainly be of benefit for purposes of product design; such connection is possible using molecular simulations. Nonequilibrium molecular dynamics enables us to observe growth for a range of temperatures around the temperature at which the maximum growth rate occurs. We present a molecular dynamics (MD) framework for measuring crystal growth rates for n-eicosane (C₂₀H₄₂, denoted C20), by simulating growth on a pre-existing crystal surface. We show that growth rates for short alkanes such as C20 are directly observable over a range of quench temperatures, for the case where the crystallization front is preceded by a supercooled amorphous region, and heat transfer occurs faster than the characteristic time for crystallization. We present data that we have acquired from these simulations through analysis of the propagation of orientational order, using the bond order parameter, and density changes, using Voronoi volumes. To determine molecular weight effects, we use the same technique to look at systems of C₅₀H₁₀₂ and C₁₀₀H₂₀₂ (denoted C50 and C100). With these higher molecular weight n-alkanes, we can also measure the occurrence of folds during crystal growth. From these MD simulations, we obtain data for the growth rate of n-alkane crystals over a range of temperatures and molecular weights. Qualitatively, we see frequent adsorption and desorption of chain segments on the surface in both C50 and C100 systems. We find evidence for a surface nucleus involving 4-5 chain segments, from multiple chains, that are approximately 20 beads long, shorter than the ultimate thickness of the chain stem in the crystal. / (cont.) We construct a general crystal growth model that can be parameterized entirely in terms of universal properties of polymer chains, described by polymer physics and chemically specific quantities that can be estimated polymer by polymer using molecular dynamics simulations. The model is an extension to polymer crystallization models to incorporate molecular weight effects, using a small number of chemically specific quantities that can be estimated from molecular dynamics simulations. It accounts for the thermodynamic driving force, using classical nucleation theory, and melt relaxation time, using WLF theory. Our model can predict rates as a function of temperature and molecular weight, up to the entanglement molecular weight. Past the entanglement molecular weight, the analysis reveals that the growth rate of alkanes and polyethylene can both be described by the same relationship. The appropriate relaxation time is used to describe the kinetic barrier to crystallization. For chains shorter than the entanglement length, this is the Rouse time. For chains longer than the entanglement molecular weight, kinetic limitations are modeled by the local relaxation of an entangled segment at the interface. Use of the model is illustrated for polyethylene crystallizing in a fiber spin line under conditions of slow cooling and fast cooling. Finally, we present a simplified framework for the study of polymer crystallization using Kinetic Monte Carlo (KMC). We have developed a general KMC algorithm for measuring growth of a polymer crystal phase during melt crystallization, based on the algorithm of Bortz et al. / (cont.) We have incorporated new moves into a general framework to allow multi-chain, three-dimensional growth and the escape of chains from the crystal to the melt, through the fold surface. We performed parametric studies on the melt-crystallization of C20 to study the effects of each energy barrier. In addition, the KMC algorithm allows us to consider the importance of individual moves in contributing to growth. We have shown, as a proof-of-concept, that this algorithm is capable of generating morphologies characteristic of several theories of secondary nucleation in polymer melts. / by Numan Waheed. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2006. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references. / While chromated copper arsenate (CCA) has proven to be exceptionally effective in protecting wood from rot and infestation, its toxic nature has led to the problem of disposal of CCA-treated lumber and remediation of waters and soils contaminated by process wastes. The active ions in water-based CCA are hexavalent chromium, divalent copper, and pentavalent arsenic. The objective of this study was to develop the underlying engineering science for remediation of aqueous CCA wastes via electrolytic deposition of neutral arsenic, chromium, and copper in order to evaluate the technical feasibility of this process. The specific approach focused on electrochemical stability analysis of the metals; development and testing of a copper sulfate reference electrode (CSE); electrolytic deposition of arsenic, chromium, and copper from model aqueous CCA wastes; and characterization of the resulting deposits. The electrochemical stability analysis of the individual components, As, Cr, and Cu, in an aqueous system was used to determined the most thermodynamically stable forms of the metals as a function of pH and electrochemical potential. This analysis predicted that under the conditions of codeposition of all three metals, hydrogen and arsine would also be produced. / (cont.) A robust and accurate CSE was designed, constructed, developed and used as a reference electrode for the electrolytic deposition experiments in this study. The potential of the CSE as a function of temperature over the range of 5 to 45 °C was measured and related to the normal hydrogen electrode potential (317 mV at 25°C, slope of 0.17 mV/°C). Electrolytic deposition was performed using working and reference electrodes specially designed and fabricated for this study. Despite the results of the electrochemical stability analysis, conditions were found experimentally where arsenic, chromium, and copper were deposited from model aqueous CCA type-C solutions over a range of concentrations without the formation of arsine or hydrogen. Three different types of deposits were observed. One type contained a ratio of metal concentrations similar to that of CCA type-C and is a good candidate for use in CCA remediation and recycling processes. This study indicated that CCA remediation via electrolytic deposition is probably feasible from an engineering perspective. / by Heather A.G. Stern. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 136-143). / In the pursuit of further miniaturization beyond Moore's law, tremendous effort has been dedicated to exploring the potential of two-dimensional (2D) materials for nanoscale electronic devices. 2D materials are a group of solid state materials that possess strong in-plane covalent bonds while individual atomic layers are held together by weak van der Waals (vdW) interactions. Hence, their bulk crystals can be exfoliated into few-layer or even atomically thin single-layers via micro-mechanical exfoliation techniques. These materials possess unique and exotic properties due to quantum confinement of importance to future electronics. However, many technical problems need to be solved to realize this goal. For example, as 2D material based devices become smaller down to the nanometer scale, the electrical contacts must also be reduced in scale which creates different characteristics from those of macroscopic counterparts. In addition, there are issues of reliability and stability with devices comprised of such materials. There is a need to understand the electronic and chemical properties of several interfaces that arise in such materials: metal-2D and 2D-2D junctions, for example. To this end, this thesis focuses on understanding nanoscale metal-2D semiconductor (SC) and 2D SC-2D SC junctions exploring: (1) electronic and optoelectronic behavior at the nanoscale junction of metal-MoS2 and dependence on the layer number (thickness), (2) realization of voltage selectable photodiodes based on a lateral in-plane MoS2-WSe2 heterojunctions, and (3) interfacial properties and (opto)electronic characteristics of a phosphorene-MoS2 vertical vdW p-n junction. The first part of this thesis explores the layer number dependent electrical characteristics of the MoS2-metal nanoscale junction using current imaging of MoS2 nanosheets consisting of regions of varying different thicknesses using conductive and photoconductive spectral atomic force microscopy (C- and PCS-AFM). The layer number dependence of the effective barrier was measured, by obtaining consecutive current images while changing bias voltages, showing it to be linear. At the same time, spatially resolved two-dimensional (2D) maps of local electrical properties are generated from simultaneously recorded local current-voltage (IV) data. Furthermore, the layer number dependent spectral photoresponse of MoS2 is investigated, which shows the highest response in single layer (1L) region. The photoresponse decreases for increasing layer number, but increases again between 4L and 1 OL due to increased light absorption. The photoresponse is also strongly dependent on the wavelength of the incident light, showing much higher currents for photon energies that are above the optical bandgap. The photoresponse in forward and reverse biases shows barrier symmetry for 1 L but asymmetry for 2, 3, and 4L, which further indicates a dominant role of the barrier on carrier transport at the junction. The second part of this thesis investigates the spatially resolved transverse electrical properties of the monolayer WSe2 -MoS2 lateral p-n heterostructures at their nanoscale junctions with metals both in the dark and under laser illumination. As in the first part of the thesis, C- and PCS-AFM, versatile tools to conveniently and efficiently interrogate layer-dependent electronic and optoelectronic characteristics in a MoS2 crystal containing regions of different thicknesses, which enables direct characterization and comparison of the different layer regions without the complexities associated with fabricating and testing of different individual field-effect transistor devices, are used for measurements. By performing current imaging using a PtIr-coated conductive tip on an ultrathin nanosheet that includes homogeneous crystals of WSe2 and MoS2 and a lateral junction region in between, many thousands of WSe2/MoS2/the junction-metal contact points form during imaging and directly compare their local properties at the same time under identical experimental conditions with the nanoscale spatial resolution. The third part of this thesis explores a new type of 2D vertical heterostructures that simultaneously possess desirable properties of constituent materials, paving the path for overcoming intrinsic shortcomings of each component material to be used as an active material in nanoelectronic devices. As a first example, a MoS 2-graphene vertical heterostructure is constructed and its charge transfer and photoluminescence (PL) at the interface are investigated. C-AFM and Raman spectroscopy show that there is a significant charge transfer between the two component materials. The PL intensity of monolayer MoS2 is noticeably quenched when in contact with a single layered graphene in comparison to that of a bare monolayer MoS2 crystal. Then, with the acquired understanding of the underlying physics at the 2D vdW heterointerfaces, the possibility of a black phosphorus (BP)-MoS2 vertical heterostructure as an ultrathin channel material for high-performance 2D (opto)electronic devices is studied. CVD-synthesized MoS2 and micromechanically exfoliated BP crystals are stacked together to form a vertical p-n heterostructure. Optical microscopy, AFM images, and Raman spectroscopy data show that the MoS2 thin films can be used as a passivation layer, protecting BP from deteriorating in ambient conditions for extended period of time or under an elevated temperature in an Ar environment. The IV characteristics of FET devices based on the vertical heterostuctures exhibit that the MoS2 layer has limited impact on superior carrier transport properties of the BP in the dark. Upon light illumination, photoconductivity of the BP-MoS2 heterostructure region increased compared to that of the bare BP region of the same flake, mainly due to the fact that a built-in electric field formed at the BP-MoS2 interface facilitates the dissociation of electron-hole pairs generated by light absorption. / by Youngwoo Son. / Ph. D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1999. / Includes bibliographical references. / by Jyotsna Iyer. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, February 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 157-162). / Reducing dependence on petroleum feedstocks motivates engineering novel conversion technologies to convert biomass as a renewable resource into target value-added products. In this thesis we developed a new pathway for the microbial synthesis of 3-hydroxyalkanoic acids as biomass derived value-added products. 3-hydroxyalkanoic acids (3HAs) find applications as monomers for biodegradable polymers and chiral pharmaceutical building blocks. One part of this thesis focused on investigating the proposed 3-hydroxyalkanoic acid synthesis platform pathway. The platform employs the reactions of the natural polyhydroxyalkanoate synthesis pathway with new substrates, taking advantage of natural enzyme promiscuity for the stereospecific synthesis of a variety of 3HAs of desired carbon chain length and substituents. Using this platform, we have now demonstrated the synthesis of five novel products: 3,4-dihydroxybutyric acid (3,4-DHBA) and 3-hydroxybutyrolactone (3HBL) as pharmaceutical building blocks and 2,3-dihydroxybutyric acid (2,3- DHBA), 3-hydroxy-4-methylvaleric acid (3H4MV) and 3-hydroxyhexanoic acid (3HH) as monomers for novel polymer applications. The synthesis of 2,3-DHBA in particular led to the identification of a novel activity associated with the thiolase enzyme and highlighted the biosynthetic capability of the platform. The experimental study of different pathway enzyme combinations offered insights into their activities and specificities to guide future enzyme selection. In another part of this thesis, we focused specifically on the hydroxyacid 3,4-DHBA and its lactone 3HBL and their synthesis from glucose as a sole carbon source by integrating the 3HA platform with the endogenous glyoxylate shunt. 3HBL has been identified as a top value-added platform chemical from biomass by the US Department of Energy due to its applications as a chiral synthon for a variety of pharmaceuticals, with an estimated wholesale cost of $450/kg. We were successful in establishing the first biosynthetic pathway for the stereospecific synthesis of 3,4-DHBA and 3HBL from glucose in this thesis, achieving up to 24% of the maximum theoretical yield and titers of the order of 1 g/L at the shake flask scale. Overcoming repression of the glyoxylate shunt and independent control of the glycolate and 3HA pathway enzyme expression using two orthogonal expression systems was critical for product synthesis. Additionally, a study of the 3HBL/DHBA fermentation at the shake flask and bench-top bioreactor scales helped gain an understanding of pH as an important factor affecting the synthesis of these products and informed approaches to improve pathway and process performance. / by Himanshu H. Dhamankar. / Ph. D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1997. / Includes bibliographical references (p. 139-147). / by John Joseph Prior, Jr. / Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1989. / Includes bibliographical references (v. 2, leaves 437-445). / by Frederick Warren Lam. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1999. / Includes bibliographical references. / Continued advances in the semiconductor industry will require the introduction of new materials and processes concurrent with shrinking device dimensions. These simultaneous demands drastically reduce margins for error and necessitate an increasingly quantitative understanding of semiconductor processes. Leaders of the semiconductor industry have recognized these challenges and featured atomistic process modeling as one of the "Difficult Challenges" for designs below 100 nm, predicted beyond the year 2006. Among these tasks, obtaining detailed, quantitative understanding of process chemistry and physics has been identified as on of the biggest hurdles. Another "chief roadblock" is linking atomistic reaction models to reactor scale process simulations. Process modeling includes simulations of reactor transport, thin film growth, morphology, and uniformity, and device feature profile evolution (how well desired features are grown). A key ingredient to processes modeling, whether atomistic or macroscopic, is knowledge of the elementary reaction pathways and chemical intermediates, and reaction thermodynamics and kinetics. Quantum chemistry methods present power tools to investigate these molecular properties for both gas phase and surface reactions. Along with pioneering efforts in applying these tools to semiconductor processes come issues such as understanding accuracy, how to approach a given problem, and defining problems into practical sizes. To this end, a combined experimental/theoretical study of aluminum chemical vapor deposition has been performed. Both conventional ab initio and more recent density functional theory methods (DFT) have been investigated and evaluated. / (cont.) Aluminum chemical vapor deposition (CVD) is a process of interest for semiconductor metalization. Aluminum has been the workhorse metal for interconnect applications since the dawning age of the semiconductor industry. Although aluminum is traditionally deposited with advanced physical vapor deposition (PVD) techniques, new deposition methods, such as CVD, are required as characteristic feature sizes move into the nanometer range. In the present study, a combined experimental/theoretical approach has been implemented to investigate both vapor phase and surface reaction pathways of the aluminum CVD precursor dimethylaluminum hydride (DMAH). DMAH is arguably the best aluminum CVD precursor available, but it has complications including a puzzling liquid and vapor phase chemistry, and an unknown surface chemistry. Detailed knowledge of the deposition process is necessary to design a robust, high yield aluminum CVD process. In the present studies, experimental observations have been taken from the literature for the vapor phase chemistry, and ultra-high vacuum (UHV) surface science studies have been employed to investigate the surface chemistry. Theoretical studies were performed by combining quantum chemistry calculations with transition state theory (TST) and micro-kinetic style estimation of rate parameters ... / by Brian Gerald Willis. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 167-173). / Biological materials such as cardiac and skin tissue exhibit the unique capacity to transduce mechanical stimuli into propagating electrical and chemical signals throughout the body. Few synthetic materials have been engineered to produce communicative chemical signals in response to mechanical input, though such synthetic material analogues could enable devices that mimic biological tissues and pressure sensitive processes whereby molecular mechanoreceptors enable rapid and localized transmission of chemical signals. In this thesis, self-oscillating polymer gels comprising N-isopropylacrylamideco- Ru(bpy) 3 are synthesized in order to elucidate chemical and mechanical (chemomechanical) coupling in synthetic, stimuli-responsive materials, and to design mechanically induced, oscillatory signaling systems. N-isopropylacrylamide-co-Ru(bpy) 3 gels represent a unique class of polymeric materials known as BZ gels, that are capable of undergoing the Belousov- Zhabotinsky (BZ) self-oscillating reaction. When submerged in stagnant solution containing chemical reactants, and in the absence of continuously applied external perturbation, the BZ gels exhibit sustained, colorful oscillations due to the changing oxidation state of Ru(bpy)3 transition metal complex. By measuring temperature profiles of the BZ gel, we showed that the swelling behavior and hydrophobicity of the gel depend on the oxidation state of covalently bound Ru(bpy) 3 . Using timelapse microscopy, we recorded the BZ oscillations and tracked the far from equilibrium chemical behavior exhibited by the gels. At constant system temperature, the BZ reaction induced cyclic changes in the osmotic pressure of the gel, resulting in periodic gel swelling and shrinking. Such volumetric changes, driven by the BZ reaction, are largest (22 %) when the edge length of the gel is relatively short (0.6 mm), and pattern formation is dominated by slow kinetics. Therefore, by quantifying the chemomechanical behavior of BZ gels, we demonstrated that the gels convert chemical oscillations into mechanical actuation. Next, we sought to design novel stimuli-responsive behavior in BZ gels by devising methods for mechanically triggering oscillations in quiescent gels. When sufficient macroscopic compressive stress was applied to submerged, non-oscillating gels, BZ oscillations were triggered and persisted until the stress was removed. To our knowledge, BZ gels represent the first synthetic hydrogel capable of producing oscillations in response to mechanical stimuli. To establish the conditions conducive to mechanical triggering, we quantified the chemical regimes for which BZ gels spontaneously oscillate or fail to oscillate. In doing so, we demonstrated that such regimes are governed by the ratio of inhibitor to activator species, which are both intermediate species that are produced throughout the reaction, providing negative and positive chemical feedback, respectively. Mechanically triggerable conditions corresponded to an intermediate ratio of reactant to inhibitor species, such that mechanical compression enabled transitions near the boundary dividing the non-oscillatory and oscillatory regimes. By varying the crosslinking density of the material, we also showed that both the required stress and strain for inducing oscillations in BZ gels increased with decreasing polymer volume fraction. Application of macroscopic, compressive stress to BZ gels caused a decrease in overall gel volume and an increase in the concentration of Ru(bpy) 3 , and oscillations were triggered at a critical concentration of Ru(bpy)3. In demonstrating that BZ gels can sense mechanical pressure and respond by transducing such energy into chemical oscillations, we have opened up new avenues of research based on mechanical sensing in BZ gels. Finally, we explore the mechanisms of synthetic "communication" in which discrete BZ gels sense mechanical stress and transmit chemical signals to neighboring gels. Specifically, we designed arrays of closely spaced gels (0.2 mm gap distance) that communicate via diffusion of activator species. We demonstrated that mechanical triggering can induce wave directionality to a set of gels that would otherwise exhibit disorder, and can propagate signals that change directions by migrating around bends without decay in signal amplitude. By introducing a node to a set of BZ gels, we showed that the BZ signal can split without attenuation, effectively doubling the system output. Last, we quantified the collision of two mechanically induced signals to show that wave collision occurs without amplification, and results in signal extinction. Taken altogether, these studies of signal propagation in BZ gels demonstrate that the underlying mechanism of BZ gel communication is governed by the diffusion of activator species. In addition to demonstrating for the first time a synthetic hydrogel that is capable of generating oscillations in response to mechanical compression, we have shown that BZ gels can propagate mechanically induced signals over long ranges and complex trajectories. Our results can be used to facilitate understanding of complex biological phenomena involving chemomechanical coupling and mechanotransduction, or to design advanced, functional materials that act as pulsating chemical or pressure sensors. / by Irene Chou Chen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 1995. / Includes bibliographical references (leaves 190-206). / by Grace Eileen Colón. / Ph.D.