Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item NANOSTRUCTURED THIN FILM POLYMER ELECTROLYTES FOR FLEXIBLE BATTERY APPLICATIONS(2009) Ghosh, Ayan; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years, the interest in polymeric batteries has increased dramatically. With the advent of lithium ion batteries being used in cell phones and laptop computers, the search for an all solid state battery has continued. Current configurations have a liquid or gel electrolyte along with a separator between the anode and cathode. This leads to problems with electrolyte loss and decreased performance over time. The highly reactive nature of these electrolytes necessitates the use of protective enclosures which add to the size and bulk of the battery. Polymer electrolytes are more compliant than conventional inorganic glass or ceramic electrolytes. The goal of this work was to design and investigate novel nanoscale polymer electrolyte flexible thin films based on the self-assembly of block copolymers. Block copolymers were synthesized, consisting of a larger PEO block and a smaller block consisting of random copolymer of methyl methacrylate (MMA) and the lithium salt of methacrylic acid (MAALi). The diblock copolymer [PEO-b-(PMMA-ran-PMAALi)] with added lithium bis(oxalato)borate, LiBC4O8 (LiBOB) salt (in the molar ratio ethylene oxide:LiBOB = 3:1) was used to form flexible translucent films which exhibited nearly two orders of magnitude greater conductivity than that shown by traditional high molecular weight PEO homopolymer electrolytes, in the absence of ceramic fillers and similar additives. The presence of the smaller second block and the plasticizing effect of the bulky lithium salt were shown to effectively reduce the crystallinity of the solid electrolyte, resulting in improved ion transporting behavior. The tailored solid self-assembled diblock copolymer electrolyte matrix also exhibits an exceptionally high lithium-ion transference number of 0.9, compared to a value between 0.2 and 0.5, shown by typical polymer-lithium salt materials. The electrolyte material also has a wide electrochemical stability window and excellent interfacial behavior with lithium metal electrode. The combination of these properties make electrolyte membranes composed of the diblock copolymer PEO-b-(PMMA-ran-PMAALi) and LiBOB salt, viable electrolyte candidates for flexible lithium ion based energy conversion/storage devices.Item Evaluation of the transcription of small RNA SgrS and glucose transporter mRNA ptsG in E. coli B and E. coli K cultures under high glucose conditions(2009) Ng, Weng Ian; Wang, Nam Sun; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Escherichia coli is commonly used as the production system for recombinant proteins. However, acetate accumulation in fermentation affects cell growth and protein yield. Recent studies have showed that the small RNA SgrS regulates the major glucose transporter mRNA ptsG in a post–transcriptional manner when the metabolic intermediate glucose–6–phosphate is accumulated intracellularly in E. coli K. Here, comparative analysis of the transcription of SgrS and ptsG is performed between E. coli B and E. coli K cultures in both shake flasks and bioreactor. Both strains expressed SgrS when grown on the non–metabolizable glucose analog α–methyl–glucoside. However, under high glucose conditions, only E. coli B showed significant expression of SgrS. This behavior is unaffected by oxygen supply and pH control. E. coli B produced less acetate on glucose than E. coli K in the bioreactor settings. This provides evidence of a possible connection between SgrS and acetate production in aerobic fermentation of E. coli.Item Self-Assembled Photoresponsive and Thermoresponsive Nanostructures(2009) Sun, Kunshan; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Responsive complex fluids based on nanostructures (e.g., micelles, vesicles and nanoparticles) have received considerable attention recently. The ability of these materials to be tuned by light or heat can have many potential applications in the areas of drug delivery, coatings, sensors, or microfluidic valves and dampers. However, most current photoresponsive and thermoresponsive formulations require the synthesis of complex organic molecules, and this prevents them from being used widely for commercial applications. In this dissertation, we seek to develop new classes of photoresponsive (PR) and thermoresponsive (TR) nanostructures based on commercially available, inexpensive precursors. In the first part of this study, we report a new PR fluid based on light-activated nanoparticle assembly. Our system consists of disk-like nanoparticles of laponite along with a surfactant stabilizer (Pluronic F127) and the photoacid generator (PAG), diphenyliodonium-2-carboxylate monohydrate. Initially, the nanoparticles are sterically stabilized by the surfactant and the result is a stable, low-viscosity dispersion. Upon UV irradiation, the PAG gets photolyzed, lowering the pH by about 3 units. In turn, the stabilizing surfactant is displaced from the negatively charged faces of the nanoparticle disks while the edges of the disks become positively charged. The particles are thereby induced to assemble into a 3 dimensional "house-of-cards" network that extends through the sample volume. The net result is a light-induced sol to gel transition, i.e., from a low, water-like viscosity to an infinite viscosity and yield stress. The yield stress of the photogel is sufficiently high to support the weight of small objects. The gel can be converted back to a sol by either increasing the pH or the surfactant content. Evidence for the above mechanism is provided from a variety of techniques, including small-angle neutron scattering (SANS). In the second part of this study, we demonstrate that laponite/PF127 mixtures also show thermogelling, i.e., the fluids transform from low viscosity sols to stiff gels upon heating above a critical temperature. This phenomenon is reversible and it requires the presence of sufficient amounts of both components. At room temperature, PF127 adsorbs onto laponite disks and stabilizes them by steric repulsion. Upon heating, the PF127 layer on the disks becomes thicker, and more importantly, PF127 micelles in the bulk solution grow significantly. Evidence for the growth of micelles is presented from SANS modeling and from transmission electron microscopy (TEM). At a distinct temperature, we believe the micelles induce depletion flocculation of the laponite particles into a gel network. Interestingly, if the PF127 concentration is increased further, the thermogelling is eliminated - this is suggested to be due to the micelles providing depletion stabilization of the particles.Item Polymer capsules as building blocks for soft, connected mesostructures(2009) George, Elijah; Raghavan, Srinivasa; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We show that polymer capsules can serve as soft building blocks for creating a range of mesoscale (0.1 to 10 mm) structures. The central innovation is a new approach for connecting spherical capsules by exploiting electrostatic complexation. Using this approach, connected structures with complex shapes can be easily assembled, and more importantly, a single connected structure can be made to have a diverse array of functions. The modular approach to shape and function is very much like using Lego bricks of different colors. The connected structures can be made responsive (capable of being actuated) by magnetic fields by including magnetic capsules within them. One motivation for creating these structures is to mimic the mechanics and motility of small creatures such as the earthworm or ant - this could eventually enable the design of autonomous biomimetic robots. In addition, soft connected structures could be employed to transport cargo such as drugs or proteins in blood vessels, or to construct valves, rotors, or mixers in microfluidic or lab-on-a-chip devices.Item A Simplified Model of Planetary Chemical Vapor Deposition Reactors(2009) Shahshahan, Negin; Adomaitis, Raymond A.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A simplified model for planetary chemical vapor deposition reactors is proposed and used to compute deposition species mole fraction and deposition rate in the reactor depletion zone. First, the modeling and optimization work performed in the literature is reviewed and their representative deposition rate profiles are extracted. Afterwards, several simplifying assumptions are applied to derive the reactor modeling equation, and the eigenfunction expansion solution is subsequently computed using a previously developed MATLAB object-oriented computational framework. The simulation result for the deposition profile is improved by modifying the inlet boundary condition, and is then compared with the previously published profiles. The MATLAB optimization toolbox is used to find the optimal deposition profile giving the best match with the published, detailed simulator profiles. Finally, an evaluation of the model consistency with the published results is given.Item PHASE BEHAVIOR AND INTERFACIAL PHENOMENA IN TERNARY SYSTEMS(2009) Subramanian, Deepa; Anisimov, Mikhail A; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase behavior in multi-component systems has a wide variety of applications in the chemical process industry. In this work, the interfaces in two-phase, three-component systems were modeled and studied. Direct calculations of the asymmetric concentration profiles near the critical points of fluid phase separation are very difficult since they are affected by mesoscopic fluctuations. In this study a "complete scaling" approach was used to model interfacial profiles for a highly asymmetric, dilute ternary mixture near the critical point of liquid-liquid separation. The symmetric order parameter profile, the density profile of the lattice gas model, was used to further calculate the asymmetric interfacial concentration profiles at the mesoscale. Fluid asymmetry has been introduced through mixing of the physical field variables into the symmetric scaling theoretical fields. The system-dependent mixing coefficients were calculated from experimental data and a mean-field equation of state, namely, the Margules model. The resultant interfacial profiles for the concentration of water across the methanol-rich and cyclohexane-rich phases show the asymmetry associated with the contribution of the entropy into the symmetric order parameter profile.Item Mesoscopic Thermodynamics in Smooth and Curved Interfaces in Asymmetric Fluids(2009) St. Pierre, Heather J.; Anisimov, Mikhail A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase separation has a significant impact on many chemical engineering applications, and since the phase transition of asymmetric one–component fluid or mixture interfaces can be gradual, or smooth, further analysis is warranted for fluid separation on the mesoscale. Complete scaling is applied to account for fluctuations in the critical region and to model the interfacial profiles of one–component and dilute binary mixtures, as well as calculate the curvature correction to the surface of tension (or Tolman length). Well–established symmetric profiles were selected for the order parameter and thermal scaling densities for use in complete scaling to model these asymmetric fluids. Real fluid asymmetry was applied to these profiles though the scaling coefficients of one–component fluids. Scaling coefficients for mixtures were introduced through the experimental critical parameters of the specific mixture; characteristics accounted for included consideration of the difference in molar volume between solvent and solute, changes in critical temperature and pressure with concentration as well as the Krichevskii parameter. Flory theory was used to approximate the scaling coefficients in dilute polymer solutions to determine the Tolman length. These results indicated that the Tolman length diverges near the critical point of separation and with an increasing degree of polymerization. As an infinite degree of polymerization is approached, the Tolman length becomes half of the width of the interface. Since fluid behavior near the critical point of separation is universal, many of the theoretical expressions shown in this work can be applied to other asymmetric systems. The results of this analysis showed that a large difference in molecular volume led to a higher degree of fluid asymmetry in polymer solutions. The concentration of added solute, specifically in dilute n–heptane–ethane solutions resulted in increased fluid asymmetry in density profiles, as well as a shift toward the density of the added solute. When considering dilute mixtures of aqueous n–hexane and n–heptane–ethane solutions, a slight increase in concentration of solute, coupled with an increasing temperature distance to the critical point of separation, yielded the greatest increase in fluid asymmetry in concentration profiles.Item Self-Assembled Photoresponsive and Thermoresponsive Fluids with Tunable Rheology(2009) Kumar, Rakesh; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fluids whose rheological properties can be tuned by light or heat (termed as photorheological (PR) or thermorheological (TR) fluids, respectively) have attracted a lot of attention as they can be useful in numerous applications such as drug delivery, coatings, sensors, and valves for microfluidic devices. However, current formulations of these fluids suffer from several limitations: in particular, they often require synthesis of complex organic molecules by elaborate procedures, and this limits the widespread use of these fluids. In this dissertation, we seek to develop and investigate new classes of PR and TR fluids based on organic molecules that are readily available and quite inexpensive. Since no new synthesis is required, these systems could prove to be more attractive for a variety of applications. In the first part of this study, we describe a new aqueous photorheological (PR) fluid based on the zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB) and the photosensitive molecule, ortho-methoxy cinnamic acid (OMCA). EDAB/OMCA fluids exhibit photogelling, i.e., a large (~ 10,000 fold) increase in viscosity upon exposure to UV radiation. We show that this photogelling is caused by the growth of long wormlike micelles in the sample. This structural change, in turn, is induced by the UV-induced isomerization of OMCA molecules from their trans to cis form. Evidence from zeta-potential studies, small-angle neutron scattering (SANS), and rheology are used to systematically reveal the molecular and microstructural mechanism for our results. In the second part of this study, we turn our attention to non-aqueous solvents and demonstrate a new class of PR fluids using such solvents. The PR effect here relies on transformations of "reverse" micellar structures formed by a well-known lipid (lecithin) in conjunction with para-coumaric acid (PCA). Lecithin/PCA fluids exhibit a substantial decrease in viscosity upon exposure to UV light (i.e., photothinning). Initially, the molecules self-assemble into long wormlike micelles, leading to highly viscoelastic fluids. Upon UV irradiation, PCA is photo-isomerized from trans to cis. This change in geometry induces a transition from long to short micelles. In turn, the solution viscosity is decreased by more than three orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. In the last study, we report a class of aqueous fluids whose viscosity increases upon heating (i.e., thermo-thickening). These fluids are mixtures of telechelic associating polymers (HEURs) and a type of supramolecules called cyclodextrins (CDs) in water. Interestingly, we observe this behavior only with a particular type of CDs, called alpha-CDs, and not with the other common CD types, i.e., beta- and gamma-CDs. These results are explained in terms of a competition between the hydrophobic end-caps and the hydrophilic backbone of the polymer for complexation with alpha-CD molecules. We have also investigated the effect of amphiphiles (single-tailed surfactants and double-tailed lipids) on the thermo-thickening. The addition of lipids substantially enhances the thermo-thickening behavior, which is explained to be due to an enhancement of the connectivity of hydrophobic junctions by lipid vesicles.Item The Effect of Hyper-Osmotic Conditions on the Growth, Metabolism, and Specific Antibody Productivity of a GS-NS0 Cell Line(2009) Brady, Stefanie Ellen; Wang, Nam S; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of cellular growth, metabolism, and monoclonal antibody production on an industrial GS-NS0 cell line to hyper-osmotic medium was studied. The GS-NS0 cell line was found to have an optimum growth rate at a medium osmolality of 350 mosm/kg and an optimum specific productivity at 450 mosm/kg. Medium osmolality was shown to affect cell size as the cell line exhibited a regulatory cell volume increase response after an initial introduction into hyper-osmotic conditions. The response of the cell line to an osmotic shift was also studied. Osmolality of the culture medium was increased, at two different time points, through the addition of NaCl. The shift in osmotic pressure was found to have a positive impact on specific productivity of the monoclonal antibody produced. A finger print of the metabolic response of the GS-NS0 cell line to increased medium osmolality was determined. The application of metabolomics to mammalian cell cultures has not been widely explored. In this study, the cells were quenched and extracted using methods previously developed for microbial and plant cultures. An increase in concentration of internal amino acids, known to be osmolytes, was found under hyper-osmotic conditions.Item Kinetics and morphology of metallocene catalyzed syndiospecific polymerization of styrene in homogeneous and heterogeneous reaction systems(2008-11-21) Han, Joong Jin; Choi, Kyu Y; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Syndiotactic polystyrene (sPS) is a semicrystalline thermoplastic polymer with many advantageous properties such as excellent heat resistance with a high melting point of 270-272oC, strong chemical resistance against acids, bases, oils and water, and low dielectric constant. The relatively fast crystallization rate makes sPS a promising material for a large number of applications in the automotive, electrical and packaging industries. In this study, the kinetics of syndiospecific polymerization of styrene is investigated through experimentation and theoretical modeling using homogeneous and heterogeneous Cp*Ti(OCH3)3/MAO catalysts. During sPS slurry polymerization, the physical phase changes of reaction mixture occur. With an increase in total solid content, sPS slurry undergoes a series of physical changes from clear liquid to a wet cake or paste-like material. A detailed reaction kinetic model based on a two-site kinetic mechanism has been developed to predict the polymerization rate and polymer molecular weight distribution. The monomer partition effect is incorporated into kinetic models to account for the nonlinear dependence of polymerization rate on the bulk phase monomer concentration. Quite satisfactory agreement between the model simulation results and experimental data has been obtained. The morphological development of nascent sPS particles during the polymerization has also been investigated. Most notably, it was found that sPS particles grow with the nanofibrillar morphology with either homogeneous or silica-supported metallocene catalyst. The analysis of nascent morphology of sPS using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDS/EDX) analysis, revealed that there is a strong correlation between the formation of sPS nanofibrillar structure and sPS crystallization. A mechanism for the growth of sPS particles is also proposed based on the experimental observations and analysis. Ultrahigh molecular weight sPS has also been synthesized in silica nanotube reactors (SNTRs) and the morphological characteristics of sPS produced in the nanotube reactors have been analyzed. A new mechanism is proposed for the formation and growth of sPS nanofibrils extruding out from the nanotube reactors. Also, a kinetic analysis is presented to interpret the observed molecular weight enhancement effect that is believed to be caused by the constrained reaction environment inside the nanotubes.