Chemical and Biomolecular Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2751

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Subject: search.filters.subject.Polymer chemistry

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    Multilayered Spheres, Tubes, and Surfaces Synthesized by "Inside-Out" Polymerization
    (2017) Zarket, Brady; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Numerous materials in nature, including eggs, onions, spinal discs, and blood vessels, have multiple layers. Each layer in these materials has a distinct composition and thereby a unique function in the overall material. Our work is motivated by the need to find a simple, versatile route for the synthesis of such multilayered materials. Toward this goal, we have devised a technique termed “inside-out polymerization” to synthesize multilayered materials with precise control over the composition and thickness of each layer. Each layer is a crosslinked polymer gel and it grows from the surface of the previous layer, with this growth being controlled by precursor molecules present in the core of the structure. Using this technique, we synthesize multilayer structures in three different geometries, as described below. First, we outline our technique and use it to create multilayered polymer capsules. In particular, we create interesting capsules with concentric layers of non-responsive and stimuli-responsive polymers. The thickness of the stimuli-responsive layer varies sharply due to the stimulus while the non-responsive layer remains at the same thickness. In addition, the permeability of small molecules through the stimuli-responsive layers is also altered. This means that these multilayered capsules could be used to conduct pulsatile release of solutes such as drugs or other chemicals. In addition, we also show that multilayered capsules exhibit improved mechanical properties compared to those of the fragile core. Next, we extend our technique to the synthesis of multilayered polymer tubes. Our technique provides precise control over the inner diameter of the tube, the number of layers in the tube wall, and the thickness and chemistry of each layer. Tubes can be patterned with different polymers either in the lateral or longitudinal directions. Patterned tubes based on stimuli-responsive polymers exhibit the ability to spontaneously change their lumen diameter in response to stimuli, or to convert from a straight to a curled shape. On the whole, these tubes mimic several features exhibited by blood vessels like veins and arteries. In our last study, we use our technique to create hair-like structures that grow outward from a base polymer gel. The diameter, length, and spacing of hairs can all be tuned. The addition of hairs serves to increase the net surface area of the base gel by nearly 10-fold. This increase is comparable to the surface area increase provided by hairs called “villi” on the inner walls of small intestines. In accordance with the increased surface area, hairy surfaces extract solutes from a solution much faster than a bare surface. We also impart stimuli-responsive properties to the hairs (e.g., magnetic properties), and we show that hairy gels can be induced to fold into tubes with hairs on the outside or inside. The latter mimics the structure of the small intestine.
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    SOFT HYDROGEL BATTERIES: THE DANIELL CELL CONCEPTUALIZED IN HYBRID HYDROGELS
    (2015) Goyal, Ankit; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Energy storage devices such as batteries are important elements in many electronic devices. Currently, researchers are seeking to create new electronic devices that are "soft", i.e., bendable and stretchable. However, the batteries that power such devices are still mostly hard structures. In the current thesis, we have attempted to develop a "soft" battery out of hydrogels. Specifically, we have made a soft version of the Daniell Cell, which is a classic electrochemical cell. Our design involves a hybrid gel composed of three distinct layers. The top and bottom layers are gels swollen with a zinc salt and a copper salt, respectively, while the middle layer is akin to a "salt bridge" between the two. The hybrid gel is made by a polymerization technique developed in our laboratory and it retains good mechanical integrity (i.e., the individual layers do not delaminate). Zinc and copper foils are then attached to the hydrogel, thus creating an overall battery, and its discharge performance is reported. One unique aspect of these gel batteries is that they can be dehydrated and stored in a dry form, whereupon they are no longer batteries. In this inactive state, the materials are safe and light to transport. Upon rehydration, the gels revert to being functional batteries. This concept could be useful for military or other applications where an emergency energy storage is needed.
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    Ultra-High Molecular Weight Nonlinear Bisphenol A Polycarbonates by Solid State Polymerization in Micro-Layers
    (2013) Baick, In Hak; Choi, Kyu Yong; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The solid-state polymerization of bisphenol A polycarbonate (BPAPC) has been studied in amorphous and partially crystallized micro-layers (SSPm) of low molecular weight prepolymers in presence of LiOH.H2O catalyst at a temperature between the glass transition temperature and the melting point. When the prepolymers (14,000 g/mol) in micro-layers of a thickness range from 5 μm to 35 μm were solid-state polymerized at 230 °C, the polymer molecular weight increased rapidly to above 100,000 g/mol, exceeding the highest molecular weight obtainable by the conventional solid-state polymerization in micro-particles. It has also been observed that the final molecular weight reached as high as 600,000 g/mol even in presence of significant stoichiometric imbalances of end group mole ratios when the prepolymer having 21,000 g/mol is used at 230°C under low pressure (10 mmHg). Most notably, amorphous prepolymer micro-layers showed significantly higher increase in molecular weight than partially crystallized prepolymer micro-layers. The chain branching and partially cross-linked structures in high molecular weight polycarbonates have been confirmed by 1H-NMR spectroscopic analysis as well as pyrolysis-gas chromatography mass spectrometry (Py-GC/MS). 13C-NMR analysis and SSP theoretical model simulation have shown that conventional linear step-growth polymerization is not responsible for the additional increase in molecular weight beyond 50,000 g/mol of polycarbonate MW. The ultra-high molecular weight is contributed to the formation of branched and partially cross-linked structures via Fries or Kolbe-Schmitt rearrangement reactions and radical recombination reaction, respectively. Micro-radical polycarbonate species can be produced via chain scission reaction and hydrogen abstraction at the solid-state polymerization temperature. The formation of cross-linked polymers by radical recombination reactions was attributed to the near complete removal of phenol (i.e. radical scavenger) from the micro-layers during the solid state polymerization. Branched structure polycarbonate was also confirmed by atomic force microscopy (AFM). The presence of branched and cross-linked polymers contributed to the insolubility of the polymer in solvents such as chloroform, tetrahydrofuran (THF), and methlylene chloride. As SSPm process extends for a long reaction time at 230°C, about 95% of the polymer was insoluble with excellent transparency (90-93% light transmission). Properties of ultra-high molecular weight nonlinear polycarbonates (SSPm PCs) have been investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and rheometer. The development of Multi-Layer Deposit and Reaction (MLDR) technique has shown that the SSPm process is not limited to 5-35μm scale. The layer thickness can be expanded while keeping the merits (e.g. high transparency, good solvent resistance, and obtaining high molecular weight in short reaction time) of the SSPm technique developed in this study.
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    Gel Formation by the Self-Assembly of Small Molecules: Insights from Solubility Parameters
    (2014) Diehn, Kevin; Raghavan, Srinivasa R; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Many small molecules can self-assemble into long fibers and thereby gel organic liquids. However, no capability exists to predict whether a molecule in a given solvent will form a gel, a thin solution (sol), or an insoluble precipitate. In this thesis, we build a framework for gelation via a common gelator based on Hansen solubility parameters (HSPs). Using HSPs, we construct 3-D plots showing regions of solubility (S), slow gelation (SG), instant gelation (IG), and insolubility (I) for DBS in different solvents. Our central finding is that these regions radiate out as concentric shells. The distance (R0) from the central sphere quantifies the incompatibility between gelator and solvent. The elastic moduli of the gels increase with R0, while the time to gelation decreases with R0. Our approach can be used to design organogels of desired strength and gelation time by judicious choice of a solvent or a blend of solvents.
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    HETEROPHASE STEP-GROWTH POLYMERIZATION IN A CONTINUOUS TUBULAR REACTOR
    (2013) Yang, Woo Jic; Choi, Kyu Y; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The interfacial process is a well-established industrial process for the production of Bisphenol A polycarbonates. However, there is a dearth of kinetic analyses of the interfacial process in a tubular reactor, which offers greater overall control of this process. In the interfacial process, Bisphenol A dissolved in a dispersed aqueous phase reacts with phosgene in the continuous organic phase producing oligomers that undergo further reaction in the organic phase to produce polymers. This process was carried out in a tubular reactor at a constant pressure of 85 PSI and a constant temperature of 35°C. The kinetics of the mass transfer and reaction and the solubility of reactants were used to develop a mathematical model of the interfacial process in a tubular reactor. The parameters were optimized using proprietary plant data and the model simulations compared to the experimental data proved to be quite accurate. The developed model was used to investigate the heterophase kinetics of this system. A key parameter controlling the interfacial process in a tubular reactor is the dispersed aqueous droplet size. The droplet size determines the total surface area for mass transfer, and decreasing this droplet size from 10μm to 5μm results in an increase of molecular weight by about 130%. Also, the mass transfer coefficient of BPA (kL2) determines whether the processes at the interface are diffusion controlled or reaction controlled. The system exhibits diffusion controlled behavior when kL2 is approximately 1.0 × 10-7 m/s. Conversely, the system exhibits reaction controlled behavior when kL2 is around 4.0 × 10-6 m/s. The chain length distribution in the interfacial process follows Flory's most probable distribution. This functional group model was then expanded to a copolymerization system for siloxane-polycarbonate copolymers. For the copolymerization process, the key parameters shown to have a significant effect on the copolymer composition (mean sequence length and sequence length distribution) is the feed composition and reactivities of the comonomers.
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    HETEROGENOUS ETHYLENE POLYMERIZATION IN A MICRO REACTOR SYSTEM
    (2013) Mahadevan, Meera; Choi, Kyu Yong; Ohadi, Michael M; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Micro reactors provide enhanced mass and heat transfer owing to their high surface area to volume ratio. These reactors offer precise control and selectivity and can be used for synthesizing specifically engineered and technically sophisticated olefin polymers. An unsteady state reactor model (using coordination reaction kinetics) was developed to study the concentration profiles of monomer, catalyst, polymer and its molecular weight distribution along the length of the reactor with time. Nano silica particles of diameter 400nm were synthesized as a support for the metallocene catalyst. Heterogeneous ethylene polymerization was carried out in tubular reactors of diameters 800 µm, 1 mm, and 2.37 mm under 2-phase flow conditions. This thesis investigates the effect of operating conditions in a micro reactor on the qualitative and quantitative properties of the polymer. The results can be extended to propose applications for synthesis of polymers with unique morphology using the inherent advantages of these reactors.
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    Synthesis of Novel Alkaline Polymer Electrolyte for Alkaline Fuel Cell Applicaitons
    (2012) Luo, Yanting; Wang, Chunsheng; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Development of the intrinsically OH- conductive polymeric electrolyte (alkaline polymer electrolyte, APE) is the critical component to enable the wide application of alkaline fuel cell (AFC) technology. Alkaline polymer electrolyte fuel cell (APEFC) based on AFC technology has been revived recently for applications in transportation and portable electronic devices due to its advantages of using non-noble metal catalysts, faster oxygen reduction in alkaline medium, and compact design. The research described in this dissertation aims to synthesize a novel APE, with controlled ionic conductivity and mechanical strength to achieve high fuel cell power density and long durability. Most APEs synthesized up to now use a modification of existing engineering polymer backbones, which are very difficult to balance its mechanical properties with its ionic conductivities. In this research, we copolymerized APE precursor polymers, namely poly (methyl methacrylate-co-butyl acrylate-co vinylbenzyl chloride) (PMBV) from three functional monomers, methyl methacrylate (MMA), butyl acrylate (BA) and vinylbenzyl chloride (VBC), where VBC was the functional group that was attached with trimethylamine (TMA) and was the OH- carrier after ion-exchanging. MMA was used for mechanical support and BA was used to alleviate the brittleness coming from MMA and VBC. We synthesized alkaline polymer electrolytes from bottom-up polymerization of these selected functional monomers using free radical solution and miniemulsion copolymerization techniques. By miniemulsion copolymerization, the properties of the obtained APEs could be precisely controlled by tuning the (1) monomer ratio, (2) glass transition temperature (Tg), (3) molecular weight (MW), and (4) crosslinking the copolymer. The increase in Tg was realized by eliminating BA from monomers, which was a low Tg component. MW was optimized through investigating binary copolymerization kinetics factors (initiator and surfactant). For crosslinking, the newly obtained poly (methyl methacrylate-co-vinylbenzyl chloride) (PMV) was crosslinked as a semi-interpenetrating network (s-IPN) to reduce water uptake and thus enhanced the mechanical strength in a humidified environment for APEFCs. After the optimization, our best quaternized PMBV (QPMBV) series APE membranes could reach a maximum power density of 180 mW/cm2 and the crosslinked QPMV APE could last 420 hours on APEFCs, which was among the best overall performance in APE technologies. In the future, we propose to use fluorinated polymer monomers to redesign the polymer backbone. Another direction in the design of APEs is to reselect the possible functional OH- carrier groups to make APEs more chemically and mechanically stable in a high pH environment. And last but not least, atomic force spectroscopy (AFM) is proposed to observe the APE nanostructure, the ionic conductive path, and the local mechanical strength by applying a small voltage between the tip and stage.
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    Polymer-Ionic Liquid Hybrid Electrolytes for Lithium Batteries
    (2012) Fisher, Aaron Steven; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Intellectual Merit: The goal of this dissertation is to investigate the electrochemical properties and microstructure of thin film polymer electrolytes with enhanced electrochemical performance. Solid electrolyte architectures have been produced by blending novel room temperature ionic liquid (RTIL) chemistries with ionically conductive polymer matrices. A variety of microstructure and electrical characterization tools have been employed to understand the hybrid electrolyte's performance. Lithium-ion batteries are limited because of the safety of the electrolyte. The current generation of batteries uses organic solvents to conduct lithium between the electrodes. Occasionally, the low boiling point and high combustibility of these solvents lead to pressure build ups and fires within cells. Additionally, there are issues with electrolyte loss and decreased performance that must be accounted for in daily use. Thus, interest in replacing this system with a solid polymer electrolyte that can match the properties of an organic solvent is of great interest in battery research. However, a polymer electrolyte by itself is incapable of meeting the performance characteristics, and thus by adding an RTIL it has met the necessary threshold values. With the development of the novel sulfur based ionic liquid compounds, improved performance characteristics were realized for the polymer electrolyte. The synthesized RTILs were blended with ionically conductive polymer matrices (polyethylene oxide (PEO) or block copolymers of PEO) to produce solid electrolytes. Such shape-conforming materials could be lead to unique battery morphologies, but more importantly the safety of these new batteries will greatly exceeds those based on traditional organic carbonate electrolytes. Broader Impacts: The broader impact of this research is that it will ultimately help push forward an attractive alternative to carbonate based liquid electrolyte systems. Development of these alternatives has been slow; however bypassing the current commercial options will lead to not only safer and more powerful batteries. The polymer electrolyte system offers flexibility in both mechanical properties and product design. In due course, this will lead to batteries unlike any currently available on the market. RTILs offer quite an attractive option and the electrochemical understanding of novel architectures based upon sulfur will lead to further potential uses for these compounds.