Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item The Hydrogel Reimagined: Gel-Derived Sponges and Sheets as Absorbents for Water, Blood, and Oil(2022) Choudhary, Hema; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer hydrogels, i.e., crosslinked networks of polymer chains swollen in water, are well-studied materials. Superabsorbent polymer (SAP) gels that can absorb more than 100x their dry weight in water are widely used in personal hygiene products – but only in the form of microscale beads. If dry SAP gels were larger, they would either take too long to swell or would be brittle solids. This dissertation seeks to reimagine polymer gels in very different physical forms: as soft sponges or foldable, fabric-like sheets. We want these macroscale dry materials to retain the ability to absorb large amounts of liquid, either aqueous or organic. In short, we would like to make polymer gels in convenient, usable forms similar to everyday absorbents like towels and sponges. The key to making gels as macroscale absorbents is to make the gels porous. In our first study, we devised a way to create porous gels by foam-templating. The approach involves in situ foaming of a monomer solution followed by fast polymerization. We generate the foams using a double-barrelled syringe that has acid and base in its two barrels. Gas (CO2) is formed at the mixing tip of the syringe by the acid-base reaction, and gas bubbles are stabilized by an amphiphilic polymer in one of the barrels. The monomers are then polymerized by ultraviolet (UV) light to form the gel around the bubbles, and the material is dried under ambient conditions to give a porous solid. We show that this dry, porous gel absorbs water at a rate of 20g/s until equilibrium is reached at ~ 300x of its weight. This is the fastest swelling and expansion ever achieved by a hydrogel. We convert the chemical potential energy from gel expansion into mechanical work: the gel is able to lift weights against gravity, with a power-density of 260 mW/kg. Next, we synthesize porous gels in the form of large sheets that resemble cloth or paper towels. For this, we polymerize thin films of the foams and ambient-dry the films after plasticization. Our gel sheets are flexible, foldable, and can be cut with scissors like fabrics. At the same time, the sheets absorb more than 30x of their dry weight in various aqueous fluids (water, blood, polymer solutions). Remarkably, these gel sheets expand as they absorb water, unlike any commercial towels. The expanded sheets retain absorbed fluid when lifted upright whereas fluid drips out of commercial absorbent sheets. Because of these superior properties, our gel sheets could be used to absorb aqueous liquids in various settings such as homes, labs and hospitals. Lastly, we design oleo-sheets, which are counterparts to the above that can absorb oils, i.e., non-polar liquids. We synthesize oleo-sheets by templating foams in which the continuous phase is non-aqueous and contains hydrophobic monomers. The oleo-sheets are hydrophobic and can selectively absorb oil from water. They show a high absorption capacity (> 50 g/g) for a range of organic solvents. The sheets can also be made magnetically responsive and an oil-soaked oleo-sheet can be lifted up by a magnet. We also fabricate a ‘Janus omni-absorbent sheet’ that has two sides: one side selectively absorbs water while the other side absorbs oil/solvents. Our oleo-sheets and omni-absorbent sheets could both be used in homes, hospitals, and various industries for cleaning up different spilled liquids.Item Synthesis and Characterization of Hemostatic Foams(2018) Rudy, Michael; Raghavan, Srinivasa; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrophobically modified (hm) derivatives of biopolymers like chitosan have the ability to coagulate blood cells and thus stop bleeding from severe injuries (i.e., achieve hemostasis). Our lab has been particularly interested in developing foams based on these polymers for use as hemostastic agents. Foams are attractive because an expanding foam at a wound site can counteract blood loss without the need for mechanical compression. The amphiphilic nature of hm-polymers also enables them to stabilize such foams. Previous work centered around a foam based on hm-chitosan (hmC) that was delivered out of a canister. To effectively combat internal hemorrhaging, we recognized the need to develop foams that could be more precisely placed at the wound site and also had greater mechanical integrity. Towards this end, this thesis describes a new class of polymeric foams that can be delivered out of a double-barreled syringe (DBS) by combining precursors in the two barrels that produce bubbles of CO2 gas in situ. Moreover, we show that by combining hmC in one barrel with a second biopolymer – hm-alginate (hmA) – in the other, we can generate foams with enhanced rheological properties compared to foams of hmC alone. This rheological enhancement is quantified in our work, and is due to electrostatic interactions between the cationic hmC and the anionic hmA chains. Preliminary studies in animal wound models also confirm that hmC-hmA foams can be precisely directed to a wound using the DBS and that these foams form effective barriers to blood loss due to their greater mechanical integrity.Item SYNTHESIS AND CONTROL OF MORPHOLOGY OF POLY METHYL METHACRYLATE AND POLY ACRYLIC ACID MICRO-PARTICLES BY THE MODIFIED SUSPENSION POLYMERIZATION TECHNIQUES(2015) Kim, Yunju Jung; Choi, Kyu Yong; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis studied the synthesis and control of morphology of two kinds of polymers by modified suspension polymerization techniques. The first polymer, poly (methyl methacrylate), is a transparent thermoplastic polymer, which is typically used in diffusing film in the backlight unit of an LCD. Also, the synthesis of micron-sized polymer particles with complex internal morphologies such as hollows, multihollows, and multiporous structures is of growing interest in many technological applications such as microelectronic displays and microencapsulation. The direct synthesis of such materials is carried out in heterogeneous processes with controlled phase separation mechanisms. In such systems, detailed knowledge of heterogeneous polymerization kinetics and phase separation phenomena is essential for investigating the process characteristics. An in situ polymerization and phase separation technique has been used to construct a ternary phase diagram for the free radical precipitation polymerization of methyl methacrylate (MMA), n-hexane, and poly(methyl methacrylate) (PMMA) system. The onset of the phase separation point during polymerization is directly monitored in real time by laser light scattering (LLS) technique for a broad range of polymer concentrations. The presented method overcomes the difficulty of determining the cloud points by titrating unreactive blends of polymer and solvent at high initial monomer concentrations that lead to high polymer concentration and high viscosity of the mixture fluid at the system phase separation point. We present the micro dispersive suspension polymerization (MDSP) process to produce complex particle morphologies in a single-stage process. MDSP is a hybrid of suspension and dispersion polymerizations. The micron-sized polymer particles are polymerized by suspension polymerization, and the internal morphology of particle is polymerized by dispersion polymerization inside the polymer particles. Varying the initial conditions for the phase separation in precipitation and dispersion polymerizations, final particles’ morphology may change from solid polymer particles to complex porous polymeric structures. In this heterogeneous process, the system evolution depends on the composition and molecular characteristics of the coexisting phases and on the characteristics of the interface. Using MDSP, we were able to develop a phase diagram to show the regions of multi-hollow/porous and core-shell/pomegranate-like poly (methyl methacrylate) (PMMA) particles. We also show that controlling morphology of polymer particles by thermodynamic and kinetic variables is technically feasible. The second polymer, poly (acrylic acid), is an absorbing polymer. Superabsorbent polymers (SAP) can absorb and retain extremely large amounts of water or aqueous solutions relative to their own mass. Partially neutralized sodium polyacrylate is industrially a very important polymer for many applications. However, in industry sodium polyacrylate is mostly manufactured by bulk polymerization, and the resultant bulk polymer is pulverized using a kneader to obtain small discrete polymer particles. It is environment-unfriendly process and the produced granules from bulk have irregular shapes, rather than a spherical shape. This study is aimed at investigating the inverse suspension polymerization of acrylic acid to make spherical polymer particles. In particular, the study is focused on how the resulting polymer morphology and characteristics are affected by the polymerization conditions. A feasible and simple technique to obtain Na-polyacrylate microparticles with sizes below 10 µm was investigated using a high shear mixing device. To maintain the stability of submicron size of aqueous droplets in the oil medium, a co-surfactant system containing Span 80 and Tween 80 was used. Neutralization of acrylic acid was proved from EDX analysis. Na-polyacrylate submicron particles were characterized by size, surface morphology, swelling capacity, and conversion. When the speed of homogenization was lowered from 3000 rpm to 1000 rpm, particles over 10 µm were obtained, but more nano-sized particles were present outside. We also developed the technique above to increase polymer particle size to tens of microns. In this process, a wrinkled and cracked surface of Na-polyacrylate particles was observed in the special environment of post treatment. Surface area, swelling capacity, and swelling speed of different morphologies and sizes were characterized and analyzed. In order to synthesize spherical Na-polyacrylate particles with smooth surface regardless of post treatment, polymerization time was progressed longer than 20 hr. Na-polyacrylate particles had a solid structure at high conversion over 0.996 after longer than 15 hrs of polymerization, which made particles maintain their shapes regardless of post treatment. When a high monomer concentration was used in this polymerization, perfectly smooth and spherical polymer particles were obtained after 9 hr, which was faster than when a lower monomer concentration was used.Item Relaxation and Stiffening Dynamics of a Single Semiflexible Polymer Chain(2006-04-27) dissanayake, inuka; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Both synthetic and biological polymers are a challenge to study because of the many features and functional roles they carry. A good understanding of the macromolecule's dynamical properties is essential for biological processes such as the cytoskeleton dynamics of actin or in creating novel materials such as biodegradable nanocomposites. Here we focus on the Brownian dynamics of single semiflexible polymer chains, speci cally the relaxation and stiffening behaviors. To date, the transient modeling of dilute solutions has concentrated mainly on flexible chains. Semiflexible polymers, with a persistence length comparable to or larger than their contour length show distinct properties in solution. Brownian dynamics simulations based on a discretized version of the Kratky-Porod chain model were employed. First, the relaxation of a bead-rod polymer chain from an initially straight configuration was followed. Through a scaling-law analysis, universal relaxation laws were determined covering all time scales. A correlation describing the properties studied by the single parameter of chain length was noticed. Based on this, we were able to confirm and explain the chain's stress and optical properties, as well as derive a nonlinear stress-optic law valid for semiflexible chains at any time period. Also, we determine the relaxation for long semiflexible chains exhibit two intermediate-time behaviors, as a result of the interplay of Brownian and bending forces on the link tensions. A second project involved the relaxation dynamics of a worm-like bead-spring chain. Existing relaxation simulations of this bead-spring model are limited to the stress behavior. Here we monitor the short and intermediate-time relaxation behaviors of a nearly extended semiflexible chain. We also look at the effects of the Kuhn length on a chain of constant length. Finally, the interesting behavior of the coil-helix-rod stiffening transition was studied. When subjected to external forces or a change in solution conditions the macromolecule may stiffen. Being able to control the chain stiffness is of technological importance especially for nanotechnology devices where the constraint of the walls limits the entropy available to the chain. We have successfully simulated the transient conformational behavior and subsequently understand the chain dynamics involved through analysis of the chain's length, width, and stress.Item Synthesis and Characterization of Diblock Copolymer Templated Iron Oxide Nanoparticles(2005-07-26) Akcora, Pinar; Kofinas, Peter; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Templating ordered assemblies of magnetic oxide nanoparticles within self-assembled diblock copolymers of varying morphologies is an important problem with a wide applicability such as in electromagnetics, optical devices, metal catalysts, medicine and biology. In this thesis, the effects of different polymer structures on particle ordering and resultant magnetic properties have been investigated using various microstructure and magnetic characterization tools. Ring-opening metathesis polymerization (ROMP) of norbornene and functionalized norbornene monomers has been used to synthesize diblock copolymers of narrow polydispersities using Grubbs' catalyst. These block copolymers can be used as templates to form inorganic nanoparticles. In this research, the structural and physical understanding of the inorganic-copolymer system was studied by small-angle neutron and x-ray scattering techniques and transmission electron microscopy. Synthesis of $\gamma$-Fe$_2$O$_3$ nanoparticles has been achieved within novel block copolymers of (norbornene)-b-(deuterated norbornene dicarboxylic) acid and (norbornene methanol)-(norbornene dicarboxylic acid). The polymer morphologies were controlled by varying the volume fractions of the constituent blocks. The pure norbornene based diblock copolymer morphologies were demonstrated by electron microscopy for the first time. Spherical, cylindrical and lamellar morphologies of these novel diblock copolymers were reported. The block ratios of the synthesized polymers were determined using gel permeation chromatography - light scattering, elemental analysis and UV-VIS spectroscopy. Solution phase doping and submersion of thin films in metal salt solutions were employed as metal doping methods and the observed nanoparticle structures were compared to those of the undoped copolymer morphologies. This project reports on the types of templating structures and dispersion of the nanoparticles. The effects of particle interactions on the microphase separation and magnetic properties were also investigated. The knowledge gained from understanding the templating mechanism in block copolymer / iron oxide nanocomposites can be applied to other similar systems for a variety of biological and catalyst applications.Item Analysis of rheological properties and molecular weight distributions in continuous polymerization reactors(2004-12-06) Dave, Kedar Himanshu; Choi, Kyu Yong; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work explores the possibility of exploiting structure-property relationships to manufacture tailor-made polymers with target end-use properties. A novel framework which aims to improve upon current industrial practices in polymerization process and product quality control is proposed. The strong inter-relationship between the molecular architecture and rheological properties of polymers is the basis of this framework. The melt index is one of the most commonly used industrial measures of a polymer's processibilty. However, this single-point non-Newtonian viscosity is inadequate to accurately reflect the polymer melt's flow behavior. This justifies monitoring the entire viscosity-shear rate behavior during the polymerization stage. In addition, the crucial role played by the polymer melt's elastic characteristics is not reflected in it's shear viscosity and so elasticity meaurements are also warranted. In this study, rheological models available in the open literature are utillized to demonstrate these critical issues at industrially relevant operating conditions. The observations made are also compared with published experimental results and found to be qualitatively similar. Two case studies are presented. The first one is the free-radical solution polymerization of styrene with binary initiators in a cascade of two CSTRs. In the second case, the solution polymerization of ethylene in a single CSTR with a mixture of two single-site transition metal catalysts is considered. The feasibility of the proposed framework to tailor the product's MWD, irrespective of the underlying reactor configuration or kinetic mechanism, is demonstrated via steady state simulations. Relative gain analysis reveals the non-linearity and interactions in the control loops. Although the main contributions of this study primarily deal with the viscoelastic behavior of linear homopolymers, potential extensions to systems involving polymers with small amounts of long chain branching or the control of other end-use properties are also discussed.