Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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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 SEQUENCE MODELING OF RAFT POLYMERIZATIONS WITH THE METHOD OF MOMENTS(2008-10-13) Zargar, Amin; Schork, Joseph; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Attempts to model the sequence structure of copolymers consisted of probabilistic functions that were incomplete and inaccurate. A novel technique to track sequence parameters is developed that determines not only copolymer composition, but sequence distribution as well. RAFT polymerizations are simulated with two independent and concurrent models to track MWD, conversion, copolymer composition, and sequence characteristics. Batch polymerizations are simulated with varying reactor conditions as a proof-of-concept to illustrate the power of the sequence model to track the composition of the polymer. Series of CSTR and PFR reactors with varying reactor conditions are then presented as applications to iteratively fine-tune copolymers with predetermined sequence and compositional structure.Item Micelles of Polybutadiene-b-Poly(Ethylene Oxide) in a Binary Solvent System(2008-05-01) Ploetz, Christopher D; Greer, Sandra C; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We studied the assembly behavior of a polybutadiene-b-poly(ethylene oxide) diblock copolymer in methanol, cyclohexane, and the corresponding partially miscible binary solvent system. Dynamic light scattering indicates that the copolymer forms coexisting spherical and cylindrical micelles in both of the pure solvents. In the binary solvent system, spherical micelles form in the methanol-rich phase for a wide range of temperatures. Conversely, micelles are present in the cyclohexane-rich phase only near the critical temperature. At the critical solvent composition, micelles form in the single phase region above the critical temperature. Size exclusion chromatography results for the binary solvent system show that the copolymer generally prefers the methanol-rich phase. The preference becomes more pronounced as temperature decreases.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 Coil-to-Helix Transition of Poly(ethylene oxide) in Solution(2004-07-30) Alessi, Michael Louis; Greer, Sandra C.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Poly(ethylene oxide) (PEO) is a simple polymer with repeating units [- C - C - O -] soluble in organic and aqueous solvents. The carbon atoms are hydrophobic; the oxygen atoms are hydrophilic and participate in hydrogen bonding. In all solvents in which PEO has previously been studied, PEO forms a coil in solution. Neutron scattering studies of PEO in isobutyric acid show that PEO undergoes a coil-to-rod transition in a solution of isobutyric acid (IBA). The stiffening is seen to progress smoothly with the addition of IBA, from a coil in D2O to a rod in pure deuterated-IBA. In addition to a solvent driven transition, a reversible rod-to-coil transition was seen to occur as a function of temperature, between 55 and 60 oC. Polarimetry experiments show that the rod formed by the PEO in solution is actually a helix, the conformation that PEO has in the solid state. It is also shown that, through the use of chiral impurities and temperature, the direction of the helix can be affected, allowing polymer folding to be influenced on a molecular level.