Browsing by Author "Alan E. Tonelli, Committee Member"

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Continuous Precipitation Polymerization of Acrylic Acid in Supercritical Carbon Dioxide
(2006-04-07) Liu, Tao; Joseph M. DeSimone, Committee Co-Chair; Alan E. Tonelli, Committee Member; Saad A. Khan, Committee Member; George W. Roberts, Committee Co-Chair
The precipitation polymerization of acrylic acid in supercritical carbon dioxide (scCO2) was carried out in a continuous stirred tank reactor. The product polymer was a white, dry, fine powder that dissolved in water. A wide range of polymer molecular weights (5 to 200 kg/mol) was obtained. The effect of the operating variables on the polymerization rate and on the polymer molecular weight was evaluated. The polymerization showed distinct deviations from the normal solution polymerization. By assuming that chain initiation occurs in the solution phase, but chain propagation and chain termination occur in the polymer phase, a 'surface polymerization model' and a 'particle polymerization model' both described the polymerization well. Scanning electron micrographs showed that three types of polymer particles were obtained: agglomerates of primary particles of about 100 nanometers in size, irregular particles of 5—20 micrometers, and spheres of 10—100 micrometers. It is speculated that the agglomerates were produced when the polymerization temperature (TP) was below the polymer glass transition temperature (Tg), the irregular particles were obtained when TP was close to Tg, and the spheres were prepared when TP was above Tg. The CO2 absorption into poly(acrylic acid) (PAA) was measured with a quartz crystal microbalance. The Tg depression by scCO2 was calculated with the Chow's equation. The calculated results lent strong support to the proposed particle formation mechanism. Cross-linking polymerization of acrylic acid in scCO2 was studied in a batch reactor at 50°C and 207 bar. All products were white, dry, fine powders. By adjusting the cross-linker concentration, water-soluble and water-insoluble PAAs were synthesized. The water-insoluble PAA was neutralized by ammonia gas and sodium hydroxide alcohol solution to make superabsorbent polymers.
Copolymerization of Vinylidene Fluoride with Hexafluoropropylene in Supercritical Carbon Dioxide
(2008-02-28) Ahmed, Tamer Samir; Joseph M. DeSimone, Committee Co-Chair; Saad A. Khan, Committee Member; Alan E. Tonelli, Committee Member; George W. Roberts, Committee Co-Chair
This thesis details research to study the copolymerization of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) in supercritical carbon dioxide (scCO2). Another objective of this thesis is to understand the origin of the bimodal molecular weight distribution (MWD) that results under certain conditions during the precipitation polymerization of poly(vinylidene fluoride) (PVDF) in scCO2. The copolymerization of VF2 with HFP was carried out in scCO2 using a continuous stirred tank reactor (CSTR). The experiments were done at 40 oC with pressure in the range of 207-400 bar using perfluorobutyryl peroxide as the free radical initiator. Four different copolymer compositions were studied: ca. 10, 23, 26, and 30 mole % HFP. The 10%-copolymer was collected as a dry free-flowing semicrystalline powder while the other compositions were amorphous elastomeric materials collected continuously using acetone. Most of the polymerizations were heterogeneous, i.e., polymer particles precipitated during the reaction. However, some were homogenous, especially in the higher range of HFP content. The effects of feed monomer concentration and reaction pressure were both explored at otherwise constant conditions. The rate of polymerization (Rp) and the number-average molecular weight (Mn) increased linearly with the total monomer concentration up to about 6 M, the highest concentration investigated. In addition, the Rp and the Mn increased with reaction pressure. The MWDs of the synthesized copolymer showed a long tail that increased to become a broad shoulder with increasing total monomer concentration. This tail decreased with HFP content in the copolymer and increased with reaction pressure. The experimental results of VF2 homopolymerization and copolymerization with HFP in scCO2 were tested against three kinetic models to determine the main locus of polymerization. The first model, the "solution polymerization" model, is based on the assumption that all the polymerization reactions place in the continuous, CO2-rich phase, with no reaction in the polymer phase. In the second model, the ?surface polymerization" model, chain initiation occurs exclusively in the continuous phase, while chain propagation and termination occur in a thin zone on the surface of the polymer particles. The third model, the "interior polymerization" model, is similar to the "surface polymerization" model, except that propagation and termination take place uniformly throughout the polymer particles. Both the surface and the interior polymerization models failed to fit the experimental results. On the other hand, the solution polymerization model was able to describe the experimental results of the polymerizations fairly well over the whole range of polymer compositions. This suggests that the CO2-rich continuous phase is the main locus of polymerization in the precipitation polymerization of VF2 homopolymer and VF2⁄HFP copolymers scCO2. Finally, a homogenous model is presented to account for the bimodal MWDs of PVDF. The model takes into account both the change of termination scheme of the polymeric radicals with chain length from chemically-controlled termination to diffusion-controlled termination and chain transfer to polymer reaction. The model was successful in accounting for the change of modality with reaction conditions such as monomer concentration, average residence time at low and high monomer concentrations, and the reaction temperature. In addition, the model could capture the occurrence of gelation, which was responsible for an inoperability region that was observed in the polymerization experiments.
Design And Study of Biodegradable Small Diameter Vascular Grafts
(2003-11-13) Agrawal, Pankaj; Peter L. Mente, Committee Member; Alan E. Tonelli, Committee Member; Bhupender S. Gupta, Committee Chair
In this research, the focus has been on designing and producing small diameter woven grafts having a biodegradable material. Woven tubes in diameters ranging from 3 to 6.5 mm are developed using a narrow width Muller loom. The material used is Polyglactin 910 biodegradable yarn of 56 denier and the structures developed are plain but with different degrees of tightness. The pick density is varied from 32 to 44 picks per inch and end density is varied by using different number of total ends. Accordingly, the experimental work in this thesis involved weaving of 5 different sets of tubular structures and examining their behaviors. Since the woven grafts needed to be heat set to develop a circular shape that was resilient, preliminary studies were conducted on the yarns to determine heat setting conditions that led to optimum set with minimum degradation in properties. Process parameters required to manufacture such structures are identified. The effects of degradation time and structural variables on the mechanical properties of the grafts are studied. The variables were the construction parameters and the days of degradation; and the properties examined were the change in mass and thickness of the graft, and the elastic recovery, compliance and porosity properties of the structures. Statistical analysis of variance is conducted to identify significant effects in several instances.
Functional Bone Tissue Engineering using Human Mesenchymal Stem Cells and Polymeric Scaffolds
(2007-12-08) Sumanasinghe, Ruwan Deepal; Behnam Pourdeyhimi, Committee Member; Elizabeth G. Loboa, Committee Chair; Martin W. King, Committee Co-Chair; Nancy Monteiro-Riviere, Committee Member; Alan E. Tonelli, Committee Member
Modification of Polymer/Polymer Interfaces using Block Copolymers and Microgels
(2006-03-13) Wei, Bin; Jan Genzer, Committee Co-Chair; John van Zanten, Committee Member; Alan E. Tonelli, Committee Member; Richard J. Spontak, Committee Chair
A guest macromolecular material, either a block copolymer (BCP) or core-shell microgel (MG) particles, has been used to stabilize a polystyrene (PS) film positioned atop an immiscible homopolymer substrate of poly(methyl methacrylate) (PMMA). Modification of the PS/PMMA interface due to interfacial partitioning of the guest macromolecule significantly increases the stability of the PS film by either slowing down or completely eliminating dewetting of the top PS layer. Our work has revealed that the dewetting mechanism of the top layer may change between nucleation and growth of holes and spinodal-like surface fluctuations, depending on the extent of interfacial heterogeneities induced by the BCP. Due to the shape retention of the MG, autophobicity is observed between the MG particles with a PS-like core and PMMA arms and a chemically identical long-chain PMMA homopolymer. This behavior is attributed to entropic exclusion of the PMMA matrix polymer from the PMMA arms of high graft density. We have demonstrated that autophobicity is strong enough to overcome the resistance of interfacial tension γAB so that the MG could be pushed from the PMMA matrix to the PS/PMMA interface, whereas the PMMA/MG surface remained free of MG, because the PMMA surface energy suppresses the surface roughening that accompanies autophobic segregation. Such MG-induced interfacial patterning in areas in contact with PS is completely reversible. Further annealing the PMMA/MG after the PS is removed permits the surface energy of PMMA to force the MG back into the PMMA substrate. Based on this reversible autophobic segregation, we have developed a simple stamping strategy of controlled and reversible patterning of MG particles on the film surface. A patterned poly(dimethyl siloxane) (PDMS) film with periodic ridges and valleys can be utilized as a stamp to control the in-plane distribution of the segregating MG particles. The MG blended in the PMMA/MG film migrates to the surface only in areas in contact with the PDMS. Further annealing with the stamp removed causes the film surface to smoothen due solely to surface tension.
Polyolefin Miscibility: Solid-State NMR Investigation of Phase Behavior in Saturated Hydrocarbon Blends
(2005-07-06) Wolak, Justyna Ewa; Jeffery L. White, Committee Chair; Alan E. Tonelli, Committee Member; Edward O. Stejskal, Committee Member; Alex I. Smirnov, Committee Member
Polyolefin blends represent a vital material field due to their economic and commercial importance. Potential new properties such as lighter weight, lower cost and higher strength, motivate research to investigate blends of saturated hydrocarbon polymers. However, many questions remain concerning how polymer chain structure and packing influence local thermodynamics, or more specifically, the interplay between enthalpy and entropy, which ultimately control bulk phase behavior. Solid-state NMR has proven to be an essential tool in these studies due to its ability to selectively observe molecular level conformations and dynamics without isotopic labeling. Combinations of basic and advanced variable temperature studies such as 1D and 2D 13C cross-polarization exchange experiments, static 2H lineshape analysis, 1H relaxation/spin-diffusion measurements, and 129Xe experiments were applied in this work. Several systems were studied, including 50/50 weight percent blends of polyisobutylene with polyethylene-co-1-butene, polyisobutylene with head-to-head polypropylene, and atactic polypropylene with the same polyethylene-co-1-butene samples. The results were used to determine a relationship between miscibility, length scales of mixing, and timescales/length scales of the glass transition.
Polystyrene Hydrogenation in Supercritical CO2-Decahydronaphthalene Using Porous Catalysts
(2010-06-28) Dong, Laura Beth; George W. Roberts, Committee Chair; Douglas J. Kiserow, Committee Co-Chair; Alan E. Tonelli, Committee Member; Ruben G. Carbonell, Committee Member
The heterogeneous hydrogenation of polystyrene (PS) was studied in a slurry batch reactor. Mixtures of supercritical carbon dioxide (scCO2) and decahydronaphthalene (DHN) were used as the solvent for the polymer. Several palladium-based porous catalysts were identified for PS hydrogenation at 150oC. Relatively high degrees of hydrogenation were obtained with monometallic palladium catalysts for the reaction conducted in neat DHN. However, when either palladium catalyst was used in scCO2-DHN, hydrogenation ceased within 15 minutes of CO2 addition to the reactor. Carbon monoxide (CO) formed via the reverse water-gas shift (RWGS) reaction and poisoned hydrogenation sites. Physical mixtures consisting of a hydrogenation catalyst and a methanation catalyst were effective in reducing CO levels. However, when the â€œsalt-and-pepperâ€ catalyst was used, aromatic ring hydrogenation levels in scCO2-DHN were consistently lower than those obtained in neat DHN. A bimetallic catalyst in which the hydrogenation and methanation functions are located on the same support was successfully used to reduce CO levels and to hydrogenate PS in scCO2-DHN. The success of the bimetallic catalyst in hydrogenating PS in scCO2-DHN over the salt-and-pepper approach was attributed to the differences in internal mass transfer resistances for PS hydrogenation and the RWGS reaction. Polymer size effects on heterogeneous PS hydrogenation were determined by varying polymer molecular weight and by using CO2 to tune polymer coil size in DHN. The ability to tune polymer coil size by varying CO2 concentration was demonstrated in high pressure dynamic light scattering experiments. The improvements in reaction rate in either neat or CO2-expanded DHN were found to be directly related to increases in PS diffusivity and decreases in polymer coil diameter, both of which are functions of polymer molecular weight and solvent quality.
Stimuli-responsive Protein-based Hydrogels by Utlizing beta-Sheet Conformation of Silk Fibroin as Cross-links
(2005-01-07) Gil, Eun Seok; Samuel M. Hudson, Committee Chair; Richard J. Spontak, Committee Member; Alan E. Tonelli, Committee Member; Richard Kotek, Committee Co-Chair
Stimuli responsive polymers can provide a variety of applications for biomedical fields such as drug delivery, biotechnology, and chromatography. The interest in these polymers has exponentially increased due to their promising potential. Stimuli-responsive polymers have been utilized in various forms: hydrogels, micelles, modified interfaces, and conjugated solutions. Among them, hydrogels have gained strong attention as biomaterials due to their biocompatibility and biodegradability in the swollen state. The introduction of stimuli-responsive characteristic into hydrogels should provide more versatile applications such as targeted drug delivery, micro or nano scale actuating valves, artificial organs responding to stimuli, and protein or DNA purification. In many applications, better biological materials are needed, particularly the incorporation of two or more functionalities into one material. One strategy is to develop interpenetrating polymer networks (INPs) in hydrogels. Novel protein-based complex hydrogels were prepared by blending gelatin (Gel) with Bombyx mori silk fibroin (SF) and introducing beta-sheet conformation of SF in their complex networks. The influence of solvent-induced SF crystallization on the properties and structures of these binary protein complexes was determined as functions of blend composition and preparation history. Rheological tests confirmed that the fine beta-sheet crystalline structure successfully governed the Gel/SF complex networks, increasing their viscoelastic properties and sustaining their physical form as hydrogels even at body temperature. The helix-coil transition of gelatin in the Gel/SF complex hydrogels was determined by DSC and rheological tests to be reversible between ambient and body temperatures, so these hydrogels exhibit reversible IPNs/semi-IPNs transitions. This reversible temperature-responsive conformational change of gelatin molecules in Gel/SF complex hydrogels could promote an abrupt swelling increase and a temperature-triggered protein release from the networks at body temperature, which could be utilized for a targeted drug delivery. These hydrogels show a temperature-responsive gelatin release profile: at 20 °C they exhibited no gelatin release and maintained their hydrogel dimensions, but at 37 °C they showed time-dependent gelatin release and their hydrogel dimensions decreased. Protein-synthetic polymer hybrid interpenetrating networks (IPNs) of poly(N-isopropylacrylamide) (PNIPAAm) with Bombyx mori (B. mori) silk fibroin (SF) are described. In these IPNs, SF has the beta-sheet crystalline structure, and shows improved storage and loss moduli. The IPN hydrogels show volume phase transition behavior at the same temperature and NaCl concentration as pure PNIPAAm hydrogels. The PNIPAAm/SF IPNs retain the swelling kinetics of PNIPAAm and show increased deswelling kinetics, with a mechanism whereby the internal water molecules are rapidly released through the induced beta-sheet networks. The IPNs with SF beta-sheet structure successfully decrease the formation of a skin layer observed in conventional PNIPAAm hydrogels. Therefore, the proposed IPN hydrogels can provide three benefits; improved mechanical property, biocompatibility, and deswelling rates.
Structural Study of Bombyx mori Silk Fibroin during Processing for Regeneration
(2005-02-28) Ha, Sung-Won; Maurice C. Balik, Committee Member; Clay Clark, Committee Member; Samuel M. Hudson, Committee Chair; Richard Kotek, Committee Member; Alan E. Tonelli, Committee Member
Bombyx mori silk fibroin has excellent mechanical properties combined with flexibility, tissue compatibility, and high oxygen permeability in the wet condition. This important material should be dissolved and regenerated to be utilized as useful forms such as gel, film, fiber, powder, or non-woven. However, it has long been a problem that the regenerated fibroin materials show poor mechanical properties and brittleness. These problems were technically solved by improving a fiber processing method reported here. The regenerated fibroin fibers showed much better mechanical properties compared to the original silk fibers. This improved technique for the fiber processing of Bombyx mori silk fibroin may be used as a model system for other semi-crystalline fiber forming proteins, becoming available through biotechnology. The physical and chemical properties of the regenerated fibers were characterized by SinTech® tensile testing, X-ray diffraction, solid state 13C NMR spectroscopy, and SEM. Unlike synthetic polymers, the molecular weight distribution of Bombyx mori silk fibroin is mono-disperse because silk fibroin is synthesized from DNA template. Genetic studies have revealed the entire amino acid sequence of Bombyx mori silk fibroin. It is known that the crystalline silk II structure is composed of hexa-amino acid sequences, GAGAGS. However, in the amino acid sequence of Bombyx mori silk fibroin heavy chain, there are present 11 chemically irregular but evolutionarily conserved sequences with about 31 amino acid residues (irregular GT~GT sequences). The structure and role of these irregular sequences have remained unknown. One of the most frequently appearing irregular sequences was synthesized by a peptide synthesizer. The three-dimensional structure of this irregular silk peptide was studied by the high resolution two-dimensional NMR technique. The three-dimensional structure of this peptide shows that it makes a turn or loop structure (distorted Ω shape), which means the proceeding backbone direction is changed 180° by this sequence. This may facilitate the β-sheet formation of the crystal forming building blocks, GAGAGS/GY~GY sequences, in fibroin heavy chain. It may also facilitate the solubilization of the fibroin heavy chain within the silk gland.
Synthesis and Characterization of Biopolymer Composites and their Inorganic Hosts
(2009-08-07) Rovira Truitt, Rosimar; Jeffery L. White, Committee Chair; Edward O. Stejskal, Committee Member; Tatyana I. Smirnova, Committee Member; Alan E. Tonelli, Committee Member
Biopolymers are biodegradable and biocompatible materials obtained from renewable sources. These polymers could have an increased impact in consumer or health applications, given a larger, more flexible range of physical properties. Targeting enhanced properties through the design of organic-inorganic hybrids requires novel synthesis routes. An in-situ polymer composite that differs from hybrids generated by simple mixing of the organic and inorganic phases, has been demonstrated here. A known ring opening polymerization catalyst was incorporated within the channels of mesoporous hosts (e.g. MCM-41). A combination of elemental, solid-state NMR, BET nitrogen adsorption, and microscopy experiments indicated that the stannous octoate catalyst was supported inside the host channels, and that a charged framework is not required for its incorporation. These Sn(Oct)2 supported mesoporous catalysts were used to prepare poly(d,l-lactide) composites. Multiple experiments, including solid state NMR, BET nitrogen adsorption, and calorimetric analysis, gave evidence that the resulting polymer forms inside the host channels. In this way, an organic-inorganic composite which grows out of the crystallites is generated in-situ. Additionally, the acid catalyzed condensation polymerization of lactic acid with micro/mesoporous materials was investigated. Results suggest that Al-SBA-15 is a potential catalyst for this type of polymerization. This approach is desirable, since the generated organic-inorganic composite would contain no impurities (i.e. metal catalysts).
Synthesis and Investigation of Chiral Polycarbodiimides with Reversible Dynamic Properties
(2010-04-21) Kennemur, Justin Glenn; Bruce M. Novak, Committee Chair; Christopher B. Gorman, Committee Member; Christian Melander, Committee Member; Alan E. Tonelli, Committee Member
Ever since the pioneering idea of macromolecules was introduced by Staudinger in the 1920â€™s, the interdisciplinary field of synthetic polymer chemistry has grown tremendously and has now emerged into the mature subject that it is today. During this maturation, polymers have globally incorporated themselves into the everyday lives of humanity. Although, in most instances, polymers have been reveled for being lightweight, strong, chemically inert, flexible, and relatively inexpensive materials, a new breed of functional polymers are now being synthesized by polymer chemists and creating frontline attention within peer-reviewed journals. These new polymers are specialty materials designed to perform a function and otherwise interact in some way with their surroundings. The breadth of functionality being investigated is too long to list, ranging from something as complicated as biomimetic or conductive polymers to something as simple as improved degradation or â€œgreenâ€ polymers. Along with these new functionalities comes new challenges faced by polymer chemists with regards to the synthesis and characterization of such systems and although many significant advances have been made, this field is generally considered to be within the early stages of discovery. This manuscript will discuss a particular class of functional polymers known as polycarbodiimides and their application as specialty materials will be outlined as well as significant advances made to the synthesis and characterization of these polymers. Focus is placed on particular types of helical polycarbodiimides containing polyarene and aliphatic pendant groups. Through chiro-optical analysis, these polymers display a low energy solvo- and thermo-controllable conformational switching which is believed to arise from reorientation of the polyarene pendant groups fixed to the helical polycarbodiimide backbone scaffold. In essence, these polymers act as molecular shutters and, through their chirality, can greatly alter the polarization of light under very mild conditions. Structure activity relationships were performed and a library of over twenty new polymers was created to further understand the limitations and driving forces behind this unique phenomenon. In addition, synthetic optimization and property relationships of a new hallmark polymer, N-(1-naphthyl)-Nâ€™-(n-octadecyl) polycarbodiimide, is explored with the inclusion of solvation, molecular weight, regioregularity, and enantiomeric studies and their effect on this switching behavior. In addition to the reversible dynamic properties of these polycarbodiimides, this manuscript will also explore new approaches towards better understanding and controlling fundamental properties of polycarbodiimides in general. Regioregularity of asymmetric carbodiimide polymers will be discussed and select polymers from within this new library reveal evidence of potentially controlling regioselectivity of these polymer systems through pendant group substituent effects. New improvements and clarification to analytical techniques for determination of the amount of regioirregularity within these polymer systems are also discussed.