An environmentally benign process, which uses supercritical carbon dioxide (sc-CO₂) as a processing aid, was developed in this work to help exfoliate and disperse nanoclay into the polymer matrices at high clay content. The process involves the use of a pressurized CO₂ chamber to assist in the exfoliation and delivery of the clay into a stream of polypropylene (PP) melt within the extruder. This CO₂ method was evaluated and compared to other conventional processing techniques. It was observed that the conventional direct-melt compounding methods, with and without the direct injection of CO₂, did not show much improvement in the mechanical properties due to their inability to adequately exfoliate the nanoparticles into the polymer matrix. The commercial RTP sample prepared using a TSE and a MA compatibilizer showed moderate improvements in the clay dispersion and properties due to high shear forces and mixing capabilities of TSE. The most improvements were seen from the technique of using the pressurized CO₂ chamber, which directly injected pre-mixed sc-CO₂ and nanoclay into the polypropylene melt during extrusion. It was observed that the mechanical properties of the PP nanocomposites prepared using the CO₂ chamber technique, especially when combined with maleic anhydride (MA) compatibilizer, outperformed those of the commercial RTP samples and those of samples prepared using conventional melt compounding techniques. WAXD and TEM data showed a good degree of exfoliation for clay concentrations as high as 6.8 wt% when the clay was expanded and mixed with CO₂. At this concentration, mechanical properties such as yield strength and modulus increased by as much as 13% and 69%, respectively, relative to the pure PP, and approximately 15% higher than those of samples prepared by direct melt compounding (without the use of CO₂). Furthermore, yield-like behavior in the viscosity and a plateau in the low-frequency behavior of storage modulus, Gâ , was also attributed to polymer-clay interaction due to strong hydrogen bonding between MA groups and the hydroxyl groups on the clay surface, not just solely to the formation of percolation network due to exfoliation between clay platelets that is commonly reported in literature for clay-filled functionalized polypropylene. / Ph. D.

Master of Science / Department of Mechanical and Nuclear Engineering / Akira T. Tokuhiro / The Secure Transportable Autonomous Reactor – Liquid Metal system combines a Generation IV nuclear reactor with an advanced Supercritical Carbon Dioxide (S-CO[subscript]2) Brayton power conversion cycle. The Brayton cycle was selected as the power conversion cycle due to its high efficiency, small turbomachinery size, and competitive cost due to reduced complexity as compared to a traditional Rankine cycle. Overall system thermal efficiency is closely tied to the performance of the precooler and recuperators. The Printed Circuit Heat Exchanger (PCHE) manufactured by Heatric is being considered for use as both the precooler and recuperator in the STAR-LM system due to its high effectiveness, wide temperature and pressure operating range, small size, and low cost. PCHEs have been used primarily in the hydrocarbon processing industry to date, and are relatively new in being considered for nuclear applications. In this study, a PCHE is investigated using S-CO[subscript]2 and water as the heat transfer media in conditions relevant to the precooler in the STAR-LM system. Experiments conducted with small temperature differences across the PCHE revealed that the heat transfer coefficient is strongly correlated with the temperature-dependent specific heat near the pseudocritical point. The STAR-LM precooler outlet temperature is near the pseudocritical point, making this region of interest to this work. Testing was conducted to determine the effect of property variation near the precooler outlet in conditions with large temperature differences in the PCHE. These tests revealed that maintaining the precooler outlet temperature near the pseudocritical point does not have a significant effect on heat transfer coefficients in the PCHE under large temperature difference test conditions. Computational Fluid Dynamics (CFD) models were developed to simulate fluid flow and heat transfer in the PCHE. A 2D, 4-channel, zig-zag model was found to reproduce the outlet temperatures to within approximately 15% relative error. The 3D straight channel model reproduced the experimental data to within 3% relative error for the cases simulated. Both of these models predicted the water side outlet temperatures to within 20% relative error.

Biocompatible and biodegradable porous materials based on silk fibroin (SF), a natural protein derived from the Bombyx mori silkworm, are being extensively investigated for use in biomedical applications including mammalian cell bioprocessing, tissue engineering, and drug delivery applications. In this work, low-pressure, gaseous CO2 is used as an acidifying agent to fabricate SF hydrogels. This low-pressure CO2 acidification method is compared to an acidification method using high-pressure CO2 to demonstrate the effect of CO2 mass transfer and pressure on SF sol-gel kinetics. The effect of SF molecular weight on the sol-gel kinetics is determined using the low-pressure CO2 method. The results from these studies demonstrate that low-pressure CO2 processing proves to be a facile method for synthesizing 3D SF hydrogels. We also determined the effect of SF solution concentration on the morphology and textural properties of SF aerogels. Changing the solution concentration from 2 wt% to 6 wt% yielded a higher surface area (260 to 308 m2/g) and different macro structure, but similar mesopore pore volume and size, and micro structure. Furthermore, we determined the effect of drying method on the morphology and textural properties of SF hydrogels gelled via CO2 acidification. Drying with supercritical carbon dioxide (scCO2) yielded an aerogel surface area five times larger than aerogels that were freeze dried. Moreover, a freeze dried hydrogel initially frozen at -20 °C had pores approximately 10 µm larger than a hydrogel initially frozen at -196 °C. The results presented here also demonstrate the potential of SF aerogels as drug delivery devices for the extended release of ibuprofen, a model drug compound. SF aerogels are loaded with ~21 wt% of ibuprofen using scCO2 at 40 °C and 100 bar. Differential scanning calorimetry of the ibuprofen-loaded SF aerogels indicates that the ibuprofen is amorphous. Scanning electron microscopy and nitrogen adsorption/desorption analysis are used to investigate the morphology and textural properties. Phosphate buffer solution (PBS) soaking studies at 37 °C and pH 7.4 reveal that the SF aerogels do not swell or degrade for up to six hours. In vitro ibuprofen release in PBS at 37 °C and pH 7.4 occurs over a six-hour period when the ibuprofen is loaded in SF aerogel discs with an aspect ratio of ~1.65 (diameter/thickness), whereas the dissolution of the same amount of pure ibuprofen occurs in 15 minutes. Furthermore, the release of ibuprofen from these SF aerogel discs are modeled using the Fu model which indicates that ibuprofen release follows Fickian diffusion for the first 65 wt% of ibuprofen release, and non-Fickian diffusion for the next 25 wt% of ibuprofen release. We also showed that SF aerogel scaffolds support in vitro human foreskin fibroblast cell attachment, proliferation, propagation, and cell seeding of different densities (10x103, 30x103, and 60x103). In summary, we created and characterized a tunable 3D SF aerogel scaffold with potential for applications in drug delivery and tissue engineering applications.

No description available.

Whey protein isolate (WPI) contains at least 90% protein and should ideally possess a bland flavor without typical dairy flavors including sweet aromatic and cooked/milky notes. However, its flavor may be highly variable due to factors including original whey source, processing and storage conditions. Novel technologies removing nonpolar compounds responsible for off-flavors and off-flavor formations are desirable. The major objective of this research was to evaluate impacts of supercritical carbon dioxide (scCO2) extraction, a known green process, on volatile profiles of WPI. A prior sub-objective was to establish an analytical technique for characterization of volatiles. Specifically, adsorption conditions in a well-established head-space solid-phase microextraction (SPME) method were used for quick and reliable assays of volatiles in WPI, using a divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) fiber. The adsorption of volatiles on the SPME fiber was studied at 21, 40 or 50 °C, each with durations of 5, 15 and 20 min, and analyzed by GC/MS. Based on the number of GC/MS peaks and the corresponding peak areas, adsorption conditions of 50 °C for 20 min were selected for subsequent studies. In the second sub-objective, GC/MS profiles of WPI were characterized after scCO2 extraction using a continuous stream of CO2 at 50 g/min, controlled at various combinations of temperature (30-65°C), pressure (7.0-30.0 MPa), and duration (10-90 min). Extractions with a higher temperature and a higher pressure for a longer time were generally more effective in removing volatiles, and most peaks on the chromatogram of the unprocessed WPI sample disappeared or were reduced very significantly after all studied extraction conditions, even at subcritical conditions of 7.0 MPa and 30 °C for 1 hour. Our findings demonstrated that supercritical or subcritical CO2 may provide a green approach to reduce volatiles in whey protein preparations for novel food applications.

For the first time stabilizer-free vinylidene fluoride (VDF) polymerizations were carried out in homogeneous phase with supercritical CO₂. Polymerizations were carried out at 140°C, 1500 bar and were initiated with di-tert-butyl peroxide (DTBP). In-line FT-NIR (Fourier Transform- Near Infrared) spectroscopy showed that complete monomer conversion may be obtained. Molecular weights were determined via size-exclusion chromatography (SEC) and polymer end group analysis by 1H-NMR spectroscopy. The number average molecular weights were below 104 g∙mol−1 and polydispersities ranged from 3.1 to 5.7 depending on DTBP and VDF concentration. To allow for isothermal reactions high CO₂ contents ranging from 61 to 83 wt.% were used. The high-temperature, high-pressure conditions were required for homogeneous phase polymerization. These conditions did not alter the amount of defects in VDF chaining. Scanning electron microscopy (SEM) indicated that regular stack-type particles were obtained upon expansion of the homogeneous polymerization mixture. To reduce the required amount of initiator, further VDF polymerizations using chain transfer agents (CTAs) to control molecular weights were carried out in homogeneous phase with supercritical carbon dioxide (scCO₂) at 120 °C and 1500 bar. Using perfluorinated hexyl iodide as CTA, polymers of low polydispersity ranging from 1.5 to 1.2 at the highest iodide concentration of 0.25 mol·L-1 were obtained. Electrospray ionization- mass spectroscopy (ESI-MS) indicates the absence of initiator derived end groups, supporting livingness of the system. The “livingness” is based on the labile C-I bond. However, due to the weakness of the C-I bond perfluorinated hexyl iodide also contributes to initiation. To allow for kinetic analyses of VDF polymerizations the CTA should not contribute to initiation. Therefore, additional CTAs were applied: BrCCl3, C6F13Br and C6F13H. It was found that C6F13H does not contribute to initiation. At 120°C and 1500 bar kp/kt0.5~ 0.64 (L·mol−1·s−1)0.5 was derived. The chain transfer constant (CT) at 120°C has been determined to be 8·10−1, 9·10−2 and 2·10−4 for C6F13I, C6F13Br and C6F13H, respectively. These CT values are associated with the bond energy of the C-X bond. Moreover, the labile C-I bond allows for functionalization of the polymer to triazole end groups applying click reactions. After substitution of the iodide end group by an azide group 1,3 dipolar cycloadditions with alkynes yield polymers with 1,2,3 triazole end groups. Using symmetrical alkynes the reactions may be carried out in the absence of any catalyst. This end-functionalized poly (vinylidene fluoride) (PVDF) has higher thermal stability as compared to the normal PVDF. PVDF samples from homogeneous phase polymerizations in supercritical CO₂ and subsequent expansion to ambient conditions were analyzed with respect to polymer end groups, crystallinity, type of polymorphs and morphology. Upon expansion the polymer was obtained as white powder. Scanning electron microscopy (SEM) showed that DTBP derived polymer end groups led to stack-type particles whereas sponge- or rose-type particles were obtained in case of CTA fragments as end groups. Fourier-Transform Infrared spectroscopy and wide angle X-ray diffraction indicated that the type of polymorph, α or β crystal phase was significantly affected by the type of end group. The content of β-phase material, which is responsible for piezoelectricity of PVDF, is the highest for polymer with DTBP-derived end groups. In addition, the crystallinity of the material, as determined via differential scanning calorimetry is affected by the end groups and polymer molecular weights. For example, crystallinity ranges from around 26 % for DTBP-derived end groups to a maximum of 62 % for end groups originating from perfluorinated hexyl iodide for polymers with Mn ~2200 g·mol–1. Expansion of the homogeneous polymerization mixture results in particle formation by a non-optimized RESS (Rapid Expansion from Supercritical Solution) process. Thus, it was tested how polymer end groups affect the particles size distribution obtained from RESS process under controlled conditions (T = 50°C and P = 200 bar). In all RESS experiments, small primary PVDF with diameters less than 100 nm without the use of liquid solvents, surfactants, or other additives were produced. A strong correlation between particle size and particle size distribution with polymer end groups and molecular weight of the original material was observed. The smallest particles were found for RESS of PVDF with Mn~ 4000 g·mol–1 and PFHI (C6F13I) - derived end groups. / Erstmalig gelang es, stabilisatorfreie Vinylidenfluorid (VDF)-Polymerisationen in homogener Phase mit überkritischem CO₂ (scCO₂) bis zu vollständigem Monomerumsatz durchzuführen. Die Homogenität während der Polymerisation wurde durch in-line Fourier-Transform Nahinfrarot Spektroskopie beobachtet. Für Polymerisationen bei 140 °C und 1500 bar wurde Di-tert-butylperoxid (DTBP) als Initiator verwendet. Es wurden Polymere mit einem Zahlenmittel der Molmasse kleiner 104 g·mol–1 und Polydispersitäten zwischen 3.1 und 5.7. erhalten. Um isotherme Reaktionen zu ermöglichen, wurden CO₂-Gehalte zwischen 61 und 83 wt.% verwendet. Die für die homogene Reaktionsführung erforderlichen hohen Drücke und Temperaturen haben keinen Einfluss auf die Mikrostruktur des Polymers. Zur Verringerung der Initiatorkonzentration wurden weitere Polymerisationen unter Verwendung von Kettentransferreagenzien (CTA) bei 120 °C und 1500 bar in homogener Phase mit scCO₂ durchgeführt. Perfluoriertes Hexyliodid als CTA ermöglicht kontrollierte radikalische Polymerisationen, wobei Polymere mit geringer Polydispersität zwischen 1.5 und 1.2 erhalten wurden. Endgruppenanalyse mit Elektronenspray-Ionisations-Massen¬spektro¬metrie (ESI-MS) zeigte, dass keine Initiatorendgruppen im Polymer enthalten sind. Diese Beobachtung unterstützt den lebenden Charakter der Polymerisationen und basiert auf einer labilen C-I-Bindung im Polymer. Aufgrund der schwachen C-I-Bindung trägt das perfluorierte Hexyliodid (C6F13I) auch zur Initiierung bei. Polymerisationen in Gegenwart von BrCCl3, C6F13Br und C6F13H zeigten, dass nur C6F13H keinen Beitrag zur Initiierung leistet. Bei 120 °C und 1500 bar wurde ein kp/kt0.5 von ~ 0.64 (L·mol−1·s−1)0.5 bestimmt, wobei kp der Wachstums- und kt der Terminierungsgeschwindigkeitskoeffizient sind. Die Kettentransfer¬konstanten (CT) bei 120°C betragen 8·10−1, 9·10−2 und 2·10−4 für C6F13I, C6F13Br und C6F13H. Die Änderung der CT-Werte lässt sich mit der zunehmenden Bindungsenergie in der Reihe C-I, C-Br und C-H erklären. Die labile C-I-Bindung ermöglicht eine Funktionalisierung des Polymers durch Click-Reaktionen. Nach Substitution der Iodid-Endgruppe durch eine Azidgruppe erfolgte eine katalysatorfreie 1,3-dipolare Cyclaoaddition mit Alkinen zu Polymeren mit 1,2,3-Triazol-Endgruppen. Dieses endfunktionalisierte PVDF besitzt im Vergleich zu konventionellem PVDF eine höhere thermische Stabilität. Nach der Expansion der Polymerisationsmischung mit scCO₂ auf Umgebungsbedingungen lag das Polymer als weißes Pulver vor, das im Hinblick auf z.B. Polymerendgruppen, Kristallinität, Gestalt und Größe der Partikel untersucht wurde. Rasterelektronenmikroskopie zeigte, dass Polymere mit DTBP-Endgruppen zu stapelförmigen Partikeln führen, während bei CTA-Fragmenten als Endgruppen schwamm- oder rosenartige Partikel erhalten wurden. Ergebnisse der FT-IR Spektroskopie und Weitwinkelröntgenbeugung zeigten, dass der höchste Gehalt an β-phasigem Material, der für die Piezoelektrizität des PVDF verantwortlich ist, für PVDF mit Initiatorendgruppen erhalten wurde. DSC (Differential Scanning Calorimetry) Messungen ergaben zudem, dass der Kristallinitätsgrad durch Endgruppen und Polymermasse beeinflusst wird. Die Expansion der homogenen Polymermischung kann als nicht-optimierter RESS-Prozess (Rapid Expanison from Supercritical Solution,) angesehen werden. Aus RESS Experimenten unter kontrollierten Bedingungen wurden jeweils nanoskalige primäre PVDF-Partikel ohne Verwendung von Lösungsmitteln, Tensiden oder anderen Additiven erhalten. Es besteht ein enger Zusammenhang zwischen einerseits der Partikelgröße und der Partikelgrößenverteilung und andererseits der Polymerkonzentration in scCO₂ vor der Expansion, bestimmt durch Polymerendgruppen und Molmassen der eingesetzten Materialien.

Supercritical fluids have unique properties which may make them ideal as reaction media for biotransformation or extractive solvents. Supercritical fluids are ideal for reducing diffusivity limitations over conventional fluids. Depending on the polarity of the fluid, a supercritical fluid can be similar to conventional organic solvents, but with few of the environmental drawbacks. The use of supercritical fluids in enzymatic research has the advantage of removing mass transport limitations so that they can act as a suitable solvent. In this study, four permeabilization techniques were compared: control, toluene, supercritical carbon dioxide, and freeze/thaw cycles. The model cell systems studied were Z. mobilis and E. coli. The cells were analyzed for lipid profiles, recovery of proteins and enzymatic activity. The use of supercritical carbon dioxide may not be the most effective of the treatments based on total protein or enzyme recovery since the greatest protein and enzyme recovery was with the freeze/thaw treatment. However, it can be selective in removing cofactors from Z. mobilis enabling sorbitol production and minimizing side reactions. In this application, supercritical carbon dioxide does show an advantage over the freeze/thaw treatment. Aspects of the mechanism of permeabilization were investigated based on the lipid profiles of the cells, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM and AFM show changes of the cell surface morphology which indicate that the treatments affect the cellular surface. The use of supercritical carbon dioxide as a reaction medium was investigated. Minute quantities of sorbitol were produced when Z. mobilis and sugars were placed in a supercritical carbon dioxide environment over a period of 24 hours.

Supercritical fluids have unique properties which may make them ideal as reaction media for biotransformation or extractive solvents. Supercritical fluids are ideal for reducing diffusivity limitations over conventional fluids. Depending on the polarity of the fluid, a supercritical fluid can be similar to conventional organic solvents, but with few of the environmental drawbacks. The use of supercritical fluids in enzymatic research has the advantage of removing mass transport limitations so that they can act as a suitable solvent. In this study, four permeabilization techniques were compared: control, toluene, supercritical carbon dioxide, and freeze/thaw cycles. The model cell systems studied were Z. mobilis and E. coli. The cells were analyzed for lipid profiles, recovery of proteins and enzymatic activity. The use of supercritical carbon dioxide may not be the most effective of the treatments based on total protein or enzyme recovery since the greatest protein and enzyme recovery was with the freeze/thaw treatment. However, it can be selective in removing cofactors from Z. mobilis enabling sorbitol production and minimizing side reactions. In this application, supercritical carbon dioxide does show an advantage over the freeze/thaw treatment. Aspects of the mechanism of permeabilization were investigated based on the lipid profiles of the cells, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM and AFM show changes of the cell surface morphology which indicate that the treatments affect the cellular surface. The use of supercritical carbon dioxide as a reaction medium was investigated. Minute quantities of sorbitol were produced when Z. mobilis and sugars were placed in a supercritical carbon dioxide environment over a period of 24 hours.

Linear Alkyl benzene Sulfonic Acid (LAS) is the main ingredient of many commercial formulations for industrial and domestic synthetic detergents. The current industrial LAS production method includes sulfonation of linear alkylbenzene (LAB) with sulfur trioxide in tubular falling film reactors. In such reactors a diluted gaseous stream of SO3 and dry air, feed gas, is contacted with liquid LAB while both reactants flow co-currently downward. The reaction is highly exothermic and product quality is primarily dependent on heat removal efficiency from the reactors, and also contact time. This research project investigates a new route for the production of LAS. This new method employs SO2 oxidation over activated carbon at 25oC to SO3, followed by the extraction of the adsorbed SO3 from the activated carbon by supercritical carbon dioxide (SCCO2). The condensed phase CO2-SO3 mixture after expansion is contacted with LAB where sulfonation of this substrate occurs to yield LAS. The new route should offer lower operating temperatures and lower feed gas SO3 concentrations in the sulfonation reaction to minimize loss of LAB to side-reactions and reduce LAS contamination (that appears as unacceptable product discoloration). The laboratory set up was designed, assembled and in total 25 experiments were carried out. Over the course of experiments a number of remedial actions were taken to improve set up functionality and reaction yield. The problems needed to be tackled included feed gas moisture removal, SO2/SO3 adsorption/desorption efficiency, homogeneous mixing of reactants and reducing the SCCO2/SO3 flow rate through LAB columns. The maximum LAB/LAS conversion obtained was 3.6 % per sulfonation column. The maximum SO3 removal efficiency from activated carbon obtained was 77%. It was also found that nitrogen gas in a specific temperature range may be used as the desorbing agent in lieu of supercritical carbon dioxide with satisfactory performance. As supplementary data, the Brauner-Emmet-Teller surface area of activated carbon type BPL 6x16 from “Calgon Carbon Corporation” was measured.

Proton exchange membrane fuel cells are one of the most promising hydrogen energy conversion devices for portable, mobile and stationary applications. For wide spread usage to produce electricity platinum loading has to be decreased by using highly active electrocatalysts. Even 10 ppm carbon monoxide or higher than 30% carbon dioxide cause performance losses via deactivation which can be diminished by using binary catalysts. The aim of this thesis is to develop new platinum based electrocatalysts with high catalytic activity and to overcome the problems due to the deactivation. platinum and platinum-ruthenium based catalysts on different carbon supports have been prepared by supercritical carbon dioxide deposition and microwave irradiation methods. By using supercritical carbon dioxide deposition platinum on Vulcan XC72R (VXR), multi wall carbon nanotube (MWCNT) and Black Pearl 2000 (BP2000) catalysts were prepared and characterized by XRD, TEM and cyclic voltammetry (CV). XRD results showed that in catalysts prepared by using supercritical carbon dioxide deposition method, the particle sizes as low as 1-2 nm can be obtained. From the CV results the electrochemical surface areas obtained were Platinum/VXR&gt / Platinum/MWCNT&gt / PlatinumBP2000. By means of the oxygen reduction reaction (ORR), the number of electrons transferred per oxygen molecule was calculated as 3.5, 3.6 and 3.7 for Platinum/BP2000, Platinum/VXR and Platinum/MWCNT, respectively. The microwave irradiation was used to prepare platinum on VX, Regal and BP2000 and platinum-ruthenium on VX. The effects of microwave duration, base concentration, carbon support used and surfactant/precursor ratios were investigated. The particle sizes of the catalysts were ranging between 2-6 nm. The prepared catalysts were characterized by XRD, XPS, and then PEMFC tests were performed. The performance was ordered as Platinum/VX&gt / Platinum/Regal&gt / Platinum/BP2000. The power losses arising from carbon dioxide in hydrogen feed were decreased by using prepared platinum-ruthenium based catalysts.