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.

Les alginates, polysaccharides extraits des algues brunes, sont des copolymères à blocs linéaires formés d'unités mannuronates (M) et guluronates (G). Leur structure dépend de la source naturelle. La complexation de cations divalents ou trivalents (sauf Mg2+) par les fonctions carboxylates conduit à la formation d'un hydrogel. Pour approfondir l'étude de la formation des gels ionotropiques d'alginate, des hydrogels et des aérogels séchés en conditions supercritiques ont été caractérisés à plusieurs échelles par différentes techniques. L'analyse orbitalaire ab initio de petits complexes a montré la formation de liaisons covalentes fortes avec les métaux de transition alors que les alcalino-terreux ne mettent en jeu que des interactions électrostatiques. Selon le rapport M/G et la nature des cations, différents régimes structuraux ont pu être identifiés par des mesures SAXS (~10-200Å) : des gels fibrillaires ou formés d'agglomérats interconnectés. Les propriétés mécaniques des hydrogels et les surfaces spécifiques des aérogels correspondants sont directement reliées à ces morphologies. La présence simultanée de plusieurs métaux dans les gels peut induire des effets coopératifs dans la formation mais aussi dans les propriétés des matériaux. De nouvelles méthodes ont été mises au point pour contrôler la synthèse de gels hétéro-cationiques, en considérant les effets d'affinité.Sur la base de cette étude, des synthèses originales de matériaux fonctionnels nano-structurés ont été développées. Certains ont été construits directement dans le réseau d'alginate en exploitant les cations de gélification ; d'autres ont nécessité le sacrifice de la matrice pour former des oxydes simples ou mixtes nanocristallins. / Alginates, polysaccharides produced by brown algae, are linear block-copolymers formed by mannuronate (M) and guluronate (G) units. Their structure depends on the natural source. The coordination of divalent or trivalent cations (except Mg2+) with the carboxylate functions leads to the formation of a hydrogel. In order to get some insight in ionotropic alginate gel formation, hydrogels and supercritical dried aerogels were analysed through various characterizations at different scales. The ab initio molecular orbital analysis on small complexes revealed strong coordination-covalent bonds in the transition metal complexes, whereas only ionic interaction occurs between the alkaline-earth cations and the carboxylates. Depending on the M/G ratio and the nature of the cation, different structural regimes were identified by SAXS analysis (~10-200Å) : well-defined fibrillar gels or gels with multiple junction nature. The mechanical properties of the hydrogels and surface areas of the resulting aerogels are directly related to these morphologies. The concomitant presence of different metals in the gel can bring cooperative effect for the formation but also for the properties of the materials. New methods to control the synthesis of hetero-cationic gels were implemented, taking into account the affinity effects. On the basis of this study, we developed new nanostructured functional materials synthesis. Some were built directly in the alginate network using the gelling cations. Others needed to sacrifice the matrix to form nanocrystalline single or mixed oxides.

Carbon has been used electrochemically in various forms for water treatment and the carbon aerogel is one of them. Carbon Aerogels (CA) are used as electrodes due to their high surface capacity and high electrical conductivity. They are also known as Carbon Nanofoams (CNF). CA electrodes attract oppositely charged ions that are nearby. This concept is known as Capacitive De-Ionization (CDI). The use of CA in CDI for water purification is well documented, but not much work has been done on regeneration of CA electrodes. Once saturated, these electrodes lose their ability to adsorb additional ions and it must be restored by regeneration. If they cannot be regenerated, they would need to be replaced, which would greatly increase the cost of the treatment they are expensive. The goal of this study is to obtain data to define optimal regeneration conditions and to develop predictive capability by examining desorption behavior of adsorbed ions on CA electrodes. This study focuses on desorption of adsorbed ions and regeneration of CA. Various experiments were conducted to explore the effects on regeneration of CA of shorting of electrodes, change of polarity of electrodes, flow speed of water over CA electrodes, and temperature of regeneration water. The optimal combination of experimental variables was identified and was used for remaining experiments that tested the effect of size, charge and mass of adsorbed ions on regeneration of CA. Also, the effect of thickness of CA and its pore size on regeneration of CA was studied. Results indicated that application of reverse potential for the first few minutes of the total regeneration time provided the greatest regeneration. Longer application of reverse potential did not result in higher regeneration. The regeneration behavior when no potential applied with and without shorting was as expected. Application of reverse potential with variable temperature or variable flow speed of water over CA surfaces provided results that were different from the ones that were obtained with no potential being applied with or without shorting of electrodes.

In this project the relation between pore size/porosity and the mechanical properties has been studied in the aerogel-like material Quartzene®. Quartzene® is a patented material produced by Svenska Aerogel AB. Density measurements were made on three different types of Svenska Aerogels ABs Quartzene® in the shape of pellets.These three types of Quartzene® is called CMS, ND and E9. The mechanical properties were studied by doing diametrical crush-tests on the pellets. Afterwards the samples were examined through SEM in order to study the structural properties like porosity and microstructure. By examining the materials in this order the group hoped to find a correlation between the mechanical properties and the pore size/porosity. Other microscopic analyses such as TEM and FIB was considered, but due to time limitation these methods were not used. Rough density measurements resulted in an estimated density of 0.82-0.88 g/cm3 for CMS, 0.28-0.30 g/cm3 for E9 and 0.21-0.22 g/cm3 for ND. The crush-tests resulted in a mean fracture stress of 0.81-0.89 MPa for CMS, 0.30 MPa for E9 and 0.20-0.21 MPa for ND. Studying the materials in SEM resulted in an observed mean pore size of 59-73 nm for CMS, 264-362 nm for E9 and 690-710 nm for ND in the mesoporous domain. A subtle relationship between density/pore size and fracture was obtained, with a higher density and smaller pores leading to a higher fracture stress. Due to the lack of data in this study, it is recommended though that this is something that should be examined further before any conclusions can be made. In general Quartzene® has shown to be a brittle material, but this study indicates that the mechanical properties could be controlled in somehow through the microstructure of the material, focusing on controlling the pore sizes. Further investigations in sintering of Quartzene® are also recommended in this study because of its promising effects on the mechanical properties shown in other studies.

In this thesis, three different types of polymer-based gels are fabricated and characterized for passive and active applications. Silica aerogel is a 3D mesoporous solid material that can be used for thermal insulation or in the biomedical industry. In this thesis, silica aerogel is cross- linked with diisocyanate to improve its strength and flexibility, which greatly opens up the range of applications. Then, soft polymer gel with tissue equivalent characteristics is fabricated to mimic the spin-lattice (T1) and spin-spin (T2) relaxation times for the magnetic resonance imaging (MRI) phantom of a liver with lesions. This study demonstrates a relationship between the composition of a gelling agent, and T1 and T2 modifiers on its dielectric, mechanical and imaging properties. Finally, an ionic electroactive polymer (EAP) that can be actuated on an electric field is fabricated, and its swelling and bending behaviours on design parameters are closely examined.

In this thesis, three different types of polymer-based gels are fabricated and characterized for passive and active applications. Silica aerogel is a 3D mesoporous solid material that can be used for thermal insulation or in the biomedical industry. In this thesis, silica aerogel is cross- linked with diisocyanate to improve its strength and flexibility, which greatly opens up the range of applications. Then, soft polymer gel with tissue equivalent characteristics is fabricated to mimic the spin-lattice (T1) and spin-spin (T2) relaxation times for the magnetic resonance imaging (MRI) phantom of a liver with lesions. This study demonstrates a relationship between the composition of a gelling agent, and T1 and T2 modifiers on its dielectric, mechanical and imaging properties. Finally, an ionic electroactive polymer (EAP) that can be actuated on an electric field is fabricated, and its swelling and bending behaviours on design parameters are closely examined.

Nanofibrillated cellulose (NFC) from wood is an interesting material constituent of high strength and high aspect ratio, which easily forms networks through interfibril secondary bonding including hydrogen bonds. This has been exploited in preparation of new materials, which extend the range of properties for existing cellulosic materials. The objective is to explore processing-structure and structure-property relationships in NFC materials. Dense networks of NFC, referred to as “nanopaper” having a random-in-the-plane orientation of the fibrils have been successfully prepared by a papermaking-like process involving vacuum filtration and water evaporation using laboratory papermaking equipment. Large, flat and transparent nanopaper sheets have thus been prepared in a relatively short time. Using the same preparation route, NFC was used to reinforce pulped wood fibers in dense network structures. NFC networks formed in the pore space of the wood fiber network give an interesting hierarchical structure of reduced porosity. These NFC/wood fiber biocomposites have greater strength, greater stiffness and greater strain-to-failure than reference networks of wood fibers only. In particular, the work to fracture (area under the stress-strain curve) is doubled with an NFC content of only 2%. The papermaking preparation route was extended to prepare nanocomposites of high NFC content with a cellulose derivative matrix (hydroxyethyl cellulose, HEC) strongly associated to the NFC. Little HEC was lost during filtration. The NFC/HEC composites have high work to fracture, higher than that of any reported cellulose composite. This is related to NFC network characteristics, and HEC properties and its nanoscale distribution and association with NFC. Higher porosity NFC nanopaper networks of high specific surface area were prepared by new routes including supercritical drying, tert-butanol freeze-drying and CO2 evaporation. Light-weight porous nanopaper materials resulted with mechanical properties similar to thermoplastics but with a much lower density and a specific surface area of up to 480 m2/g. Freeze-drying of hydrocolloidal NFC dispersions was used to prepare ultra-high porosity foam structures. The NFC foams have a cellular foam structure of mixed open/closed cells and “nanopaper” cell wall. Control of density and mechanical properties was possible by variation of NFC concentration in the dispersion. A cellulose I foam of the highest porosity ever reported (99.5%) was prepared. The NFC foams have high ductility and toughness and may be of interest for applications involving mechanical energy absorption. Freeze-drying of NFC suspended in tert-butanol gave highly porous NFC network aerogels with a large surface area. The mechanical behavior was significantly different from NFC foams of similar density due to differences in deformation mechanisms for NFC nanofiber networks. / QC 20110406

Traitement et séchage de sol-gels est une pratique utilisée couramment dans l'industrie en raison du grand nombre d'applications existantes pour les produits en aval. Parmi eux, un produit spécifique capte une grande attention en raison de son élevé potentiel principalement pour l’application dans l'isolation thermique résidentielles, industrielles et même l'aérospatiale. Ce type de produit, appelé aérogel est un matériau dont la structure nanoporeuse marque une très faible densité et de conductivité thermique. La production d'aérogel peut être réalisée par diverses méthodes en fonction de la technique de séchage utilisée pour évacuer le liquide retenu dans les pores de gel: Tourner autour du point triple, franchissant la ligne d'évaporation ou contourner le point critique. L'objet de cette thèse consiste à fournir la base technologique nécessaire pour le développement d'un programme de la commercialisation industrielle des aérogels. A cet effet, la poursuite soutiendra sur la compréhension et l'identification de la présente maturité technologique, le développement et la mise à l'échelle d'un processus et de produits optimisés, et l'évaluation d'un plan d'affaires pour la production commerciale d'aérogels. L'environnement dans lequel s’effectue cette thèse se compose de deux domaines différents. Une consisté dans la conception, construction et opération des équipements spécifiques au sein de SEPAREX, et l'autre était dirigé dans la participation et la gestion de quatre projets européens du 7ème framework program: AEROCOINs, AerSUS, HIPIN et RESEEPE. Tous engagée dans la recherche et le développement de divers matériaux à base d’aérogels pour des applications dans l'isolation et la rénovation des bâtiments et dans l'isolation des vaisseaux spatiaux et des satellites. SEPAREX a participé en tant qu'expert dans le procédé de séchage de ces matériaux et aussi comme expert dans la conception et la mise à l'échelle d'équipements pour le processus concerné / Processing and drying of sol-gels is a widely practice used in industry due to the large number of existing applications for the downstream products. Among them, a specific product captures a great attention due to its high potential mainly for thermal insulation as residential, industrial and even aerospace applications. This product type, called aerogel, is a nanoporous material whose structure having extremely low density and thermal conductivity. Aerogel production can be carried out by various methods depending on the drying technique used to evacuate the liquid retained within the gel pores: Turning around the triple point, crossing the evaporation line or turning around the critical point. The purpose of the present thesis is to provide the technological basis required for the development of a program for the industrial commercialization of aerogels. For this purpose, the survey shall cover the understanding and identification of the present technology maturity, the development and scaling of an optimized process and products, and evaluation of a commercial business plan for the production of aerogels. The environment in which takes place this thesis is comprised in two different areas. One consisted in designing, construction and operation of a specific equipment within SEPAREX, and the other one was led in participation and management of four European projects of the 7th framework program: AEROCOINs, AerSUS, HIPIN and RESEEPE. All of them engaged in the research and development of diverse aerogels-based materials for applications in insulation and retrofitting of buildings and insulation of spacecraft and satellites. SEPAREX participated as expert in the drying process of such materials and also as expert in the design and scale-up of equipment for the process therefore

Progress in nanotechnology has promoted an increasing interest in the rational design of the emerging hierarchical aerogels, which represents a second stage of the NC-based aerogel research. By fine-tuning the surface properties of the backbones, metallic hierarchical aerogels are able to address the growing demands of advanced electrocatalysts. In this dissertation, three types of metallic hierarchical aerogels were designed by introducing different nanostructures (i.e. hollow, porous/dendritic and core-shell) and alloy effects (with noble or transition metals) into the aerogels. Thus, as a proof-of-concept for fuel cells, advanced electrocatalytic performances have been achieved on the resulting metallic hierarchical aerogels towards both anode (oxidation of ethanol) and cathode (reduction of oxygen) reactions. First, alloyed PdxNi hollow nanospheres with controlled composition and shell thickness were utilized as building blocks for the design of hierarchical aerogels. The combination of transition-metal doping, hollow interior, as well as the 3D aerogel structure make the resulting aerogels promising electrocatalysts for ethanol oxidation with a mass activity up to 5.6-fold higher than that of the Pd/C. Second, continuously shape-engineering of the building blocks (ranging from hollow shells to dendritic shapes) was achieved by the synthesis of a series of multimetallic Ni-PdxPty hierarchical aerogels. By optimization of the nanoscale morphology and the chemical composition, the Ni-Pd60Pt40 aerogel exhibits remarkable electrocatalytic activity for oxidation of ethanol. Moreover, the particle growth mechanism underlying the galvanic replacement was revealed in terms of nanowelding of the nanoparticulate reaction intermediates based on experimental and theoretical results. Third, a universal approach was demonstrated for core-shell structuring of metallic aerogels by coating of an ultrathin Pt shell on a composition-tunable Pd-based alloyed core. Their activities for oxygen reduction exhibit a volcano-type relationship as a function of the lattice parameter of the core substrate. Largely improved Pt utilization efficiency was accomplished based on the core-shell motifs, as the mass activity reaches 5.25 A mg-1Pt which are 18.7 times higher than those of Pt/C. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e. an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination “locks in” the inherent properties of the NCs, so that the beneficial genes obtained by nano-engineering are retained in the resulting monolithic hierarchical aerogels. These results expand the exploitation approach of the electrocatalytic properties of aerogels into morphology control of their NBBs and are of great importance for the future development of aerogels for many other electrochemical reactions.

abstract: A new nanoparticle deposition technique, Aerosol Impaction-Driven Assembly (AIDA), was extensively characterized for material structures and properties. Aerogel films can be deposited directly onto a substrate with AIDA without the long aging and drying steps in the sol-gel method. Electron microscopy, pore size analysis, thermal conductivity, and optical measurements show the nanoparticle (NP) films to be similar to typical silica aerogel. Haze of nanoparticle films modeled as scattering sites correlates strongly with pore size distribution. Supporting evidence was obtained from particle sizes and aggregates using electron microscopy and small-angle X-ray scattering. NP films showed interlayers of higher porosity and large aggregates formed by tensile film stress. To better understand film stress and NP adhesion, chemical bonding analyses were performed for samples annealed up to 900 °C. Analysis revealed that about 50% of the NP surfaces are functionalized by hydroxyl (-OH) groups, providing for hydrogen bonding. Ellipsometric porosimetry was used to further understand the mechanical properties by providing a measure of strain upon capillary pressure from filling pores. Upon annealing to 200 °C, the films lost water resulting in closer bonding of NPs and higher Young’s modulus. Upon further annealing up to 900 °C, the films lost hydroxyl bonds while gaining siloxane bonds, reducing Young’s modulus. The application of ellipsometric porosimetry to hydrophilic coatings brings into question the validity of pore size distribution calculations for materials that hold onto water molecules and result in generally smaller calculated pore sizes. Doped hydrogenated microcrystalline silicon was grown on crystalline silicon NPs, as a test case of an application for NP films to reduce parasitic absorption in silicon heterojunction solar cells. Parasitic absorption of blue light could be reduced because microcrystalline silicon has a mix of direct and indirect bandgap, giving lower blue absorption than amorphous silicon. Using Ultraviolet Raman spectroscopy, the crystallinity of films as thin as 13 nm was determined rapidly (in 1 minute) and non-destructively. A mono-layer of nanocrystals was applied as seeds for p-doped microcrystalline silicon growth and resulted in higher crystallinity films. Applications of the method could be explored for other nanocrystalline materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020