Supercritical CO2 Drying Research Articles

Silk fibroin aerogels: potential scaffolds for tissue engineering applications

Silk fibroin (SF) is a natural protein, which is derived from the Bombyx mori silkworm. SF based porous materials are extensively investigated for biomedical applications, due to their biocompatibility and biodegradability. In this work, CO2 assisted acidification is used to synthesize SF hydrogels that are subsequently converted to SF aerogels. The aqueous silk fibroin concentration is used to tune the morphology and textural properties of the SF aerogels. As the aqueous fibroin concentration increases from 2 to 6 wt%, the surface area of the resultant SF aerogels increases from 260 to 308 m2 g−1 and the compressive modulus of the SF aerogels increases from 19.5 to 174 kPa. To elucidate the effect of the freezing rate on the morphological and textural properties, SF cryogels are synthesized in this study. The surface area of the SF aerogels obtained from supercritical CO2 drying is approximately five times larger than the surface area of SF cryogels. SF aerogels exhibit distinct pore morphology compared to the SF cryogels. In vitro cell culture studies with human foreskin fibroblast cells demonstrate the cytocompatibility of the silk fibroin aerogel scaffolds and presence of cells within the aerogel scaffolds. The SF aerogels scaffolds created in this study with tailorable properties have potential for applications in tissue engineering.

Mechanically robust aerogels derived from an amine-bridged silsesquioxane precursor

A novel bridged silsesquioxane (BSQ) monomer was synthesized from 3-(2,3-epoxypropoxy)propyltrimethoxysilane and (3-aminopropyl)trimethoxysilane based on the reaction between epoxy and amine groups. The monomer was used to fabricate aerogels with excellent mechanical properties via sol–gel processes followed by supercritical CO2 drying. The influence of the sol–gel conditions on the chemical structure, morphology and mechanical property of the aerogels was investigated by NMR, TGA, SEM, nitrogen sorption analysis and DMA. BSQ aerogels prepared from the optimal sol–gel procedure showed excellent properties with high mechanical performance (compression modulus of 20.4 MPa) and low density (0.22 g cm−3), which are of great advantage for practical applications. Further, SiO2/BSQ composite aerogels with significantly improved mechanical performance than SiO2 aerogels can be obtained by co-condensing of BSQ monomer and tetraethoxysilane. The abundant hydroxyl groups in the BSQ precursor are believed to contribute to the robustness of BSQ and SiO2/BSQ composite aerogels due to the strengthened skeleton via the intermolecular H-bonds.

Synthesis and characterization of functionalized metal-organic frameworks.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis and Properties of Step-Growth Polyamide Aerogels Cross-linked with Triacid Chlorides

We report the first synthesis of step-growth aromatic polyamide (PA) aerogels made using amine end-capped polyamide oligomers cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC). Isophthaloyl chloride (IPC) or terephthaloyl chloride (TPC) were combined with m-phenylenediamine (mPDA) in N-methylpyrrolidinone (NMP) to give amine-capped polyamide oligomers formulated with up to 40 repeat units. Addition of the cross-linker, BTC, typically induces gelation in under 5 min. Solvent exchange of the resulting gels into ethanol followed by supercritical CO2 drying gives colorless aerogels with densities ranging from 0.06 to 0.33 g/cm3, compressive moduli between 5 and 312 MPa, and surface areas as high as 385 m2/g. Dielectric properties were also measured in the X-band frequency range. It was found that relative dielectric constant decreased with density as seen with other aerogels with the lowest relative dielectric constant being 1.15 for aerogels with densities of 0.06 g/cm3. Because of their superior mechanical properties, these aerogels can be utilized in a number of aerospace related applications, such as insulation for rovers, habitats, deployable structures, and extravehicular activity suits, as well as low dielectric substrates for antennas and other electronics. Because of potentially lower cost relative to polyimide and other polymer aerogels, they also have potential for use in more terrestrial applications as well, such as insulation for refrigeration, building and construction, and protective clothing.

Continuous extraction rate measurements during supercritical CO2 drying of silica alcogel

We investigated the kinetics of supercritical CO2 (SCCO2)-based drying of silica aerogels, a common, but time consuming and energy intensive step in their manufacture. An apparatus was developed to continuously measure alcohol extraction rates from alcogels as a function of key process variables by two redundant techniques. Kinetics data are reported for the drying of 2.5mm, 5mm, and 7.5mm thick annular alcogels by pumping SCCO2 through a 10mm-thick concentric annulus surrounding their exterior. The SCCO2 was at a temperature of 323K and a pressure of 12.4MPa and its mass flow rate varied from 1kg/h to 5kg/h. Gel thickness and SCCO2 flow rate were both shown to significantly effect drying rate and required drying time. The results of a conjugate mass transfer model assuming pure diffusion in the alcogel compared favorably with the data when the composition dependence of molecular diffusivity was captured utilizing available correlations.

Pharmacologically active hydrogels derived from silicon glycerolates and chitosan

Novel hybrid chitosan-containing hydrogels were obtained from various functionalized silicon glycerolates by the sol—gel method. According to SEM images of aerogels prepared by supercritical drying in liquid CO2, chitosan serves as a template in the structuring of hybrid hydrogels. The hydrogels obtained are nontoxic, exhibit pronounced hemostatic activity, and are promising for medical use.

Self-formation of 3D interconnected macroporous carbon xerogels derived from polybenzoxazine by selective solvent during the sol–gel process

Polybenzoxazine (PBZ)-based carbon xerogel has been synthesized by a sol–gel process and carbonization. By different solvents, the microstructure of the porous carbon can be tailored for a wide range of desired properties. In addition, a new aspect to produce 3D interconnected macroporous carbon xerogels by selective solvent via self-formation is introduced. Dimethylformamide (DMF), dioxane, and isopropanol are separately used as a solvent during a sol–gel process. The SEM micrographs reveal different structures of carbon xerogel depending on the type of solvent used. Using DMF as a solvent during a sol–gel process and ambient pressure drying, the carbon xerogel shows a similar porous structure to that of a PBZ-based carbon aerogel obtained through supercritical CO2 drying. In the DMF system, a short gelation time is observed (1.15–3 h) due to the fast ring-opening polymerization accelerated by DMF resulting in the formation of 3D interconnected macroporous structure without using any template. Comparing the rates of cluster growth between DMF and dioxane systems, the rate of cluster growth in dioxane system is slower than that of DMF system, implying good miscibility between PBZ and dioxane. Moreover, microporous spherical particles are obtained from the isopropanol system due to the self-micelle-like formation.

Highly Porous Silica Glasses and Aerogels Made Easy: The Hypersaline Route

Through the addition of inorganic salts as porogens during an alcohol-based sol-gel process high surface area silica glasses and aerogels with varying pore sizes are synthesized. In the presence of ZnCl2 and CaCl2 respectively, tetraethyl orthosilicate (TEOS) dissolved in ethanol hydrolyzes also in the absence of any acid or base catalysts, eventually forming high surface area monolithic silica networks. The porosity can be controlled by the gel ageing step as well as the salt nature. Two different gel drying conditions were investigated, i.e., room temperature (RT) air drying at ambient pressure and supercritical (sc) CO2 drying. While a reference solution which only contains ethanol and TEOS does not gel at all, the addition of ZnCl2 leads to mesoporous aerogels with high surface areas of 1400 m2 g−1 after sc CO2 drying. Interestingly, the utilization of CaCl2 however, produces aerogel-like materials of high porosity for both drying methods under preservation of the nano-and macroscopic features. Thereby, it is shown that inorganic salts can influence the structure formation during the sol-gel process where the type of salt controls the framework connectivity/stability and tunes the porosity towards smaller pore sizes than usually obtained. At the same time, the herein presented hypersaline aerogel-synthesis is very simple, fast and in the case of CaCl2 drying can even be performed in air

Significant Improvement of Thermal Stability for CeZrPrNd Oxides Simply by Supercritical CO2 Drying

Pr and Nd co-doped Ce-Zr oxide solid solutions (CZPN) were prepared using co-precipitation and microemulsion methods. It is found that only using supercritical CO2 drying can result in a significant improvement of specific surface area and oxygen storage capacity at lower temperatures for CZPN after aging at 1000°C for 12 h in comparison with those using conventional air drying and even supercritical ethanol drying. Furthermore, the cubic structure was obtained in spite of the fact that the atomic ratio of Ce/(Ce+Zr+Pr+Nd) is as low as 29%. The high thermal stability can be attributed to the loosely aggregated morphology and the resultant Ce enrichment on the nanoparticle surface, which are caused by supercritical CO2 drying due to the elimination of surface tension effects on the gas-liquid interface.

Synthesis of an organic conductive porous material using starch aerogels as template for chronic invasive electrodes

We report the development of an organic conducting mesoporous material, as coat for invasive electrodes, by a novel methodology based on the use of starch aerogel as template. The poly(3,4-ethylenedioxythiophene) (PEDOT) aerogel was synthesized by polymerization of 3,4-ethylenedioxythiophene within a saturated starch aerogel with iron (III) p-toluenesulfonate (oxidizing agent) and subsequent removal of the polysaccharide template, followed by supercritical CO2 drying. The chemical structure and oxidation state of the resulting material were studied by Raman spectroscopy. The morphology and surface properties of the obtained nanoporous material were investigated by scanning electron microscopy (SEM), micro computed tomography (μCT) and nitrogen adsorption–desorption techniques. The composition and thermal behaviour were evaluated by energy dispersive spectroscopy (EDS) and thermogravimetric analysis (TGA) respectively. A preliminary biocompatibility test verified the non-cytotoxic effects of the PEDOT aerogel. The large surface area and wide pore size distribution of the PEDOT conductive aerogel, along with its electrical properties, enable it to be used as extracellular matrix scaffold for biomedical applications.

Contamination Control in Supercritical CO2 Drying Process for Nano-Scale Memory Manufacturing

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Mistaken for a parent: A series of observations on a UK neonatal unit part 3

Multi-walled carbon nanotube (MWCNT)/resorcinol–formaldehyde (RF) composite gels were prepared by sol–gel polymerization of RF monomers in a suspension of carbon nanotubes in water, followed by CO2 supercritical drying. X-ray microtomography showed that the suspensions of MWCNTs were stable and that the distribution of nanotubes throughout the RF matrix was uniform. The level of MWCNT dispersion in the composites was evaluated by microscopic methods (optical microscopy, SEM and TEM). Micrographs showed that, at low MWCNT concentration (<0.26 v%), the nanotube bundles were completely exfoliated, leading to a homogeneous material at the nanometer scale. On the contrary, at higher concentrations, aggregates of MWCNTs with diameters up to 50 μm were observed. N2 sorption analysis showed that the carbon nanotube aggregation may result in a decrease of BET surface area of the composites. Thermal conductivity of the nanocomposites was measured by TPS hot disk method and the data were consistent with effective medium theory (EMT). At high MWCNT loadings, it was possible to estimate the volume fraction of separated and aggregated carbon nanotubes based on the deviation of the experimental conductivity data from EMT predictions.

Characterization of multi-walled carbon nanotube dispersion in resorcinol–formaldehyde aerogels

Multi-walled carbon nanotube (MWCNT)/resorcinol–formaldehyde (RF) composite gels were prepared by sol–gel polymerization of RF monomers in a suspension of carbon nanotubes in water, followed by CO2 supercritical drying. X-ray microtomography showed that the suspensions of MWCNTs were stable and that the distribution of nanotubes throughout the RF matrix was uniform. The level of MWCNT dispersion in the composites was evaluated by microscopic methods (optical microscopy, SEM and TEM). Micrographs showed that, at low MWCNT concentration (<0.26 v%), the nanotube bundles were completely exfoliated, leading to a homogeneous material at the nanometer scale. On the contrary, at higher concentrations, aggregates of MWCNTs with diameters up to 50μm were observed. N2 sorption analysis showed that the carbon nanotube aggregation may result in a decrease of BET surface area of the composites. Thermal conductivity of the nanocomposites was measured by TPS hot disk method and the data were consistent with effective medium theory (EMT). At high MWCNT loadings, it was possible to estimate the volume fraction of separated and aggregated carbon nanotubes based on the deviation of the experimental conductivity data from EMT predictions.

Contamination Control in Supercritical CO2 Drying Process for Nano-Scale Memory Manufacturing

The control of defects on the wafer is a critical task for nano-scale memory devices to achieve high yields. Even though supercritical CO2 drying technology has been available for more than 10 years, its application in 300mm wafer fabrication process still lacks an effective solution for the control of particles and metallic contaminant generated. To address this, the authors studied the CO2 purification process and found that most of particles on the wafer after supercritical CO2 treatment were found to be organic contaminants from the CO2 raw material. A CO2 purification system with an activated carbon absorber demonstrated superior performance for removing organic contaminants from the CO2. The metallic contaminants, coming from the supercritical chamber’s stainless steel parts, were decreased by aging of the supercritical fluid at a high temperature. A dense metal oxide film generated by aging with IPA-added supercritical CO2 at a high temperature was shown to prevent corrosion of the stainless steel surface. Knowledge and control of contaminations at the nano level will soon enable supercritical CO2 drying in 300mm nano-scale memory manufacturing.

Water Extractable Arabinoxylan Aerogels Prepared by Supercritical CO2 Drying

Water extractable arabinoxylan (WEAX) aerogels were prepared by extracting the solvent from the alcogels (WEAX hydrogels with an alcohol as the solvent) with carbon dioxide under supercritical conditions. WEAX aerogels were characterized using scanning electron microscopy and adsorption and desorption nitrogen isotherms. The micrographs indicate a heterogeneous porous network structure in WEAX aerogel. Adsorption/desorption nitrogen isotherms of this material were type IV, which confirm that this material possess a mesoporous structure. WEAX aerogels rehydration capability was evaluated and the water absorption mechanism was determined. The WEAX aerogels water absorption mechanism was non-Fickian (n = 0.54).

Monolithic germanium oxide aerogel with the building block of nano-crystals

In the present paper, we have prepared germanium oxide aerogel using the sol–gel method followed by CO2 supercritical drying. Scanning electron microscopy showed a fine microstructure of the as-prepared aerogel. Results from the nitrogen adsorption and desorption data exhibited that the aerogel had attractive specific surface area and typical pore size distribution of aerogel. X-ray diffraction indicated that the aerogel was composed of nano-crystals with hexagonal structure. X-ray photoelectron spectroscopy confirmed the existence of oxygen vacancies. At room temperature, interesting strong blue luminescence was observed at 425nm under excitation wavelength at 280nm. We proposed an explanation for the phenomenon.

Conservation of waterlogged archaeological corks using supercritical CO2 and treatment monitoring using structured-light 3D scanning

Archaeological waterlogged cork is one the most unpredictable archaeological materials to conserve. Over the years, various techniques designed to conserve waterlogged wood have been applied to cork with less than satisfactory results. These techniques include freeze drying with or without consolidant, air-drying and silicone oil treatment. Alternatively, recent studies demonstrated that methanol exchange followed by supercritical CO2 drying can overcome most of the limitations of the latter techniques when applied to organic waterlogged materials. In 2005, a joint research project was initiated between the Warren Lasch Conservation Center (WLCC) and Parks Canada to evaluate the use of supercritical CO2 drying on significant archaeological corks and composite artifacts from several shipwrecks. This paper will discuss the drying process of the various corks and the techniques employed to monitor their appearance and dimensions, namely conventional measurement techniques and structured-light 3D scanning combined with three-dimensional inspection.

Effect of Drying Processes on the Texture of Silica Gels

Peculiar xerogels and aerogels constituted by a silica network, made of spherical fully dense silica particles having the same size, are investigated by adsorption of nitrogen at 77.4 K. Comparison of sorption data between materials dried via different methods, gentle drying at room temperature, alcohol supercritical drying and CO2 supercritical drying, shows that the specific surface area is associated to the particle sizes and necks established between them during drying and not to the sample density. The dissolution-redeposition of silica, which occurs in the alcohol supercritical drying process, induces a decrease of specific surface area and consequently an increase in the mechanical properties comparatively to CO2 supercritical drying. Investigating pore volume measurements as a function of dwell time, which is the interval of time allowing a pressure change of 0.01%, we corroborate that for compliant materials the full volume can not be detected because of capillary stresses. So the time required to perform correct measurements of the pore volume decreases with sample bulk density increase and elastic properties increase. All these experiments qualitatively corroborate the theory proposed previously.

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO2) aerogels were synthesized using sol–gel process followed by supercritical CO2 drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO2 aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m2/g, a porosity of 95.1 % and a pore volume of 2.59 cm3/g. The mass fraction of SiC in C/SiC aerogels is 31 %.

Stretchable and Strong Cellulose Nanopaper Structures Based on Polymer-Coated Nanofiber Networks: An Alternative to Nonwoven Porous Membranes from Electrospinning

Nonwoven membranes based on electrospun fibers are of great interest in applications such as biomedical, filtering, and protective clothing. The poor mechanical performance is a limitation, as is some of the electrospinning solvents. To address these problems, porous nonwoven membranes based on nanofibrillated cellulose (NFC) modified by a hydroxyethyl cellulose (HEC) polymer coating are prepared. NFC/HEC aqueous suspensions are subjected to simple vacuum filtration in a paper-making fashion, followed by supercritical CO(2) drying. These nonwoven nanocomposite membranes are truly nanostructured and exhibit a nanoporous network structure with high specific surface area, as analyzed by nitrogen adsorption and FE-SEM. Mechanical properties evaluated by tensile tests show high strength combined with remarkably high strain to failure of up to 55%. XRD analysis revealed significant fibril realignment during tensile stretching. After postdrawing of the random mats, the modulus and strength are strongly increased. The present preparation route uses components from renewable resources, is environmentally friendly, and results in permeable membranes of exceptional mechanical performance.