Mark A. McHugh

Solubility of vinylidene fluoride polymers in supercritical CO2 and halogenated solvents

The cloud-point behaviors of poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-22 mol % hexafluoropropylene) (VDF–HFP22) are reported at temperatures up to 250 °C and pressures up to 3000 bar in supercritical CO2, CHF3, CH2F2, CHClF2, CClF3, CH3CHF2, CH2FCF3, CHF2CF3, and CH3CClF2. The molecular weight of PVDF has a smaller effect on the cloud point than the solvent quality. Cloud-point pressures for both fluoropolymers decrease as the solvent polarizability, polar moment per molar volume, and density increases. However, it is extremely difficult to dissolve either fluoropolymer in CClF3, which has a large polarizability and a small dipole moment. CO2 is an effective solvent because it complexes with the CF dipole at low temperatures where energetic interactions fix the phase behavior. In addition, polymer architecture has a strong impact on the cloud-point pressure. VDF–HFP22 has lower cloud-point pressures than PVDF in all solvents because it has a larger free volume that promotes facile interactions between the solvent and the polymer segments. Cloud-point data are also reported for amorphous poly(tetrafluoroethylene-co-x mol % 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole) (TFE–PDDx ; x = 65 and 85) in CO2. These data provide an interesting comparison to the PVDF–CO2 and VDF–HFP22–CO2 systems because TFE–PDD65 and TFE–PDD87 have very high glass-transition temperatures of 160 and 240 °C, respectively.

Solubility behavior of ethyl cellulose in supercritical fluid solvents

Abstract Solubility data to 180xa0°C and 1200 bar are reported for ∼1.0 wt.% ethyl cellulose (50% ethoxyl content, 2.5 average degree of substitution) (EC) in neat supercritical fluid (SCF) chlorodifluoromethane (F22); difluoromethane; 1-chloro-1,1-difluoroethane; 1,1-difluoroethane; and dimethyl ether (DME). The pressures needed to dissolve EC in the polar fluorocarbons decreases with increasing solvent size. The exception in this trend is F22 which is the best fluorocarbon solvent of the series likely due to its ability to hydrogen bond to the oxygens in EC. Data are also reported for EC in CO 2 with up to 30 wt.% ethanol and methanol showing that, on a weight basis, methanol is a much better cosolvent although on a mole basis methanol is only slightly better. DME is the highest quality solvent for EC of the series of SCF solvents investigated. Although the EC+DME system exhibits lower critical solution temperature behavior similar to the EC+F22 system, EC dissolves in DME at lower temperatures and pressures compared with F22. Solution density data at the phase boundaries are reported for the EC+SCF solutions. The EC+DME solutions exhibit the lowest densities which suggests that EC-DME cross interactions are very strong and likely dominated by hydrogen bonding.

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 6u2009wt%, the surface area of the resultant SF aerogels increases from 260 to 308u2009m(2)u2009g(-1) and the compressive modulus of the SF aerogels increases from 19.5 to 174u2009kPa. 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.

CO2-assisted synthesis of silk fibroin hydrogels and aerogels.

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 3-D SF hydrogels.

Pressure assisted stabilization of biocatalysts at elevated temperatures: Characterization by dynamic light scattering

The effect of pressure, at elevated temperatures, is reported on the activity and stability of a thermophilic endo‐β‐glucanase from the filamentous fungus Talaromyces emersonii. The production of reduced sugars after treatment at different temperatures and pressures is used as a measure of the activity and stability of the enzyme. The activity of the enzyme is maintained to higher temperatures with increasing pressure. For example, the relative activity of endo‐β‐glucanase decreases to 30% after 4u2009h at 75°C and 1u2009bar, whereas it is preserved at 100% after 6u2009h at 75°C and 230u2009bar. High‐pressure dynamic light scattering is used to characterize the hydrodynamic radius of the enzyme as a function of pressure, temperature, and time. At higher temperature the hydrodynamic radius increases with time, whereas increasing pressure suppresses this effect. Changes in the hydrodynamic radius are correlated with the activity measurements obtained at elevated pressures, since the changes in the hydrodynamic radius indicate structural changes of the enzyme, which cause the deactivation. Biotechnol. Bioeng. 2013; 110: 1674–1680.

Limited polysulfone solubility in supercritical dimethyl ether with THF and DMF cosolvents

Abstract Reported in this short communication are the conditions needed to dissolve up to 1.5 wt.% polysulfone (PSU) (Mw=35u2008000; Mn=16u2008000) in supercritical fluid (SCF) solvents plus cosolvents. Initial experiments showed that PSU does not dissolve in neat CO2, propane, butane, dimethyl ether (DME), chlorodifluoromethane, or difluoroethane to temperatures as high as 200xa0°C and pressures of 2100 bar. However, PSU solubility is observed for DME with the addition of 24–65 wt.% tetrahydrofuran (THF) or N,N-dimethyl formamide (DMF), two polar liquid solvents that readily dissolve PSU at room conditions even though PSU melts at ∼188xa0°C. DMF is a better cosolvent for PSU since DMF has a much higher dipole moment than THF, it has a higher molar density at room temperature, and it has a higher critical temperature which implies that DMF has a higher cohesive energy density than THF. With ∼24 wt.% cosolvent added to DME, the single-phase region extends to 40xa0°C lower temperatures with DMF compared with THF. Likewise, the cloud-point pressures for 0.15 wt.% PSU in DME with 56 wt.% DMF are ∼400 bar lower than those for a solution with approximately the same overall concentrations but with THF. The solution densities for PSU in DME+THF range from 0.90 to 0.65 g/cm3, depending on THF concentration and the solution densities for PSU in DME+DMF range from 0.53 to 0.66 g/cm3, also depending on DMF concentration. Compared with THF solutions, lower DMF solution densities are observed since it takes lower pressures to obtain a single-phase when DMF is used as a cosolvent. The experimental data demonstrate that DME should be considered an anti-solvent to knock PSU out of solution, since significant amounts of cosolvent are needed to dissolve PSU in DME.

Chemical Oxygen Generation

While pressurized oxygen in tank form, as well as oxygen concentrators, are ubiquitous in civilian healthcare in developed countries for medical use, there are a number of settings where use of these oxygen delivery platforms is problematic. These settings include but are not limited to combat casualty care and healthcare provided in extreme rural environments in undeveloped countries. Furthermore, there are a number of settings where delivery of oxygen other than the pulmonary route to oxygenate tissues would be of value, including severe lung injury, airway obstruction, and others. This paper provides a brief overview of the previous and current attempts to utilize chemical oxygen production strategies to enhance systemic oxygenation. While promising, the routine use of chemically produced oxygen continues to pose significant engineering and physiologic challenges.

Equation of state density models for hydrocarbons in ultradeep reservoirs at extreme temperature and pressure conditions

The necessity of exploring ultradeep reservoirs requires the accurate prediction of hydrocarbon density data at extreme temperatures and pressures. In this study, three equations of state (EoS) models, Peng-Robinson (PR), high-temperature high-pressure volume-translated PR (HTHP VT-PR), and perturbed-chain statistical associating fluid theory (PC-SAFT) EoS are used to predict the density data for hydrocarbons in ultradeep reservoirs at temperatures to 523 K and pressures to 275 MPa. The calculated values are compared with experimental data. The results show that the HTHP VT-PR EoS and PC-SAFT EoS always perform better than the regular PR EoS for all the investigated hydrocarbons.