43 research outputs found
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Polyimides and their derivatives for gas separation applications
High performance polymers are of interest for high temperature gas separations, especially for the sequestration of carbon dioxide. A new family of high performance imide polymers (VTEC, RBI Inc.) has been identified as a material class containing the potential building blocks needed for a successful membrane capture material. The VTEC polyimides possess the desired thermal properties (up to 500 °C) and are robust and flexible even after multiple thermal cycles (up to 400 °C). A critical variable when working with the glassy polymers is their moisture content. It has been found that water entrapped within the polymer matrix (either as hydration molecules attached to salts in the polymer, left over solvent, or physisorbed) can also cause the polymer to change dramatically. Additionally presence of molecular water in the polymer’s void volume has been validated through Positron Annihilation Lifetime (PAL) spectroscopy. In this presentation, polymer characterization and gas-separation testing results will be discussed
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Membrane Transport Behavior and the Lability of Chloride on Polyphosphazenes Bearing Bulky Substituents
Polyphosphazenes are an intriguing class of inorganic polymers where much of their functionality is derived from pendant groups attached to phosphorus. The backbone of the polymer consists of alternating phosphorus and nitrogen atoms where the bonding is conventionally drawn as alternating double and single bonds. Orbital nodes are located at each phosphorus atom resulting in electron delocalization between phosphorus atoms, but not through them. Thus, the polymer backbone has a high degree of flexibility where halogens or other leaving groups can be effectively displaced with nucleophiles. In this paper, the first known example of a polyphosphazene with large quantities of non-labile chloride substituents induced by neighboring group steric effects will be discussed. This example is the result of the substitution of poly[bis-chlorophosphazene] with the sodium salt of 3,5-di-tert-butylphenol where only 60% of the chlorines were displaced. This contrasts with the 100% substitution observed with other phenols (phenol, 4-tert-butylphenol, 3-methylphenol, etc.)
Drastic enhancement of carbon dioxide adsorption in fluoroalkyl-modified poly(allylamine)
Polyamine-based carbon dioxide sorbents suffer from a seesaw relationship between amine content and amine efficiency. High polyamine loadings equate to increased amine contents, but often at the expense of amine efficiency. Carbon dioxide mass transport in compact polymers is severely limited, especially at ambient temperature. High polymer contents curtail diffusion pathways, hindering CO2 from reaching and reacting with the numerous amine functions. Here, we overcome this issue using poly(allylamine) (PAA) grafted with short fluoroalkyl chains and then cross-linked with C60. As experimentally evidenced by positron annihilation lifetime spectroscopy, the incorporation of fluoroalkyl chains generates free volume elements that act as additional diffusion pathways within the material. The inclusion of void volume in fluoroalkyl-functionalized PAA sorbents results in radically increased CO2 uptakes and amine efficiencies in diluted gas streams at room temperature, including simulated air. We speculate that the hydrophobic fluorinated functions interfere with the strong amine hydrogen bonding network disrupting and consequently altering the packing and conformation of the polymer chains. The evidence presented here is a blueprint for the development of more efficient amine-based CO2 sorbents. This journal i
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CO2 Separation Using Thermally Optimized Membranes: A Comprehensive Project Report (2000 - 2007)
This is a complete (Fiscal Years 2000–2006) collection of the Idaho National Laboratory’s (INL) research and development contributions to the project, “CO2 Separation Using Thermally Optimized Membranes.” The INL scientific contribution to the project has varied due to the fluctuations in funding from year to year. The focus of the project was polybenzimidazole (PBI) membranes and developing PBI compounds (both substitution and blends) that provide good film formation and gas separation membranes. The underlying problem with PBI is its poor solubility in common solvents. Typically, PBI is dissolved in “aggressive” solvents, like N,N-dimethylacetamide (DMAc) and N methylpyrrolidone (NMP). The INL FY-03 research was directed toward making soluble N-substituted PBI polymers, where INL was very successful. Many different types of modified PBI polymers were synthesized; however, film formation proved to be a big problem with both unsubstituted and N-substituted PBIs. Therefore, INL researchers directed their attention to using plasticizers or additives to make the membranes more stable and workable. During the course of these studies, other high-performance polymers (like polyamides and polyimides) were found to be better materials, which could be used either by themselves or blends with PBI. These alternative high-performance polymers provided the best pathway forward for soluble high-temperature polymers with good stable film formation properties. At present, the VTEC polyimides (product of RBI, Inc.) are the best film formers that exhibit high-temperature resistance. INL’s gas testing results show VTEC polyimides have very good gas selectivities for both H2/CO2 and CO2/CH4. Overall, these high-performance polymers pointed towards new research areas where INL has gained a greater understanding of polymer film formation and gas separation. These studies are making possible a direct approach to stable polymer-based high-temperature gas separation membranes. This report is separated into several sections due to the complexity of the research and the variation with the development of better high-temperature, gas separation membranes. Several fiscal years are combined because the research and development efforts within those areas crossed fiscal year boundaries
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Separation of Minor Actinides from Lanthanides by Dithiophosphinic Acid Extractants
The selective extraction of the minor actinides (Am(III) and Cm(III)) from the lanthanides is an important part of advanced reprocessing of spent nuclear fuel. This separation would allow the Am/Cm to be fabricated into targets and recycled to a reactor and the lanthanides to be dispositioned. This separation is difficult to accomplish due to the similarities in the chemical properties of the trivalent actinides and lanthanides. Research efforts at the Idaho National Laboratory have identified an innovative synthetic pathway yielding new regiospecific dithiophosphinic acid (DPAH) extractants. The synthesis provides DPAH derivatives that can address the issues concerning minor actinide separation and extractant stability. For this work, two new symmetric DPAH extractants have been prepared. The use of these extractants for the separation of minor actinides from lanthanides will be discussed
Constraint methods for determining pathways and free energy of activated processes
Activated processes from chemical reactions up to conformational transitions
of large biomolecules are hampered by barriers which are overcome only by the
input of some free energy of activation. Hence, the characteristic and
rate-determining barrier regions are not sufficiently sampled by usual
simulation techniques. Constraints on a reaction coordinate r have turned out
to be a suitable means to explore difficult pathways without changing potential
function, energy or temperature. For a dense sequence of values of r, the
corresponding sequence of simulations provides a pathway for the process. As
only one coordinate among thousands is fixed during each simulation, the
pathway essentially reflects the system's internal dynamics. From mean forces
the free energy profile can be calculated to obtain reaction rates and insight
in the reaction mechanism. In the last decade, theoretical tools and computing
capacity have been developed to a degree where simulations give impressive
qualitative insight in the processes at quantitative agreement with
experiments. Here, we give an introduction to reaction pathways and
coordinates, and develop the theory of free energy as the potential of mean
force. We clarify the connection between mean force and constraint force which
is the central quantity evaluated, and discuss the mass metric tensor
correction. Well-behaved coordinates without tensor correction are considered.
We discuss the theoretical background and practical implementation on the
example of the reaction coordinate of targeted molecular dynamics simulation.
Finally, we compare applications of constraint methods and other techniques
developed for the same purpose, and discuss the limits of the approach
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Correlations of Polyimides and Blended Polyimides for High Temperature Gas Separations
High performance polymers are of interest for high temperature gas separations, especially for the sequestration of carbon dioxide. A new family of high performance imide polymers has been identified as a successful membrane capture material. VTEC polyimides possess desired thermal properties (up to 500 °C) along with being robust and flexible even after multiple thermal cycles (up to 400 °C). Polyimides (PI) are excellent materials for high selectivity for smaller kinetic diameter gases such as H2 and CO2; however, they have low fluxes. We blended small amounts of different polymers with VTEC polyimide, which changes the fluxes. Another critical problem when working with glassy polymers is their moisture content. It has been found that water entrapped within the polymer matrix (left over from the solvent, or physisorbed) can also cause the polymer to change dramatically. Additionally presence of molecular water in the polymer’s void volume has been validated through Positron Annihilation Lifetime (PAL) spectroscopy. In this presentation, polymer characterization and gas-separation testing results will be discussed