Christopher J. Orme

Characterization of gas transport in selected rubbery amorphous polyphosphazene membranes

Gas permeabilities for six different gases have been evaluated for a series of closely related polyphosphazenes. Polyphosphazenes are attractive polymers for use as gas separation membranes due to their inherent chemical, thermal, and radiation stability. Additionally, polyphosphazenes may be tailored for specific chemical affinities. In this report, polyphosphazenes with three different pendant groups with varied hydrophilicity were characterized for gas permeation. All polymers were characterized as having modest permeabilities for methane, oxygen, nitrogen, helium, and hydrogen. These gases were not observed to have a significant interaction with the polymer structure and transport is attributed to segmental chain motion. Carbon dioxide was found to have a significant intermolecular interaction with the polymer and the permeability was observed to be proportional to the percentage of hydrophilic 2-(2-methoxyethoxy)ethanol on the backbone. Thus, we report a promising method for the development of CO2 selective membranes.

Testing of polymer membranes for the selective permeability of hydrogen

Selective gas barriers are of prime importance in thin polymer membranes. The focus of this work was to find a polymer membrane that allows the transport of H2 and acts as a barrier to CO2 and chlorinated organics. Membrane screening has included the following testing: single gas permeability measurements, mixed gas separations, and polymer physical characterization. Single gas permeability measurements were made using the time-lag method for five gases (H2, O2, N2, CO2, and CH4). Permeability coefficients and selectivities for the gas pair H2/CO2 are presented. Mixed gas separations were performed to measure actual separation factors for H2/CCl4 and to determine the effects on hydrogen permeability caused by exposing polymers to chlorinated hydrocarbons. The results of basic polymer characterization, such as polymer density and glass transitions, are addressed. #The submitted manuscript has been authored by a contractor of the U.S. Government under contract No. DE-AC07-99ID13727. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.

Pervaporation of water-dye, alcohol-dye, and water-alcohol mixtures using a polyphosphazene membrane

A novel phosphazene heteropolymer (HPP) was synthesized that contained three differing pendant groups: 2-(2-methoxyethoxy)ethanol (MEE), 4-methoxyphenol, and 2-allylphenol. The resulting polymer is an amorphous elastomer with good film forming properties where MEE and 4-methoxyphenol pendant groups influenced the hydrophilicity and the solvent compatibility of the polymer. Sorption studies were performed to characterize the polymer in terms of Hansen solubility parameters. Additionally, group contributions were used to predict the Hansen parameters for the polymer and these data compared favorably with the observed solubility behavior with 15 solvents that ranged from hydrocarbons to water. Homopolymers synthesized from MEE and 4-methoxyphenol were also studied for solubility revealing different behaviors with each representing a limit in hydrophilicity; MEE formed a water-soluble hydrophilic polymer and 4-methoxyphenol yielded a hydrophobic polymer. Membranes formed from HPP were characterized for use as pervaporation membranse using five different feeds: water–dye, methanol–dye, 2-propanol–dye, water–2-propanol, and water–methanol. Fluxes of methanol and isopropanol were greater than for water. For the alcohol–water separations, the alcohol was the favored permeate in all cases with higher fluxes observed for higher alcohol feed concentrations, however, separation factors declined.

Concentration dependent speciation and mass transport properties of switchable polarity solvents

Tertiary amine switchable polarity solvents (SPS) consisting of predominantly water, tertiary amine, and tertiary ammonium and bicarbonate ions were produced at various concentrations for three different amines: N,N-dimethylcyclohexylamine, N,N-dimethyloctylamine, and 1-cyclohexylpiperidine. These amines exhibit either osmotic or non-osmotic character as observed through forward osmosis, which led to this study to better understand speciation and its influence on water transport through a semi-permeable membrane. For all concentrations, several physical properties were measured including viscosity, molecular diffusion coefficients, freezing point depression, and density. Based on these measurements, a variation on the Mark–Houwink equation was developed to predict the viscosity of any tertiary amine SPS as a function of concentration using the amines molecular mass. The physical properties of osmotic SPS, which are identified as having an amine to carbonic acid salt ratio of ∼1 : 1, have consistent concentration dependence behavior over a wide range of concentrations, which suggests osmotic pressures based on low concentrations freezing point studies can be extrapolated reliably to higher concentrations. The observed physical properties also allowed the identification of solution state speciation of non-osmotic SPS, where the amine to carbonic acid salts ratio is significantly greater than one. These results indicate that, at most concentrations, the stoichiometric excess of amine is involved in solvating a proton with two amines.

Chemical Separations Using Shell and Tube Composite Polyphosphazene Membranes

Abstract Several applications of modular shell and tube polyphosphazene coated membrane units are reported in this paper. These modules were used to measure the mixed-gas separation properties of poly[bis(phenoxy)phosphazene] based polymers on a larger scale. Transport behavior was determined using the variable volume technique. The test gas mixture was SO2/N2 at temperatures between 80[ddot]C and 270[ddot]C. Transport of these gases was found to be a sorption controlled process. Several organic-aqueous and organic-organic separations have been performed using the polyphosphazene coated shell and tube modules. The separations include: methylene chloride/water, acetic acid/water, isopropyl alcohol/water, glycerol/water, and hexane/soy oil. The membranes were prepared using slip casting techniques. The results of these studies show that polyphosphazene membranes can effectively be used to separate acid gases and organic chemicals from various waste streams in harsh, chemically aggressive environments.

Formation of pervaporation membranes from polyphosphazenes having hydrophilic and hydrophobic pendant groups: Synthesis and characterization

A series of new polyphosphazene polymers were synthesized using three different pendant groups with the goal of probing structure-function relationships between pendant group substitution and polymer swelling/water flux through thin dense films. Formation of polymers with relative degrees of hydrophilicity was probed by varying the stoichiometry of the pendant groups attached to the phosphazene backbone: p-methoxyphenol, 2-(2-methoxyethoxy)ethanol, and o-allylphenol. The polymers in this study were characterized using NMR, thermal methods, and dilute solution light-scattering techniques. These techniques revealed that the polymers were amorphous high polymers (M w = 10 5 -10 7 ) with varying ratios of pendant groups as determined by integration of the 1 H- and 31 P-NMR spectra. Thin dense film membranes were solution-cast with azo-bis(cyclohexane)carbonitrile included in the matrix and crosslinked using thermal initiation.

Preparation and characterization of membranes with adjustable separation performance using polyphosphazene membranes

Abstract Preparation of polymeric membranes with dynamically adjustable separation properties is a new area of research. The approach described herein involves adjusting the permeate pressure of a swollen polymeric membrane to alter its separation properties. For example, during the separation of methylene blue from isopropyl alcohol, pressure on the permeate side was increased such that both methylene blue and alcohol were passed, resulting in no separation. The permeate pressure then was decreased and only colorless isopropyl alcohol passed through the membrane. A subsequent increase in the pressure on the permeate side results in no separation once again demonstrating that the effects were fully reversible. In a mixed dye experiment in isopropyl alcohol, methylene blue and rose bengal were separated with the lower-molecular-weight methylene blue permeating with isopropyl alcohol while the rose bengal was retained in the feed. The new membranes are distinguished by improved control of separation paramet...

Density functional theory analysis of the impact of steric interaction on the function of switchable polarity solvents

A density functional theory (DFT) analysis has been performed to explore the impact of steric interactions on the function of switchable polarity solvents (SPS) and their implications on a quantitative structure-activity relationship (QSAR) model previously proposed for SPS. An X-ray crystal structure of the N,N-dimethylcyclohexylammonium bicarbonate (Hdmcha) salt has been solved as an asymmetric unit containing two cation/anion pairs, with a hydrogen bonding interaction observed between the bicarbonate anions, as well as between the cation and anion in each pair. DFT calculations provide an optimized structure of Hdmcha that closely resembles experimental data and reproduces the cation/anion interaction with the inclusion of a dielectric field. Relaxed potential energy surface (PES) scans have been performed on Hdmcha-based computational model compounds, differing in the size of functional group bonded to the nitrogen center, to assess the steric impact of the group on the relative energy and structural properties of the compound. Results suggest that both the length and amount of branching associated with the substituent impact the energetic limitations on rotation of the group along the N-R bond and NC-R bond, and disrupt the energy minimized position of the hydrogen bonded bicarbonate group. The largest interaction resulted from functional groups that featured five bonds between the ammonium proton and a proton on a functional group with the freedom of rotation to form a pseudo six membered ring which included both protons.

Humidified Gas Stream Separations at High Temperatures Using Matrimid 5218

Most industrially relevant high temperature gas separations (≥150°C) of either carbon dioxide (flue gas) or hydrogen (syn-gas) must be performed in the presence of water vapor. At ambient temperatures, water vapor can permeate easily through most polymeric membranes and can influence the permeation of other gases through interaction with the polymer, such as swelling and clustering. At higher temperatures, water vapor can be destructive to polymer membranes by changing the polymer structure that can result in diminished gas separation performance. Little data has been reported on the influence of water vapor in gas separations at >100°C because most polymers are not stable at temperature. Many high performance (HP) polymers are able to endure high temperatures and aggressive chemical conditions. For example, polyimides are promising HP polymers that effectively separate permanent gases at temperatures higher than 150°C under dry conditions. In this report, the analysis of selected HP polymers in humidified gas streams (2–4 vol% water) shows that they can perform modest separations at ambient temperatures. In general, it was observed that water vapor permeability is greater than other tested gases. Additionally, the permeabilities of the analyte gases were somewhat influenced by the presence of humidity and their selectivities were significantly lower, as compared to corresponding experiments performed in the absence of water. To elucidate the role of water vapor in gas transport, energy of activation of permeation (Ep) values were obtained for Matrimid 5218 from 30–200°C in a humidified mixed gas stream, and it was found that the selectivities are nearly identical to dry gas streams at 150°C. This data suggests that water vapor functions as a gas and only slightly decreases selectivity of the other gases at elevated temperatures. As a result, economic wet gas separations may be possible using these materials if the gas stream is kept at higher temperature (≥150°C), which is assisted by the inherent stability of the membranes.

Gas Permeation Testing Results from the Mixed Waste Focus Area Improved Hydrogen Getter Program

Abstract: The gas permeabilities of more than 20 polymers were measured using pure and mixed gas techniques. The motivation was to determine potential materials that could be used to protect hydrogen getter particles from poisons while permitting sufficient hydrogen rates to enable the getters use in TRUPACT types of containers. A rate of five barrers or larger is needed. Of the materials screened in the pure gas tests, more than 15 qualified. Nine materials qualified in the mixed gas tests, but of the nine only three had high CCI4 rejection rates and four others would greatly reduce the transport of the CCl4.