Frederick F. Stewart

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.

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.

Phosphazene monomers from the regiospecific reaction of tert-butylhydroquinone with hexachlorocyclotriphosphazene: A new composite material precursor

A new synthesis of a phosphazene-based polymer precursor is described. tert-Butylhydroquinone was found to react with hexachlorocyclotriphosphazene in the presence of a base, 4-picoline, in cyclohexane to yield a regiospecifically substituted hexa-tert-butylcyclotriphosphazene. This reaction proceeds more rapidly to completion and in a higher yield than data previously reported for analogous cyclotriphosphazenes. Nuclear magnetic resonance spectroscopy and thermal methods (differential scanning calorimetry and thermogravimetric analysis) were employed to characterize this material, and it is observed that the bifunctional tert-butylhydroquinone substitutes only at the least hindered phenolic hydroxyl under the reported reaction conditions. This conclusion clarifies published data that suggests that without the tert-butyl substituent on the hydroquinone ring, crosslinking between phosphazene rings occurs to some extent.

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.

Electron beam crosslinking of fluoroalkoxy, methoxyethoxyethoxy, and substituted phenoxy polyphosphazenes: Physical and chemical characterization and comparison to a thermally induced free radical process and ionic complexation

Electron beam, thermal free radical, and cationic complexation mechanisms have been employed to investigate crosslinking in selected polyphosphazenes. In polyphosphazenes functionalized with o-allylphenol to facilitate free radical crosslinking, maximum crosslink density was achieved after 10 min at 130°C utilizing benzoyl peroxide as an initiator. Electron beam radiation was found to give an increased crosslink density with increased dose. The dose–crosslink density relationship observed for a aryloxyphosphazene terpolymer PPXP also was seen in poly[bis(2,2′-(methoxyethoxy)ethoxy)phosphazene] (MEEP). However, with two lots of a fluoroalkoxyphosphazene an initial crosslink density was achieved at a lower electron beam exposure with no additional crosslink density observed with increasing dose. These measurements are observations of net crosslinking, which is the result of crosslinking processes balanced by chain scission processes. DSC revealed that neither thermal- nor electron beam-initiated crosslinking cause any significant change in the Tg of the polymer. Metal ion complexation with MEEP consistently gave Tg values that were higher than MEEP. The Tg values measured for both MEEP and the lithium-complexed MEEP were unaffected by electron beam irradiation. These data suggest the location of lithium complexation may be at the nitrogen lone electron pair on the backbone, representing a new mechanism of lithium complexation in phosphazenes.

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...

Improved method for the isolation and purification of water-soluble polyphosphazenes

We report in this article an improved procedure to isolate and purify representative water-soluble polyphosphazenes that dramatically reduces the time and equipment involved, while maintaining or exceeding the yields and purity reported in the literature for these polymers obtained using dialysis methods. This technique takes advantage of the phase transition behavior exhibited by some hydrophilic polymers, namely that associated with the lower critical solubility temperature (LCST). The polymers used in this study were poly[bis-(2-(2-methoxyethoxy)ethoxy)phosphazene], MEEP (1), and two new water-soluble polymers. These polymers are similar to MEEP, yet they contain a small percentage of a crosslinkable pendant group; either 2-hydroxyethyl allyl ether (2), or o-allyl phenol (3). The observed behavior was quite different for these two polymers than that found for MEEP, and is a direct consequence of the pendant group substitution patterns. Although the homopolymer MEEP yielded a single sharp LCST point, the two heteropolymers exhibited this phase transition over a broader temperature range. Further, fractionation of polymer 3, based on pendant group speciation, was possible due to the more hydrophobic nature of the phenol.

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.

Synthesis and Characterization of Mixed-Substituent P- n -Propyl-N-trimethylsilylphosphoranimines

One approach to the synthesis of polyphosphazenes is the condensation polymerization of phosphoranimines. In this work, several novel P-n-propyl-N-trimethylsilylphosphoranimines have been synthesized and characterized. Modifications to the literature synthetic routes were required to obtain the precursor phosphines. The N-trimethylsilylphosphoranimines were obtained though oxidation of the phosphine with bromine and then subsequent nucleophilic displacement using lithium phenoxide. These phosphoranimines were stable for long periods of time under dry inert conditions. NMR analyses revealed complex splitting patterns beyond typical coupling due to the stereocenter at phosphorus. We report several approaches to the n-propyl containing phosphines and phosphoranimines.

Reactions and polymerization of hexa-[3-tert-butyl-4-hydroxyphenoxy]cyclotriphosphazene : A new method for the preparation of soluble cyclomatrix phosphazene polymers

Novel cyclomatrix phosphazene-containing polyester polymers were synthesized through the reaction of a polyhydroxylated cyclotriphosphazene and a bifunctional acid chloride. To demonstrate the chemistry of the free hydroxyl of hexa-[3-tert-butyl-4-hydroxyphenoxy]cyclotriphosphazene, nucleophilic displacement reactions were performed with both acetic anhydride and alkyl chlorides. This work compares favorably to literature data for the chemistry of hexa-[4-hydroxyphenoxy]cyclotriphosphazene, whose hydroxyl is not hindered by an adjacent substituent. The hindered site of hexa-[3-tert-butyl-4-hydroxyphenoxy]cyclotriphosphazene was found to react with bidentate acid chlorides to yield new high polymers. The phosphazene-containing polyesters were observed to have good solubility in polar organic solvents. Characterization of these new materials was performed using dilute solution laser light scattering techniques, thermal analysis, and NMR spectroscopy.