Chapter 12
Membrane Separations Using Functionalized Polyphosphazene Materials 1
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Frederick F. Stewart , Christopher J. Orme , Mason K. Harrup , Robert P. Lash , Don H . Weinkauf , and John D. McCoy 1
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Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2208 Departments of ChemicaI Engineering and Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, N M 87801 2
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Polyphosphazenes are intriguing materials with potential application as membranes for selective mass transport. In this study, several polyphosphazenes, differing only i n ratios of three different pendant groups, were synthesized and formed into supported thin dense film membranes. A balance of hydrophilic/hydrophobic behaviors was achieved through varying die amount of hydrophilic 2-(2methoxyethoxy)ethanol (MEE) attached to the polymer. The remaining sites were substituted with hydrophobic 4methoxyphenol with a small amount of 2-allylphenol added to provide a cross-linking moiety. Resulting polymers were studied using swelling determinations and pervaporation.
© 2004 American Chemical Society
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178 Solubility control in membrane processes offers the ability to tailor a membrane to provide a high affinity for a specific permeate while rejecting other chemical species. Thus, separations are determined by the intermolecular interactions between specific permeates and the polymer membrane substrate. Maximizing these interactions then becomes the goal. Addition of functional groups to polymers that interact with specific permeates can be performed through chemical synthesis. In this study, polyphosphazenes have been employed to serve as a stable "platform" for chemical synthesis through attachment of pendant groups with specific functionality.
Background Polyphosphazenes can be thought of as hybrid organic-inorganic polymers with a backbone comprised of alternating phosphorus and nitrogen atoms. Each phosphorus is pentavalent resulting in two pendant groups per mer (Figure 1). Typical pendant groups consist of organic nucleophiles (1). Selection of the pendant groups is critical in determining the physical and chemical properties of the resulting polymer. In this work, several polyphosphazenes with differing chemical properties are discussed. Homopolymers, phosphazenes with only one type of pendant group, vary widely. For example, poly[bis-(2-(2methoxyethoxy)phosphazene] (MEEP) is a hydrophilic elastomer, whereas poly[bis-4-methoxyphenoxyphosphazene] is a semi-crystalline solid material.
(HPP General Structure)
Figure 1. Structures for three polyorganophosphazenes studied in this work The general structure for HPP is a representation of both HPP1 and HPP2. To provide an enhanced level of control over the properties of phosphazenes, the use of blended pendant group mixtures has been employed to yield HPP polymers (Figure 1). For example, hydrophobic 4-methoxyphenol
179 was added to a polymer containing hydrophilic M E E with the resultant polymer properties dependent on the relative loading of each pendant group (2). Higher levels of M E E in the polymer matrix yielded membranes with high affinities for polar permeates. Recent data for gas transport show a clear linear correlation between the amount of M E E on the polymer and the permeability of C 0 , suggesting a strong solubility interaction between the polymer and the gas (3). Synthesis of polymers a priori is inherently inefficient. A n accurate description of solubility behavior is necessary such that polymers may be designed and synthesized for transport of specific permeates. Tools that recently have been explored to describe the solubility behavior of blended pendant group phosphazenes are Hanson solubility parameters (2). Hanson parameters provide a numerical method to describe solubility such that estimations of mutual solubility can be made using the chemist's "rule of thumb" that like materials dissolve like materials. Thus, polymer membranes and permeates with similar Hanson parameters should exhibit mutual solubility resulting in increased solubility driven transport. Three Hanson parameters are used to characterize the types of interactions that are possible for solvents in terms of molecular forces: 1) hydrogen bonding (8 ), 2) polarity (8 ), and 3) dispersion (8
