Membrane Materials Science - ACS Symposium Series (ACS

1 Membrane Materials Science An Overview

Downloaded by BOSTON UNIV on July 22, 2013 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch001

DOUGLAS R. LLOYD Department of Chemical Engineering, Separations Research Program, Center for Polymer Research, The University of Texas at Austin, Austin, TX 78712

Material science aspects of synthetic polymeric membranes are presented in this survey. The objective is to place each of the subsequent chapters of this volume into proper perspective. Therefore, frequent reference is made to the accompanying chapters and, where necessary, to alternative information sources. By way of introduction, this chapter considers in turn: material selection, material characterization and evaluation, membrane preparation, membrane characterization and membrane evaluation. Membrane module design and manufacture, transport phenomena and process performance are introduced in the discussion only as they pertain to membrane materials science. Following this introduction, the various chapters of this volume are previewed. Membrane science i s a phrase that encompasses a vast array of topics (1). The common thread that t i e s together the various aspects of membrane science i s that each one deals to some extent with the study of permeation and permeable media. More s p e c i f i c a l l y , the permeable medium or membrane is a phase between two phases. The role of the membrane may be either to change the composition of a solution on the basis of r e l a t i v e permeation rates (membrane separation processes), to p h y s i c a l l y or chemically modify the permeating species (ion-exchange membranes and biofunctional membranes), to conduct e l e c t r i c current, to prevent permeation (packaging and coating) or to regulate the rate of permeation (controlled release). Functionally, membranes may be either passive or reactive depending upon the membrane's a b i l i t y to a l t e r the chemical nature of the permeating species. Membranes can also be categorized as being either neutral or charged according to t h e i r i o n i c nature. S t r u c t u r a l l y , membranes can be categorized as being e i t h e r non-porous (that i s , membranes i n which the membrane phase i s continuous, such as dense polymeric films and l i q u i d

0097-6156/ 85/0269-0001 $06.50/ 0 © 1985 American Chemical Society In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


2

MATERIALS SCIENCE O F SYNTHETIC M E M B R A N E S

membranes) or porous to various extents (for example, porous polymeric films i n which both the polymer matrix and the void spaces are continuous. Further categorization according to the porous structure of these co-continuous membranes i s discussed below under the heading "Membrane Preparation"). The various chapters i n t h i s book discuss the materials science of synthetic polymeric membranes f o r separations, conductive membranes and reactive membranes. Membrane performance i s often measured by the a b i l i t y of the membrane to prevent, regulate or f a c i l i t a t e permeation. The rate of permeation and the mechanism of transport depend upon the magnitude of the d r i v i n g force, the s i z e of the permeating molecule r e l a t i v e to the s i z e of the available permanent or dynamic transport corridor and the chemical nature (dispersive, polar, i o n i c , etc.) of both the permeant and the polymeric membrane material. Membrane science can a r b i t r a r i l y be divided into seven intimately related categories: material s e l e c t i o n , material characterization and evaluation, membrane preparation, membrane characterization and evaluation, transport phenomena, membrane module design and process performance. This chapter and those to follow emphasize the materials science aspects of synthetic polymeric membranes; that i s , the s e l e c t i o n , characterization and evaluation of membrane materials as w e l l as the preparation, characterization and evaluation of membranes. Transport phenomena, membrane module design and process performance enter the discussion only as these topics pertain to materials science. Material Selection. In designing a membrane, one must f i r s t determine the physicochemical nature of the permeants (gas, vapor, l i q u i d , s o l i d ; dispersive, polar, i o n i c ; reactive, i n e r t ; physical size and shape). The membrane s c i e n t i s t then has two materials science controls over permeation: membrane material s e l e c t i o n and membrane preparation procedures. Membrane material selection allows control over the nature and magnitude of the permeant-membrane physicochemical interactions. Choice of membrane material also determines the packing density and segment mobility of the polymer chains which comprise the s o l i d regions of the membrane. Membrane preparation procedures determine membrane morphology and the extent to which physical considerations, such as s t e r i c hindrance, influence the rate of permeation. Together, material s e l e c t i o n and membrane preparation procedures influence the mechanism of transport, membrane s t a b i l i t y and membrane performance. In the past, membrane material s e l e c t i o n f o r gas separations (2) and l i q u i d separations has often followed either an Edisonian or a common sense approach. For example, almost every polymer that can be formed into a f i l m has been characterized i n terms of gas permeability (at least f o r a few common gases). In t h i s volume, Chern et a l . (3) and Lloyd and Meluch (4) discuss membrane material selection i n terms of physicochemical interactions for gas mixture separations and l i q u i d mixture separations, respectively. Hoehn (5) presents s i m i l a r arguments for membrane material s e l e c t i o n i n general. In comparison to the wide v a r i e t y of polymers investigated

In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


1. LLOYD

Membrane Materials Science

3

for t h e i r gas transport properties, the number of polymers studied for t h e i r l i q u i d transport properties i s somewhat l i m i t e d . However, a survey of recent l i t e r a t u r e and patents (4) indicates that an ever-increasing number of polymers, copolymers and blends are being considered as p o t e n t i a l membrane materials f o r l i q u i d mixture separations. I t i s i n t e r e s t i n g to compare the number of materials which have reached commercialization for l i q u i d separations and gas separations. Lloyd and Meluch (4) l i s t i n excess of twenty-seven polymers, copolymers and blends currently being used f o r l i q u i d separation membranes. However, only polysulfone and a v a r i e t y of c e l l u l o s e acetates and s i l i c o n e based polymers have attained commercial success for gas mixture separations (6). In addition to the guidelines suggested by Chern et a l . (3) and by Lloyd and Meluch (A), Ward et a l . (7) point out that biocompatibility and s t e r i l i z a b i l i t y need to be considered when selecting membrane materials for therapeutic applications. Each of these chapters (3-7), plus the chapter by Cadotte (8), demonstrates that one can molecularly engineer polymer chemical structures to obtain membrane materials which t h e o r e t i c a l l y can achieve the desired control over permeability. A l t e r n a t i v e l y , the ideas developed (3-5) can simply serve as c r i t e r i a by which one can s e l e c t , from a l i s t of available polymers, those membrane materials with the greatest p o t e n t i a l f o r achieving the desired control over permeability. Material Characterization and Evaluation. Physicochemical considerations can be u s e f u l i n membrane material s e l e c t i o n . However, i t would be b e n e f i c i a l i f one could experimentally v e r i f y that the proper choice has been made p r i o r to undertaking the often d i f f i c u l t tasks of membrane preparation and characterization. In addition, i t i s frequently b e n e f i c i a l to have f u l l y characterized the polymer p r i o r to forming the membrane. In t h i s context, material c h a r a c t e r i z a t i o n r e f e r s to obtaining information related to the i n t r i n s i c properties of the polymer rather than s t r u c t u r a l or physical properties of the membrane which i s to be subsequently formed from the polymer. I t i s often useful to characterize the polymer i n terms of the following: molecular architecture or bulk properties (molecular weight c h a r a c t e r i s t i c s , degree of branching, chemical f u n c t i o n a l i t y and, i f necessary, copolymer or blend composition), polymer solution properties (polymer s w e l l a b i l i t y , polymer s o l u b i l i t y and solution v i s c o s i t i e s ) , thermal properties (glass t r a n s i t i o n temperature and c r y s t a l melting temperature), chemical stability and, i f appropriate, biocompatibility. Physical properties of the polymers, such as mechanical strength and extent of c r y s t a l l i z a t i o n , often depend upon sample preparation procedures and are ultimately r e f l e c t e d i n the properties of the membrane. As such, they are discussed below under the heading of Membrane Characterization and Evaluation. Lloyd and Meluch (4) summarize several methods of evaluating p o t e n t i a l membrane materials f o r l i q u i d separations without actually preparing membranes. They point out that once a membrane or f i l m has been formed, i t i s often d i f f i c u l t to d i s t i n g u i s h the i n t r i n s i c properties of the polymer from the s t r u c t u r a l c h a r a c t e r i s t i c s of the membrane, which are themselves dependent


4



upon preparation procedures. Therefore, they emphasize those methods which characterize membrane materials i n terms of t h e i r physicochemical interactions with small molecules, without requiring the formation of membranes or dense f i l m s . Similar procedures (for example, inverse gas chromatography) can be used to characterize the physicochemical interactions which occur during gas and vapor transport. Membrane Preparation. The a b i l i t y of a membrane to regulate permeation l i e s not only i n the selection of an appropriate membrane material, but also i n the physical structure of the membrane. The physical structure and the physical properties of a membrane can be d i r e c t l y related to membrane preparation procedures. Depending upon the membrane preparation procedure and based on information obtained from membrane characterization, membranes are often c l a s s i f i e d as having one of the following structures: (i) Non-porous, Dense or Homogeneous. These membranes consist of closely packed polymer chains with a uniform, continuous packing density throughout the system. The void spaces between macromolecules, and thus the corridors f o r transport, are beyond the current l e v e l of o p t i c a l and electron microscopic resolution. Transport through these i n t e r s t i c i a l spaces i s often discussed i n terms of dynamic free volume and energy requirements f o r polymer segmental motion. That i s , the transport mechanism i s considered to be one of d i s s o l u t i o n i n the " l i q u i d " or "swollen" membrane phase and d i f f u s i o n down the chemical p o t e n t i a l gradient. The properties of dense films are often equated with the i n t r i n s i c properties of the bulk polymer. However, t h i s comparison must be made with caution because chain packing, and thus transport properties, may be dependent upon thermal history or solution history during f i l m preparation and upon the attainment of equilibrium p r i o r to characterization. Dense films can be prepared by melt extrusion, compression molding or solution casting. ( i i ) Porous or Heterogeneous. In contrast to the dense membranes i n which the polymer forms e s s e n t i a l l y a continuous phase, porous membranes are comprised of co-continuous phases of polymer and interconnecting voids that serve as transport corridors. Depending on the size of the voids and the interactions which occur during transport, the mechanism of transport may be considered as one of convection, d i f f u s i o n or a combination of these mechanisms. The size and d i s t r i b u t i o n of these voids vary with membrane preparation procedure and are somewhat a r b i t r a r i l y distinguished as finely porous, microporous and macroporous. A l t e r n a t i v e l y (9), the terms ultragel and microgel have been suggested to indicate membranes of d i f f e r e n t pore size; " u l t r a " indicating smaller pores (0.1-0.5 Um), "micro" indicating larger pores (1-2 ym). The exact pore size or range of pore sizes that are described by these various terms are poorly defined and d i f f e r from author to author. Porous membranes may be symmetric (that i s , uniformly porous throughout the structure), anisotropic (that i s , pore size varies continuously i n one direction) or skinned, i n t h i s review, the terms skinned and asymmetric are used interchangeably to describe membranes which consists of a thin, relatively dense layer supported by a porous support layer. Symmetrically porous membranes often r e t a i n



1. LLOYD


solutes and suspended matter within the membrane structure; thus, the term depth f i l t e r i s used to describe these membranes. On the other hand, highly anisotropic and skinned membranes reject solutes and suspended matter at the surface; thereby avoiding membrane clogging or blinding problems. Consequently, skinned, and to a lesser extent anisotropic, membranes have become the membranes of choice f o r l i q u i d phase and gas phase separations. Since permeation i s inversely related to membrane thickness, the advantage of the skinned membrane over the homogeneous membrane l i e s i n the extreme thinness of the skin ( t y p i c a l l y < 1.0 ym), and thus the a b i l i t y to achieve high fluxes without any loss i n s e l e c t i v i t y . Whether the skin layer of a membrane i s dense or f i n e l y porous often depends on the method of membrane preparation. Skin layers deposited from solution or plasma onto a porous support (8,10), especially those intended f o r gas separations, are probably homogeneous. Skin layers r e s u l t i n g from phase inversion processes (9,11,12) may be f i n e l y porous or dense, depending upon casting solution composition, casting conditions and post-formation thermal or chemical treatments. The presence or absence of pores, e s p e c i a l l y below the l e v e l of electron microscope resolution, continues to be a point of debate. Regardless of o r i g i n or structure, i t i s t h i s skin which exerts the greatest influence over the permeation properties of the membrane. The skin and the porous support layer may be comprised of the same material and produced i n one casting operation. In t h i s case, the asymmetric membrane may be referred to as an integrally-skinned microgel or an integrallyskinned ultragel depending on the size of the pores i n the support layer. A l t e r n a t i v e l y , the skin and the support may be comprised of d i f f e r e n t materials and produced i n separate steps. These nonintegrally-skinned structures are referred to as composite membranes (8) or dynamic membranes (10) depending on the method by which and the time at which the skin i s deposited on the support layer. Porous and asymmetric membranes are t y p i c a l l y produced v i a a phase inversion process (9,11-13). The asymmetry and porosity (that i s , pore size, pore s%ze d i s t r i b u t i o n and pore volume) of the f i n a l s o l i d i f i e d g e l are d i r e c t l y related to the structure of the metastable " s o l u t i o n " immediately p r i o r to gelation (6,9,Π-13). Often a post-formation treatment i s used to improve membrane performance c h a r a c t e r i s t i c s by p h y s i c a l l y or chemically modifying the membrane (6,9,11). One example of a physical modification i s the thermal annealing of c e l l u l o s e acetate membranes. The e f f e c t of t h i s thermal treatment on the physical structure of the membrane i s a l t e r n a t i v e l y viewed as increasing the c r y s t a l l i n i t y or decreasing the porosity. Perhaps a combination of these effects occurs. In either case, the result i s a decrease i n flux and an increase i n solute r e j e c t i o n with increasing temperature and duration of the thermal treatment. An example of a chemical modification i s the sulfonation of hydrophobic membrane materials to increase h y d r o p h i l i c i t y and thus increase water permeability (14). Usually these post-formation treatments are conducted p r i o r to mounting the membrane i n the membrane module. However, Grodzinsky and co-workers (15) have reported that the physical structure of a variety of charged polymeric membranes can be


5


6


temporarily and controllably altered a f t e r mounting the membrane i n the module. By imposing an e l e c t r i c f i e l d on the membrane, the authors found that i t i s possible to modify the permeation rates of both charged and neutral species. Presumably the applied e l e c t r i c f i e l d causes the transport corridors to change i n s i z e due to e l e c t r o s t a t i c interactions within the membrane matrix. When discussing membrane preparation, not only must the physical structure be considered, but one must also consider the membrane form or shape. In an e f f o r t to combat concentration p o l a r i z a t i o n and membrane fouling and to maximize the membrane surface area per unit module volume, membranes are produced i n the form of f l a t sheets (used either i n plate-and-frame or s p i r a l wound modules), supported and unsupported tubes, and hollow f i b e r s . Although much of the technology associated with membrane development and membrane production i s closely guarded as proprietary information, some of the d e t a i l s are beginning to appear i n the l i t e r a t u r e (6^9-13,26-20). Membrane Characterization and Evaluation. Following the s e l e c t i o n , characterization and evaluation of the material and the preparation of the membrane, the next steps are to characterize the membrane i n terms of physical properties and to evaluate the membrane i n terms of performance. In t h i s context, membrane c h a r a c t e r i z a t i o n refers to obtaining information about the physical structure of the membrane phase and c h a r a c t e r i s t i c s of the bulk polymer. Pusch and Walch (21) have c r i t i c a l l y reviewed the commonly used techniques to investigate porous structure (pore s i z e , pore s i z e d i s t r i b u t i o n , pore volume, anisotropy, etc.) and molecular structure (rubbery versus glassy, c r y s t a l l i n i t y , chain segment arrangement and mobility, e t c . ) . Their l i s t of methods f o r characterizing the porous structure includes microscopy, low-angle X-ray scattering, bubble point determination, mercury i n t r u s i o n , f l u i d permeation measurements and molecular weight cut-off determinations. For characterizing the bulk polymer properties, Pusch and Walch l i s t low-energy neutron scattering and d i f f r a c t i o n , X-ray scattering and d i f f r a c t i o n , infra-red absorption, nuclear magnetic resonance, thermomechanical analysis, d i f f e r e n t i a l thermoanalysis, d i f f e r e n t i a l scanning calorimetry, as w e l l as gas permeation and sorption methods. For membranes that are used i n an aqueous environment, the structure of the water within the membrane can be characterized v i a water sorption isotherms, nuclear magnetic resonance, infra-red absorption and by determining the heat capacity of the water within the membrane. These water-structure studies provide insight into the porous nature of the membrane and into the strength of the water-membrane i n t e r a c t i o n s . In r e l a t i n g membrane structure to permeability, Pusch and Walch (21) make extensive use of an excellent c o l l e c t i o n of electron photomicrographs. In t h i s volume Kyu (22) c r i t i c a l l y evaluates the techniques available f o r characterizing ion-exchange membranes i n terms of polymer structure and f u n c t i o n a l i t y . Frequently, only the properties of the skin layer of asymmetric membranes are of importance, because the skin layer i s considered to play a dominant r o l e i n determining membrane



1. LLOYD


7

performance. In t h i s volume, Smolders and Vugteveen (23) as well as Zeman and Tkacik (24) present a number of techniques f o r characterizing the porous structure of the skin layer of u l t r a f i l t r a t i o n membranes. Membrane evaluation may r e f e r to determining the a b i l i t y of the membrane to achieve i t s desired function; f o r example, to separate the components of a mixture. A l t e r n a t i v e l y , membrane evaluation may r e f e r to i d e n t i f y i n g a suitable application f o r an e x i s t i n g membrane. A survey of the l i t e r a t u r e reveals that there are few standard procedures f o r evaluating membrane performance. This lack of uniformity i n evaluation procedures, plus the i n a b i l i t y to confidently predict performance under a s p e c i f i c set of operating conditions on the basis of experiments conducted under d i f f e r e n t conditions, r e s t r i c t s comparisons of available membranes and performance data. Therefore, membrane performance must be evaluated f o r each application under the conditions of use. Depending on the s p e c i f i c membrane a p p l i c a t i o n , membrane evaluation might include measurements of f l u x , r e j e c t i o n or separation f a c t o r , ion-exchange capacity, membrane f u n c t i o n a l i t y or a c t i v i t y , membrane degradation (physical and chemical), membrane f o u l i n g , membrane compaction, release rates and b a r r i e r properties. These membrane properties are d i r e c t l y related to the choice of membrane material, the membrane preparation procedures and the f i n a l application conditions. Overview of This Volume The various chapters i n t h i s book address the topics of material s e l e c t i o n , characterization and evaluation as w e l l as membrane preparation, characterization and evaluation. At the expense of neglecting membranes f o r applications such as controlled release and impermeable b a r r i e r s , t h i s book focuses on synthetic membranes for separation processes as w e l l as active membranes and conductive membranes. While many of the concepts developed herein can be extrapolated to other applications, the interested reader i s referred elsewhere f o r s p e c i f i c d e t a i l s (for example, controlled release (25-30), coating and packaging b a r r i e r s (31-33), contact lenses (34,35), d e v o l a t i l i z a t i o n (36), ion-selective membrane electrodes (37-42) and membranes i n electrochemical power sources (43)). Membrane Material Selection and Evaluation. Chern et a l . (3) and Lloyd and Meluch (4) propose possible approaches to the problem of selecting the appropriate membrane material to separate gas mixtures and l i q u i d mixtures, respectively. Chern et a l . discuss membrane permeation and separation i n terms of thermodynamic s o l u b i l i t y and k i n e t i c mobility of the permeant gases i n the polymer phase and how each of these i s influenced by the molecular structure of the glassy polymeric membrane material. Correlations that apply i n the absence of strong p l a s t i c i z a t i o n or swelling interactions between the penetrant gases and the membrane are discussed i n terms of inherent polymer and penetrant properties to r a t i o n a l i z e the s o l u b i l i t y and mobility contributions to the permselection process. Advantages of focusing on "mobility



8


s e l e c t i o n " rather than " s o l u b i l i t y s e l e c t i o n " are emphasized by these authors. They point out how the chemical nature of the polymer, and thus polymer segmental motion and packing, can be used to select membrane materials f o r gas mixture separations. The authors suggest that a loosely packed glassy polymer with s u f f i c i e n t cohesive energy and a r i g i d p l a s t i c i z a t i o n - r e s i s t a n t backbone i s i d e a l l y suited to achieve both high flux and high s e l e c t i v i t y . S t r u c t u r a l l y modified p o l y ( a r y l ethers), polyimides, polyamides, polycarbonates, polyesters and polyurethanes appear to be l i k e l y candidates to a t t a i n these goals. Chern et a l . (3) suggest that modification of the above simple concepts are necessary when one or more of the permeants p l a s t i c i z e s the membrane material. The authors discuss these and other "environmental" considerations and the importance of evaluating the membranes under f i n a l useage conditions. In contrast to the separation of r e l a t i v e l y inert gas mixtures, the separation of l i q u i d mixtures i s strongly influenced by both the e f f e c t i v e size of the permeants and the chemical interactions which occur within the membrane phase. That i s , both thermodynamic s o l u b i l i t y or p a r t i t i o n i n g and k i n e t i c mobility or transport are d i r e c t l y influenced by physical as w e l l as chemical factors. Lloyd and Meluch (4) discuss membrane material s e l e c t i o n i n terms of the e f f e c t i v e size of the permeants r e l a t i v e to each ether and r e l a t i v e to the s i z e of the membrane transport corridor. The transport corridor size i s subsequently related to polymer molecular structure and membrane preparation procedures. In t h i s sense, the discussion c l o s e l y resembles that of Chern et a l . (3). However, Lloyd and Meluch add the often more s i g n i f i c a n t factor of chemical interactions near and within the membrane phase. The authors demonstrate the p o s s i b i l i t y of selecting an appropriate membrane material on the basis of the dispersive, polar and hydrogen-bonding character of both the available membrane materials and the solution components to be separated. Having selected an appropriate membrane material on t h i s thermodynamic basis, membrane performance i s ultimately related to the k i n e t i c s of transport through the physical structure of the membrane, which i n turn i s related to membrane preparation procedures. Lloyd and Meluch also discuss methods f o r characterizing and evaluating membrane materials p r i o r to membrane formation. Hoehn (5) reviews the development of aromatic polyamide (aramid), polyamide-hydrazide, polyhydrazide and polyimide membranes f o r l i q u i d and gas phase separations. Hoehn discusses the structure-property relationships of these membranes i n terms of polymer chemical composition (primary or Level I c h a r a c t e r i s t i c s ) , i n terms of s t e r i c e f f e c t s r e s u l t i n g from chain packing and chain f l e x i b i l i t y (secondary or Level II) and i n terms of membrane morphology, such as asymmetry, anisotropy and skinning (Levels I I I and IV). I t i s Level I that Lloyd and Meluch (4) use to e s t a b l i s h t h e i r material s e l e c t i o n index on the basis of dispersive, polar and hydrogen bonding considerations. Structure Level II i s beyond the l e v e l of detection v i a electron microscopy, and determines the size of the transport corridor i n dense membranes and the skin layer of asymmetric membranes. As such, Hoehn's Structure Level II can be related to the corridor size parameter i d e n t i f i e d by Lloyd



1. LLOYD


9

and Meluch (4) and to the mobility s e l e c t i v i t y discussed by Chern et a l . (3). Levels I and II are i n t r i n s i c properties of the polymer; Levels III and IV represent physical structure c h a r a c t e r i s t i c s on a microscopic l e v e l and are dependent upon membrane preparation procedures for i n t e g r a l l y - and nonintegrallyskinned membranes, respectively. As such, Structure Level III relates to the chapters by Finken (6), Resting (9), Strathmann (11), Kamide and Manabe (12), Hiatt et a l . (13) and Cabasso (44), while Level IV relates to the chapters by Cadotte (8), Spencer (10), Cabasso (44) and Lee et a l . (45). Ward, Feldhoff and K l e i n (7) demonstrate that i n addition to some demanding separation performance requirements (which are primarily attributable to membrane preparation procedures and transport mechanisms), therapeutic applications such as hemodialysis, hemofiltration and plasmapheresis impose some unique material s e l e c t i o n r e s t r i c t i o n s . S p e c i f i c a l l y , one must consider protein adsorption, biocompatibility (that i s , thrombogenicity, complement a c t i v a t i o n and t o x i c i t y ) and the s t e r i l i z a b i l i t y of the membrane. In selecting a membrane material for any of these therapeutic l i q u i d separations, the biocompatibility and s t e r i l i z a b i l i t y r e s t r i c t i o n s must be considered i n addition to the guidelines established by Lloyd and Meluch (4). Thrombogenicity and complement a c t i v a t i o n are d i r e c t l y related to the physicochemical interactions between the polymer and the various components of blood. T o x i c i t y i s related to the chemical f u n c t i o n a l i t y of the polymer or any r e s i d u a l low molecular weight compounds i n the polymer. S t e r i l i z a b i l i t y i s a function of both the thermal properties of the polymer, such as glass t r a n s i t i o n temperature and c r y s t a l l i n e melting temperature, and the chemical s t a b i l i t y of the polymer to ethylene oxide exposure and gamma irradiation. Additional requirements of o p t i c a l c l a r i t y and biodegradability must also be taken into account i n selecting membrane materials for other applications such as contact lenses and controlled release devices, respectively. Ward et a l . review several membrane materials currently i n use for therapeutic l i q u i d separations and discuss t h e i r advantages and disadvantages for s p e c i f i c applications. The authors also point out that for continued growth i n the use of membranes for therapeutic l i q u i d separations a better understanding of blood-membrane physicochemical interactions i s required. This knowledge would be of great value i n predicting biocompatibility, and thus i n s e l e c t i n g membrane materials. The authors also point out the need to refine membrane preparation procedures to more accurately control pore size and to produce membranes of narrow pore size d i s t r i b u t i o n . In the biomedical applications outlined by Ward et a l . C7), more so than i n any other separation application of synthetic polymeric membranes, the goal i s to mimic natural membranes. S i m i l a r l y , the development of l i q u i d membranes and biofunctional membranes represent attempts by man to imitate nature. Liquid membranes were f i r s t proposed for l i q u i d separation applications by L i (46-48). These l i q u i d membranes were comprised of a t h i n l i q u i d f i l m s t a b i l i z e d by a surfactant i n an emulsion-type mixture. While these membranes never attained widespread commercial success, the concept did lead to immobilized or supported liquid membranes. In



10


these membranes, the transport occurs i n a stationary l i q u i d phase which has been fixed within a porous membrane f o r purposes of s t a b i l i t y . A v a r i e t y of techniques have been used to support the l i q u i d membrane phase including immobilization within porous inert "membranes" (for example, common f i l t e r paper, n i t r o c e l l u l o s e f i l t e r s , c e l l u l o s e acetate films and hollow f i b e r s , track-etched polycarbonate membranes, polytetrafluoroethylene membranes, p o l y v i n y l chloride membranes and polypropylene membranes such as those discussed by Hiatt et a l . (13)) as w e l l as within i o n exchange membranes and between two r e l a t i v e l y dense films i n the form of a "sandwich." In any of these forms, the membranes can be used f o r either gas or l i q u i d mixture separations. Way, Noble and Bateman (49) review the h i s t o r i c a l development of immobilized l i q u i d membranes and propose a number of s t r u c t u r a l and chemical guidelines f o r the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (50 volume % ) , mean pore size (

Membrane Materials Science - ACS Symposium Series (ACS

Recommend Documents