POLY ELECTR OlY TE COMPLEXES ALAN S. MICHAELS
lonically bonded polymeric network-structures, readily synthesized f r o m linear polyelectrobtes, Possess unusual physical and chemical properties not found in conventional polymers.
These new
materials are finding equally unusual practical applications and have generated much interest in widely diverse scient&
and engineering jields
olymer science and technology as we know it today P has been focused primarily upon the synthesis, structure-properties relationships, and applications of secondary valence-bonded linear macromolecular systems (thermoplastics) and of covalently bonded, network systems (thermosetting or cross-linked plastics). Polymeric structures within which the primary macromolecular elements are held together by coulombic or ionic linkages have received relatively little attention by the materials specialists. Curiously, however, interest in ionically bound organic polymeric structures has developed to a high degree among biochemists and biophysicists, in light of the fact that the majority of biological polymers appear to be ionically associated in living systems. The extraordinary complexity and instability of biological systems appear to have discouraged the applied polymer scientist and engineer from considering synthetic, ionically cross-linked polymeric materials as products of practical and commercial utility. As will be shown below, polymeric structures of this sort can be readily synthesized and are found to possess a fascinating array of physical, chemical, and 32
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
electrical properties, far different from those of conventional polymers, which can be put to use in a host of unusual and practical applications. Among the earliest definitive studies of ionically interacting organic polymeric substances were those of Bungenberg de Jong and coworkers at the University of Leyden in the 1930’s and ’40’s ( 7 ) . Out of their interest in biocolloidal systems, they exhaustively examined the interactions between several natural hydrophilic (watersoluble) polymers in aqueous media. They found that certain water-soluble polymers containing ionizable functions as a part of their structure (so-called polyelectrolytes) were capable of coreacting with one another in water solution to form “complex coacervates”separate (usually liquid) phases containing less water and more polymer than the overlying solution. Generally, coacervation occurred when one of the polymer components was negatively charged in solution-Le., a polyanion-and the other, positively charged-i.e., a polycation. The nature and extent of such reactions were found to be sensitive to such variables as temperature, pH, and the ionic strength of the solution phase. A typical example of such a “coacervating” system is acid gelatin (a polycation) and gum arabic (a polyanion). I t is of interest that this particular system has been utilized by B. K. Green and his associates at National Cash Register Co. in the development of “microencapsulated” organic liquids, and in coatings for NCR’s “carbonless” carbon paper system. Interactions between various synthetic and natural polyelectrolytes with opposite signs of charge have received close scrutiny since the investigations of the Leyden group (2). However, the major emphasis has been on the solution chemistry of the reactions and not on the properties of the reaction products. Most of the polyelectrolytes examined thus far have been weakly acidic polyanions (algin, polyacrylate salts) and weakly basic polyanions (polyvinyl amine, polyethyleneimene) .
Polysalt structures are hard, amorphous, transparent, and homogeneous These materials yield gellike or quasiliquid coacervates of indefinite chemical composition and high water content with little demonstrable utility. The first reported study of an interaction between a strongly acidic polyanion [sodium poly(styrene sulfonate)] and strongly basic polycation [poly(vinyl methyl pyridinium) chloride] in aqueous solution was by Fuoss and Sad& in 1949 (3). They found that the interaction between these two polyelectrolytes yielded a colloidal precipitate rather than a gelatinous coacervate, and that the reaction occurred rapidly to yield a product containing essentially stoichiometric equivalents of the component polyions. In 1961, Michaels and Miekka (5) reported on the interaction between sodium poly(styrene sulfonate) (NaSS) and poly(viny1 benzyl trimethyl ammonium) chloride (VBTAC), and on the compcsition and properties of the resulting precipitate or “polysalt.” When two relatively high molecular weight polymers (mol. wt. > 150,000) were reacted in dilute aqueous solution, a precipitate was formed which contained almost exactly stoichiometric proportions of the component polyions, and furthermore contained virtually none of the counterions initially associated with the individual polymers--i.e., Na+ and C1-. This precipitate was found to be infusible and insoluble in all common solvents. The composition of the polysalt was independent of the relative proportions in which the component polymers were mixed, and of the order or rate of addition. A more recent study of the reaction by conductometric techniques has confirmed these observations (6). A satisfactory explanation for the high specificity and completeness of the polycation-polyanion interaction is not yet available. I t is clear, however, that a nearly perfectly stoichiometric, microion-free complex of two species of high molecular weight linear macromolecules is a statistically improbable structure within which the individual molecules should exist in highly strained conformations. Furthermore, the driving force for the reaction is evidently not energetic in origin, because the process appears to be nearly athermal. The most l i i d y explanation for the phenomenon lies in the ‘‘escaping tendency” of the microions associated with each separate polyion. When a strongly ionized polyelectrolyte is dissolved in water, the individual ions hydrate and diffuse apart from one another, attempting to distribute themselves homogeneously throughout the solvent. They are constrained from doing so. however, by the covalent bonds holding the polymer chain together. As a consequence, the chain, with its pendant ionic groups of common charge, assumes a highly extended statistically improbable configuration, and the microcounterions are electrostatically constrained to occupy a small volume of the solvent adjacent to the chain. This constrainment is clearly evidenced by the electrical conductance of a polyelectrolyte solution 34
INDUSTRIAL AND ENGINEERING CHEMISTRY
-30
. Q 1
Figure 3. Solvant-compositionphase diogram for “neutral” VBTASSpolysdt in the t m r y q s t m , NaBr-attone-water at 25O C.
i
\ -I
Figure 4. Tha
variation of thc dielectric constants of “ncutraP’ VBTASSpolysalt with h f r e q u m y and thc NoBr conUnt
S.Michaels is President of Amicon Gorp. of Cambridge, Mass., and is part-time Professor of ‘Chmical Engineering at MIT. He snved as Chairman of this year’s I@EC Symposium on Nucleation Phmomcna and i u s been actiue in membrane &chnologv for a numbn of years. AUTHOR ALan
which is far lower than that of a simple electrolyte solution of corresponding ionic concentration. Thus, when two highly extended oppositely charged polyelectrolyte molecules meet in solution, the interaction of any pair of polyion sites releases a microanion and a microcation which are then free to diffuse into the body of the solvent. This reaction will rapidly continue from site to site between the polymer chains until all microions are released, provided that the entropy increase upon microion liberation exceeds any entropy decrease upon collapse and condensation of the polyion-polyion pair. Strong suggestion that the reaction is indeed entropically driven is afforded by the observation that the polyion complex swells markedly in concentrated aqueous electrolyte solutions and may actually dissolve in such solutions at elevated temperatures. O n the other hand, the entropy argument is evidently not a completely adequate explanation in that the reaction of the hydrogen and hydroxyl forms of polyanion and polycation yields neither a stoichiometric nor a complete reaction product (6). It thus appears that the microions themselves in some fashion moderate the polyion interaction and allow rearrangements of chain segments to take place soon after the chains come into contact. The molecular architecture of the stoichiometric polyion complex has not yet been clarified. At first it was suggested (5) that, because the component polyelectrolyte molecules before reaction are in a highly extended chain conformation, they may zipper together to form a ladder polymer network as illustrated in Figure 1. However, recent studies of the creep and stress relaxation properties of the complex as a function of temperature and water and salt content have yielded results which do not support this model. Rather, it appears that the complex contains completely random, ionically interlinked chains of the component polyions and resembles (Figure 2), in some respects, a scrambled salt or glassy ionic solid such as might be produced by rapidly quenching a molten electrolyte. According to this model, the ionic sites within the network are more or less homogeneously distributed in space, as are ions in concentrated electrolyte solutions, but are sufficiently close together to interact strongly with one another. Because of the lack of directionality of ionic bonds, coupled with the restraints placed upon individual ion movement by their attachment to polymer chain backbones, the complex would be expected to (and indeed does) exhibit physical properties transitional between those of ionic crystalline solids and amorphous organic polymers. This investigation might have terminated with mere observations of the reaction chemistry, were it not for the important concurrent discovery that the intractable polysalt precipitate could be dissolved (or, alternatively, the component polyelectrolytes codissolved without reaction) in selected ternary solvent mixtures comprising water, a water-miscible organic solvent-e.g., acetone-
and a strongly ionized simple electrolyte-e.g., NaBr. The solubilizing activity of these so-called shielding solvents is believed to be caused by loosening of the polyion linkages of the complex by the ions of the electrolyte and the interaction of the organic solvent component with the organophilic backbone of the polymer chain. As Figure 3 shows, there is a small region in the solvent composition field where the component polyions remain in solution to yield a homogeneous, transparent, viscous sirup. These solutions can be used for the preparation of homogeneous films, fibers, coatings, and formed objects from the polysalt, because by suitable alteration of the solution composition (by drying and/or washing), a solid structure containing only the polymer and water can be produced. Furthermore, by varying the relative proportions of the component polyelectrolytes in solution, it is possible to produce nonstoichiometric complex structures exhibiting either cation exchange or anion exchange capacity. Properties of Polyion Complexes
Polysalt structures produced in this fashion are homogeneous, transparent, amorphous resins, extremely hard and brittle when dry, and either leathery or rubberlike when water-wet. Their water absorptivity is extremely sensitive to their polyion composition : The stoichiometric or “neutral” complex has relatively low water absorption (typically 30% water by weight when saturated), while polyanion- or polycation-rich complexes (depending on their excess polyion content and the nature of the associated counterions) may absorb up to ten times their dry weight of water. All resin types show extraordinarily specific absorptivity for water relative to most other common liquids-e.g., alcohols, glycols, ketones, etc. I n this respect, they more closely resemble ionic solidse.g., salts, acids, bases-than organic polymers. The maximum water absorption of the neutral complex corresponds to fewer than three water molecules per ion in the structure, indicating that virtually all the water present is probably ion hydration water. When immersed in aqueous electrolyte solutions of increasing concentration, the neutral complex becomes progressively more rubberlike and swells, absorbing both electrolyte and water. The capacity of a given electrolyte to plasticize the resin in this fashion is dependent upon the capacity of one or both ions of the electrolyte to associate with the ionic groups in the polymer; for example, calcium chloride and sodium bromide are more active swelling agents than sodium chloride. Nonstoichiometric complexes, on the other hand, behave in electrolyte solutions much as conventional ion exchange resins : they exchange counterions readily with those in solution, shrink in dilute electrolyte solutions by Donnan exclusion, and reswell in concentrated electrolytes, as do their “neutral” counterparts. The water- and electrolyte-plasticized complex can be elastically and plastically deformed, and, under suitable conditions, cold-drawn and oriented, as conventional VOL. 5 7
NO. 1 0 O C T O B E R 1 9 6 5
35
I
5.0
2.0
-3
1.0
RWW
Figure 6. The
m1 VBTA-
Figwe 5. Vmiotion of the loss factors of “ncutrol” VBTA-SS polysalt m’th the frequency and the NOBr contcnt
omintion of the loss tangents of “ncutroP’ SS polysalt m~thfrequflcrand NOBr COW
thermoplastic resins; after such manipulation, the structure can be frozen by washing to remove the electrolyte. Monofilaments prepared in this fashion appear to have reasonable strength when wet, but are brittle when dry. The electrical properties of polyion complexes are anomalous (4). The stoichiometric complex, when completely free of extraneous electrolyte, exhibits a high d.c. resistivity (ca. 10’O ohm cm.); a low kequency (100 c.P.s.) dielectric constant of about 50 when watersaturated and about 5 when dry; and a high frequency (10s c.P.s.) dielectric constant of 2 to 3 in the wet and dry states. When doped with simple electrolytes-e.g., NaBr-however, the low frequency dielectric constant of the complex rises to high values (in excess of lOO,OOO), as dws the loss factor, and both parameters are extremely sensitive to water content, salt content, and frequency over some six decades. This is illustrated in Figures 4 to 6. Nonstoichiometric complexes free from extraneous electrolyte behave similarly; however, the magnitudes of the dielectric constant and Iw factor and their frequency dependency are highly sensitive to the nature of the microcounterion present in the structure. For example, hydrogen and sodium forms of plyanion-rich complex exhibit much larger dielectric constants, and much stronger frequency variation, than does the calcium form of the same resin (7). Curiously, even when doped with electrolytes to concentration levels approaching 1.0 molar, the complexes exhibit low d.c. conductivities-ca. to 10-1 mho cm.-despite their high a.c. loss conductivities.
This unusual behavior has been explained (6) by postulating that microions sorbed into the complex become isolated within microscopic noninterconnecting domains where they are surrounded by polyion segments of opposite charge. Ions within such domains are electrically polarizable and (if adequately mobile) can give rise to the enormous dielectric constants which are observed. If the complexes are equilibrated with highly concentrated electrolyte solutions where they become somewhat swollen, their anomalous dielectric properties vanish and they become excellent ionic d.c. conductors. Typically, their conductivities under these conditions are of the order of 1.5 to 2.5 times higher than those of the solution with which they are in contact--evidently because of the higher ion population density within the resin relative to the external solution. Thin films of polyion complex have also shown unusual transport characteristics when used as dialysis membranes (5). Simple electrolytes such as NaCl diffuse through the membrane rapidly, whereas higher molecular weight solutes--e.g., methylene blue hydrochloride are nearly completely prevented from doing so. In summary, the distinguishing and unique properties of polyion complexes which basic studies have brought to light over the past five years are as follows: Physicochemical properties Insolubility in common solvents Infusibility Plasticizability by water and electrolytes Highly specific, but limited water absorptivity
36
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Transparency of flexible solids (when wet) Selectivity of ion sorption and ion exchange properties Electrical properties High dielectric constant and loss factor when wet and ion-doped ; broad dielectric dispersion Dielectric properties very sensitive to moisture and ion content Low d.c. conductance up to rather high electrolytedopant levels Extremely high d.c. conductance in contact with concentrated aqueous electrolytes Transport properties Highly permeable to water Highly permeable to electrolytes and other watersoluble microsolutes Impermeable to macrosolutes Applications and Uses
During the past two years, much effort has been expended in determining whether the properties of polyion complexes enumerated above can be profitably utilized in practical devices and materials, and for the solution of important materials and process problems. The results of this development effort have been both fruitful and exciting and merit description here. Dialysis and Ultrafiltration Membranes. Neutral polyion complex membranes have been evaluated as reverse osmosis membranes for the desalination of sea water (8) and, more recently, as dialysis and ultrafiltration membranes for the purification and/or concen-
TABLE I.
Film Type I-YLL
I-YL I-YM I-YH
I-XL I-XM I-XH UM-1 UM-2 UM-3 Cellophane (untreated)d Cellulose acetatee a
Charge Neutral Neutral Neutral Neutral Anionic Anionic Anionic Neutral Neutral Anionic
tration of aqueous solutions containing colloids and macrosolutes as well as microsolutes. The most striking and important property of these materials for these applications is their extraordinarily high permeability to water. A 1-mil thick film of neutral complex, when subjected to 3.5% NaCl solution at 1500 p.s.i., will transmit roughly 8 gal. of water per day per sq. ft. of membrane area; this is approximately 15 times higher than the specific water permeability of cellulose diacetate-the polymer now receiving greatest attention for use in reverse osmosis membranes. The salt retention characteristics of polyion complex membranes are, however, inferior to those of cellulose acetate-of the order of 50 to boy0 retention us. 97 to 98% for the latter polymer. Techniques for consolidating or tightening the polyion complex structure are now being evaluated in an effort to improve salt retention characteristics without sacrifice in water permeability (8). When compared with regenerated cellulose (cellophane) dialysis/ultrafiltration membranes, polyion complex membranes again show decidedly higher (10- to 50-fold) water permeabilities, and equal or better retention characteristics for solutes of molecular weight 500 or greater. Recently, techniques have been developed for accurately controlling the submicroscopic network structure of the complex film, with attendant precise control over water permeability and solute cutoff characteristics. Ultrafiltration membranes showing water permeabilities of 80 to 100 gal./day/sq. ft. at 100 p.s.i. pressure, yet with rejection efficiencies for raffinose in excess of 90%) have been produced in this fashion. At the present
DIAPLEX (POLYION COMPLEX) MEMBRANES TRANSPORT CHARACTERISTICS
Water Content Gm ./Gm Resin 0.4 1.3 2.2 3.5 1.3 2.8 6.5 8.0 8.0 c
*..
.
Water Permeability at 100 P.s.i.:(Cc.) ( M i l ) , NaCl Sq. Cm. Sec. X 106 3.0 50 340 5.5 110.0 460.0 100Ob 330b 125b
5 0 0 0 0 0 0 1 5 25
7.0
0
810
0.7
I
% Solute Retention Eflciency at 100 P.s.~. I Ruchsin Carbowax
Urea
Sucrose
Raflnose
Red
1500
6000
BSAa
5
50 10 0 0 5 0 0 15 60 100
80
... ...
... ...
0
100 98 85
0 20 0 0
75 99 50 18
... ...
20 90 100
...
50
100 100
100 100
100 100 100
100 100 100 100 100 100 100 100 100 100
...
*..
...
100
100
100
100
100
-~ 0 0 0 0 0 0
1 5 * .
.
95 ... I -
99
30
I
100
...
... ...
... ...
... *.. ...
B S A = bovine serum albumin. Anisotropic membrane-barrier thickness unknown; water frux in cc./sq. cm. sec. at 100 p.s.i. Membrane shows 700% retention of phenylalanine at solution pH's greater than 7, 25% retention at isoelectric point. Henderson, W. E., and Sliepceuich, C. M., Chem. Eng. Process Symposium, Ser. No. 24, Vol. 55, 7959. Sourirajan, S., Ind. Eng. Chem. Fundamentals, Vol. 2, No. 7, February 1963. VOL. 5 7
NO.
10
OCTOBER
1965
37
Nonstoichiometric complexes behave in solution much as TABLE II. MOISTURE-VAPOR PERMEABILITIES OF “IOPLEX” (POLYION COMPLEX)-LOADED PLASTIC FILMS Gm./l00 sq. I“. Day/Mil MVP, ai 3 5 0 c. (RH):100% 47
Thermoplastic urethane (E& 5703,7595; 5740,25%))-
108 228
dvmt Cast
Thamoplastie urethane (EBtane 5703, 65%; 5740, 35%)-
*
d Figwg 7. Ekcrnn micrograph of microporous polyion compbx powder trrcd i o impart m’stllrc vapor pmncnbility i o plastic film nd &‘q5
time, these membranes are being used for the ultrafiltration of biological fluids such as urine and blood (where, because of their high permeability and resistance to fouling, they outperform conventional ultrafilters by a wide margin), and for the purification of wasteand food-processing streams. Typical performance data for some of these membranes are shown in Table I. Controlled microstructure polyion complex membranes also exhibit far higher dialysis coefficients for microsolutes-e.g., simple electrolytes, urea, dextrose, etc.-than do conventional dialysis membranes such as cellophane. In general, relative to cellophane or corresponding thickness, diffusion rates are higher by a factor of 5 to 10. Considerable interest has consequently developed in the use of these membranes in hemodialyzers (artificial kidneys) and hemooxygenators (aitificial lungs), whose efficiency is limited by existing membrane materials which provide the major resistance to mass transfer. Battery Separators a n d Fuel Cell Membranes. Polyion complexes, either in the form of unsupported film or as a saturant within the pores of a fibrous sheet, serve as excellent separators in primary and secondary batteries, and as solid electrolyte materials in certain fuel cells. The resins are stable in concentrated acids (sulfuric, phosphoric) and alkalies (KOH, NaOH) up to moderate temperatures, and (as noted above) exhibit low d.c. resistivities. Inasmuch as the complexes are homogeneous nonporous gels, they are effective bamers to gas transport between electrode compartments and discourage treeing of electrcde metals. Moreover, their extremely fine-textured microstructure retards diffusive and electrical transport of all but the smallest ionic 38
INDUSTRIAL AND ENGINEERING CHEMISTRY
rolvcnt cast
m
1.5
53 164
20
1.5
30
1.5 1.5
PVC plaatiaol (B.P.Goodrich Gton121); 117PHRWP (thee-rOU milled and I d at 165-C.)
1.5
10
4.7 4.6 4.4
266
416 20.4
59.4 105.5
species (usually hydrogen or hydroxyl ions) in the electrolyte, thereby minimizing interelectrode transfer of undesirable components. Solutions of the resins (containing appropriate electrolytes) can also be applied as coatings directly to the electrode surfaces; this procedure eliminates the need for a separator or membrane per se, and facilitates cell assembly. Compact batteries of fuel cells can be made in this fashion. Moisture-Breathable Plastic Composites. The unusually high moisture permeability of the polyion complexes, coupled with their water insolubility, has prompted their evaluation as additives to conventional film-forming and -coating resins for the purpose of developing plastic products with high moisture-vapor transmissivity (MVT). Results of this evaluation have been most encouraging. A complex in the form of fine powder (under 10 microns), when physically blended with such film-forming resins as solution grade vinyls, thermoplastic urethane elastomers, vinyl plastisols, and acrylics, yields composites which exhibit MVT values three to five times higher than those of the matrix resin, at weight loadings of 20% (Table 11). Effects of powder incorporation on the important physical properties of the matrix resin are minor. Recently, formulation techniques have been developed for producing a microporous powder of extremely low bulk density (Figure 7); this material is about twice as effective in raising MVT levels as the nonporous powder, giving a fivefold improvement at 15% loading, and over tenfold at 20%. The introduction of polyion complex as a pigmentliie tiller into plastics has thus opened the door to the manufacture of low cost breathable sheet goods for use as upholstery, wearing apparel, leather substitutes in
though they were conventional ion exchange resins footwear, surgical dressings, etc. As an additive to protective coating formulations, it provides a selectively moisture-permeable film which prevents blistering and adhesive failure on hydrophilic or water-transmissive substrates like wood and masonry. Breathable rubber or urethane elastomer gloves can be made by incorporating the complex into dip-coating latices or solutions. Curiously, the air permeability of plastics is reduced by polyion complex incorporation, despite the marked increase in MVT; as a consequence, complex-containing elastomers are now being evaluated as components of space suit laminates, where low air leakage and high moisture leakage for temperature control are desired. Certain other unusual properties of polyion complexcontaining composites have suggested equally unusual applications for these products: Because of their lossy dielectric characteristics, the complexes generate much beat when irradiated by a high radio frequency field. Thus, incorporation of a small amount af particulate complex in a low power factor polymer makes it possible to heat the product rapidly by dielectric means. This property is being considered for use in the manufacture of multilayer laminates, where it is desired to heat cure the interlayer adhesive rapidly without beating and degrading the other elements of the laminate. Also, as might be expected, polyion complexes have extraordinary affinity for most acidic and basic textile dyes; the addition of pigmentary-grade complex to fiber spinning melts or dopes, for the purpose of imparting
ready dyeability to such refractory fiber-forming polymers as the polyolefins, is now under evaluation. Electrically Conductive a n d Antistatic Coatings. Electrolyte-doped polyion complex solutions have been developed which can be applied to most plastic, ceramic, and metallic surfaces and dried to yield transparent, nontacky, adherent, flexible films from 0.1 to 20 mils in thickness which exhibit extremely high d.c. and a.c. conductivities. On insulating surfaces, 1-mil coatings exhibit surface conductivities of the order of lo* to 10' ohms/sq. cm. and show relatively little conductance variation with temperature and relative humidity. These coatings can be used as an adhesive layer between glass or plastic sheets to produce a transparent, electrically conductive laminate for lighting or heating purposes. Formulations are also being evaluated as transparent conductive base coats for electrophotographic printing on glass and other insulating substrates. When filled with opacifying pigments such as clay, titania, or zinc oxide, these polyion complex formulations yield attractive electrically conductive coatings for paper; surface resistances of the order of 10' to 10' ohms/sq. cm., depending on coating weight, are obtained which vary by a factor of about ten over the 10 to 90% relative humidity range. These coatings are under evaluation for electrostatic photocopy applications. When filled with graphite or pigmentary-grade metals such as copper or silver, the formulations yield electronically conductive coatings with resistivities in the
IO'
2.2 x
IO'
10 '
Id
1.4 x IO'
ULl CMI(uIlMMwI IYIUIWlVN€HlStU Lll€Rl
Figurc 8. R~rirtrmccc-imped~c-c=p~'ta~c vanation with thc rclatiw humidity for a thinfrlm polyion complex
Figure 9. Rcsistmm-upm'tamc variation with salt comcntralion in a p o w solurionfor n thinjlm polyion comprlcx s w r V O L 5 7 NO. 1 0 O C T O B E R
1965
39
range of 1 to 10 ohms/sq. cm. Such coatings offer promise as easily applied, inexpensive base coats for electroplating plastics. When applied from solutions as extremely thin (0.1 mil) coatings or finishes on bulk plastics, plastic film, or textile fibers, the polyion complexes serve as highly effective antistatic agents even at low relative humidities. Their antistatic activity does not appear to correlate with their surface or bulk resistivity, but rather with the relative proportions of polycation and polyanion used to prepare the complex. The effectiveness of a given composition of complex also varies with the substrate to which it is applied. Because these coatings are water- and organic-solvent-insoluble, they are attractive prospects for application to textiles, optical components, and the like. Medical a n d Surgical Prosthetic Materials. Within the past nine months, considerable interest has developed in the possible use of polyion complexes as tissue substitutes in the human body. This interest has developed largely because of the close resemblance of the complex to connective tissue such as collagen and its extremely high permeability to water and most of the microsolutes found in body fluids. Transparent, flexible polyion complex film is under evaluation in animals as a corneal substitute. The material can be implanted in the eye without tissue reaction or rejection, vascularization of peripheral living tissue does not take place as it does with most other plastics, and the material retains its transparency over the lifetime of the animal. Furthermore, the complex can be readily sterilized by autoclaving without loss in clarity. The apparent compatibility of polyion complex with tissues of the eye has prompted its evaluation as a material for preparation of scleral contact lenses. I t is believed that a transparent, highly permeable material of this sort may be the answer to a permanent contact lens which will not have to be removed to allow proper irrigation of the corneal surface. More recently, polyion complex solutions have been under evaluation for potting or encapsulating aneurysms is small arteries to prevent further distension of the weakened arterial wall. When contacted with water or biological fluids, these solutions form rubbery gels which adhere reasonably well to living tissue. Surprisingly, the host animal accepts these solutions without evident injury; the resulting gel is neither irritating nor necrotic to surrounding tissue, and the capsule formed from the gel appears to remain unaffected by the organism. If clinical studies currently in progress continue to confirm these early observations, broader evaluation of polyion complexes as prosthetic materials-e.g., as synthetic cartilage, membrane substitutes, blood vessel prostheses, etc.-is contemplated. An additional and important observation which has been made is that certain hydrous polyion complex gels (specifically, those containing excess polycation), when applied as coatings to glass or plastics, provide a surface which, when contacted with blood, significantly retards coagulation or clotting. Such coatings are being 40
INDUSTRIAL A N D ENGINEERING CHEMISTRY
evaluated in various extracorporeal devices through which blood is circulated-e.g., heart-lung machinesand upon plastic and metallic components of prosthetic appliances such as heart valves where surface coagulation of blood is a recurrent problem. Teflon and Dacron fabric arterial grafts are also being impregnated with these nonthrombogenic gels to determine whether fibrin buildup and gradual narrowing of the lumen of the graft can be minimized or eliminated. Environmental Sensors and Chemical Detectors. The extreme sensitivity of the dielectric properties of polyion complexes to moisture and simple electrolytes has suggested their use as sensing elements in electronic detection devices. For example, a n extremely thin film of complex deposited on a metallic substrate and incompletely vacuum-metalized on its exposed surface forms a capacitor whose dielectric is accessible to its environment. Such a device functions as an extremely sensitive, rapid-responding humidity detector; typical variations in resistance, capacitance, and impedance with relative humidity are shown in Figure 8. Maximum sensitivity occurs in the low (0 to 30% RH) relative humidity range, where most conventional humidity sensors are least sensitive. The same device can function as a moisture detector for organic liquids. Neutral polyion complex, when immersed in dilute (0 to 100 p.p.m.) saline solutions, undergoes marked changes in a.c. capacitance and resistance with small variations in electrolyte concentration (Figure 9). I n addition, the frequency dependence of its capacitance and resistance is sensitive to the nature of the ions present in solution. I t is thus possible to produce detectors capable of analyzing aqueous solutions containing a variety of ionic species, with respect to both ion type and concentration. At the present time, nearly all the above-mentioned applications for these unusual materials are in relatively early stages of development, and their ultimate use potential is still far from being adequately assessed. I n many respects, our current state of knowledge of polyion complex structure-properties relationships and utility is reminiscent of that of semiconductor materials in the 1940’s; it is likely that, as they are subjected to more intensive and sophisticated scientific scrutiny, an ever broader spectrum of special properties and useful applications will evolve. Even today, however, these polymers show promise of occupying an important place in materials technology, performing functions which cannot properly be served by other organic or inorganic solid substances. REFERENCES (1) Bungenberg de Jong, H . G., “Colloid Science,” H. R. Kruyt, ed., Vol. 11, Chap. VIII, pp. 232-55; IX, pp. 259-330; X, pp. 335-429; XI, 433-80, Elsevier. Amsterdam. 1949. (2) Deuel, H., Solms, J., Dengler, A,, Helv. Chim. Acta 3 6 , 1671 (1953). (3) Fuoss, R . M., Sadek, H., Science 110, 552 (1949). (4) Michaels, A . S., Falkensrein, G. L., Schneider, X. S., J . Phys. Chem. 69 (1965). (5) Michaels, .4. S., Miekka, R. G., Ibid.,6 5 , 1765 (1961). (6) Michaels, A . S . , Mir, L., Schneider, K.S., X d . , 69 (1965). (7) Opp, D., “Dielectric Properties of Polyelectrolyte Complexes,” M. S . thesis in Chemical Engineering, Massachusetrs Instirute of Technology, May 1 9 6 5 , (8) “Polyelectrolyte Com lex Films as Reverse-Osmosis Desalinarion Membranes,” Final Rept. io U. S. 8 e p t . of Interior, Office of Saline Water, Contract KO. 14-01-0001-315 .
