Anion exchange membranes and process for making

Abstract

Embodiments of the present invention provide for anion exchange membranes and processes for their manufacture. The anion exchange membranes described herein are made the polymerization product of at least one functional monomer comprising a tertiary amine which is reacted with a quaternizing agent in the polymerization process.

Claims

1. A process of producing an ion exchange membrane, comprising: saturating porous regions of a porous substrate having a porosity of at least about 45% with a first solution comprising vinylbenzyl chloride, benzyl chloride, and a tertiary amine monomer selected from the group consisting of vinylimidazole and vinylcarbazole, the porous substrate having a thickness of greater than about 20 microns and less than about 55 microns; and heating the saturated porous substrate to forma crosslinked ion exchange polymer in the porous regions; and conditioning the porous substrate having the crosslinked ion exchange polymer in a second solution comprising sodium chloride for at least thirty minutes.

2. The process of claim 1, wherein the first solution further comprises at least one polymerization initiator selected from the group consisting of organic peroxides, 2,2′-azobis[2,(2-imidazolin-2-yl)-propane] dihydrochloride, a,a′-azoisobutyronitrile, 2,2′-azobis(2-methylpropioaminidine) dihydrochloride, 2,2′-azobis[2,(2-imidazolin-2-yl)-propane], and dimethyl 2,2′-azobis(2-methylpropionate).

3. The process of claim 2, wherein the first solution further comprises at least one polymerization inhibitor selected from the group consisting of 4-methoxyphenol and 4-tert-butyl catechol.

4. The process of claim 1, wherein the porous substrate is comprised of polypropylene, high molecular weight polyethylene, ultrahigh molecular weight polyethylene or polyvinylidene fluoride.

5. The process of claim 1, wherein the first solution further comprises a solvent selected from the group consisting of butanol, propanol, dipropylene glycol, dimethylacetamide, and N methylpyrrolidone.

6. The process of claim 1, wherein heating the porous substrate having porous regions saturated with the first solution comprises heating the porous substrate in the absence of oxygen.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 presents a schematic representation of the construction of a membrane test cell and reference electrode as described in the accompanying examples.

DETAILED DESCRIPTION

(2) International Application #PCT/US 10/46777 incorporated in its entirety by reference describes a method of making ion exchange membranes produced by polymerizing one or more monofunctional ionogenic monomers with at least one multifunctional monomer in the pores of a porous substrate. (c) As described herein the inventor has found that by using functional monomers having a tertiary amine group with a quaternizing chemical, anion exchange membranes of low resistance high permeability and good chemical resistance can be made. The quaternary ammonium functional groups are strongly basic and ionized to act over the pH range of 0 to 13 allowing a broad operational range. Of particular utility are vinyl compounds having nitrogen containing rings.

(3) Preferred tertiary amine monomers are vinylimidazole and vinylcarbazole. The tertiary amine containing monomer is polymerized with at least one crosslinking monomer and at least one quaternizing agent and one or more polymerization initiators to form the ionogenic polymer in the pores of the porous substrate.

(4) The tertiary amine containing monomer may be copolymerized with at least one secondary functional monomer such as but not limited to; vinylbenzyltrimethylammonium chloride, trimethylammonium ethylmethacyrlic chloride, methacrylamidopropyltrimethylammonium chloride, (3-acrylamidopropyl)trimethylammonium chloride, 2-vinylpyridine, and 4-vinylpyridine, at least one crosslinking monomer and at least one quaternizing agent, and one or more polymerization initiators.

(5) Furthermore, either of these methods may be done with at least one added non-functional secondary monomer such as but not limited to; styrene, vinyl toluene, 4-methylstyrene, t-butyl styrene, alpha-methylstyrene, methacrylic anhydride, methacrylic acid, n-vinyl-2-pyrolidone, vinyltrimethoxysilane, vinyltriethoxysilane, vinyl-tris-(2-methoxyethoxy)silane, vinylidene chloride, vinylidene fluoride, vinylmethyldimethoxysilane, 2,2,2,-trifluoroethyl methacrylate allyamine, vinylpyridine, maleic anhydride, glycidyl methacrylate, hydroxyethylmethacrylate, methylmethacrylate, or ethylmethacrylate.

(6) The at least one crosslinker is preferably divinylbenzene or ethylene glycol dimethacrylate.

(7) Optionally, The at least one crosslinker may be chosen from propylene glycol dimethacrylate, isobutylene glycol dimethacrylate, Octa-vinyl POSS® (Hybrid Plastics, OL1160) (C.sub.16H.sub.24O.sub.12Si.sub.8), Octavinyldimethylsilyl POSS® (Hybrid Plastics, OL1163) (C.sub.32H.sub.72O.sub.20Si.sub.16), Vinyl POSS® Mixture (Hybrid Plastics, OL1170) ((CH.sub.2CH).sub.n(SiO.sub.1.5).sub.n, wherein n=8,10, or 12), Trisilabolethyl POSS® (Hybrid Plastics, SO1444) (C.sub.14H.sub.38O.sub.12Si.sub.7), Trisilanolisobutyl POSS® (Hybrid Plastics, SO1450) (C.sub.28H.sub.66O.sub.12Si.sub.7), Trisilanolisooctyl POSS® (Hybrid Plastics, SO1455) (C.sub.56H.sub.122O.sub.12Si.sub.7), Octasilane POSS® (Hybrid Plastics, SH1310) (C.sub.16H.sub.56O.sub.20Si.sub.16), Octahydro POSS® (Hybrid Plastics, SH1311) (C.sub.16H.sub.56O.sub.20Si.sub.16), epoxycyclohexyl-POSS® cage mixture (Hybrid Plastics, EP0408) ((C.sub.8H.sub.13O).sub.n(SiO.sub.1.5).sub.n, wherein n=8, 10, or 12), glycidyl-POSS® cage mixture(Hybrid Plastics, EP0409) ((C.sub.6H.sub.11O.sub.2).sub.n(SiO.sub.1.5).sub.n, wherein n=8, 10, or 12), methacryl POSS® Cage Mixture (Hybrid Plastics, MA0735) ((C.sub.7H.sub.11O.sub.2).sub.n(SiO.sub.1.5).sub.n, wherein n=8, 10, or 12), or Acrylo POSS® Cage Mixture (Hybrid Plastics, MA0736) ((C.sub.6H.sub.9O.sub.2).sub.n(SiO.sub.1.5).sub.n, wherein n=8, 10, or 12). Solvents found useful are N-propanol and dipropylene glycol. Similar hydroxy containing solvents, such as alcohols, for example isopropanol, butanol; diols such as various glycols, or polyols, such as glycerine may be useful in some cases. Additionally aprotic solvents such as N-methylpyrrolidone and dimethylacetamide may be used. These are given as examples, not to be limiting to a practitioner. Dipropylene glycol is a preferred solvent.

(8) Free radical initiators useful for the present invention include, but are not limited to; benzoyl peroxide (BPO), ammonium persulfate, 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2-methylpropionamidine)dihydrochloride, 2,2′-Azobis[2-(2-imidazolin-2yl)propane]dihydrochloride, 2,2′-Azobis[2-(2-imidazolin-2-yl)propane] and dimethyl 2,2′-azobis(2-methylpropionate). (d) A person skilled in the art of membrane development and manufacturing will realize that this convenient laboratory method can be adapted to other laboratory scaled methods and may be scaled up to continuous manufacturing. For example, the substrate pore filling or saturation may be done at a slightly elevated temperature (>40° C.) to reduce air solubility, or this step could be done after a mild vacuum treatment of the substrate sample submerged in the formulation solution. Substrate samples may be presoaked and then placed on the polyester or similar sheet and covered with a covering sheet and smoothed out to remove air bubbles. Several presoaked pieces may be layered and then placed on the polyester or similar sheet and covered with a covering sheet and smoothed out to remove air bubbles. (e) Rather than heating in an oven, the saturated substrate sandwich may be placed on a heated surface at a temperature sufficient and for a time necessary to initiate and complete polymerization. Alternate methods for initiation of the polymerization reaction may be used. Ultraviolet light or ionizing radiation, such as gamma radiation or electron beam radiation may be used to initiate the polymerization reaction.

(9) Low resistance reduces the electrical energy required to desalinate and lowers operating cost. Specific membrane resistance is measured in Ohm-centimeters (Ω cm). A more convenient engineering measure is area resistance, Ohm-cm.sup.2 (Ω cm.sup.2). Area resistance may be measured by using a cell having two electrodes of known area, platinum or black graphite are typically used, with the membrane sample of known area between them in an electrolyte solution. The electrodes do not touch the membrane. Membrane resistance is estimated by subtracting the electrolyte resistance without the membrane from the test result with the membrane in place. The resistance may also be measured by determining a voltage vs. current curve in a cell having two well stirred chambers separated by the membrane. A calomel electrode measures the potential drop across the membrane. The slope of the potential drop vs. current curves, which may be obtained by varying voltage and measuring current. Electrochemical impedance may also be used. In this method, alternating current is applied across the membrane. Measurement at a single frequency gives data relating to electrochemical properties of the membrane. By using frequency and amplitude variations, detailed structural information may be obtained. Herein, resistance will be defined by the methods described in the Experimental section.

(10) Permselectivity refers to the relative transport of counterions to co-ions during electrodialysis. For an ideal cation exchange membrane only positively charged ions would pass the membrane, giving a permselectivity of 1.0. Permselectivity is found by measuring the potential across the membrane while it separates monovalent salt solutions of different concentrations. The method and calculations used herein are described in the Experimental section.

(11) To meet these initial goals the inventors developed a type of composite ion exchange membrane in which a cross-linked polymer having charged ionic groups attached is contained in the pores of a microporous membrane substrate. The porous membrane substrate is preferably less than about approximately 155 microns thick, more preferably less than about approximately 55 microns thick. Substrate membranes having porosity greater than about 45% are preferred, with those having porosities greater than about 60% more preferred. In the most preferred embodiments, the substrate membranes have porosities greater than about 70%. Preferred substrate membranes have a rated pore size of from about approximately 0.05 microns to about approximately 10 microns, with a more preferred range of from about approximately 0.1 microns to about approximately 1.0 microns. Most preferred porous substrates have a rated pore size of from about approximately 0.1 microns to about approximately 0.2 microns.

(12) Microporous membrane supports may be manufactured from polyolefins, polyolefin blends, polyvinylidene fluoride, or other polymers. A class of preferred substrates comprises thin polyolefin membranes manufactured primarily for use as battery separators. A more preferred substrate class are thin battery separators manufactured from ultrahigh molecular weight polyethylene (UHMWPE).

(13) To produce the desired ion exchange membranes, the inventors developed a method of placing the crosslinked charged polymer in the pores of the substrate by polymerizing the crosslinked polymer in these pores. The method involved saturating the porous substrate with a solution of charged monomer, multifunctional monomer, (e.g., a crosslinking agent) and polymerization initiator. Herein we use the term ionogenic monomer to mean a monomer species having at least one charged group covalently attached. The charged group can be positively charged or negatively charged. In an embodiment, the crosslinked polymer was produced by polymerizing a multifunctional charged monomer. The Polymerization was initiated by heat or by UV light, preferably with a polymerization initiator such as a free radical initiator. Monofunctional monomers are monomers which have a single site for carrying forward the polymerization reaction. Multifunctional monomers have more than one polymerization reaction site and so can form networked or crosslinked polymers.

(14) The following laboratory method was used to investigate formulation and process effects by producing small coupons for resistivity and permselectivity testing. Porous membrane substrate 43 mm diameter coupons were die cut. Somewhat larger discs (50 mm or 100 mm diameter) of transparent polyester sheets were also die cut. A 105 mm aluminum weighing boat was typically used to hold a set of coupons. The coupons were sandwiched between two polyester film discs. First, substrate coupons were thoroughly wetted with a monomer solution to make up a test sample. This was done by adding the formulated solution to the aluminum boat, and immersing a polyester film disc with a substrate coupon layered on it into the solution so that the porous support is saturated. The saturated support was then removed from the monomer solution and placed on a piece of polyester film. Air bubbles were removed from the coupon by, for example, smoothing or squeezing the coupon with a convenient tool, such as a small glass rod, or by hand. A second polyester disc was then layered on top of the first coupon and smoothed to have complete surface contact between the coupon and the lower and upper polyester film layers. A second porous substrate was then layered on the upper polyester film and the saturation, smoothing and addition of an over layer of polyester film repeated to give a multilayer sandwich of two coupons and three protective polyester film layers. A typical experimental run would have a multilayered sandwich of 10 or more saturated substrate coupon layers. The rim of the aluminum boat can be crimped down to hold the disc/coupon assembly if required.

(15) The boat and assembly were then placed in a sealable bag, typically a zip-lock polyethylene bag and a low positive pressure of inert gas, usually nitrogen, added before sealing the bag. The bag containing the boat and coupon assembly is placed into a oven at 80° C. for up to about 60 minutes. The bag is then removed and cooled, and the now reacted ion exchange membrane coupons are placed in 0.5N NaCl solution at 40° C.-50° C. for at least 30 minutes, with a soak in NaCl solution of up to 18 hours being found satisfactory.

EXPERIMENTAL EXAMPLES

(16) The following examples are meant to illustrate the extent of effort expended in developing the subject membranes. The finding resulted in showing that ion exchange membranes having the desired properties could be made and that improvements are possible with further experimentation. These results are meant to be illustrative and to indicate developmental directions to those skilled in the art of membrane development and associated arts and not to be limiting as to the extent of the matter disclosed herein.

(17) Properties and suppliers of the supports used are given in Table 1 below.

(18) TABLE-US-00001 TABLE 1 Substrates Used Porous Substrates Rated pore Thickness Trade name Manufacturer Material Size microns Porosity % APorous H6A APorous HDPE 0.1 52 68 Billerica MA APorous S14 HDPE 0.1 84 77 Ahlstrom Wall Township, Polyester 200 (Hollytex) New Jersey Teklon HPIP32 Entek UHMWPE 32 48 Lebanon, OR Delpore 6002 Delstar meltblown Middleton DE Delstar Stratex Delstar 3.65L-G Middleton DE Novatexx 2413 Freudenberg Spunlace 558 Hopkinsville, KY polyester Celgard EZ2090 CELGARD PP Charlotte NC Celgard EZ2590 CELGARD PP 32 45 Charlotte NC Solupor 16P5A Lydall Filtration UHMWPE 0.5 115 83% Rochester NH Solupor Lydall Filtration UHMWPE 0.9 120 85% 16P10A Rochester NH

(19) Representative porous substrates were tested for baseline permselectivity and resistance. They are pre-washed using isopropanol-ethanol and D.I. water each for 5 minutes, then they were rinsed by 0.5 N NaCl (aq) 3 times testing Table 2 below shows the results of area resistance in Ohm cm.sup.2 of AEM thus made and their apparent permselectivity %:

(20) TABLE-US-00002 TABLE 2 Characteristics of selected substrates Description R (Ohm cm.sup.2) Apparent Permselectivity % Teklon HPIP 0.593 57.24 Solupor 16P1OA 2.192 57.38 Aporous H6A 0.152 57.54 Celgard EZ-2590 0.788 57.54 Celgard EZ-2090 1.159 57.38

Example 1

(21) In a 4 oz jar with 17.08 g of 1-vinylimidazole, 9.14 g of vinylbenzyl chloride, 5.39 g of divinylbenzene (80%), 16.61 g of benzyl chloride, 20.65 g of dipropylglycol (DPG) and 0.40 g Vazo-64 were combined with stirring. A clear brown solution formed immediately. Substrates Solupor 16P10A, 16P05A, Teklon, Aporous S14, Celgard EZ2090, EZ2590, Novatexx 2431ND, Delstar 6SLG, Ahlstrom 3329, Delpore DP3924-80PNAT, Delpore 6002-20PNAT were soaked the solution for 1 hour to assure complete pore filling. Then they were sandwiched between Mylar disks, air bubbles between Mylar disks removed by pressure and the sandwiched substrates placed in an aluminum weighting dish. The dish was loaded into a Ziploc® bag, the bag pressurized with nitrogen gas and placed in an 80° C. oven for 1 hour. The membrane coupon thus made was placed in 0.5 N NaCl (aq) for conditioning. Table 3 below shows the results of area resistance in Ohm cm.sup.2 of AEM thus made and their apparent permselectivity %:

(22) TABLE-US-00003 TABLE 3 R Apparent Substrates (Ohm cm.sup.2) Perms % Ahsltrom 3329 200 micron thick 31.75 92.62 Aporous S14 2.145 93.11 Celgard X021 (EZ2590)  32 micron thick 2.68 92.78 Teklon HPIP  32 micron thick 5.00 94.26 Solupor 16P05A 115 micron thick 2.55 92.95 Solupor 16P1OA 200 micron thick 3.55 92.62 Delpore6002-20PNAT nonwoven 3.64 89.01 Novatexx 2431ND NO 7.51 73.03 Delstar 6SLG nonwoven 12.62 87.70 Celgard EZL2090 3.29 90.52 Delpore6002-20PNAT 2nd coupon 2.35 89.86 nonwoven

Example 2

(23) In a 4 oz jar with 15.71 g of 1-vinylimidazole, 25.47 g of vinylbenzyl chloride, 13.25 g of DPG and 0.42 g Vazo-64 were combined with stirring. A clear brown solution formed immediately. Substrates Solupor 16P10A, 16P05A and Teklon HPIP, were soaked the solution for 1 hour to assure complete pore filling.

(24) Then they were sandwiched between Mylar disks, air bubbles between Mylar disks removed by pressure and the sandwiched substrates placed in an aluminum weighting dish. The dish was loaded into a Ziploc bag, the bag pressurized with nitrogen gas and placed in an 80° C. oven for 1 hour. The membrane coupon thus made was placed in 0.5 N NaCl (aq) for conditioning. Table 4 below shows the results of area resistance in Ohm cm.sup.2 of AEM thus made and their apparent permselectivity %.

(25) Also shown are commercially available ion exchange membranes AMX and CMX from Astom-Japan. Both are 125 microns thick.

(26) TABLE-US-00004 TABLE 4 R (Ohm Apparent Description cm.sup.2) Permselectivity % Teklon HPIP  32 micron thick 6.55 91.64 Solupor 16P1OA 120 micron thick 3.54 92.62 Astom AMX (anion exchange membrane) 3.13 96.07 Astom CMX (cation exchange membrane 2.37 106.50

(27) The results in Tables 3 and 4 show that membranes made by the inventive method has approximately equivalent properties to much thicker membranes. Thinner membranes allow for increased number of membranes per module or housing volume and therefore more productivity per unit volume.

Example 3

(28) In a 4 oz jar with 17.12 g of 1-vinylimidazole, 20.00 g of vinylbenzyl chloride, 16.00 g of benzyl chloride, 11.02 g of DPG and 0.51 g Vazo-64 were combined with stirring. A clear brown solution formed immediately. Substrates Solupor 16P10A, and Teklon (HPIP, 32 micron, single layer) were soaked in the solution for 1.5 hour to assure complete pore filling.

(29) Then they were sandwiched between Mylar disks, air bubbles between Mylar disks removed by pressure and the sandwiched substrates placed in an aluminum weighting dish. The dish was loaded into a Ziploc® bag, the bag pressurized with nitrogen gas and placed in an 80° C. oven for 40 minutes. The membrane coupon thus made was placed in 0.5 N NaCl (aq) for conditioning. Table 5 below shows the results of area resistance in Ohm cm.sup.2 of AEM thus made and their apparent permselectivity %.

(30) Also shown is a commercially available ion exchange membranes AMX from Astom-Japan. Its thickness is 125 microns thick.

(31) TABLE-US-00005 TABLE 5 Apparent Description R(Ω cm.sup.2) Permselectivity % Teklon HPIP  32 micron 2.33 95.09 Solupor 16P10A 120 micron 2.17 95.57 Astom AMX (anion exchange membrane) 2.73 94.55

Example 4

(32) In a 4 oz jar with 8.55 g of 1-vinylimidazole, 10.01 g of vinylbenzyl chloride, 1.02 g of divinyl benzene (80%), 12.01 g of benzyl chloride, 5.61 g of DPG and 0.31 g Vazo-64 were combined with stirring. A clear brown solution formed immediately. Substrates Solupor 16P10A, and Teklon HPIP (single layer), Aporous H6A and Celgard EZ2590, were soaked in the solution for 75 minutes to assure complete pore filling.

(33) Then they were sandwiched between Mylar disks, air bubbles between Mylar disks removed by pressure and the sandwiched substrates placed in an aluminum weighting dish. The dish was loaded into a Ziploc® bag, the bag pressurized with nitrogen gas and placed in an 80° C. oven for 47 minutes. The membrane coupon thus made was placed in 0.5 N NaCl (aq) for conditioning. Table 6 below shows the results of area resistance in Ω cm.sup.2 of AEM thus made and their apparent permselectivity %.

(34) Also shown is a commercially available ion exchange membranes AMX from Astom-Japan. Its thickness is 125 micron.

(35) TABLE-US-00006 TABLE 6 Apparent Description R(Ohm cm.sup.2) Permselectivity % Teklon HPIP 32 micron 4.24 95.21 Solupor 16P10A, 120 micron  2.62 94.71 Aporous H6A, 52 micron 2.55 95.04 Celgard EZ2590, 27 micron 1.98 94.55 Astom AMX (anion exchange 2.73 94.55 membrane)

Example 5

(36) In an 8 oz jar with 30.7 g of 1-vinylimidazole, 17.2 g of vinylbenzyl chloride, 42.5 g of benzyl chloride, 11.8 g Divinylbenzene (80%), 27.0 g of DPG and 0.41 g Vazo-64 were combined with stirring. A clear brown solution formed immediately. Coupons of substrates Solupor 16P05A were soaked in the solution for 2 hour to assure complete pore filling.

(37) Then they were sandwiched between Mylar disks, air bubbles between Mylar disks removed by pressure and the sandwiched substrates placed in an aluminum weighting dish. The dish was loaded into a Ziploc® bag, the bag pressurized with nitrogen gas and placed in an 80° C. oven for 1 hour. The membrane coupon thus made was placed in 0.5 N NaCl (aq) for conditioning. Table 7 below shows the results of area resistance in Ω cm.sup.2 of AEM thus made and their apparent permselectivity %.

(38) Also shown is a commercially available ion exchange membranes AMX from Astom-Japan. Its thickness is 125 micron.

(39) TABLE-US-00007 TABLE 7 R (Ohm Apparent Description cm.sup.2) Permselectivity % Solupor 16P05A, 115 micron 2.60 95.00 Astom AMX (anion exchange membrane) 3.10 95.08

Example 6

(40) In a 20 ml vial with 3.43 g of 1-vinylimidazole, 3.0 of vinylbenzyl chloride, 1.0 gm EPO409 (Hybrid Plastics, glycidyl-POSS® cage mixture CAS #68611-45-0), 3.2 g of benzyl chloride, 2.20 g of DPG and 0.10 g Vazo-64 were combined with stirring. A clear brown solution formed immediately. Coupons of substrate Solupor 16P10A were soaked in the solution for 0.5 hour to assure complete pore filling.

(41) Then they were sandwiched between Mylar disks, air bubbles between Mylar disks removed by pressure and the sandwiched substrates placed in an aluminum weighting dish. The dish was loaded into a Ziploc® bag, the bag pressurized with nitrogen gas and placed in an 90° C. oven for 1 hour. The membrane coupon thus made was placed in 0.5 N NaCl (aq) for conditioning. Table 8 below shows the result of area resistance in Ohm cm.sup.2 of AEM thus made and their apparent permselectivity %.

(42) Also shown is a commercially available ion exchange membranes AMX from Astom-Japan having a thickness is 125 micron.

(43) TABLE-US-00008 TABLE 8 R(Ohm Apparent Description cm.sup.2) Permselectivity % Solupor 16P10A, 120 micron 2.59 93.92 Astom AMX (anion exchange membrane) 2.42 93.59
Experiment Procedures for Membrane Area Resistivity and Apparent Permselectivity Characterization

(44) The membrane resistance and counter ion transport number (permselectivity) can be measured using an electrochemical cell. This bench top experiment provides us with a very effective and quick experiment using a small piece of sample. The equipment and procedure are described here.

(45) Experiment Preparation

(46) (1) Solartron 1280 electrochemical measurement unit

(47) The Solartron 1280 electrochemical measurement unit enables us to apply a current between the two platinum electrodes on the two sides of the cell and to measure the voltage drop across membrane. It has 4 connectors: work electrode (WE), counter electrode (CE), Reference electrodes (RE1 and RE2). The CE and WE are used to apply current and RE1 and RE2 to measure the voltage drop. (2) Reference electrodes

(48) Reference electrodes (see the insert in FIG. 1) used for membrane characterization are made in R&D lab. ¼″ glass tubing is softened, then bent and drawn to the form shown. A porous plug is inserted in the tip to control solution flow to a low rate.

(49) Silver/silver chloride wire is freshly made for each day's testing. A current of 2-3 mA was supplied and controlled by a power supplier and an ampere meter to a platinum wire cathode and silver wire anode immersed in a 0.1N HCl solution. After several minutes, the sliver wire starts to turn black, indicating the formation of AgCl layer on the surface. The solution used inside the reference electrode tubing is 1.0M KCl solution. Since the solution will leak through the porous tip, constant addition of KCl is a necessary (˜every 20 min) during experiment. (3) Membrane test cell

(50) FIG. 1 shows the detailed electrochemical testing cell construction used for the experiment to measure resistance and counter ion permselectivity of the membrane. The membranes are cut into disc using a die cutter. The reference electrodes are used to monitor the voltage drop across the testing membrane and the 2 platinum discs are used to provide a current through the membrane. The cylindrical path of the cell has a cross section area of 7.0 cm.sup.2 (4) Solutions

(51) All the solutions need to be prepared with quantitative level as indicated by their significant figures. These includes 0.500N NaCl, 1.0N HCl and 1.0N NaOH (caustic, using plastic container or volumetric flask). The 0.5N Na.sub.2SO.sub.4 is used to feed the electrode compartments without evolution of chlorine gas.

(52) III. Measurement Procedures

(53) (2) Resistance measurement

(54) Resistance here refers to area resistance Ω-cm.sup.2. The measurement contains 3 steps. (a) Set up electrode positions: Prior to a measurement, the reference electrode horizontal positions are set. To set reference electrode position, a rigid plastic disc is used as a stand-in for the membrane. Each reference electrode is adjusted to just touch the plastic disc and their position fixed by two set screws. (b) Measure the solution conductivity: The plastic disc was then removed and the two reference electrodes moved to 1.0 cm apart by removing the two 0.50 mm plastic blocks. The voltage drop between the two reference electrodes is recorded at an applied a current (˜10-50 mA) by the Solartron 1280. The distance of the 2 reference electrodes(1.00 cm here), the current density (10.00 mA) and voltage (to 0.1 mV precision) used to obtain the conductivity of the solution (0.50 N NaCl typically. (c) Measuring membrane resistance: The membrane sample is then placed by the sample slider and the voltage and current measured again. The resistance of membrane is the total resistance less the solution resistance measured in procedure (b) (3) Counter ion Permselectivity (Transport number)

(55) The measurement procedures are: (a) Reference electrode position is set as described by part(a) of Resistance measurement. The reference electrodes position may be approximate since the voltage measured in this test is theoretically independent of the distance, but it is recommended that the position be located as reproducibly as possible. (b) Solutions: After emplacing the sample membrane with the slider, pour 0.500N NaCl solution in the right part of the cell separated by the testing membrane and 0.250N NaCl on the left side of the cell. (c) Measuring the voltage: the voltage was measured (without applying current) using a voltage meter attached to the platinum electrodes and data were entered the spreadsheet to obtain counter ion permselectivity.
IV. Sample Calculations: C=conductivity (siemens/cm) ρ=resistance (ohms/cm) R=resistivity (ohm-cm.sup.2 or Ω.Math.cm.sup.2) A=area (cm.sup.2) U, V=measured voltage (mV) I=measured current (mA) L=distance between reference electrodes (1) Conductivity of the 0.500 M NaCl at 10.00 mA current and 33.1 mV measured for a reference electrode distance of 1.00 cm, the conductivity of solution:

(56) $C=\frac{1}{\rho }=\frac{L}{R}=\frac{L}{\frac{U}{I}\times A}=\frac{1.00\mathrm{cm}}{\frac{33.1\mathrm{mV}}{10.0\mathrm{mA}}\times 7.00{\mathrm{cm}}^2}=0.0432S\text{/}\mathrm{cm}$ (2) Area resistance of the membrane calculation needs to subtract the solution resistance. For a CMX membrane with a measured potential of 38.0 mV, the area resistance is:

(57) $R=\frac{\left(38.1-33.1\right)\mathrm{mV}}{10.0\mathrm{mA}}\times 7.00{\mathrm{cm}}^2=3.42\Omega .Math.{\mathrm{cm}}^2$ (3) Permselectivity (transport number) of anion(+) or anion(−) membrane T.sub.± is obtained by:

(58) $V=\left(2T_{\pm }-1\right)\frac{\mathrm{RT}}{F}\ln \frac{a_L}{a_R}$
Which rearranges to;

(59) $2T_{\pm }=1+\mathrm{VF}/\mathrm{RT}\left(\ln \frac{a_R}{a_L}\right)$
Where V is measured voltage by the reference electrodes, R is gas constant (8.314 Joule.Math.K.sup.−1.Math.mole-.sup.1) T is Kelvin temperature of solution, F is Faraday constant (96480 coulomb/mole) and a .sub.R and a .sub.L are concentration (activity format) of the solution on the two sides of the membrane in the cell.