Ind. Eng. Chem. Res. 2000, 39, 2485-2490
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Recovery of Monomethylhydrazine Liquid Propellant by Pervaporation Technique† S. Sridhar, R. Ravindra, and A. A. Khan* Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Monomethylhydrazine (MMH) is a well-known liquid propellant used as fuel for launching of rockets and satellites. The desalted reaction liquor of MMH contains about 2 wt % of the propellant (balance water), which also forms an azeotrope with water at 35% MMH concentration. Removal of water to obtain pure MMH by conventional separation methods such as distillation is hazard prone and expensive. The potential of the membrane-based pervaporation technique has been investigated in this study for enriching aqueous solutions of MMH. Ethylcellulose membrane, which is selected on the basis of estimations of Flory-Huggins interaction parameters and Hansen’s solubility parameter, gives promising results and is resistant to chemical attack by MMH. The effect of operating parameters such as feed concentration and membrane thickness on flux and selectivity are evaluated. Finally aging studies have been carried out to determine the life of the membrane in the propellant medium. Introduction Monomethylhydrazine (MMH) is one of the basic materials used in a pure state in space technology for rocket and satellite applications. MMH has many other important applications, such as oxygen scavenger for boiler feedwater, starting material for dye intermediates, catalysis for polymerization reactions etc.1 The reaction liquor of MMH contains less than 2% propellant, 8% salt, and more than 90% water by weight; therefore, obtaining high-purity MMH from its reaction liquor is a challenging task, specially because of the high alkalinity (pH > 12) and the strongly reducing and hydrolyzing nature of the propellant. Due to its tendency for strong hydrogen bonding, MMH also forms an azeotrope with water at a composition of 35 wt % of the fuel,1 which needs to be processed to obtain propellant grade material. Conventional separation processes such as extractive or azeotropic distillation are prohibitively energy intensive and expensive besides being extremely hazardous due to the explosive nature of the propellant. Therefore the search for an alternate economical and highly safe separation method is of considerable importance. The membrane-based pervaporation (PV) technique is an economical separation method compared to conventional processes for specific separations involving azeotropic and close boiling mixtures due to its high separation factor and flux rates.2 Moreover, PV is safe and environmentally clean and requires less installation space and capital cost. A number of research and industrial applications use this technique for the dehydration of alcohols,3-5 for separation of isomeric compounds6,7 and for separation of mixtures of saturated hydrocarbons8 and extraction of valuable organics present in trace concentrations in aqueous streams. In the present case, especially, pervaporation is more advantageous than other membrane methods such as * To whom correspondence should be addressed. Phone: +91-40-7173626. Fax: +91-40-7173757, 7173387. E-mail:
[email protected]. † IICT Communication No. 4423.
reverse osmosis (RO), nanofiltration (NF), and electrodialysis (ED) since certain pervaporation membranes are capable of withstanding highly alkaline solutions of pH greater than 12 such as MMH-water, mixtures whereas RO, NF, and ED membranes disintegrate in such a medium. Since the requirement in this study is for a very safe technology, heating is to be totally avoided, and hence the vapor permeation method was not employed. The authors have so far successfully applied pervaporation technology for desalting the MMH reaction liquor.9 In the present study, dehydration of the desalted reaction liquor containing 2% MMH (balance water) has been carried out to enrich the propellant to 90+% concentrations. Literature indicates only one similar previous study in this direction by Dytnerskij10 et al. who used cellulose ester membranes for the dehydration of dimethylhydrazine. The ultimate objective of the present research is to find a safe and energy efficient method for breaking the MMH-water azeotrope. Dense membranes of ethylcellulose (EC) were used for the dehydration of MMHwater solutions by pervaporation. The effect of operating parameters such as membrane thickness and feed concentration on the separation performance of the membrane was investigated. Theory of Pervaporation The mechanism of separation by pervaporation is schematically shown in Figure 1. In the PV process, the feed mixture is contacted with a nonporous permselective membrane.2 Separation is in general explained by a solution-diffusion mechanism which involves the steps of sorption into, diffusion through, and desorption from the membrane.11,12 The driving force for separation is created by maintaining a pressure lower than the saturation pressure on the opposite side of the membrane. In pervaporation the flux J of a given species, for example, faster permeating component i of a binary
10.1021/ie990776c CCC: $19.00 © 2000 American Chemical Society Published on Web 06/14/2000
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Figure 1. Schematic of laboratory pervaporation unit: (a) manifold of laboratory-scale pervaporation system; (b) blow-up of the pervaporation cell assembly.
liquid mixture comprising i and j, is given by13
Ji )
Qi sat (xpi - ypper) l
(1)
where Qi represents the permeability coefficient which is also the product of solubility (SI) and diffusivity (DI) of component i, whereas x and y are its mole fraction concentrations in feed and permeate, respectively. psat i is the saturation vapor pressure of i, and pper is the pressure on the permeate side, whereas l stands for membrane thickness. The membrane selectivity is the ratio of the permeability coefficients and can be calculated from their respective concentrations as given below
R)
( ) ( )( )
Qi Si Di y(1 - x) ) ) Qj S j Dj x(1 - y)
(2)
where 1 - x and 1 - y represent the feed and permeate concentrations of the slower permeating component j. Polymer Selection Selection of the polymer materials for separation is based mainly on three important aspects: the polymer should have high chemical resistance (compatibility), sorption capacity, and good mechanical strength in the solution. Additionally it should have good interaction, preferably with one of the components of the mixture for separation. In general, selection of polymers compatible with the mixtures for separation is based on the Hansen solubility parameters (∆) and Flory-Huggins interaction parameters (χ). The compatibility among water (component 1), propellant (component 2), and polymer (component 3) is indicated by the relationship14
∆ ) [(dd,1 - dd,2)2 + (dp,1 - dp,2)2 + (dh,1 - dh,2)2]1/2 (3) where dd, dp, and dh are vectors corresponding to
dispersive, polar, and hydrogen bonding contributions, and ∆ is the distance between end points of the vectors. The greater the compatibility between any two components, the smaller will be the magnitude of ∆. The Flory-Huggins interaction parameter (χ) also signifies the compatibility of the components with the polymer. The binary interaction parameters, χ1,3 and χ2,3 between the components and the polymer can be determined from
χ1,3 ) -[ln(1 - νp) + νp]/νp2
(4)
where νp is volume fraction of the polymer (3) which is calculated as 1 - a, where “a” represents the solubility of the liquid in the polymer determined experimentally as {liquid cc/(liquid cc + polymer cc)}, where “cc” is cubic centimeters.15. “i” represents component 1 or 2. Again the smaller the value of χ (close to 0.5 but not below), the greater will be the interaction. The authors investigated the performance of chitosan membrane whose solubility parameter16 was calculated to be 43.04 J1/2/cm3/2, which is closer to that of water17 (47.9 J1/2/cm3/2) than MMH17 (21.17 J1/2/cm3/2) but obtained poor selectivities probably due to extensive interand intramolecular hydrogen bonding between chitosan and MMH. The hydrophobic poly(dimethylsiloxane) (PDMS) could not be used to extract MMH exclusively from the dilute reaction liquor since it degraded in the high-pH medium. Other chemically compatible membranes made of polypropylene and polystyrene gave negligible flux. In the present case emphasis has been placed on neutral and basic polymers such as ethylcellulose (EC) because of its expected high chemical resistance as well as hydrophilic nature toward highly alkaline and reducing solution. Experimental Section Materials. MMH was supplied by VSSC-ISRO, Trivandrum, India. Ethylcellulose (powdered form; ethoxy content, 48-49%) and toluene (LR grade) were purchased from Loba Chemie, Mumbai, India, and were
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used as received. Mn (63 156) and Mw (89 448) of the ethylcellulose used in this study were determined by the GPC method using polystyrene as the standard. Membrane Preparation. Approximately 12 wt % clear solution of EC in toluene was prepared and cast on nonwoven membrane support fabric fixed onto a clean glass plate. The solvent from the cast film was initially evaporated at atmospheric pressure at room temperature (30 ( 5 °C) for 1 h to obtain a dense membrane which was then vacuum-dried for a period of 5 h at ambient temperature in an oven to remove the remaining traces of solvent. Laboratory Pervaporation Unit. This section describes the experimental setup that was used to carry out pervaporation studies for dehydration of MMHwater mixtures. A schematic diagram of the experimental facility is shown in Figure 1, which includes the pervaporation manifold (Figure 1a) and blowup of the permeability cell assembly (Figure 1b). The pervaporation cell consisted of two bell-shaped B-24 size glass column reducers/couplers clamped together with external padded flanges by means of tie rods to give a vacuum tight arrangement. The top half was used as the feed chamber, and the bottom one worked as the permeate chamber. The membrane was supported by a stainless steel porous plate which was embedded with an SS mesh/screen. Teflon gaskets were fixed by means of high-vacuum silicone grease on either side of the membrane, and the sandwich was placed between the two glass column couplers and secured tightly. The effective area of the membrane in the PV cell was 19.4 cm2. After fixing the membrane, the cell was installed in the manifold and connected to the permeate line by means of a B-24 cone which was fixed on one side to a high-vacuum glass valve followed by a glass condensor trap which consisted of a small detachable collector. The trap was placed in a Dewar flask containing a dry iceacetone mixture (-50 °C) for condensing the permeate vapors. A 0.75 HP rotary vacuum pump was used to maintain the permeate side pressure, which was measured with an Edward’s Mcleod gauge (scale, 10-0.01 mmHg). High-vacuum rubber tubing was used to connect the various accessories to the experimental manifold. All glass cone-socket joints were fixed with good quality high-vacuum grease (Dow Corning). Pervaporation Procedure. Dry membrane film was fixed over the porous SS support, and the pervaporation cell assembly was clamped and installed in the manifold. Initially no feed solution was introduced in the cell, and the vacuum pump was turned on to detect the presence of pinholes in the membrane or any other leakage present in the manifold and cell assembly. Experiments were begun only after a steady-state vacuum of at least 0.05 Torr could be obtained with the system. The vacuum pump was then switched off, and a known volume of feed liquid (70 mL) was poured into the top-half chamber of the cell, allowing it to soak the membrane. Evacuation was again applied and data collection started once a steady-state concentration profile was attained. Since MMH is highly hygroscopic in nature, effective sealing of the feed chamber was ensured. Most of the experiments were carried out with the feed stirred continuously throughout the experiments to reduce concentration polarization. The authors have already explored the effect of concentration polarization (mass-transfer resistances on the feed side liquids) by estimating the membrane resistance and
desorption resistance of MMH and water in EC in a separate publication.18 The same volume of the feed material was introduced in each run to avoid any experimental disturbances. At the beginning of each run, a dry membrane was mounted in the cell and the permeate pressure was kept constant throughout the run. Permeate was collected for a duration of 5-6 h. Tests were carried out at room temperature (30 ( 2 °C) and repeated twice using fresh feed solution to check for reproducibility. The effect of operating parameters such as feed composition and membrane thickness on the separation performance and flux of the membrane was studied in detail. The collected permeate was weighed after allowing it to attain room temperature in a Sartorius electronic balance (accuracy, 10-4 g) to determine the flux and then analyzed by the iodometric titration method to evaluate the membrane selectivity. Analytical Procedure. The Penneman method19 of employing potassium iodate (KIO3) as titrant was extensively used for the quantitative analysis of MMH liquid propellant in feed and permeate samples. Visual Titration. Reagent grade KIO3 was heated to 120 °C for 3-4 h with vacuum-drying to remove the residual moisture present in the hygroscopic chemical; then 21.4 g of it was dissolved in 1 L of water in a volumetric flask. The solution was then standardized by titration against 0.5-0.6 g of primary arsenic trioxide in 6 N HCl taken in a conical flask using 10 mL of chloroform as indicator. A known weight (w) of the sample of the feed/ permeate containing MMH was pipetted out into a 50 mL volumetric flask containing a 25 mL aliquot of 0.1 M KIO3 solution. The solution was then transferred into a 250 mL glass stoppered flask containing 30 mL of 12 N HCL and 5-10 mL of chloroform. The purple colored contents in the 250 mL stoppered flask were titrated against 0.1 M KIO3 solution taken in the buret. After addition of each drop, the flask was stoppered and shaken vigorously. The end point was reached when the purple colored solution turned yellow for the first time accompanied by change of the organic layer (chloroform) color from purple to colorless. The volume of KIO3 consumed (v) was noted, and the titration was repeated with fresh sample to get consistent readings. The quantity of MMH present in the sample was then calculated as follows
MMH wt % ) vM(mw)(100)/(w)(1000) where M is the molarity of KIO3 used for the titration and mw is the molecular weight of MMH (46.08). The accuracy of the analysis was confirmed to be about (1% by carrying out titrations with anhydrous hydrazine as well as synthetically prepared standard solutions of MMH-water of different compositions. Results and Discussions Hansen and Flory-Huggins Parameters. The Hansen’s solubility and Flory-Huggins interaction parameters for EC-water and EC-MMH systems were determined from eqs 3 and 4, respectively. By substituting the dd, dp, and dh values of water, MMH, and the polymer17 in eq 3, the Hansen values were found to be 9.8 and 30.0 for MMH and water, respectively. FloryHuggins interaction values calculated from eq 4 were 1.39 and 2.5 for EC-MMH and EC-water systems, respectively. These results indicate that EC has more
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Figure 2. Effect of ethylcellulose membrane thickness on selectivity and water flux.
affinity toward MMH than water since both ∆ and χ values are smaller in the case of MMH. Due to the greater affinity between MMH and EC, the propellant molecules would be held strongly within the polymer matrix as a result of which its diffusion could be retarded. The interaction of water molecules with EC functional groups is weaker, which allows the loosely held water molecules to flow more freely through the membrane barrier. Process Parameters. (a) Membrane Thickness. The effect of membrane thickness on water flux and selectivity is shown in Figure 2. The thickness was varied from 25 to 120 µm with the feed composition fixed at the azeotropic point (35% MMH) and permeate vacuum at 0.05 mmHg throughout. In pervaporation, one side of the membrane is in a swollen state due to contact with the mixture and the other downstream face is in a dry state due to continuous evacuation; i.e., the membrane will always be in pseudoequilibrium.2 The swollen membrane layer acts like a fluid and allows all the permeating components to diffuse through, whereas the dry layer is the one which restricts the flow of the penetrants. Every dense pervaporation possesses a dry downstream layer during operation, and the thickness of this dry layer increases with increasing total membrane thickness causing a rise in resistance to mass transfer. Hence the water flux as well as the overall flux reduced with increasing membrane thickness. In normal practice, the selectivity is not greatly influenced by the thickness of the membrane as long as the barrier is a dense nonporous one. A similar observation could be made from Figure 2, where only a marginal increase in R from 5 to 5.8 occurred. (b) Feed Concentration. Figures 3 and 4 are pictorial representations of the effect of the MMH concentration on pervaporation selectivity and water flux of the EC membrane of thickness 35 µm at a downstream pressure of 0.05 mmHg. Explanations for observed changes in the two important membrane parameters could be based on membrane swelling and various forms of interactions taking place between the penetrating liquids and functional groups of the polymer such as water-water, MMH-MMH, MMH-water, ECwater, EC-MMH, water-MMH-EC. At low feed concentrations of MMH, most of the hydroxyl groups of EC form hydrogen bonding types of interactions with water. In the bulk phase a small number of water molecules interact with the small concentration of MMH molecules, whereas the remaining water molecules bind together as H2O-H2O which could also be called the uninteracted free state of water. Since excessive amounts of free water are available to the hydrophilic membrane, more and more H2O molecules are transported freely
Figure 3. Effect of MMH feed concentration on selectivity of EC membrane (permeate vacuum, 0.05 mmHg; membrane thickness, 35 µm).
Figure 4. Effect of MMH feed concentration on water flux (permeate vacuum, 0.05 mmHg; membrane thickness, 35 µm).
through the barrier, resulting in high concentration of water in the permeate and consequently high flux and selectivity. As the MMH feed concentration increases, the membrane swelling also increases because the equilibrium percent sorption of MMH in EC is about 15.66%, which is much higher than that of water (3.4%).20 The increased swelling plasticizes the upstream layer of the membrane which now acts as a fluid phase and does not restrict the passage of any of the two permeating components up to the dry downstream layer. Now an increasing number of MMH molecules are interacting with the dry downstream layer, thus increasing propellant flux besides restricting water flux to some extent causing a drop in selectivity. As the MMH concentration is further increased, it forms an azeotrope with water at 35 wt % propellant concentration.1 Though the bond strength between MMH and water is very strong at this point, the reduced interaction between the liquids and the membrane is strong enough to break the azeotrope, as proven by widely different solubility coefficient values20 for water and MMH which were found to be 1.13 × 10-3 and 3.89 × 10-3 g g-1 mmHg-1, respectively. The membrane swells to a smaller extent due to lesser interaction, thus reducing the flux. The selectivity value is relatively lower than those obtained for other feed compositions due to the increasingly significant diffusion of the MMH-water interacting pair through the membrane. The higher diffusion coefficient of water,20 which is 2.57
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membrane, which proved that the membrane had retained its original mechanical strength. Conclusions
Figure 5. FTIR spectra of aged EC membrane soaked in MMHwater azeotrope for (a) 1 month and (b) 6 months and of (c) the unsoaked film.
× 10-8 cm2/s compared to the 9.5 × 10-9 cm2/s of MMH, ensures that R remains reasonably high. For very high feed concentrations of 75% and above, a marginal rise in flux and selectivity could be observed. The membrane swelling is at its highest level due to maximum sorption of MMH in the polymer matrix, and this condition is characterized by a large swollen upstream portion and considerably reduced dry downstream layer thickness, resulting in enhancement of total as well as individual fluxes. The MMH-water bond strength is now much weaker than at the azeotropic point, and, moreover, the H2O molecule requires less energy to get desorbed since its desorption resistance in the EC membrane is 1.493 × 1012 mol2‚N-1‚ M-2‚s-1 compared to that of MMH (1.135 × 1013 mol2‚N-1‚M-2‚s-1), which is more strongly bound to the polymer functional groups as proved by solubility and interaction parameters as well as mass-transfer resistances.18 (c) Aging Phenomenon. Aging is a phenomenon of changes in internal structure of a substance with time and is an extremely important factor with respect to polymeric materials, especially polymeric membranes. Aging studies were conducted by soaking small strips of exact thickness and size of dry EC membrane in MMH-hydrate azeotrope. Periodic evaluations of mechanical strengths and degradation studies of the soaked membranes were followed by tensile testing (Universal Testing Machine) and FTIR analysis.21 FTIR spectra in Figure 5a,b of aged EC films exposed for 1 and 6 months, respectively, in MMH-hydrate were found to be similar to the spectra of unsoaked EC membrane in Figure 5c. A comparison clearly indicates that the membrane did not degrade even after 6 months of exposure in the MMH-hydrate environment. The ultimate tensile strength of EC membrane soaked for 6 months and then dried was found to be 27.7 kg/cm2 compared to 28.25 kg/cm2 of the original untreated
Based on Hansen and Flory-Huggins parameters, ethylcellulose was found to be useful for the dehydration of MMH-water solutions. Pervaporation experiments demonstrated that ethylcellulose membrane has good potential for breaking MMH-water azeotrope and dehydrating mixtures of various other compositions. The greater interaction and sorption of MMH coupled with its slower diffusion rate made EC more selective toward water. High selectivities and water flux were obtained at low MMH feed concentrations, and the values dropped gradually at higher concentrations due to increased membrane swelling and availability of greater numbers of MMH molecules than the EC membrane. With increasing membrane thickness, selectivity remained more or less constant, whereas flux expectedly decreased due to increasing resistance to flow. Aging studies confirmed that the EC membrane can be continuously used over 6 months for the separation of the MMH-water azeotrope without any perceptible loss of strength or damage to its internal structure. It would be uneconomical to process the dilute desalted reaction liquor of 2% MMH by pervaporation since large amounts of water would have to be vaporized, but the technique would be very economical for breaking the azeotrope and reasonably cost effective for dehydrating higher concentrations of MMH. Besides, if safety is the most important criterion, as it is in the present case, pervaporation appears to be the most viable alternative. In actual practice, a pervaporationdistillation hybrid process would be the most ideal solution to this separation problem wherein PV could be used to pass over the azeotropic barrier after which the mixture could be distilled. Acknowledgment The authors thank the Vikram Sarabhai Space CenterIndian Space Research Organization, India, for funding the studies under their RESPOND program. The Membrane Separations Group of IICT is indebted to Veratec Co., Walpole, MA, for supplying Memback nonwoven membrane support fabric. Literature Cited (1) Schmidt, E. W. Hydrazine and Its Derivatives; John Wiley and Sons: New York, 1984. (2) Huang, R. Y. M. Pervaporation Membrane Separation Processes; Elsevier Science Publishers: Amsterdam, The Netherlands, 1991. (3) Uragami, T.; Saito. M. Permeation and separation characteristics of alcohol solutions through hydrophilic polymer membrane. Abstracts of International Congress on Membrane and Membrane Process: I. C. O. M. 87, Tokyo, Japan, June 8-12, 1987; pp 576-577. (4) Huang, R. Y. M.; Jarvis, N. R. Separation of liquid mixtures using polymer membranes. J. Appl. Polym. Sci. 1970, 14, 2341. (5) Neel, J.; Aptel, P.; Clement, R. Basic aspects of pervaporation. Desalination 1985, 53, 297. (6) Choo, C. Y. Membrane Permeation. Adv. Petroleum Chem. 1962, 6, 73. (7) Baddour, R. F.; Michaels, A. S.; Bixler, H. J.; De Filippi, R. P.; Barrie, J. A. Transport of liquid in structurally modified polyethylene. J. Appl. Polym. Sci. 1964, 8, 897.
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(8) Huang, R. Y. M.; Fels, M. Separation of organic liquid mixtures by permeation process with graft copolymer membranes. Chem. Eng. Prog., Symp. Ser. 1969, 65, 52. (9) Ravindra, R.; Khan, A. A. Processing of liquid propellant reaction liquors by pervaporation. J. Appl. Polym. Sci. 1999, 72, 141. (10) Dytnerskij, Ju. I. Membranprozesse Zur Trennung Flussiger Gemische, VEB; Deutscher Verlag fur Grundstoffindustrie: Leipzig, 1977. Cited in: Rautenbach, R.; Albrecht, R. Membrane Process; John Wiley & Sons: Chichester, U.K., 1989; p 368. (11) Mulder, M. H. V.; Smolders, C. A. On the mechanism of separation of ethanol/water mixture by pervaporation. I. Calculation of concentration profiles. J. Membr. Sci. 1984, 17, 289. (12) Aptel, P.; Challard, N.; Cuny, J.; Neel, J. Application of pervaporation processes to the separation of azeotropic mixtures. J. Membr. Sci. 1976, 1, 271. (13) Crank, J.; Park, G. S. Diffusion in Polymers; Academic Press: New York, 1968. (14) Rigbi, Z. Prediction of swelling of polymer in 2 and 3 component solvent mixtures. Polymer 1978, 1, 1229. (15) Pervaporation separation of binary organic-aqueous mixtures using modified blended polymer membranes: A theoritical and experimental investigation; University of Waterloo: Waterloo, Ontario, Canada, 1989.
(16) Ravindra, R.; Khan, A. A. Solubility parameter of Chitin/ Chitosan. Carbohyr. Polym. 1998, 36, 121. (17) Barton, A. F. M., Ed. CRC Handbook of Solubility Parameters and Other Cohesive Parameters; CRC Press: Boca Raton, FL, 1983. (18) Ravindra, R.; Sridhar, S.; Kameswara Rao, A.; Khan, A. A. Pervaporation of water, hydrazine and monomethylhydrazine using ethylcellulose membranes. Polymer 2000, 41, 2795. (19) Penneman, R. A.; Audrieth, L. F. Quantitative determination of hydrazine Anal. Chem. 1948, 20, 1058. (20) Ravindra, R.; Khan, A. A. A Qualitative Evaluation of Water and Monomethyl Hydrazine in Ethylcellulose Membrane. J. Appl. Polym. Sci. 1999, 72, 689. (21) Ravindra, R.; Krovvidi, K. R.; Khan, A. A.; Kameswara Rao, A. FTIR, diffusivity, selectivity, and aging studies of interactions of water, hydrazine, and hydrazine hydrate with ethylcellulose membrane. Macromolecules 1997, 30, 3228.
Received for review October 26, 1999 Revised manuscript received April 3, 2000 Accepted April 7, 2000 IE990776C
