Ind. Eng. Chem. Res. 1995,34, 263-274
263
Silica-Treated Ceramic Substrates for Formation of Polymer-Ceramic Composite Membranes Maryam Moaddeb and William J. Koros* Department of Chemical Engineering, The University of Texas, Austin, Texas 78712
Fabrication of composite membranes from a highly porous substrate and a thin polymeric permselective layer allows achieving both high flux and selectivity in a single structure. In practice, formation of such composites is difficult due to the presence of large pores on the surface of highly permeable substrates. A novel approach to the formation of composite membranes was examined in this work to avoid this difficulty with large, hard-to-coat pores. Anopore microporous aluminum oxide membranes with 2000 A pores were used as model systems and were treated with 100 A spherical colloidal silica particles to reduce their pore diameter while maintaining low flow resistance through them. The silica-treated ceramics were then used as substrates for formation of thin defect-free polymer-ceramic composite membranes by a solutioncoating method. The treatment protocols, characterization of the treated substrates, and gas permeation results for the composites formed are discussed.
Introduction Composite membranes in which a thin layer of a selective material is supported on a highly porous nonselective substrate are especially attractive for gas separation applications because of the combination of high flux and high selectivity achievable. The selective layer (skin layer) and the substrate may be made from organic or inorganic materials or a combination of the two. The ultimate goal of formation of an ultrathin defect-free skin layer on the substrate is realized by optimization of the properties of each layer. Defects refer t o pinholes present in the skin layer which could allow viscous flow of a gas (Pinnau and Wind, 1991). Organic composite membranes with polymeric skin layer and substrates have been formed and investigated for many years (Riley et al., 1971;Ward et al., 1976; Cabasso and Lundy, 1986;Pinnau, 1989;Bikson and Nelson, 1989;Nelson, 1990;Le Roux and Paul, 1992). These membranes are typically formed by deposition of a dilute solution of a polymeric material on the substrate. Membranes used as substrates are often chemically resistant asymmetric membranes, typically formed by method described by Loeb and Sourirajan (Loeb and Sourirajan, 19631,with surface porosities of about 5% (Stratham, 19901,tortuosities of 4-5, and surface pore sizes of less than 50 A (Loeb and Sourirajan, 1963;Rezac and Koros, 1992). The low surface porosity and small pore diameters a t the interface in such membranes improve the ability to form a defect-free coating on the substrate by preventing the solution from “falling” through the pores but increase the resistance of the substrate. Another commonly practiced method for formation of composite membranes is the use of a highly permeable polymer such as silicone rubber as an intermediate layer (also referred to as a gutter layer) between the substrate and the permselective layer or as a coating for covering the defects of the skin layer. The silicone rubber layer is often necessary even when asymmetric substrates are used (Henis and Tripodi, 1981; Lundy and Cabasso, 1989). Despite its high permeability, the silicone rubber coating, in most cases, acts as an additional resistance and drops the overall performance of the composite membrane.
*
To whom correspondence should be addressed. 0888-5885l95l2634-0263$09.00I0
Ceramic membranes are becoming attractive for gas separation applications because of the recent developments in their formation and possibility of modification of their pore size by sol-gel processes (Lin and Burggraaf, 1993). The high thermal and chemical resistance of these materials make them especially attractive for applications in harsh environments. Sol-gel processes for formation of aluminum oxide membranes, for instance, result in inorganic asymmetric membranes with pore sizes in the range of 40 A (Hsieh et al., 1988). Many researchers have attempted to further reduce the pore size of such membranes by sol-gel techniques, chemical vapor deposition, or modification of the pore surface chemical properties to achieve separation by molecular sieving (Kitao and Asaeda, 1991; Lin and Burggraaf, 1992;Lin and Burggraaf, 1991;Miller and Koros, 1990). In most such cases, in effect, an inorganic composite membrane with a thin layer of silicon dioxide supported on an aluminum oxide substrate is formed (Uhlhorn et al., 1992). These efforts have resulted in fabrication of membranes with high fluxes and selectivities for some gas pairs; nevertheless, very precise techniques required to form a thin separating layer without any cracks but with a narrow pore size distribution limit wide applicability of such membranes. When separation is achieved by molecular sieving, even a small crack can be detrimental in the overall performance of the membrane. Recently, fabrication of polymer-ceramic composite membranes on asymmetric ceramic substrates was reported (Wilson et al., 1990;Rezac and Koros, 1992). This unique combination allows one to take advantage of the superior thermal and chemical resistance of the ceramic substrates as well as the excellent separation capability of high performance glassy polymeric membranes. The asymmetric inorganic substrate utilized in the works of Wilson, et al. (Wilson et al., 1990) and Rezac and Koros (Rezac and Koros, 1992)had a highly ordered “honey comb” structure of capillary pores (200 A in diameter on one surface) with approximately 50% surface porosity and unit tortuosity. The performance of the composite membranes fabricated on these ceramics was entirely determined by the selectivity and permeance of the permselective layer, because of the insignificant flow resistance in the utilized substrates. For solution coating of some lower molecular weight 0 1995 American Chemical Society
264 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Table 1. Comparison of Flux and Selectivity for Various m e s of Composite Membranes
flux (GPUO) composite type as-received Anopore PDMSVAnopore as-received AnoporeC 6FDA-IPDA2/Anopore PCVAnopore polysulfone (PSFjd PMPIPSF PDMSPSF PMP/(PDMSPSF)
Nz > 100 000 272.4 > 100 000 8.4 16.2 3000 910 160 9.74
0 2
selectivity CH4
COZ
OdNz
COdCH4
844.69
2611.44
2.0 (2.2Ib
3.1
ref Wilson et al., 1990
> 100 000
556.14 ’100 000 38.2 15.5 850 352 38
4.6 (5.1) 1.0 (4.8)
Rezac and Koros, 1992
0.94 (4.2) 2.2 (2.2) 3.9 (4.21
Le Roux and Paul, 1992
a 1 GPU = 1 x cm3(STP)/(cmHgcm2 8 ) . Numbers in parentheses are dense film selectivities reported in the referenced papers. Composite membranes in this work were formed by solution deposition on the as-received Anopore membrane. Thickness of the 6FDAIPDA layer is reported to be 0.16 pm. Composite membranes in this work were formed by spin coating. The PMPPSF composite was formed from a 0.25% solution and has three layers of PMP coating. The PMP/(PDMSIPSF) composite was formed by placing three layers of PMP (0.25% solution) by spin coating onto a PSF substrate that was previously coated with PDMS. (1)Polydimethyl siloxane, PO,= 600 Barrers, P N = ~ 280 Barrers. (2) Poly(hexafluorodianhydride isopropylidinedianiline). (3) Bisphenol-A polycarbonate. (4) Poly(4methyl-l-pentene).
polymers; however, even the 200 A pores proved to be too large and made the formation of a defect-free composite challenging or impossible (Rezac and Koros1992). In Table 1, permeability and selectivity of various types of composite membranes discussed are reported to bring the issues addressed into perspective and provide a quantitative means of comparison of the various techniques. This table is by no means comprehensive but sufficiently illustrates some advantages of monodispersed pore size ceramic substrates over traditional polymeric ones. Of course, economic and fragility of the ceramics remain serious issues to contend with in practical applications. Examples of flux through asreceived ceramic and polysulfone (PSF) substrates as well as flux through the silicone rubber (polydimethyl siloxane or PDMS) coated substrates are given. It is evident that the ceramic substrate utilized in the work of Wilson (Wilson et al., 1990) and Rezac (Rezac and Koros, 1992)has a much higher flux than the polymeric substrate used by Le Roux (Le Roux and Paul, 1992) prior and after coating with silicone rubber while the same oxygenhitrogen separation factor is achieved in each case. Rezac and Koros (Rezac and Koros, 1992) used high-performance glassy polymers on the Anopore substrate without an intermediate layer and obtained excellent results in most cases, but not always (see Table 1). On the other hand, as reported by Le Roux and Paul (Le Roux and Paul, 1992) direct spin-coating of poly(Cmethyl-l-pentene) (PMP) on an as-received polysulfone support resulted in a nonselective membrane. In this work we demonstrate the feasibility of formation of thin defect-free polymer-ceramic composite membranes from ceramic substrates modified by solgel techniques. We first reduce the pore size of the ceramic substrate by causing colloidal silica particles to gel inside the pores and near the surface and then deposit the skin layer on the modified substrate. Modification of support morphology by this technique results in a substrate with a pore size in the range suitable for coating with a dilute solution of a high-performance polymer, yet with a relatively low flow resistance. Such unique properties are realizable due to the highly porous nature of the dried silica network and its concentration near the pore entrances. Since a polymeric material is used as the thin separating layer, the need for the precise chemistry often required in sol-gel process for reduction of pore size to molecular sieving range is eliminated.
Theory The overall molar flux of a gas, Ni, through a composite membrane is determined by the contributions of the resistances of support and the substrate as described by the series resistance model (Henis and Tripodi, 1981; Pinnau et al., 1988):
Ni=
APi
[ ) ( lskin
Pi,skin
+
lsupport
)
(1)
Pi,support
In eq 1, Api is the partial pressure driving force of component i across the membrane, I represents the thickness, and Pi is the permeability of component i in each layer. The (Z/P)i terms are measures of the resistance in each layer. Clearly, maximum flux through the composite is realized only for membranes in which the (Z/P)su,,portis negligible compared to that of the permselective layer. In such cases, the expression for the overall flux through the composite essentially reduces to the expression for flux of a gas through a polymeric membrane as described by the solutiondiffusion mechanism (Hellums and Koros, 1989):
where Pskin,the permeability of the permselective layer, measured in Barrers (1 Barrer = 1 x (cm3(STP) cm)/(cmHg cm2 SI), represents the overall permeability of the composite. When the resistance of the composite to gas permeation lies within the permselective layer, the ability of the composite membrane to separate gas A from a mixture of A and B, a m ,may be determined from the ratio of the permeability of pure gas A in the skin to the permeability of pure gas B in the skin layer (Pinnau et al., 1988):
(3) If eq 3 holds, the thickness of the permselective layer for a defect-free composite can be estimated from a knowledge of the permeance of the composite membrane, (P/Z)composite, and the permeability of the dense film of the polymer, P, from the expression
(4) Permeance, or pressure normalized flux, is typically
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 266 reported in gas permeation units (GPU) (1GPU) = 1 x cm3(STP)cm)/(cmHg cm2 s)). In this work composite membranes that exhibited 90% of the selectivity of the dense film were considered free of defects. For such composites, the thickness of the permselective layer was determined using the overall flux of the composite (from experimental measurements) and the nitrogen permeability of the dense film. Previous work showed this estimate of the thickness from gas permeation measurements to be in good agreement with the thickness observed under an electron microscope (Rezac and Koros, 1992). Flow type in a porous substrate depends on the magnitude of the radius of its pores. Nonselective viscous flow occurs in large-pored substrates (larger than the mean free path of the molecule). Knudsen flow occurs when the radius of the pore is smaller than the mean free path of the molecules (Hines and Maddox, 1985; Koros, 1990). In this type of flow the separation is based on the inverse square root ratio of the molecular weights of the two gases. Knudsen diffusion through the porous substrate is essentially nonselective compared to the polymeric membrane, and its contribution to the overall flux of the composite is often negligible (Koros, 1990). Further reduction of pore size is undesirable since it increases the resistance of the substrate. Surface diffusion due to surface adsorption of molecules can contribute t o the overall flux a t certain temperatures and pressures, specifically for a high sorbing gas such as COz. When surface diffusion is of importance, total permeability deviates from the gas phase permeability predicted from expressions for Knudsen and viscous flow (Keizer et al., 1988). Pore size modification in our work aimed at reduction of pore size t o achieve Knudsen flow in the substrate. Surface diffusion was expected to be negligible at temperature and pressures of our experiments. However, Knudsen flow in the modified substrates was verified by comparison of the selectivity of the silicatreated substrates for various gas pairs with the theoretical Knudsen selectivity. Also, for pure Knudsen flow a pressure normalized plot of flux vs mean pressure should give a straight line with a slope of zero for a low sorbing gas such as helium (Dullien, 1992; Uchytil et al., 1992). In this work, we modified 2000 8, (DIA) pores of asymmetric microporous substrates. Our key objective was t o produce a thin, small-pored but low resistance uniform gelled silica layer at the entrance to the large 2000 8, pores. It was expected that subsequent deposition of a rate limiting polymer film on this asymmetrically formed silica on alumina support would yield a low-resistance composite with the intrinsic selectivity of the organic polymers selected for study.
Sol-Gel Process for Modification of the Substrate Colloidal silica is a dispersion of silicone dioxide particles in an aqueous o r organic dispersing medium. Their synthesis and properties are extensively researched and well documented (Larbot et al., 1988; Myers, 1991). The polymerization reaction used in formation of these colloids results in the formation of suspended spherical particles in a state of high total surface energy, because of the large interfacial area of contact between the particles and the dispersing medium (Larbot et al., 1988). Such systems are inherently unstable and, unless stabilized, tend to form aggregates.
Stabilization of colloids is achieved by electrostatic repulsion between the particles of like charges by adjusting pH and electrolyte levels (Myers, 1991; Brinker and Scherer, 1990). On the other hand, an existing colloidal solution may be destabilized either by changing the pH of the solution to eliminate surface charges or simply by evaporation of the dispersing medium. Both actions promote collisions between the particles and formation of siloxane bonds at sites with unreacted hydroxyl groups, ultimately leading t o the development of a three-dimensional gel network. The final pore size and the pore volume of the dried three-dimensional gel network depends on the type of solvent and its rate of evaporation. Rapid evaporation of the solvent gives insufficient time for condensation reactions between the reactive sites on the particles; thereby resulting in a relatively weak nascent network which easily collapses to yield small-pore diameters with a high likelihood of cracks in the dried network. Slow drying processes allow more time for condensation reactions, and a stiffer more expanded large-pored network with fewer cracks (Brinker et al., in press). In this study both evaporation of the dispersing medium and change of pH were used to induce gelation of the sol. Qualitatively, the effectiveness of the silica treatment for reduction of pore diameter was investigated by the scanning electron microscope (SEMI. However, determination of pore size distribution and the largest pore size were done by a progressive liquid displacement technique or the bubble point test (Kesting, 1985).
Experimental Section I. Materials. Colloids. A colloidal dispersion of 100 8, silicon dioxide particles in 2-propanol, IPA-ST, kindly provided by the Nissan Chemical Ind. was used in this work. The 30 wt % sol has a pH range of 3-5 and was used as received. According to the manufacturer, this colloid is stabilized by electrostatic stabilization using sodium hydroxide as the electrolyte. Substrates. Chemically resistant microporous Anopore aluminum oxide filters prepared by Anotec Separations were used as the ceramic supports in this work. These membranes are flat and circular (47mm DIA). They are 60 pm thick, have about 50% surface porosity, and unit tortuosity. The asymmetric membranes with 200 8, pores on one surface and 2000 8, pores on the opposite surface are prepared by anodic oxidation (Furneaux et al., 1985; OSullivan and Wood, 1970). The 200 8, pores are approximately 1pm long. The bulk of the membrane; however, consists of highly uniform 2000 8, pores. Figure 1shows a schematic representation of the structure of the Anopore ceramic as well as images of the surface with 2000 8, pores and the cross section of an as-received ceramic taken by an electron microscope. It was our objective to gel the IPA-ST colloid at the entrance to the large 2000 8, pores. Polymers. Four glassy polymers from three different family of polymers were used in the formation of composite membranes. The selected polymers were poly(hexafluorodianhydride-isopropylidinedianiline) (GFDA-IPDA),poly(hexafluorodianhydride-methylenedianiline) (6FDA-MDA),tetramethylhexafluoro polysulfone (TMHFPSF), and bisphenol A polycarbonate (PC). The structure of the repeat unit of the polymers used and some of their pertinent properties are shown in Table 2.
266 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995
d = 200A
Figure 1. (a, top) Schematic representation of the Anopore aluminum oxide asymmetric ceramic. (b, middle) Surface with 2000 A pores of an as-received asymmetric ceramic. (c, bottom) Cross section of an as-received asymmetric ceramic.
The selection of these polymers was primarily based on their attractive gas transport properties and the size of their hydrodynamic diameter. It has been demonstrated that formation of a defect-free coating on a microporous ceramic is possible when the hydrodynamic diameter of the polymer coil in the dilute polymeric solution is larger or of the same order of magnitude as the diameter of the pore on the substrate (Rezac and Koros, 1992). The polycarbonate sample used in this work has a bimodal distribution of molecular weights resulting in a distribution of molecular diameters which range from 90 to 290 A (Rezac and Koros, 1992). This polymer was used primarily as a test for the pore size, since despite the presence of some polycarbonate chains in the range of 290 A ,it is essentially not possible to coat this olymer on a surface with pores larger than about 90 (Rezac and Koros, 1992). The polymers used in this work were either purchased or supplied by various donors. Properties of dense films of these polymers have been investigated (Hellums, 1990;Kim,
A:
1987;McHattie, 1990; Ruiz-Treviiio, 1994). Polymer densities and glass transition temperatures (Tg)had been previously measured in a density gradient column and by a Perkin-Elmer differential scanning calorimeter (DSC 7)at a heat rate of 20 "C/min. The hydrodynamic diameters were calculated based on the measurement of the retention time from a gel permeation chromatograph (Rezac and Koros, 1992). Solvents and Gases. Reagent grade methylene chloride was used without further purification as the solvent for the polymers. Ultrahigh-purity compressed oxygen, nitrogen, helium, and methane and instrument grade C02 were purchased from Linde and were used without further purifications. 11. Equipment. Gas Permeation Experiments. The gas permeability and selectivity of the silica-treated substrates and the composite membranes were determined from measurements of pure gas fluxes. Measurements were done using Millipore test cells at 25 "C and atmospheric pressure downstream unless otherwise noted. The volumetric flow rate was measured by a soap-bubble flow meter. For the majority of the work reported here the gas permeation area is 13.2 cm2. In a few instances, it was necessary to mask the composite with aluminum tape to improve its mechanical integrity and to cover imperfectly coated areas near the circumference of the ceramic. In such cases, the permeation area was reduced to approximately 4.0 cm2. Pure gases were applied to the upstream side of the membrane at a fured pressure. Once at steady-state, the flow rate of the permeate was determined with the bubble flow meter. Upstream pressures of around 70 psia were typically applied for the composites, but the uncoated silica-treated substrates were exposed to lower upstream pressures due to their lower flow resistance and their fragile nature. Incremental Liquid Displacement Test. This test is well described in the literature (Kesting, 1985),and its concept is based on the inverse relationship between the capillary pressure, p, and the pore diameter, d, as depicted in eq 5, where y u is the liquid-liquid surface
tension and is the contact angle. To estimate the pore size in the treated asymmetric membranes, the membrane was first completelywetted with butanol-saturated water in a Millipore test cell. Then, about 5 mL of butanol-saturated water was poured on one surface and the test cell was closed. The nonwetting fluid, water-saturated butanol, was brought into contact with butanol-saturated water on the membrane by opening the valve connected to the butanol reservoir. Initially, water-saturated butanol was pressurized with helium at a very low pressure corresponding to a pore size larger than 2000 & and the flow of water through the membrane was measured. Flow stops when butanol comes in contact with the membrane surface and the water in the pores. The test continued by incrementally increasing the pressure and measuring the steady-state flow rate. The convective flux of pure butanol through the membrane was measured in a separate experiment.
Experimental Procedure After a series of preliminary experiments two protocols were selected for pore size modification, one by solvent evaporation and the other by changing the pH.
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 267 Table 2. Polymers Used for Formation of ComDosite Membranes polymer structure of the repeat unit Tg("C) GFDA-IPDA
310
1.352
da (A) 192
GFDA-MDA
304
1.400
230
TMHF'PSFb
243 233
1.286
280 =A40
McHattie, 1990 Ruiz-Treao, 1994
PC
150
1.200
92 (290)
Hellums, 1990
~ _ _ _ _
L
density (p/cm3) ~
ref Kim, 1989
Kim. 1987
-'n
Values of hydrodynamic diameter from Rezac and Koros, 1992. The PC sample shows a bimodal distribution of molecular weights and therefore a distribution of hydrodynamic diameters (Rezac and Koros, 1992). The "MHFPSF sample used in this study is similar to the polymer studied by Ruiz-Trevifio (1994). The molecular weight and hydrodynamic diameter of the sample studied in this work was estimated using the value of the intrinsic viscosity reported by Ruiz-"revifio ([VI = 0.34d u g measured in chloroform at 25 "C (RuizTrevhio, 1994);[ q ] = 0.82 d u g (McHattie, 1990)). a
Treatment of the Ceramic Substrates by Evaporation of 2-Propanol. The asymmetric substrates were first rinsed with 2-propanol and water to remove surface impurities. After drying, the ceramic was placed on a support that only contacted the rims of the filter. Then, 1mL of IPA-ST was deposited on the surface with 2000 A pores, and the 2-propanol was allowed to evaporate in a controlled environment. When all the solvent had evaporated, the excess silica on the surface of the ceramic was removed and the surface was carefully cleaned with a Kimwipe tissue. The amount of silica deposited in the pores was determined by weighing the ceramic with a Mettler AE 163 (accuracy & 0.0001g) balance before and after the treatment. Two more deposits of silica on the same surface were made following the same procedure. Treatment of the Ceramic Substrates by Changing the pH. After cleaning the surface with water and 2-propanol and drying the ceramic, enough 1M solution of hydrochloric acid was deposited on the surface with 2000 A pores to fill up the pores in the substrate. The excess acid on the surface of the ceramic was removed to minimize surface agglomeration. Then, 1mL of the IPA-ST (pH = 4) was deposited on the ceramic. The presence of the 1 M HC1 inside the pores reduces the pH of the sol to about 2 due to diffusive exchange. At the isoelectric point of silica (pH= 2) electrostatic particle repulsion is suppressed and agglomeration occurs. After drying, the excess silica was removed from the surface with a Kimwipe tissue and the same procedure was repeated one more time. In the final step, to ensure no defects were present on the surface, 1mL of IPA-ST was deposited on the surface, and the 2-propanol was allowed to evaporate. The weight change of the ceramic was monitored before and after each step. Preparation of the Composites. Composites were formed by a solution deposition method. Dilute solu-
tions of polymer in methylene chloride (0.1 w t % unless specified) were prepared. The solutions were stirred on a stirring plate in tightly sealed bottles for at least 6 h. Prior to use, all polymer solutions were filtered either by vacuum filtration using 10 pm Teflon filters or by a syringe and 0.2 pm PTFE microfilters. The solutions were used soon after preparation to avoid a significant change in the solution concentration due t o solvent evaporation. The silica-treated substrates were placed on a leveled Teflon surface, and then 1 mL of the polymeric solution was placed on the surface of the substrate and the substrate was covered so that evaporation of the solvent occurred in a controlled environment. The membranes were allowed to dry in air for about 24 h and then in a vacuum oven at 80 "C to 100 "Cfor at least 4 h prior to testing. The amount of the polymer deposited on the support was determined by weighing the ceramic before and after coating.
Results and Discussion Analysis of the Evaporation-Induced Gelation Protocol for Pore Size Reduction. Three sets of ceramics corresponding to one, two, or three deposits of silica were prepared and analyzed. Duplicate or triplicate samples of each set of conditions were prepared to determine the reproducibility of the results. Qualitative SEM analysis of the surface of the treated ceramics revealed defects as large as 1700 A on the surface of the ceramics with one and two deposits. No large pores were detected on the surface of the ceramics with three deposits of silica (Figures 2a-d). Since a very small area of the membrane is viewed by the electron microscope, it is premature to conclude that the surface after the third deposit is thoroughly free of defects. Gas permeation results for composite membranes, discussed later, further clarify this point. The
268 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 m
Pigare2. (a, top left) Surface and (b, middle leR) cross section of an Anopore ceramic after the first step of evaporation-inducedgelation protocol. (c, bottom left) Surface after the second step of the evaporation-inducedgelation protocol. (d, top right) Surface and (e, bottom right) cross section of the ceramic after the third step of the evaporation-induced gelation protocol.
images of the cross section after the fist and third deposits (Figures2b,e) are similar, suggestingthat after the first deposit most of the silica is placed near the surface. Clearly, a layer of silicon dioxide particles covers the surface of the ceramic after three deposits of silica. To determine the approximate depth of this layer, the cross section of the substrates treated with three deposits of silica were scanned with the electron microscope to detect the presence of silicon and aluminum. The scans were done from the modified surface toward the opposite
surface and vice versa. As the electron beam hits the modified surface the silicon signal rises but then drops as the beam moves across the cross section (see Figure 3a). Similarly, the cross section was scanned for aluminum. In this case, there is also a rise in the aluminum signal immediately after the electron beam hits the modified surface; however, the aluminum signal remains constant throughout the cross section as expected. The essentially immediate rise of the signals for both elements a t the silica-treated surface suggests the presence of at most an ultrathin (50.2 pm) silica
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 269 Table 3. Resalts for the Weight Gained by the Ceramics Treated by Evaporation of 2-Propanol wtgained wtgained wtgained aftertwo afterthree initial afterone membrane wt? (g) deposit (g) deposits (g) deposits (g) one depositb 0.1945 0.0320 two deposits6 0.2006 0.0320 0.0331 three depositif 0.2050 0.0340 0.0350 0.0350
1
a Accuracy fO.OO1 g. Values are averaged for three ceramics. Values are averaged for two ceramics.
Table 4. Flux in GPU of Various Gases and Selectivities for the Ceramics Treated by Evaporation of 2-Propanol
onedeposiv 3005 2793 7507 3001 0.929 twodepositso 3040 2835 7224 2960 0.933 threedepositsb 2852 2674 6812 2823 0.937 Knudsen flow 0.935
I 100 000 GPU for the as-received filter); however, these flow rates are still high enough to allow the assumption of negligible resistance in the modified substrate compared to the permselective layers comprised of the polymers examined in this study. The separation factor for various gas pairs in the silicatreated ceramics matches those of Knudsen flow except for C O D 2 and HdC02 selectivities. As noted earlier, surface diffision for a high sorbing gas such as CO2 may contribute to its permeation. Deviations of C O D 2 and HdC02 from the theoretical Knudsen selectivities for these gas pairs are consistent with the effects of surface diffision. This may be verified by testing the membrane at higher temperatures. Overall, for the low sorbing gases tested here, Knudsen diffusion seems to be the primary mechanism of transport. Plots of pressure normalized flux vs average pressure also resulted in a straight line with a slope of zero; indicating pure Knudsen flow in these silica-treated substrates (Figure 4).
Incremental liquid displacement tests were performed to determine the pore size distribution. A plot for the flow of butanol against water vs. pore diameter is shown in Figure 5. Pore sizes ranging from 200 A to as small as 50 A were detected. To detect pores smaller than 50 A with the butanol-water system, the membrane must be placed under pressures as high as 250 psig. The ceramic substrates usually crack under such pressures; thus it was not ossible to obtain any data for pores smaller than 50 . A flow rate of 0.0002 mumin was measured at a pressure of 9.6 psig corresponding to a pore size of 1100A Such a small flow rate is indicative
1
270 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 Table 5. Reeults for the Weight Gained by Ceramics Treated by Changing the pHQof the So1 wtgained wtgained wtgained initial after one after two after three membrane 4 (g) deposit (g) deposits (g) deposits (g) s t e l~b 0.1955 0.0374 step 2b 0.1978 0.0427 0.0413 ~ t e p 3 ~ 0.2138 0.0480 0.0471 0.0480 a
About 0.0050 g weight loss was observed in some cases due to the interaction of hydrochloric acid with aluminum oxide. Accuracy fO.OO1g. Average of values for two ceramics. Average of values for four ceramics.
L
4.5
2
2.1
2.2
2.3
2.4
2.5
2.6
P x 1 0 5 (Pa)
Figure 4. Illustration of Knudsen flow from a plot of pressure normalized flux of He vs arithmetic average of upstream and downstream pressures for ceramic substrates after the final step of evaporation-induced gelation protocol. 0.007
I
Table 6. Flux in GPU of Various Gases and Selectivities for the Ceramics Treated by Changing the pH of the Sol@ ceramic support NZ 02 He C02 aoa, acoa, aHe/col step1 5752 5333 13479 5283 0.927 2.34 0.918 2.55 step2 2878 2713 7185 2494 0.943 2.50 0.867 2.88 step3 2044 1973 5429 1841 0.965 2.66 0.900 2.92 0.935 2.64 0.798 3.32 Knudsen flow
I
=50A 0.006 0.005
x
1
I
0
0.004
E:
0.003 0.002 0.001
to
I
1:
0
'
0
'
1
200
c
I
I
400
j
I , ,
600
I , ,
800
1
lo00 1200
Pore diameter (A)
Figure 5. Incremental liquid displacement test for asymmetric ceramics after the final step of the evaporation-inducedgelation protocol.
of a very small population for pores of this size, if any, since 0.0002 mumin may be considered essentially negligible within experimental uncertainty. Yet, the presence of even one pore of such a large magnitude is detrimental in the formation of a defect-free skin layer. Although observations noted above imply that no large defects are present after the third deposit, this issue is more rigorously demonstrated by the ability to form a defect-free skin layer on the modified substrates from a dilute polymeric solution. These results are shown in a subsequent section. The stability of the treatment for the modified ceramics was examined by placing them in various solvents including caustic KOH for up to 4 days and monitoring their weight change. The deposited silica was not removed by the solvents. Analysis of the pH Change Induced Gelation Protocol for Pore Size Reduction. Properties of three sets of ceramics modified following this procedure were evaluated after each step. The results for the weight gained and the flux of various gases through these treated ceramics are presented in Tables 5 and 6. The overall weight change for the ceramics modified by changing the pH (designated as type B substrates) is slightly higher than the overall weight change for the ceramics modified by evaporation of the solvent (designated as type A Substrates). Although long-term contact of aluminum oxide with HC1 could result in damage to the ceramic, it was expected that no significant damage
Values reported are averaged for three ceramics. 1 GPU = 1 cmS(STP)/(cmHgs em2).
was induced to the structure of the ceramic as a result of its short-term contact with the small amount of the acid. This was verif'ied by gas permeation measurements and by examining the surface and the cross section of the ceramic under the electron microscope before and after contact with 1 M HC1. Weight gain results for this protocol also indicate a trivial amount of silica uptake in the steps following the first deposit. However, as the SEM pictures indicate these steps are significant for the crack-free and complete coverage of the porous ceramic substrate. SEM pictures of the surface of a ceramic modified with one pH change induced gelation step followed with one deposit of IPA-ST revealed the presence of large pores on the modified surface; while the images of the ceramics with two pH change induced gelation steps followed with one deposit of IPA-ST appeared free of any defects (see Figure 6). Several sections of the surface were carefully and thoroughly examined for the presence of cracks or defects; none was detected. The second pH induced gelation is critical for plugging up the pores left aRer the first step. Since the size of the primary particle used in our work is 100 8, , after the first step of the treatment the silica particles only penetrate into defects that are larger than 100 8,. The presence of acid promotes gelation of the silica particles inside these defects. Aggregation of the silica particles is immediately observed as IPA-ST is added to the ceramic surface wetted with the 1M HC1 solution, thus making the path for evaporation of 2-propanol molecules more tortuous. This process may also be retarded by hydrogen bond formation between the alcohol and water. As illustrated in Figure 7, the rate of evaporation of 2-propanol after a change in pH of the sol is slower than the rate of evaporation of 2-propanol without a pH change. This slower rate of evaporation gives silica particles more time t o aggregate and form a stronger network. As a result, the formed structure better withstands the tension caused by evaporation of 2-propanol, thereby reducing the possibility of crack formation (Brinker et al., 1994) compared t o the purely evaporation induced gelation protocol. The cross section of type B substrates were also scanned for silicon and aluminum following the same outlines discussed for type A substrates. A rise in the
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 271
2
1
1.s
1.6
1.8
2.2
2
2.4
2.6
P IO-5 (pa) Figure 8. Illustration of Knudsen flow from a plot of pressure normalized flux of He vs arithmetic average of upstream and downstream pressures for ceramic substrates after the final step of pH-induced gelation protocol. Error bars are smaller than the points. 0.016 0.014
' n
-2
0.0 12 0.01
0.008 0.006
Figure 6. (a, top) Surface of a ceramictreated with one pH change step and one evaporation step. (b, bottom) Surface of a ceramic after the final step of the pH change induced gelation protocol.
E o
P A evaporationrate aftapH change P A evapmtionrate without pH change
0 0
2000
4Ooo
6OOo
m
loo00 1200 14Ooo
Time (=)
Figure 7. Comparison of the rate of evaporation of 2-propanol from IPA-ST with and without a pH change.
signals for both elements was observed immediately as the beam hit the modified surface suggesting that in this type of modification, similar to the previous case, the silica is primarily placed inside the pores and only an ultrathin layer of deposit exists on the surface. The flux of various gases and selectivity of type B substrates a h r the final stage of the treatment are listed in Table 6. At the final step of the treatment, the gas fluxes are lower for type B substrates than the fluxes at the final step for type A substrates. Therefore,
i c
50
100
150
200
250
diameter (A>
Figure 9. Incremental liquid displacementtest for asymmetric ceramics after the final step of the pH-induced gelation protocol.
the resistance of the substrate was increased more by the pH change induced treatment than by the solvent evaporation treatment suggesting a denser silica network. Knudsen flow in the modified substrate was verified from a plot of pressure normalized flux versus mean pressure (see Figure 8). Calculations using the series resistance model (eq 1) confirmed that the resistance of both types of silicatreated substrates examined in this work for all gases tested is less than 6% of the overall resistance of the composite membranes formed from the polymers studied. In fact, for nitrogen and oxygen the resistance of the substrate is on average less than 0.3% of the overall resistance, and for helium it is about 1% of the overall resistance. The resistance of the support should be less than 10% of the overall resistance of the composite membrane to achieve acceptable gas transport properties (Pinnau and Koros, 1991). Since the resistance of the silica-treated substrates was low enough to allow formation of a composite membrane with the intrinsic properties of the polymeric skin layer, no further attempts were made to change the treatment protocols to obtain higher flow rates in the modified substrates. Resistance of the silica-treated substrates may be further reduced by inhibiting the diffusion of silica particles into the pores of the ceramics, thereby decreasing the thickness of the silica layer deposited inside the pores. This can be achieved by
272 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Table 7. Gas-TransportProperties of Composites Formed from Substrates Modified by the Two Different Protocols composite 6FDA-IPDA type Ab (A = 3.43 cm2) type Be dense film 6FDA-MDA type A type B dense film TMHFPSF type A type B dense film PC type A (A = 4.91 cm2) type B (A = 4.15 cm2) dense film
Nz
0 2
He
coz
aofi2
aHm2
WOD,
aHe/co2
la@m)
1.97 3.43 1.39 1.3
12.45 19.74 7.84 6.7
109.8
79.68
55.76
40.47
1.38
64.7 59.2
42.7 30.0
6.32d 5.76 5.64 5.1
46.55 45.5
30.72 23.1
1.52 2.37
0.66 0.38 0.94
1.51 0.88 0.8
9.03 5.47 4.6
82.2 52.4 50.0
55.2 35.76 19.3
5.98 6.20 5.7
54.44 59.41 62.5
36.56 40.54 24.13
1.49 1.47 2.59
0.53 0.91
4.35 4.01 4.5
23.39 20.76 28.25
18.58 16.55 18.0
1.26 1.25 1.57
0.61 0.54
4.55d 4.16d 4.8
31.93 40.39 39.4
18.07 22.31 21.25
1.77 1.82 1.91
1.18 0.92
6.53 7.37 4.0 0.28 0.36 0.33
28.4 29.6 18.0 1.26 1.5 1.6
152.8 153.0 113.0 8.94 14.54 13.0
121.3 122.0 72.0 5.06 8.03 6.8
a Thickness of the permselective layer was estimated for defect-free composites from nitrogen permeability of the dense film. * Type A: composites formed from ceramics modified by solvent evaporation. Type B: composites formed from ceramics modified by pH change. dComposite was masked with aluminum tape. Permeation area for the masked composites is given in parentheses. Dense film permeabilities and selectivities were measured a t 35 “C. Permeabilities in Barrers. References: Kim, 1987; McHatie, 1990; Hellums, cm3(STP)/(cmHgcm2 s), 1 Barrer = 1 x cm3(STP) cm/(cmHg cm2 9). 1990. 1 GPU = 1 x
increasing the viscosity of the colloid by addition of a polymer (McHenry, et al., 1993). Incremental liquid displacement tests resulted in similar results for the type B substrates with a sharp increase in flow rate a t about 56 fi (see Figure 9). Overall, the evidence suggest that the pH induced pore size modification results in a uniform and crack-free deposition of the silica particles inside the pores. Formation of Polymer-Ceramic Composites from the Silica-TreatedSubstrates. Analysis of the evaporation induced gelation protocol and pH change induced gelation protocol suggest that the substrates at the final step of each treatment procedure are the only ones suitable for coating with a polymeric solution. Therefore, these modified substrates referred to as type A (evaporation induced) and type B (pH induced) were used in the composite membrane formation following the previously outlined procedure. The permeance and permselectivity of the composite membranes formed from the four different polymers studied on both types of substrates are listed in Table 7. Dense film selectivities and permeabilities are also reported for comparison. In this work composite membranes that exhibit approximately 90% of the selectivity of the dense film are considered essentially free of defects. This selectivity is determined for the composite membrane without any posttreatment of the permselective layer (Rezac et al., 1993; Pinnau and Wind, 1991). As reported in Table 7, thin (ranging from 0.4 to 1.0pm) defect-free composite membranes were formed from the four different polymers on the two types of silica-treated ceramics. However, the yield of defectfree composites formed on the substrates modified by the pH treatment was higher than the yield of the defect-free composites formed on the substrates treated by solvent evaporation. Approximately 85% of the GFDA-IPDA and GFDA-MDA composites formed from the substrates modified by changing the pH were defectfree; whereas lower yields (Mi0-60%) were obtained for composites formed on type A substrates. Two main factors influence the quality of the polymeric coating on any ceramic substrate: the pore size distribution of the substrate and the polymer deposition process. Defectfree composite membranes are not always formed even if a ceramic substrate with a narrow pore size distribution and reasonably small pore sizes (for instance 100-
200 A pores) is utilized. The presence of any foreign particles in the polymeric solution, for instance, could easily result in pinholes and prevent the formation of a thin defect-free skin layer. In the case of polymerceramic composites formed on type A substrates, defective composites were formed even when extreme care was given to preparation and deposition of the polymeric solution. In this case, defects in the silica-treated substrate rather than problems in the coating step were more likely the cause of the lower yield of formation of defect-free composite membranes. The much higher yield of defect-free membranes for the composites formed on type B substrates is a clear indication that this protocol for modification of the pore size of the ceramics is preferred for uniformly reducing the pore size. It is not surprising that formation of defect-free composite membranes from the PC and TMHFPSF samples used in our work proved to be quite challenging. In effect, by choosing to modify the surface with 2000 A pores with silica and to coat the modified ceramic with these low molecular weight polymers, we examined the treatment protocol for the “worst” possible case. Still, about 50% yield was obtained for composites formed on type B substrates. However, poor results were obtained for composites formed on type A substrates, again suggesting less perfection in the pore size achievable via this protocol. The results for the composites formed on type B substrates are impressive considering that no posttreatment was used to enhance the permselectivity of these membranes. Higher yields of defect-free membranes can be obtained by application of such treatments (Rezac et al., 1993; Pinnau and Wind, 1991). For practical gas separation applications, it is desirable to have defectfree ultrathin ( < 0 . 2 pm) permselective layers. The primary goal of this work was to illustrate the feasibility of this novel approach for formation of composite membranes for an extreme case (modification of 2000 fi pores). Clearly, the feasibility of the concept has been demonstrated with composite membranes with permselective layers as thin as 0.38 pm exhibiting 90% of the selectivity of the dense film without any posttreatments. No significant dimculties are expected in formation of skin layers of lower thickness; especially from the high molecular weight polymers such as GFDA-
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 273 IPDA and GFDA-MDA on type B substrates. In fact, the pore size protocol discussed here facilitates formation of ultrathin skin layers by plugging up defects on the substrate. Referring back to Table 1 and the data reported by Le Roux and Paul (Le Roux and Paul, 1992), the asreceived PSF membrane used in their work exhibited an N2 flux of 3000 GPU. A 1.7 pm layer of PDMS was necessary to coat this substrate (Le Roux and Paul, 1992). The pore size of this asymmetric membrane is not reported; however, based on its nitrogen flux a pore size between 50 and 100 A (closer to 50 A) is assumed for this asymmetric membrane. Coating 59 pm long, 2000 A (DIA) pores requires a much thicker layer of silicone rubber (possibly a 60 pm thick layer). Assuming a conservative value of 30 pm for the thickness of the PDMS layer required to coat the 2000 A pores, and assuming that it is possible to form a 0.38 pm defectfree permselective layer of GFDA-IPDA with the nitrogen and oxygen fluxes reported in Table 7 on this precoated substrate, we can use the series resistance model to estimate the N2 and 0 2 fluxes for this composite. Such a calculation indicates a 27% drop in the NOflux and about a 50% drop in 0 2 flux for the silicone rubber treated substrates compared to the fluxes reported in Table 7 for the 0.38 pm GFDA-IPDA on the silica-treated substrate. The overall oxygednitrogen selectivity also drops to 3.97 instead of 5.1 (the intrinsic selectivity of GFDA-IPDA). Clearly, the silica treatment is a much more attractive alternative for modification of the substrate morphology than the use of silicone rubber. Utilization of the reported silica treatments in practical applications requires further fine tuning of the procedures involved. Economic considerations clearly play an important role in selection of ceramic versus polymeric substrates such as microporous polysulfone membranes. Consideration of brittleness and higher cost of ceramic substrates over polymeric membranes may tilt the balance toward polymeric membranes, despite the attractive thermal and chemical properties of ceramic substrates. The next logical step would be to extend the results of this work with “model”ceramic supports to the more cost effective polymeric substrates. This work is currently under investigation.
Summary The feasibility of fabrication of thin defect-free composite membranes from a variety of high performance polymeric materials on inorganic substrates treated with silica sols is demonstrated. The 2000 A pores of aluminum oxide membranes were reduced by causing colloidal silica particles to gel inside the pores. Gelation was induced by evaporation of the solvent and by changing the pH of the sol. Qualitative and quantitative analysis of the modified substrates verify that the particles are placed inside the 2000 A pores and primarily near the modified surface. Gelation by changing the pH appears to result in a tighter, crack-free network than the evaporation induced gelation. The technique illustrated for excessively large pores may easily be extended to substrates with smaller pore sizes or for plugging up defects; thereby, promoting defectfree fabrication of ultrathin composite membranes with the intrinsic properties of the polymeric membrane. Acknowledgment The authors gratefully acknowledge the financial support provided by the State of Texas Advanced
Technology Program and the Separations Research Program of the University of Texas at Austin. Special thanks go to Nissan Chemical Ind, Ltd., Specialty Chemical Division, for kindly providing the colloidal silica used in this study.
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IE940241D @
Abstract published in Advance ACS Abstracts, November
1, 1994.
