Ind. Eng. Chem. Res. 1993,32, 1921-1926
1921
MATERIALS AND INTERFACES Aging of Thin Polyimide-Ceramic and Polycarbonate-Ceramic Composite Membranes Mary E. Rezac, Peter H. Pfromm, Lora M. Costello, and William J. Koros. The University of Texas, Department of Chemical Engineering, Austin, Texas 78712
The gas-transport properties of several polymer-ceramic composite membranes with glassy polymer layers having effective thicknesses of approximately 4000 A exhibit strongly time-dependent behavior. The membranes, prepared from polyimide and polycarbonate, demonstrated decreases in gas flux of 40-60% in the first 20 days following manufacture. The pure-gas selectivity of these membranes for the He/N2 separation increased over the same period to values approximately 50% higher than those of a thick film of the same material. Substructure compaction of these membranes is nonexistent, unlike integral-asymmetric polymeric membranes, so transport property changes are less ambiguous to interpret. We suggest a true glassy state drift in the properties of the selective layer of the composite membranes as the cause for our observations. Our observations show that transport properties determined on thick films of glassy polymers may differ significantly from the properties of thin, gas permeation membranes.
Introduction The process of physical aging in polymer films has been extensively documented (Struik, 1978; Tant and Wilkes, 1981; Matsuoka, 1981; Bubeck et al., 1984; Lee and McGarry, 1990; Bartos et al., 1990; Moe et al., 1988a and 1988b; Koros and Chern, 1987). The majority of these studies have focused on the effect of aging on the physical properties, specifically,mechanical strength and modulus. Very few studies have evaluated the effects of time on the gas-transport properties of polymeric membranes or films. Investigations of transport properties of polymer films with realistic thicknesses (4 pm) for practical gasseparation processes are quite rare in the literature. The manufacturing of thin films that are defect-free, as determined by gas permeation, that do not require coating or posttreatment to achieve gas selectivity and that can be tested a t realistic gas pressures for permeation is exceedingly difficult. Recently, Pinnau (1991) reported on time-dependent fluxes and gas selectivities of ultrathin asymmetric membranes made from 6FDA-IPDA polyimide. Increased activation energies of permeation and gas selectivities higher than those of thick isotropic films were also reported for integral-asymmetricmembranes (Pinnau et al., 1991; Pinnau, 1991; Pfromm et al., 1993). In a more detailed study of the time-dependent permeation properties of ultrathin asymmetricpolycarbonatemembranes,Pfromm et al. (1992) showed that the nitrogen flux decreased by about 50% in the first ten days, while the single gas oxygen/ nitrogen selectivity increased above the value for thick isotropic films. The following factors or combinations thereof have been considered as the reasons for the above observations: (1) True glassy-state drift in the properties of the selective skin of the membrane resulting from a slow densification of this layer of the membrane. (2) Compaction of the open-cell support structure to a point where the resistance of the substructure becomes significant. ~
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* To whom correspondence should be addressed.
(3) The presence of residual casting solvents in the membranes and their subsequent loss. Pfromm and co-workers (1992) have attempted to eliminate all but the first reason listed above by careful experimentation. However, it is inherently difficult to separate the properties of the ultrathin membrane skin of asymmetric membranes from influences by the substructure. Therefore, the results of the present study provide an interesting addition to the literature on aging of membranes made from glassy polymers. Through the evaluation of polymer-ceramic composite membranes,the possibility of substructure compaction is eliminated. Further, the substrate does not absorb or release casting solvents. Only one low boiling solvent is used. Casting dopes for integrally skinned asymmetricmembranes often contain high-boiling solvents or nonsolvents. Also, the problem of solvents trapped in the porous substructure of integrally skinned membranes is circumvented by using a ceramic support. The polymer layer is a dense film of polymer approximately 1000-6000 A thick. The very simple structure of the polymer-ceramic composite membranes provides for an easy evaluation of the effects of aging on the separating polymer layer. The developmentof these membranes has been detailed previously (Rezac and Koros, 1992). The selectivities of the polymer-ceramic composite membranes reported by Rezac and Koros were generally a few percent less than that of a dense film of the same polymer. In light of the current results on the aging of these membranes, the selectivities originally reported can be seen as a minimum limit. The values of selectivity reported were for films aged only 1day. In retrospect, if these membranes would have aged for only a few more days, the selectivities would have, almost certainly, been higher than those reported for the dense, isotropic films. The transport of gases through composite membranes has been described by the resistance model (Rogers et al., 1957;Henis and Tripodi, 1981;Lopez et al., 1986; Pinnau et al., 1988). When the resistance to gas flow lies primarily in the dense polymer layer, and the resistance in the highly
0888-5885/93/2632-1921$04.00/0 0 1993 American Chemical Society
1922 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
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Figure 1. Structure of the GFDA-IPDA polyimide repeat unit.
porous support layer is negligible, as it is here, the pressurenormalized flux of the membrane is (P/l)i = Ni/AAp (1) where Ni is the volumetric flow of gas i in cm3 (STP)/s, A p is the total partial pressure difference across the polymer layer in cmHg for gas i, A is the cross sectional area of the polymer surface available for gas transport in cm2, 1 is the thickness of the dense polymer layer in cm, and P is the permeability coefficient of the polymer layer for the transport of gas i, usually given in the unit of 1 Barrer = 1 X 10-lo cm3 (STP) cm/cm2 s cmHg. The pure-gas selectivity of a composite membrane is simply the ratio of pressure-normalized fluxes for each component: aj,j = (P/OJ(P/l)j (2) If the polymer layer is essentially defect-free, the thickness of this layer can be estimated from eq 3 using the permeability of the polymer for each gas as determined from a dense, isotropic film of the polymer with a thickness which is mechanically measurable, generally >25 pm:
1 = Pi/(P/l)i (3) This can, of course, only be done if it is assumed that the permeability of the dense isotropic film and the thin polymer layer of the composite are identical. As the results from this study will suggest, these values may not be the same and the estimation of polymer layer thickness using eq 3 is completed to provide order-of-magnitude estimates only. Experimental Section Materials. Polymers. Results are presented for two polymers which have been prepared and tested both as dense films and as polymer-ceramic composite membranes. Results for a polyimide and a polycarbonate material are presented here. The polyimide, poly(hexafluorodianhydride isopropylidinedianiline) [GFDAIPDAI was purchased from Boron Biologicals, Durham, NC. The polymer was synthesized according to the procedure of Husk et al. (1988)and shows a glass transition temperature of approximately 310 "C. The structure of the GFDA-IPDA polyimide repeat unit is shown in Figure 1. The polycarbonate studied was a highly substituted, temperature-resistant material, tetramethylhexduorobisphenol-A polycarbonate [TMHFPC]. This material was synthesized in our labs and its properties characterized (Hellums, 1990). The glass transition temperature for this material is 208 "C. The structure of the TMHFPC repeat unit is shown in Figure 2. Ceramic Supports. Microporous Anopore aluminum oxide filters prepared by Anotech Se arations with an average surface pore diameter of 200 and a bulk pore diameter of 2000 A were used as the supports. The inorganic membrane has a highly ordered "honeycomb" structure of capillary pores (Furneaux et al., 1989),which are essentially cylindrical and straight. These asymmetric
w
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Figure 2. Structure of the tetramethylheduorobisphenol-A polycarbonate [TMHFPC] monomer.
membranes exhibit a high porosity and low tortuosity producing minimal resistance to gas flow. These membranes were used as received without further treatment. Gases. Compressed gases with purities of greater than 99.9% were purchased from Linde and used without further purification. Oxygen, nitrogen, and helium were used as test penetrants. Dense Film Preparation. A 4 wt % solution of polyimide, GFDA-IPDA, in methylene chloride was prepared and filtered. This solution was poured into a stainless steel casting ring supported on a glass plate. Solvent was allowed to evaporate from the film for a period of 24 h at room temperature. Following this, the clear film was removed from the casting plate by immersion in water. The film was dried in a vacuum oven at 25 "C for 24 h, 100 "C for 24 h and finally 250 "C for 24 hours. The film was cooled in the oven from 250 to 25 "C over a period of several hours. The film was stored under atmospheric conditions between tests. Polycarbonate dense film preparation is detailed by Hellums (1990) and follows the same general procedure used for the GFDA-IPDA film. Composite Membrane Preparation. Composite membranes were prepared by casting solutions of approximately 0.1 wt % polymer in methylene chloride directly onto a porous ceramic support. Solvent was allowed to evaporate over 1h. The membranes were subsequently air dried for a minimum of 12 h, and, finally, dried in a vacuum oven at 100 "C for 1 h before testing. A detailed description of this preparation procedure has been given previously (Rezac and Koros, 1992). Differential scanning calorimetry (DSC) studies of the films and composite membranes indicated the absence of solvent in all samples. Due to the composite nature of these membranes, however, we feel it is not possible to obtain meaningful information on the solvent content of the polymer layer from DSC. The membranes are composed of less than 2 vol % polymer and greater than 98 vol % ceramic support. Therefore, a DSC run on the composite is strongly influenced by the inert ceramic and subtle changes due to the evaporation of solvent from the polymer layer may not be observable in this situation. To complement the DSC studies, other methods were used. First, the time required for the solvent to be removed from the membrane was estimated using the well-known "half-time" relationship. The half-time, t0.6, of solvent sorption or desorption from a polymer sample is the time required for 50% of the maximum uptake value of a penetrant molecule to adsorb into or desorb from the polymer matrix. The time required for removal of more than 99 % of a sorbed molecule from the matrix of a polymer can be estimated as equal to 9 half-times. The half-time can be calculated for a slab of polymer with double-sided removal of solvent from eq 4 (Crank and Park, 19681, where 1 is the polymer layer t,, = O.0492l2/D
(4)
thickness and D the appropriate diffusion coefficient. It is necessary to obtain an estimate of the diffusion coefficient, D, of methylene chloride in polyimide and
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1923 polycarbonate in order to evaluate eq 2. This coefficient has been estimated from data available for molecules of similar size in polyvinyl chloride (Koros and Hellums, 1989). The values for cyclohexane, propanol, and butane in PVC are about 8 X 10-13cm2/s, at 25 "C (Koros and Hellums, 1989). These penetrants have similar van der Waals volumes to that of methylene chloride (Berens and Hopfenberg, 1982). PVC has a much lower diffusion coefficient for permanent gases compared to the polyimide and polycarbonate studied here. Similar trends should apply to low-concentration diffusion of penetrants such as methylene chloride. Nevertheless, to get a highly conservative estimate of the time required for removal of methylene chloride from polyimide, this value has been decreased by a factor of 50. Thus, the diffusion coefficient cm2/s. Using this value used in calculations was 1.6 X in eq 2,the time required for removal of more than 99% of the meth lene chloride from a membrane with a polymer layer 4000 thick is approximately 12h if all drying occurs at 25 "C. Drying times would be expected to decrease as the drying temperature increases. Therefore, after drying the membranes for a total of 14 h, partially at elevated temperatures, it is reasonable to believe that there is no measurable residual solvent remaining in the films. As further confirmation that the casting solvent had indeed desorbed from the cast membranes, the technique of head space gas chromatography (HSGC) was employed. This highly sensitive technique was used to measure the concentration of methylene chloride or any other volatile component present in a dried polymer-ceramic composite membrane sample. The technique is capable of detecting methylene chloride to a level of a few parta per billion (Kolb et al., 1985). Using this technique, the concentration of methylene chloride in three samples was measured. The first was prepared, air dried for 12 h, and then, dried in a vacuum oven at 100 "C for 1 h. The second and third samples were prepared as the first. Then, they were held in air with about 50% relative humidity a t 25 "C for 20 days and 12months, respectively. The dried samples were placed in sample vials with a total volume of 20 cm3. The samples were allowed to equilibrate in the vials at 65 "C for a total of 24 h. Following this equilibration period, the concentration of methylene chloride in the vapor phase was measured. No methylene chloride was detected in any of the samples. MembraneTesting. The pure gas pressure-normalized fluxes reported for helium, nitrogen, and oxygen were obtained at 25 "C for membrane samples with an area of 13.85 cm2. This area represents the total surface area of the polymer layer exposed to feed gas. The substrate is approximately 50 7% porous with the nonporous areas being impermeable to gas flow. The diameter of the largest impermeable section is approximately 300 A (Furneaux et al., 1989). Analysis of two-dimensional diffusion provided by Keller and Stein (1967)indicates that, for the present geometry, the diffusional paths are such that using the total surface area of the polymer layer as the area available for permeation will introduce an error of less than about 3% for a polymer layer thickness of greater than 1000A. This analysis also supports the assumption of double sided desorption used in the previous section. All testa were completed with an upstream pressure of 50 psig and atmospheric pressure downstream. Volumetric gas flow rates were determined with a soap bubble flow meter. The high volumetric flow rates of the membranes preclude any back diffusion of gases or vapors from the bubble flowmeter through the porous substructure of the membranes to the active skin layer. Inaccuracies due to
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Table 1. Effects of Physical Aging on the Permeability Coefficients and Selectivities in a GFDA-IPDA Film. Ap = 32 psi, T = 35 OC, Film Thickness = 71 fim Permeability coefficient.
0 5
1.69 1.66
7 G 71.8
43 43
water vapor developing in the flow meter or back diffusion of gases through the soap film are negligible for the same reason. Care was taken to fully purge the permeate volume with the gas to be measured. Pure gases were always tested in the order of increasing permeability so that any potential residues on the feed side remaining after the usual repeated purge cycleswould decrease the pressure normalized fluxes of the faster gases, thereby decreasing the calculated selectivities and giving a "worst case" value of the selectivity. Dense Film Testing. The permeabilities of helium, oxygen, and nitrogen for solution-cast films of polyimide and polycarbonate were measured at 25 and 35 "C for the polyimide and 35 "C for the polycarbonate. The upstream pressure was 32 psia for the polyimide and 44 psia for the polycarbonate. The permeation data were obtained using the standard permeation techniques employed in our labs (O'Brien et al., 1986). The downstream pressure of less than 10 mmHg was negligible relative to the upstream pressure. To correct the data for polycarbonate to 25 "C, activation energies measured over the range 35-150 "C were employed. Sample Storage. Following each series of testa completed, the membranes were stored in polypropylene Petri dishes. The dishes were composed of two pieces, a bottom, and a lid. The dishes protected the fragile membranes from mechanical breakage but were not air tight and allowed ambient air to pass over the membranes freely. The membranes were stored in their dishes under ambient conditions, approximately 50 % relative humidity and 25 "C, until the next series of testa.
Results and Discussion Polyimide Materials. Dense Film. The gas transport properties of a dense film of polyimide as a function of time are presented in Table I. For the time measured, there is no significant change in the gas transport properties of the polymer. Previous reports by Kim indicate that the dense films of polyimide show some time dependence over a period of 1 year (Kim, 1987). This may be the case with our materials, as well. However, for the first 5months, this sample did not show significant differences in either permeability or selectivity for the heliumtnitrogen gas pair. Composite Membranes. While the time dependent behavior of "thick" (>25pm) dense films of polymer is interesting, the important consideration for membrane separators is the behavior of very thin films of the material as would be present in integrally skinned asymmetric membranes or composite membranes. This study has focused on evaluating the time-dependent transport properties of a number of polymer-ceramic composite membranes. The study of these membranes is quite interesting due to the following factors. First, the membranes are composed of a dense film of polymer supported on a highly porous ceramic support. Polymer layer thicknesses are typically on the order of 1000-6000 A. These estimates were obtained from gas permeation measurements made on membranes 1 day old. These values are in fair agreement with the thickness
1924 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 80
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Figure 3. Percent of pressure-normalizedflux of helium (O), oxygen (o),and nitrogen (A) for a6FDA-IPDA-ceramiccompoeitemembrane as a function of time. Percent of pressure-normalizedflux = (flux on day of testing/original flux)*lOO%. Ap = 50 psi, T = 25 O C . Table 11. Gas-Transport Properties and Selectivities in a GFDA-IPDA Composite Membrane as a Function of Timea gas flux, P/1 [Wcc(STP)/cmZs cmHg] ideal selectivity a aging time (days) NZ Oz He OdNz He/Oz He/NZ 4.1 0.8
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estimated from scanning electron microscopy measurements, and with the thickness estimated from material balance calculations (Rezac and Koros, 1992). Since a ceramic support is used in these studies, the question of compaction of the substructure is eliminated. Further, since the total volume of polymer present in the membrane is only that present in the thin separating layer, the total volume of solvent which must be removed from the cast membrane is very small. The polymer layer thickness is well defined and only a single low-boiling solvent is used in the casting step. The rate of removal of this solvent can be calculated as described above and the presence of residual solvent can be measured using a sensitive HSGC method. Thus, the simple geometry of the polymerceramic composite membrane allows us to followthe aging of these materials and be assured that compaction of the substrate or the presence of residual casting solvents are not contributing to the overall gas permeation properties of the membrane. The gas flux properties for a polyimide-ceramic composite membrane normalized by the initial value [(P/l)l (P/l)o]as a function of time are presented in Figure 3 and the absolute values of a few selected points presented in Table 11. The estimated thickness for this membrane, based on the nitrogen gas flux and eq 3, was 3300 8, on the first day of testing. It is evident from Figure 3 that the gas flux properties of the membrane decrease quite rapidly in the first few days following production. The membrane continues to show a decrease in gas flux over the entire test period. After 20 days, the flux of the membrane for helium is only 50 % that which was measured on the first day. The oxygen and nitrogen flux values fall to 26 96 and 20 ?4, respectively. Because the rate of flux decay is gas specific, the selectivity of the membrane also changes over time. The helium/nitrogen, helium/oxygen, and oxygenlnitrogen selectivities as a function of time are presented in Figure
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Figure 4. Pure-gas heliumhitrogen, heliumloxygen, and oxygen/ nitrogen selectivity (fromtop to bottom) for a 6FDA-IF'DA-ceramic composite membrane as a function of time. Ap = 50 psi, T = 25 O C . Dense film selectivity measured by the authors on a film 5 months old.
4. The reference "dense-film" values which are reported
in these figures were measured by the authors 5 months after the dense film had been dried. Even though the properties of the dense materials do change with time, the change observable in 3 weeks can be neglected in terms of this discussion. Figure 4 indicates that the selectivity of the membrane for these gas pairs measured on the day the membranes were prepared was somewhat lower than that which was achieved with the dense polymer film. However, as the membrane ages, the selectivities for all three gas pairs increase. After only 2 days, the oxygen/nitrogen selectivity had surpassed that achieved in the dense film.The helium/ nitrogen and helium/oxygen selectivitiesfor the membrane are higher than those of the dense film after 5 days of aging. The selectivity of the membrane for all three gas pairs continues to increase over the entire test period. However, the rate of this increase is clearly slowing with time. The gas flux and selectivities achieved by this membrane after 20 days of aging are summarized in Table 11. The results presented here support the suggestion by Pfromm et al. (1992) and Pinnau (1991) that the time dependence may be due to a decrease in the fractional
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1925 ;
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Figure 5. Percent of pressure-normalizedflux of helium (e),oxygen (a),and nitrogen (A)for a TMHFPC-ceramiccomposite membrane aa a function of time. Percent of pressure-normalizedflux = (flux on day of testing/original flux).lOO%. Ap = 50 psi, T = 25 O C .
free volume and increased cohesive energy density of the polymer caused by more efficient chain packing. This decrease would result in an effective tightening and densification of the polymer matrix. Thus, the gas flux of the largest gas molecules would be affected the most. The gas flux of helium would be affected the least by this type of change. If a densification of the selective layer of the membrane occurs, the flux of all gases would be expected to decrease and the selectivity of the material for the separation of helium from nitrogen would be expected to increase. This behavior is, indeed, seen in Figures 3 and 4 and Table 11. TMHFPC Membranes and Films. Dense-film measurements for TMHFPC were completed at 35 "C and corrected to 25 "C with activation energies measured by the authors. These measurements were completed on films which were approximately 3 months old. Unfortunately, no long-term aging studies have been completed on dense films of this material. The gas-transport properties of a composite membrane prepared from this material were evaluated as a function of time. The estimated thickness for this membrane, based on the nitrogen gas flux and eq 3, was 4700 A on the first day of testing. The normalized gas fluxes for helium, nitrogen, and oxygen are presented in Figure 5. This figure indicates that the behavior of the polycarbonate membrane is similar to that of the polyimide described above; the flux of the membrane decreases rapidly following preparation and then levels out at some dramatically reduced value. The TMHFPC membrane tested was not removed from its test cell between tests. This reduced the possibility of failure or damage due to handling and allowed for a very long test. The total time of this test is approximately 2 years. It is clear, that the majority of the flux loss is experienced in the first month following preparation and the decrease in flux during the remaining time is moderate. As with the polyimide membrane, the rate of change in gas flux is a function of the gas which is being tested. Therefore, the pure-gas selectivities of the TMHFPC membrane also change with time. These properties are displayed in Figure 6 for the helium/nitrogen, helium/ oxygen, and oxygenlnitrogen gas pairs. The selectivity of the membrane for the separation of helium from either oxygen or nitrogen was slightly below that of a dense film of TMHFPC on the first day of testing. However, after less than 5 days of aging, the selectivity of the membrane for these gas pairs had exceeded that of the dense film. As the membrane continues to age, the selectivity for all gas
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Figure 6. Pure-gas helium/nitrogen, helium/oxygen, and oxygen/ nitrogen selectivity (from top to bottom) for a TMHFPC-ceramic composite membrane as a function of time. Ap = 50 psi, T = 25 OC. Dense film selectivity measured by the authors on a 3-month-old film. Table 111. Gas-Transport Properties and Selectivities in a TMHFPC Composite Membrane as a Function of T i m e gas flux,PI1 ( 1 Vcc(STP)/cm*s cmHg) ideal selectivitya aging time N2 0 2 He OJNn HelOl He/Nl (days) 0 20 87 260 515
dense film (Ap = 44 psi)
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21
30 33 34 35 25
Selected data from Figures 5 and 6. Ap = 50 psi; T = 25 OC; dense film thickness = 75 pm, age of dense film at tasting = 3 months.
pairs increases. The selectivity of the membrane for oxygenlnitrogen was higher than that of a dense film of TMHFPC on the first day of testing. The selectivity of this gas pair did not increase significantly over the entire test. The exact values of the increase are reported in Table 111. These data for the aging process of a TMHFPC-ceramic composite membrane are in good agreement with that
1926 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
provided for the polyimide-ceramic membrane. The controlling phenomenon is believed to be a tightening of the polymer matrix caused by physical aging of the polymer. The detailed time dependent behavior of the polymerceramic composite membranes is provided for a single membrane sample for each polymer studied. However, the properties of a large number of samples have been measured after approximately six months of aging. These samples consistently exhibit He/N2 selectivities which are 20-100% higher than those of a dense film of the same material.
Conclusions The gas-transport properties of defect-free polymerceramic composite membranes as a function of time have been reported for two polymers, 6FDA-IPDA polyimide and tetramethylhexafluoropolycarbonate,TMHFPC. The polymer layers had effective thicknesses estimated from gas flux measurements of approximately 4000 A. The pressure-normalized fluxes of helium, nitrogen, and oxygen all decrease rapidly with time for these membranes. Typically, the flux of these gases is about 40% of the initial flux after 1 month of aging. The selectivities of these membranes increase with time for the gas pairs helium/ nitrogen, helium/oxygen, and oxygen/nitrogen. On the first day of testing, the selectivities of the membranes were typically less than that of dense f i i of the same materials. However, in less than 7 days, the selectivities of the membranes had surpassed that of dense films of material. The selectivities of the membranes continued to increase with aging and could easily reach values of up to 50% greater than that of the dense films. Due to the ceramic nature of the substrate layer, compaction of this layer with a concurrent increase in the resistance to gas flow is not a factor in the observed phenomena. Rather, the behavior is consistent with physical aging of the material and a concurrent densification of the selective separating layer. It is clear from our results that the determination of gas-transport properties of glassy polymers for gas separations should take place on samples that are comparable to the thin effective layers used in actual gas-separation processes. Important design parameters like protracted time dependence of gas selectivities and fluxes, which is not due to any contamination or process conditions, can be overlooked if work is restricted to unrealistically thick polymer films.
Acknowledgment The authors would like to thank Mr. Edward Simpson for assistance with the HSGC and Mr. Ken Achacosa for technical assistance. This material is based in part upon work supported by the Governor’s Energy Management Center-State of Texas Energy Research in Applications Program under Contract No. 003658-101.
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