J. Phys. Chem. 1995,99, 13187-13192
13187
Incorporation of Zeolites in Polyimide Membranes Ivo F. J. Vankelecom, Edouard Merckx, Mark Luts, and Jan B. Uytterhoeven" Centrum voor Oppervlaktechemie en Katalyse, Kardinaal Mercierlaan 92, 3001 Leuven, Belgium Received: October 27, 1994; In Final Form: March 7, 1995@
The incorporation of zeolites in PI (polyimide) is investigated, aiming at higher fluxes and selectivities in membrane applications. Remarkable differences are found in comparison with the earlier reported incorporation of zeolites in PDMS (poly(dimethylsi1oxane)): the adhesion between the PI and the zeolite is bad, resulting in weak membranes. The sorption of xylenes in the membrane is used to study the impact of sorption and diffusion in potential membrane applications. The preparation method used induces anisotropy in the membrane,
Introduction
In contrast to elastomers,'-8 the incorporation of fillers in glassy polymers has not yet been described in the literature for membrane purposes. The incorporation of zeolites in polyimides was chosen here, as this polymer has the inherent ability to separate xylenes9 It is expected that the incorporation of p-xylene selective zeolites in this polymer would preserve or even improve the p-xylene selectivity of the polymer and simultaneously increase the low fluxes characteristic for polyimide membranes, due to both an increased sorption in the zeolite and an enhanced diffusion through its pore system. Furthermore, possessing outstanding thermal,'O.l mechanical, and chemical'* properties, this polymer is generally seen as a very promising tool in future membrane ~eparati0ns.I~ So far, the interest for polyimides in membrane separations has been almost exclusively focused on gas ~ e p a r a t i o n ' ~and - ~ ~less on p e ~ a p o r a t i o n . * ~The - ~ ~dominant role played by diffusion in processes involving polyimides was proven several times.23,26,29 First, the unfilled PMDA-ODA will be characterized with reference to commercially available PI. Then the incorporation of zeolites will be studied using xylene sorption and several physical methods. Finally, some preliminary pervaporation results will be shown. Experimental Section Membrane Preparation. Preparation of Pure Polyimide Membranes. A 15 wt % solution of polyamic acid was prepared by adding 2.631 g of ODA (4,4'-oxydianiline, Janssen Chimica, 98%) to 30 mL of NMP (N-methyl-2-pyrrolidone, Aldrich, 99.9%). This was stirred for 15 min at room temperature, after which an equimolecular amount of 2.838 g of Ph4DA (pyromelletic dianhydride, Janssen Chimica, 99%) was added at 0 "C. Stirring was continued for several hours during which the solution temperature slowly increased as a consequence of the exothermic polymerization reaction. The air, entrapped in the solution during mixing, was removed by applying vacuum. After cooling the solution to room temperature, it was cast on a glass plate with a thickness of 250 pm. Removal of the solvent and curing took place in a programmed vacuum oven. After 1 h at 25 "C, the temperature was raised at a rate of 10 "C/h to 70 "C, which was maintained for 6 h. After a further increase of 30 "Ch, the final temperature of 220 "C was reached. It was kept constant for 3 h. The solvent was used as such, while the monomers were purified by vacuum sublimation at 60 'C. @
Abstract published in Advance ACS Abstracts, August 1, 1995.
0022-365419512099-13187$09.00/0
Apart from these membranes, prepared from monomers, other membranes were prepared starting from commercial solutions of polyamic acids (Du Pont De Nemours), listed in Table 1. Preparation of Zeolite Containing Membranes. The zeolites (described elsewheres) were stored in a desiccator at 80% relative humidity. After weighing, the zeolites were carefully dried ovemight at 180 "C, since the slightest amount of sorbed water could cause hydrolysis of the polyamic acid, decreasing the molecular weight of the polymer chains. The zeolites were then dispersed in NMP for 1 h in an ultrasonic bath, as a 15 wt % dispersion. Afterward, 10 g of the already prepared 15 wt % polyamic acid was added. This was stirred for 1 h. Entrapped air bubbles were removed under vacuum. The casting and curing of the membrane happened as described above for pure polyimide membranes. The zeolite content is expressed in volume percent as (weight of zeolite)ldz,o,it, (weight of zeolite)ldz,o,it,
+ (weight of polymer)/dpolymer
in which d is the density. A density of 1.27 mWg was taken for zeolite Y and 1.76 mWg for all other zeolites. Membrane Characterization. Thickness, Tensile Strength, Density, Sorption, and Contact Angle Measurements. The determination of these parameters was done as described elsewhere .* IR Measurements. For the IR measurements, ultrathin membranes were prepared to reduce the intensities of the peaks in the spectra. This was realized by reducing the slit when casting the membrane. The spectra were recorded on a Nicolet 680 spectral workstation coupled to a 730 FT-IR spectrometer. WAXD. Measurements for WAXD were done on a Rigaku Rotoflex FTP 300 RC with Cu K a radiation (11 = 1.541 87 nm) and a Ni filter. The anode was a Rigaku rotating anode with a slit collimator. Reflection was recorded for 28 between 5" and 30". The recorder speed was 5"/min, and a recording was taken every 0.05'. From the crystalline part of the film, the intracrystalline distances ( d spacings) could be calculated using Bragg's law. Pervaporation. An equimolecular xylene mixture was pervaporated at 74 "C using the apparatus described elsewhere.8 Fluxes are normalized to a membrane thickness of 100pm, and selectivity is expressed as
S=
Nl(1 - N ) nl(1 - n)
0 1995 American Chemical Society
13188 J. Phys. Chem., Vol. 99, No. 35, 1995
Vankelecom et al.
TABLE 1: List of the Monomers and Solvents of the Commercial Polyamic Acid Used polymer dianhydride diamine PI 2540 PMDA ODA 0
solvent NMP + aromatic hydrocarbons
0
II
II
II
0
0
PI 2560
benzophenonetetracarboxylicacid dianhydride 0 0
oxydianiline
H
0
PI 2611
0
biphenyl dianhydride
TABLE 2: Thickness, Density, and Tensile Strength of the Commercial Polyimide Kapton H-100, the Polyimide Membranes Prepared from the Commercial Samples, and the Self-prepared PMDA-ODA membrane thickness density tensile strength (g/cm3) (N/mm2) @m) type PI 2540 24 1.3901 97.2 PI 2560 64 1.3660 36.5 PI 261 1 24 1.4453 157.9 PMDA-ODA 18 1.4050 79.6 Kapton H-100 25 1.4278 113.9
+ in-phenylenediamine 2
N
NMP + xylene- 1-methoxy-2-propanol
q
NHz
NMP
1,4-~henylenediamine
-A
3500
3000
2500
I
2000
I500
Wave number ("1)
where N is the weight percent in permeate and n is the weight percent in feed. The amount of each isomer in the mixtures was determined using gas chromatography (Hewlett Packard 5890, Chrompack WCOT Fused Silica, CP-WAX-SZCB, HP3392A integrator). The analysis was isothermal at 110 "C using nitrogen as carrier gas. Results and Discussion Unfilled Polyimide Membranes. Synthesis. DMA (N,Ndimethylacetamide, Janssen, 99+%) and NMP were tested. Even though DMA is lower boiling (61-63 "C at 12 " H g ) than NMP (80-81 "C at 10 " H g ) and more easily removed during curing, NMP gave better results. NMP forms hydrogen bonds with the polymer chains.30 This enhances the viscosity of the polymer solution remarkably, compared to solutions with the same monomer concentration in DMA. The polyamic acid was synthesized in an ice bath, which was necessary to reach the desired polymer chain length and the ultimate membrane strength. Once polymerization was started, the ice was no longer needed, since the high viscosity in the cooled solution limited the further reaction between the already formed oligomers. This synthesis at low temperature permitted the addition of the total amount of dianhydride to the diamine solution at once. The polyamic acid was stored at -18 "C without any significant loss of viscosity. Before casting on the glass plate, the solution was brought to room temperature to obtain a uniform film thickness over the total length of the film. Density. Polymer densities are summarized in Table 2. Using the PI 261 1, a membrane with an even surface could be prepared. The film prepared from the PI 2560 solution, containing xylenes as solvent, resulted in a membrane with an uneven surface. Probably, the low boiling xylenes gave rise to small holes in the membrane, as a consequence of xylene boiling in the applied vacuum at the initial temperatures. It resulted in
Figure 1. FT-IR recording of a self prepared PMDA-ODA film.
a brittle membrane. It is difficult to compare the densities of the commercial samples PI 2560 and PI 2611 with the others listed, since they are built up by other monomers. As mentioned in the l i t e r a t ~ r e , ~ the ' - ~ ~monomer combination has a strong influence on the polymer properties. PI 2540, on the other hand, contains the same monomers as the self prepared membrane and the Kapton H-100 film. The density of this monomer combination is very well documented in the literature: 1.400,20 1.407,27and 1.395 g/cm3.37 Clearly, the density of the selfprepared PMDA-ODA membrane coincides with these values. Okamoto et a1.I8 found a density between 1.4040 and 1.4029 g/cm3 ,for the same polymer solution, but with different imidization temperatures. This might provide a possible e ~ p l a n a t i o nfor ~ ~the . ~ high ~ density of Kapton H-100. Probably the final curing for Kapton H-100 took place at higher temperatures. Indeed, PI 2540 is the starting material for the preparation of Kapton H-100. When using our imidization procedure on this product instead of the company's procedure, the resulting density of the PI 2540 membrane equals the one found for the self-synthesized membrane (Table 2). Tensile Strength. For the membranes prepared from the commercial polyamic acids, the density results are reflected in their tensile strengths (Table 2): the denser the membrane, the stronger it is, due to stronger interchain interactions in the better packed polymers. IR Measurements. To check the degree of imidization, an IR spectrum was recorded between 1500 and 3500 em-', the region in which the most important imide- and polyamic acid bands occur (Figure 1). Characteristic absorption bands of polyamic acid and polyimides have been l i ~ t e d . Traces ~ ~ , ~ of ~ polyamic acid should give bands at 3200 cm-I (OH stretch of the carboxylic acid3s) and at 3350 cm-I (NH stretch of the
J. Phys. Chem., Vol. 99, No. 35, 1995 13189
Zeolites in Polyimide Membranes
-
Remaining weight
0-
: -0 5
0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 10 15 20 25 30
I5
10
20
Time (h)
Figure 3. Removal of the solvent for a 20 wt % borosilicate membrane as a function of the membrane synthesis time and the oven temperature programming. 1.60
1.55
1.50
l . : . : . : . : ' : ' : ' : ' : . : ' : ' : ' : ' ~ . : . : . : . : ' : . : . ~ 5 IO I5 20 25 30
Figure 2. (a, top) Comparison between the WAXD measurements of Kapton H- 100 and the self-prepared PMDA-ODA. (b, bottom) Comparison between the top and the bottom side of the membrane, containing 30 wt % borosilicate.
$.;i
1.45 1.40
135 1.24 1.25 1.20 0
5
10 15 20 25 30 35 40
0
IO 20 30
40 50
0
10 20 30 40
Zeolite content (wt%)
TABLE 3: d Spacings and 28 Values Obtained from the WAXD Spectra of Kapton H-100 and the Self-prepared PMDA-ODA Dolvmer Kauton PMDA-ODA 14.75 6.0
14.00
6.3
secondary amide group30). This last band is slightly visible in the spectrum. The small shoulder at 1670 cm-' (originating from a hydrogen-bonded amide30)also reveals the existence of a negligible amount of polyamic acid. , WAXD Measurements. In Figure 2, the commercial Kapton film and the self prepared PMDA-ODA film are compared, and the calculated interchain distances are listed in Table 3. It is clear that the distances between the chains in the commercial polyimide (d = 6.0 A) are smaller than those in the self prepared polyimide (d = 6.3 A). The small d spacing of the Kapton film confirms the better packing for this material than for the self prepared membranes, as pointed out already before. These values seem to differ from the value reported by Kim et al.*O of 4.6 8, for a PMDA-ODA membrane with a density of 1.40 g/cm3. Zeolite-Filled Polyimide Membranes. Synthesis. A 15 wt % solution of polyamic acid in NMP was found to be optimal. Indeed, more solvent would produce a better zeolite dispersion but cause more difficulties in the removal of the solvent. On the other hand, less solvent rendered the removal of the solvent more easy but made the dispersion of the zeolite more difficult. Furthermore, the less solvent used, the more viscous the solution and the more easy the casting was. The removal of the solvent was followed experimentally by weighing a cast 20 wt % borosilicate membrane together with the glass plate every hour of the synthesis period. The results are shown schematically in Figure 3 . At a temperature of 200 "C, solvent removal seemed to be accomplished completely. The weight loss in percent was calculated in relation to the sum of polyamic acid and solvent. The graph shows that a plateau of 6 h on 70 "C could have been sufficient, but 8 h was taken since higher zeolite loadings would probably result in a retarded solvent removal. After the plateau, only 8% of the solvent remained in the membrane. Other temperatures than 70 O C were tested as well.
Figure 4. Densities of the zeolite filled polyimide membranes as a function of the zeolite content for the different zeolites.
Lower plateau values resulted in an insufficient solvent removal before imidization started. At higher temperatures, imidization was started already before solvent removal was completed. It resulted in a brown color of the membrane, while a wellimidized membrane had a bright yellow color. Since the solvent is more easily removed from the polymer than from the zeolite, the preparation of pure polyimide membranes would probably be possible in a shorter plateau time, but for reasons of better comparison, they were prepared under identical conditions. Density. Figure 4 clearly shows that membrane density changes as zeolite is incorporated. Out of membrane and zeolite densities, a theoretical density of the composite membranes was calculated, assuming a perfect adhesion between the zeolite and the polymer. The experimental as well as the theoretical values for zeolite containing membranes are given in the figure. For low borosilicate contents, a very slight increase in experimental density can be observed, but from 20 wt % on, a strong deviation from the theoretical value is seen. This can be explained by the creation of voids in the membrane, occurring at the zeolitemembrane interface. Due to their strongly differing chemical nature, the adhesion between the zeolite particles and the polyimide is probably weak. Maybe even some zeolite aggregates appear at higher loadings. As the polyimide chains are very rigid, their close packing is disturbed in the vicinity of the zeolite particles, which is once more leading to voids. Silicalite containing polyimide membranes could be prepared for loadings up to 60 wt %. Unfortunately, these high loadings resulted in membrane pieces that were too brittle to perform characterizations on. However, density clearly increases until a loading of 30 wt %, even though the increase is lower than expected theoretically. It can be concluded that silicalite disperses better in the polyimide than borosilicate. Whether this is attributed to the more hydrophobic character of the silicalite, its bigger particle size or its different shape was not further investigated. A totally different effect is seen when zeolite Y is incorporated. First of all, the theoretical calculations for zeolite Y containing polyimides are somewhat different since zeolite Y-in contrast to the two MFI-type zeolites mentioned
13190 J. Phys. Chem., Vol. 99, No. 35, 1995 100
,
Borosilicate
Vankelecom et al. Zeolite Y
Silicnlite
p-xylene
M\
SO
m-xylene
@xylene
250
70 2M)
-
3 I
J
Theoretical
10
150
!.
Experimental
50 0
0 10 20 30 40 50 60
5 10 I5 20 25 30 35 40
0 10 20 30
0
..
Zeolite content (wt4h)
Figure 5. Experimental and the recalculated tensile strengths of the zeolite-containing polyimide membranes as a function of the zeolite content. 250
-r
I
.
w t % borosilicate
Figure 7. Sorption of xylenes in borosilicate containing membranes with the first measurement after 1 week (indicated as “first” on the graph) and the last measurement after 6 weeks (indicated as “last” on the graph) (* = incomplete sorption as the last measurement was obtained after only 3 weeks). A
,
,
,
,
,
,
I
, 0 First measurement
h l
I
0
I
2
3
4
5
6
7
8
Sorption time (weeks)
Figure 6. Sorption of the three xylene isomers as a function of time in a polyimide containing 20 wt % borosilicate.
above-has a lower density than the polyimide. By consequence, the theoretical density decreases as zeolite content increases. The experimental results, on the contrary, show a strong increase in density with increasing zeolite content. A possible explanation is the invasion of the zeolite channels by the polyimide chains. Indeed, the large 12-ring membered pores (7.4 A) of the zeolite probably allow the sorption of the polymer chains, having just the same effect as mentioned for zeolite Y filled PDMS membranes.8 Tensile Strength. Tensile strength measurements for zeolite containing membranes are recalculated to the real polymer volume as explained in Vankelecom et al.* It is clear from Figure 5 that void formation is much limited for silicalite up to a zeolite content of 20 wt %, while borosilicate containing membranes lose their strength already at 10 wt %. Once again, this shows the bad dispersion of the borosilicate in the polyimide. In spite of the large scatter on the results for zeolite Y filled membranes, the invasion of the chains in the zeolite seems to result in an increased membrane strength for the low zeolite contents, indicating once again that zeolite Y actually acts as a physical cross-linker. At high loadings, tensile strength decreases, but to a lesser extent than in the case of borosilicate and silicalite. Sorption. In Figure 6, the time evolution for the sorption of the three different xylene isomers in a zeolite containing membrane (20 wt % borosilicate) is shown. Already after 1 week, p-xylene has reached its equilibrium sorption capacity, while it takes about 4 weeks for m-xylene and 7 weeks for o-xylene. This clearly shows how difficult it is for o-xylene to sorb in the membrane. Once the equilibrium is reached, sorption capacities for the three isomers are the same. This means that, for a possible application of such composite polyimides in the separation of xylene isomers by pervaporation or gas separation, diffusion will determine the selectivity. Furthermore, the
pxylene
m-xylene
o-xylene
Figure 8. Sorption of the xylene isomers in Kapton H-100, PI 2611, and a zeolite Y-containing polyimide after 1 week (first measurement) and after 6 weeks (last measurement) (* = sorption measured after 6 weeks, but not yet at equilibrium).
process will probably be extremely slow. This is also illustrated in Figure 7, where no difference occurs between the final sorption capacities of the different isomers in membranes with different borosilicate loadings. It is striking that these equilibrium sorptions are almost the same, regardless of the zeolite content. Since the borosilicate sorbs about 0.12 mL of p-xylenelg and the polyimide 0.23 mL/g, a steady decrease of the sorption is expected as zeolite content increases. However, Figure 7 shows that this is not the case. The creation of voids in the polymer explains this. In spite of these defects, the first measurement after only one week sorption clearly results in a certain selectivity for p-xylene. So, there still seems to be an intact polyimide layer at the surface of the membrane responsible for a selective sorption. This confirms the fact that diffusion is the rate-limiting factor in the sorption of polyimide membranes, as stated already in the literature by several a ~ t h o r s . ~ ~ , ~ ~ , ~ ~ In Figure 8, the sorption of the three xylene isomers is followed as a function of time for two pure polyimide films and one film containing zeolite Y. Taking into consideration the high density found for the PI 261 1 membranes, the very low sorption values in Figure 8 were expected. For both the Kapton film and the zeolite Y-containing polyimide, the slow sorption-even for p-xylene-and the large and remaining difference between p-xylene and o-xylene are striking. For pervaporation, this might suggest a certain sorption selectivity in these membranes, apart from the diffusion selectivity already found in the earlier mentioned membranes. Substituting zeolite Y in a polyimide membrane should normally increase the sorption capacity, since zeolite Y sorbs about 0.47 mL/g. The observation of the opposite confirms the cross-linking once more. Sorption Out of a Xylene Mixture. In Figure 9, the total sorption out of a xylene mixture for three membranes containing
Zeolites in Polyimide Membranes A
I
J. Phys. Chem., Vol. 99, No. 35, 1995 13191 I
I
Week I
0
NaY
CHV-2802
Horo
I3oro (para)
Figure 9. Comparison between the sorption capacities (after 1 and 5 weeks) obtained from a xylene mixture in three polyimide membranes containing IO wt 96 zeolite. On the right, the sorption capacity of the same IO wt 96 borosilicate containing polyimide membrane is given out of pure p-xylene.
0 wt%
35 w%
30
so
Wl%
Wl%
horo
2802
2802
Figure 10. Contact angle as a function of the zeolite incorporation (2802 refers to zeolite CBV-2802).
different zeolites is depicted. For borosilicate, the equilibrium sorption capacity of p-xylene out of a mixture is compared with the sorption of pure p-xylene, and no difference is found at equilibrium. After 1 week, however, the sorption out of the mixture is much lower than the sorption out of the pure p-xylene. This means that the two other xylenes slow down the sorption of p-xylene. The zeolite Y containing membrane seems to sorb most slowly, even not yet being at equilibrium after 5 weeks. Since the zeolite acts as a cross-linker in this polymer, this is not really surprising. WAXD. When comparing the WAXD recordings shown in Figure 2, some new peaks arise due to the zeolite incorporation: the strong polyimide peak found for pure polyimide is almost completely replaced by the new peaks originating from the zeolites. The crystallinity of the film seems to be destroyed immediately, once the zeolite is added. This is not surprising, since the zeolite particles interrupt the crystalline packing of the polymer chains. WAXD also suggests a difference between the upper side of the membrane and the bottom side. Contact Angle Measurements. In Figure 10, the contact angles are shown, measured at both sides of the membrane, the upper side meaning the side that was in contact with the air during synthesis. In general, the polyimide membranes are hydrophilic with a clear difference between the upper and the bottom side, the latter being most hydrophilic. It is this side for which contact angle is varying when zeolite is added. The more silicalite is added, the higher the contact angle becomes. This suggests a sinking of the hydrophobic zeolite in the membrane, confirming the WAXD observations. SEM. Figure 1 la shows the bottom side of a 10 wt % borosilicate PMDA-ODA membrane. It confirms the sinking of the zeolite particles as they are clearly visible, while they were completely absent in the picture of the upper membrane side, showing a uniform surface without any pattern. In spite of the sinking, every single zeolite particle is still covered by polymer as can be seen in Figure 1 Ib, showing the bottom side
Figure 11. SEM pictures showing (a, top) the upper side of a 10 wt 96 borosilicate PMDA-ODA membrane and (b, bottom) a view of a cross section of the bottom side of a 20 wt ?6 CBV-2802 PMDAODA membrane.
TABLE 4: Flux (g/(m2 h)), Thickness, and Selectivities at 74 "C
Kapton H-100 PMDA-ODA 25 wt 9%Boro PMDA-ODA
+
s
s
PA4
P/O
S P/(M+O)
flux thickness (100pm) Qcm)
1.34 1.87 1.20 1.38
1.56 1.28
0.075 0.279
25
1.03 1.03
1.03
0.206
20
18
of the membrane. So actually, an asymmetric membrane is formed with an integral polymer skin on both sides. Pervuporution. The Kapton H- 100 membrane showed paraselectivity (Table 4), while no selectivity at all was found for the self-prepared PMDA-ODA. As fluxes were very low, this lack of selectivity was proven not to be caused by pinholes or defects in the membrane. These observations imply that the interchain distance measured by WAXD is extremely important. Indeed, it was found in Table 3 that the interchain distance ia the self-prepared polyimide was 6.3 A,compared to the 6.0 A for the commercial polyimide. When borosilicate is incorporated in the membrane, an improved selectivity compared to the self prepared PMDA-ODA can be observed (Table 4) in spite of the presence of voids. It proves that the incorporation of a selective zeolite in the polymer can indeed induce selectivity on the whole system. Conclusions
For the incorporation of zeolites, the use of NMP as solvent, high-purity reagents, and a low-temperature polymerization were
13192 J. Phys. Chem., Vol. 99, No. 35, 1995 required. Adhesion between PI and the zeolite was found to be bad, showing an essential difference between this rigid glassy polymer and the flexible elastomer described before.8 Contact angle measurements, SEM, and WAXD revealed the existence of an asymmetry over the membrane due to the sedimentation of the incorporated zeolite particles. In spite of this sedimentation, an integral polymer skin was found on both sides of the membrane, resulting in a selective uptake of p-xylene from a xylene mixture. Xylene sorption seemed to be a very slow process, especially for m-xylene and o-xylene, providing the prediction of low membrane fluxes in the separation of xylenes. High loadings of silicate and borosilicalite especially created voids in the polymer, whereas zeolite Y acted as a cross-linker on the polymer. In a following report, an improved incorporation of zeolites in PI will be published. Acknowledgment. We are grateful to the Belgian Govemment for support in the form of a IUAP-PAI grant on Supramolecular Catalysis. I.F.J.V. acknowledges a fellowship as Research Assistant from the Catholic University of Leuven, Belgium. References and Notes (1) AI-ghamdi, A. M. S.; Mark, J. E. Polym. Bull. 1988, 20, 537542. (2) Barrer, R. M.; James, S. D. J. Phys. Chem. 1960, 64, 417-421. (3) Bartels-Caspers, C.; Tusel-Langer, E.; Lichtenthaler, R. N. J. Membr. Sci. 1992, 70, 75-83. (4) Jia, M.-D.; Peinemann, K.-V.; Behling, R.-D. J. Membr. Sci. 1991, 57, 289-296. (5) Jia, M.-D., Peinemann, K.-V., Behling, R.-D. J. Membr. Sci. 1992, 73, 119-128. (6) Paul, D. R.; Kemp, D. R. J. Polym. Sci., Polym. Symp. 1973, No. 41, 79-93. (7) te Hennepe, J. Ph.D. Thesis, University of Twente, The Netherlands, 1988. (8) Vankelecom, I. F. J.; Scheppers, E.; Heus, R.; Uytterhoeven, J. B. J. Phys. Chem. 1994, 98, 12390-12396. (9) McCandless, F. P.; Downs, W. B. J. Membr. Sci. 1987, 30, 111116. (10) Bell, V. L. J. Polym. Sci. 1976, 14, 225-235. (11) Sroog, C. E. Encyclopedia of Polymer Science & Technology; Wiley: New York, 1969; Vol. 11, pp 247-272. (12) Wallach, M. L. J. Polym. Sci. Part A-2 1969, 7, 1995-2004. (13) Studt, T. R&D Mag. 1992 (Aug), 30-34. (14) Coleman, M. R.; Koros, W. J. J. Membr. Sci. 1990,50,285-297. (15) Mi, Y.; Stem, S. A.; Trohalaki, S. J. Membr. Sci. 1993, 77, 4148. (16) O’Brien, K. C.; Koros, W. J. J. Membr. Sci. 1988, 35, 217-230.
Vankelecom et al. (17) Tanaka, K.; Kita, H.; Okamoto, K.; Nakamura, A,; Kusuki, Y. Polym. J. 1989, 21, 127-135. (18) Okamoto, K.; Tanaka, K.;Yokoshi, 0.;Kita, H. J. Polym. Sci., Part E : Polym. Phys. 1989, 27, 643-654. (19) Okamoto, K.; Tanaka, K.; Kita, H.; Nakamura, A,; Kusuki, Y. J. Polym. Sci., Parr B: Polym. Phys. 1989, 27, 1221-1233. (20) Kim, T. H.; Koros, W. J.; Husk, G. R.; O’Brien, K. C. J. Membr. Sci. 1988, 37, 45-62. (21) Sykes, G. F.; St. Clair, A. K. J. Appl. Polym. Sci. 1986.32, 37253735. (22) Tanaka, K., Kita, H.; Okamoto, K.; Nakamura, A,; Kusuki, Y. J. Membr. Sci. 1989, 47, 203-215. (23) Kim, T. H.; Koros, W. J.; Husk, G. R. J. Membr. Sci. 1989, 46, 43-56. (24) Matsumoto, K.; Minamizaki, Y.; Xu, P. Membrane 1992, 17, 395402. (25) Petrenko, K. D.; Zamulina, L. I.; Pedosenko A. V.; Privalko, V. P. Gas Sorption Purv 1990, 4, June, 87-90. (26) Kim, T. H.; Koros, W. J.; Husk, G. R.; O’Brien, K. C. J . Appl. Polym. Sci. 1987, 34, 1767-1771. (27) Okamoto, K.; Tanihara, N.; Watanabe, H.; Tanaka, K.; Kita, H.; Nakamura, A.; Kusuki, Y.; Nakagawa, K. J. Membr. Sci. 1992, 68, 5363. (28) Huang, R. Y. M.; Feng, X. Sep. Sci. Technol. 1992, 27, 15831597. (29) Semenova, S. I.; Ohya, H.; Higashijima, T.; Negishi,Y. J. Membr. Sci. 1992, 67, 29-37. (30) Snyder, R. W.; Thomson, B.; Bartges, B.; Czemiawski, D.; Painter, P. C. Macromolecules 1989, 22, 4166-4172. (31) Bessonov, M. I.; Kuznetsov, N. P. In Polyimides: Synthesis, Characterization and Applications; Mittal, K. L., Ed.; Plenum Press: New York, 1984; Vol. 1, pp 385-399. (32) Fryd, M. In Polyimides: Synthesis, Characterization and Applications; Mittal, K. L., Ed.; Plenum Press: New York, 1984; Vol. 1, pp 377383. (33) Harris, F. W.; Norris, S. 0.;Lanier, L. H.; Reinhardt, B. A,; Case, R. D.; Varaprath, S.; Padaki, S. M.; Torres, M.; Feld, W. A. In Polyimides: Synthesis, Characterization and Applications; Mittal, K. L., Edd.; Plenum Press: New York, 1984; Vol. 1, 3-16. (34) Bell, V. L.; Stump, B. L.; Gager, H. J. Polym. Sci. 1976, 14, 22752292. (35) Zubkov, V. A.; Birshtein, T. M.; Milevskaya, I. S. Vysokomol. Soedin. 1975, A17, 1955-1961. (36) Sandler, R. S.; Karo, W. Polymer Synthesis; Academic Press: New York, 1974; Vol. 1, Chapter 9, pp 214-262. (37) Stem, S. A.; Mi, Y.; Yamamoto, H.; St. Clair, A. K. J. Polym. Sci., Polym. Phys. Ed. 1990, 27, 1887. (38) O’Brien, K. C.; Kor0s.W. J.; Husk, G. R. Polym. Eng. Sci. 1987, 27, 211-217. (39) Sroog, C. E.; Endrey, A. L.; Abramo, S. V.; Ben, C. E.; Edwards, W. M.; Olivier, K. L. J. Polym. Sci., Part A 1965, 3, 1373-1390. (40) Navarre, M. In P olyimides: Synthesis, Characterization and Applications; Mittal, K. L., Ed.; Plenum Press: New York, 1984; Vol. 1, pp 429-442. JP942901Y
