Polymer Membranes for Gas and Vapor Separation - American

Chapter 5

Aging of Gas Permeability in Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and PTMSP-Poly(tert-butylacetylene) Blends 1

1

1

2

T. Nakagawa , T. Watanabe , M . Mori , and K. Nagai 1

Department of Industrial Chemistry, Meiji University, Higashi-mita, Tama-ku, Kawasaki 214-0033, Japan Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695

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2

The effect of aging on gas permeability in glassy p o l y [ l (trimethylsilyl)-1-propyne] (PTMSP) was investigated in terms of the dual mode sorption and transport theory. The decrease in the gas permeability coefficient depended not only on the decrease in the hole saturation constant of Langmuir adsorption (C' ), which is related to the volume of the microvoids, but also on the decrease in the gas permeability coefficient in the Henry's law mode (k D ). The decrease in C' originated from the relaxation of nonequilibrium excess volume in the Langmuir mode, which is called physical aging. To stabilize the permeability of PTMSP, poly(tert-butyl acetylene) (PTBA), whose solubility parameter resembles that of PTMSP, was synthesized and PTMSP/PTBA blend membranes were prepared. The mixing of each polymer was excellent, and no microphase separation was observed for a blend containing 20 vol % PTBA (PTMSP/PTBA 80/20). The effect of P T B A content on C' , gas permeability, and also on aging of gas permeability was investigated. Although C'H values and gas permeability decreased with an increase in P T B A content, the stability of gas permeability is maximized in the PTMSP/PTBA 80/20 blend. The spin-lattice relaxation time (T ) of carbon atoms was also a maximum for the PTMSP/PTBA 80/20 blend, which means that molecular motion is slowest for this composition. The T value of the carbons in the methyl groups of the trimethylsilyl group was very low compared with T values of carbons in the backbone. These T values did not change during aging for the PTMSP/PTBA 80/20 blend. The oxygen permeability of this 80/20 blend membrane is 6,140 Barrers at 30°C. H

D

D

H

H

1

1

1

1

Poly[l-(trimethysilyl)-l-propyne] (PTMSP) is a glassy polymer which was first synthesized by Masuda and Higashimura's group (1,2). PTMSP has the highest gas permeability, diffusivity, and solubility of all nonporous polymeric membranes, though the selectivity is very low. Both the high diffusivity and solubility coefficients of gases come from the large excess free volume of PTMSP. The biggest

68

© 1999 American Chemical Society

In Polymer Membranes for Gas and Vapor Separation; Freeman, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

69 problem, however, is a decrease in its gas permeability with time (i.e. physical aging) and upon absorption of low volatility organic vapors (i.e., pump oil vapor). The absorption of low volatility organic vapors can be prevented by careful experimental protocol. However, the effect of physical aging on gas permeability must be minimized by modification of PTMSP. In this paper, physical aging of gas permeability properties of PTMSP is described using the dual mode sorption and transport model. This model is also used to rationalize the increase in stability of gas permeation properties as a result of blending PTMSP with poly(terr-butyl acetylene) (PTBA).


Experimental Polymer Synthesis. PTMSP was synthesized according to Masuda's method(i,2). The polymerization was performed in toluene under a dry nitrogen atmosphere for 24 h at 80°C. T a C l was used as a catalyst. The polymerization of rm-butyl acetylene was performed in toluene under a dry nitrogen atmosphere at 30°C for 24 h. The catalyst was M0CI5. The chemical structure of the monomers, 1-trimethylsilyl-1-propyne and t-butylacetylene, and the corresponding polymers, PTMSP and PTBA, are shown in Figure 1. 5

Membrane Preparation. A l l PTMSP, PTBA and PTMSP/PTBA blend polymer membranes were cast on a horizontal glass plate from polymer solutions in toluene. They were immersed in methanol until just before beginning experimental measurements to prevent hysteresis of the membranes. Gas Sorption and Permeability. Gas sorption isotherms at 35°C and elevated pressure were determined by a gravimetric method using a C A H N 2000 electronic micro-balance. Gas permeabilities were measured using an adaptation of the constant volume, variable pressure technique, which employed an M K S Baratron model 310HBS-100SP pressure transducer for upstream pressures below 1 atm and at temperatures in the 30-90°C range. A n M K S Baratron model 370HA pressure transducer was used for permeability measurements at 35°C and upstream pressures as high as 40 atm. NMR Measurements. High resolution, solid-state N M R measurements were performed using a JEOL JNM400 spectrometer at 30°C. The spin-lattice relaxation time (Ti) of C and 29Si were obtained using the CP Tj pulse sequence at a spinning speed of 6 kHz (3). 1 3

Theoretical Background A quantitative description of penetrant solution and diffusion in microheterogeneous media has evolved over the past forty years and has become known as the dual mode sorption theory (4). Originally, this theory postulated that two concurrent modes of sorption are operative in a microheterogeneous medium. Nonlinear sorption isotherms can be decomposed into a linear part that accounts for normal solution (Henry's law-type domain) and a nonlinear Langmuir-type domain that accounts for immobilization of the penetrant molecules at fixed sites in the medium. Meares concluded that the glassy state contains a distribution of microvoids frozen into the structure as the polymer is cooled through its glass transition temperature (5-7). In the glassy state, segmental motion of the polymer chains is restricted, resulting in fixed microvoids or "holes" at the molecular level as shown in Figure 2. These microvoids in the glassy state immobilize a portion of the penetrant molecules by entrapment or adsorption.


70


ÇH

ÇH

3

CH Si-CH CH TMSP r

3

3

8 0

°

c

H

3

CH ^i-CH CH PTMSP r

3

3

CH ^-CH CH TBA r

3

3 0

°

c

3

Figure 1. Chemical structure of the monomers, 1-trimethylsilyl-1-propyne and t-butyl acetylene, and the corresponding polymers, PTMSP and FTB Α.

Figure 2. Schematic pictorial representation of dual-mode sorption.


71 Based on this model, sorption isotherms are described using the following equation (4):

C=C

D +

C =k p+Ç^ H

(D

D

where C is the total concentration of gas dissolved in the polymer [cm (STP)/cm (polymer)], Co is the gas concentration in the Henry's law mode, and C is the gas concentration in the Langmuir mode. The parameters of the model are ko, which is the Henry's law solubility constant, b, which is the hole affinity constant, and C'H, which is the hole saturation constant. The gas pressure in contact with the polymer sample is p. The three parameters of the dual mode sorption theory, ko, C'H, and b, are determined by analyzing an isothermal plot of C versus p. For gas sorption at low pressure, where bp « 1, the sorption isotherm in Equation 1 reduces to the following linear expression, 3

3


H

C = (k + C b)p D

(2)

H

Generally, solubility coefficients of the same gas in different amorphous polymers do not vary widely. Therefore, C'H is roughly proportional to the amount of microvoids in the polymers. At sufficiently high pressure, the microvoids become saturated and will no longer sorb additional penetrant molecules. When bp » I, sorption in the microvoids reaches the saturation limit, C'H, and Equation 1 again reduces to a linear form, C=kp +C D

(3)

H

As shown by Equation 3, ko can be obtained from the slope of the sorption isotherm at high pressure, and C'H can be determined from the y intercept of the high pressure data, b can then be determined from the slope of the low pressure data as indicated in Equation 2. If the penetrants sorbed in both the Henry's law and Langmuir modes diffuse independently through both domains, the permeability coefficient of the glassy membrane is expressed as follows (8-10): Ρ = ΚΏ

Β

+

^Ά

(4)

l + bp where Do is the diffusion coefficient in the Henry's law mode, and DH is the diffusion coefficient in the Langmuir mode. Plots of permeability coefficients versus (1 + bp)' should be linear. The intercept and slope of the line correspond to koDo and C'nbDn, respectively. Once the equilibrium parameters from the dual mode sorption theory are known, the two diffusion coefficients, Do and DH, can be determined. 1

Results and Discussion Effect of Physical Aging on Gas Permeability. Literature values of the oxygen permeability coefficient of PTMSP are strikingly variable. At room temperature, the minimum and maximum values are 3,000 and 12,000 Barrers (11,12). The unusually In Polymer Membranes for Gas and Vapor Separation; Freeman, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

72


wide variation in O2 permeability makes PTMSP a remarkable material. In this regard, the effect of storage time in vacuum at 30°C on gas permeability is shown in Figure 3. The permeability coefficients of each gas decreased by 50-60 % over the course of 2 weeks. This phenomenon is typical for aging of PTMSP membranes whose thicknesses are 100-200 μπι. There are two reasons for the decrease in gas permeability coefficients with time: (1) absorption of low volatility pump oil vapor in the PTMSP membrane and (2) physical aging. Even if a liquid nitrogen trap is used between the permeation cell and an oil rotary vacuum pump or oil diffusion pump in the gas permeation apparatus, PTMSP still absorbs oil vapor and permeability coefficients decrease. A plasticizer such as dioctylphthalate, which is contained in the rubber gasket of the permeation cell, also decreases the gas permeability (13). The effect of catalysts (14), absorbed oil vapor (12,15,16), temperature (17) and storage in air or under vacuum (12,17,18), on the decrease in PTMSP permeability has also been reported. Characterization of Aged P T M S P membranes. Physical aging is a very special phenomenon in glassy polymers. It is well known that annealing below the glass transition temperature is effective for reducing the nonequiliblium excess volume of glassy polymers. Annealing causes a decrease in the nonequiliblium excess volume, and this decrease then causes a reduction in diffusivity and solubility (19-22). The nonequiliblium excess volume is the volume of the Langmuir mode, which can be evaluated using C'H values. The C'H value of PTMSP calculated from the sorption isotherm of CO2 at 35°C, which was first reported by Ichiraku et al. (24), was 112.9 cm (STP)/cm (polymer). This value is fifteen times higher than that of glassy polystyrene (23,24) and is the highest C 0 C'H value of any known polymer. One of the authors reported decreases in ^-spacing (13) and C'H (12) of PTMSP membranes upon aging. Upon aging, based on density measurements, wide-angle X ray diffraction, and positron annihilation lifetime spectroscopy studies, Yamporskii and co-workers also inferred a decrease in the free volume of PTMSP(i#). The CO2 permeability coefficient measured at 35°C for a PTMSP membrane annealed at 75°C for varying lengths of time is shown in Figure 4. The measurement was performed by increasing the upstream pressure of CO2 from 1 to 40 atm, and then decreasing it from 40 to 1 atm. The permeability coefficient was lower at higher pressures, which is consistent with the dual sorption and partial immobilization transport theory expressed by Equation 4. Figure 4 shows no plasticization of PTMSP by CO2 even at the highest pressure (40 atm), but a small hysteresis effect was observed. The relationship between C O permeability coefficient and (1 + bp)is shown in Figure 5. As already mentioned, physical aging derives from relaxation of the nonequiliblium excess volume. This causes a decrease in C'H and DH- It is quite clear, however, that the decrease in the y-intercept in Figure 5 signifies a decrease in koDp in the Henry's mode. Therefore, the relaxation of the nonequiliblium excess free volume in glassy PTMSP not only affects the Langmuir mode but also the segmental motion in the Henry's law mode. We previously characterized the molecular motion of carbon atoms in the main chain and side chain of PTMSP using C spin-lattice relaxation time (3). During aging, changes in T i values of the side chain carbons were smaller than those of the backbone chain carbons. This result suggests that molecular motions, such as twisting, of the backbone carbons were most strongly influenced by aging. 3

3

2

1

z

1 3


73

L=0.0O61cm

m ζ Ε

§1

10

it Downloaded by CORNELL UNIV on June 15, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0733.ch005

S CL 101 Ο 10

20

30

Time [days] Figure 3. Effect of aging time at 30°C on the gas permeability coefficient of PTMSP. Upstream pressure for each of these measurements is 20-30 cmHg. Aging protocol: stored under vacuum at 30°C. 4

0

10

20

30

40

Pressure [atm] Figure 4. Effect of thermal treatment time at 75°C in nitrogen on a weekly basis for one month on the CO2 permeation isotherm of PTMSP at 35°C. Ο · : 1 week Δ A : 2 weeks • • : 3 weeks < > • : 4 weeks. The open and closed symbols represent data taken during increasing and decreasing pressure, respectively.



74


75


Gas Permeability and Aging of Blend Membranes of PTMSP and PTBA. Although the gas permeability of PTMSP is the highest of all polymeric materials, its decrease over time is remarkable. Therefore, stabilization of gas permeability in PTMSP is required. Many papers have reported modifications of PTMSP to control physical aging. Such methods include filling PTMSP with poly(dimethylsiloxane) oil (25), halogenation (26-28), copolymerization with phenylpropyne (29), blending with poly(phenylpropyne) (30), and laminating with poly(dimethylsiloxane) film (16). In this paper, the gas permeability and molecular motions of blend membranes composed of PTMSP and PTBA are reported. Characterization of PTMSP/PTBA Blend Membranes. Figure 6 shows wideangle X-ray diffraction patterns of the blend membranes. The content of P T B A (in volume %) was gradually increased from 0 % (PTMSP) to 100 % (PTBA). The PTBA membrane exhibits a peak at 17.8 degrees 2Θ in blend membranes containing 20 % or more P T B A . P T B A also exhibits a peak at 9.8 degrees. The 9.8 and 17.8 degree peaks suggest the average intersegmental and intrasegmental distances, respectively. P T B A is an amorphous, glassy polymer, like PTMSP. PTMSP exhibits an amorphous halo peak centered at 11.2 degrees 2Θ. Figure 7 shows the effect of PTBA content on C'H (obtained from CO2 sorption) and oxygen permeability coefficient at 30°C. As expected, both C' and 0 permeability decrease monotonically upon increasing the concentration of P T B A since P T B A has a lower fractional free volume than PTMSP. H

2

Gas Permeability and Oxygen/Nitrogen Selectivity. Figure 8 shows the effect of P T B A content on the permeability coefficient of oxygen and the ideal separation factor for oxygen and nitrogen at 30°C. The P T B A oxygen permeability coefficient is 160 Barrers, which is about 25% of the value for poly(dimethylsiloxane), the most permeable rubbery polymer. It is very helpful to understand the effect of the difference in side chain bulkiness on sorption and transport properties. In this regard, the trimethylsilyl group and methyl group in PTMSP enlarge the intersegmental distance and, therefore, free volume relative to that in PTBA. From Figure 7, the decrease in the permeability coefficient of oxygen is almost linear with the decrease in C'H> The compatibility of PTMSP with P T B A is evaluated by the relationship between the logarithm of gas permeability of the blends and the vol % of P T B A (3133). To calculate the volume fraction of PTBA in the blend requires the density of each component. In this study, the density of PTMSP and P T B A were determined to be 0.74 and 0.83 g/cm , respectively. Typical representative equations for the permeability of homogeneous and microphase separated blends are expressed by Equations 5 and 6, respectively: 3

InP = φιΙηΡι + ç^ln/^ p

. ρ

Ρ +2Ρ +2φ (Ρ -Ρ ) />+2/>+0 (/>-/>) 2

1

5

( )

ι

2

ι

2

2

where P i is the permeability coefficient of component 1, P2 is that of component 2, and ψ\ and q>2 are the volume fractions of components 1 and 2, respectively. In Equation 6, components 1 and 2 refer to the continuous and discontinuous phases, respectively. The straight dashed line in Figure 8 is the theoretical line corresponding to Equation 5 and the solid curve corresponds to Equation 6, when PTMSP and



76


77


200

^ χ

PTBA content [vol %] Figure 7. Effect of P T B A content on the 0 permeability coefficient at 3 0 ° C and the hole saturation constant (C'H) of CO2 in the blend membranes at 35°C. Upstream pressure for O permeation experiments is 2 0 - 3 0 cm Hg. C ' is calculated from the C 0 isotherms up to 3 0 atm. 2

Z

H

2

Figure 8. Effect of P T B A content on the 0 permeability coefficient and the O2/N2 ideal separation factor at 3 0 ° C . Upstream pressure for permeation 2

experiments is 2 0 - 3 0 cm Hg.


78 PTBA are the continuous phase and discontinuous phase, respectively. The solubility parameters of PTMSP and P T B A are 7.74 and 7.45 (cal/cm ) , respectively (34), and the degree of polymerization of PTBA is known to be slightly less than that of PTMSP. Thus the formation of a phase-separated blend composed of P T B A domains in a PTMSP matrix was not expected. When the blend ratio of PTMSP/PTBA is 80/20, gas permeability follows the straight line, which is consistent with a homogeneous (Le., non-phase separated) blend. Figure 9 shows the time dependence of the permeability coefficients of N , 0 , and C 0 of the PTMSP/PTBA 80/20 blend membrane, which was kept in an evacuated vessel at 30°C between permeability measurements. The decrease in permeability coefficients with time has been reduced relative to that of pure PTMSP. The influence of aging time on oxygen permeability coefficients of the blend membranes is shown in Figure 10 as a function of the initial permeability of the blend. The 80/20 PTMSP/PTBA membrane is the best based on the balance of the permeability coefficient and its stability. C 0 sorption isotherms for the blend membranes at 35°C are shown in Figure 11, and the dual-mode sorption parameters are summarized in Table I. C'H decreases with an increase in P T B A content. The C'H value does not directly correlate with the stability of permeability, because the C'H value is a measure of the total volume of the nonequiliblium excess volume (Le., the microvoids), and it is the size distribution of the microvoids which is important for stabilization. The bigger microvoids easily decay with aging. The initial C'H value of 128 cm (STP)C02/cm polymer for the 80/20 blend membrane was not changed after aging this blend for 2 weeks. 3


2

2

1/2

2

2

3

3

Table I Dual-mode sorption parameters for CO2 in PTMSP, PTBA, and PTMSP/PTBA blend membranes Membrane

K

D

< 2) ^ H

b

r

3)

PTMSP

0.937

149

0.0347

80/20 Blend

0.673

128

0.0440

40/60 Blend

0.738

61.6

0.0728

PTBA

0.714

36.4

0.0924

3

1) cm (STP)/(cm^(polymer)-atm) 2) cm (STP)/ cm (polymer) 3) atm" 3

3

1

1 3

Solid-state N M R of P T M S P / P T B A Blend Membranes. The CP/MAS C N M R spectrum of P T B A is shown in Figure 12. The peak assignments are also shown in this figure. The two peaks at 60-90 ppm and 180-210 ppm are spinning side bands (SSB). In the liquid state, the N M R signal of the methyl groups (a) in the t-butyl moiety is split into two parts due to the difference in the configuration of the polymer chains. However, a single peak is observed in the solid state spectrum presented in Figure 12. In Figure 13, the C P / M A S C N M R spectra of several PTMSP/PTBA blends are shown. Each chemical shift coincides with the CP/MAS spectra of the PTMSP and P T B A membranes. No changes in chemical shift values were observed with variations in P T B A content. The intensity of the peak in the 1 3


79

Έ =

«ε ο ο

I* § οι


δ

I f

ι

|10-6|

.ο 9

il

10r7l

5 10 Time [days]

Ο

15

Figure 9. Effect of time under vacuum on the gas permeability coefficients at 30°C of PTMSP/PTBA 80/20 membrane. Upstream pressure is 20-30 cm Hg. 100 Gas: 0

i i

80/20 Blend

80 Ξ "I S Ό δ ? Ë S φ c Ou « α.

2

so 95/5 Blend ' •

PTBA 70 h

PTMSP 60/40 Blend

60

40/60 Blend

50 I

• • ...ml

10"

. .

• • ••••••I

10"

8

10"

7

6

10"

5

Initial permeability coefficient [cm (STP)-cm/(cm -s-cm Hg)] 3

2

Figure 10. Effect of time under vacuum on the decrease of oxygen permeability of blend PTMSP/PTBA membranes at 30°C. Upstream pressure is 20-30 cm Hg.



80

Figure 12. Solid-state CP/MAS SSB=Spinning side band.

1 3

C N M R spectrum of P T B A membrane.


81


95/5 Blend

80/20 Blend

40/60 Blend

250 200 150 100 50 Figure 13. Solid-state CP/MAS membranes.

1 3

0

-50

ppm

C - N M R spectra of PTMSP/PTBA blend


82


backbone chain is very weak for the 40/60 blend membrane because of the higher PTBA content. Molecular Motion of PTMSP/PTBA Blends Before and After Aging. The authors have already reported the influence of aging on the molecular motion of PTMSP, copolymer membranes and blend membranes composed of 1-trimethylsilyl1-propyne and 1-phenylpropyne (35). The motions of the carbons in the main chains changed upon aging, but those of the side chain groups did not change. Figures 14 and 15 show the relationship between P T B A content and carbon spin-lattice relaxation time (Ti) of the PTMSP side-chain and backbone carbon atoms, respectively. The effect of P T B A content on C'H is also shown in these figures. T i of the methyl group carbon atoms (a) in the side-chain of PTMSP became longer with increases in P T B A content. This result indicates that the molecular motion of the trimethylsilyl groups became slower. On the other hand, T i values for the carbon atoms of the other methyl group (b) and the backbone chain show a maximum at 20 vol % PTBA, which suggests perfect mixing of PTMSP and PTBA. These results coincide with the maximum stability of the gas permeability coefficient (cf. Figure 10). Conclusions PTMSP exhibits significant decreases in gas permeability with age. During aging, the nonequilibrium excess volume of glassy PTMSP relaxes. The reasons for the decrease in gas permeability were not only relaxation of the nonequilibrium excess volume in the Langmuir mode, but also reduction of the permeability coefficient in the Henry's law mode. This latter effect appears to be correlated with relaxation of the nonequilibrium excess volume. Blending PTMSP with P T B A is a novel strategy to improve the stability of the gas permeability of PTMSP membranes by controlling the excess free volume, which is characterized by C'H- The compatibility of PTMSP and P T B A is excellent. The 80/20 blend exhibited perfect mixing based on the relationship between gas permeability and volume content of PTBA and based on the molecular motions of the blend as characterized by C T i . The stability of gas permeability was also best for the 80/20 blend, and the oxygen permeability coefficient of this blend is 6,140 Barrers at 30°C. The nonequilibrium excess volume hardly decreased with age. The molecular motions of the trimethylsilyl group were not influenced by aging. However, motions of the carbon atoms in the backbone were reduced after aging. The spacing between the polymer segments in PTMSP/PTBA blend membranes is wide enough to permit motions of the side-chain after aging. 13

Literature Cited 1. Masuda, T.; Isobe, E.; Higashimura, T.; Takada, K. J. Amer. Chem. Soc., 1983, 105, 7473. 2. Masuda, T.; Isobe, E.; Higashimura, T. Macromol. 1985, 18, 841. 3. Nagai, K.; Watanabe, T.; Nakagawa, T. Polymer J., 1996, 28, 933. 4. Vieth, W. R.; Howell, J. M.; Hsieth, J. H . J. Memb. Sci., 1976, 1, 177. 5. Meares, P. J. Amer. Chem. Soc., 1954, 76, 3415. 6. Meares, P. Trans. Farad. Soc., 1957, 53, 101. 7. Meares, P. Trans. Farad. Soc., 1958, 54, 40.



83

Figure 15. Relationship between ρ χ β Α content and backbone C'H in PTMSP/PTBA blend membranes.

1 3

C T i and



84 8. Paul, D. R.; Koros, W. J. Polym. Sci.: Part B: Polym. Phys., 1976, 14, 675. 9. Koros, W. J.; Paul, D . R.; Rocha, A . A. J. Polym. Sci.: Part B: Polym. Phys., 1976, 14, 687. 10. Koros, W. J.; Patton, C. J.; Felder, R. M . ; Finder, S. J. J. Polym. Sci.: Part B: Polym. Phys., 1980, 18, 1485. 11. Odani, H.; Masuda, T. In Polymers for Gas Separation; Toshima, Ν. Ed., V C H Publishers, Inc.: New York, 1992; Chap. 4, pp107-144. 12. Nagai, K.; Nakagawa, T. J. Memb. Sci., 1995, 105, 261. 13. Nakagawa, T.; Saito, T.; Asakawa, S.; Saito, Y . Gas Sep. Purif, 1988, 2, 3. 14. Nagai, K.; Nakagawa, T. J. Memb. Sci., 1995, 105, 261. 15. Witchey-Lakshmanan, L . C.; Hopfenberg, H . B.; Chern, R. T. J. Memb. Sci., 1990, 48, 321. 16. Asakawa, S.; Saito, Y.; Waragi, K.; Nakagawa, T. Gas Sep. Purif., 1989, 3, 117. 17. Shimomura, H.; Nakanishi, K.; Odani, H.; Kurata, M.; Masuda, T.; Higashimura, T. Kobunshi Ronbunshu, 1986, 43, 747. 18. Ysampol'skii, Y . P.; Shantorovich, V . P.; Chernyakovskii, F. P.; Kornilov, A . I.; Plate, N. A . J. Appl. Polym. Sci., 1993, 47, 85. 19. Chen, A . H.; Paul, D. R. J. Appl. Polym. Sci, 1979, 24, 1539. 20. Wonders, A . G.; Paul, D. R. J. Memb. Sci., 1979, 5, 63. 21. Hachisuka, H.; Kito, H.; Tsujita, Y.; Takizawa, Α.; Kinoshita, T. Polym. J., 1988, 35, 1333. 22. Moe, M. B.; Koros, W. J.; Paul, D. R. J. Polym. Sci.: Part B: Polym. Phys., 1988, 26, 1931. 23. Hachisuka, H.; Tsujita, Y.; Takizawa, Α.; Kinoshita, T. Polym. J., 1989, 21, 681 24. Ichiraku, Y . ; Stern, S. Α.; Nakagawa, T. J. Memb. Sci., 1987, 34, 5. 25. Nakagawa, T.; Fujisaki, S.; Nakano, H.; Higuchi, A . J. Memb. Sci., 1994, 94, 183. 26. Nagai, K.; Higuchi, Α.; Nakagawa, T. J. Appl. Polym. Sci., 1994, 54, 1207. 27. Nagai, K.; Mori, M.; Watanabe, T.; Nakagawa, T. J. Polym. Sci.: Part B: Polym. Phys., 1997, 35, 119. 28. Langsam, M.; Anand, M.; Karwacki, E. J. Gas Sep. Purif, 1988, 2, 162. 29. Nagai, K.; Higuchi, Α.; Nakagawa, T. J. Appl. Polym. Sci., 1994, 54, 1353. 30. Nakagawa, T.; Watanabe, T.; Nagai, K. ACS Symposium Series, 1998, in press. 31. Paul, D. R. J. Memb. Sci., 1966, 22, 360. 32. Barrie, J. Α.; Ismail, J. B. J. Memb. Sci., 1983, 13, 197. 33. Barnabeo, A . E.; Creasy, W. S.; Robeson, L . M. J. Polym. Sci.: Part A: Polym. Chem., 1975, 13, 1979. 34. Fedors, R. F. Polym. Eng. Sci., 1974, 14, 143. 35. Nagai, K.; Mori, M.; Watanabe, T.; Nakagawa, T. J. Polym. Sci.: Part B: Polym. Phys., 1997, 35, 119.


Polymer Membranes for Gas and Vapor Separation - American

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