Surface fluorination of poly (phenylene oxide) composite membranes Part I. Transport properties

journal

ELSEVIER

of

Journal of Membrane Science 90 ( 1994) 2 1-35

Surface fluorination of poly ( phenylene oxide) composite membranes Part I. Transport properties J.D. Le Rouxa, D.R. Paula+, J. Kampabyl, R.J. Lagowb ‘Department of Chemical Engineering, Center for Polymer Research, The University of Texas, Austin, TX 78712, USA bDepartment of Chemistry, Centerfor Polymer Research, The University of Texas, Austin, TX 78712, USA (Received August 27, 1993; accepted in revised form December 20, 1993)

Abstract

The effect of surface fluorination on the gas transport properties of composite membranes, comprising an inert porous ceramic support and a selective layer consisting of poly ( phenylene oxide), was examined. A small reactor volume permitted the treatment time and the fluorine feed concentration to be investigated independently. The gas transport properties of the treated membranes were evaluated for six gases (N,, 02, CH4, Hz, He and COZ), in terms of permeance (P/I or pressure-normalized flux) and the ideal selectivity for eight pairs of these gases. It was generally found that fluorination at different fluorine feed concentrations and reaction times reduced the permeante of all of the gases. The permeance of the lighter gases (He and H,) was reduced by a smaller factor than that of the heavier gases ( N2 and CH4). Fluorination increased the selectivity of He and Hz relative to N2 or CH4 by a small factor, but reduced the selectivity of O2 and CO2 relative to N2 or CH4. When the membranes were coated with a layer of poly( dimethylsiloxane ) (PDMS) subsequent to fluorination, the permeance decreased, considerably more for N2 and CH4 than for the other gases. Surface coating also substantially increased the selectivities of all the gas pairs. The largest gains in selectivity after fluorination and coating were found at the higher concentration (0.1% F,) and intermediate treatment times of 3 to 5 min. Based on these results, surface coating with PDMS is recommended as a post-treatment step in the fluorination process. Key work Gas separation; Composite membranes; Fluorination; Surface modification; Poly(2,6_dimethyl-1,4-phenylene

1. Introduction

Controlled direct surface fluorination has been employed to alter the gas transport properties of both thick and thin-skinned polymer membranes where the selective separation layer is *Corresponding author. ‘Current address: Southwest Research Institute, nio, TX, USA.

San Anto-

oxide)

formed from either a rubbery or a glassy polymer [ l-71. Detailed studies have been reported on the gas phase fluorination of the glassy polymers poly [ l- (trimethylsilyl)- 1-propyne ] (PTMSP ) [ 41 and polysulfone (PSF) [ 1,3]. In the case of PTMSP both thick films and hollow fiber composite membranes were used. For PSF, one study considered the fluorination of thick films [ 3 1, but integrally-skinned asymmetric membranes were predominantly used because they allow a better

0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO376-7388(94)00005-J

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J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 21-35

assessment of any improvements that may have commercial potential [ 1,3 1. These membranes were formed with very thin and relatively defectfree skins using a dry/wet phase inversion process [ 8,9 1. Under relatively mild fluorination conditions which minimized damage to the treated surface, fluorination treatment resulted in a significant increase in the ideal selectivity of O2 over N2; simultaneously the selectivities of He and Hz relative to Nz or CH, were increased by factors of 2 to 10. In all cases a gain in selectivity was accompanied by an often substantial decrease in the pressure-normalized gas flux or permeance (P/1) of the membrane. Particularly in the case of PSF, where the permeability of the untreatedpolymer is relatively low, the treated membrane may be unacceptably slow despite the improved selectivity [ 11. In a recent study [ 1 ] asymmetric PSF membranes were fluorinated in a well-mixed reactor where the treatment conditions at the membrane surface were similar to that of the feed, so that the effects of fluorine concentration and fluorination time could be investigated independently. These results showed that fluorination

times of 5 min at concentrations of 0.02% F2 in He yielded the highest selectivities of He and Hz relative to Nz or CH4 but that shorter times were required to optimize the separation of O2 and COZfrom Nz or CH4. Loss of selectivity at higher fluorine concentrations or longer treatment times was attributed to damage at the polymer surface because of the harsh nature of the fluorination treatment. This was supported by the observation that selectivities were invariably improved when the membrane surface was coated with a layer of poly (dimethylsiloxane ) ( PDMS ) after fluorination treatment. The present study investigates the fluorination treatment of composite membranes with a selective layer formed from the glassy polymer poly ( 2,6-dimethyl- 1,Cphenylene oxide ) , known as poly (phenylene oxide) or PPO. Table 1 compares the gas transport properties of dense PPO films with those of PSF, PTMSP and PDMS. It is clear that the permeability coefficients of PPO for all six gases shown are significantly higher than for PSF, but lower than for PTMSP. Since the selectivities show the opposite trend, PPO represents the typical trade-off in transport

Table 1 Transport properties for dense films of poly (phenylene oxide) and other relevant polymers Polymer

T’

Permeability

(barrers)b

N2

02

Selectivity

Ref.

(“C) Cb

PDMS

25

281

604

PTMSP”

25 30

6745 4970

10040 7730

PPO

25 25

3.8 2.96

25 25 25

0.205 0.250 0.179

PSF

15.8 13.2

1.18 1.3 1.15

He

H2

17000 13000

co2

02/N2

649

354

3230

2.15

14

16150

6600

33100 28000

1.49 1.56

4 10

4.16 4.46

13 this workd

5.76 5.2 6.43

12 11 this workd

3.71

0.39 0.158

“Temperature at which properties were measured. bl barrer= lo-r0 cm3(STP) cm/cm’s cmHg. “Poly [ I-(trimethylsilyl)-1-propyne]. dN2, O2 and CH4 measured at 6 atm and He, Hz and CO2 at 2 atm.

112.5 86.9

19.3 10.8 11.54

77.8 61.5

10.64

75.5 56.5

7.06 5.70 6.07

J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 21-35

properties found for many glassy polymers [ 15 1. An objective of this work was to produce fluorinated PPO membranes with improved selectivities and overall permeances which are higher than fluorinated PSF membranes; the permeance is, of course, inversely related to the skin thickness. In addition, the PPO composite membranes were prepared by spin coating the polymer onto a highly porous inorganic substrate, according to a method developed recently [ 16 1. In both integrally-skinned asymmetric membranes and composite membranes with polymeric supports, the porous support could contribute signiflcantly to the gas transport properties of the membrane and may itself be affected by fluorination. The method of forming composite membranes by spin coating can potentially be used to prepare membranes from a variety of polymers which may not readily be formed into asymmetric membranes. By employing a relatively inert and highly porous alumina support, the intention was to minimize interference of the substrate and isolate the effects of treating the selective skin. Although the thickness of this layer was an order of magnitude greater than those reported for asymmetric PSF membranes [ 1,9], the fluorination depth (200 to 500 A) [ 3 ] is expected to be of the same order. Thus the proportion of fluorinated to unfluorinated polymer in the skin region could be of the order of 10% or smaller which approaches the optimum thickness derived from theoretical considerations [ 1,2]. Finally, the relatively simple structure of the PPO repeat unit, compared to PSF, was also expected to facilitate analysis of the fluorinated layer, as discussed in Part II of this study [ 17 1. 2. Experimental 2.1. Formation of composite membranes

The PPO composite membranes were formed by spin coating a dilute solution of the polymer directly onto microporous ceramic substrates. The microporous aluminum oxide microfilters ( AnoporeTM made by Anotec Separations) are

23

asymmetric with an average surface pore diameter of 200 A, a bulk pore diameter of 2000 8, and a molecular weight cutoff of 1O5daltons; the bulk porosity is 30-35% and the surface pore density of the order of IO+” cmm2 [ 181. These membranes are manufactured electrochemically by anodic oxidation of aluminum and have narrowly distributed hexagonal pores of very low tortuosity [ 19 1. The PPO (supplied by the courtesy of Dr. D.W. White of General Electric Company) had a relatively high intrinsic viscosity in chloroform of 1.5 dl/g (at 25 “C). Using reported parameters [ 201, the Mark-Houwink equation was used to estimate a molecular weight of - 290,000. The spin-coating solution comprised 2.0% (by weight) of PPO in 1,l ,Ztrichloroethane. A photoresist spin coater (Series EClOlD) from Headway Research Inc. (Garland, TX) was used to apply the PPO solution. The ceramic substrate membrane was taped to a 13 x 13 cm glass plate which in turn was centered on the vacuum chuck of the spin coater. After flooding the membrane with - 5 ml of polymer solution, the spinner was immediately activated and allowed to rotate at a preselected spinning speed of 250 rpm for 60 s; maximum speed was reached within 3 s. By the end of the spinning cycle the coated substrate appeared dry but was further air-dried for several hours and then dried under vacuum at 80” C for at least 16 h to ensure complete removal of solvent. Prior to solvent treatment and gas permeation measurements, the circular composite membranes (4.7 cm in diameter) were masked peripherally with adhesive aluminum foil to expose a permeation area of 5.1 cm2; masking helped to minimize damage during handling. All membranes were treated with liquid cyclohexane by drawing a liquid-saturated sponge applicator across the exposed surface of the membrane, followed by vacuum drying at 80°C. It was found that this after-treatment with a weak solvent repaired minor defects. The solvent treatment usually raised the selectivity of the final membrane to well above the value measured for thick films shown in Table 1. The unfluorinated membranes of this study had an average 0z/N2 selectivity of 5.0 with a standard deviation of 0.3.

J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 21-35

24

The subject of solvent treatment of these PPO membranes is considered in greater detail by Rezacet al. [21]. At this point membrane samples were screened by measuring their O2 and N2 fluxes and the ideal selectivity at 24’ C. The O2 permeance (Table 1) gave an indication of the skin thickness and the Oz/N2 selectivity was used to gauge the integrity of the skin. A membrane was considered defectfree if it had an 0z/N2 selectivity equal to or greater than 4.4, i.e., that of thick dense PPO films prepared by solution casting, as shown in Table 1. The final membranes had a composite structure comprising the microporous ceramic support covered by a substantially defect-free PPO skin with apparent thicknesses ranging from 0.45 to 0.60 pm. To minimize variability, mem-

0

0.02

0.04

0.06

Fluorine concentration

0.08

0.10

[vol%]

branes within a narrow and Oz/N2 selectivities nation. After measuring erties, the membrane fluorination treatment

range of skin thicknesses were selected for fluoritheir gas transport propdiscs were prepared for as described below.

2.2. Fluorination and surface coating Prior to fluorination the composite membranes were mounted on a solid aluminum backing using adhesive aluminum foil, so that only the PPO skin surface was exposed. The masked membranes were fluorinated in a stainless steel fluorination reactor with a reaction chamber volume of 14.6 cm3. The feed gas consisted of commercially available 2% (by volume ) F2 in He which was further diluted to the desired concen-

0.120

0.02

0.04

0.06

Fluorine concentration

u.lJx

0.10

0.12

[vol%]

Fig. 1. The effect of fluorine feed concentration at a fixed fluorination time of 2 min on the permeance (P/l) of PPO composite membranes for six gases: ( 0 ) untreated membranes, ( 0 ) after fluorination and ( 0 ) after coating the fluorinated membrane surface with a layer of PDMS. Solid lines are intended to emphasize trends in the data. The units of permeance are GPU or gas permeation units [ 1 GPLJ= 10e6 cm3 ( STP) /cm* s cmHg1.

J.D. Le Roux et al. /Journal ofMembraneScience

0

0.02

0.04

0.06

Fluorine concentration

0.08

0.10

0.12 0

[vol%]

0.02

25

90 (1994) 21-35

0.04

0.06

Fluorine concentration

0.08

0.10

0.12

[vol%]

Fig. 2. Effect of fluorine feed concentration for a fixed fluorination and lines have the same meanings as in Fig. 1.

time of 2 min on the selectivity of six gas pairs. The symbols

tration with pure He. For the small reactor, the fluorine concentration in the feed approaches the concentration at the membrane surface within 1 min of treatment; after this the treatment time and fluorine concentration at the membrane surface can be varied independently. The increased F2 concentration in the reactor at short times necessitated the use of extremely low feed concentrations to prevent overfluorination of the membrane. Oxygen and nitrogen are present as impurities in the feed gas, an issue which is discussed in detail in Part II [ 17 1. Background and further experimental details of the fluorination process are given elsewhere [ 11. The gas transport properties of the membranes were measured before and after coating their surfaces with a layer of PDMS. As noted from its gas transport properties listed in Table

1, PDMS is highly permeable and has a relatively low Oz/Nz selectivity compared to PPO. These properties make it well suited for caulking minor skin defects [ 22,231. A 2% (by weight) solution of PDMS in n-heptane was applied to the membrane surface by means of a sponge applicator and allowed to dry at ambient temperature for several hours and then overnight in a vacuum oven at 80°C to remove all traces of solvent. 2.3. Gas permeation measurements The permeability coefftcients of the six gases Nz, 02, CH4, Hz, He and CO2 were measured independently for a thick PPO film at 25 ‘C. Table 1 contains these and other values from the literature. It is apparent that the permeabilities from

26

J.D. Le Roux et al. /Journal OfMembraneScience 90 (1994) 21-35

bility coefficient of O2 at 25 ‘C. It should be emphasized that this is an apparent skin thickness averaged over the area of the membrane. Thus it accounts for regions of the skin with non-uniform thickness, intrusion of polymer into the pores of the substrate, and for any resistance to the permeation of O2 in the porous substructure.

8

.O

6

-2 8 g

5 4

3. Gas transport 3.1. Variablefeed composition

ot~I~I”~‘~“1 0

0.02

0.04

0.06

0.08

0.10

0.12

Fluorine concentration lvol%J

Fig. 3. The effect of fluorine feed concentration for a fixed fluorination time of 2 min on the selectivities of the 02/N2 and NJCH, gas pairs. The symbols have the same meanings as in Fig. 1.

different sources differ considerably and the authors’ measured values were used to calculate selectivities for reference purposes, as well as the apparent skin thicknesses of the PPO layers of the membranes. For each membrane the permeation rates of these six gases were measured before and after fluorination, and after coating with PDMS. The membrane was mounted in a commercially available permeation cell (Millipore Corporation, Bedford, MA). After thorough purging of both the upstream and downstream parts of the cell, the volumetric gas flow rates were determined using a soap bubble flow meter at feed gas pressures in the range of 2 to 4 atm. These measurements yielded the pressurenormalized flux (P/Z) from which the ideal selectivities were calculated for several gas pairs. The thickness of the PPO selective layer was estimated from the measured P/l and the permea-

As discussed above, the small volume of the fluorination reactor permits the independent investigation of the effects of fluorination time and concentration of fluorine in the reactor feed gas. Each of the composite PPO membranes were formed separately and there is naturally some deviation from the targeted thickness. To minimize this source of variability, membranes within a relatively narrow range of skin thicknesses were selected for treatment and usually several membranes were treated at each set of fluorination conditions. The average apparent skin thickness was 4,800 8, and only defect-free membranes with an 02/N2 selectivity above 4.4 were used, in fact, as mentioned above, the average selectivity was 5.0 2 0.3. The permeance for each of the six experimental gases was measured before and after fluorination treatment, and again after the membrane had been coated with PDMS. Preliminary experiments and experience with the fluorination of asymmetric polysulfone membranes [ 1 ] indicated the use of low fluorine concentrations in the feed gas. Figs. 1 to 3 show the gas transport properties when a set of membranes was treated for 2 min at various fluorine feed concentrations. The individual data points are shown to illustrate the unavoidable variation in the results; this can be ascribed to variabilities within both the gas phase fluorination treatment itself and among the individually formed membranes. Solid lines in the figures are to illustrate trends in the data more clearly. Fig. 1 shows the permeance (P/Z) of the six gases as a function of fluorine feed concentration before and after fluorination and after surface

27

J.D. L.e Roux et al. /Journal of Membrane Science 90 (I 994) 21-35

012345

6

7

8

9

Fluorination time [min]

10

0

1234567

8

9

10

Fluorination time [min]

Fig. 4. The effect of treatment at different fluorination times for a constant feed concentration of 0.02% F2 on the permeance (P/ I). The symbols and lines have the same meanings as in Fig. 1.

coating. In the interest of brevity, P/Z is reported in gas permeation units (GPU), where 1 GPU= 1OS6cm3 (STP ) /cm2 s cmHg. As a control, three of the membranes were not fluorinated but were only coated with PDMS. For all gases, as shown by the solid symbols, increased fluorine concentration consistently results in decreased permeance after fluorination. Higher concentrations result in lower P/Z values for all but the two slowest gases, N2 and CH4, where the decrease appears to be independent of the concentration. After surface coating He, Hz, O2 and CO2 again respond similarly and P/I is lowered by a small but roughly constant factor; thus the trend lines for the fluorinated and coated membranes are approximately parallel on the semilogarithmic scale. On the other hand, for N2 and CH4, surface coating reduces the permeance after fluorination by a progressively increasing factor

as fluorine concentration is increased. After coating, the trend appears to be broadly similar for all six gases. These results could be due to the opposing effects of polymer degradation or surface damage occurring simultaneously with the creation of a layer of fluorinated polymer at the membrane surface. Thus, a more highly fluorinated polymer with a lower permeability may be formed at progressively higher fluorine concentrations. However, at the increasingly harsh reaction conditions minor damage could increase the permeability by creating defects in the skin, possibly stemming from polymer chain scission in thin or weak regions of the skin. It appears that the scale of the defects are such that the gases with larger kinetic diameters (N2 and CH4) are most affected. Subsequent coating with PDMS may then serve to repair or caulk these defects, which could

28

J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 21-35

0

I

2

3

4

5

Fluorination

6

7

8

9

time [min]

10

0

I

2

3

4

5

Fluorination

6

7

8

9

10

time [min]

Fig. 5. The selectivity before ( 0 ) and after ( 0 ) fluorination and after coating with PDMS ( 0 ) for the six gas pairs shown in Fig. 2 at different fluorination times and a constant feed concentration of 0.02% F2. The symbols and lines have the same meanings as in Fig. 1.

explain why the responses of all the gases follow similar decreasing trends after coating. Figs. 2 and 3 show ideal selectivities (P/I ratios) for 8 pairs of the gases described in Fig. 1. With few exceptions, fluorination at the lowest fluorine concentration (0.02%) increases the selectivity of the membrane, while higher concentrations yield progressively lower selectivities. After coating with PDMS, however, the trend is generally reversed and considerably improved selectivities are realized at higher fluorine concentrations. The gas pairs where He or H2 is the faster gas relative to N2 or CH4, show a progressive increase in selectivity to values of around 300-500 for He/CH, and 700-900 for HJCH4 at concentrations of 0.08 and 0.1% Fz. In Fig. 3 the Oz/N2 selectivity after fluorination apparently declines rapidly at high concentrations, but

after surface coating the selectivity could be raised above 7 for several membranes. The results in Figs. 2 and 3 can also be interpreted&r terms of the opposing effects of creating a more selective fluorinated polymer versus damage to the fluorinated layer as proposed above. For all but the mildest treatment conditions, fluorination lowers the selectivity due to damage, and the selectivity is restored by surface coating and considerably improved over that of the unfluorinated membrane. Since the permeability of the slower gases (N, and CH4) is most affected by the coating, the addition of the PDMS surface layer has a significant effect on the selectivity but only a modest effect on the productivity of membranes as measured in terms of the permeance of the faster gas. Thus, the actual properties of the fluorinated membranes are only

J.D. L.eRoux et al. /Journal ofMembrane Science 90 (1994) 21-35

00 0

I

2

3

4 Fluorination

5

6

7

time

[min]

8

9

10

Fig. 6. The selectivity before ( 0 ) and after ( 0 ) fluorination and after coating with PDMS (0 ) for 02/N2 and N&H4 at the same conditions as those for Figs. 4 and 5.

approached after coating, since it is assumed that the PDMS layer is thin enough to have only a negligible effect on the overall transport properties of the membrane and that its main function is to repair the defects. The above results illustrate the importance of the surface coating posttreatment step as an integral part of the amelioration of glassy PPO membranes by surface fluorination. 3.2. Variable treatment time Another fluorination strategy is to vary the treatment time while maintaining a constant concentration of fluorine in the feed gas. Based on the above results for fluorination at different fluorine feed concentrations, concentrations of 0.02 and 0.1% were selected for further investigation. At a fluorination time of 2 min the lower

29

concentration generally yielded the smallest loss of permeability and the best selectivities before coating. On the other hand, after fluorination and coating, the greatest gains in selectivity were found at high fluorine concentrations, although this was accompanied by a greater loss of productivity. Figs. 4 to 9 show the response of permeance (P/I) and a range of selectivities when treatment time was varied at these two concentrations. Fig. 4 shows a progressive decrease in the permeance of all six gases after fluorination at 0.02% Fz as the treatment time is increased from 2 to 10 min. A further decrease is apparent after the membranes have been coated with PDMS and it appears that this post-treatment again has a greater effect on the permeability of the slower gases, N2 and CH4. The relative variation in the permeability of the treated membranes to different gases is more readily apparent from Figs. 5 and 6 which show the accompanying changes in selectivity. For He and H2 relative to Nz or CH4 in Fig. 5, the ideal selectivities increase due to fluorination and, as before, show a substantial increase after coating. A progressive increase is observed up to a fluorination time of 5 min, followed by a slight decline at the longest time of 10 min. For example, at times above 2 min the He/N, selectivity was increased from an average of 40.1 t 4.3 (standard deviation) before treatment to well over 200 after fluorination and coating. A smaller but still significant improvement from 47.2 + 4.4 before coating to 100 and higher was achieved for Hz/ Nz. The selectivities for He/CH, and H2/CH4 showed improvements of similar or greater magnitudes. On the other hand, the response of the gas pairs containing O2 or CO2 as the fast gas is somewhat different. For these pairs fluorination for longer than 2 min showed a general decrease in selectivity which became more pronounced at longer fluorination times. Surface coating, however, was of critical importance, since coating consistently raised the selectivity of most treated membranes to values well above those of the unfluorinated membranes. For 02/N2, selectivities of between 6 and 8 were obtained, compared to those of the unfluorinated membranes which

J.D. Le Roux et al. /Journal

Fluorination time

ofMembrane

[min]

Science 90 (1994) 21-35

Fluorination time [mini

Fig. 7. The effect of treatment at different fluorination times for a constant feed concentration I). The symbols and lines have the same meanings as in Fig. 1

have an average value of about 5. Similarly the selectivity of the gas pair C02/CH4 was increased from an average of 16.72 1.6 before fluorination to about 35-45 after fluorination and coating. Figs. 7 to 9 show the same permeance and selectivities as above for treatment times of 2, 3 and 5 min at a constant fluorine feed concentration of 0.1%. Compared to P/lat the lower concentration of 0.0296, the same general trends are observed in Fig. 7; for most membranes fluorination substantially lowers the permeance and coating with PDMS results in a further but smaller decrease. The effect of coating is again more pronounced for N2 and CH,. This is also reflected in the selectivities shown in Figs. 8 and 9, where high selectivities are obtained for He and Hz relative to N2 or CH,. The small gain in selectivity after fluorination is considerably in-

of 0.1% F2 on the permeance

(I’/

creased after coating. Again, the selectivities of the gas pairs containing O2 or CO2 as the fast gas (Fig. 9) are only increased when fluorination treatment is followed by coating. Compared to fluorination at a lower concentration of 0.02% F2, the results at 0.1% show no significant dependence on treatment time. This may result from the severity of the damage to the membrane surface at the higher concentration. At lower concentrations, the damage is small at short times but greater when the membrane surface has a longer exposure to fluorine. It has been suggested that the net effect of increased fluorination time is to increase the thickness of the fluorinated portion of the skin [ 1 ] and this seems to be reflected in the progressive decrease in permeance with time. At higher fluorine concentrations and longer times the damage may be sufficiently severe to counterbalance the effect of

J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 21-35

0

1

2

3

4

5

Fluorination time [min]

6

0

1

2

3

4

31

5

6

Fluorination time [min]

Fig. 8. The selectivity before ( 0 ) and after ( 0 ) fluorination and after coating with PDMS ( 0 ) for six gas pairs at different fluorination times and a constant feed concentration of 0.1% F2.

any increase in the thickness of the fluorinated layer. These issues are investigated analytically inPart [17]. 4. Summary and evaluation 4.1. Fluorination of composite PPO membranes In this paper the effect of surface fluorination treatment on the gas transport properties of composite membranes comprising an inert porous ceramic support and a selective layer consisting of PPO, was examined. Part II [ 17 ] which follows, describes the characterization of the fluorinated layer by means of two complementary analytical methods: X-ray photoelectron spectroscopy and dynamic contact angle analy-

sis. The gas transport properties of the treated membranes were evaluated for six gases, in terms of productivity (i.e., pressure-normalized flux, P/Z or permeance) and the ideal selectivity for eight gas pairs. It was found generally that fluorination at different fluorine feed concentrations and reaction times reduced the permeance of all of the gases. The P/l of the lighter gases (He and Hz) was reduced by a smaller factor than that of the heavier gases. When the membranes were coated with a layer of PDMS subsequent to fluorination, the permeance was again observed to decrease, considerably more for N2 and CH4 than for the other gases. The effect of fluorination on the selectivity was found to be somewhat more complex. Fluorination increased the selectivity of He and Hz relative to N2 or CH, by a small factor, but lowered

J.D. Le Roux et al. /Journal ofMembrane Science 90 (I 994) 21-35

.o

2.0

-

1.5

-

1.0

-

D 0

.B P d

.

1 0.5

0

-

o-.l.l.l.1~1.0

I

El

.

* 8

0

2 Fluorination

3

4

5

h

time [minj

Fig. 9. The selectivity before (0 ) and after (0 ) fluorination and after coating with PDMS (0 ) for 02/N2 and NZ/CH4 at the same conditions as those for Figs. 7 and 8.

the selectivity of O2 and CO2 relative to Nz or CH4. Surface coating after fluorination, however, yielded substantial gains in the selectivities for all these gas pairs. To provide an overview of the effect of the various treatment conditions, the Oz/Nz and He/N2 selectivities of fluorinated and coated membranes for all of the treatment conditions are summarized graphically in Fig. 10. The highest gains in selectivity are found at the higher concentration (0.1% F2) and intermediate times of 3 to 5 min. Reference to Figs. 5 and 8 confirms that all selectivities, with the possible exception of He/N,, start to decrease at times exceeding 5 min. The results for He/CH4 and Hz/N2 suggest that He/N, may also be close to its maximum selectivity in this time range. Comparison of Figs. 4 and 7 shows that the productivities of the coated membranes are essentially similar at treatment conditions which

Fig. 10. The average selectivities of He/N2 and 02/N2 after fluorination and coating with PDMS, as a function of fluorine feed concentration and treatment time.

yield high selectivities, i.e., at 0.02% F2 for 8 to 10 min and at 0.1% F2 for 2 to 5 min of fluorination. The results illustrate that surface coating with PDMS has a relatively small additional effect on the productivity of the faster gases, but substantially increases the selectivities of fast gases relative to slow gases. A post-treatment step of this nature can therefore be recommended as an adjunct to the fluorination process. 4.2. Fluorination

of glassy polymers

The overall effect of the surface fluorination of PPO composite membranes is conveniently shown in terms of the relation between their selectivity and the permeance (P/Z) before and after treatment. Figs. 11 and 12 show the selectivity of He, H2 and O2 relative to N2 versus the

J.D. L.e Roux et al. /Journal ofMembrane Science 90 (1994) 21-35

_ l* .

’ 80

33

l.

I

.

4 :

PPO composite membranes

. I

4

He Permeance [GPU]

10

1

1

40

0, Permeance [GPU]

Fig. 12. The relation between the O2 permeability and 02/N2 selectivity for all PPO membranes treated under different conditions. The symbols and box have the same meanings as in Fig. 11. The horizontal solid line represents the selectivity of thick PSF.

L

PPO composite membranes . I111111

IO 30

70

100

300

700

H, Petmeance [GPU]

Fig. 11. The relation between permeance and selectivity for all PPO membranes treated under different conditions, in terms of the selectivity before (0 ) and after (0 ) fluorination and after coating with PDMS ( q ). Open boxes show the range of properties reported for PSF membranes before and after fluorination, followed in some cases by coating with PDMS [ 1,2 11. The lower limit of the range for unfluorinated PSF is bounded by the average selectivities for thick films of PSF while the permeance range is determined only from the values of asymmetric PSF membranes.

permeance of the faster gas, for all of the membranes of this study, regardless of the specific treatment conditions. The range in permeance of the untreated membranes can be attributed largely to variations in thickness between the unfluorinated PPO membranes. The broadened range of permeances after fluorination and coating is due to the effect of different treatment conditions which affect the permeance in different ways, as discussed above. For comparative pur-

poses these figures also contain the ranges of the transport properties of fluorinated and unfluorinated thin-skinned asymmetric and thick film PSF membranes reported in a previous study [ 11. The results for PSF membranes show those membranes which were most improved by treatment, including some which were only fluorinated and some which were coated with PDMS after fluorination. From Fig. 11 it is apparent that the He/N, selectivity is slightly increased by fluorination and then further increased by coating with PDMS, with a concomitant loss of permeance. Although the trend is somewhat similar for the HZ/N2 pair, the loss of productivity is much greater and the gain in selectivity much smaller after fluorination; the effect of coating the fluorinated membranes is also more pronounced. In Fig. 13, the same trend is apparent for the Oz/Nz pair, but here the loss in productivity is accompanied by a substantial loss in selectivity. An eventual gain in selectivity is only realized after coating. It appears that the response to fluorination and the selectivity relative to N2 is related to the kinetic diameter of the faster gas, with the smaller He molecule being less affected by the fluorination treatment and, therefore, having a greater permeability and selectivity. As discussed above, the performance of He, Hz and O2 relative to N2

34

J.D. Le Roux et al. /Journal

ofMembrane

after fluorination and before coating may be related to physical defects in the skin caused by the treatment. A comparison in Fig. 12 of PPO membranes with unfluorinated PSF membranes shows that the latter membranes have higher selectivities. The productivities (P/I) are of the same order, although the permeability coefficient of PSF is an order of magnitude lower than that of PPO (cf. Table 1). This is due to the ten-fold difference in apparent thickness between the asymmetric PSF membranes ( -0.05 ,um) and the composite PPO membranes ( -0.5 pm). With higher selectivities and comparable permeance, the asymmetric PSF membranes (after only fluorination and without surface coating treatment) outperform the PPO membranes (after fluorination and coating). For the 02/N2 pair in Fig. 12, the selectivity of the unfluorinated PSF membranes of the previous study [ 1 ] is comparable to that of the fluorinated and coated PPO membranes, generally within a higher range of permeance. The only potential advantage to be gained from using PPO membranes would be if they could be formed with thinner separation layers or skins. On the whole, however, it can be concluded that optimized fluorination followed by surface coating is capable of improving the performance of membranes formed by different methods from these two glassy polymers.

5. Acknowledgments The authors thank James DeYoung and Alberto Ruiz-Trevifio for their technical assistance, as well as Dr. D.W. White of General Electric Company, Schenectady, NY for supplying the high molecular weight PPO. This work was supported by the Texas Advanced Technology Program under Grant No. 1607, by the Separations Research Program at the University of Texas, Austin, TX, and by the CSIR, Pretoria, South Africa. 6. References [ 11J.D. Le Roux, D.R. Paul, J. Kampa and R.J. Lagow, Modification

of asymmetric polysulfone membranes by

Science 90 (1994) 21-35

mild surface fluorination. Part I. Transport properties, J. Membrane Sci., in press. [ 2 ] J.M. Mohr, D.R. Paul, T. Mlsna and R.J. Lagow, Surface fluorination of composite membranes. Part I. Transport properties, J. Membrane Sci., 55 ( 199 1) 13 l148. [ 3 ] J.M. Mohr, D.R. Paul, I. Pinnau and W.J. Koros, Surface fluorination of polysulfone asymmetric membranes and films, J. Membrane Sci., 56 ( 1991) 77-98. [ 41 M. Langsam, M. Anand and E.J. Karwacki, Substituted propyne polymers. I. Chemical surface modification of poly [ 1- (trimethylsilyl )- I-propyne ] for gas separation membranes, Gas Sep. Purif., 2 (1988) 162-170. [ 5 ] M. Langsam, Fluorinated polymeric membranes for gas separation processes, US Pat. 4,657,564, assigned to Air Products and Chemicals Inc., PA, 1987. [ 6 ] C.L. Kiplinger, D.F. Persico, R.J. Lagow and D.R. Paul, Gas transport in partially fluorinated low-density polyethylene, J. Appl. Polym. Sci., 31 (1986) 2617-2626. [ 7 ] M. Langsam, Polytrialkylgermylpropyne polymers and membranes, US Pat. 4,759,776, assigned to Air Products and Chemicals Inc., PA, 1988. [ 8 ] K.V. Peinemann and I. Pinnau, Method for producing an integral asymmetric gas separating membrane and the resultant membrane, US Pat. 4,746,333, assigned to Kemforschungszentrum Karlsruhe GmbH, 1988. [ 91 I. Pinnau, Skin Formation of Integral-asymmetric Gas Separation Membranes made by Dry/wet Phase Inversion, Ph.D. Dissertation, University of Texas at Austin, 1991. [ lO]Y. Ichiraku, S.A. Stem and T. Nakagawa, An investigation of the high gas permeability of poly( l-trimethylsilyl-1-propyne), J. Membrane Sci., 34 (1987) 5-18. [ 11lH.J. Bixler and O.J. Sweeting, Barrier properties of polymer films, in O.J. Sweeting (Ed.), The Science and Technology of Polymer Films, Vol. II, Wiley, New York, 1971, pp. l-130. 12 ] Y. Nagase, A. Naruse and K. Matsui, Chemical modification of polysulphone. 2. Gas and liquid permeability of polysulphone/polydimethylsiloxane graft copolymer membranes, Polymer, 3 1 ( 1990) 12 l- 125. [ 131s. Pauly, Permeability and diffusion data, in J. Brandrup and E.H. Immergut (Eds. ), Polymer Handbook, Wiley, New York, 1989. [ 14]H. Yasuda and K.J. Rosengren, Isobaric measurement of gas permeability of polymers, J. Appl. Polym. Sci., 14 (1970) 2839-2877. [ 15 ] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membrane Sci., 62 (1991) 165-185. [ 161J.D. Le Roux and D.R. Paul, Preparation of composite membranes by a spin coating process, J. Membrane Sci., 74(1992)233-252. [ 17 ] J.D. Le Roux, D.R. Paul, J. Kampa and R.J. Lagow, Surface fluorination of poly( phenylene oxide) composite membranes. Part II. Characterization of the fluorinated layer, J. Membrane Sci., 90 ( 1994) 37-53.

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[21]M.E.Rezac, J.D.LeRoux, H.Chen,D.R.PaulandW.J. Koros, Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes, J. Membrane Sci., 90( 3) ( 1994) in press. [22]J.M.S. Henis and M.K. Tripodi, Composite hollow fiber membranes for gas separation: the resistance model approach, J. Membrane Sci., 8 ( 1981) 233-246. [23]J.M.S. Henis and M.K. Tripodi, Multicomponent membranes for gas separation, US Pat. 4,230,463 ( 1981).