Modification of asymmetric polysulfone membranes by mild surface fluorination Part I. Transport properties

Journal of Membrane Science, 94 (1994) 121-141

121

Elsevier Science B.V., Amsterdam

Modification of asymmetric polysulfone membranes mild surface fluorination Part I. Transport properties

by

J.D. Le ROUX’,D.R. Paul*,*, J. Kampab and R.J. Lagowb eDepartment of Chemical Engineering, Center for Polymer Research, The University of Texas at Austin, Austin, TX 78712 (USA) bDepartment of Chemistry, Center for Polymer Research, The University of Texas at Austin, Austin, TX 78712 (USA)

(Received June 7,1993; accepted in revised form September 14,1993)

Abstract Flat sheet integral asymmetric polysulfone membranes were subjected to gas phase surface fluorination in a well-mixed reactor. Short residence times ensured a constant fluorine concentration in the reactor (i.e., at the membrane surface) after 1 min of operation, so that the fluorination time and the fluorine feed concentration could be investigated independently. Fluorination conditions were optimized to yield an improvement in the selectivity of the gas pairs 0,/N,, Hz/Na, H&HI, He/N,, He/CH, and CO,/ CHI with a varying decrease in the permeability of all gases. For example, at 0.02% Fz the 0,/N, selectivity was improved by up to 50% at fluorination times of 2 min. However, 5 min of fluorination produced the optimum (more than ten-fold) increase in the He/CH, selectivity with an average decrease in He permeance (P/I) of only 27%. Low fluorine feed concentrations of 0.02-0.04% gave optimum results and higher concentrations resulted in a sharp decline in the selectivity of all gas pairs. Good transport properties were also obtained by subjecting membranes to successive fluorination cycles of short duration at 0.02% Fz. Two cycles of 1 min each produced optimum results. Membranes coated with highly permeable silicone rubber (PDMS ) after fluorination invariably showed an increase in selectivity with a relatively small additional loss of permeability. Key words: gas separations;

membranes;

membrane

preparation

Introduction Polymeric gas separation membranes should ideally comprise a highly permeable porous substructure supporting a thin homogeneous and defect-free separating layer or skin. The maximum selectivity of the membrane is determined by the skin material, while the produc% whom correspondence should be addressed.

0376-7388/94/$07.00

and structure;

surface fluorination;

asymmetric

polysulfone

tivity or gas flux through the membrane depends both on the intrinsic permeability of the skin material and its thickness. The gas separation properties of such membranes can be improved by controlled fluorination of the membrane surface [l-7]. As will be discussed in Part II of this study, such treatment creates a layer of partially fluorinated, polar polymer at the skin surface (shown schematically in Fig. 1) which typically decreases the flux but in-

0 1994 Elsevier Science B.V. All rights reserved.

SSDI 0376-7388(93)E0153-B

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J.D. Le Roux et al. 1 J. Membrane Sci. 94 (1994) 121-141

creases the permselectivity for a number of gas pairs. Surface fluorination has been described for the modification of homogeneous solid films of polyethylene [ 11, substituted polypropynes, poly [ l- (trimethylsilyl)propyne] such as (PTMSP) [ 3,4,8], poly (4-methyl-1-pentene) (PMP) [ 6,9-111, ethyl cellulose, polystyrene, aromatic polyethers and certain copolymers [ 21. Surface fluorination has also been applied to flat composite membranes with a PMP selective layer [6], to composite hollow fiber membranes coated with PTMSP [ 41 and to flat integrally-skinned asymmetric polysulfone (PSF) membranes [ 71. All of these studies employed gas phase fluorination at ambient temperature and near atmospheric pressure, where the surface of the membrane was exposed to a relatively dilute mixture of elemental fluorine in an inert carrier gas, helium [ 1,2,6] or nitrogen [ 31. Fluorine contact times ranged from 2 min to 90 h. Mohr et al. [ 6,7] employed a well-mixed cylindrically-shaped reactor. For the surface fluorination of PTMSP, Langsam et al. [ 3,4] used a glass plate containing a solution-coated polymer film which was placed in a turbulent flow, narrow channel plate reactor of unspecified dimensions. Other reported studies do not provide any details of the fluorination reactor or its operation which is unfortunate since the reactor configuration and operation determine the concentration of fluorine at the membrane surface as a function of time. This makes it possible to distinguish feed conditions from the acfluorinated layer +

selective skin layer

Fig. 1. Schematic of an integrally-skinned asymmetric membrane, showing an idealized fluorinated layer formed by surface treatment with molecular fluorine.

tual time-dependent conditions at the membrane surface that directly influence the nature of the fluorinated layer. Based on theoretical considerations there is an optimum thickness of the fluorinated layer and it is an objective of this work to understand and control the fluorination process to exploit this fact. Langsam [3] showed that fluorination of PTMSP films and membranes raised the O,/ N, selectivity by a factor of 3.3 and considerably improved the selectivity for He and Hz over CH,. These gains were accompanied, as usual, by substantial losses in productivity. They employed fluorination times of up to 60 min using a gaseous mixture of 0.1 or 0.5% F, in a N, carrier gas. Mohr et al. [ 71 fluorinated asymmetric PSF membranes using a 2,200 cm3 reactor operated in an unsteady-state mode where the fluorine concentration inside the reactor differed from that of the feed. The feed gas consisted of 2% F2 in a He carrier gas and fluorination times of 0, and 5 min were investigated by comparison of the permeance (or pressure normalized flux) and the permselectivity of each membrane before and after fluorination. They found that fluorination improved the selectivity for the H,/CH, gas pair 2 to 5 fold, while the 02/Nz selectivity generally decreased. The permeances of H, and 0, decreased by factors ranging from 1.2 to 2.9. They estimated the depth of the fluorinated layer to be 250 to 500 A for fluorination times of 2 and 5 min in their reactor. Although no improvement in 0,/N, selectivity was observed, this study demonstrated the potential benefits of fluorinating asymmetric PSF membranes. It seems that their treatment was too harsh to realize the maximum benefits of surface fluorination, and the current re-examination emphasizes milder fluorination conditions, i.e., a lower fluorine concentration at the membrane surface. In this research, flat sheet asymmetric polysulfone membranes were surface fluorinated in

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

a small reactor ( 14.6 ml). Because of this small volume, steady-state conditions in the gas phase of the reactor could be achieved within 1 min, after which the concentration of fluorine inside the reactor approached that of the feed gas. This made it possible to better distinguish between the effects of varying fluorination time and fluorine concentration. The primary objective was to determine fluorination conditions for asymmetric PSF membranes that optimize their permeation and gas separation properties. A special attempt was made to improve the selectivity of the membrane for the 02/Nz gas pair. Another objective was to distinguish the effects of two different fluorination strategies: the systematic variation of either fluorination time or fluorine feed concentration, with all other process variables kept constant. A third strategy was to perform successive fluorination cycles at mild conditions. Finally, the effect of small skin defects in the unfluorinated and fluorinated membranes was corrected by a post-treatment where a rubbery polymer was coated onto the membrane surface. Background The fluorination process Direct reaction of materials with fluorine may be carried out in the liquid or gaseous phase. The reaction between elemental fluorine and organic substrates is highly exothermic, to the extent that early attempts at direct fluorination often led to explosions or degradation of the substrate [ 12-141. Diatomic fluorine replaces the hydrogen atom of a carbon-hydrogen bond and liberates hydrogen fluoride in an overall reaction which releases 103.4 kcal/mol of energy. The average carbon-carbon bond strength is only 36 kcal/mol and organic molecules are thus easily fractured, unless the release of energy can be controlled or dispersed [ 131. This can be achieved by strategies which

123

reduce the reaction rate or rapidly remove the excess energy during fluorination, so that the intermediate fluorination reactions may, in principle, occur sequentially rather than simultaneously. These strategies include operating at low temperatures and at low fluorine concentrations, at least during the initial stages of fluorination. As fluorine groups are added to the carbon skeleton, adjacent fluorination sites become sterically shielded against subsequent attack and reaction conditions can then be stepped up as required. An increasing fluorine concentration has the disadvantage, however, that the fluorination environment and the extent of reaction become difficult to control. The objective of the fluorination step is to create a fluorinated polymer with good gas separation properties, to control the depth of fluorination and to minimize damage to the membrane structure and skin. This can in principle be achieved by operating at short times and low fluorine concentrations in the reactor. Defects formed under such conditions may be corrected by the application of a surface coating. Improved fluorination techniques and the use of moderating agents have made selective fluorination a practical possibility by which specific fluorinated short-chain organics can be created [ 14-161. Controlled fluorination has also been used to change the structure of polymers. By regulating fluorination conditions, partially or perfluorinated structures can be created at the surfaces of solid polymers or throughout fine powders, for instance to improve refractive index, barrier properties or surface wetting [ 13,16,17]. Fluorine treatment of gas separation membranes has been shown to yield improved selectivity but decreased productivity [l-7]. However, the gas phase surface fluorination of polymers is relatively nonselective. Studies of the nature of fluorinated PMP have shown that the amount of fluorine incor-

J.D. LQ Roux et al. /J. Membrane Sci. 94 (1994) 121-141

124

porated into the polymer varies approximately linearly with time for unsteady-state reactor operation [ 61 and that various fluorinated polymer structures are formed in the surface region [4,10,11]. The high reactivity of fluorine and its relatively slower rate of diffusion suggest that a relatively distinct fluorinated front exists below the polymer surface at short fluorination times. As fluorination time increases, this front not only moves deeper into the polymer, but the nature of the fluorinated species throughout the layer may change, i.e., more fluorine atoms are added [ 111. In particular, new fluorinated structures may be formed nearer the surface where the polymer was exposed to fluorine longer. Other reactions are also possible, e.g., when oxygen is present in the feed it may be incorporated into the polymer [7,10,16]. The concept of a uniform fluorinated layer, while useful for understanding the gas transport issues involved, may only be an approximation of reality. The validity of this approximation will be considered in a subsequent paper dealing with the analysis of the fluorinated region.

where PA and PB are the intrinsic permeability coefficients of layers A and B, and ZAand Znare their respective thicknesses. The ideal separation factor for gas i relative to gas j is tplz)i

(2)

aG=oj

This model is illustrated in Fig. 2 for the gas pair H&H,. The points and solid line indicate the familiar trade-off between selectivity and permeability for a number of typical polymers [ 201. The broken line represents the performance of a composite structure consisting of a layer of polymer A (e.g., fluorinated polymer) and a layer of polymer B (e.g., unfluorinated polymer) when the thickness of A relative to the total thickness (A+B) is varied. The optimum fluorination depth is achieved when the fluorinated region comprises between 1% and 10% of the skin thickness. This argument is developed in detail by Mohr et al. [6,7] and can 150

150

loo

Fluorinated composite structure If it is assumed that fluorination proceeds from the surface as a sharp front, then the skin of the membrane can be represented as a composite structure comprising a sublayer of unfluorinated polymer overlaid by a fluorinated layer. If the layers have different permeation properties, the selectivity, cy, and the permeante or pressure normalized flux (P/Z) of such a structure can be described by a series resistance model [ 6,18,19]. The permeance (P/Z) of a single gas i through composite layers A and B is given by

(1)

50

0

I 1

0 10

100

500

H, Permeability [Barrers] Fig. 2. Illustration of experimentally observed trade-off between selectivity and permeability for the H&HI gas pair. The solid line describes the experimental data (solid points) for several polymer types of some interest for membrane formation. The broken line shows the calculated advantage of a composite membrane structure and the percentages indicate the relative contribution of polymer A to the total thickness of the skin.

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

be used to approximate the behavior of a fluorinated membrane. Whatever the thickness of the fluorinated layer, the productivity of the fluorinated membrane is greatest when the overall thickness of the skin region is minimized. Of course, the skin must be free of defects in order to provide the intrinsic selectivity of the polymer. On the whole the model provides a useful conceptual framework for analyzing the fluorination process, even if the assumptions of a uniform fluorinated layer and negligible substructure resistance are not entirely accurate. In general, both the depth and the composition of the fluorinated layer determine the transport properties, i.e., the permselectivity and permeability, of the fluorinated membrane. On the one hand, therefore, it is important to determine whether the depth can be controlled by increasing the fluorination time at a fixed fluorine concentration or by varying the F2 concentration for a fixed contact time. On the other hand, each fluorination strategy affects the nature of the fluorinated species formed at different depths. The effects of the various fluorination strategies are the subject of this study. Experimental Asymmetric membranes

Asymmetric polysulfone membranes were solution cast using a dry/wet phase inversion process [21,22]. The casting solution contained 12.2 wt% polysulfone, 20.4 wt% 1,1,2trichloroethane and 53.0 wt% dichloromethane as solvents, and 14.4 wt% 2-methyl-2-butanol as non-solvent. The polysulfone was Udel P1700 (Amoco Corp.) with a weight average molecular weight of 40,000 g/mol and a number-average molecular weight of 20,400 g/mol as determined by membrane osmometry [ 22 1. Each membrane sheet was formed by first

125

casting the solution onto a glass plate at 25°C with a knife gap of 250 pm. A stream of air saturated with moisture was then blown over the surface for several seconds. As a result of this forced convection, the surface of the membrane became turbid and opaque, signaling the onset of phase separation. After a free-convection period in still air, the nascent membrane was quenched in a methanol bath and remained there for at least 1 h to complete the solvent exchange process. The washed membrane was air-dried for at least 16 h and finally dried in a vacuum oven at 80” C for several hours to remove all traces of solvent. At this point membrane samples were screened by measuring their 0, and N, fluxes and selectivity at 24 oC. The 0, permeance gave an indication of the skin thickness and the O,/ N2 selectivity was used to gauge the integrity of the skin. A membrane was considered defectfree if it had an 0,/N, selectivity equal to or greater than 6.4, i.e., that of thick dense polysulfone films prepared by solution casting, as shown in Table 1. It was found that minor defects could be repaired by exposing the membrane surface briefly to a weak solvent; cyclohexane, followed by vacuum drying at 80°C. The solvent treatment usually raised the selectivity of the final membrane to well above the intrinsic value for thick films of PSF (Table 1), so that the unfluorinated membranes of this study had an average 02/Nz selectivity of 7.2 with a standard deviation of 0.39. The subject of solvent treatment is considered in greater detail elsewhere [ 23 1. The final membranes exhibited an asymmetric structure comprising a substantially defectfree skin from 0.05 to 0.1 pm thick and supported by a graded porous substructure. To minimize variability, membranes of similar skin thicknesses and 0,/N, selectivities were selected for fluorination. Membrane discs 4.8 cm in diameter were masked with adhesive alumi-

126

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

TABLE 1 Transport properties for dense films of polysulfone and poly (dimethylsiloxane Polymer

T” (“C)

PDMS

25

PSF

24 25 25 35 25

) Ref.

Permeability (Barrers ) b CK

N2

02

281

604

0.165 0.205 0.250 0.25 0.179

1.05 1.18 1.3 1.4 1.15

0.39 0.265 0.158

H2

He

CCZ

649

354

3230

19.3 10.8 _ 11.54

13.0 10.64

7.06 5.70 5.60 6.07

24 22 25 27 26’ c.d

‘Temperature at which properties were measured. bl Barrer= 10-lo cm3 (STP) cm/cm2 s cmHg. ‘N2, 0, and CH, measured at 6 atm and He, H, and COz at 2 atm. dPresent work.

num foil to expose a permeation area of 5.1 cm’. These masked membranes were used for gas permeation tests and then subjected to fluorination treatment as described below. Fluorination After measuring their gas transport properties, the membrane discs were mounted onto a solid aluminum backing using adhesive aluminum foil so that only the skin surface was exposed. The masked membranes were placed on the support grid of the stainless steel fluorination reactor, shown in Fig. 3, with the exposed surface facing upward. The fluorination gas passed through a baffled inlet port into a reaction chamber with a volume of 14.6 cm3. The functions of the baffle were to promote rapid mixing of the fluorination gas and to prevent direct impingement of the gas onto a central portion of the membrane surface. The reactor was provided with a bypass, so that it could be isolated from the feed gas stream. Prior to fluorination the reaction chamber was purged with pure helium to remove all traces of air, moisture and other impurities. The gas feed line was then filled with the fluorine-containing feed

gas premixed to the required concentration by diluting commercially available 2% Fz in He with pure He. A gas flow rate of 50 cm3/min was maintained throughout. The supplier indicated that the fluorine source used to generate the 2% mixture is of a guaranteed purity of 97% and that the impurities are mainly oxygen and nitrogen in a 50: 50 ratio. This oxygen has been shown to participate in the fluorination reaction [ 11. Fluorination was carried out by shunting the reactor into the gas feed line for the required time. The exhaust gas from the reactor was passed through packed alumina columns to deactivate residual fluorine and then bubbled through a liquid seal containing 1.5 inches of mineral oil. Consequently, the actual operating pressure in the reactor was about 1.6 mmHg (0.03 psi) above atmospheric pressure. Upon removal of the membranes from the reactor, their exposed (un-masked) areas were observed to bulge upward, particularly after extended periods of fluorination. This may result from exposure of the masked membrane to helium during the purge and fluorination stages. Helium is believed to permeate through the membrane into the space between the mem-

J.D. L.e Roux et al. /J. Membrane Sci. 94 (1994) 121-141

127

Fluorine/helium feed gas

/-

Reactor volume of 14.6 ml Membrane masked to expose surface only Screw cap closure Viton’” O-ring Stainless steel body

Membrane k

support

Baffle

Exhaust gas

Fig. 3. Schematic representation of the fluorination reactor used in this study.

brane and the aluminum backing. A pressure greater than that in the reactor could then build up in this space, since air at atmospheric pressure trapped there before fluorination cannot permeate out as fast as helium permeates in. This bulging may result in defects at the perimeter of the exposed membrane surface which is also the inner edge of the mask. The relevance of these observations will be discussed in the section dealing with the treatment of skin defects. An unsteady-state mass balance on a wellmixed continuous flow reactor can be integrated to yield the following relation between feed concentration, cf, and the concentration in the reactor c at any time t: c=cf[l-exp

(-;t)]

(31

where V is the volume of the reaction chamber and Q is the feed gas flow rate. Thus, at a constant feed rate the time-dependent concentration profile in the reactor is determined by the reactor volume. In Fig. 4 the profile for the 14.6 cm3 reactor of the current study is compared to that of the 2,200 cm3 reactor used by Mohr et al. [ 71. It is evident that for the larger reactor

the fluorine concentration increases with time during the period normally employed and approaches the feed concentration, cf, after several multiples of the average residence time ( V/ Q= 44 min). For the small reactor, the fluorine concentration approaches steady-state within 1 min, after which the fluorination time and fluorine concentration in contact with the membrane surface can be varied independently. The increased fluorine concentration in the reactor at small times necessitated the use of extremely low feed concentrations to prevent overfluorination of the membrane. Gas permeation measurements The permeability coefficients of the six gases Ng, 02, CHI, Hz, He and CO, were first measured independently for a thick PSF film at 25°C. Table 1 contains these and other values from the literature; one set of values at 35°C has been included for comparison. It is apparent that the permeabilities from different sources differ considerably and we used our measured values to calculate selectivities for reference purposes. The Nz and O2 permeabilities and Oz/Nz selectivity of 6.4 compared well

J.D. Le Roux et al. 1 J. Membrane Sci. 94 (1994) 121-141

128

Volume

= 14.7 c m3

Volume

= 14.7 c m3

_

,:’

Volume

”

0

20

40

60

Fluorination

80 time

100

120

140

[min]

160

0

1

2 3 4 5 Fluorination time

= 2,200 c m3

6 [min]

7

8

9

Fig. 4. Comparison of the fluorine concentration versus time in a reactor with a volume of 2,200 cm3 [6,7] and one with a volume of 14.6 cm3 (used in the current study) for a feed flow rate of 50 cm3/min. The small reactor reaches constant (feed) fluorine concentration after - 1 min, whereas the large reactor requires - 160 min.

with those reported by Pinnau [ 221. Permeation rates for these six gases were then measured independently before and after fluorination for each membrane. These measurements yielded values for P/1 and from these the ideal selectivities were calculated for several gas pairs. The thickness of the asymmetric membrane skin was estimated from the measured P/Z and the permeability coefficient of O2 at 24°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 and also for any other resistance to the permeation of 0, in the porous substructure. Single-step modification Effect of fluorination time

As discussed above, the small reactor volume used in this study made it possible to investigate the effect of fluorination time and fluorine concentration independently. An attempt was made to minimize the effect of variable skin thickness and morphology of the asymmetric membranes by using membranes from the same

batch within a narrow range of apparent skin thickness. The permeance of the six gases was measured for each membrane before and after fluorination treatment. Preliminary experiments indicated that extremely low fluorine concentrations and short fluorination times were required to minimize damage to the polymer by overfluorination. Figures 5 to 7 show the effect of varying fluorination time at a constant fluorine concentration in the reactor of 0.02% (by volume) in He. Solid lines show the average properties of the unfluorinated membranes and indicate trends in the data after fluorination. The error bars in Fig. 5 represent the standard deviation from the average values obtained from at least three membranes tested for each set of fluorination conditions; in other figures the error bars were omitted in favor of showing the variability in the data. The issue of variability is discussed below. In general terms, the effect of increasing fluorination time was to decrease the permeante (P/Z) and to increase the selectivity relative to the original unfluorinated membranes. As shown in Fig. 5, there is a steady decline in the permeance of all gases up to a fluorination time of 5 min, after which the permeance effec-

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

129

I’ ” ” ’ n” ” ” ” ” ” “I

I

0

Fluorination time [min]

1

2

3

4

I

I.

5

I

6

I

I

I

7

I.

8

I

9

I

I,

10

Fluorination time [min]

Fig. 5. The effect of treatment at different fluorination times for a constant feed concentration of 0.02% F, on the permeance (P/l). Open symbols represent untreated membranes and solid symbols the results after fluorination. The error bars indicate standard deviation for each point. The solid lines are intended tc emphasize trends in the data. Permeance is given in GPU or gas permeation units [ 1 GPU = 10e6 cm3 (STP) /cm* s cmHg]

tively levels off. With the exception of He, P/Z is significantly reduced at fluorination times exceeding 2 min. For example, fluorination for 1 min results in an average reduction of the permeance for N2 and CH, by factors of 1.8 and 1.5,while 5 min of fluorination reduced the permeance to 10 and 6% of the unfluorinated _

values for the same gases. By contrast, the permeance of He was only reduced to 73% of its unfluorinated value after 5 min of fluorination. As seen most clearly for He, P/l after fluorination depends not only on the fluorination conditions, but also to some extent on the P/Z before treatment. This illustrates the stringent

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

130

could be realized at fluorination times of 5 min while retaining He permeance at 73% of the level prior to fluorination. As seen in the lower half of Fig. 6, fluorination increases the N,/CH, selectivity with optimum results at N 5 min, although there is considerable variability from membrane to membrane. The CO,/CH, selectivity is improved only by mild fluorination at short times. Effect of fluorine concentration

0

I

2

3

4

5

6

7

8

9

1011

Fluorination time [mini

Fig. 6. The effect of treatment at different fluorination times and a constant feed concentration of 0.02% F2 on the selectivity of the 0,/N, and N&H, gas pairs. The symbols and lines have the same meanings as in Fig. 5.

experimental requirement that as far as possible fairly similar membranes should be selected for comparison. Figures 6 and 7 show the accompanying changes in selectivity. Clearly the ideal selectivity for all gas pairs, including 0,/N,, can be increased by mild fluorination at short times. The selectivities for He or H, relative to N2 or CH, increase markedly up to a fluorination time of 5 min and then effectively level off at longer times. By contrast, 0,/N, selectivity has a maximum at 2 min of fluorination and then decreases at longer fluorination times. For example, for one membrane this meant an increase in selectivity from 6.2 before fluorination to 9.5 after fluorination. However, this improvement was accompanied by a severe reduction in permeability, i.e., the O2permeance was reduced to 10% of its original value. On the other hand, He/CH4 selectivities above 1000

A set of membranes were exposed to feed streams with different concentrations of fluorine ranging from 0.02 to 0.15% (by volume) for a constant time of 2 min. The unfluorinated membranes had defect-free skins ranging in thickness between 600 and 840 A (based on 0, permeance) and average selectivities of 7.48 (std. dev. =0.2) for 0,/N, and 150 (std. dev. = 14) for He/CH,. Figures 8 and 9 show that fluorination at low fluorine concentrations invariably results in enhanced selectivity while at higher concentrations there is generally a reduction or no improvement. For 0,/N,, the greatest improvement occurs at about 0.02 to 0.04% F, after which the selectivity falls below that of the unfluorinated membranes. At these low concentrations, 0,/N, selectivities of close to 10 were obtained for some membranes, although not all membranes were improved. The selectivities for He, H, or CO2 relative to N, or CHI are also highest at the lowest fluorine concentrations of 0.02 to 0.04%; higher concentrations up to 0.1% F2 raise only the He/N, and He/CH, selectivities above those of the unfluorinated membranes. Figure 10 shows the permeance (P/Z) after fluorination of six gases as a function of fluorine feed concentration; the values of the untreated membranes are also shown for comparison. For all gases, increased fluorine

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Sci. 94 (1994) 121-141

0

10

0

l

0

I,,,,,,,,,,,,,,,,,,, 0

1

2

3

4

5

6

7

8

9

Fluorination time [min]

10

-A

0

1

2

3

4

5

6

7

8

9

10

Fluorination time (min]

Fig. 7. The selectivity before and after fluorination for six other gas pairs at the same conditions as for Figs. 5 and 6. The symbols and lines have the same meanings as in Fig. 6.

concentration results in decreased permeance up to a concentration of N 0.1%. At the highest fluorine concentration of 0.15% the permeance appears to increase markedly for 02, Nz and CH,. At low concentrations (0.02 to 0.04% ) the more rapid decrease of P/l for the slow gases (CH, and N,) relative to that of the fast gases results in improved selectivities, as shown in Figs. 8 and 9. At intermediate concentrations (0.06-0.1% F2) the permeabilities of both slow

and fast gases are reduced but without a concomitant increase in selectivity. A high fluorine concentration of 0.15% results in membranes with selectivities close to that expected for Knudsen diffusion and large increases in fluxes for all but the fast gases. Evidently, this is the result of polymer degradation caused by the highly exothermic fluorine/hydrogen substitution reaction leading to defects in the membrane.

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

132

.B .*

&

3

6 4

2.0 1

=I$ N,/CH,

:

0.5 1

1 0

t*

0

’ n ”

0.02

0.04

”

0.06

’ * ’ m”

0.08

0.10

0.12

’ 0.14

nA 0.16

Fluorineconcenustion [vol%] Fig. 8. Effect of feed concentration for a fixed fluorination time of 2 min on the selectivity for the 0,/N, and N&H, gas pairs. The symbols and lines have the same meanings as in Fig. 6.

It appears that mild fluorination conditions yield the best results for all gas pairs considered. This confirms our premise that the results of prior studies on fluorination, particularly of PSF [ 71, could be improved by working at mild conditions to avoid overfluorination. As found by Mohr et al. [ 71 and as reported in the previous section, considerable scatter of the data is evident. Discussion It is apparent from the above that fluorination of asymmetric membranes can lead to quite variable results. This variability may result from several factors inherent in both the membrane formation and surface fluorination pro-

cesses. The majority of the membranes tested in this study had an apparent skin thickness of the order of 700 A. The inherent variability in actual skin thickness and subsurface morphology for such thin skins may be amplified by the fluorination treatment. For instance, an essentially defect-free skin may be so thin in places that even mild fluorination causes damage leading to minor skin lesions. The depth of fluorination is of the same order of magnitude as the homogeneous skin region (several hundred Angstroms ) [ 71 and variations in skin thickness will result in large local variations in the fractional depth of fluorination. The 0,/N, gas pair is notoriously difficult to separate due to the similarity in size and electronic structures of the molecules; only a narrow range of treatment conditions were found to modify the polymer sufficiently to cause a large difference in the permeability of these two gases. On the other hand, the selectivity of He and H2 over CH, and Nz is dictated mainly by the changes in the permeability of the slower gas, since the fast gases are much less affected by fluorination. Likewise, since fluorination creates skin defects through which Knudsentype diffusion probably takes place, the permeability of the slow gases are increased by a larger factor. This is observed at the highest fluorine concentration of 0.15%. Mild fluorination, at a low effective fluorine concentration of 0.02%, seems to minimize damage to the skin. At this concentration both the selectivities for He or Hz relative to N2 or CH, and their permeances level off at fluorination times of 5 min and longer. Considering the approximate fluorination rates determined by Mohr et al. [ 71 for PSF, it appears possible that 5 min of fluorination is sufficient for the fluorination front to pass through the entire skin which is only N 700 A thick. Longer fluorination times would not increase the skin thickness, but may gradually increase the extent of fluorination. Another possibility is that

Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

8:

,_0

0.02

0.04 0.06 0.08 0.10 0.12 Fluorine concentration [vol%]

0.14

0.16 0

0.02

0.04 0.06 0.08 0.10 0.12 Fluorine concentration [vol%]

0.14

0.16

Fig. 9. Effect of feed concentration for a fixed fluorination time of 2 min on the selectivity for the six gas pairs shown in Fig. 7. The symbols and lines have the same meanings as in Fig. 6.

the thickness of the fluorinated region continues to increase as a function of fluorination time, but that the resulting decrease in permeante is balanced by damage which tends to increase permeance. Such damage may be due either to the formation of defects or to erosion at the surface where the membrane has had the longest exposure to fluorine. The former effect establishes a parallel diffusion pathway

(Knudsen diffusion) which should decrease the selectivity; a result which is not observed in Figs. 6 and 7 but is seen at high fluorine concentrations in Figs. 8 and 9. If the effect of surface erosion matches the increase in the thickness of the fluorinated region, then both selectivity and permeance would effectively remain constant. This result is observed in Figs. 6 and 7 at fluorination times exceeding 5 min.

J.D. k Roux et al. /J. Membrane

134

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Fig. 10. The effect of feed concentration at a fixed fluorination time of 2 min on the permeance fluorination for the gases in Figs. 8 and 9. The symbols have the same meanings as in Fig. 6.

Evidently, for short fluorination times at fluorine concentrations exceeding 0.06%, the effects of surface damage outweigh those due to the creation of undamaged fluorinated skin and selectivity decreases markedly. The above discussion considers only changes in the physical structure of the skin without regard to the changing chemical nature of the polymer in the fluorinated region. The solubility and permeability of different gases may re-

0.10

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0.14

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Fluorine concentration [vol%] (P/l)

before and after

spond differently to the modified polymer environment when progressively more fluorine atoms are added to different parts of the PSF repeat group. In homogeneous fluorine-containing polymers, permeability and selectivity are not monotonically related to the number of fluorine atoms in the polymer repeat group. For example, the permeabilities of poly (vinyl fluoride ) and poly (vinylidene fluoride ) are appreciably lower and their 02/N2 and He/N2 se-

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

lectivities higher than those of both polyethylene and poly (tetrafluoroethylene) [ 1,27-291. In this example the fluorine content of the unfluorinated, partially fluorinated and perfluorinated polyethylene seems to affect the chain packing and conformation, and ultimately the transport properties. In the present study it appears that the mild fluorination conditions do not produce highly fluorinated, certainly not perfluorinated, PSF species. It is therefore considered unlikely that the increase in permeability and decrease in selectivity at higher fluorine concentrations are due to the creation of perfluorinated species. These complex issues will be considered in Part II which reports an investigation of the chemical nature of the fluorinated skin region. Based on Fig. 2, a fluorinated composite structure might be expected to have better transport properties than other polymers which lie on the usual trade-off curve. Figure 11 shows the Hz/N2 selectivity versus H2 permeability for those membranes fluorinated at mild conditions which did not cause marked damage. These include treatment with fluorine at feed concentrations of 0.02 and 0.04% for various times. The absolute permeability was obtained by multiplying the permeance (P/Z) of the fluorinated membrane by the effective skin thickness of the unfluorinated membrane calculated from the 0, flux. For reference purposes, a generalized trade-off curve (like the corresponding line from Fig. 2 for the H,/CH, pair) is plotted as the lower solid line. The other solid line in Fig. 11 is the so-called ‘upper bound defined by Robeson [ 201 above which virtually no permeability data currentZyexists. The family of curves (broken lines) were predictions using eqns. (1) and (2 ) for a composite layer comprising unfluorinated PSF and a fluorinated layer at various lower permeabilities assumed, as a first approximation, to lie on the trade-off line (see the assumed values listed in Table 2). While these calculated lines encom-

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H, Permeability[Bsrrers] Fig. 11. The relation between the H, permeability and H,/ N2 selectivity for all of the membranes fluorinated at 0.02% (0 ) and 0.04% ( •TJ) fluorine for various times. The top solid line indicates the ‘upper bound’ defined by Robeson [ 201 and the bottom solid line represents the conceptual ‘trade-off line for a number of polymers commonly studied for gas separation. Each broken line shows the properties of a hypothetical composite structure as shown in Fig. 2. Different curves (listed as (a) to (d) in Table 2) result when the fluorinated polymer of the composite structure is located at different positions on the trade-off line. Further details are provided in the text. TABLE 2 Transport parameters used to compute the predicted curves in Fig. 11 Permeability (Barrers)

H2

Selectivity Hz/N*

N2

10.8 0.115 Unfluorinated PSF” Assumed value for fluorinated layer: 2.0 1.33 10-Z Curve (a) 0.4 1.68 10-3 Curve (b) 0.025 4.75 10-5 Curve (c) 0.008 1.10 10-5 Curve (d)

94 150 238 527 729

“Calculated from the average properties of the unfluorinated membranes.

pass the data, similar predictions could be made assuming the fluorinated layer has properties above the arbitrary trade-off curve but below Robeson’s upper bound. There is no way to de-

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J.D. LQ Roux et al. /J. Membrane 5%. 94 (1994) 121-141

duce from these data and the model what the actual characteristics of the fluorinated layer are. The fact that the data do not lie on a welldefined composite curve is due to the experimental scatter observed most clearly in Figs. 5 to 7. This scatter can be attributed to such factors as the presence of potential localized imperfections in the skin which are exposed dursubstructure ing fluorination, variable resistance, variations in the overall skin thickness resulting in an inconsistent ratio of fluorinated to unfluorinated skin regions and the presence of different fluorinated species with slightly different transport properties. By comparing Figs. 5 to 7 with Figs. 8 and 9, it becomes clear that fluorination conditions have to be carefully optimized to achieve the most beneficial improvements for asymmetric PSF membranes. The fluorine concentrations must be kept at substantially lower levels than those used earlier [7] in order to achieve any improvement in OS/N2 selectivity or maximum benefits for gas pairs like He/CH, and H2/CH4. Even at these low fluorine concentrations, reaction times must be held below certain limits to avoid losing the gains in selectivity that are clearly possible. Multistep

modification

Surface coating after fluorination

Asymmetric gas separation membranes often have small defects or lesions in their skins which reduce selectivity. Such membranes can be coated with a highly permeable polymer to caulk the defects and restore much of the intrinsic selectivity of the original polymer with only a modest loss of productivity. A surface coating of poly (dimethylsiloxane ) (PDMS) has been successfully applied to improve slightly defective asymmetric PSF membranes [ 19,221. Fluorination appears to create or to

exacerbate existing defects in the membrane skin which can lead to loss of selectivity. Other processes ancillary to fluorination may also cause damage. As mentioned earlier, the He carrier gas caused the exposed area of the membrane to bulge upwards during fluorination. In this state the membrane is stretched and vulnerable to damage, particularly around the perimeter where it is in contact with the aluminum foil used to mask the membrane. It is possible that such stretching could cause minor damage to the skin in some cases and add to the observed variance in transport properties. These considerations prompted us to investigate the effect of applying a PDMS coating to a number of membranes fluorinated under similar conditions. Starting membranes with a wide range of transport properties were chosen to determine the limitations of the coating treatment. The unfluorinated membranes had O,/ N, selectivities between 6.1 and 7.9 with apparent skin thicknesses ranging from 450 to 1050 A. Each membrane was fluorinated with 0.02% F, in He for 4 min, then sponge-coated with a 2% solution of PDMS in n-heptane and dried overnight in a vacuum oven at 80’ C. Permeation measurements were performed before and after fluorination, and after application of the PDMS coating. Figure 12 shows the effect of fluorination and surface coating on the permeance and selectivity of these membranes. At these relatively harsh conditions, fluorination caused a consistent decrease in the permeance of every membrane, yet rarely increased the selectivity for OJN,. The subsequent PDMS coating led to a considerable increase in selectivity with only a small additional loss of permeance. The three membranes with the highest unfluorinated selectivities (6.7-7.9 for OJN, and 90-150 for HJCH,) also yielded the highest selectivities after fluorination and coating (9.1-9.9 for O,/ Nz and 370-580 for H.&H,). These results substantiate the notion that

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

137

substantial practical advantages for the fluorination process. Successive fhrinations

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10

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yield the best results for 0,/N, separation. However, when the fluorination time is decreased to 1 min at 0.02% only modest im-

provements in selectivity are obtained. Conceptually, the intensity of the fluorination treatment can be increased in three ways, as shown schematically in Fig. 13. The options of increasing the fluorination time or fluorine concentration have already been discussed. A third strategy is to fluorinate the same membrane successively under very mild conditions. After each exposure to fluorine, the reactor is flushed with the inert carrier gas (helium), so that for each exposure to fluorine the reactor required 1 min to reach steady-state condi-

Fig. 12. The change in the OJN, and H,/CHI transport properties of unfluorinated (0 ) membranes which were fluorinated with 0.02% F2 for 4 min ( 0 ) and then surface coated with a layer of PDMS in n-heptane ( q ).

fluorination, even under the relatively mild conditions employed here, tends to cause minor damage to the membrane skin due to overfluorination. Whatever the origin of these defects, it appears that most can effectively be corrected by the PDMS coating. The unfluorinated membranes with lower initial selectivities may already contain tiny defects which further deteriorate under the influence of fluorination and cannot be completely caulked by the coating. The defects occupy such a small fraction of the membrane area that their presence is detected only by the decrease in selectivity to a value below that of the unfluorinated membrane. In general, it may be concluded that the relatively simple coating procedure has

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Fluorination time (min) Fig. 13. The time-dependent fluorine concentration in the reactor for three fluorination strategies: (a) variation of fluorination residence time at constant F, feed concentration; (b) variation of F, feed concentration at a constant fluorination time; and (c) multiple fluorination cycles of 1 min duration at a constant F, feed concentration.

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

138

tions. Effectively, each cycle of exposure and subsequent flushing of fluorine can be seen as a ‘pulse’ of fluorination treatment. As illustrated, the maximum (steady-state) fluorine concentration is only attained periodically and for short times before the end of each cycle. This pulsed treatment also differs from continuous (single cycle) fluorination in that the hydrogen fluoride formed during the reaction may be purged from the membrane after each cycle. HF itself may react aggressively with the polymer and prompt removal after a fluorination cycle may minimize such damage to the polymer. The effect of successive 1 min fluorination cycles at 0.02% Fz is shown in Fig. 14 where the membranes are characterized by their 0,/N, selectivity and 0, permeance (P/I). As expected, each fluorination cycle or pulse causes a decrease in the permeance. This is accompanied by a step-wise increase in selectivity for the first two fluorinations, after which the selectivity declines. A similar pattern emerges when the same membranes are characterized in terms of their selectivity for the gas pairs HZ/ CH, and He/CH,. After the second fluorination, at optimum conditions, the selectivity for

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2

4

6

8

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HJCH, ranges from 290 to 350, and for He/ CH, the selectivities are between 500 and 700. In these instances the P/Z for He was reduced on average by only 22% while for H, the average decrease was 52%. After the third fluorination cycle, the selectivity generally begins to decline while the permeance continues to decline. The fact that P/l does not increase at this point suggests a very low level of defects. Successive fluorinations were also performed at 0.02% F2 with pulse durations of 2 and 4 min. For 2 min contact times, the second fluorination cycle produced variable results: for some membranes the selectivity increased considerably while for others the treatment proved too harsh and a lower selectivity resulted. This result emphasizes that small differences between individual membranes are magnified by fluorination treatment. Four minute pulses never produced membranes with properties superior to those of the unfluorinated membranes; thus, this cycle is clearly too long. The above results show that fairly consistent amelioration of membrane properties can be achieved using two successive fluorinations at the mildest conditions of which this experimental system was capable. It should be noted that when the successive fluorination strategy is employed, no post-treatment step appears to be necessary if fluorination is optimized. This can be ascribed to a combination of at least two effects. Successive ‘pulsed’ fluorine exposures of the order of the reactor residence time (1 min here) result in a lower net exposure to fluorine at the polymer surface than one continuous exposure of the same duration. In addition, the first fluorination creates polar fluorine moieties on the polymer in the membrane surface region and these may restrict the access of fluorine during subsequent fluorination cycles.

O2 Pennesnce (P/l) [GPU] Fig. 14. Variation in the 02/Nz transport properties of membranes which were treated by multiple fluorination cycles of 1 min duration at a constant feed concentration of 0.02% Fz.

The role of quality of the unfluorinated asymmetric membrane It has been shown that a slightly

defective

139

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

membrane can be improved after fluorination by application of a PDMS coating. Reference to Figs. 8 and 9 shows that the membranes which had the highest selectivities before fluorination often had the highest final selectivities. This raises the question of whether an initially slightly defective membrane is improved or detrimentally affected by fluorination and what the effect of subsequent surface coating will be. This matter has considerable practical significance, since large areas of asymmetric membranes which are both thin and completely defect-free are not readily prepared, particularly on an industrial scale. To address this issue, four slightly defective PSF membranes were fluorinated for different times and coated with PDMS. The results are shown in Fig. 15. The unfluorinated membranes had 0,/N, selectivities ranging from 4.7 to 5.7, slightly below the 6.3 value obtained from solution cast PSF films. The low selectivity of these membranes was attributed to minor surface defects. Fluorination at 0.02% F, was performed for dif-

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0, Fcrmeance[GPU] Fig. 15. Effect of fluorination and surface coating on the Oz/N2 transport properties of untreated slightly defective membranes with selectivities below the intrinsic selectivity of unfluorinated PSF. The solid line shows the dense film selectivity of PSF. The fluorination time in minutes at 0.02% F, is shown beside each point. The symbols have the same meanings as in Fig. 12.

ferent times and the resulting changes in O,/ N, selectivity and 0, permeance are shown in Fig. 15. In all cases, both the selectivity and the permeance decreased after fluorination. After coating the fluorinated membranes with PDMS, the permeance again decreased but an increase in selectivity was observed. Comparison of the open symbols in Figs. 6 to 9 with those of Fig. 15 shows that the final selectivities of the initially defective membranes are similar in magnitude to that of the defect-free unfluorinated membranes used in previous experiments. Moreover, as shown in Fig. 15, the combined treatment significantly reduced the productivity of the defective membranes. This leads to the speculation that fluorine attacks the polymer surrounding the defects, thereby increasing the size, but perhaps not the number, of defects. The larger defects significantly reduce selectivity and should tend to increase the permeance somewhat for the fast gases. However, it appears that this slight increase in permeance is off-set by a more substantial decrease due to fluorination of the defect-free portion of the membrane surface. We cannot rule out the possibility that fluorination creates new defects, especially in regions of stress or other vulnerability. In any case, it can be concluded that the condition of the membrane before fluorination significantly affects the outcome of the overall treatment. Fluorination appears to be of limited benefit unless the membranes are initially defect-free. Often defect-free asymmetric membranes can only be formed at the expense of productivity, i.e., by increasing the effective skin thickness. Conclusions Flat sheet integral asymmetric polysulfone membranes were subjected to gas phase surface fluorination in a well-mixed reactor. A short reactor residence time ensured that constant fluorine concentration in the reactor (i.e., at the

140

membrane surface) could be achieved after 1 min of operation. Thus it was possible to vary the fluorination time independently of the fluorine concentration in the feed, so that these two variables could be rationally investigated. Fluorination conditions were optimized to yield an improvement in the selectivity of the gas pairs 0,/N,, Hz/Nz, H&Hd, He/N,, He/CH, and CO,/CH, with a varying decrease in the permeability of all gases. It was found that different conditions were required for the optimization of different gas pairs. At a fluorine feed concentration of 0.02% (by volume), the O,/ N2 selectivity was improved by up to 50% at fluorination times of 2 min, whereas the other gas pairs yielded their highest selectivities at 5 min of fluorination. On average these conditions produced a more than ten-fold increase in the He/CH, and H&H, selectivities while the permeance (P/1 or pressure normalized flux) for He and H, decreased on average by 27 and 65%, respectively. Under conditions of optimum selectivity the P/Z of the other four gases decreased to less than 10% of their values before fluorination. Increasing the fluorination time at a constant feed concentration was found not to be equivalent to increasing the feed concentration while fluorinating for a constant time. The latter strategy produced best results at low feed concentrations of 0.02-0.04% F,. Higher concentrations resulted in a sharp decline in the selectivity of all gas pairs. There was a net decrease in selectivity for the Oz/Nz pair, while the H2/N2, H2/CHI, He/N2 and He/CH, selectivities still showed a net improvement over those of the unfluorinated membranes up to concentrations of 0.10% F,. For concentrations below 0.10% the decline in selectivity was accompanied by a decline in the permeance; a concentration of 0.15% was probably high enough to cause severe damage to the membrane skin with the formation of defects and a consequent increase in permeance.

J.D. Le Roux et al. /J. Membrane Sci. 94 (1994) 121-141

To exploit the observation that mild fluorination conditions produced favorable results, membranes were subjected to successive fluorination cycles using short fluorination times at a feed concentration of 0.02% Fz. Two cycles of 1 min each were found to produce optimum results; for more or longer cycles the 02/Nz selectivity showed a decrease. The success of this strategy emphasizes the general conclusion that surface fluorination of asymmetric PSF membranes yields best results at mild conditions. It was also found that membranes coated with a layer of highly permeable silicone rubber (PDMS ) after fluorination invariably showed an increase in selectivity with a relatively small additional loss of permeability. This suggests that fluorination produces minor skin lesions or unfavorable surface morphologies which can be corrected by application of a highly permeable yet relatively non-selective coating. The fluorination of slightly defective membranes consistently resulted in decreased selectivity and permeance. Subsequent coating with PDMS restored some of the selectivity but did not result in an overall improvement in the membrane properties. Acknowledgments

The authors thank James DeYoung and Alberto Ruiz-Trevifio for their technical assistance. This work was supported by the Texas Advanced Technology Program under Grant No. 1607, by the Separations Research Program at the University of Texas at Austin, USA, and by the CSIR, Pretoria (South Africa). References 1

C.L. Kiplinger, D.F. Persico, R.J. Lagow and D.R. Paul, Gas transport in partially fluorinated low-density polyethylene, J. Appl. Polym. Ski., 31 (1986) 26172626.

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C.C. Chiao, Surface modified gas separation membranes. US Pat. 4,828,585, assigned to The Dow Chemical Company, Midland, USA, 1989. M. Langsam, Fluorinated polymeric membranes for gas separation processes, US Pat. 4,657,564, assigned to Air Products and Chemicals Inc., PA, USA 1987. M. Langsam, M. Anand and E.J. Kawacki, Substituted propyne polymers. I. Chemical surface modification of poly [ I- (trimethylsilyl)propyne] for gas separation membranes, Gas Sep. Purif., 2 (1988) 162170. M. Langsam, Polytrialkylgermylpropyne polymers and membranes, US Pat. 4,759,776, assigned to Air Products and Chemicals, Inc., PA, USA 1988. 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 (1991) 131-148. 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. M. Langsam and L.M. Robeson, Substituted propyne polymers. Part II. Effects of aging on the gas permeability properties of poly [ 1- (trimethylsilyl ) propyne ] for gas separation membranes, Polym. Eng. Sci., 29 (1989) 44-54. J.M. Mohr, Surface Fluorination of Gas Separation Membranes, Ph.D. Dissertation, University of Texas at Austin, 1990. J.M. Mohr, D.R. Paul, Y. Taru, T. Mlsna and R.J. Lagow, Surface fluorination of composite membranes. Part II. Characterization of the fluorinated layer, J. Membrane Sci., 55 (1991) 149-171. J.M. Mohr, D.R. Paul, Y. Taru, T. Mlsna and R.J. Lagow, XPS characterization of surface fluorinated poly(4-methyl-1-pentene), J. Appl. Polym. Sci., 42 (1991) 2509-2516. V. Grakauskas, Direct liquid phase fluorination of organic compounds, Intra-Sci. Chem. Rep., 5 (1971) 85104. R.J. Lagow and J.L. Margrave, Direct fluorination: A “new” approach to fluorine chemistry, Prog. Inorg. Chem., 26 (1979) 161-210. S.T. Purrington and B.S. Kagen, The application of elemental fluorine in organic synthesis, Chem. Rev., 86 (1986) 997-1018. F. Cacace, P. Giacomello and A.P. Wolf, Substrate selectivity and orientation in aromatic substitution by molecular fluorine, J. Am. Chem. Sot., 102 (1980) 3511-3515.

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