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Surface fluorination of poly (phenylene oxide) composite membranes
-
journal of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 90 ( 1994) 37-53
Surface fluorination of poly (phenylene oxide) composite membranes Part II. Characterization of the fluorinated layer J.D. Le Rouxa, D.R. Paula,*, M.F. Arendtb, Y. Yuan”, I. Cabassoc “Department of Chemical Engineering, Centerfor Polymer Research, The University of Texas, Austin, TX 78712, USA bDepartment of Chemistry The University of Texas, Austin, TX 78712, USA ‘Polymer Research Institute, State University ofNew York, Syracuse, NY 13210, USA
(Received August 27, 1993; accepted in revised form December 20, 1993)
Abstract Composite membranes, comprising a selective layer of poly( 2,6-dimethyl-1,Cphenylene oxide) on a microporous ceramic support, were surface fluorinated under mild conditions. The treatment parameters, fluorine feed concentration and treatment time, were each varied independently while the other parameter was kept constant. The fluorinated region was characterized using dynamic contact angle analysis and X-ray photoelectron spectroscopy (XPS). The fluorine to carbon ratio increased as a function of treatment time, but showed little variation when the feed concentration was varied. A relatively high oxygen to carbon ratio was observed which showed little change over the range of fluorination conditions. Angle-dependent XPS showed the concentration of fluorine species to be higher at greater depths below the surface. Pluorination times of 5 to 7 min minimized the water contact angle and maximized the total and polar surface free energies. Water contact angles decreased after fluorination but showed no definite trend as a function of feed concentration. The composition of the fluorinated region is discussed in relation to the gas transport properties of membranes fluorinated under similar conditions (Part I of this study). Key words: Poly(2,6-dimethyl-l+phenylene oxide); Gas separation; Composite membranes; Fluorination; Surface modification; X-ray photoelelectron spectroscopy; Dynamic contact angle analysis
1. Introduction The gas transport properties of a gas separation membrane can be modified by exposing its surface to low concentrations of elemental fluorine in an inert diluent gas, such as helium or nitrogen. The fluorine in the treatment feed gas, and also oxygen which is often present in the re*Corresponding author. 0376-7388/94/$07.00
action environment, react with the polymer at the surface to produce an oxyfluorinated layer [ l41. The chemical nature and depth of the newly formed layer depend on the treatment conditions: predominantly the fluorine concentration at the polymer surface and the exposure time. This work involves the fluorination of composite membranes comprising a selective skin of poly (2,6-dimethyl- 1,Cphenylene oxide ) (PPO) supported on a substrate which is effectively in-
0 1994 Elsevier Science B.V. All rights reserved
SSDI0376-7388(94)00006-K
38
J.D. Le Roux et al. /Journal ofMembrane Science 90 (I 994) 37-53
ert towards fluorination. Fluorination was performed in a well-mixed reactor with a volume small enough that the composition of the reaction gas at the membrane surface approaches that in the feed [ $61. This permits the fluorination time and fluorine concentration at the surface to be varied independently. The structure of the PPO repeat unit is shown in Fig. 1, together with the atomic ratios of its three constituent elements. In the initial stages of fluorination, hydrogens are replaced by the more bulky and electron-rich fluorine atoms. The relatively simple structure of PPO contains only two different potential carbon-hydrogen sites for such a substitution: at the 3,5-positions on the aromatic ring and on the methyl pendant groups. Other fluorination or oxidation reactions could involve a loss of aromaticity, aromatic ring opening, chain scission at the ether linkages in the polymer backbone and cross-linking reactions. Unfortunately, the highly exothermic nature of fluorination complicates the control of these reactions and the interpretation of analytical results. Part I of this study [ 51 described the gas transport properties of the fluorinated composite PPO membranes. The objective of this paper (Part II ) is to investigate the nature of the fluorinated layer and to relate this information to the observed transport properties. To this end two complementary analytical techniques were employed: Xray photoelectron spectroscopy (XPS) and dynamic contact angle analysis (DCA ) . Generally, DCA yields information on the uppermost surface of the membrane while XPS probes the immediate subsurface region to a depth of - 60-90 A in polymers [ 2,7,8 1. Both XPS and DCA have been applied for the analysis of fluorinated polymers, separately and in combination, to complement each other [2,4,8-l 51. The background Number uf atoms:
2.1. Membrane formation and fluorination treatment
=X
1
Alumic elemental ratio: CHJ
2. Experimental
Hydrogen = 8 Oxygen =
0 :A w
CdiXl
and theory relevant to these two analytical methods are discussed elsewhere [ 1,3]. In the gas transport study (Part I) [ 5 ] composite membranes were formed by spin coating a selective layer of PPO onto a microporous ceramic (aluminum oxide) ultrafiltration membrane with a very low resistance to gas permeation [ 16 1. For analysis of the fluorinated layer (Part II), composite laminates were formed by spin coating a thin layer of PPO onto a solid aluminum metal support. In contrast with the microporous ceramic supports, the aluminum support is not brittle and this facilitated sample preparation for both of the analytical techniques employed.
”
WC = I.0 OK
= 0.125
Fig. 1. Chemical structure of poly ( phenylene oxide )
Composite laminates for fluorination were prepared by spin coating a dilute solution of PPO onto circular disks of rigid aluminum foil. The spin-coating solution consisted of PPO dissolved in reagent grade 1,1,2-trichloroethane at a concentration of 2.0% by weight. The PPO used for this purpose (PPO-34 from General Electric Company) had a weight average molecular weight of 32,900 g/mol and a number average molecular weight of 17,900 g/mol, as determined by light scattering. The intrinsic viscosity in chloroform (25°C) was 0.34 dl/g. A higher molecular weight PPO was used to prepare membranes for transport measurements [ 5 1. However, due to the limited supply of that material, this more standard PPO was employed here where skin integrity is not such a critical issue. A photoresist spinner (Series EClOlD) from Headway Research Inc. (Garland, TX) was used to apply the PPO solution. The aluminum substrate 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 aluminum substrate with - 5 ml of polymer solution the spinner was immediately activated and allowed to rotate at 800 rpm for 60 s; maximum speed was
J.D. L.e Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
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 oC for at least 16 h to ensure complete removal of solvent. Thermal analysis of delaminated thick PPO films formed under similar conditions showed no evidence of residual solvent. The circular composite laminates (4.7 cm in diameter) were the same size as the composite membranes used in Part I [ 5 1. Although PPO has a tendency to crystallize during quiescent solution casting, the polymer films were clear and differential scanning calorimetry (DSC) confirmed the absence of any crystallinity. 2.2. X-ray photoelectron spectroscopy (XPS) The XPS spectra were obtained with a VG 1000 ESCA spectrometer using MgKa exciting radiation ( 1253.6 eV). Typically, the X-ray gun was operated at 15 kV and 22 mA with a sample chamber vacuum of less than 5 x 10m9Torr. Pure graphitic carbon at a full width at half-maximum ( fwhm) of 1.4 ? 0.1 eV was used for calibration. All samples showed charging effects and the binding energies were referenced to the hydrocarbon C,, core-level peak at 285 eV. Previous studies have shown that this corresponds closely to the shift of gold-sputtered samples referenced to the AQ,,~ level at 84 eV. Except where indicated, XPS spectra were recorded at an electron emission angle (0) of 90” to give the maximum sampling depth of N 60-90 A. All samples were mounted onto the sample probe with doublesided adhesive tape. The peak for each of the sampled elements C, 0 and F consisted of the average value of five 30-s scans over the appropriate binding energy range at a pass energy of 50 eV. Some analyses were duplicated at a pass energy of 20 eV and the peak shapes were essentially similar. Sensitivity factors for the quantitative analysis were determined from homogeneous samples of poly (tetrafluoroethylene ) (PTFE ) and poly (vinylidene fluoride) (PVDF) for fluorine and from solution cast PPO for oxygen. The factors were: 0.17 for carbon, 0.38 for oxygen and 1.Ofor fluorine.
39
Table 1 contains a listing of relevant binding energy shifts with respect to the C1, binding energy, as reported in the literature for fluorinated and oxidized hydrocarbon species. These data were derived from XPS studies of both pure monomeric compounds and polymers, only two of which refer specifically to PPO in its unfluorinated and oxidized forms [ 15,171. Generally, the addition of each fluorine to a carbon produces an additive primary shift of N 2.7-3.0 eV and each oxygen attached to a carbon causes a shift of N 1.2-1.5 eV. However, the interpretation of spectra of polymers with aromatic and aliphatic carbons is complicated by the relatively large secondary shifts when fluorine and oxygen are added to neighboring carbons [ 15,18,19,2326 ]. Since the primary binding energy shift due to the formation of one fluorine bond ( N 3.0 eV) is virtually the same as that observed for two oxygen bonds, the monofluorinated and carbonyl carbons are not readily distinguished by deconvolution of the C1, peak. From an XPS study of monomeric benzene derivatives [ 251 the primary and secondary binding energy shifts caused by addition of substituents, such as -F, -CH3 and -CF,, to carbons of the aromatic ring appear to differ significantly from those resulting from addition to aliphatic carbons. For instance, in monomeric benzotrifluoride the shift for aromatic C-CF3 was reported to be 1.9 eV [ 25 ] compared to values of N 1.5 eV for both ether bonded carbons and C-CF, of secondary aliphatic carbons (Table 1). This brief discussion serves merely to emphasize that the interpretation of the spectra of oxyfluorinated PPO should be approached with caution and that certain approximations are necessarily involved. Deconvolution of the Cl, peaks was performed using a non-commercial computer routine. Considering the range of potential oxyfluorination reaction products of the PPO repeat unit and the numerous possible binding energy allocations that can be made (cf. Table 1 ), an exhaustive deconvolution was not attempted. Instead, based on the anticipated fluorinated and oxidized species, the Cl, spectra were resolved into only three widely spaced and fairly broad component peaks. The first of these represents
40
J. D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
Table 1 Binding energy shifts relative to the Cr. peak at 285 eV Representative functional species
Binding energy shift (eV)
Comment
Ref.
C-C/C-H
0.0
hydrocarbon
0-C% C-CF,
1.9
aromatic C-CFs
1.5kO.4
aliphatic C-CF.
3.OkO.4
general
reference peak
2.8
18 9, 17 9 18 18
GCF
2.8-4.5 1.5-1.9 2.6-2.9
cyclic, non-aromaticb cyclic, non-aromatic’
9, 19, 20 9, 19, 20
Cx =CFz
6.OkO.4 4.1-4.5
general cyclic, non-aromaticb
9,17 9, 19, 20
8.4kO.7
general
17 9,20
0-o.Q
17
Em-
8.8 C-O
1.5
c=o/o-c-o o-c=0
2.9 4.0
n--K*
7.0
17,21,22 17,21,22 aromatic shake-up satellite
17
“Ortho-,meta- and para-substitued benzene. bUnsaturatedportion of a cyclic, non-aromatic ring system. “Saturatedpart of a cyclic, non-aromatic ring system.
the binding energies of the unfluorinated carbons of the PPO unit, including the secondary shifts as discussed above. The other two peaks were shifted by approximately 3 and 5 eV, respectively, and account for fluorinated and oxidized carbon species. The binding energy of hydrocarbon was fixed at 285 eV and a ratio of 30% Lorentzian:70% Gaussian was assumed for the peak shapes. 2.3. Dynamic contact angle analysis (DCA) Advancing and receding contact angles for water and methylene iodide ( CH212 ) on fluorinated and unfluorinated PSF membranes were determined by the Wilhelmy plate technique [ 27,28 ] using a Cahn Dynamic Contact Angle Analyzer (DCA Model 322). The analyzer was
thermostated at 20’ C with a glass slide having a perimeter of 48.3 mm. In principle, a solid sample is progressively immersed and then withdrawn from a well-defined probe liquid while continuously recording its weight change as a function of immersion depth. Sample preparation involved cutting and securing two identically shaped portions of a PPO sample back-to-back to create a rectangular sample fluorinated on both surfaces. The fluorinated surface was washed several times with distilled deionized water and dried before measurements. This sample was fixed statically to the electrobalance of the analyzer and a computer-controlled platform supporting a beaker of the probe liquid was then scanned upwards at a constant rate to gradually immerse the sample. The tangent of the meniscus formed at the liquid-solid-
41
J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
vapor interface defines the dynamic contact angle as the interface passes over the solid surface. A plot of the overall force on the sample as a function of immersion depth yields advancing and receding contact angles (0, and 0,) from the parallel immersion and withdrawal traces [ 3,27,28 1. Average values for 0, and 0, are evaluated by extrapolation of the slopes to zero immersion depth using a linear regression, and each reported contact angle is the average of at least three measurements. To determine the accuracy of the measurements, seven DCA samples were prepared from the same fluorinated membrane and measured independently; several such tests yielded a maximum standard deviation of 5%. This small variation also confirmed that the feed gas was well mixed in the reaction chamber leading to an even treatment over the entire membrane surface. Following Owens and Wendt [ 29 1, the average advancing contact angles (0,) measured for water and methylene iodide were used to calculate the polar and dispersive components (7: and y,”) of the solid surface free energy by solution of the following equation l+c0sB=2J$$)+z&($)
10 min
5 min
3 min
2 min
1 min
(1)
where ylyis the vapor-liquid surface tension, and the polar and dispersive components of the liquid surface free energy (rf and yP ) are known. The surface tensions of water and CH212 have been reported as 72.8 and 50.8 dynes/cm, respectively; the dispersive and polar components for water are 22.5 and 50.3 dynes/cm and for CHzIz they are 48.5 and 2.3 dynes/cm [ 30,3 11. The theoretical and practical aspects of this method are discussed in greater detail elsewhere 131. 3. Analysis by X-ray photoelectron spectroscopy (xps) 3.1. Variable treatment time Fig. 2 shows the shape of the C1, peak of PPO as a function of fluorination time when a con-
Fig. 2. The C,, peak of the XPS spectra of PPO after treatment with 0.02% F2 for various fluorine exposure times.
stant fluorine feed concentration of 0.02% is used. All samples were corrected for charging and the peaks are aligned according to the position of the hydrocarbon peak at a binding energy of 28 5 eV, which coincides with the slope of the peak on the low binding energy side of each composite peak. The unjluorinated PPO peak was resolved into two component peaks with a peak area ratio of 3: 1. These peaks represent the contributions of the two ether-bonded carbons and the other six ahphatic and aromatic carbons of each repeat unit [ 15 1. Actually the situation is more complex. Reference to Fig. 1 shows that each PPO repeat unit has four pairs of carbons which all have different electronic environments. An XPS study of pure toluene [ 25 ] indicates that three
42
J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 3 7-53
of these pairs may have different binding energies. Clark and Thomas [ 17 ] report three component peaks for PPO which suggests that the unsubstituted aromatic carbons and the methyl carbons can be grouped together. For the analysis of fluorinated PPO the contributions of all the unfluorinated carbon species were grouped together, due to the multiple secondary shifts which may result from different configurations of fluorine and oxygen added randomly to the polymer. After fluorination the shape of the C,, peak changes considerably. Compared to unfluorinated PPO, a distinct new component peak evolves at a binding energy shift of - 3 eV. As fluorination time increases the new peak becomes increasingly prominent and, at the longest time of 10 min, is the dominant feature. Qualitatively, this feature is associated with the formation of monofluorinated carbon (C-F) and carbonyl (C=O) moieties. The widened base at the high binding energy side of the composite peak suggests the presence of a second component peak which can be assigned to difluorinated carbon (CF,) and to more highly oxidized carbon. According to Table 1, the C’F2 peak is expected at a shift of between 4.3 and 6 eV, depending on whether the carbon is single or double bonded, i.e., whether it is in an aliphatic or aromatic environment. In addition, the carbon shift for the carboxylic acid structure (0-C=O) is expected at -4 eV. The feature in this binding energy region apparently does not change substantially as a function of fluorination time. There is clearly no peak at a shift of around 8.4 eV which would indicate the presence of trifluorocarbon species (CF,) and evidently the pendant methyl groups are not perfluorinated under these treatment conditions. A similar result was reported for the fluorination of PSF membranes which were fluorinated under similar conditions [ 3 1. On the other hand, the formation of both CF2 and CF3 species were reported by Mohr et al. for the fluorination of poly (4-methyl- 1-pentene ) [ 2 ] and by Langsam et al. for poly [ 1- (trimethylsilyl ) - 1-propyne ] [ 12 1. Their polymers contained only aliphatic carbon species, and fluorination was carried out at higher concentra-
tions and in quite different fluorination reactor environments. To supplement the qualitative observations based on the peak shapes in Fig. 2, Fig. 3 shows the results of the deconvolution of the C,, peaks into three component peaks. The solid lines in the figures are intended only to emphasize trends in the data. The relative binding energy shifts and ultimate fwhm of the peaks are given in Table 2. Judging by their widths, each peak encompasses a range of minor binding energy shifts due to the various possible electronic environments. The unfluorinated carbon species in Fig. 3 are designated by open symbols while fluorinated or oxidized species formed during this process are represented by the solid symbols shown in the legend. As observed in Fig. 2, the area contribution of the unfluorinated carbon peak ( [a] ) decreases with treatment time, while the contributions from the other two peaks ( [b] and [c] ) gradually increase. These latter peaks represent the mono- and difluorinated carbons and carbony1 species. It should be pointed out that the deconvolution analysis locates the CF2 peak ( [c ] ) at a binding energy shift of - 4.8 + 0.2 eV. Assuming that the PPO repeat unit remains intact, this implies one difluorinated carbon per loop
,
,
,
,
,
,
.
,
,
,
,
,
CFaWor C=O
0 20
. . L;
-
OY’ 0
” 12
” 3
4
5
1’ 6
” 7
8
9
’
I
IO
II
Fluorination time [min]
Fig. 3. The concentration of various carbon species from deconvolution of the C,, peak of PPO before and after treatment with 0.02% F2 for various treatment times. The peaks are defined in Table 2. Carbon species to which either fluorine or oxygen have been added during treatment are designated by open symbols (0 ), and unmodified carbons are represented by solid symbols ( 0, + ) .
43
J.D. Le Roux et al. /Journal ofMembrane Science 90 (1994) 37-53 Table 2 C,, peak deconvolution:
constituent
Peak
Designated species
Binding energy shift (eV)’
fwhmb
[cl
C-C, C-H, C-O, C-CF, CF, c=o CF;
o-o.4 2.69 I? 0.06 4.66 f 0.04
2.28 f 0.07= 2.28 + 0.07 2.28 f 0.07
peak designations
‘Relative to the Cr. peak at 285 eV. bFull width at half-maximum. “Standard deviation. dCould include carboxylic acids (0 = C-O) at a shift of _ 4.0 eV (see text ) .
repeat unit which may be located either at a site on the carbon ring (shifted by 4.1-4.5 eV) or on a pendant aliphatic carbon (shifted by 6 eV). Clearly, if the aromatic rings or the polymer backbone are disrupted during fluorination, many other possibilities present themselves and qualitative analysis is complicated considerably. In summary, increased treatment time results in the progressive addition of fluorine and oxygen atoms to create increasingly polar structures. On the other hand, longer exposure to the reaction gas also increases the likelihood of damage and other uncontrolled reactions at the polymer surface. The fact that Figs. 2 and 3 show only a smaII increase in peak [c] after 5 min of treatment could indicate either that no further addition takes place or that the addition of the CF and C = 0 species is balanced by a simultaneous loss of these species. The issue of damage or disruption of the polymer structure will be considered again below. Fig. 4 shows the F/C and O/C atomic ratios as a function of fluorination time. These ratios were obtained from the integrated areas of the fluorine, oxygen and carbon peaks, taking into account the atomic sensitivity factors of these elements. The solid horizontal line in the lower graph indicates the O/C ratio of 0.125 for PPO before fluorination (cf. Fig. 1). The F/C ratio increases with treatment time with a trend similar to that of peaks [b] and [c] in Fig. 3. The O/ C ratio increases by a factor of N 2.5 due to fluorination. This ratio remains largely independent of treatment time at an average value of 0.32 and the slight decline at 10 min is probably not significant. As reported in other studies [ 1,3,4,12 1,
0.8 0.7 .::
0.6
d
0.5
‘i
0.4
4
0.3
s
0.2 0.1
::
j * .o
0.3
E s
0.2
v 0
, 0.1
O/C ratio [or unmated PF’O
-
0
0
I I
-
I,,,,,,,,, 2
3
4
5
6
7
8
9
10
11
12
Fluorination time [min] Fig. 4. Fluorine to carbon and oxygen to carbon atomic ratios of PPO after various exposure times at a fluorine feed concentration of 0.02% F2. The solid line in the lower graph shows the theoretical O/C ratio for untreated PPO.
these results confirm that fluorination is accompanied by oxidation as evidenced by the addition of a constant amount of oxygen apparently independent of the fluorination conditions.
44
J.D. Le Roux et al. /Journal ofMembrane Science 90 (1994) 37-53
3.2. Variablefluorine concentration
To investigate the effect of various fluorine feed gas concentrations on the composition of the fluorinated layer, a constant treatment time of 2 min was employed. Fig. 5 shows the shape of the C,, peak of PPO as a function of fluorine feed concentration during treatment for a constant treatment time of 2 min. As discussed for Fig. 2, the feature which appears at a binding energy shift of N 3 eV is associated with the formation of monofluorinated (C-F) and carbonyl (C= 0) carbon species. A CF2 peak is present at a shift of N 5 to 6 eV, and again no trifluoromethyl ( CF3 ) groups are apparent. There appears to be
0.04%
F2
0.02%
F2
no significant qualitative change in the peak shape at fluorine concentrations exceeding 0.04%. Fig. 6 is a graphical representation of the deconvolution of the Ci, peak into its constituent peaks as defined in Table 2. The contribution of unfluorinated carbons (peak [a] ) decreases considerably due to fluorination but shows little variation at different concentrations. The effectively constant contribution of peak [c] indicates that only one CF2 moiety is formed per repeat unit, irrespective of the fluorine feed concentration. The contribution from peak [b 1, which represents both CF and carbonyl species, is increased by fluorination treatment but remains essentially independent of fluorine concentration. Thus, the results of the deconvolution confirm the qualitative effects observed in Fig. 2. In general, it appears that about half of the carbon atoms in each repeat unit react either with fluorine or with oxygen. Fig. 1 shows that the unreacted carbons could be the four carbons without any hydrogens which can only be fluorinated by loss of aromaticity, pendant groups or chain connectivity. Fig. 7 shows the F/C and O/C atomic ratios at different concentrations of fluorine in the feed. Both atomic ratios increase markedly due to fluorination with only small variations at the dif-
I
0
0.02
I
I
0.04
,
,
0.06
Fluorine concentration
Fig. 5. The C,, peak of the XPS spectra of PPO after 2 min of treatment at various fluorine feed concentrations.
,
,
0.08
,
,
0.1
0.12
(~0176 F, in He)
Fig. 6. The concentration of the carbon species after deconvolution of the C,, peak of PPO before and after 2 min of treatment at various fluorine feed concentrations. The peaks and symbols are the same as in Fig. 3.
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
0.60L 0.50
,
I
,
,
,
,
,
,
,
,
,
,
,
,
,
,
.
-
0.60 0.50
,
,
,
,
,
,
1
O/C ratio for unueavzd PPO
0
0.02 Fluorine
0.04
0.06
0.08
concentration
[vol%
0.1
0.12
F,/He]
Fig. 7. Fluorine/carbon and oxygen/carbon atomic ratios of PPO after 2 min of treatment at various fluorine feed concentrations. The solid line in the lower graph shows the theoretical O/C ratio for untreated PPO.
ferent fluorine concentrations. The F/C ratio lies between 0.37 and 0.48 which represents between 3 and 4 fluorines per PPO repeat unit. The apparent maximum F/C at 0.06% F2 can also be identified in Figs. 5 and 6. If this maximum is experimentally significant, it may indicate the highest fluorine concentration the polymer can tolerate before the addition of fluorine is balanced by its removal due to damage or disruption of the polymer structure. In the lower graph the O/C ratio increases from the untluorinated experimental value of 0.14 to around 0.36 after fluorination, which represents the addition, on average, of two oxygens to each repeat group. As in Fig. 4, the addition of oxygen appears to be effectively independent of the reaction conditions. In an XPS investigation of the natural weath-
45
ering of PPO by surface oxidation, Dilks [ 15 ] concluded that the pendant methyl groups are converted to carboxylic acids with little change to other parts of the polymer. In an investigation of fluorinated PTMSP by surface sensitive infrared techniques, Langsam et al. [ 121 mention the presence of a strong carboxyl band and report O/C ratios of 0.12 to 0.17. On the assumption that the aromatic ring and the polymer backbone remain intact during fluorination, the above evidence seems to indicate that the oxidation which accompanies fluorination may occur predominantly at the pendant methyl groups. Thus, an unfluorinated methyl group is converted progressively into more highly oxidized forms, such as alcohols, aldehydes and eventually carboxylic acid, the formation of peroxides is also possible. At the same time, the addition of fluorine probably occurs by substitution of hydrogens both at the methyl groups and on the two exposed aromatic carbon sites. The presence of oxygen after fluorination treatment has been reported by several independent researchers [ 1,3,4, lo- 12,321. Following a careful investigation of this issue, Mohr et al. [ 1 ] concluded that the oxygen is introduced as an impurity in the gaseous feed. Personal communications with the supplier of the fluorine feed gas [ 33 ] indicated that technical grade fluorine gas was specified to have a purity of > 97% and that the contaminants are likely to include mostly oxygen and nitrogen. Two other sources of oxygen must also be considered. The first is atmospheric oxygen and/or moisture which may be present in the sample before fluorination and is not completely removed by purging the reactor with pure helium prior to treatment. Another possibility is that exposure to the atmosphere after fluorination allows oxygen to react with reactive radicals remaining at the polymer surface. To some degree all of these sources of oxygen may contribute to the high O/C ratios observed after fluorination. 3.3. Angle-dependentjilm properties XPS spectra can be recorded with the detector positioned at different take-off or electron emis-
J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
46
[FJ = 0.02~01% Time = 10 min.
[FJ = 0.02~01% Time = 2 min.
[FJ = 0.08~01% Time = 2 min.
Take-d allgl.C:
Take-otf angle.
90”
90”
55”
400
25”
15O
I 280
I
1 285
”
1 290
I 295
I’
I
280
285
”
(
I
12
I
290
I
I 295
I”” 280
11 285
c
‘1’ 290
“I 295
Binding Energy [eV] Fig. 8. The C,, spectra of PSF at electron take-off angles of 1S”, 25”, 40”, 55” and 90” for three sets of fluorination
sion angles, while its angle with respect to the Xray source remains constant. This has the effect of varying the probing depth between a maximum of -60 8, below the sample surface at a take-off angle of 90” and a depth of - 15 A at an angle of 15’. Fig. 8 shows the C,, spectra at takeoff angles of 15”, 25”, 40”, 55” and 90” recorded at fluorination conditions of 0.02% F2 for 2 and 10 min, and at 0.08% F2 for 2 min. These take-off angles were used to estimate the variation in the composition of the fluorinated film at different depths below the surface. Qualitatively, in all three series there is a decrease in the height of the shoulder at 3 eV, which represents the C-F and C=O feature as a function of decreasing probe depth or take-off angle. The extent of decrease appears to be less in the two series fluorinated for 2 min (at 0.02% and 0.08% F,) compared to the series for 10 min. These observations show that the composition of the fluorinated layer is not uniform, but has a lower fluorine and oxygen content nearer the surface than at a greater depth. Since the phe-
conditions.
nomenon appears to be time related, it may result from damage to the polymer structure in the near surface region which has been exposed to the fluorinating agent for a longer time. Thus, 10 min of exposure should have a more severe effect than shorter times. Similar observations were reported in a previous study [ 31 for the depthof fluorinated PSF dependent analysis membranes.
4. Dynamic contact angle analysis (DCA) 4. I. Water and methylene iodide contact angles The contact angles of polar and non-polar liquids, and the surface free energies calculated from them, provide information on the surface constitution of a fluorinated polymer composite to a depth of - 10 %i. Surface analysis, therefore, complements XPS which probes the region to - 60-90 A, depending on the escape depth of the polymer. Tables 3 and 4 and Figs. 9 and 10 show
J. D. L.e Roux et al. /Journal
of Membrane Science 90 (I 994) 3 7-53
Table 3 Advancing and receding contact angles for PPO fluorinated at different fluorine concentrations Fluorination treatment Time’
Water
[Fib
47
for a constant time
Methylene iodide
Advancing
Receding
Hysteresis
Advancing
Receding
Hysteresis
68.7 15.1 6.6 F c c c
30.3 71.2 70.6 c E c c
58.8 42.3 52.4 59.3 54.1 56.1 60.5
41.8 22.3 31.0 33.3 18.8 34.0 14.5
17.0 20.0 21.4 26.0 35.3 22.7 46.0
0
0
99.0
2
0.02
2 2 2 2
0.04 0.06 0.08 0.10
92.3 77.2 88.6 80.5 80.5 86.8
“Treatment time (min ) . bFluorine concentration in the feed (~01% F2 in He). “Could not be reliably measured. Table 4 Advancing and receding contact angles for PPO fluorinated for different times at a constant fluorine concentration Fluorination treatment Time’
Water
Methylene iodide
[Flb
Advancing
Receding
Hysteresis
Advancing
Receding
Hysteresis
0
0
99.0
1 2
0.02 0.02
3 5
0.02 0.02
68.7 14.8 15.1 6.6 c
30.3 63.6 17.2 70.6 F
c
c
12.4
57.4
7
0.02 0.02 0.02
88.4 92.3 71.2 76.4 61.8 69.8 67.8 72.1 83.6
58.8 58.6 42.3 52.4 51.3 48.7 51.2 52.2 54.4 54.3
41.8 35.1 22.3 31.0 22.3 18.3 23.9 15.8 25.6 14.6
17.0 23.5 20.0 21.4 30.0 30.4 27.3 36.4 28.8 39.7
8 10
E
c
E
c
‘Treatment time (min). bFluorine concentration in the feed (~01% F2 in He). “Could not be reliably measured.
the advancing and receding contact angles (6, and 0,) for water and methylene iodide (CHJ,) on the PPO composites under different treatment conditions. Table 5 compares the advancing contact angles for these two liquids on PPO with those reported for a number of polymers which have aromatic, aliphatic and polar moieties. The advancing contact angles for water and CH212 on PPO are considerably higher than for polysulfone (PSF) and polystyrene, and closer to that of polyethylene. This could be due to the exposed pendant methyl groups of PPO which
tend to mask the contribution of the aromatic ring in the backbone and impart a predominantly aliphatic character to the PPO; conversely the aromatic features of PSF and polystyrene are more exposed. Fig. 9 shows the advancing contact angles ( 19,) of water and CHzIz on PPO as a function of treatment time for a constant fluorine concentration of 0.02%. The 6, for water decreases from 99” before fluorination to a minimum of 68” at 5 and 7 min and then increases at longer fluorination times. The trend for the non-polar liquid
48
J.D. Le Roux et al. /Journal ofMembrane Science 90 (1994) 37-53
ditions, in this case 0.02% Fz and treatment times of 5 and 7 min. On the other hand, the interaction with the non-polar liquid CHzIz is hardly influenced by the treatment conditions employed here. The possible physical and chemical origin of these effects will be considered below in conjunction with the calculated surface free energies. 4.2. Contact angle hysteresis 012345678
9
10
11
Fluorination time [mm] Fig. 9. The advancing contact angles for water (0 ) and methylene iodide (CH&) ( 0 ) as a function of fluorine exposure time at a fluorine feed concentration of 0.02% F2.
100
2
,
,
80
,
. ,
,
,
,
,
,
,
,
.
f
01 0
0.02
0.04
0.06
0.08
0.10
0.12
Fluorine concentration [vol% F,/He] Fig. 10. The advancing contact angles for water ( 0 ) and methylene iodide (CH&) ( q ) after 2 min of treatment at various fluorine feed concentrations.
CHzIz is similar, though 0, varies only by a few degrees at the most. Fig. 10 relates the advancing contact angles for water and CH212 to the fluorine feed concentration at a constant treatment time of 2 min. Here, the #, for water generally decreases due to fluorination but apparently shows no significant trend as a function of concentration. The 0, for CH21z appears to be virtually unaffected by the fluorination treatment. These results indicate that the wettability or adhesion of the polymer surface for water, a polar (hydrogen-bonding) liquid, can be maximized by selecting the proper fluorination con-
Hysteresis, the difference between the advancing and receding contact angles, could be due to a variety of surface imperfections such as macroscopic roughness, surface heterogeneity and sorption of the probe liquid [ 35-371. For those cases where 0, could be measured with confidence, the hysteresis of the water and CH212 contact angles for PPO under different fluorination conditions is shown in Tables 3 and 4. As observed for PSF previously [ 3 1, both liquids exhibit considerable hysteresis for both the unfluorinated and fluorinated PPO samples. Table 3 indicates a general increase in hysteresis for the CH212 contact angle as a function of treatment time: hysteresis increases from 17 ’ for unfluorinated PPO to - 40” after 10 min of fluorination. This effect is due mostly to a decrease in 19,,since the advancing contact angle shows little variation. A similar trend of increasing hysteresis as a function of increasing fluorine concentration is apparent from Table 4. Where the hysteresis of the water contact angles could be measured, the effect is even more pronounced than for CHzIz and high values are evident after fluorination treatment. It is believed that these effects may be related to microscopic surface roughness caused by etching or other damage to the polymer during fluorination. Thus, harsher treatment conditions (i.e., longer treatment times or higher concentrations of fluorine) result in greater damage, increased roughness and a concomitant increase in hysteresis. 4.3. Surface free energy The total surface free energy ( rs) and its polar and dispersive components (ySp and 7,” ) were determined from the contact angles for water and
49
J. D. L.e Roux et al. / Journal of Membrane Science 90 (I 994) 3 7-53 Table 5 Contact angles and surface energies for various polymers Polymer
Ref.
Advancing contact at@e,@. (“)
Surface energy (dynes/cm)
Water
Total (YS)
Polar (Y.P)
Dispersive (r.d)
CHJ2
Poly ( phenylene oxide )
99.0
58.8
29.44
0.55
28.9
Polysulfone
90.7
42.4
38.1
0.94
37.6
94
53.9
93.6 80 82 108 91 70 80.4 80
54.3 49 63 88/77 35 41 36.8 41
33.1 35.7135.3 30.1 36.7 30.3 18.6118.4 42.0140.7 47.0146.5 41.3144.6 40.2141.1
1.1 0 0 5.4 7.1 0.5l1.7 0.616.1 6.2/14 3.519 4.3111.5
32.0 35.7135.3 30.1 31.3 23.2 19.1l20.0 41.4134.5 40.8132.5 37.8135.6 35.9129.6
(PSF)
Polyethylene linear/branched Polypropylene Poly (vinyl fluoride ) Poly (vinylidene fluoride) Poly (tetrafluoroethylene) Polystyrene Polyamide (Nylon-6,6) PETa PMMAb
this work this work 29 34 34 29 29 29,34 29,34 29,34 29,34 29,34
‘Poly(ethylene terephthalate). bPoly (methyl methacrylate ) .
methylene iodide according to the method of Owens and Wendt [ 3,291. For unfluorinated PPO the value of JJ~was calculated as 29.4 dynes/ cm. Comparison with the ySvalues for the polymers listed in Table 5 shows that the value of PPO ranks well below that of PSF (38.1 dynes/ cm) and polystyrene (42.0 dynes/cm) but is closer to that of polypropylene (30.1 dynes/cm) which is aliphatic and has one pendant methyl group. As pointed out in the above discussion of the advancing contact angles of these polymers, the surface of PPO appears to have more of an aliphatic than an aromatic character. Generally, the lower the surface free energy of the polymer, the less will be its tendency to interact with a polar liquid (such as water) and the larger its contact angle will be. Figs. 11 and 12 show the total surface energy and its polar and dispersive components at the different fluorination treatment conditions: fluorine concentration and treatment time. In Fig. 11 the total surface energy ( yS) initially increases and then decreases as a function of fluorination time with a maximum increase of - 10 dynes/
28 =----a_* @ w" 8
f!
:
Dispersive. -
26:.
0.
12,:.
0 --..._,
-..,O . . ..
_,,.
Igo: :b
s 6-
0 ,_
.o
..
0
‘.
‘.
Polar
4 - :' - .d 2 7.' OQi.'~','.','.""',i.I,_ 0 123 4 5 678
0:
91011
Fluorination time [min]
Fig. 11. The total free surface energy (y.) (m) and its polar ( 0 ) and dispersive ( 0 ) components ( y.P and yt ) as a funo tion of exposure time at a fluorine feed concentration of 0.02% Fz.
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
50 40
,
38 _36 -
,
,
,
,
,
,
,
q /S---m-i__
34 32 -
,
,
,
: Total
,I’
“p
\
\
Sl
-
-
24 Dispersive
-
IO 8 6
--
4
_ I _ ,.I..
2
0
OV~?,‘~“‘~‘~ 0 0.02
0.
. ..O
0
0.04
0.06
0.08
Polar
-
-0
_
0.10
0.12
Fluorine concentration [vol%] Fig. 12. The total free surface energy ( y,) ( n ) and its polar ( 0 ) and dispersive ( 0 ) components (r,P and 7: ) after 2 min of treatment at various fluorine feed concentrations.
cm at about 5 to 7 min. The polar component of the free surface energy (yf ) has a trend similar to that of the total surface energy, with a maximum located at 5 to 7 min of treatment. By contrast, the dispersive component (7,” ) exhibits a very small but steady decrease with increasing fluorination time. Fig. 12 shows the effect of fluorine concentration at a constant treatment time of 2 min. Compared to unfluorinated PPO, there is a significant increase in the total and polar surface energies after 2 min of fluorination at 0.02% F2. At higher fluorine concentrations ys decreases gradually, while 7,” shows no definite trend. On the other hand, the dispersive component (7:) decreases slowly but steadily as a function of increased fluorine concentration. These results show that the polar surface energy contribution is more sensitive to the various treatment conditions than the dispersive contribution. Similarly, the contact angle of water, a polar liquid, is more sensitive to treatment than that of methylene iodide which is non-polar. The XPS results discussed above show that both oxygen and fluorine are added to PPO in the surface region and it would not be unrealistic to assume that this also happens on the upper surface which is analyzed by DCA. Both oxygen and fluorine have higher electronegativities than
the original carbon and hydrogen of the unfluorinated polymer. Substitution of hydrogen by fluorine and oxygen can, therefore, be expected to affect the polar and dispersive nature of the PPO in different ways. Addition of these electronegative species will decrease the ease with which electrons can be displaced by the electric fields of other molecules, i.e., the polarizability [ 381, and will tend to lower the dispersive force interactions between adjacent polymers which, in turn, would be reflected in a lower dispersive contribution to the surface free energy. This is illustrated in Table 5 where the progressively fluorinated series, comprising polyethylene fluoride ) poly (vinyl (PVF), (PE), poly (vinylidene fluoride) (PVDF) and poly (tetrafluoroethylene) (PTFE ), exhibits a steady decrease in 7:’ as fluorine is added to the PE backbone. In Fig. 12, ps also decreases gradually as more polar species are added at higher concentrations and longer treatment times. The polar surface energy contribution ( 7: ) , on the other hand, depends on the magnitude of localized dipole moments in the polymer molecule. The total dipole moment increases when fluorine or oxygen is added to the PPO repeat unit in an unsymmetric fashion causing an unbalanced charge distribution. This is illustrated by reference to the fluorinated PE series in Table 5 where the polarity of the polymer increases from PE to PVF and PVDF, with progressively more unsymmetrically arranged fluorines. When further fluorines are added, they balance the effect of those already present and reduce the polarity, as shown for PTFE. Reference to Table 5 also shows that the polar surface energy contributions of PET, Nylon 6,6 and PMMA, all of which contain carbonyl groups, are considerably larger than those of most fluorine containing polymers, with the exception of PVDF. The addition of oxygen is certainly expected to markedly increase the polarity of a polymer. In Fig. 11 the initial increase in yP up to 5 min of fluorination may be related to the unsymmetric addition of both fluorine and oxygen. The subsequent decrease in r,P could then indicate the balancing of these dipoles and a consequent loss of polarity. Other factors may also contribute to the ob-
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
served effects. Physical and chemical degradation at all but the mildest fluorination conditions may include cross-linking, disruption of the phenyl rings, chain scission, or the loss of surface pendant groups containing fluorine and oxygen. Such processes could produce roughness by etching of the polymer surface or by the creation of defects and cannot be discounted in the light of the large hysteresis effects discussed above. The net result of increased surface roughness is to increase the advancing contact angle [ 36,371 as seen for water in Fig. 9. Thus, as the severity of the fluorination treatment increases, the polymer surface becomes more polar, less polarizable and the surface roughness may be increased. These effects could all contribute to some extent to the observed results. Finally, it should be pointed out that similar trends were found for the fluorination treatment of asymmetric polysulfone membranes [ 3 1.
5. summary 5.1. Nature of the fluorinated region
Surface fluorination treatment of a polymer membrane leads to a fluorinated layer whose depth and chemical nature depend on fluorination conditions, viz. fluorine feed concentration and treatment time. The XPS results show that even at the mildest conditions the chemical composition of this layer is substantially different from that of the unfluorinated polymer, while further increments in fluorination time and fluorine feed concentration did not markedly change the polymer composition. However, DCA analysis of the upper surface region indicated progressive incremental changes in the water contact angle and surface free energy, in response to the same range of treatment conditions. Depth dependent XPS analysis (Fig. 8) showed a lower concentration of oxyfluorinated groups closer to the surface; this effect being more pronounced at longer fluorination times. The overall conceptual physical and chemical model consistent with these results appears to be that the thickness of the fluorinated region de-
51
pends primarily on the duration of the fluorination treatment. Due to the inherently harsh nature of gas phase fluorination, this effect is accompanied by damage or disruption of the polymer, particularly at the outer most surface at higher fluorine concentrations and longer treatment times. This damage is inferred from increased roughness at the surface and a lower concentration of fluorine closer to the surface, an effect which is more pronounced at longer fluorination times. Apart from the thickness of the fluorinated region, the results shown in Figs. 5 to 7 suggest that the fluorine and oxygen concentrations on the PPO repeat unit plateau at a feed concentration of 0.04% F2. This is confirmed by a similar leveling off of the polar surface energy contribution (Fig. 12 ) and was also observed for the surface fluorination of asymmetric polysulfone membranes under similar conditions [ 3 1. Fluorine and oxygen are probably first added at the most accessible sites and successive additions become increasingly difficult. It is also possible that these species are both added and eliminated during fluorination at all but the lowest fluorine concentrations in the feed. Both the DCA and gas transport results indicate changes to the polymer as a result of increasing the fluorine concentration which appears to support this contention. 5.2. Gas transport The effects of fluorination on the gas transport properties of composite PPO membranes were discussed in an accompanying paper [ 5 1. It was shown that increasing either the fluorine feed concentration or the reaction time reduced the permeance (P/I) of all gases tested; the lighter gases (He and HZ) were reduced by a smaller degree than the heavier gases. When the membranes were coated with a layer of poly ( dimethylsiloxane ) ( PDMS ) subsequent to fluorination, the permeance decreased again, but by a much larger factor for Nz and CH4 than for the other gases. These observations are consistent with the notion that the addition of polar fluorine and oxygen groups creates a fluorinated polymer with a low permeability and that longer
52
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
treatment times increase the thickness of the fluorinated region causing a progressive decrease in permeance. This tendency is not evident until the membranes have been coated with PDMS to neutralize the effect of surface defects caused by fluorination damage. These concepts are confirmed when the ideal selectivities of several gas pairs are considered in response to fluorination conditions [ 5 1. It was found that fluorination increased the selectivity of He and Hz relative to N2 or CH4 by a small factor but lowered the selectivity of O2 and CO* relative to Nz or CH4. Surface coating after fluorination, however, yielded substantial gains in the selectivities of all these gas pairs. The highest gains before coating were found at the mildest treatment conditions (0.02% and 1 to 2 min of fluorination), while the best overall selectivities after coating were achieved under more severe fluorination conditions (0.1% for 3 to 5 min). Under mild conditions damage to the polymer appeared to be slight and the benefits of fluorination were, therefore, more apparent. This was also found in a related study of the fluorination of PSF membranes [ 3,6]. After the coating had restored the actual transport properties of the treated membranes, the best results were obtained at harsher treatment conditions and a higher degree of fluorination. The commercial viability of the surface fluorination of membranes formed from glassy polymers, such as PPO and PSF, presents a two-fold optimization problem. On the one hand, the treatment process should maximize the beneficial effects of fluorination and minimize the detrimental effects due to damage. In this regard, the application of a suitable caulking layer (e.g., PDMS) as a post-treatment step is considered essential and the benefit of increasing the selectivity clearly outweighs the small concomitant loss in productivity. On the other hand, even successful fluorination treatment remains subject to the inevitable loss of productivity in exchange for any gain in selectivity. In this regard, the emphasis remains on the production of membranes with the thinnest possible (yet defect-free) skins to enhance productivity. Another approach would be to fluorinate polymers
with a higher permeability, though the gain in selectivity may then be lower.
6. Acknowledgments The authors thank Prof. Paul Ho, Dr. Steve Anderson and John Hall-Pellerin for their assistance with XPS deconvolution. Thanks are also due to Dr. Joel Kampa and James DeYoung for carrying out the fluorination treatment. This work is based in part upon work supported by the Texas Advanced Technology Program under Grant No. 1607, by the Separations Research Program at the University of Texas at Austin, TX, and by the CSIR, Pretoria, South Africa.
7. References [ 1] 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. MembraneSci.,55 (1991) 149-171. [2]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. [ 31 J.D. Le Roux, D.R. Paul, M.F. Arendt, Y. Yuan and I. Cabasso, Modification of asymmetric polysulfone membranes by mild surface fluorination. Part II. Characterization of the fluorinated surface, J. Membrane Sci., in press. [4]L.J. Hayes, Surface energy of fluorinated surfaces, J. Fluorine Chem., 8 ( 1976) 69-88. [ 5] J.D. Le Roux, D.R. Paul, J. Kampa and R.J. Lagow, Surface fluorination of poly ( phenylene oxide) composite membranes. Part I. Transport properties, J. Membrane Sci., 90 (1994) 21-35. [6]J.D. Le Roux, D.R. Paul, J. Kampa and R.J. Lagow, Modification of asymmetric polysulfone membranes by mild surface fluorination. Part I. Transport properties, J. Membrane Sci., in press. [ 71 D. Briggs, Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Heyden and Son, London, 1978. [ 81 D.T. Clark, ESCA applied to polymers, Adv. Polym. Sci., 24(1977)125-188. [9]D.T. Clark and D. Shuttleworth, Plasma polymerization. II. An ESCA investigation of polymers synthesized by excitation of inductively coupled RF plasma in perfluorobenzene and pertbtorocyclohexane, J. Polym. Sci., Polym. Chem. Ed., 18 ( 1980) 27-46.
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 37-53 [ 1OlC.L. Kiplinger, D.F. Persico, R.J. Lagow and D.R. Paul, Gas transport in partially fluorinated low-density polyethylene, J. Appl. Polym. Sci., 3 1 ( 1986) 26 17-2626. [ 111J.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. [ 12]M. Langsam, M. Anand and E.J. Karwacki, Substituted propyne polymers. I. Chemical surface modification of poly [ I-(trimethylsilyl)propyne] for gas separation membranes, Gas Sep. Purif., 2 ( 1988 ) 162- 170. [ 13lH.J. Leary Jr. and D.S. Campbell, ESCA studies of polyimide and modified polyimide surfaces, ACS Symp. Ser., 162 (1981) 419-433. [ 141 M. Millard, J. Bums and H. Sachdev, Mild direct fluorination of polymers studied by X-ray photoelectron spectroscopy, in K.L. Mittal (Ed.), Physicochemical Aspects of Polymer Surfaces, Vol. 2, Plenum Press, New York, 1983, pp. 773-791. [ 15]A. Dilks, X-ray photoelectron spectroscopy for the investigation of polymer surfaces, ACS Symp. Ser., 162 (1981) 293-317. [ 161J.D. Le Roux and D.R. Paul, Preparation of composite membranes by a spin coating process, J. Membrane Sci., 74(1992)233-252. [ 171 D.T. Clark and H.R. Thomas, Application of ESCA to polymer chemistry. XVII. Systematic investigation of the core levels of simple homopolymers, J. Polym. Sci., Polym. Chem. Ed., 16 (1978) 791-820. [ 18lD.T. Clark, D. Kilcast, D.B. Adams and W.K.R. Musgrave, An ESCA study of the molecular core binding energies of the fluorobenzenes, J. Electron Spectrosc., 1 (1972173) 227-250. [ 19lD.T. Clark, R.D. Chambers and D.B. Adams, An ESCA study of the fluoride-ion-induced trimerization product from perfhiorocyclobutene, J. Chem. Sot. Perkin Trans. 1, (1975) 647-650. [ 201 D.T. Clark, W.J. Feast, D. Kilcast, D.B. Adams and W.E. Preston, Charge distributions in dodecafluorotricyclo[ 5,2,2,0z6]undeca-2,5,8-triene and tetradecafluorotricyclo[6,2,2,0z’]dodeca-2,6,9-triene as determined by ESCA, J. Fluorine Chem., 2 ( 1972/73) 199-202. [ 2 1 ]D.T. Clark and H.R. Thomas, Applications of ESCA to polymer chemistry. X. Core and valence energy levels of a series of polyacrylates, J. Polym. Sci., Polym. Chem. Ed., 14 (1976) 1671-1700. [ 22 ] D.T. Clark and A. Dilks, ESCA applied to polymers. XV. RF glow-discharge modification of polymers, studied by means of ESCA in terms of a direct and radiative en-
53
ergy-transfer model, J. Polym. Sci., Polym. Chem. Ed., 15(1977)2321-2345. [23]D.T. Clark, B.J. Cromarty and A. Dilks, A theoretical investigation of molecular core binding and relaxation energies in a series of oxygen-containing organic molecules of interest in the study of surface oxidation of polymers, J. Polym. Sci., Polym. Chem. Ed., 16 ( 1978) 3173-3184. [ 24lA.G. Shard and J.P.S. Badyal, Surface oxidation of polyethylene, polystyrene, and PEEK the synthon approach, Macromolecules, 25 (1992) 2053-2054. [ 25 ] D.T. Clark, D. Kilcast and W.K.R. Musgrave, Molecular core binding energies for some monosubstituted benzenes, as determined by X-ray photoelectron spectroscopy, J. Chem. Sot. Chem. Commun., ( 197 1) 5 16518. [26]D.T. Clark, D. Kilcast, D.B. Adams and 1. Scanlan, On the core and valence energy levels of fluorobenzene, J. Electron Spectrosc., 1 (1972/73) 153-159. [ 271 J. Domingue, Probing the chemistry of the solid/liquid interface, Am. Lab., (October 1990) 50-55. [ 281 J. Hiipken and M. Moller, Low surface energy polystyrene, Macromolecules, 25 (1992) 1461-1467. [ 29lD.K. Owens and R.C. Wendt, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci., 13 ( 1969) 1741-1747. [ 30lE.N. Dalal, Calculation of solid surface tensions, Langmuir, 3 (1987) 1009-1015. [ 3 1 ] S. Wu, Calculations of interfacial tension in polymer systems, J. Polym. Sci., C34 ( 1971) 19-30. [ 321 M. Anand, R.E. Cohen and R.F. Baddour, Surface characterization of plasma-fluorinated polymers, ACS Symp. Ser., 162 (1981) 353-370. [33]M. Langsam and E.J. Karwacki, Air Products and Chemicals, Inc., personal communication, 1993. 341s. Wu, Polymer Interface and Adhesion, Marcel Dekker, New York, 1982. 35lD.W. Dwight and W.M. Riggs, Fluoropolymer surface studies, J. Colloid Interface Sci., 47 (3) ( 1974) 650-660. 36lR.H. Dettre and R.E. Johnson Jr., Contact angle hysteresis. II. Contact angle measurements on rough surfaces, Adv. Chem. Ser., 43 (1964) 135-144. 37lR.E. Johnson Jr. and R.H. Dettre, Contact angle hysteresis. I. Study of an idealized rough surface, Adv. Chem. Ser., 43 (1964) 112-135. [ 38lJ.M. Prausnitz, R.N. Lichtenthaler and E. Gomes de Azevedo, Molecular Thermodynamics of Fluid Phase Equilibria, Prentice-Hall, Englewood-Cliffs, NJ, 1986.
journal of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 90 ( 1994) 37-53
Surface fluorination of poly (phenylene oxide) composite membranes Part II. Characterization of the fluorinated layer J.D. Le Rouxa, D.R. Paula,*, M.F. Arendtb, Y. Yuan”, I. Cabassoc “Department of Chemical Engineering, Centerfor Polymer Research, The University of Texas, Austin, TX 78712, USA bDepartment of Chemistry The University of Texas, Austin, TX 78712, USA ‘Polymer Research Institute, State University ofNew York, Syracuse, NY 13210, USA
(Received August 27, 1993; accepted in revised form December 20, 1993)
Abstract Composite membranes, comprising a selective layer of poly( 2,6-dimethyl-1,Cphenylene oxide) on a microporous ceramic support, were surface fluorinated under mild conditions. The treatment parameters, fluorine feed concentration and treatment time, were each varied independently while the other parameter was kept constant. The fluorinated region was characterized using dynamic contact angle analysis and X-ray photoelectron spectroscopy (XPS). The fluorine to carbon ratio increased as a function of treatment time, but showed little variation when the feed concentration was varied. A relatively high oxygen to carbon ratio was observed which showed little change over the range of fluorination conditions. Angle-dependent XPS showed the concentration of fluorine species to be higher at greater depths below the surface. Pluorination times of 5 to 7 min minimized the water contact angle and maximized the total and polar surface free energies. Water contact angles decreased after fluorination but showed no definite trend as a function of feed concentration. The composition of the fluorinated region is discussed in relation to the gas transport properties of membranes fluorinated under similar conditions (Part I of this study). Key words: Poly(2,6-dimethyl-l+phenylene oxide); Gas separation; Composite membranes; Fluorination; Surface modification; X-ray photoelelectron spectroscopy; Dynamic contact angle analysis
1. Introduction The gas transport properties of a gas separation membrane can be modified by exposing its surface to low concentrations of elemental fluorine in an inert diluent gas, such as helium or nitrogen. The fluorine in the treatment feed gas, and also oxygen which is often present in the re*Corresponding author. 0376-7388/94/$07.00
action environment, react with the polymer at the surface to produce an oxyfluorinated layer [ l41. The chemical nature and depth of the newly formed layer depend on the treatment conditions: predominantly the fluorine concentration at the polymer surface and the exposure time. This work involves the fluorination of composite membranes comprising a selective skin of poly (2,6-dimethyl- 1,Cphenylene oxide ) (PPO) supported on a substrate which is effectively in-
0 1994 Elsevier Science B.V. All rights reserved
SSDI0376-7388(94)00006-K
38
J.D. Le Roux et al. /Journal ofMembrane Science 90 (I 994) 37-53
ert towards fluorination. Fluorination was performed in a well-mixed reactor with a volume small enough that the composition of the reaction gas at the membrane surface approaches that in the feed [ $61. This permits the fluorination time and fluorine concentration at the surface to be varied independently. The structure of the PPO repeat unit is shown in Fig. 1, together with the atomic ratios of its three constituent elements. In the initial stages of fluorination, hydrogens are replaced by the more bulky and electron-rich fluorine atoms. The relatively simple structure of PPO contains only two different potential carbon-hydrogen sites for such a substitution: at the 3,5-positions on the aromatic ring and on the methyl pendant groups. Other fluorination or oxidation reactions could involve a loss of aromaticity, aromatic ring opening, chain scission at the ether linkages in the polymer backbone and cross-linking reactions. Unfortunately, the highly exothermic nature of fluorination complicates the control of these reactions and the interpretation of analytical results. Part I of this study [ 51 described the gas transport properties of the fluorinated composite PPO membranes. The objective of this paper (Part II ) is to investigate the nature of the fluorinated layer and to relate this information to the observed transport properties. To this end two complementary analytical techniques were employed: Xray photoelectron spectroscopy (XPS) and dynamic contact angle analysis (DCA ) . Generally, DCA yields information on the uppermost surface of the membrane while XPS probes the immediate subsurface region to a depth of - 60-90 A in polymers [ 2,7,8 1. Both XPS and DCA have been applied for the analysis of fluorinated polymers, separately and in combination, to complement each other [2,4,8-l 51. The background Number uf atoms:
2.1. Membrane formation and fluorination treatment
=X
1
Alumic elemental ratio: CHJ
2. Experimental
Hydrogen = 8 Oxygen =
0 :A w
CdiXl
and theory relevant to these two analytical methods are discussed elsewhere [ 1,3]. In the gas transport study (Part I) [ 5 ] composite membranes were formed by spin coating a selective layer of PPO onto a microporous ceramic (aluminum oxide) ultrafiltration membrane with a very low resistance to gas permeation [ 16 1. For analysis of the fluorinated layer (Part II), composite laminates were formed by spin coating a thin layer of PPO onto a solid aluminum metal support. In contrast with the microporous ceramic supports, the aluminum support is not brittle and this facilitated sample preparation for both of the analytical techniques employed.
”
WC = I.0 OK
= 0.125
Fig. 1. Chemical structure of poly ( phenylene oxide )
Composite laminates for fluorination were prepared by spin coating a dilute solution of PPO onto circular disks of rigid aluminum foil. The spin-coating solution consisted of PPO dissolved in reagent grade 1,1,2-trichloroethane at a concentration of 2.0% by weight. The PPO used for this purpose (PPO-34 from General Electric Company) had a weight average molecular weight of 32,900 g/mol and a number average molecular weight of 17,900 g/mol, as determined by light scattering. The intrinsic viscosity in chloroform (25°C) was 0.34 dl/g. A higher molecular weight PPO was used to prepare membranes for transport measurements [ 5 1. However, due to the limited supply of that material, this more standard PPO was employed here where skin integrity is not such a critical issue. A photoresist spinner (Series EClOlD) from Headway Research Inc. (Garland, TX) was used to apply the PPO solution. The aluminum substrate 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 aluminum substrate with - 5 ml of polymer solution the spinner was immediately activated and allowed to rotate at 800 rpm for 60 s; maximum speed was
J.D. L.e Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
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 oC for at least 16 h to ensure complete removal of solvent. Thermal analysis of delaminated thick PPO films formed under similar conditions showed no evidence of residual solvent. The circular composite laminates (4.7 cm in diameter) were the same size as the composite membranes used in Part I [ 5 1. Although PPO has a tendency to crystallize during quiescent solution casting, the polymer films were clear and differential scanning calorimetry (DSC) confirmed the absence of any crystallinity. 2.2. X-ray photoelectron spectroscopy (XPS) The XPS spectra were obtained with a VG 1000 ESCA spectrometer using MgKa exciting radiation ( 1253.6 eV). Typically, the X-ray gun was operated at 15 kV and 22 mA with a sample chamber vacuum of less than 5 x 10m9Torr. Pure graphitic carbon at a full width at half-maximum ( fwhm) of 1.4 ? 0.1 eV was used for calibration. All samples showed charging effects and the binding energies were referenced to the hydrocarbon C,, core-level peak at 285 eV. Previous studies have shown that this corresponds closely to the shift of gold-sputtered samples referenced to the AQ,,~ level at 84 eV. Except where indicated, XPS spectra were recorded at an electron emission angle (0) of 90” to give the maximum sampling depth of N 60-90 A. All samples were mounted onto the sample probe with doublesided adhesive tape. The peak for each of the sampled elements C, 0 and F consisted of the average value of five 30-s scans over the appropriate binding energy range at a pass energy of 50 eV. Some analyses were duplicated at a pass energy of 20 eV and the peak shapes were essentially similar. Sensitivity factors for the quantitative analysis were determined from homogeneous samples of poly (tetrafluoroethylene ) (PTFE ) and poly (vinylidene fluoride) (PVDF) for fluorine and from solution cast PPO for oxygen. The factors were: 0.17 for carbon, 0.38 for oxygen and 1.Ofor fluorine.
39
Table 1 contains a listing of relevant binding energy shifts with respect to the C1, binding energy, as reported in the literature for fluorinated and oxidized hydrocarbon species. These data were derived from XPS studies of both pure monomeric compounds and polymers, only two of which refer specifically to PPO in its unfluorinated and oxidized forms [ 15,171. Generally, the addition of each fluorine to a carbon produces an additive primary shift of N 2.7-3.0 eV and each oxygen attached to a carbon causes a shift of N 1.2-1.5 eV. However, the interpretation of spectra of polymers with aromatic and aliphatic carbons is complicated by the relatively large secondary shifts when fluorine and oxygen are added to neighboring carbons [ 15,18,19,2326 ]. Since the primary binding energy shift due to the formation of one fluorine bond ( N 3.0 eV) is virtually the same as that observed for two oxygen bonds, the monofluorinated and carbonyl carbons are not readily distinguished by deconvolution of the C1, peak. From an XPS study of monomeric benzene derivatives [ 251 the primary and secondary binding energy shifts caused by addition of substituents, such as -F, -CH3 and -CF,, to carbons of the aromatic ring appear to differ significantly from those resulting from addition to aliphatic carbons. For instance, in monomeric benzotrifluoride the shift for aromatic C-CF3 was reported to be 1.9 eV [ 25 ] compared to values of N 1.5 eV for both ether bonded carbons and C-CF, of secondary aliphatic carbons (Table 1). This brief discussion serves merely to emphasize that the interpretation of the spectra of oxyfluorinated PPO should be approached with caution and that certain approximations are necessarily involved. Deconvolution of the Cl, peaks was performed using a non-commercial computer routine. Considering the range of potential oxyfluorination reaction products of the PPO repeat unit and the numerous possible binding energy allocations that can be made (cf. Table 1 ), an exhaustive deconvolution was not attempted. Instead, based on the anticipated fluorinated and oxidized species, the Cl, spectra were resolved into only three widely spaced and fairly broad component peaks. The first of these represents
40
J. D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
Table 1 Binding energy shifts relative to the Cr. peak at 285 eV Representative functional species
Binding energy shift (eV)
Comment
Ref.
C-C/C-H
0.0
hydrocarbon
0-C% C-CF,
1.9
aromatic C-CFs
1.5kO.4
aliphatic C-CF.
3.OkO.4
general
reference peak
2.8
18 9, 17 9 18 18
GCF
2.8-4.5 1.5-1.9 2.6-2.9
cyclic, non-aromaticb cyclic, non-aromatic’
9, 19, 20 9, 19, 20
Cx =CFz
6.OkO.4 4.1-4.5
general cyclic, non-aromaticb
9,17 9, 19, 20
8.4kO.7
general
17 9,20
0-o.Q
17
Em-
8.8 C-O
1.5
c=o/o-c-o o-c=0
2.9 4.0
n--K*
7.0
17,21,22 17,21,22 aromatic shake-up satellite
17
“Ortho-,meta- and para-substitued benzene. bUnsaturatedportion of a cyclic, non-aromatic ring system. “Saturatedpart of a cyclic, non-aromatic ring system.
the binding energies of the unfluorinated carbons of the PPO unit, including the secondary shifts as discussed above. The other two peaks were shifted by approximately 3 and 5 eV, respectively, and account for fluorinated and oxidized carbon species. The binding energy of hydrocarbon was fixed at 285 eV and a ratio of 30% Lorentzian:70% Gaussian was assumed for the peak shapes. 2.3. Dynamic contact angle analysis (DCA) Advancing and receding contact angles for water and methylene iodide ( CH212 ) on fluorinated and unfluorinated PSF membranes were determined by the Wilhelmy plate technique [ 27,28 ] using a Cahn Dynamic Contact Angle Analyzer (DCA Model 322). The analyzer was
thermostated at 20’ C with a glass slide having a perimeter of 48.3 mm. In principle, a solid sample is progressively immersed and then withdrawn from a well-defined probe liquid while continuously recording its weight change as a function of immersion depth. Sample preparation involved cutting and securing two identically shaped portions of a PPO sample back-to-back to create a rectangular sample fluorinated on both surfaces. The fluorinated surface was washed several times with distilled deionized water and dried before measurements. This sample was fixed statically to the electrobalance of the analyzer and a computer-controlled platform supporting a beaker of the probe liquid was then scanned upwards at a constant rate to gradually immerse the sample. The tangent of the meniscus formed at the liquid-solid-
41
J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
vapor interface defines the dynamic contact angle as the interface passes over the solid surface. A plot of the overall force on the sample as a function of immersion depth yields advancing and receding contact angles (0, and 0,) from the parallel immersion and withdrawal traces [ 3,27,28 1. Average values for 0, and 0, are evaluated by extrapolation of the slopes to zero immersion depth using a linear regression, and each reported contact angle is the average of at least three measurements. To determine the accuracy of the measurements, seven DCA samples were prepared from the same fluorinated membrane and measured independently; several such tests yielded a maximum standard deviation of 5%. This small variation also confirmed that the feed gas was well mixed in the reaction chamber leading to an even treatment over the entire membrane surface. Following Owens and Wendt [ 29 1, the average advancing contact angles (0,) measured for water and methylene iodide were used to calculate the polar and dispersive components (7: and y,”) of the solid surface free energy by solution of the following equation l+c0sB=2J$$)+z&($)
10 min
5 min
3 min
2 min
1 min
(1)
where ylyis the vapor-liquid surface tension, and the polar and dispersive components of the liquid surface free energy (rf and yP ) are known. The surface tensions of water and CH212 have been reported as 72.8 and 50.8 dynes/cm, respectively; the dispersive and polar components for water are 22.5 and 50.3 dynes/cm and for CHzIz they are 48.5 and 2.3 dynes/cm [ 30,3 11. The theoretical and practical aspects of this method are discussed in greater detail elsewhere 131. 3. Analysis by X-ray photoelectron spectroscopy (xps) 3.1. Variable treatment time Fig. 2 shows the shape of the C1, peak of PPO as a function of fluorination time when a con-
Fig. 2. The C,, peak of the XPS spectra of PPO after treatment with 0.02% F2 for various fluorine exposure times.
stant fluorine feed concentration of 0.02% is used. All samples were corrected for charging and the peaks are aligned according to the position of the hydrocarbon peak at a binding energy of 28 5 eV, which coincides with the slope of the peak on the low binding energy side of each composite peak. The unjluorinated PPO peak was resolved into two component peaks with a peak area ratio of 3: 1. These peaks represent the contributions of the two ether-bonded carbons and the other six ahphatic and aromatic carbons of each repeat unit [ 15 1. Actually the situation is more complex. Reference to Fig. 1 shows that each PPO repeat unit has four pairs of carbons which all have different electronic environments. An XPS study of pure toluene [ 25 ] indicates that three
42
J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 3 7-53
of these pairs may have different binding energies. Clark and Thomas [ 17 ] report three component peaks for PPO which suggests that the unsubstituted aromatic carbons and the methyl carbons can be grouped together. For the analysis of fluorinated PPO the contributions of all the unfluorinated carbon species were grouped together, due to the multiple secondary shifts which may result from different configurations of fluorine and oxygen added randomly to the polymer. After fluorination the shape of the C,, peak changes considerably. Compared to unfluorinated PPO, a distinct new component peak evolves at a binding energy shift of - 3 eV. As fluorination time increases the new peak becomes increasingly prominent and, at the longest time of 10 min, is the dominant feature. Qualitatively, this feature is associated with the formation of monofluorinated carbon (C-F) and carbonyl (C=O) moieties. The widened base at the high binding energy side of the composite peak suggests the presence of a second component peak which can be assigned to difluorinated carbon (CF,) and to more highly oxidized carbon. According to Table 1, the C’F2 peak is expected at a shift of between 4.3 and 6 eV, depending on whether the carbon is single or double bonded, i.e., whether it is in an aliphatic or aromatic environment. In addition, the carbon shift for the carboxylic acid structure (0-C=O) is expected at -4 eV. The feature in this binding energy region apparently does not change substantially as a function of fluorination time. There is clearly no peak at a shift of around 8.4 eV which would indicate the presence of trifluorocarbon species (CF,) and evidently the pendant methyl groups are not perfluorinated under these treatment conditions. A similar result was reported for the fluorination of PSF membranes which were fluorinated under similar conditions [ 3 1. On the other hand, the formation of both CF2 and CF3 species were reported by Mohr et al. for the fluorination of poly (4-methyl- 1-pentene ) [ 2 ] and by Langsam et al. for poly [ 1- (trimethylsilyl ) - 1-propyne ] [ 12 1. Their polymers contained only aliphatic carbon species, and fluorination was carried out at higher concentra-
tions and in quite different fluorination reactor environments. To supplement the qualitative observations based on the peak shapes in Fig. 2, Fig. 3 shows the results of the deconvolution of the C,, peaks into three component peaks. The solid lines in the figures are intended only to emphasize trends in the data. The relative binding energy shifts and ultimate fwhm of the peaks are given in Table 2. Judging by their widths, each peak encompasses a range of minor binding energy shifts due to the various possible electronic environments. The unfluorinated carbon species in Fig. 3 are designated by open symbols while fluorinated or oxidized species formed during this process are represented by the solid symbols shown in the legend. As observed in Fig. 2, the area contribution of the unfluorinated carbon peak ( [a] ) decreases with treatment time, while the contributions from the other two peaks ( [b] and [c] ) gradually increase. These latter peaks represent the mono- and difluorinated carbons and carbony1 species. It should be pointed out that the deconvolution analysis locates the CF2 peak ( [c ] ) at a binding energy shift of - 4.8 + 0.2 eV. Assuming that the PPO repeat unit remains intact, this implies one difluorinated carbon per loop
,
,
,
,
,
,
.
,
,
,
,
,
CFaWor C=O
0 20
. . L;
-
OY’ 0
” 12
” 3
4
5
1’ 6
” 7
8
9
’
I
IO
II
Fluorination time [min]
Fig. 3. The concentration of various carbon species from deconvolution of the C,, peak of PPO before and after treatment with 0.02% F2 for various treatment times. The peaks are defined in Table 2. Carbon species to which either fluorine or oxygen have been added during treatment are designated by open symbols (0 ), and unmodified carbons are represented by solid symbols ( 0, + ) .
43
J.D. Le Roux et al. /Journal ofMembrane Science 90 (1994) 37-53 Table 2 C,, peak deconvolution:
constituent
Peak
Designated species
Binding energy shift (eV)’
fwhmb
[cl
C-C, C-H, C-O, C-CF, CF, c=o CF;
o-o.4 2.69 I? 0.06 4.66 f 0.04
2.28 f 0.07= 2.28 + 0.07 2.28 f 0.07
peak designations
‘Relative to the Cr. peak at 285 eV. bFull width at half-maximum. “Standard deviation. dCould include carboxylic acids (0 = C-O) at a shift of _ 4.0 eV (see text ) .
repeat unit which may be located either at a site on the carbon ring (shifted by 4.1-4.5 eV) or on a pendant aliphatic carbon (shifted by 6 eV). Clearly, if the aromatic rings or the polymer backbone are disrupted during fluorination, many other possibilities present themselves and qualitative analysis is complicated considerably. In summary, increased treatment time results in the progressive addition of fluorine and oxygen atoms to create increasingly polar structures. On the other hand, longer exposure to the reaction gas also increases the likelihood of damage and other uncontrolled reactions at the polymer surface. The fact that Figs. 2 and 3 show only a smaII increase in peak [c] after 5 min of treatment could indicate either that no further addition takes place or that the addition of the CF and C = 0 species is balanced by a simultaneous loss of these species. The issue of damage or disruption of the polymer structure will be considered again below. Fig. 4 shows the F/C and O/C atomic ratios as a function of fluorination time. These ratios were obtained from the integrated areas of the fluorine, oxygen and carbon peaks, taking into account the atomic sensitivity factors of these elements. The solid horizontal line in the lower graph indicates the O/C ratio of 0.125 for PPO before fluorination (cf. Fig. 1). The F/C ratio increases with treatment time with a trend similar to that of peaks [b] and [c] in Fig. 3. The O/ C ratio increases by a factor of N 2.5 due to fluorination. This ratio remains largely independent of treatment time at an average value of 0.32 and the slight decline at 10 min is probably not significant. As reported in other studies [ 1,3,4,12 1,
0.8 0.7 .::
0.6
d
0.5
‘i
0.4
4
0.3
s
0.2 0.1
::
j * .o
0.3
E s
0.2
v 0
, 0.1
O/C ratio [or unmated PF’O
-
0
0
I I
-
I,,,,,,,,, 2
3
4
5
6
7
8
9
10
11
12
Fluorination time [min] Fig. 4. Fluorine to carbon and oxygen to carbon atomic ratios of PPO after various exposure times at a fluorine feed concentration of 0.02% F2. The solid line in the lower graph shows the theoretical O/C ratio for untreated PPO.
these results confirm that fluorination is accompanied by oxidation as evidenced by the addition of a constant amount of oxygen apparently independent of the fluorination conditions.
44
J.D. Le Roux et al. /Journal ofMembrane Science 90 (1994) 37-53
3.2. Variablefluorine concentration
To investigate the effect of various fluorine feed gas concentrations on the composition of the fluorinated layer, a constant treatment time of 2 min was employed. Fig. 5 shows the shape of the C,, peak of PPO as a function of fluorine feed concentration during treatment for a constant treatment time of 2 min. As discussed for Fig. 2, the feature which appears at a binding energy shift of N 3 eV is associated with the formation of monofluorinated (C-F) and carbonyl (C= 0) carbon species. A CF2 peak is present at a shift of N 5 to 6 eV, and again no trifluoromethyl ( CF3 ) groups are apparent. There appears to be
0.04%
F2
0.02%
F2
no significant qualitative change in the peak shape at fluorine concentrations exceeding 0.04%. Fig. 6 is a graphical representation of the deconvolution of the Ci, peak into its constituent peaks as defined in Table 2. The contribution of unfluorinated carbons (peak [a] ) decreases considerably due to fluorination but shows little variation at different concentrations. The effectively constant contribution of peak [c] indicates that only one CF2 moiety is formed per repeat unit, irrespective of the fluorine feed concentration. The contribution from peak [b 1, which represents both CF and carbonyl species, is increased by fluorination treatment but remains essentially independent of fluorine concentration. Thus, the results of the deconvolution confirm the qualitative effects observed in Fig. 2. In general, it appears that about half of the carbon atoms in each repeat unit react either with fluorine or with oxygen. Fig. 1 shows that the unreacted carbons could be the four carbons without any hydrogens which can only be fluorinated by loss of aromaticity, pendant groups or chain connectivity. Fig. 7 shows the F/C and O/C atomic ratios at different concentrations of fluorine in the feed. Both atomic ratios increase markedly due to fluorination with only small variations at the dif-
I
0
0.02
I
I
0.04
,
,
0.06
Fluorine concentration
Fig. 5. The C,, peak of the XPS spectra of PPO after 2 min of treatment at various fluorine feed concentrations.
,
,
0.08
,
,
0.1
0.12
(~0176 F, in He)
Fig. 6. The concentration of the carbon species after deconvolution of the C,, peak of PPO before and after 2 min of treatment at various fluorine feed concentrations. The peaks and symbols are the same as in Fig. 3.
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
0.60L 0.50
,
I
,
,
,
,
,
,
,
,
,
,
,
,
,
,
.
-
0.60 0.50
,
,
,
,
,
,
1
O/C ratio for unueavzd PPO
0
0.02 Fluorine
0.04
0.06
0.08
concentration
[vol%
0.1
0.12
F,/He]
Fig. 7. Fluorine/carbon and oxygen/carbon atomic ratios of PPO after 2 min of treatment at various fluorine feed concentrations. The solid line in the lower graph shows the theoretical O/C ratio for untreated PPO.
ferent fluorine concentrations. The F/C ratio lies between 0.37 and 0.48 which represents between 3 and 4 fluorines per PPO repeat unit. The apparent maximum F/C at 0.06% F2 can also be identified in Figs. 5 and 6. If this maximum is experimentally significant, it may indicate the highest fluorine concentration the polymer can tolerate before the addition of fluorine is balanced by its removal due to damage or disruption of the polymer structure. In the lower graph the O/C ratio increases from the untluorinated experimental value of 0.14 to around 0.36 after fluorination, which represents the addition, on average, of two oxygens to each repeat group. As in Fig. 4, the addition of oxygen appears to be effectively independent of the reaction conditions. In an XPS investigation of the natural weath-
45
ering of PPO by surface oxidation, Dilks [ 15 ] concluded that the pendant methyl groups are converted to carboxylic acids with little change to other parts of the polymer. In an investigation of fluorinated PTMSP by surface sensitive infrared techniques, Langsam et al. [ 121 mention the presence of a strong carboxyl band and report O/C ratios of 0.12 to 0.17. On the assumption that the aromatic ring and the polymer backbone remain intact during fluorination, the above evidence seems to indicate that the oxidation which accompanies fluorination may occur predominantly at the pendant methyl groups. Thus, an unfluorinated methyl group is converted progressively into more highly oxidized forms, such as alcohols, aldehydes and eventually carboxylic acid, the formation of peroxides is also possible. At the same time, the addition of fluorine probably occurs by substitution of hydrogens both at the methyl groups and on the two exposed aromatic carbon sites. The presence of oxygen after fluorination treatment has been reported by several independent researchers [ 1,3,4, lo- 12,321. Following a careful investigation of this issue, Mohr et al. [ 1 ] concluded that the oxygen is introduced as an impurity in the gaseous feed. Personal communications with the supplier of the fluorine feed gas [ 33 ] indicated that technical grade fluorine gas was specified to have a purity of > 97% and that the contaminants are likely to include mostly oxygen and nitrogen. Two other sources of oxygen must also be considered. The first is atmospheric oxygen and/or moisture which may be present in the sample before fluorination and is not completely removed by purging the reactor with pure helium prior to treatment. Another possibility is that exposure to the atmosphere after fluorination allows oxygen to react with reactive radicals remaining at the polymer surface. To some degree all of these sources of oxygen may contribute to the high O/C ratios observed after fluorination. 3.3. Angle-dependentjilm properties XPS spectra can be recorded with the detector positioned at different take-off or electron emis-
J.D. Le Roux et al. /Journal of Membrane Science 90 (I 994) 37-53
46
[FJ = 0.02~01% Time = 10 min.
[FJ = 0.02~01% Time = 2 min.
[FJ = 0.08~01% Time = 2 min.
Take-d allgl.C:
Take-otf angle.
90”
90”
55”
400
25”
15O
I 280
I
1 285
”
1 290
I 295
I’
I
280
285
”
(
I
12
I
290
I
I 295
I”” 280
11 285
c
‘1’ 290
“I 295
Binding Energy [eV] Fig. 8. The C,, spectra of PSF at electron take-off angles of 1S”, 25”, 40”, 55” and 90” for three sets of fluorination
sion angles, while its angle with respect to the Xray source remains constant. This has the effect of varying the probing depth between a maximum of -60 8, below the sample surface at a take-off angle of 90” and a depth of - 15 A at an angle of 15’. Fig. 8 shows the C,, spectra at takeoff angles of 15”, 25”, 40”, 55” and 90” recorded at fluorination conditions of 0.02% F2 for 2 and 10 min, and at 0.08% F2 for 2 min. These take-off angles were used to estimate the variation in the composition of the fluorinated film at different depths below the surface. Qualitatively, in all three series there is a decrease in the height of the shoulder at 3 eV, which represents the C-F and C=O feature as a function of decreasing probe depth or take-off angle. The extent of decrease appears to be less in the two series fluorinated for 2 min (at 0.02% and 0.08% F,) compared to the series for 10 min. These observations show that the composition of the fluorinated layer is not uniform, but has a lower fluorine and oxygen content nearer the surface than at a greater depth. Since the phe-
conditions.
nomenon appears to be time related, it may result from damage to the polymer structure in the near surface region which has been exposed to the fluorinating agent for a longer time. Thus, 10 min of exposure should have a more severe effect than shorter times. Similar observations were reported in a previous study [ 31 for the depthof fluorinated PSF dependent analysis membranes.
4. Dynamic contact angle analysis (DCA) 4. I. Water and methylene iodide contact angles The contact angles of polar and non-polar liquids, and the surface free energies calculated from them, provide information on the surface constitution of a fluorinated polymer composite to a depth of - 10 %i. Surface analysis, therefore, complements XPS which probes the region to - 60-90 A, depending on the escape depth of the polymer. Tables 3 and 4 and Figs. 9 and 10 show
J. D. L.e Roux et al. /Journal
of Membrane Science 90 (I 994) 3 7-53
Table 3 Advancing and receding contact angles for PPO fluorinated at different fluorine concentrations Fluorination treatment Time’
Water
[Fib
47
for a constant time
Methylene iodide
Advancing
Receding
Hysteresis
Advancing
Receding
Hysteresis
68.7 15.1 6.6 F c c c
30.3 71.2 70.6 c E c c
58.8 42.3 52.4 59.3 54.1 56.1 60.5
41.8 22.3 31.0 33.3 18.8 34.0 14.5
17.0 20.0 21.4 26.0 35.3 22.7 46.0
0
0
99.0
2
0.02
2 2 2 2
0.04 0.06 0.08 0.10
92.3 77.2 88.6 80.5 80.5 86.8
“Treatment time (min ) . bFluorine concentration in the feed (~01% F2 in He). “Could not be reliably measured. Table 4 Advancing and receding contact angles for PPO fluorinated for different times at a constant fluorine concentration Fluorination treatment Time’
Water
Methylene iodide
[Flb
Advancing
Receding
Hysteresis
Advancing
Receding
Hysteresis
0
0
99.0
1 2
0.02 0.02
3 5
0.02 0.02
68.7 14.8 15.1 6.6 c
30.3 63.6 17.2 70.6 F
c
c
12.4
57.4
7
0.02 0.02 0.02
88.4 92.3 71.2 76.4 61.8 69.8 67.8 72.1 83.6
58.8 58.6 42.3 52.4 51.3 48.7 51.2 52.2 54.4 54.3
41.8 35.1 22.3 31.0 22.3 18.3 23.9 15.8 25.6 14.6
17.0 23.5 20.0 21.4 30.0 30.4 27.3 36.4 28.8 39.7
8 10
E
c
E
c
‘Treatment time (min). bFluorine concentration in the feed (~01% F2 in He). “Could not be reliably measured.
the advancing and receding contact angles (6, and 0,) for water and methylene iodide (CHJ,) on the PPO composites under different treatment conditions. Table 5 compares the advancing contact angles for these two liquids on PPO with those reported for a number of polymers which have aromatic, aliphatic and polar moieties. The advancing contact angles for water and CH212 on PPO are considerably higher than for polysulfone (PSF) and polystyrene, and closer to that of polyethylene. This could be due to the exposed pendant methyl groups of PPO which
tend to mask the contribution of the aromatic ring in the backbone and impart a predominantly aliphatic character to the PPO; conversely the aromatic features of PSF and polystyrene are more exposed. Fig. 9 shows the advancing contact angles ( 19,) of water and CHzIz on PPO as a function of treatment time for a constant fluorine concentration of 0.02%. The 6, for water decreases from 99” before fluorination to a minimum of 68” at 5 and 7 min and then increases at longer fluorination times. The trend for the non-polar liquid
48
J.D. Le Roux et al. /Journal ofMembrane Science 90 (1994) 37-53
ditions, in this case 0.02% Fz and treatment times of 5 and 7 min. On the other hand, the interaction with the non-polar liquid CHzIz is hardly influenced by the treatment conditions employed here. The possible physical and chemical origin of these effects will be considered below in conjunction with the calculated surface free energies. 4.2. Contact angle hysteresis 012345678
9
10
11
Fluorination time [mm] Fig. 9. The advancing contact angles for water (0 ) and methylene iodide (CH&) ( 0 ) as a function of fluorine exposure time at a fluorine feed concentration of 0.02% F2.
100
2
,
,
80
,
. ,
,
,
,
,
,
,
,
.
f
01 0
0.02
0.04
0.06
0.08
0.10
0.12
Fluorine concentration [vol% F,/He] Fig. 10. The advancing contact angles for water ( 0 ) and methylene iodide (CH&) ( q ) after 2 min of treatment at various fluorine feed concentrations.
CHzIz is similar, though 0, varies only by a few degrees at the most. Fig. 10 relates the advancing contact angles for water and CH212 to the fluorine feed concentration at a constant treatment time of 2 min. Here, the #, for water generally decreases due to fluorination but apparently shows no significant trend as a function of concentration. The 0, for CH21z appears to be virtually unaffected by the fluorination treatment. These results indicate that the wettability or adhesion of the polymer surface for water, a polar (hydrogen-bonding) liquid, can be maximized by selecting the proper fluorination con-
Hysteresis, the difference between the advancing and receding contact angles, could be due to a variety of surface imperfections such as macroscopic roughness, surface heterogeneity and sorption of the probe liquid [ 35-371. For those cases where 0, could be measured with confidence, the hysteresis of the water and CH212 contact angles for PPO under different fluorination conditions is shown in Tables 3 and 4. As observed for PSF previously [ 3 1, both liquids exhibit considerable hysteresis for both the unfluorinated and fluorinated PPO samples. Table 3 indicates a general increase in hysteresis for the CH212 contact angle as a function of treatment time: hysteresis increases from 17 ’ for unfluorinated PPO to - 40” after 10 min of fluorination. This effect is due mostly to a decrease in 19,,since the advancing contact angle shows little variation. A similar trend of increasing hysteresis as a function of increasing fluorine concentration is apparent from Table 4. Where the hysteresis of the water contact angles could be measured, the effect is even more pronounced than for CHzIz and high values are evident after fluorination treatment. It is believed that these effects may be related to microscopic surface roughness caused by etching or other damage to the polymer during fluorination. Thus, harsher treatment conditions (i.e., longer treatment times or higher concentrations of fluorine) result in greater damage, increased roughness and a concomitant increase in hysteresis. 4.3. Surface free energy The total surface free energy ( rs) and its polar and dispersive components (ySp and 7,” ) were determined from the contact angles for water and
49
J. D. L.e Roux et al. / Journal of Membrane Science 90 (I 994) 3 7-53 Table 5 Contact angles and surface energies for various polymers Polymer
Ref.
Advancing contact at@e,@. (“)
Surface energy (dynes/cm)
Water
Total (YS)
Polar (Y.P)
Dispersive (r.d)
CHJ2
Poly ( phenylene oxide )
99.0
58.8
29.44
0.55
28.9
Polysulfone
90.7
42.4
38.1
0.94
37.6
94
53.9
93.6 80 82 108 91 70 80.4 80
54.3 49 63 88/77 35 41 36.8 41
33.1 35.7135.3 30.1 36.7 30.3 18.6118.4 42.0140.7 47.0146.5 41.3144.6 40.2141.1
1.1 0 0 5.4 7.1 0.5l1.7 0.616.1 6.2/14 3.519 4.3111.5
32.0 35.7135.3 30.1 31.3 23.2 19.1l20.0 41.4134.5 40.8132.5 37.8135.6 35.9129.6
(PSF)
Polyethylene linear/branched Polypropylene Poly (vinyl fluoride ) Poly (vinylidene fluoride) Poly (tetrafluoroethylene) Polystyrene Polyamide (Nylon-6,6) PETa PMMAb
this work this work 29 34 34 29 29 29,34 29,34 29,34 29,34 29,34
‘Poly(ethylene terephthalate). bPoly (methyl methacrylate ) .
methylene iodide according to the method of Owens and Wendt [ 3,291. For unfluorinated PPO the value of JJ~was calculated as 29.4 dynes/ cm. Comparison with the ySvalues for the polymers listed in Table 5 shows that the value of PPO ranks well below that of PSF (38.1 dynes/ cm) and polystyrene (42.0 dynes/cm) but is closer to that of polypropylene (30.1 dynes/cm) which is aliphatic and has one pendant methyl group. As pointed out in the above discussion of the advancing contact angles of these polymers, the surface of PPO appears to have more of an aliphatic than an aromatic character. Generally, the lower the surface free energy of the polymer, the less will be its tendency to interact with a polar liquid (such as water) and the larger its contact angle will be. Figs. 11 and 12 show the total surface energy and its polar and dispersive components at the different fluorination treatment conditions: fluorine concentration and treatment time. In Fig. 11 the total surface energy ( yS) initially increases and then decreases as a function of fluorination time with a maximum increase of - 10 dynes/
28 =----a_* @ w" 8
f!
:
Dispersive. -
26:.
0.
12,:.
0 --..._,
-..,O . . ..
_,,.
Igo: :b
s 6-
0 ,_
.o
..
0
‘.
‘.
Polar
4 - :' - .d 2 7.' OQi.'~','.','.""',i.I,_ 0 123 4 5 678
0:
91011
Fluorination time [min]
Fig. 11. The total free surface energy (y.) (m) and its polar ( 0 ) and dispersive ( 0 ) components ( y.P and yt ) as a funo tion of exposure time at a fluorine feed concentration of 0.02% Fz.
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
50 40
,
38 _36 -
,
,
,
,
,
,
,
q /S---m-i__
34 32 -
,
,
,
: Total
,I’
“p
\
\
Sl
-
-
24 Dispersive
-
IO 8 6
--
4
_ I _ ,.I..
2
0
OV~?,‘~“‘~‘~ 0 0.02
0.
. ..O
0
0.04
0.06
0.08
Polar
-
-0
_
0.10
0.12
Fluorine concentration [vol%] Fig. 12. The total free surface energy ( y,) ( n ) and its polar ( 0 ) and dispersive ( 0 ) components (r,P and 7: ) after 2 min of treatment at various fluorine feed concentrations.
cm at about 5 to 7 min. The polar component of the free surface energy (yf ) has a trend similar to that of the total surface energy, with a maximum located at 5 to 7 min of treatment. By contrast, the dispersive component (7,” ) exhibits a very small but steady decrease with increasing fluorination time. Fig. 12 shows the effect of fluorine concentration at a constant treatment time of 2 min. Compared to unfluorinated PPO, there is a significant increase in the total and polar surface energies after 2 min of fluorination at 0.02% F2. At higher fluorine concentrations ys decreases gradually, while 7,” shows no definite trend. On the other hand, the dispersive component (7:) decreases slowly but steadily as a function of increased fluorine concentration. These results show that the polar surface energy contribution is more sensitive to the various treatment conditions than the dispersive contribution. Similarly, the contact angle of water, a polar liquid, is more sensitive to treatment than that of methylene iodide which is non-polar. The XPS results discussed above show that both oxygen and fluorine are added to PPO in the surface region and it would not be unrealistic to assume that this also happens on the upper surface which is analyzed by DCA. Both oxygen and fluorine have higher electronegativities than
the original carbon and hydrogen of the unfluorinated polymer. Substitution of hydrogen by fluorine and oxygen can, therefore, be expected to affect the polar and dispersive nature of the PPO in different ways. Addition of these electronegative species will decrease the ease with which electrons can be displaced by the electric fields of other molecules, i.e., the polarizability [ 381, and will tend to lower the dispersive force interactions between adjacent polymers which, in turn, would be reflected in a lower dispersive contribution to the surface free energy. This is illustrated in Table 5 where the progressively fluorinated series, comprising polyethylene fluoride ) poly (vinyl (PVF), (PE), poly (vinylidene fluoride) (PVDF) and poly (tetrafluoroethylene) (PTFE ), exhibits a steady decrease in 7:’ as fluorine is added to the PE backbone. In Fig. 12, ps also decreases gradually as more polar species are added at higher concentrations and longer treatment times. The polar surface energy contribution ( 7: ) , on the other hand, depends on the magnitude of localized dipole moments in the polymer molecule. The total dipole moment increases when fluorine or oxygen is added to the PPO repeat unit in an unsymmetric fashion causing an unbalanced charge distribution. This is illustrated by reference to the fluorinated PE series in Table 5 where the polarity of the polymer increases from PE to PVF and PVDF, with progressively more unsymmetrically arranged fluorines. When further fluorines are added, they balance the effect of those already present and reduce the polarity, as shown for PTFE. Reference to Table 5 also shows that the polar surface energy contributions of PET, Nylon 6,6 and PMMA, all of which contain carbonyl groups, are considerably larger than those of most fluorine containing polymers, with the exception of PVDF. The addition of oxygen is certainly expected to markedly increase the polarity of a polymer. In Fig. 11 the initial increase in yP up to 5 min of fluorination may be related to the unsymmetric addition of both fluorine and oxygen. The subsequent decrease in r,P could then indicate the balancing of these dipoles and a consequent loss of polarity. Other factors may also contribute to the ob-
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
served effects. Physical and chemical degradation at all but the mildest fluorination conditions may include cross-linking, disruption of the phenyl rings, chain scission, or the loss of surface pendant groups containing fluorine and oxygen. Such processes could produce roughness by etching of the polymer surface or by the creation of defects and cannot be discounted in the light of the large hysteresis effects discussed above. The net result of increased surface roughness is to increase the advancing contact angle [ 36,371 as seen for water in Fig. 9. Thus, as the severity of the fluorination treatment increases, the polymer surface becomes more polar, less polarizable and the surface roughness may be increased. These effects could all contribute to some extent to the observed results. Finally, it should be pointed out that similar trends were found for the fluorination treatment of asymmetric polysulfone membranes [ 3 1.
5. summary 5.1. Nature of the fluorinated region
Surface fluorination treatment of a polymer membrane leads to a fluorinated layer whose depth and chemical nature depend on fluorination conditions, viz. fluorine feed concentration and treatment time. The XPS results show that even at the mildest conditions the chemical composition of this layer is substantially different from that of the unfluorinated polymer, while further increments in fluorination time and fluorine feed concentration did not markedly change the polymer composition. However, DCA analysis of the upper surface region indicated progressive incremental changes in the water contact angle and surface free energy, in response to the same range of treatment conditions. Depth dependent XPS analysis (Fig. 8) showed a lower concentration of oxyfluorinated groups closer to the surface; this effect being more pronounced at longer fluorination times. The overall conceptual physical and chemical model consistent with these results appears to be that the thickness of the fluorinated region de-
51
pends primarily on the duration of the fluorination treatment. Due to the inherently harsh nature of gas phase fluorination, this effect is accompanied by damage or disruption of the polymer, particularly at the outer most surface at higher fluorine concentrations and longer treatment times. This damage is inferred from increased roughness at the surface and a lower concentration of fluorine closer to the surface, an effect which is more pronounced at longer fluorination times. Apart from the thickness of the fluorinated region, the results shown in Figs. 5 to 7 suggest that the fluorine and oxygen concentrations on the PPO repeat unit plateau at a feed concentration of 0.04% F2. This is confirmed by a similar leveling off of the polar surface energy contribution (Fig. 12 ) and was also observed for the surface fluorination of asymmetric polysulfone membranes under similar conditions [ 3 1. Fluorine and oxygen are probably first added at the most accessible sites and successive additions become increasingly difficult. It is also possible that these species are both added and eliminated during fluorination at all but the lowest fluorine concentrations in the feed. Both the DCA and gas transport results indicate changes to the polymer as a result of increasing the fluorine concentration which appears to support this contention. 5.2. Gas transport The effects of fluorination on the gas transport properties of composite PPO membranes were discussed in an accompanying paper [ 5 1. It was shown that increasing either the fluorine feed concentration or the reaction time reduced the permeance (P/I) of all gases tested; the lighter gases (He and HZ) were reduced by a smaller degree than the heavier gases. When the membranes were coated with a layer of poly ( dimethylsiloxane ) ( PDMS ) subsequent to fluorination, the permeance decreased again, but by a much larger factor for Nz and CH4 than for the other gases. These observations are consistent with the notion that the addition of polar fluorine and oxygen groups creates a fluorinated polymer with a low permeability and that longer
52
J.D. Le Roux et al. /Journal of Membrane Science 90 (1994) 3 7-53
treatment times increase the thickness of the fluorinated region causing a progressive decrease in permeance. This tendency is not evident until the membranes have been coated with PDMS to neutralize the effect of surface defects caused by fluorination damage. These concepts are confirmed when the ideal selectivities of several gas pairs are considered in response to fluorination conditions [ 5 1. It was found that fluorination increased the selectivity of He and Hz relative to N2 or CH4 by a small factor but lowered the selectivity of O2 and CO* relative to Nz or CH4. Surface coating after fluorination, however, yielded substantial gains in the selectivities of all these gas pairs. The highest gains before coating were found at the mildest treatment conditions (0.02% and 1 to 2 min of fluorination), while the best overall selectivities after coating were achieved under more severe fluorination conditions (0.1% for 3 to 5 min). Under mild conditions damage to the polymer appeared to be slight and the benefits of fluorination were, therefore, more apparent. This was also found in a related study of the fluorination of PSF membranes [ 3,6]. After the coating had restored the actual transport properties of the treated membranes, the best results were obtained at harsher treatment conditions and a higher degree of fluorination. The commercial viability of the surface fluorination of membranes formed from glassy polymers, such as PPO and PSF, presents a two-fold optimization problem. On the one hand, the treatment process should maximize the beneficial effects of fluorination and minimize the detrimental effects due to damage. In this regard, the application of a suitable caulking layer (e.g., PDMS) as a post-treatment step is considered essential and the benefit of increasing the selectivity clearly outweighs the small concomitant loss in productivity. On the other hand, even successful fluorination treatment remains subject to the inevitable loss of productivity in exchange for any gain in selectivity. In this regard, the emphasis remains on the production of membranes with the thinnest possible (yet defect-free) skins to enhance productivity. Another approach would be to fluorinate polymers
with a higher permeability, though the gain in selectivity may then be lower.
6. Acknowledgments The authors thank Prof. Paul Ho, Dr. Steve Anderson and John Hall-Pellerin for their assistance with XPS deconvolution. Thanks are also due to Dr. Joel Kampa and James DeYoung for carrying out the fluorination treatment. This work is based in part upon work supported by the Texas Advanced Technology Program under Grant No. 1607, by the Separations Research Program at the University of Texas at Austin, TX, and by the CSIR, Pretoria, South Africa.
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