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Surface fluorination of composite membranes. Part II. Characterization of the fluorinated layer
Journal of Membrane
Science,
Elsevier Science Publishers
55 (1991)
149-171
149
B.V., Amsterdam
Surface fluorination of composite membranes. Part II. Characterization of the fluorinated layer J.M. Mohr, D.R. Paul*, Y. Taru** Department of Chemical Engineering, Austin, Austin, TX 78712 (U.S.A.)
Center for Polymer
Research,
The University
of Texas at
T.E. Mlsna and R.J. Lagow Department of Chemistry, Center for Polymer Austin, TX 78712 (U.S.A.)
Research,
The University
of Texas at Austin,
(Received February 23,199O; accepted in revised form July 30,199O)
Abstract The fluorinated surface layer of membranes exposed to a dilute stream of fluorine gas has been characterized. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy attenuated total reflection (FTIR-ATR) were used to determine the concentration and profile of reacted fluorine in poly(4-methyl-1-pentene) membranes after exposure to fluorine. The thickness of the fluorinated layer as a function of reaction time was calculated using the results of bulk elemental analysis and of XPS. The permeability of this layer was determined using the calculated thickness and the (P,/l,) values reported previously. It is shown that the permeability of the fluorinated material is two orders of magnitude lower than that of the original poly(4methyl-1-pentene ). A simple diffusion limited reaction model is able to give a good estimate of the thickness of the fluorinated layer as a function of fluorine reaction time. Keywords: membrane preparation tion; poly(4-methyl-1-pentene)
and structure;
composite
membrane;
gas separation;
fluorina-
Introduction Gas phase fluorination of polymer films or membranes results in a thin layer of fluorinated material at the contacted surface. This causes a substantial change in the gas transport properties as reported in the accompanying paper [l] dealing with poly (4-methyl-1-pentene) (PMP) materials. In this work, 50 ,um films of PMP and composite membranes with a PMP permselective *Author to whom correspondence should be addressed. **Current address: Musashi Institute of Technology, 28-l Tamazatsumi Tokyo 158, Japan.
0376-7388/91/$03.50
0 1991-
Elsevier Science Publishers
B.V.
I-chome,
Setagaya-ku,
150
layer were fluorinated for times varying from 2.5 min to 96 hr. The structure of PMP is shown below.
fCH,-CH
j CH, CH
CH%H3 Upon exposure to molecular fluorine, some of the hydrogen atoms will be replaced with fluorine atoms while other reactions such as chain scission or crosslinking may follow. After the immediate surface is fluorinated, the reaction proceeds to progressively deeper layers below the surface. This paper deals with characterization of this fluorinated layer using various techniques. The objective here is to learn how much fluorine is incorporated into the PMP structure by reaction and to gain some idea about the depth of the fluorinated layer as a function of the time the surface was exposed to fluorine. A subsequent paper will deal with a more detailed analysis of the placement of fluorine atoms in the PMP repeat unit. The fluorinated polymer created by this type of process has been shown to be restricted to a relatively thin surface layer, perhaps no deeper than 1000 A [ 2-51. To characterize this thin layer, two surface sensitive techniques, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR), were used. In addition to the surface analyses, bulk elemental analysis was done to determine the chemical composition of the sample as a whole after various fluorine exposure times. The latter information can also be used to estimate the depth of fluorination as a function of treatment time by using a simplified diffusion-controlled model for the reaction process. Finally, solubility tests and thermal analysis were done to complement the above techniques. Background and theory XPS, also known as electron spectroscopy for chemical analysis (ESCA), is particularly suited to examine the fluorinated polymer layer since it only probes a short distance into the surface of solids. The surface sensitivity of XPS is a consequence of the short mean free path, or escape depth, of electrons in a solid. XPS experiments measure the kinetic energy of electrons ejected by interactions of a molecule with a beam of soft X-rays. For the case of a substrate material covered with a continuous overlayer of thickness d, the intensity of a core-level signal from the overlayer, Ii, is expected as:
151
Ii = Ii,,
[-4&e)]
(1)
where Ii,, is the intensity arising from an infinitely thick layer of the overlayer material, Si is the electron escape depth, and 8 is the electron emission angle relative to the plane of the sample surface [6-g]. When 0 is 90”) the depth from which the signal arises is governed only by the escape depth of the electrons. Equation ( 1) shows that when d/6i= 1,2, and 3,li/li,, = 0.63,0.87, and 0.95, respectively, when f3is 90”. Hence, to a reasonable approximation, the depth of material actually sampled is given by 36;. The escape depth Sidepends on the material and on the kinetic energy of the electrons with values typically on the order of lo-30 A for polymer materials [8,10,11]. Thus, in the XPS experiments, information is obtained from about the top 90 A of the sample. In this research, XPS was used to determine the chemical composition of this layer and to gain some idea about the depth profile of the fluorine within this layer for PMP composite membranes. However, we found that the depth of fluorination extended beyond the 90 A sampling depth of XPS. To characterize the fluorinated layer beyond 90 A and to ultimately determine the thickness of this layer, other analyses were required. One means of doing this is with ion beam sputtering, where the surface of the sample is eroded with argon ions. Alternate cycles of sputtering and of surface analysis gives compositional information as a function of depth below the surface, a technique called depth profiling [ 71. This technique is accompanied by several problems, such as the preferential ejection of certain electrons which leads to chemical changes in the surface [ 121. Perhaps the largest uncertainty is knowing at what depth the analysis is being done after several cycles of sputtering and analysis; i.e. knowing the sputtering rate of the material. Despite these drawbacks, the fluorinated composite membranes were depth profiled to give a general indication of the fluorine concentration as a function of depth. FTIR-ATR is a technique to obtain the infrared spectrum of species located near the surface of a sample and has been used to characterize interfacially formed membranes [ 13,141, to determine the thickness of reacted surface layers [ 15,161, and to determine the rate of fluorination reactions [ 171. In an ATR experiment, an evanescent IR beam penetrates into the sample to a depth that depends on several factors, but is actually 2-3 orders of magnitude greater than that probed by XPS. The depth of penetration of the IR beam, d,, is given as (18):
(2)
This expression links the effective sampling depth, dp, of an IR beam of wave-
152
length ;1into a sample of refractive index rz2that is in contact with a reflecting element of refractive index n, when the effective angle of incidence is @ For PMP with a refractive index of 1.463 [ 191 and for a KRS-5 reflecting element with a refractive index of 2.38 and at $ equal to 45 O,c&/A is 0.1913. As 3,scans from 3 ,um to 10 ,um (3300 cm-’ to 1000 cm-‘) dp varies from 0.57 pm to 1.9 ,um. Actually, the IR beam is penetrating fluorinated PMP, and it would be more correct to use a refractive index value for fluorinated PMP to determine de. Comparison of the refractive index of polyethylene (1.49) to poly (vinylidene fluoride) (1.42) and to poly(tetrafluoroethylene) (1.35) [20] suggests that fluorination will result in a decrease in refractive index. Therefore, d,, for fluorinated PMP is likely to be slightly less than that calculated for PMP. In order to characterize the surface fluorinated PMP membranes with FTIRATR, the fluorinated layer must comprise a large enough fraction of the 0.571.9 pm penetration depth to be detected. The composite membranes used for the XPS characterization had a permselective layer of PMP ranging in thickness from 0.2 to 20 ,um, with the majority of samples in the 0.7-2 pm range. These membranes were fluorinated for 2.5-15 min. In these composite membranes the PMP layer and the fraction of it that is fluorinated is too small to be detected in an FTIR-ATR experiment. For this reason PMP films 50 pm thick were fluorinated for times ranging from 15 min to 96 hr. The fluorinated layer in these samples was thick enough to be detected. The FTIR-ATR spectra is too coarse to determine detailed changes in the molecular structure, however, the gross progress of the fluorination can easily be detected and the thickness of the fluorinated layer can be estimated. The fluorinated 50 pm PMP films were submitted for bulk elemental analysis. The carbon, hydrogen, fluorine, and oxygen concentrations in the sample as a whole were determined as a function of exposure time. Assuming the fluorination reaction progresses as a sharp front dividing completely reacted material from unreacted material (see Fig. 1A in accompanying paper [ 1]), then the depth to which the fluorination reaction has penetrated, If, can be approximated by: 21f
(% fluorine),
1 - (% fluorine),
(3)
Here, I is the thickness of the film (50 ,um) and the factor of 2 arises because these films were fluorinated from both sides. The weight percent fluorine after fluorination time t, ( % fluorine) t, is determined from bulk elemental analysis. The term (% fluorine), is the weight percent fluorine behind the moving front and is assumed not to change with reaction time. Ideally this value would correspond to replacing every available hydrogen atom with a fluorine atom. The PMP repeat unit has 12 hydrogen atoms and 6 carbon atoms. Thus, the theoretical maximum fluorine concentration occurs when there are 12 fluorine at-
153
oms to every 6 carbon atoms, a fluorine to carbon atomic ratio of 2. This atomic ratio corresponds to 76 wt.% fluorine. XPS indicates that this limit is not reached even at the surface of the membrane after the longest fluorination time. The limit indicated by XPS was 69.7 wt.% fluorine, and this was used as ( % fluorine ) _ Experimental Materials The 50 ,um PMP films were obtained from Mitsui Petrochemicals, Ltd, and were designated TPX-X22 @. The composite membranes consisted of three layers; a polysulfone microporous support, a 1 pm silicone rubber interlayer, and a permselective PMP layer. Preparation of the polysulfone microporous support and application of the silicone rubber layer is described elsewhere [ 211. The polysulfone-silicone rubber membrane was coated with PMP, obtained from Scientific Polymer Products. The polymer was applied from a dilute solution in cyclohexane. The membranes were fluorinated with a 2% fluorine in helium gas stream. Details of the fluorination procedure and of the reactor are described in the preceding paper [ 11. Equipment 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 13 kV and 20 mA and the sample chamber was evacuated to less than 5 x lo-’ Torr. The gold 4f7,2 level at 84 eV binding energy was used for calibration and had a full width half maximum (FWHM) of 1.18+0.1 eV. The spectra were taken with the electron emission angle at 90” to give the maximum sampling depth of approximately 90 A. In one experiment, spectra were recorded with the electron emission angle at 7’) 20’) 30’) and 50 o to collect spectra at sampling depths less than 90 A. Ion sputtering was done with an argon ion gun at 1 kV and an ion current density of 9 @/cm’. The sputtering rate of PMP was determined by casting a thin layer of the polymer onto a polished nickel surface. The thickness of the polymer film, 2800 A, was measured with an AlphaStep 200 profilometer. The polymer film was sputtered until nickel appeared in the XPS spectra. A sputtering rate of 11.5 A/min was determined, which is consistent with other values reported [ 121. Samples were mounted onto the sample probe with double sided adhesive tape. Analysis times were kept short to minimize radiation damage to the sample. The sensitivity factors used were: carbon, 0.2; oxygen, 0.61; and fluorine, 1.0. A Digilab FTS 15/90 FTIR spectrometer and a continuously variable Spectra-Tech Model 301 ATR attachment were used for FTIR-ATR measure-
154
ments. One hundred scans were obtained at a resolution of 2 cm-‘. A KRS-5 internal reflection element was used in the ATR accessory at an incident angle of 45”. Results and discussion Composite membranes XPS
Broad scan spectra for untreated PMP and for PMP after 5 min fluorination are shown in Fig. 1. The primary feature in the unfluorinated spectrum is the C (1s) peak at 285 eV. A small oxygen peak at 533 eV is present, probably due to surface oxidation of the PMP. The spectrum of the fluorinated PMP has a less intense C (1s) peak at 288 eV, an 0 (1s) peak at 533 eV, and a F (1s) peak at 695 eV. The small peak at 34 eV corresponds to the F (2s) electron. Hydrogen is not detected by XPS at the core level electron for this element is also the valence level [ 81. Spectra were recorded for membranes after fluorination times of 1-15 min. The carbon peak from each spectra is shown in Fig. 2. The carbon peak for unfluorinated PMP is narrow and well defined. After just one minute of fluorination, the peak broadens and shifts to a slightly higher binding energy. An additional peak at 288.2kO.2 eV is due to monofluorocarbon
5 min fluorination
1. I I I I I / 0 100 200 300 400 I
/
I
I A---J , , , / , , I,, 500 600 700 800 900 1000
BindingEnergy(eV) Fig. 1.XPS broad scan spectra of untreated PMP and after 5 min fluorination.
155
280
292 288 284 Binding Energy (eV)
296
Fig. 2. Carbon peak of the XPS spectra of PMP after various fluorine exposure times.
environments [ 22,231. The shoulder at 291.5 + 0.7 eV arises from difluorocarbon groups and the bump at 293 eV, seen most clearly in the 12.5 min peak, is due to trifluorocarbon environments [ 231. Deconvolution of the carbon peak into its component peaks can be done to determine the contribution of CF, CFP, and CF3 groups as will be described in a subsequent paper. The concentrations of carbon, fluorine, and oxygen were determined from the spectra at each fluorination time and the results are shown in Fig. 3 as the atomic ratio of fluorine to carbon versus fluorination time. The ratio of fluorine atoms to carbon atoms after 1 min is 0.64 and this increases to 1.0 after 15 min of fluorine exposure. A fluorine to carbon ratio of 1.0 indicates that one-half of the 12 hydrogen atoms of the PMP repeat unit have been replaced by fluorine atoms. PMP is semicrystalline at room temperature. A recent study has shown that gases up to approximately 4 A in molecular diameter are able to diffuse within the PMP crystal, which has gaps of approximately this size [ 241. The fluorine molecule has a diameter of 3.653 A [25] and the fluorine atom has a diameter of 2.70 A [ 26,271. Presumably then, the fluorine atom would
156
- 0.09
6
-0.08
z v
-0.07
E Y! 3
0.6
.g -0.06
J+ I,, 6
-0.05
0
8
Fluorination
,I, 10
,.l?l, 12 14
16
y” .o E 7
_I0.04
Time (mm)
Fig. 3. Fluorine to carbon and oxygen to carbon atomic ratios of PMP after various fluorine exposure times.
be able to enter the crystalline fraction of the polymer and react. The extent to which this might occur depends on the diffusivity of fluorine in the crystalline fraction, which probably is much less than that in the amorphous fraction. Fluorine is very reactive and this will further limit the amount of fluorine that enters the crystalline fraction. Hence, fluorination of the amorphous fraction would be favoured, yet the possibility of the crystalline fraction being fluorinated exists. Thermal analysis of films fluorinated for 90 hr, to be discussed later, suggests that the crystalline regions are fluorinated. Furthermore, the level of fluorine content achieved at the surface and the chemical nature of its placement in the PMP repeat unit give even more convincing evidence of this. Oxygen is present in the XPS spectra in amounts greater than can be attributed to atmospheric contamination. The oxygen to carbon atomic ratio, shown in Fig. 3, remains relatively constant with treatment time. Higher than expected levels of oxygen after fluorination have been observed by others [ 2,3]. Several sources of the oxygen may be considered. One possibility is that long lived radicals in the fluorinated surface react with atmospheric oxygen or water, while another is some source of contamination in the fluorination process. To determine the source of oxygen in this study a controlled experiment was done. A membrane was placed in the fluorination chamber and the reactor was purged for several hours with pure helium to remove all atmospheric oxygen and moisture. The membrane was fluorinated for 5 min, after which the reaction chamber was flushed with helium to remove the fluorine gas. The reactor was then placed in an argon environment where it was opened to remove the fluorinated membrane. A portion of this membrane was loaded into the XPS sample chamber under an argon blanket, so that it was never exposed to the atmosphere. Another portion of the membrane was removed from the argon environment and allowed to sit in the atmosphere overnight before loading it into the XPS
157
sample chamber. The results of the XPS spectra are summarized in Table 1. The oxygen contents of these membranes are the same, suggesting that the source of oxygen is present during the fluorination reaction and that the membranes does not react with atmospheric oxygen after fluorination. Because the reactor was thoroughly purged with helium prior to fluorination, it is unlikely that atmospheric oxygen in the reactor is the source. Commercially available fluorine commonly contains some oxygen as an impurity and this is the most probable source of the oxygen in the fluorinated membranes. The XPS spectra shown above were taken with the electron emission angle, 13,at 90’ so that the sampling depth is at its maximum of N 90 A. By using smaller electron emission angles the composition of the membrane at depths less than 90 A can be estimated. Figure 4 shows the fluorine to carbon atomic ratio as a function of sin 8 for membranes after 1, 5, and 15 min fluorination. At the top of this figure is a scale showing the approximate depth of the layer TABLE 1 Results of XPS experiment to determine the source of oxygen in surface fluorination* Unfluorinated (control)
Atomic %
5 min fluorination
99 0 1
Carbon Fluorine Oxygen
air
argon
46 48 6
42 52 6
“After 5 min fluorination, a part of the membrane was exposed to the atmosphere while the other part was kept under an argon blanket. Sampling Depth
(A)
1.21,
0.71 0.0
1 0.2
’ 0.4
’ 0.6
’ 0.8
’ 1.0
1
Fig. 4. Atomic ratio of fluorine to carbon as a function of electron emission cngle in the XPS experiment. As the emission angle varies the sampling depth varies from - 10 A to - 90 A.
158
that is sampled at electron emission angle 0. The scale was calculated based on the approximation that 95% of the signal arises from a depth of 36isin 8, where a value of 30 A was used for Si, the escape depth. From these results it is clear that fluorination extends more deeply than the maximum sampling depth (&9O”C) possible with XPS. There is evidently some spectral variation in the fluorine content within this layer, but on a relative basis the variation is not great. There is very little difference in the results obtained after 5 and 15 min of fluorination. This suggests that the immediate surface reaches a maximum attainable level of fluorination within 1 to 5 min and changes very little thereafter. Argon ion sputtering was used to profile the membrane beyond the maximum sampling depth of the XPS. The sputtering rate of PMP was first determined to establish the relationship between the analysis depth and sputtering time. A thin film of PMP was cast onto a polished nickel substrate and the thickness of the film was determined to be 2800 A with a profilometer. The polymer surface was sputtered with an argon ion gun and analyses were made periodically to watch for the appearance of nickel in the XPS spectra. Eventually, nickel begins to appear in the signal. The nickel/carbon atomic ratio extrapolates to a breakthrough time of 244 min as shown in Fig. 5. Thus, an average sputtering rate of 2800 A/244 min= 11.5 A/min was established. Membranes fluorinated for 2 min and for 15 min were depth profiled using argon ion sputtering. Figure 6 shows the fluorine to carbon atomic ratio vs. sputtering time. The amount of fluorine beyond 50 min of sputtering is constant, but this may be an artifact of the sputtering process, as the polymer is not clearly stripped away from the surface. The time at which the fluorine concentration ceases to drop sharply and begins to level off is likely to correspond closely to the actual depth of fluorination. Using the sputtering rate
Sputtering Conditions - Argon ion gun, 20111.4 .s 2 .v E z 8 f G 3 z Is z
0.03
ion current density =
0.02
0.01
Sputter Time (min)
Fig. 5. Atomic ratio of nickel to carbon vs. argon ion sputtering time for a 2800 i thick PMP film cast onto a nickel substrate.
159
Sputter Time (min) I 0
I
240
I
I
460
I
I
690
I
I
920
I
I
I
1150
I
1380
Estimated Thickness of Fluorinated Layer (A)
Fig. 6. Depth profile by argon ion sputtering of PMP membranes fluorinated for ,2 min and for 15 min. Estimated thickness of fluorinated layer based on a sputtering rate of 11.5 A/min.
determined for unfluorinated PMP, this corresponds to a depth of 140 A for the 2 min sample and 490 A for the 15 min sample. Because of the artifacts and uncertainties involved in depth profiling with ion sputtering, these results must be interpreted with caution. Other methods described below were used to gain further estimates of the thickness of the fluorinated layer. Thick film (50 pm) To profile the fluorinated region beyond the 90 A that can be probed by XPS directly, FTIR-ATR and bulk elemental analysis methods were employed. To use these techniques the fluorinated layer must be thick enough to be detected, and for this reason thick films (50 pm) were fluorinated for long times. These films were fluorinated from both sides for times ranging from 15 min to 96 hr. FTIR-ATR Figure 7 shows the FTIR-ATR spectrum of unfluorinated PMP. The peaks in the region 2950 cm-‘-2850 cm-’ arise from stretching and bending vibrations of C-H bonds in CH3, CH2, and CH groups. The adsorption at 1470 cm-’ is associated with the deformation vibrations in CH, and CH, groups [ 28,291. The third feature of interest is the peak at 1170 cm-l with a shoulder at 1145 cm-‘. This is characteristic of gem-dimethyl groups [ 291, corresponding to the CH ( CHB) 2 portion of the pendant group in the PMP repeat unit. Spectra after 30 min, 2 hr, and 5 hr fluorine exposure are also shown in Fig. 7. The intensity of the bands in the 2950 cm-‘-2850 cm-’ region decreases with fluorination, until this band disappears after 5 hr of fluorination. The CF, CF2, and
160
2 hours fluorination
5 hours fluorination
r 4000
3500 3000 2500 2000 Wavenumbers
1500
1000
500
(cm-‘)
Fig. 7. FTIR-ATR scans of untreated PMP and after 30 min, 2 hr, and 5 hr of fluorine exposure.
depth of penewat~on =
1.3pnl
Fluorination Time (hours)
Fig. 8. Absorbance of the 2950 cm-’ and of the 1470 cm-’ bands in the FTIR-ATR spectra of PMP vs. fluorination time. The depth of penetration of the IR beam into the sample surface is calculated via eqn. ( 2 ) .
161
CF3 stretching vibrations occur in the 1400 cm-l-lOOO cm-’ region [ 281. The intensity and broadness of the peak in this area increases with fluorination time, eventually masking the other peaks in the region. The FTIR-ATR spectra are too coarse to relate to specific structural changes in the PMP repeat unit, but information concerning the rate and depth of fluorination can be derived. Figure 8 shows the absorbance of the 2950 cm-’ and the 1470 cm-’ bands as a function of fluorination time. The intensity of these bands decreases with fluorination as the CH bonds are replaced with CF. The steeper decline in the 2950 cm-l absorbance with time reflects the more shallow sampling depth at this wavelength. At 2950 cm-l the depth of penetration is 0.65 pm whereas at 1470 cm-l the penetration depth is 1.3 ,um. Lines drawn tangent to the maximum rate of change and to the baseline intersect at approximately the time when the depth of fluorination is equal to the penetration depth of the evanescent IR beam. The tangent lines intersect at 2.2 hr for the 2950 cm-l absorbance, while this occurs at 5 hr for the 1470 cm-’ absorbance. XPS and bulk elemental analysis XPS and bulk elemental analyses were used to calculate the thickness of the fluorinated layer via eqn. (3). The 50 pm PMP films were submitted to Galbraith Laboratories, Inc. (Knoxville, TN) for bulk elemental analysis and Table 2 summarizes the weight percent of carbon, fluorine, hydrogen, and oxygen found. These weight percents were converted to atomic percents, and the fluorine to carbon atomic ratio is shown as a function of fluorination time in Fig. 9. XPS was used to determine the fluorine content at the surface of each film. The results, also shown in Fig. 9, indicate that the fluorine to carbon ratio at the surface is nearly constant after 2 hr of fluorination. The large difference between the XPS and the bulk analyses at short fluorination times indicates TABLE 2 Bulk elemental analysis of PMP after various fluorination times to both sides of the film Fluorination time
(hr) 0
0.25 0.50 1 2 5 96
Element (wt.% ) Carbon
Hydrogen
Fluorine
Oxygen
86.1 85.4 85.6 84.3 84.4 82.2 63.1
13.9 14.3 14.2 14.4 14.3 13.7 9.1
0.02 0.15 0.43 1.48 1.80 4.0 27.0
0.39 < 0.50 co.50 0.68 0.80 1.4 B
.
“Unable to determine oxygen in presence of high percentage fluorine.
162
Fluorination Time (hours)
Fig. 9. Fluorine to carbon atomic ratio in the surface of fluorinated PMP and in the sample as a whole (determined by bulk elemental analysis). indicates the maximum ratio for PMP.
(determined by XPS) The dashed line at 2
that the fluorine is originally confined near the surface. As the fluorination time increases the bulk analysis approaches the XPS result since the fluorination extends more deeply into the sample. The dashed line in Fig. 9 indicates the maximum possible ratio of fluorine atoms per carbon atom for PMP. This occurs when the 12 hydrogen atoms on the PMP repeat unit are each replaced by a fluorine atom. This theoretical maximum of 2 is not reached even after 96 hr of fluorination. The surface ratio of fluorine atoms per carbon atom reaches a maximum of 1.65 after 5 hr of fluorination and remains at this value even after 96 hr of fluorination. The maximum fluorine to carbon atomic ratio of 1.65 corresponds to 69.7 wt.% fluorine, and this was used as the limiting fluorine concentration, (% fluorine),, in eqn. (3). The weight percent fluorine determined from XPS is based on the carbon, fluorine, and oxygen content only, since hydrogen is not measured by XPS. However, at these long fluorine exposure times the hydrogen content should be negligible. The weight percent fluorine as a function of fluorination time, (% fluorine),, was determined by bulk elemental analysis (Table 2). The thickness of the fluorinated layer calculated by this approach is shown in Fig. 10 by the open points. The thickness of the fluorinated layer is 0.054 pm (540 A> after 15 min of fluorination and 9.7 pm after 96 hr of fluorine exposure. The closed circles at 2 min and 15 min in Fig. 10 indicate the thickness of the fluorinated layer determined by XPS ion sputtering. For the 15 min sample, the thickness of 460 A determined by ion sputtering agrees with the calculated thickness of 540 A. The two triangles in Fig. 10 are the thickness of the fluorinated layer at 2.2 hr and 5 hr fluorination time determined by FTIR-ATR. Considering all the limitations of these different meth-
163
Fluorination Time (min)
Fig. 10. Thickness of the fluorinated layer vs. fluorination time. Thickness values for bulk elemental analysis were calculated via eqn. (3 ) .
ods and their simplifying assumptions, the agreement among them is quite remarkable. The data in Fig. 10 suggest that the thickness of the fluorinated layer is directly proportional to fluorination time. For diffusion controlled processes, one generally expects the depth to be proportional to the square root of time [ 30-331; however, the situation in this research is slightly different in that the gas phase fluorine concentration inside the reactor is time dependent. A model for the system used here is developed in the Appendix, and it shows that the thickness of the fluorinated layer should be linearly related to fluorination time in the early stages and eventually approach the square root of time regime after rather long fluorination times. Table 3 summarizes the thickness of the fluorinated layer for membranes for reaction times of 15 min and less, extrapolated from the line through the data in Fig. 10. In the gas transport studies described in the preceding paper, the permeation resistance [or (P/Z) = (permeability coefficient) / (thickness of layer) ] of the fluorinated PMP layer was determined for the composite membranes as a function of fluorination time. Having estimated the thickness of the fluorinated layer, the absolute permeability of the fluorinated layer for each gas can be estimated using the previously determined (PJZ,) values. The permeabilities so determined are listed in Table 3 for six gases. For comparison, the permeability of PMP is also shown. After 2.5 min of fluorination the permeability of this layer to most gases is about one to two orders of magnitude less than that of PMP. The values for methane and carbon dioxide are nearly two orders of magnitude less. The permeability coefficients calculated for the fluorinated layer after 5 min are even lower. The permeability values after 5, 10, and 15 min fluorination are remarkably similar. The closeness of these
164 TABLE
3
Thickness of the fluorinated layer, l,, for PMP films after various fluorination from the line in Fig. 10 Fluorination
time
times as determined
Permeability”,d
lr (pm)
(min)
N2
02
CH,
HZ
He
CC*
8.5
31.6
20.1
120.8
88.2
107.8
2.5 5 10
0.011 0.025 0.048
1.8 0.088 0.087
7.8 0.44 0.41
0.40 0.059 0.066
12.5 5.9 8.8
9.6 7.5 _c
5.2 2.4 2.1
15
0.054
0.048
0.20
0.050
3.2
7.5
1.1
Ob
“Units: lo-“’ bTPX-X22@
cm3-cm/set-cm’-cmHg. at 35°C.
“Helium data not available for the 10 min exposure time. dPermeability of the fluorinated layer determined from ( Pf/lf) values at 25’ C reported in preceding paper ( 1).
values suggests that there is very little change in the material behind the reaction front after fluorination times beyond 5 min. This agrees with the XPS data in Fig. 4 which also indicate that the reaction at the surface is complete after about 5 min of fluorination. Longer fluorination times result in a fluorinated layer that has essentially the same extent of fluorination as shorter times, the difference in (I’,/&) is primarily due to the difference in depth of fluorination. The dashed line in Fig. 11 shows the typical trade-off between selectivity
60-
PMP o
0’0.1
‘,I.,,’
1
~~111’ 10
‘,,.,,’
100
-I
1
Fig. 11. Carbon dioxide/methane selectivity against carbon dioxide permeability for typical polymers (dashed line) and PMP before and after 5 and 10 min of fluorine exposure.
165
2.5
3.0
3.5
4.0
Kinetic Diameter (A)
Fig. 12. Permeability of coefficient as a function of kinetic diameter of the gas for polyethylene (PE), poly(4-methyl-1-pentene) (PMP), poly(vinylidenefluorine) (PVDF) and fluorinated PMP (F-PMP). The closed data points for F-PMP are for 5 (m) and 15 (0) min fluorination times and define a band for this polymer.
and permeability for polymer materials using the gas pair carbon dioxide and methane as an example, taken from Fig. 2 in the accompanying paper [ 11. The data points are for PMP before fluorination and after 5 and 10 min of fluorine treatment. Untreated PMP falls slightly below the curve; however, PMP fluorinated for 5 or 10 min falls on and above the trade-off line. In Fig. 12 the permeability vs. the kinetic diameter [ 341 of six gases for PMP and for fluorinated PMP is shown along with polyethylene [35] and poly (vinylidene fluoride) [36]. The closed data points denote the 5 and 15 min fluorinated PMP and define a band that represents fluorinated PMP. The permeabilities of polyethylene and poly (vinylidene fluoride) differ by about two orders of magnitude for the six gases compared. This is also the case for PMP and fluorinated PMP. It is also interesting to note the similarity in the shapes of the curves for poly (vinylidene fluoride) and fluorinated PMP. The permeability of the fluorinated PMP layer responds to gas molecular size in much the same way as other polymers. This layer has similar selectivity as other polymers with this permeability. Solubility and thermal analysis PMP is soluble in a number of solvents at or just below their boiling point. After fluorination, the 50 pm PMP films are not soluble in cyclohexane or in chloroform, two of the solvents used to prepare PMP membranes. Dimethylformamide is a solvent for polyvinylfluoride; however, fluorinated PMP is not soluble in it. The lack of solubility indicates either that the surface is crosslinked or that the fluorinated PMP is simply not soluble in these solvents.
166
Differential scanning calorimetry was done on specimens cut from the 50 ym PMP films that were fluorinated for 5 and 96 hr. There was no observable change in the melting or glass transition temperatures relative to the unfluorinated PMP. However, there was a reduction in the endotherm area of the 96 hr fluorinated sample. The endotherm area, or heat of fusion, for the untreated PMP was 17.5 J/g. The sample fluorinated for 5 hr had a heat of fusion of 17.1 J/g. After 96 hr of fluorination the heat of fusion was 11.5 J/g. Using 61.9 J/g for the heat of fusion of the 100% crystalline polymer [ 24,371 this corresponds to 28.3% and 28.0% crystallinity for the untreated and the 5 hr fluorinated PMP. The 96 hr fluorinated sample is then 18.6% crystalline. This decrease in crystallinity suggests that the fluorination disrupts the crystalline fraction of the polymer, perhaps by reacting within the crystal structure itself. Conclusions
A combination of surface sensitive techniques and bulk elemental analysis was used to determine the nature of the fluorinated layer in surface fluorinated PMP. From XPS, the elemental composition within the top 90 A of the surface was found and how the composition varies within this sampling depth was examined by angular dependent studies. It was found that fluorination extends beyond the 90 A layer. As a result, three additional methods were used to gain information about the thickness of the fluorinated layer as a function of fluorination time, viz. argon ion sputtering, FTIR-ATR, and bulk elemental analysis. Agreement among the three methods was quite good and the data suggest that the thickness of the fluorinated layer increases linearly with fluorination time. This linear relationship is mathematically consistent with a simple model for a diffusion controlled reaction when the surface concentration of the reactant is driven in time as done in this work. The permeability of the fluorinated layer was estimated from the (P&) values reported in the preceding paper and the estimated thickness of the fluorinated layer. The permeability of fluorinated PMP is one to two orders of magnitude lower than untreated PMP and has a selectivity typical of other polymers of this permeability. The benefit of fluorination of gas separation membranes for achieving a better combination of selectivity and productivity stems for the spatial heterogeneity of the fluorinated structure as described in the first paper in this series. Acknowledgement
This material is based in part upon work supported by the Texas Advanced Technology Program under Grant No. 1607 and by the Separations Research Program at the University of Texas at Austin. We also acknowledge Jeff Cook of the University of Texas at Austin for his contribution in the XPS studies.
167
References 1 2 3
4 5
6 7 8
9 10 11
12 13 14 15 16
17
18 19 20 21 22
J.M. Mohr, D.R. Paul, T. Mlsna and R.J. Lagow, Surface fluorination of composite membranes. Part I. Transport properties, 55 (1991) 131-148. C.L. Kiplinger, D.F. Persico, R.J. Lagow and D.R. Paul, Gas transport in partially fluorinated low-density polyethylene, J. Appl. Polym. Sci., 31 (1986) 2617-2626. 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. Purification, 2 (1988) 162-170. J.F. Gentilcore, M.A. Trial0 and A.J. Woytek, Single-step barrier coating for containers, Plast Eng., 34 (9) (1978) 40-43. W.J. Koros, V.T. Stannett and H.B. Hopfenberg, Estimation of the effective permeability of thin surface layers created by exposure of polyethylene to fluorine, Polym. Eng. Sci., 22 (12) (1982) 738-745. D. Briggs, Handbook of X-Ray and Ultraviolet Photoelectron Spectroscopy, Heydon and Son, London, 1978, Chapt. 4. J.C. Rivikre, in: D. Briggs and M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley, NY, 1983, Chapt. 2. D.T. Clark and W.J. Feast, Application of electron spectroscopy for chemical applications (ESCA) to studies of structure and bonding in polymeric systems, J. Macromol. Sci., Revs. Macromol. Chem., C12(2) (1975) 191-286. D.T. Clark, ESCA applied to polymers, Adv. Polym. Sci., 24 (1977) 125-188. M.P. Seah and W.A. Dench, Quantitative electron spectroscopy of surfaces: A standard data base for electron inelastic mean free paths in solids, Surf. Interface Anal., 1 (1979) 2-11. D.T. Clark and H.R. Thomas, Application of ESCA to polymer chemistry. XVI. Electron mean free paths as a function of kinetic energy in polymeric films determined by means of ESCA, J. Polym. Sci., Polym. Chem. Ed., 15 (1977) 2843-2867. D.E. Williams and L.E. Davis, in: L.H. Lee (Ed.), Characterization of Metal and Polymer Surfaces. Vol. 2. Polymer Surfaces, Academic Press, New York, NY, 1977, pp. 53-71. C.R. Bartels, K.L. Kreuz and A. Wachtel, Structure-performance relationships of composite membranes: Porous support densification, J. Membrane Sci., 32 (1987) 291-312. C.R. Bartels, A surface science investigation of composite membranes, J. Membrane Sci., 45 (1989) 225-245. H.G. Tompkins, The physical basis for analysis of the depth of absorbing species using internal reflection spectroscopy, Appl. Spectrosc., 28(4) (1974) 335-341. K. Ohta and R. Iwamoto, Experimental proof of the relation between thickness of the probed surface layer and absorbance in FTIR/ATR Spectroscopy, Appl. Spectrosc., 39 (3) (1985 ) 418-425. C.S. Blackwell, P.J. Degen and F.D. Osterholtz, Internal reflectance spectroscopy of reacted surfaces: Fluorinated polyethylene and polypropylene, Appl. Spectrosc., 32 (5) (1978) 480484. N.J. Harrick, Internal Reflection Spectroscopy, Interscience Publishers, New York, NY, 1967. Mitsui Petrochemicals (America) Ltd., New York, NY, technical bulletin, Nov. 1986. D.W. Van Krevelen, Properties of Polymers, Their Estimation and Correlation with Chemical Structure, Elsevier/North-Holland Inc., New York, NY, 1976, p. 575. I. Pinnau, J.G. Wijmans, I. Blume, T. Kuroda and K.-V. Peinemann, Gas permeation through composite membranes, J. Membrane Sci., 37 (1988) 81-88. D.T. Clark, W.J. Feast, W.K.R. Musgrave and I. Ritchie, Application of ESCA to polymer chemistry. Part IV. Surface fluorination of polyethylene. Application of ESCA to the examination of structure as a function of depth, J. Polym. Sci., Polym. Chem. Ed., I3 (1975) 857890.
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D.T. Clark, W.J. Feast, D. Kilcast and W.K.R. Musgrave, Applications of ESCA to polymer chemistry. III. Structures and bonding in homopolymers of ethylene and fluoroethylene and determination of the compositions of fluoro copolymers, J. Polym. Sci., Polym. Chem. Ed., 11(1973)389-411. A. Puleo, D.R. Paul and P.K. Wong, Gas sorption
and transport
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poly(4-
methyl-1-pentene), Polymer, 30 (1989) 1357. R.B. Bird, W.E. Stewart and E.N. Lightfoot, Transport Phenomena, John Wiley, New York, NY, 1960,744. P.W. Atkins, Physical Chemistry, Oxford University Press, Great Britain, 1982, 750.
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J. Emsley, The Elements, Oxford University Press, New York, NY, 1989. L.J. Bellamy, The Infra-red Spectra of Complex Molecules, Halsted Press, New York, NY,
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32 33 34 35 36 37 38
and P.H. Solomon,
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London, 1940, Chap. VIII. F. Booth, A note on the theory of surface diffusion reactions, Trans Faraday Sot., 44 (1948) 796-801. J. Crank, The Mathematics of Diffusion, 2nd edn., Oxford University Press, New York, NY, 1975,305. D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, John Wiley and Sons, New York, NY, 1974,650. H.J. Bixler and O.J. Sweeting, in: O.J. Sweeting (Ed.), The Science and Technology of Polymer Films, Vol. II, John Wiley and Sons, New York, NY, 1971. M.J. El-Hibri, Effects of molecular orientation and processing conditions on gas sorption and transport in various polymers, Ph.D. Dissertation, University of Texas at Austin, 1986. P. Zoller, H.W. Starkweather, Jr. and G.A. Jones, The heat of fusion of poly(4-methyl pentene-1), J. Polym. Sci., Part B, 24 (1986) 1451-1458. D.R. Paul and S.K. McSpadden, Diffusional release of a solute from a polymer matrix, J. Membrane
Sci., 1 (1976)
33-48.
Appendix
The results presented in this and the accompanying paper [l] support the notion that fluorination of PMP is essentially diffusion limited which has allowed us to successfully unify a variety of observations in terms of a fluorinated layer that increases in thickness with reaction time. It would be very useful to
.2
J
l-
reacted
5 2 B ._8 8 2
dissolved
c,
LL
0
Fig. 13. Illustration layer.
+
x
of the fluorine reaction front and the concentration
profile in the fluorinated
169
be able to predict the rate of advancement of the reaction front, and it is the purpose of this Appendix to show that a relatively simple diffusion model can be useful for this purpose. First, we must address a basic question of chemical mechanism concerning the identity of the species whose diffusion controls the advancement of the reaction front. The apparent reactant is molecular fluorine, F,; however, most plausible mechanisms envision its dissociation product, atomic fluorine radicals, or Fe, as an intermediate that actually reacts with the polymer. Thus, the important question is whether this step in the reaction sequence is far from or near the locus of reaction with the polymer since this influences which diffusion process, i.e. F, or F., governs. Normally one would not expect F. to be very long-lived in the matrix of a polymer with available hydrogens; however, the region of diffusion is the fluorinated layer where all readily reactive hydrogens have already been removed by reaction. In the model development that follows we treat this in a general way and return to the question later when we attempt to quantify it. We assume a sharp front at which reaction occurs instantaneously as illustrated in Fig. 13. The concentration of chemically bound fluorine atoms is r (to be determined from XPS measurements) behind the reaction boundary at x = Zfand is zero in front of the reaction boundary. The concentration of the unbound and mobile reactant is c. At this reaction front we have: (4) where D is the diffusion coefficient of the reactive species in the fluorinated layer while n is a stoichiometric coefficient that is 2 if the reactive species is F, and is unity if it is F.. A pseudo-steady state approximation is usually adequate for such moving boundary problems [ 381 which allows eqn. (4) to be replaced by:
DC s=-lf
rdl,
(5)
n dt
For our reactor, the partial pressure of fluorine in the gas phase changes with time according to a continuous stirred tank model (CSTR): Pt=Pi[l-eXP(
-:)I
(6)
where pi is the partial pressure in the inlet or feed and z is the residence time in the reactor (44 min). Thus, the concentration of mobile fluorine in the polymer at the surface is:
cs= SPt
(7)
170
where S is the Henry’s law solubility followed by integration gives:
coefficient.
Combining
eqns. (5)-(7)
(8) where P= DS is the permeability coefficient of the diffusing reactant. For small t/z when the pressure in the reactor is rising linearly in time, eqn. (8) reduces to: nPPi
If=
-g
(9)
\ifor a linear increase in If with reaction at pi, eqn. (8) reduces to: 2npPi t
If=
;
time. For large t/7 when pL is constant
(10)
: which is the more familiar square root of time dependence. Our fluorination of thin membranes falls in the regime described by eqn. (9), whereas our fluorination of thick (50pm) films will extend into the intermediate region. Owing to our long r, we never fully reach the limit described by eqn. (10). The XPS results in Fig. 9 suggest that there are about 1.64 fluorine atoms per carbon atom, or 69.7 wt.% fluorine, in the fluorinated layer which gives an estimated Tof 0.098 moles F/cm”. One might argue that at very short reaction times (see Fig. 3) a somewhat lower value off should be used, but this turns out to be a very minor issue. In all our experiments, pi was equal to 0.02 atm and r was 44 min. The results in Fig. 12 permit us to estimate the permeability of the fluorinated layer to the diffusion limiting reactant. If this is F,, then n = 2 and we can read off the permeability corresponding to the collision diameter of F, [ 251, 3.65 A, or about that of NS, to get P= 0.09barrers. Substituting these values into eqn. (8) gives the lower line in Fig. 14. It has the same slope and shape as the data points, reproduced from Fig. 10, but it is too low by nearly an order of magnitude. If atomic fluorine controls the process, then n= 1 and we need a higher permeability owing to its smaller size. The fluorine atom is reported to have a collision diameter of 2.70 A [26,27]. Its size evidently lies between that for helium, 2.60 A, and hydrogen, 2.89 A [34]. For this calculation, we use the estimated permeability for helium given in Fig. 12, or P= 7.5 barrers. With this value we calculate the top line in Fig. 14 which agrees remarkably well with the data points. This then is some evidence for F. being the species whose diffusion controls this process; however, some independent chemical evidence is required before any such conclusion is justified. A more detailed and sophis-
Thickness
of Fluorinated Layer (pm)
Science,
Elsevier Science Publishers
55 (1991)
149-171
149
B.V., Amsterdam
Surface fluorination of composite membranes. Part II. Characterization of the fluorinated layer J.M. Mohr, D.R. Paul*, Y. Taru** Department of Chemical Engineering, Austin, Austin, TX 78712 (U.S.A.)
Center for Polymer
Research,
The University
of Texas at
T.E. Mlsna and R.J. Lagow Department of Chemistry, Center for Polymer Austin, TX 78712 (U.S.A.)
Research,
The University
of Texas at Austin,
(Received February 23,199O; accepted in revised form July 30,199O)
Abstract The fluorinated surface layer of membranes exposed to a dilute stream of fluorine gas has been characterized. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy attenuated total reflection (FTIR-ATR) were used to determine the concentration and profile of reacted fluorine in poly(4-methyl-1-pentene) membranes after exposure to fluorine. The thickness of the fluorinated layer as a function of reaction time was calculated using the results of bulk elemental analysis and of XPS. The permeability of this layer was determined using the calculated thickness and the (P,/l,) values reported previously. It is shown that the permeability of the fluorinated material is two orders of magnitude lower than that of the original poly(4methyl-1-pentene ). A simple diffusion limited reaction model is able to give a good estimate of the thickness of the fluorinated layer as a function of fluorine reaction time. Keywords: membrane preparation tion; poly(4-methyl-1-pentene)
and structure;
composite
membrane;
gas separation;
fluorina-
Introduction Gas phase fluorination of polymer films or membranes results in a thin layer of fluorinated material at the contacted surface. This causes a substantial change in the gas transport properties as reported in the accompanying paper [l] dealing with poly (4-methyl-1-pentene) (PMP) materials. In this work, 50 ,um films of PMP and composite membranes with a PMP permselective *Author to whom correspondence should be addressed. **Current address: Musashi Institute of Technology, 28-l Tamazatsumi Tokyo 158, Japan.
0376-7388/91/$03.50
0 1991-
Elsevier Science Publishers
B.V.
I-chome,
Setagaya-ku,
150
layer were fluorinated for times varying from 2.5 min to 96 hr. The structure of PMP is shown below.
fCH,-CH
j CH, CH
CH%H3 Upon exposure to molecular fluorine, some of the hydrogen atoms will be replaced with fluorine atoms while other reactions such as chain scission or crosslinking may follow. After the immediate surface is fluorinated, the reaction proceeds to progressively deeper layers below the surface. This paper deals with characterization of this fluorinated layer using various techniques. The objective here is to learn how much fluorine is incorporated into the PMP structure by reaction and to gain some idea about the depth of the fluorinated layer as a function of the time the surface was exposed to fluorine. A subsequent paper will deal with a more detailed analysis of the placement of fluorine atoms in the PMP repeat unit. The fluorinated polymer created by this type of process has been shown to be restricted to a relatively thin surface layer, perhaps no deeper than 1000 A [ 2-51. To characterize this thin layer, two surface sensitive techniques, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR), were used. In addition to the surface analyses, bulk elemental analysis was done to determine the chemical composition of the sample as a whole after various fluorine exposure times. The latter information can also be used to estimate the depth of fluorination as a function of treatment time by using a simplified diffusion-controlled model for the reaction process. Finally, solubility tests and thermal analysis were done to complement the above techniques. Background and theory XPS, also known as electron spectroscopy for chemical analysis (ESCA), is particularly suited to examine the fluorinated polymer layer since it only probes a short distance into the surface of solids. The surface sensitivity of XPS is a consequence of the short mean free path, or escape depth, of electrons in a solid. XPS experiments measure the kinetic energy of electrons ejected by interactions of a molecule with a beam of soft X-rays. For the case of a substrate material covered with a continuous overlayer of thickness d, the intensity of a core-level signal from the overlayer, Ii, is expected as:
151
Ii = Ii,,
[-4&e)]
(1)
where Ii,, is the intensity arising from an infinitely thick layer of the overlayer material, Si is the electron escape depth, and 8 is the electron emission angle relative to the plane of the sample surface [6-g]. When 0 is 90”) the depth from which the signal arises is governed only by the escape depth of the electrons. Equation ( 1) shows that when d/6i= 1,2, and 3,li/li,, = 0.63,0.87, and 0.95, respectively, when f3is 90”. Hence, to a reasonable approximation, the depth of material actually sampled is given by 36;. The escape depth Sidepends on the material and on the kinetic energy of the electrons with values typically on the order of lo-30 A for polymer materials [8,10,11]. Thus, in the XPS experiments, information is obtained from about the top 90 A of the sample. In this research, XPS was used to determine the chemical composition of this layer and to gain some idea about the depth profile of the fluorine within this layer for PMP composite membranes. However, we found that the depth of fluorination extended beyond the 90 A sampling depth of XPS. To characterize the fluorinated layer beyond 90 A and to ultimately determine the thickness of this layer, other analyses were required. One means of doing this is with ion beam sputtering, where the surface of the sample is eroded with argon ions. Alternate cycles of sputtering and of surface analysis gives compositional information as a function of depth below the surface, a technique called depth profiling [ 71. This technique is accompanied by several problems, such as the preferential ejection of certain electrons which leads to chemical changes in the surface [ 121. Perhaps the largest uncertainty is knowing at what depth the analysis is being done after several cycles of sputtering and analysis; i.e. knowing the sputtering rate of the material. Despite these drawbacks, the fluorinated composite membranes were depth profiled to give a general indication of the fluorine concentration as a function of depth. FTIR-ATR is a technique to obtain the infrared spectrum of species located near the surface of a sample and has been used to characterize interfacially formed membranes [ 13,141, to determine the thickness of reacted surface layers [ 15,161, and to determine the rate of fluorination reactions [ 171. In an ATR experiment, an evanescent IR beam penetrates into the sample to a depth that depends on several factors, but is actually 2-3 orders of magnitude greater than that probed by XPS. The depth of penetration of the IR beam, d,, is given as (18):
(2)
This expression links the effective sampling depth, dp, of an IR beam of wave-
152
length ;1into a sample of refractive index rz2that is in contact with a reflecting element of refractive index n, when the effective angle of incidence is @ For PMP with a refractive index of 1.463 [ 191 and for a KRS-5 reflecting element with a refractive index of 2.38 and at $ equal to 45 O,c&/A is 0.1913. As 3,scans from 3 ,um to 10 ,um (3300 cm-’ to 1000 cm-‘) dp varies from 0.57 pm to 1.9 ,um. Actually, the IR beam is penetrating fluorinated PMP, and it would be more correct to use a refractive index value for fluorinated PMP to determine de. Comparison of the refractive index of polyethylene (1.49) to poly (vinylidene fluoride) (1.42) and to poly(tetrafluoroethylene) (1.35) [20] suggests that fluorination will result in a decrease in refractive index. Therefore, d,, for fluorinated PMP is likely to be slightly less than that calculated for PMP. In order to characterize the surface fluorinated PMP membranes with FTIRATR, the fluorinated layer must comprise a large enough fraction of the 0.571.9 pm penetration depth to be detected. The composite membranes used for the XPS characterization had a permselective layer of PMP ranging in thickness from 0.2 to 20 ,um, with the majority of samples in the 0.7-2 pm range. These membranes were fluorinated for 2.5-15 min. In these composite membranes the PMP layer and the fraction of it that is fluorinated is too small to be detected in an FTIR-ATR experiment. For this reason PMP films 50 pm thick were fluorinated for times ranging from 15 min to 96 hr. The fluorinated layer in these samples was thick enough to be detected. The FTIR-ATR spectra is too coarse to determine detailed changes in the molecular structure, however, the gross progress of the fluorination can easily be detected and the thickness of the fluorinated layer can be estimated. The fluorinated 50 pm PMP films were submitted for bulk elemental analysis. The carbon, hydrogen, fluorine, and oxygen concentrations in the sample as a whole were determined as a function of exposure time. Assuming the fluorination reaction progresses as a sharp front dividing completely reacted material from unreacted material (see Fig. 1A in accompanying paper [ 1]), then the depth to which the fluorination reaction has penetrated, If, can be approximated by: 21f
(% fluorine),
1 - (% fluorine),
(3)
Here, I is the thickness of the film (50 ,um) and the factor of 2 arises because these films were fluorinated from both sides. The weight percent fluorine after fluorination time t, ( % fluorine) t, is determined from bulk elemental analysis. The term (% fluorine), is the weight percent fluorine behind the moving front and is assumed not to change with reaction time. Ideally this value would correspond to replacing every available hydrogen atom with a fluorine atom. The PMP repeat unit has 12 hydrogen atoms and 6 carbon atoms. Thus, the theoretical maximum fluorine concentration occurs when there are 12 fluorine at-
153
oms to every 6 carbon atoms, a fluorine to carbon atomic ratio of 2. This atomic ratio corresponds to 76 wt.% fluorine. XPS indicates that this limit is not reached even at the surface of the membrane after the longest fluorination time. The limit indicated by XPS was 69.7 wt.% fluorine, and this was used as ( % fluorine ) _ Experimental Materials The 50 ,um PMP films were obtained from Mitsui Petrochemicals, Ltd, and were designated TPX-X22 @. The composite membranes consisted of three layers; a polysulfone microporous support, a 1 pm silicone rubber interlayer, and a permselective PMP layer. Preparation of the polysulfone microporous support and application of the silicone rubber layer is described elsewhere [ 211. The polysulfone-silicone rubber membrane was coated with PMP, obtained from Scientific Polymer Products. The polymer was applied from a dilute solution in cyclohexane. The membranes were fluorinated with a 2% fluorine in helium gas stream. Details of the fluorination procedure and of the reactor are described in the preceding paper [ 11. Equipment 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 13 kV and 20 mA and the sample chamber was evacuated to less than 5 x lo-’ Torr. The gold 4f7,2 level at 84 eV binding energy was used for calibration and had a full width half maximum (FWHM) of 1.18+0.1 eV. The spectra were taken with the electron emission angle at 90” to give the maximum sampling depth of approximately 90 A. In one experiment, spectra were recorded with the electron emission angle at 7’) 20’) 30’) and 50 o to collect spectra at sampling depths less than 90 A. Ion sputtering was done with an argon ion gun at 1 kV and an ion current density of 9 @/cm’. The sputtering rate of PMP was determined by casting a thin layer of the polymer onto a polished nickel surface. The thickness of the polymer film, 2800 A, was measured with an AlphaStep 200 profilometer. The polymer film was sputtered until nickel appeared in the XPS spectra. A sputtering rate of 11.5 A/min was determined, which is consistent with other values reported [ 121. Samples were mounted onto the sample probe with double sided adhesive tape. Analysis times were kept short to minimize radiation damage to the sample. The sensitivity factors used were: carbon, 0.2; oxygen, 0.61; and fluorine, 1.0. A Digilab FTS 15/90 FTIR spectrometer and a continuously variable Spectra-Tech Model 301 ATR attachment were used for FTIR-ATR measure-
154
ments. One hundred scans were obtained at a resolution of 2 cm-‘. A KRS-5 internal reflection element was used in the ATR accessory at an incident angle of 45”. Results and discussion Composite membranes XPS
Broad scan spectra for untreated PMP and for PMP after 5 min fluorination are shown in Fig. 1. The primary feature in the unfluorinated spectrum is the C (1s) peak at 285 eV. A small oxygen peak at 533 eV is present, probably due to surface oxidation of the PMP. The spectrum of the fluorinated PMP has a less intense C (1s) peak at 288 eV, an 0 (1s) peak at 533 eV, and a F (1s) peak at 695 eV. The small peak at 34 eV corresponds to the F (2s) electron. Hydrogen is not detected by XPS at the core level electron for this element is also the valence level [ 81. Spectra were recorded for membranes after fluorination times of 1-15 min. The carbon peak from each spectra is shown in Fig. 2. The carbon peak for unfluorinated PMP is narrow and well defined. After just one minute of fluorination, the peak broadens and shifts to a slightly higher binding energy. An additional peak at 288.2kO.2 eV is due to monofluorocarbon
5 min fluorination
1. I I I I I / 0 100 200 300 400 I
/
I
I A---J , , , / , , I,, 500 600 700 800 900 1000
BindingEnergy(eV) Fig. 1.XPS broad scan spectra of untreated PMP and after 5 min fluorination.
155
280
292 288 284 Binding Energy (eV)
296
Fig. 2. Carbon peak of the XPS spectra of PMP after various fluorine exposure times.
environments [ 22,231. The shoulder at 291.5 + 0.7 eV arises from difluorocarbon groups and the bump at 293 eV, seen most clearly in the 12.5 min peak, is due to trifluorocarbon environments [ 231. Deconvolution of the carbon peak into its component peaks can be done to determine the contribution of CF, CFP, and CF3 groups as will be described in a subsequent paper. The concentrations of carbon, fluorine, and oxygen were determined from the spectra at each fluorination time and the results are shown in Fig. 3 as the atomic ratio of fluorine to carbon versus fluorination time. The ratio of fluorine atoms to carbon atoms after 1 min is 0.64 and this increases to 1.0 after 15 min of fluorine exposure. A fluorine to carbon ratio of 1.0 indicates that one-half of the 12 hydrogen atoms of the PMP repeat unit have been replaced by fluorine atoms. PMP is semicrystalline at room temperature. A recent study has shown that gases up to approximately 4 A in molecular diameter are able to diffuse within the PMP crystal, which has gaps of approximately this size [ 241. The fluorine molecule has a diameter of 3.653 A [25] and the fluorine atom has a diameter of 2.70 A [ 26,271. Presumably then, the fluorine atom would
156
- 0.09
6
-0.08
z v
-0.07
E Y! 3
0.6
.g -0.06
J+ I,, 6
-0.05
0
8
Fluorination
,I, 10
,.l?l, 12 14
16
y” .o E 7
_I0.04
Time (mm)
Fig. 3. Fluorine to carbon and oxygen to carbon atomic ratios of PMP after various fluorine exposure times.
be able to enter the crystalline fraction of the polymer and react. The extent to which this might occur depends on the diffusivity of fluorine in the crystalline fraction, which probably is much less than that in the amorphous fraction. Fluorine is very reactive and this will further limit the amount of fluorine that enters the crystalline fraction. Hence, fluorination of the amorphous fraction would be favoured, yet the possibility of the crystalline fraction being fluorinated exists. Thermal analysis of films fluorinated for 90 hr, to be discussed later, suggests that the crystalline regions are fluorinated. Furthermore, the level of fluorine content achieved at the surface and the chemical nature of its placement in the PMP repeat unit give even more convincing evidence of this. Oxygen is present in the XPS spectra in amounts greater than can be attributed to atmospheric contamination. The oxygen to carbon atomic ratio, shown in Fig. 3, remains relatively constant with treatment time. Higher than expected levels of oxygen after fluorination have been observed by others [ 2,3]. Several sources of the oxygen may be considered. One possibility is that long lived radicals in the fluorinated surface react with atmospheric oxygen or water, while another is some source of contamination in the fluorination process. To determine the source of oxygen in this study a controlled experiment was done. A membrane was placed in the fluorination chamber and the reactor was purged for several hours with pure helium to remove all atmospheric oxygen and moisture. The membrane was fluorinated for 5 min, after which the reaction chamber was flushed with helium to remove the fluorine gas. The reactor was then placed in an argon environment where it was opened to remove the fluorinated membrane. A portion of this membrane was loaded into the XPS sample chamber under an argon blanket, so that it was never exposed to the atmosphere. Another portion of the membrane was removed from the argon environment and allowed to sit in the atmosphere overnight before loading it into the XPS
157
sample chamber. The results of the XPS spectra are summarized in Table 1. The oxygen contents of these membranes are the same, suggesting that the source of oxygen is present during the fluorination reaction and that the membranes does not react with atmospheric oxygen after fluorination. Because the reactor was thoroughly purged with helium prior to fluorination, it is unlikely that atmospheric oxygen in the reactor is the source. Commercially available fluorine commonly contains some oxygen as an impurity and this is the most probable source of the oxygen in the fluorinated membranes. The XPS spectra shown above were taken with the electron emission angle, 13,at 90’ so that the sampling depth is at its maximum of N 90 A. By using smaller electron emission angles the composition of the membrane at depths less than 90 A can be estimated. Figure 4 shows the fluorine to carbon atomic ratio as a function of sin 8 for membranes after 1, 5, and 15 min fluorination. At the top of this figure is a scale showing the approximate depth of the layer TABLE 1 Results of XPS experiment to determine the source of oxygen in surface fluorination* Unfluorinated (control)
Atomic %
5 min fluorination
99 0 1
Carbon Fluorine Oxygen
air
argon
46 48 6
42 52 6
“After 5 min fluorination, a part of the membrane was exposed to the atmosphere while the other part was kept under an argon blanket. Sampling Depth
(A)
1.21,
0.71 0.0
1 0.2
’ 0.4
’ 0.6
’ 0.8
’ 1.0
1
Fig. 4. Atomic ratio of fluorine to carbon as a function of electron emission cngle in the XPS experiment. As the emission angle varies the sampling depth varies from - 10 A to - 90 A.
158
that is sampled at electron emission angle 0. The scale was calculated based on the approximation that 95% of the signal arises from a depth of 36isin 8, where a value of 30 A was used for Si, the escape depth. From these results it is clear that fluorination extends more deeply than the maximum sampling depth (&9O”C) possible with XPS. There is evidently some spectral variation in the fluorine content within this layer, but on a relative basis the variation is not great. There is very little difference in the results obtained after 5 and 15 min of fluorination. This suggests that the immediate surface reaches a maximum attainable level of fluorination within 1 to 5 min and changes very little thereafter. Argon ion sputtering was used to profile the membrane beyond the maximum sampling depth of the XPS. The sputtering rate of PMP was first determined to establish the relationship between the analysis depth and sputtering time. A thin film of PMP was cast onto a polished nickel substrate and the thickness of the film was determined to be 2800 A with a profilometer. The polymer surface was sputtered with an argon ion gun and analyses were made periodically to watch for the appearance of nickel in the XPS spectra. Eventually, nickel begins to appear in the signal. The nickel/carbon atomic ratio extrapolates to a breakthrough time of 244 min as shown in Fig. 5. Thus, an average sputtering rate of 2800 A/244 min= 11.5 A/min was established. Membranes fluorinated for 2 min and for 15 min were depth profiled using argon ion sputtering. Figure 6 shows the fluorine to carbon atomic ratio vs. sputtering time. The amount of fluorine beyond 50 min of sputtering is constant, but this may be an artifact of the sputtering process, as the polymer is not clearly stripped away from the surface. The time at which the fluorine concentration ceases to drop sharply and begins to level off is likely to correspond closely to the actual depth of fluorination. Using the sputtering rate
Sputtering Conditions - Argon ion gun, 20111.4 .s 2 .v E z 8 f G 3 z Is z
0.03
ion current density =
0.02
0.01
Sputter Time (min)
Fig. 5. Atomic ratio of nickel to carbon vs. argon ion sputtering time for a 2800 i thick PMP film cast onto a nickel substrate.
159
Sputter Time (min) I 0
I
240
I
I
460
I
I
690
I
I
920
I
I
I
1150
I
1380
Estimated Thickness of Fluorinated Layer (A)
Fig. 6. Depth profile by argon ion sputtering of PMP membranes fluorinated for ,2 min and for 15 min. Estimated thickness of fluorinated layer based on a sputtering rate of 11.5 A/min.
determined for unfluorinated PMP, this corresponds to a depth of 140 A for the 2 min sample and 490 A for the 15 min sample. Because of the artifacts and uncertainties involved in depth profiling with ion sputtering, these results must be interpreted with caution. Other methods described below were used to gain further estimates of the thickness of the fluorinated layer. Thick film (50 pm) To profile the fluorinated region beyond the 90 A that can be probed by XPS directly, FTIR-ATR and bulk elemental analysis methods were employed. To use these techniques the fluorinated layer must be thick enough to be detected, and for this reason thick films (50 pm) were fluorinated for long times. These films were fluorinated from both sides for times ranging from 15 min to 96 hr. FTIR-ATR Figure 7 shows the FTIR-ATR spectrum of unfluorinated PMP. The peaks in the region 2950 cm-‘-2850 cm-’ arise from stretching and bending vibrations of C-H bonds in CH3, CH2, and CH groups. The adsorption at 1470 cm-’ is associated with the deformation vibrations in CH, and CH, groups [ 28,291. The third feature of interest is the peak at 1170 cm-l with a shoulder at 1145 cm-‘. This is characteristic of gem-dimethyl groups [ 291, corresponding to the CH ( CHB) 2 portion of the pendant group in the PMP repeat unit. Spectra after 30 min, 2 hr, and 5 hr fluorine exposure are also shown in Fig. 7. The intensity of the bands in the 2950 cm-‘-2850 cm-’ region decreases with fluorination, until this band disappears after 5 hr of fluorination. The CF, CF2, and
160
2 hours fluorination
5 hours fluorination
r 4000
3500 3000 2500 2000 Wavenumbers
1500
1000
500
(cm-‘)
Fig. 7. FTIR-ATR scans of untreated PMP and after 30 min, 2 hr, and 5 hr of fluorine exposure.
depth of penewat~on =
1.3pnl
Fluorination Time (hours)
Fig. 8. Absorbance of the 2950 cm-’ and of the 1470 cm-’ bands in the FTIR-ATR spectra of PMP vs. fluorination time. The depth of penetration of the IR beam into the sample surface is calculated via eqn. ( 2 ) .
161
CF3 stretching vibrations occur in the 1400 cm-l-lOOO cm-’ region [ 281. The intensity and broadness of the peak in this area increases with fluorination time, eventually masking the other peaks in the region. The FTIR-ATR spectra are too coarse to relate to specific structural changes in the PMP repeat unit, but information concerning the rate and depth of fluorination can be derived. Figure 8 shows the absorbance of the 2950 cm-’ and the 1470 cm-’ bands as a function of fluorination time. The intensity of these bands decreases with fluorination as the CH bonds are replaced with CF. The steeper decline in the 2950 cm-l absorbance with time reflects the more shallow sampling depth at this wavelength. At 2950 cm-l the depth of penetration is 0.65 pm whereas at 1470 cm-l the penetration depth is 1.3 ,um. Lines drawn tangent to the maximum rate of change and to the baseline intersect at approximately the time when the depth of fluorination is equal to the penetration depth of the evanescent IR beam. The tangent lines intersect at 2.2 hr for the 2950 cm-l absorbance, while this occurs at 5 hr for the 1470 cm-’ absorbance. XPS and bulk elemental analysis XPS and bulk elemental analyses were used to calculate the thickness of the fluorinated layer via eqn. (3). The 50 pm PMP films were submitted to Galbraith Laboratories, Inc. (Knoxville, TN) for bulk elemental analysis and Table 2 summarizes the weight percent of carbon, fluorine, hydrogen, and oxygen found. These weight percents were converted to atomic percents, and the fluorine to carbon atomic ratio is shown as a function of fluorination time in Fig. 9. XPS was used to determine the fluorine content at the surface of each film. The results, also shown in Fig. 9, indicate that the fluorine to carbon ratio at the surface is nearly constant after 2 hr of fluorination. The large difference between the XPS and the bulk analyses at short fluorination times indicates TABLE 2 Bulk elemental analysis of PMP after various fluorination times to both sides of the film Fluorination time
(hr) 0
0.25 0.50 1 2 5 96
Element (wt.% ) Carbon
Hydrogen
Fluorine
Oxygen
86.1 85.4 85.6 84.3 84.4 82.2 63.1
13.9 14.3 14.2 14.4 14.3 13.7 9.1
0.02 0.15 0.43 1.48 1.80 4.0 27.0
0.39 < 0.50 co.50 0.68 0.80 1.4 B
.
“Unable to determine oxygen in presence of high percentage fluorine.
162
Fluorination Time (hours)
Fig. 9. Fluorine to carbon atomic ratio in the surface of fluorinated PMP and in the sample as a whole (determined by bulk elemental analysis). indicates the maximum ratio for PMP.
(determined by XPS) The dashed line at 2
that the fluorine is originally confined near the surface. As the fluorination time increases the bulk analysis approaches the XPS result since the fluorination extends more deeply into the sample. The dashed line in Fig. 9 indicates the maximum possible ratio of fluorine atoms per carbon atom for PMP. This occurs when the 12 hydrogen atoms on the PMP repeat unit are each replaced by a fluorine atom. This theoretical maximum of 2 is not reached even after 96 hr of fluorination. The surface ratio of fluorine atoms per carbon atom reaches a maximum of 1.65 after 5 hr of fluorination and remains at this value even after 96 hr of fluorination. The maximum fluorine to carbon atomic ratio of 1.65 corresponds to 69.7 wt.% fluorine, and this was used as the limiting fluorine concentration, (% fluorine),, in eqn. (3). The weight percent fluorine determined from XPS is based on the carbon, fluorine, and oxygen content only, since hydrogen is not measured by XPS. However, at these long fluorine exposure times the hydrogen content should be negligible. The weight percent fluorine as a function of fluorination time, (% fluorine),, was determined by bulk elemental analysis (Table 2). The thickness of the fluorinated layer calculated by this approach is shown in Fig. 10 by the open points. The thickness of the fluorinated layer is 0.054 pm (540 A> after 15 min of fluorination and 9.7 pm after 96 hr of fluorine exposure. The closed circles at 2 min and 15 min in Fig. 10 indicate the thickness of the fluorinated layer determined by XPS ion sputtering. For the 15 min sample, the thickness of 460 A determined by ion sputtering agrees with the calculated thickness of 540 A. The two triangles in Fig. 10 are the thickness of the fluorinated layer at 2.2 hr and 5 hr fluorination time determined by FTIR-ATR. Considering all the limitations of these different meth-
163
Fluorination Time (min)
Fig. 10. Thickness of the fluorinated layer vs. fluorination time. Thickness values for bulk elemental analysis were calculated via eqn. (3 ) .
ods and their simplifying assumptions, the agreement among them is quite remarkable. The data in Fig. 10 suggest that the thickness of the fluorinated layer is directly proportional to fluorination time. For diffusion controlled processes, one generally expects the depth to be proportional to the square root of time [ 30-331; however, the situation in this research is slightly different in that the gas phase fluorine concentration inside the reactor is time dependent. A model for the system used here is developed in the Appendix, and it shows that the thickness of the fluorinated layer should be linearly related to fluorination time in the early stages and eventually approach the square root of time regime after rather long fluorination times. Table 3 summarizes the thickness of the fluorinated layer for membranes for reaction times of 15 min and less, extrapolated from the line through the data in Fig. 10. In the gas transport studies described in the preceding paper, the permeation resistance [or (P/Z) = (permeability coefficient) / (thickness of layer) ] of the fluorinated PMP layer was determined for the composite membranes as a function of fluorination time. Having estimated the thickness of the fluorinated layer, the absolute permeability of the fluorinated layer for each gas can be estimated using the previously determined (PJZ,) values. The permeabilities so determined are listed in Table 3 for six gases. For comparison, the permeability of PMP is also shown. After 2.5 min of fluorination the permeability of this layer to most gases is about one to two orders of magnitude less than that of PMP. The values for methane and carbon dioxide are nearly two orders of magnitude less. The permeability coefficients calculated for the fluorinated layer after 5 min are even lower. The permeability values after 5, 10, and 15 min fluorination are remarkably similar. The closeness of these
164 TABLE
3
Thickness of the fluorinated layer, l,, for PMP films after various fluorination from the line in Fig. 10 Fluorination
time
times as determined
Permeability”,d
lr (pm)
(min)
N2
02
CH,
HZ
He
CC*
8.5
31.6
20.1
120.8
88.2
107.8
2.5 5 10
0.011 0.025 0.048
1.8 0.088 0.087
7.8 0.44 0.41
0.40 0.059 0.066
12.5 5.9 8.8
9.6 7.5 _c
5.2 2.4 2.1
15
0.054
0.048
0.20
0.050
3.2
7.5
1.1
Ob
“Units: lo-“’ bTPX-X22@
cm3-cm/set-cm’-cmHg. at 35°C.
“Helium data not available for the 10 min exposure time. dPermeability of the fluorinated layer determined from ( Pf/lf) values at 25’ C reported in preceding paper ( 1).
values suggests that there is very little change in the material behind the reaction front after fluorination times beyond 5 min. This agrees with the XPS data in Fig. 4 which also indicate that the reaction at the surface is complete after about 5 min of fluorination. Longer fluorination times result in a fluorinated layer that has essentially the same extent of fluorination as shorter times, the difference in (I’,/&) is primarily due to the difference in depth of fluorination. The dashed line in Fig. 11 shows the typical trade-off between selectivity
60-
PMP o
0’0.1
‘,I.,,’
1
~~111’ 10
‘,,.,,’
100
-I
1
Fig. 11. Carbon dioxide/methane selectivity against carbon dioxide permeability for typical polymers (dashed line) and PMP before and after 5 and 10 min of fluorine exposure.
165
2.5
3.0
3.5
4.0
Kinetic Diameter (A)
Fig. 12. Permeability of coefficient as a function of kinetic diameter of the gas for polyethylene (PE), poly(4-methyl-1-pentene) (PMP), poly(vinylidenefluorine) (PVDF) and fluorinated PMP (F-PMP). The closed data points for F-PMP are for 5 (m) and 15 (0) min fluorination times and define a band for this polymer.
and permeability for polymer materials using the gas pair carbon dioxide and methane as an example, taken from Fig. 2 in the accompanying paper [ 11. The data points are for PMP before fluorination and after 5 and 10 min of fluorine treatment. Untreated PMP falls slightly below the curve; however, PMP fluorinated for 5 or 10 min falls on and above the trade-off line. In Fig. 12 the permeability vs. the kinetic diameter [ 341 of six gases for PMP and for fluorinated PMP is shown along with polyethylene [35] and poly (vinylidene fluoride) [36]. The closed data points denote the 5 and 15 min fluorinated PMP and define a band that represents fluorinated PMP. The permeabilities of polyethylene and poly (vinylidene fluoride) differ by about two orders of magnitude for the six gases compared. This is also the case for PMP and fluorinated PMP. It is also interesting to note the similarity in the shapes of the curves for poly (vinylidene fluoride) and fluorinated PMP. The permeability of the fluorinated PMP layer responds to gas molecular size in much the same way as other polymers. This layer has similar selectivity as other polymers with this permeability. Solubility and thermal analysis PMP is soluble in a number of solvents at or just below their boiling point. After fluorination, the 50 pm PMP films are not soluble in cyclohexane or in chloroform, two of the solvents used to prepare PMP membranes. Dimethylformamide is a solvent for polyvinylfluoride; however, fluorinated PMP is not soluble in it. The lack of solubility indicates either that the surface is crosslinked or that the fluorinated PMP is simply not soluble in these solvents.
166
Differential scanning calorimetry was done on specimens cut from the 50 ym PMP films that were fluorinated for 5 and 96 hr. There was no observable change in the melting or glass transition temperatures relative to the unfluorinated PMP. However, there was a reduction in the endotherm area of the 96 hr fluorinated sample. The endotherm area, or heat of fusion, for the untreated PMP was 17.5 J/g. The sample fluorinated for 5 hr had a heat of fusion of 17.1 J/g. After 96 hr of fluorination the heat of fusion was 11.5 J/g. Using 61.9 J/g for the heat of fusion of the 100% crystalline polymer [ 24,371 this corresponds to 28.3% and 28.0% crystallinity for the untreated and the 5 hr fluorinated PMP. The 96 hr fluorinated sample is then 18.6% crystalline. This decrease in crystallinity suggests that the fluorination disrupts the crystalline fraction of the polymer, perhaps by reacting within the crystal structure itself. Conclusions
A combination of surface sensitive techniques and bulk elemental analysis was used to determine the nature of the fluorinated layer in surface fluorinated PMP. From XPS, the elemental composition within the top 90 A of the surface was found and how the composition varies within this sampling depth was examined by angular dependent studies. It was found that fluorination extends beyond the 90 A layer. As a result, three additional methods were used to gain information about the thickness of the fluorinated layer as a function of fluorination time, viz. argon ion sputtering, FTIR-ATR, and bulk elemental analysis. Agreement among the three methods was quite good and the data suggest that the thickness of the fluorinated layer increases linearly with fluorination time. This linear relationship is mathematically consistent with a simple model for a diffusion controlled reaction when the surface concentration of the reactant is driven in time as done in this work. The permeability of the fluorinated layer was estimated from the (P&) values reported in the preceding paper and the estimated thickness of the fluorinated layer. The permeability of fluorinated PMP is one to two orders of magnitude lower than untreated PMP and has a selectivity typical of other polymers of this permeability. The benefit of fluorination of gas separation membranes for achieving a better combination of selectivity and productivity stems for the spatial heterogeneity of the fluorinated structure as described in the first paper in this series. Acknowledgement
This material is based in part upon work supported by the Texas Advanced Technology Program under Grant No. 1607 and by the Separations Research Program at the University of Texas at Austin. We also acknowledge Jeff Cook of the University of Texas at Austin for his contribution in the XPS studies.
167
References 1 2 3
4 5
6 7 8
9 10 11
12 13 14 15 16
17
18 19 20 21 22
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Appendix
The results presented in this and the accompanying paper [l] support the notion that fluorination of PMP is essentially diffusion limited which has allowed us to successfully unify a variety of observations in terms of a fluorinated layer that increases in thickness with reaction time. It would be very useful to
.2
J
l-
reacted
5 2 B ._8 8 2
dissolved
c,
LL
0
Fig. 13. Illustration layer.
+
x
of the fluorine reaction front and the concentration
profile in the fluorinated
169
be able to predict the rate of advancement of the reaction front, and it is the purpose of this Appendix to show that a relatively simple diffusion model can be useful for this purpose. First, we must address a basic question of chemical mechanism concerning the identity of the species whose diffusion controls the advancement of the reaction front. The apparent reactant is molecular fluorine, F,; however, most plausible mechanisms envision its dissociation product, atomic fluorine radicals, or Fe, as an intermediate that actually reacts with the polymer. Thus, the important question is whether this step in the reaction sequence is far from or near the locus of reaction with the polymer since this influences which diffusion process, i.e. F, or F., governs. Normally one would not expect F. to be very long-lived in the matrix of a polymer with available hydrogens; however, the region of diffusion is the fluorinated layer where all readily reactive hydrogens have already been removed by reaction. In the model development that follows we treat this in a general way and return to the question later when we attempt to quantify it. We assume a sharp front at which reaction occurs instantaneously as illustrated in Fig. 13. The concentration of chemically bound fluorine atoms is r (to be determined from XPS measurements) behind the reaction boundary at x = Zfand is zero in front of the reaction boundary. The concentration of the unbound and mobile reactant is c. At this reaction front we have: (4) where D is the diffusion coefficient of the reactive species in the fluorinated layer while n is a stoichiometric coefficient that is 2 if the reactive species is F, and is unity if it is F.. A pseudo-steady state approximation is usually adequate for such moving boundary problems [ 381 which allows eqn. (4) to be replaced by:
DC s=-lf
rdl,
(5)
n dt
For our reactor, the partial pressure of fluorine in the gas phase changes with time according to a continuous stirred tank model (CSTR): Pt=Pi[l-eXP(
-:)I
(6)
where pi is the partial pressure in the inlet or feed and z is the residence time in the reactor (44 min). Thus, the concentration of mobile fluorine in the polymer at the surface is:
cs= SPt
(7)
170
where S is the Henry’s law solubility followed by integration gives:
coefficient.
Combining
eqns. (5)-(7)
(8) where P= DS is the permeability coefficient of the diffusing reactant. For small t/z when the pressure in the reactor is rising linearly in time, eqn. (8) reduces to: nPPi
If=
-g
(9)
\ifor a linear increase in If with reaction at pi, eqn. (8) reduces to: 2npPi t
If=
;
time. For large t/7 when pL is constant
(10)
: which is the more familiar square root of time dependence. Our fluorination of thin membranes falls in the regime described by eqn. (9), whereas our fluorination of thick (50pm) films will extend into the intermediate region. Owing to our long r, we never fully reach the limit described by eqn. (10). The XPS results in Fig. 9 suggest that there are about 1.64 fluorine atoms per carbon atom, or 69.7 wt.% fluorine, in the fluorinated layer which gives an estimated Tof 0.098 moles F/cm”. One might argue that at very short reaction times (see Fig. 3) a somewhat lower value off should be used, but this turns out to be a very minor issue. In all our experiments, pi was equal to 0.02 atm and r was 44 min. The results in Fig. 12 permit us to estimate the permeability of the fluorinated layer to the diffusion limiting reactant. If this is F,, then n = 2 and we can read off the permeability corresponding to the collision diameter of F, [ 251, 3.65 A, or about that of NS, to get P= 0.09barrers. Substituting these values into eqn. (8) gives the lower line in Fig. 14. It has the same slope and shape as the data points, reproduced from Fig. 10, but it is too low by nearly an order of magnitude. If atomic fluorine controls the process, then n= 1 and we need a higher permeability owing to its smaller size. The fluorine atom is reported to have a collision diameter of 2.70 A [26,27]. Its size evidently lies between that for helium, 2.60 A, and hydrogen, 2.89 A [34]. For this calculation, we use the estimated permeability for helium given in Fig. 12, or P= 7.5 barrers. With this value we calculate the top line in Fig. 14 which agrees remarkably well with the data points. This then is some evidence for F. being the species whose diffusion controls this process; however, some independent chemical evidence is required before any such conclusion is justified. A more detailed and sophis-
Thickness
of Fluorinated Layer (pm)