Langmuir 1991, 7, 2146-2152
2146
Surface Selective Chemical Modification of Fluoropolymer Using Aluminum Deposition Neil B. McKeown, Peter G. Kalman,* Rana Sodhi,+Alex D. Romaschin, and Michael Thompson* Max Bell Research Centre, Toronto General Hospital, 200 Elizabeth Street, Toronto, Ontario M4C 1 G6, Canada Received November 26,1990. In Final Form: April 15, 1991 The surface of poly(tetrafluoroethy1ene)in both skived and expanded forms has been modified by sequential deposition of aluminum from the vapor phase followed by aqueous removal after each step. Maximum introduction of new surface functional groups occurred after three depositions of the metal. Angular-dependent X-ray photoelectron spectroscopy revealed that the depth of optimal modification is restricted to 3 nm from the surface and that the oxygen is present in the form of hydroxyl groups. Secondary ion mass spectrometry confirmedthat the modified material is mostly saturated hydrocarbon. The advancing contact angle for water was reduced from 114O on virgin polymer to 70° on modified surfaces. Finally, scanning electron microscopy showed that the delicate fibrils of the expanded polymer used in vascular grafts are in no way altered by the deposition process.
Introduction Chemical modification of the surface of various polymeric materials can alter a number of properties such as wettability,l adhesive joint strength,2 protein adhesion? and the ability to form covalent bonds between the surface and suitably reactive compound^.^ Polymers composed of fluorocarbons, and in particular poly(tetrafluor0ethylene) (PTFE),exhibit characteristics typical of low surface energy materials including low wettability and poor adhesive joint strength when bonded directly with many metals, other polymers, and commonly encountered adhesive~.~The property of interest to this group is associated with the thrombogenic reactions that can occur between the surface of expanded PTFE (ePTFE) and blood. This behavior has been blamed for the poor performance of small diameter synthetic arterial substitutes made from this material.6 Covalent attachment of organic species to a biomaterial can provide a useful route to altering the physical and biologic properties of that surface. With respect to PTFE, the lack of indigenous reactive functional groups (e.g. hydroxyl, amino, etc.) has meant that primary surface modification is required before such species can be immobilized onto the polymer surface. The introduction of primary reactive species onto PTFE has been effected by both chemical7s8andgas plasma treatmenkg Included in the latter category is a recent paper which used a plasma composed of water and hydrogen to achieve a hydroxylated surface.'O A selective introduction of surface hy-
* Authors to whom correspondence should be addressed. Present address: Surface Science Unit, Centre for Biomaterials, University of Toronto, 170 College St., Toronto, Ontario M5S 1A1, Canada. (1) Schonhorn, H. Polym. Eng. Sci. 1977, 17, 440. (2) Skeist, I. In Handbook ofddhesiues, Van Nostrand Reinhold: New York, 1990; p 86. (3) Horbett, T. A. In Proteins at Interfaces; Brash, J. L., Horbett, T. A. Eds.; American Chemical Society: Washington, DC, 1987; p 239. (4) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988, 21, 2318. (5) Dwight, D. W.; Riggs, W. M. J. ColloidInterface Sci. 1974,47,650. (6) Formichi, M. J.; et al., Ann. Vascular Surg. 1988, 2, 14. (7) Benderly, A. A. J. Appl. Polym. Sci. 1962, 6, 221. (8)Rye, R. R.; Kelber, J. A. Appl. Surf. Sci. 1987, 29, 397. (9) Moria, M.; Occhiello, E.; Garbassi, F. Lanamuir 1989,5872. and references therein. (10) Vargo, T. G.; Hook, D. J.; Litwiler, K. S.; Bright, F. W.; Gardella, J. A. Polym. Mater. Sci. Eng. 1990, 62, 259. f
droxyl groups has been reported that uses a treatment involving the benzoin dianion, followed by reaction with borane and finally, oxidation with aqueous sodium peroxide solution.11J2 Cruder modifications have been performed by treatment of the PTFE surface with solutions of alkali metals in ammonia5J3 or sodium/anthracene complex in ether Alkali-metal amalgams have also been shown to reduce PTFE,14J6as has sodium metal when deposited onto the polymer surface.le Unfortunately, these reductive treatments seem to share a common mode of reaction and produce an electrically conducting carbonaceous layer on the fluoropolymer. The depth of this layer cannot be easily controlled because of the high rate of the reaction which produces a large difference between the maximum and minimum depth of defluorinated material.11J2 Generally, in all the reductive reactions mentioned above, the fluoropolymer is darkened in color, indicating the presence of a substantial thickness of light-absorbing material, and furthermore the morphology of the surface is changed drastically. Recently, however, it has been shown that a degree of control over the etching depth could be achieved by prior cross-linking of the polymer using irradiation techniques.17J8 It has been reported previously,that the surface of PTFE can be modified by placing molten PTFE in contact with aluminum metal foil which had been previously activated by treatment with acid.lg A similar modification to the surface of fluoroethylenepropylene (FEP) could be brought about by the deposition of a thin layer of aluminum metal onto the polymer.20 In both cases,the modifiedfluoropolymer was exposed when the metal was dissolved in dilute aqueous sodium hydroxide solution. The resulting surface layer was reported to be composed of an oxygen-containing hydrocarbon material. The present paper is concerned with an appraisal of the (11) Costello, C. A.; McCarthy, T. J. Macromolecules 1984,17,2940. (12) Costello, C. A.; McCarthy, T. J. Macromolecules 1987,20,2819. (13) Chakrabarti, N.; Jacobus, J. Macromolecules 1988,21,3011. (14) Jansta, J.;Doueek, F. P.; Riha, J. J.Appl. Polym. Sci. 1976,19, 3201. (15) Jansta, J.; Dousek, F. P.; Electrochim. Acta 1973, 18, 673. (16) Leonard, E. C.; Erwin, L. U.S. Patent No. 4 855 018, 1989. (17) Rye, R. R.; Shinn, N. D. Langmuir 1990,6, 142. (18) Rye, R. R. Langmuir 1990,6, 338. (19) Ryan, F. W.; Schonhom H. U S . Patent No. 3 635 938, 1972. (20) Roberts, R. F.; Ryan, F. W.; Schonhorn, H.; Sessler, G. M.; West, J. E. J. Appl. Polym. Sci. 1976, 20, 255.
0743-7463I 9 1/24O1-2146%O2.5O/O 0 1991 American Chemical Societv
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Chemical Modification of Fluoropolymer
aluminum deposition method in order to ascertain whether it is more surface selective than other technqiues and therefore to establish its possible applicability to the modification of the delicate microfibrillar structure of ePTFE. Experimental Section Materials and Reagents. Skived virgin PTFE (Warehouse Plastics, Toronto), Teflon FEP (Norton Canada Inc.), and ePTFE (W. L. Gore and Associates,Flagstaff, AZ) were cleaned by Soxhlet extraction using tetrahydrofuran (THF) as the cleansing solvent for at least 4 h prior to use. This process allowed for the creation of contaminant-free polymer surfaces, as assessed by X-ray photoelectron spectroscopy (XPS). Aluminum metal wire (1mm diameter) and sodium hydroxide were obtained from Aldrich Chemical Co., as were dichlorodimethylsilane (DCDMS) and (aminopropy1)triethoxysilane (APTES) which were purified by distillation. Toluene was distilled over sodium. All water used was first doubly distilled and further purified by a Milli-Q filter system. Equipment. Aluminum deposition was achieved by using a Key High Vacuum metal deposition unit. The parameters used during deposition were those described by Roberts et al." The pressure in the deposition chamber during deposition was 2 X lo+' mbar and the distance between the tungsten filament and the polymer target was adjusted to 0.3 m. The rate of deposition was maintained at approximately 0.2 nm s-'. All XPS spectra were acquired with a Leybold MAX 200 XPS system employing an unmonochromatized Mg X-ray source with photoelectrons sampled from an area 1 mm in diameter. Each sample was analyzed at angles of 90°, 60°, 45O, and 30' relative to the electron detector. The survey spectra (binding energy range, 0-lo00 eV) were obtained by using a pass energy of 192 eV. The apparent elemental surface composition was calculated from the satellite subtracted survey spectra normalized for constant transmission using the software provided by the manufacturer. The sensitivity factors employed in these calculations (F(1s) = l , C(1s) = 0.34, O(1s) = 0.78, Si(2p) = 0.4, N(1s) = 0.54) were empirically derived for the Max 200 spectrometer by Leybold and gave a ratio of carbon to fluorine of 1:2 for clean PTFE. The high-resolution spectra (binding energy range, 310-280 eV) were acquired with a pass energy of 48 eV and then peak fitted by using the software provided with the spectrometer. Damage of PTFE was reported recently due to unmonochromatic (and monochromatic)X-raysources.*l* In order toconfim that the observed modification of the fluoropolymers was caused only by the aluminum deposition process, a 90° survey scan was repeated after the angular dependence survey and high-resolution analyses. The X-ray damage was previously measured by the decrease in intensity of the fluorine 1s peak. The repeat elemental analysis of the polymer surfaces, however, indicated that the decrease in intensity of this peak due to the prior XPS analyses was, at most, only around 3% for unmodified PTFE and less for modified samples. The positive and negative secondary ion mass spectrometry (SIMS) spectra were obtained on a Leybold SIMSJSNMS 200 module attached to the MAX 200 system. In order to generate the spectra a 5-kV Ar+beam was rastered over 2 mm X 2 mm area. The ion beam was produced by a Leybold IQE 12/38 ion gun of approximately 300 pm diameter. The pressure in the ion gun chamber was 3 X 10" mbar and an electron emission current of 0.1 mA was used to produce the ions. These settings were consistent with static SIMS conditions. The positive SIMS spectrum was optimized by maximizingthe signal on increasingly higher masses (31,69,and93 amu). Once optimized,the spectrum was collected on a new area. A minimal amount of charge compensation was required from an electron flood gun. The negative ion spectra were optimized on masses 19 and 38 amu. Scanning electron micrographs were obtained with a Hitachi 5-570 system; the operating voltage was 18 kV. The ePTFE samples were gold coated in a Polaron gold deposition unit. (21) Chaney, R.; Barth, G.Freseniua' 2.A w l . Chem. 1987,329,143. (22) Wheeler, D.R.; Pepper, S. V. J. Vac. Sci. Technol. A. 1990,8, 4046.
Advancing water contact angles were measured on a RemyHart goniometer at room temperature. Procedures. In order to modify PTFE, FEP, and ePTFE, between 3 and 4 nm of aluminum metal were deposited on the polymer surfaces using the conditions described above. The reverse side of each of the polymer samples was left unmetaliid in order to allow it to be used as a control. In the case of FTFE or FEP, the metalized polymer was first immersed in an ultrasonically agitated aqueous sodium hydroxide solution (0.1 M) to dissolve the aluminum and then washed with copious quantities of water. However, this technique did not remove all the metal from ePTFE due to the inability of the aqueous solution to penetrate into the pores of the hydrophobic material. Prewetting the metalized ePTFE by immersion of the sample into ethanol overcame this difficulty. The ethanol was displaced from the sample with pure water under ultrasonic conditions prior to removal of the metal by the sodium hydroxide solution. The modified ePTFE samples were then washed thoroughly with water and dried in an oven at 50 OC. Sequential depositions and removals of aluminum were performed by using exactly the same protocols. Finally, for the purpose of silanization, samples of ePTFE modified by three depositions of aluminum were placed in anhydrous toluene (50 mL) and the silane (APTES or DCDMS) added (0.5 mL). The reaction was allowed to proceed for 12 h under anhydrous conditions. The polymer samples were then removed to a Soxhlet extractor and cleaned by using THF as the solvent.
Results and Discussion XPS Analysis of Aluminum-Modified PTFE. Deposition and subsequent removal of a thin film of aluminum cause a significant change in the appearance of the XPS wide scan spectrum of PTFE. New peaks at 532 and 285 eV binding energy, assigned to O(1s) and hydrocarbon C(ls), respectively, are found in addition to the expected signals originating from fluorine (Is,689 eV) and fluorocarbon (C(ls),292 eV). No spectra taken on the reverse side of any sample exhibited these changes. The degree ofmodification observed in our work is not as high as that found by Roberta et aL20 in an earlier study of FEP. However, precise comparison between the two studies is not possible, because the angle of photoelectron detection was not specified in the earlier report. The relative intensity of the O(1s) and hydrocarbon (1s) peaks compared to those of the F(1s) and fluorocarbon (1s) signals can be increased significantly by repeating the aluminum deposition and removal process (Table I and Figure 1). Optimal modification is achieved after three such applications, but a fourth treatment decreased the degree of modification. The high-resolution C( le) spectra acquired after three aluminum applications (Figure 2) suggest that four types of carbon species can be distinguished, with the two largest peaks at 285.0 and 292.2 eV, respectively. The smaller peak, found at a position around 3.5 eV higher binding energy than the hydrocarbon peak, correspondsto the expected binding energy of a polymeric carbon bonded to both a hydrogen and a fluorine atom. This assignment was based on experimental data for a fluorine-containing polymer speciesz3in addition to XPS analyses of ion beamz4 and hydrogen plasma modified PTFE s~rfaces.~5The remaining peak in the C(1s) spectrum has a binding energy of 286.7 eV which is characteristic of a carbon attached via a single covalent bond to an oxygen atom in, for example, a hydroxyl group. Evidence to support this assignment was obtained by (23) Clark, D. T.; Feast, W. J.; Kilcaet, D.; Mugrave, W. K. R. J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 389. (24) Takahagi, T.; Iehitani, A. Macromolecules 1987,20, 404. (25) Clark, D. T.; Hutton, D. R. J. Polym. Sci., Part A: Polym. Chem. 1987,25, 2643.
McKeown et ai.
2148 Langmuir, Vol. 7, No. 10,1991 '
Table I. Elemental Composition of Modified FEP, PTFE, and ePTFE Obtained from XPS Analysis no.of detector A1 deps angle, deg C, % F, % 32.2 66.4 FEPb 0 1 51.7 40.8 FEPb 36.1 62.1 1 90 FEP 38.2 59.3 60 45 38.0 59.3 38.8 58.8 30 33.6 66.3 PTFEC 0 1 90 37.2 60.6 PTFE 60 37.0 60.2 37.9 59.2 45 38.6 58.4 30 41.4 53.9 PTFE 2 90 60 43.3 35.1 45.7 48.9 45 49.1 44.6 30 3 90 56.0 36.1 PTFE 56.8 35.0 60 57.2 33.5 45 64.8 24.7 30 PTFE 4 90 49.5 45.3 51.0 44.0 60 51.8 42.1 45 53.8 40.3 30 32.5 67.4 ePTFEc 0 ePTFE 1 90 37.9 59.9 47.1 48.5 ePTFE 2 90 53.8 36.2 3 90 ePTFE 60 53.1 39.7 52.1 41.5 45 50.3 42.7 30
11m.
0, % 1.4 7.4 1.8 2.5 2.7 2.4 0.0 2.2 2.6 2.9 3.0 4.7 5.5 5.4 6.3 7.8 8.1 9.4 10.5 5.2 5.0 6.1 6.2
sample
0
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From high-resolution C(1s) spectra. b Calculated from values
given in ref 20. e Average for survey spectra with detector at 90°,60°,
45O,and 30'. % Composition
70 T
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Figure 2. High-resolutionXPS spectra of modified PTFE after three applicationsof aluminum metal: (a)photoelectron takeoff angle = 90°; (b) photoelectron takeoff angle = 30'. Table 11. Oxygen Content of Modified PTFE Surface from XPS Survey Spectra Compared to 286.7 eV Peak in C(1s) High-&-solution Spectra detector angle, deg 90 30
% of 286.7 eV peak in C 1s spectra 11.8 15.3
%C in survey spectra 56.0 64.8
% of 286.7 eV peak scaled to survey spectra 6.6 9.9
% 0 in
survey 7.8 9.9
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:k
section), the predominant species are present in the ratios ~~
0
0
1
2
3
4
No. of Aluminum Depositions
Figure 1. XPS elemental analysisof aluminum-modifiedPTFE. Photoelectrons collected at 90'. scaling the relative intensity of this peak with respect to the total carbon content measured in the survey spectra of the same modified PTFE sample, at the same angle of electron detection. It can be seen (Table 11) that there is excellent agreement between the intensity of the peak at 286.7 eV, once adjusted for the amount of carbon observed in the corresponding survey spectrum, and the relative amount of oxygen measured directly from the same survey spectrum. An estimate of the chemical composition of the modified material can be obtained directly from the intensities of the non-CFz peaks in the C(ls) XPS spectra. The relative intensities of these signals are constant regardless of the angle of photoelectron detection. Assuming that the peak at 285 eV originates solely from saturated hydrocarbon moieties rather than unsaturated hydrocarbon or carbonaceous material (see relevant discussion in the SIMS
CH,:COH:CFH = 7:2:1 Accordingly, the simplest estimate of the elemental stoichiometry for the surface-modified material is Cd&FOz. The spectra of modified PTFE and FEP exhibit considerable angular dependence (Tables I and Figure 2). This is especially apparent in the intensity of the CFz signal relative to the other peaks in the high-resolution carbon spectra. The width and position of this peak were also dependent on the angle a t which the photoelectrons were collected. The CFz peak shift to a lower binding energy (291.7 eV), observed in the spectrum acquired a t the more surface selectivetakeoff angle (30'1, likely results from the detection of more photoelectrons from CF2 groups which are not adjacent to other fluorocarbons. Such groups are undoubtedly present at the boundary between modified and unmodified polymer. The results of the angular dependence studies indicate that the depth of modification is significantly less than the sampling depth of the XPS analysis, which has been defined previously as 3 times the mean free path of the relevant photoelectrons.26 The mean free path of the carbon (1s) photoelectron has been estimated, by Ashely= (26) Andrade, F. D. In Sur/ace and Znter/acial Aspects ofBiomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985;pp 105-
140.
Chemical Modification of Fluoropolymer
Langmuir, Vol. 7, No. 10, 1991 2149
Table 111. Degree of Modification (t)Estimated from Intensity of XPS CFt Peaks (from Eauation 1) . electron twoAl threeAl fourAl detector one Al angle, deg deposition depositions depositions depositions 90 0.21 0.62 0.88 0.73 60 0.21 0.61 0.91 0.69 45 0.23 0.61 0.91 0.69 30 0.21 0.62 0.85 0.69
Table IV. Advancing Water Contact Angles for Variable Deposition of Aluminum on PTFE
no. of Al depositions
contact angle, deg 114f 2
~~
av
0.22
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No. AI. Depositions
Figure 3. Degree of modification of PTFE, as a result of sequential aluminum depositions. Calculated by using eq 1. 90”
A
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!
I 1
I 1
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I
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No. of Aluminum Depositions Figure 5. Dependenceof cos (water contact angle) on the number of aluminum depositions onto PTFE.
this model is
IB = IBo [(I- f )
Figure 4. A representation of a cross section through the microfibrillar structure of ePTFE. During aluminum deposition shielding effects result in unmodified microfibrils which contribute to the XPS analysis only when the photoelectron takeoff angle is less than 90°.
to be around 3 nm for most polymers. Therefore, the etching depth of the aluminum deposition technique is certainly less than 10 nm. The regular trends observed in the angular-dependent XPS analyses, especially in the intensity of the CF2 peak, indicates that the modified PTFE overlayer is fairly well defined. Clearly, any carbon-containing species other than CF2, by definition, is part of the modified material. Thus, a good indication of the degree of modification to the surface of the PTFE, after each deposition of aluminum, can be obtained from measuring the decrease in the contribution of the CF2 peak (Table I) to the carbon (1s) XPS spectrum taken from that surface. A more quantitative estimate of the degree of modification can be obtained by applying the “partial coverage model” to the relative intensity of the CF2 peak. This model has been used previously to relate the measured intensity of XPS peaks, from elemenp or chemical species particular to the underlying substrab to the coverage of an overlayer resulting from adsorption or surface modifi~ation.~6,~7 The equation derived from (27)Paynter, R. W.;Ratner, B.D.;Horbett, T. A.; Thomas,H. R.J.
Colloid Interface Sci. 1984,101,233.
+ f exp(-dlX, cos 811
(1) for an element or chemical species found only in the substrate, where for this case IBis the measured intensity of the CF2 peak relative to the intensity of the totalcarbon (1s) spectrum, Im is the intensity of the CF2 peak relative to the intensity of the total carbon (1s) species on clean unmodified PTFE (=loo%),& is the inelastic mean free path for the carbon (1s)photoelectron within the modified overlayer (=3 nm), 8 is the photoelectron takeoff angle, d is the depth of the overlayer, andf is the fractionalcoverage of the overlayer or the degree of modification. If it is assumed that the modified material is uniformly distributed over the polymer surface, f can be defined as the degree of modification of the overlayer rather than the fractional coverage of the overlayer. Thus, a value of f = 0 implies that no modified material is present on the surface whereas f = 1would suggest that only modified polymer (i.e. non-CF2 material) exists within the overlayer of depth d. Values between 0 and 1give the degree to which the CF2 species have been replaced by modified material during the modification process. The calculated degree of modification for each of the aluminum-treated surfaces should be independent of the angle at which the peak intensity data were accumulated. This proved to be the case only when the depth of modification (d) was taken to lie between 2.5 and 3.0 nm. The degree of modification, within a 3 nm deep layer, of these PTFE surfaces is given in Table I11 and shown in Figure 3. It is apparent from Table I that the XPS spectra acquired from aluminum-modified ePTFE display very different angular dependence propertiesto those obtained from unexpanded PTFE. Unlike the smooth skived PTFE, the apparent contributionto the spectra from photoelectrons originating from the modified layer decreases as the electron detector is placed at a smaller angle relative to the plane of the surface of the polymer. This reverse angular phenomenon is clearly associated with the morphology of the ePTFE and in particular the microfibrillar structure (see SEM section). During the aluminum
McKeown et al.
2150 Langmuir, Vol. 7, No.10,1991 a-
40000-
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intensity (cps) lin
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Figure 6. StaticSIMSspectra: (a)positive ions SIMSspectrum of unmodified FEP;(b)positive ions SIMS spectrum of FEP modified by 1 deposition and removal of aluminum; (c) negative ion SIMS spectrum of unmodified FEP;(d) negative ion SIMS spectrum of FEP modified by 1 deposition and removal of aluminum. Table V. Summary of Static SIMS of Unmodified and Aluminum-Treated FEP positive ions negative ions FEP treated FEP FEP treated FEP mass ion mass ion mass ion mass ion 12 c 12 c 1 H 27 C2H3 12 C 12 c 29 C2H6 13 CH 31 CF 31 CF 14 CHz 39 FzH 16 0 41 C3H6 17 OH 43 CsH, 19 F 19 F 50 CFz 50 CFz 24 C2 24 CZ 55 CaH, 25 CzH 57 C4H9 26 C2H2 62 CiFz 31 CF 31 CF
deposition process the metal is expected to be deposited on fibrils located at the surface of the ePTFE sample. Metal, during vacuum deposition, travels in a straight trajectory directly from the heating filament. Thus, fibrils which are not located at the surface would be shielded from the path of the aluminum and would, therefore, not be modified. During the XPS analysis of the modified ePTFE when the electron detector is placed normal to the surface of the sample (i.e. a t the same angle from which the aluminum was deposited), only photoelectrons from
the fibrils which were in the direct path of the aluminum are observed. However, when the electron detector is placed a t a different angle, photoelectrons may be collected from the fibrils which are shielded from the metal (see Figure4). Nonetheless, it is clear that the basic aluminumpolymer reaction on the ePTFE is identical with that obtained by using flat PTFE. The advancing contact angles on the modified smooth PTFE are given in Table IV. The values of cos (contact angle) for the aluminum-modified PTFE surfaces correlate well with the estimate of the degree of modification of the ePTFE surface, obtained from the XPS data (compare Figures 3 and 5). SIMS Analysis of Modified FEP. SIMS is an analytical technique well suited to the surface analysis of polymers and can often give structural information which cannot be derived from XPS. For example, the binding energy of saturated, unsaturated and carbonaceous carbon (Is) photoelectrons are all around 285 eV and therefore cannot be differentiated directly by XPS. In the earlier study by Roberts et aL20on aluminum-modified FEP, unsaturation was tested for by treating the modified surface with bromine. No bromine was subsequently detected by XPS and thus it was concluded that the carbon peak located a t 285 eV originated from saturated hydrocarbon. SIMS was used in an attempt to confirm this finding. The positive and negative static SIMS spectra of unmodified FEP and FEP modified by a single application of aluminum metal are shown in Figure 6 and represented in Table V. The positive ion spectrum of the modified FEP exhibits a number of additional peaks not attributable
Langmuir, Vol. 7, No. 10,1991 2151
Chemical Modification of Fluoropolymer
Figure 7. Electron micrographs: (a) unmodified ePTFE; (b) ePTFE modified by three depositions and removals of aluminum. Table VI. XPS Elemental Analysis of Silanized ePTFE silanizing agent C(ls), % F(ls), % O(ls),s % N(ls), % Si(21>), %
DCDMS APTES
40.8 56.3
18.7 14.1
21.3 16.1
8.2
19.1 5.4
to the fluoropolymer. The new peaks are typical of hydrocarbon polymers (e.g. polyethylene). The peak at 41 daltons, for example, can be assigned to the cyclopropyl cation, which is characteristically the most intense peak found in a positive SIMS spectrum of a pure hydrocarbon polymer.28 The negative ion spectrum of aluminum-modified FEP, in addition to the fluorine and fluorocarbon ions seen both in this spectrum and that of the unmodified polymer, displays elemental ions of hydrogen and oxygen as well as a distinct hydroxyl ion. There are also intense peaks corresponding to small negatively charged hydrocarbon fragments at 13, 14,25, and 26 daltons.28 The observed hydrocarbon peaks in the SIMS spectra of the modified FEP in part form the basis for the estimate of the elemental stoichiometry given in the discussion of the XPS data. Modified ePTFE. The electron micrograph of ePTFE (magnified 5000X) modified by three depositions (and removals) of aluminum together with a micrograph of unmodified ePTFE (magnified 4900X) are given in Figure 7. These clearly show that the surface microfibrils are unchanged by the aluminum deposition process. The few damaged fibrils exhibited by both surfaces are probably due to mechanical damage resulting from the physical manipulation of the samples prior to obtaining the SEM. XPS analysis of the modified ePTFE surfaces after treatment with DCDMS and APTES (Table VI) shows that the organosilaneswere immobilizedonto the polymer (28) Briggs, D.;Brown, A,; Vickerman, V. C. In Handbook of Static
SZMq J. Wiley and Sons: New York, 1989; p 24.
surface on the side of the aluminum modification. No silane is observed on the reverse unmodified side of the ePTFE. The immobilized silanes are probably not in the form of a uniform monolayer due to the tendency of such compounds to polymeri~e.~~,~O However, the polymers are quite intractable in organic solvent strongly indicating that the silanes are, at least to some extent, covalently bound to the ePTFE surface. Work is in progess on the derivatization of the modified ePTFE. Mechanism of Fluoropolymer Modification by Aluminum. The mechanism of the formation of the oxygen-containing hydrocarbon layer can be understood by considering the analogous reaction between aluminum metal and simple fluorocarbons. It has been shown that tetrafluoromethane can react with activated aluminum to form aluminum fluoride.31 Indeed, tetrafluoromethane has been used to etch the surface of aluminum Evidence for the formation of an aluminum fluoride type of species at the PTFE/aluminum interface has been obtained by a XPS analysis of a FEP sample with a thin covering of aluminum metal reported p r e v i ~ u s l y . ~ In~ addition to the expected peak arising from the bulk fluorocarbon, another peak was found in the fluorine (1s) region which was consistent with the presence of an aluminum/ fluoride species. When the aluminum layer is dissolved by the application of the sodium hydroxide solution, the thin layer of metal fluoride is also removed, exposing a reactive carbonaceous surface layer. This can (29) Moses, P. R.; Weir, L. M.; Lennox, J. C.; Finklea, H. 0.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978,50,576. (30) Kallury, K. M. R.; K r d , U. J.; Thompson, M. A d . Chem. 1988,
60, 169. (31) Opalovsky, A. A.; Labkov, E. V.; Torosyan, S. S.; Dzhambek, A. A., J. Therm. Anal. 1979,15,67. (32) Horowitz, C. M.; Melngailis, J. J. Vac. Sci. Technol. 1981, 19, 1408. (33) Roberts, R. F.; Schonhorn, H., A.C.S. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1975, 16, 146.
2152 Langmuir, Vol. 7, No. 10,1991
undergo reaction readily with the basic aqueous solution to form the observed oxygen-containinghydrocarbon layer.
Conclusions Aluminum deposition and removal has been shown to be an excellent method for introducing active hydroxyl groups onto fluoropolymer surfaces in a highly surface selective process. The surface density of hydroxyl groups may be controlled by the number of sequential aluminum applicationsand estimated from the degree of introduction of the modified material which has been shown to contain around one hydroxyl group per five carbon atoms. For example, the optimal modification obtained after three applicationsof aluminum has been estimated at 0.9. Thus, for this surface the hydroxyl density would be around 18% ! (0.9 X 0.2 X 100%). Similarly, the hydroxyl density for a fluoropolymer surface modified by one and two aluminum applications would be on the order of 5 76 and 12 !% , respectively. It would appear from preliminary silanization reactions that the hydroxyl groups may be used to covalently immobilize organic speciesonto the fluoropolymer surface. This methodology, therefore, allows for the
McKeown et ai. possibility of producing well-controlled chemical modifications, both initially by the extent of the modification brought about by the aluminum deposition and ultimately by the chemistry which can be performed on the modified surface. The technique is applicable to ePTFE and in this case the procedures can be thought of as being “doubly” surface selective because only surface microfibrils are modified to a depth of only 3 nm. It is clear that the aluminum modification method will give greater control over the surface properties of ePTFE, which may help to prepare a vascular graft with better blood compatibility, while retaining the excellent mechanical properties of such grafts.
Acknowledgment. Support for this work from The Physicians’ Services Incorporated Foundation, Willowdale, Ontario (Grant No. 904,The Ontario Centre for Materials Research, and The Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Registry NO. PTFE, 9002-84-0; Al,7429-90-5.
