Xenon difluoride plasma fluorination of polymer surfaces

Polymer 52 (2011) 5250e5254

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Xenon difluoride plasma fluorination of polymer surfaces S.H. Wheale, J.P.S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, England, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 July 2011 Received in revised form 4 September 2011 Accepted 7 September 2011 Available online 19 September 2011

Xenon difluoride (XeF2) plasma treatment of a series of polymers containing different repeat units gives rise to varying levels of surface fluorination. Alkene and aromatic CeH bonds appear to be more susceptible towards reaction compared to their sp3 counterparts. The extent of fluorine incorporation can be accounted for in terms of a structureebehaviour relationship derived from extended Huckel molecular orbital calculations. Comparison with CF4 plasma modification shows that XeF2 electrical discharges are more effective at fluorinating polymer surfaces. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Surface Fluorination Plasma

1. Introduction Fluorinated polymer surfaces are widely sought after for their low surface energies [1] and chemical inertness [2]. Conventional methods include casting films from solution, and direct gas fluorination of the substrate. The former tends to produce solvent waste, whereas the latter can present safety hazards. Plasma based techniques are alternatives, they can either be used to deposit coatings via plasma polymerization of fluoromonomers, or initiate surface fluorination via the excitation and dissociation of non-polymerizable feed gases [1e20], e.g. CF4 [7,13,16,17], SF6 [13,18], C2F6 [16,17], and F2 [4]. Normally, surface modification is preferred over thin film deposition, since it offers faster processing speeds, lower consumption of monomer, and less waste products. Another potential reagent for plasma fluorination is xenon difluoride (XeF2). At room temperature it is a white solid with a vapour pressure of 4.5 mbar [21], and can be used for etching silicon [22e28] and silicon compounds [24,25,29], as well as for the removal of hydrocarbon residue from silicon surfaces [23]. XeF2 is also renowned as an effective fluorinating reagent for many solution phase organic reactions [29e31] (e.g. addition across carbonecarbon double [31]/triple bonds [30]; hydrogen replacement [30,31]; electrophilic substitution [31]; fluorination

* Corresponding author. E-mail address: [email protected] (J.P.S. Badyal). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.09.027

of organoelements [28]; fluorodecarboxylation [30], etc.). In the case of XeF2 electrical discharges, a number of potential advantages are evident compared to existing plasma fluorination routes: firstly the precursor is a solid compound and therefore much cheaper and safer to transport; also no chemically reactive by-products from the feed gas can interfere with the surface reactions, since xenon is chemically inert towards polymer substrates (whereas other feed gases, e.g. CF4, SF6 etc., all generate additional reactive intermediate species, e.g. CFx, SFx respectively); another attribute is that the reaction by-products can be easily scrubbed and recycled, hence providing environmental merits. In this article we examine the structuree behaviour relationship of a range of polymers towards nonisothermal XeF2 plasma fluorination. This comprises evaluating the susceptibility of different types of structural repeat unit towards reaction under identical experimental conditions.

2. Experimental Small strips of additive-free (þ99.9%) low density polyethylene (ICI), polypropylene (ICI), nylon 6,6 (Goodfellows), polystyrene (Huntsman), polyetheretherketone (ICI), polyethylene teraphthalate (Hoechst), polycarbonate (General Electric Plastics), polyethersulfone (Westlake Plastics Company) and polysulfone (Westlake Plastics Company) were ultrasonically washed in a 50:50 mixture of isopropyl alcohol and hexane for 30 s and dried in air. Polyisoprene (Shell, þ99.9%) was dissolved in toluene (2% w/v) and spin coated onto glass slides. XPS and infrared spectroscopies verified the correct polymer structures [32,33]. Xenon difluoride

S.H. Wheale, J.P.S. Badyal / Polymer 52 (2011) 5250e5254

(99%, Fluorochem) and carbon tetrafluoride (99.7%, Air Products) were used as feed gases for surface plasma fluorination treatments. Experiments were carried out in an electrodeless cylindrical glass plasma reactor (diameter 5 cm, volume 500 cm3, base pressure of 6  103 mbar and a leak rate of better than 2  105 cm3 s1) enclosed in a Faraday cage [34]. This was fitted with a gas inlet and a thermocouple pressure gauge. All joints were grease free. RF power from a 13.56 MHz generator was connected via an LC matching unit to a copper coil (6 cm diameter, 10 turns, and spanning 10 cm) wound around the glass reactor. Prior to each experiment, the inside of the chamber was lined with polyethylene film in order to prevent etching of the glass walls by reactive XeF2 plasma species. A typical experimental run comprised placing a strip of polymer onto a polyethylene coated glass substrate holder located in the centre of the coils, followed by evacuation to base pressure. XeF2 stored in a PTFE-lined steel reservoir was heated to 80  C (melting point 129  C) [35] and introduced into the plasma reactor via a needle valve at a pressure of 0.1 mbar and a flow rate of approximately 1.9 cm3 min1 (i.e. at least 99% overall purity). The system was purged for 5 min, and then the electrical discharge ignited at 20 W for a duration of 1 min. Upon termination of plasma treatment, XeF2 was allowed to continue to pass over the substrate surface for a further 5 min. Afterwards, the apparatus was

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evacuated back down to base pressure and vented to air. These experiments were repeated using a glass slide substrate in order to check for the absence of etching and subsequent redeposition of polyethylene from the reactor lining:e no fluorinated coating was detected for either XeF2 or CF4. Control experiments where polymer substrates were exposed to XeF2 for similar times in the absence of an electrical discharge indicated negligible surface fluorination (XPS F:C ratio ¼ 0.11  0.01 for polyethylene, and 0.16  0.01 for polystyrene, with no oxygen being detected). These very low levels of detected surface fluorine species are most likely to have arisen from the reaction of XeF2 with trace amounts of background water in the plasma chamber to give reactive HF [36]. In the absence of plasma excitation, no reaction was found to occur between any of the polymer substrates and CF4 gas. The possibility of chain degradation during surface modification was considered by rinsing the plasma treated polymer substrates with solvents in which the parent polymer was insoluble e no change in the level of surface fluorination was found as measured by XPS. A thermocouple probe was used to check that the substrates remained at room temperature during each plasma exposure. Surface characterization prior to and immediately following plasma fluorination was carried out using X-ray photoelectron spectroscopy (XPS). A Kratos ES300 electron spectrometer equipped with a 200 W non-monochromatic Mg Ka X-ray source (1253.6 eV) and a concentric hemispherical electron analyzer operating in the fixed retarding ratio mode (FRR, 22:1) was used to acquire XPS spectra. Photoemitted core level electrons were collected with a take-off angle of 30 from the substrate normal, which corresponds to a sampling depth of approximately 1e2 nm for the C(1s) envelope. XPS peaks were fitted using a Marquardt minimization computer program with Gaussian components having equal full-width-at-half-maximum (FWHM) and linear background subtraction [37]. Elemental concentrations were calculated using instrument sensitivity factors determined from chemical standards, C(1s):O(1s):S(2p):N(1s):F(1s) equals 1.00:0.55:0.54:0.74:0.67 respectively. X-ray beam damage caused less than 2% change in the F:C ratio during a typical XPS scan. This was sufficiently small, that no discernible change in the C(1s) envelope was observable. Attenuated total reflectance (ATR) infrared spectra of the plasma modified polymer surfaces were acquired using a diamond ATR accessory (Graseby Specac, Golden Gate) fitted to a Perkin Elmer Spectrum One FTIR instrument. Spectra were recorded at

Fig. 1. C(1s) XPS spectra of: (a) clean polymers; and (b) XeF2 plasma treated polymers (20 W, 1 min).

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Table 1 A comparison between XeF2 and CF4 plasma treatment of various polymers (20 W, 1 min). Polymer

Clean

Fluorination

Experimental

Polyethylene Polypropylene Nylon 6,6 Polyisoprene Polystyrene Polysulfone Polyethersulfone Polyetheretherketone Polycarbonate Polyethylene teraphthalate a

Theoretical

XeF2 Plasma

Theoreticala

CF4 Plasma

O:C

S:C/N:C

O:C

S:C/N:C

F:C

O:C

S:C/N:C

F:C

O:C

S:C/N:C

F:C

O:C

S:C/N:C

0 0 0.15 0 0 0.16 0.21 0.15 0.20 0.41

0 0 0.13 0 0 0.05 0.10 0 0 0

0 0 0.17 0 0 0.15 0.25 0.16 0.19 0.40

0 0 0.17 0 0 0.04 0.08 0 0 0

1.56 1.41 1.43 1.67 1.59 1.52 1.53 1.29 1.57 1.23

0.02 0.03 0.06 0.02 0.08 0.08 0.10 0.12 0.10 0.24

0 0 0.14 0 0 0.01 0.03 0 0 0

1.50 1.40 1.42 1.49 1.40 1.48 1.51 1.13 1.29 0.98

0.03 0.02 0.05 0.02 0.05 0.16 0.26 0.17 0.20 0.41

0 0 0.08 0 0 0.01 0.04 0 0 0

2.00 2.00 1.83 1.60 1.00 0.81 0.67 0.63 0.93 0.83

0 0 0.16 0 0 0.16 0.25 0.16 0.19 0.40

0 0 0.16 0 0 0.04 0.08 0 0 0

Calculated on the basis of straightforward fluorine substitution of CeH bonds.

a resolution of 4 cm1 across the 700e4000 cm1 wavelength range using a liquid nitrogen cooled MCT detector. 3. Results The C(1s) XPS spectra of untreated polyethylene (PE) and polypropylene (PP) showed a single peak at 285.0 eV corresponding to eCxHye, Fig. 1(a). Polyisoprene (PIP) and polystyrene (PS) displayed an additional pep* shake-up satellite feature at 291.6 eV [33,38]. The C(1s) spectra of nylon 6,6 (N-66), polyetheretherketone (PEEK), polyethylene teraphthalate (PET), polycarbonate (PC), polysulfone (PSF) and polyethersulfone (PES) all contained oxidized carbon functionalities [33]: e.g. carbon adjacent to carboxylate (CeCO2) at 285.7 eV, ether linkage (eCeOe) at 286.6 eV, carbonyl (eC]O) at 287.9 eV, carboxylate (eOeC]O) at 289.0 eV, or carbonate (eOeCOeOe) at 290.4 eV. In the case of

Fig. 2. C(1s) XPS spectra of: (a) clean polyethylene; and (b) XeF2 plasma treated polyethylene (20 W, 1 min).

polyethersulfone (PES) and polysulfone (PSF), carbon centres attached to sulfone groups (CeSO2e) at 285.6 eV were included [33]; whilst carbon bonded to nitrogen (eCeNH) at 286.0 eV and carbon located in an amide environment (NHeC]O) at 288.0 eV had to be taken into consideration for nylon 6,6 (N-66) [33]. The experimental elemental ratios for the clean samples were in close agreement with theoretically predicted values, Table 1. XeF2 plasma treatment gave rise to extensive surface fluorination, Table 1. This series of experiments was then repeated using a CF4 electrical discharge operating under the same processing conditions (at a pressure of 0.1 mbar and a flow rate of approximately 1.9 cm3 min1). In both cases, 1 min exposure times were found to give rise to limiting levels of chemical modification. Variable take-off angle measurements showed uniform fluorination throughout the XPS sampling depth (approximately 1e2 nm), with a small amount of oxygen incorporation at the surface (w3%) due to the reaction of trapped surface free radicals with the atmosphere during sample transfer from the plasma chamber to the XPS spectrometer [18]. The C(1s) spectra taken for each of the polymers following XeF2 plasma fluorination are shown in Fig. 1(b). Additional fluorinated carbon environments were evident: CeCFn at 286.6 eV, CF at 287.8 eV, CFeCFn at 289.3 eV, CF2 at 291.2 eV and CF3 at 293.3 eV [18,39], Fig. 2. The components below 285.0 eV are associated with the X-ray satellite lines. A broad F(1s) peak located at approximately 689 eV was observed following plasma exposure. In the case of polysulfone and polyethersulfone, the S(2p) peaks were strongly attenuated upon surface fluorination with no discernible shift in core level binding energy value. Whilst the N(1s) peak for nylon remained at approximately 400 eV, but was slightly broader.

Fig. 3. Comparison of experimentally measured levels of XeF2 and CF4 plasma fluorination (20 W, 1 min).

S.H. Wheale, J.P.S. Badyal / Polymer 52 (2011) 5250e5254

F•

H

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F H

H

H

C

C

• C

C

H

H

H

H

F•

F

H

C

C

H

H

Scheme 1. Atomic fluorine attack at saturated hydrocarbon sites.

Fig. 3 clearly illustrates that XeF2 plasmas are more effective than CF4 at surface fluorination; this difference is more pronounced for polyisoprene, polystyrene, polyetheretherketone, polyethylene teraphthalate, and polycarbonate, Table 1. In all cases, over 95% of fluorination was found to occur within the first 10 s of plasma treatment. A variation in feed-gas pressure between 0.05 and 0.30 mbar did not alter the reported trends. No change was detectable in ATR-FTIR spectra following plasma surface fluorination, which is consistent with previous reports in the literature for plasma modification of polymer surfaces [40] (including CF4 plasma fluorination [41,42]). Therefore surface modification is less than the ATR-FTIR sampling depth of 1e2 mm [43]. This is consistent with the 5e50 nm depth reported in the literature for plasma modification of polymer surfaces [44].

Fig. 4. Comparison between theoretically calculated (based on fluorine substitution of CeH bonds, Scheme 1) and experimentally measured levels of XeF2 plasma fluorination.

fluorine atoms [2,12]. Extended Huckel molecular orbital calculations [3] predict that F atoms can participate in reactions at polymer surfaces; these include hydrogen substitution and/or addition across carbonecarbon double bonds, Schemes 1 and 2 respectively. In the case of saturated polymers, the initiation step encompasses the thermodynamically favoured substitution of hydrogen by fluorine to form HF by-product [3], Scheme 1. Based on this assumption, the XPS F:C ratios for each treated polymer substrate can be compared with the theoretically expected value corresponding to straightforward replacement of CeH bonds by CeF, Scheme 1 and Fig. 4. It is clearly evident that polymers containing carbonecarbon double bonds experience a far greater degree of fluorination than might be expected on this basis. Hence, further fluorination via addition must be occurring for polymers containing carbone carbon double bonds, Scheme 2. Such an enhancement can be explained in terms of the overall trajectory of a fluorine atom incident upon a polymeric surface being influenced by interactions between valence p orbitals of the fluorine atom and the molecular orbitals of the polymer: Theoretical calculations based on a three

4. Discussion XeF2 molecules can undergo fragmentation in the vicinity of an oscillating electric field to generate fluorine atoms via electron/ion impact and VUV photodissociation processes. Similarly, electrical discharge excitation of CF4 can also produce chemically reactive

Scheme 2. Atomic fluorine attack at unsaturated hydrocarbon functionalities (alkenes and aromatics).

Fig. 5. Percentage of carbon atoms in >C]C< bond environments (including aromatic) versus percentage conversion (where 100% conversion corresponds to just fluorine substitution of CeH bonds; hence values greater than 100% must reflect some fluorine addition across >C]C< double bonds).

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orbital interaction model predict that the affinity of a fluorine atom towards a polymer surface is governed by the energy gap between the highest occupied and the lowest unoccupied molecular orbitals of the polymer [3]. Saturated polymers have a large energy gap, and therefore fluorine atoms approaching alkyl groups experience a high-energy trajectory; whereas the gap between the highest occupied and lowest unoccupied molecular orbitals is much smaller for alkene and phenyl functionalities, thereby offering a greater probability for reaction. Furthermore, fluorination across carbonecarbon double bonds is strongly favoured from a thermodynamic perspective. Supporting evidence is presented in Fig. 5, which shows that there is a good correlation between the proportion of carbon atoms located in >C]CC]C< or aromatic bonds exert the predominant influence upon the overall level of surface fluorination. The F:C ratios are at least an order of magnitude greater than the O/C, S/C and N/C ratios. Therefore although the O/ C, S/C and N/C ratios vary independently, they do not detract from the overall stated conclusion. In the case of the oxygen-containing polymer substrates (polyetheretherketone, polycarbonate, and polyethylene teraphthalate) a comparison of XeF2 versus CF4 plasma fluorination shows that if there is a higher F/C ratio for the former treatment, then there is a correspondingly lower O/C ratio. Lower O/C ratios can be attributed to there being a greater likelihood of oxygen abstraction from the polymer surface by fluorine atoms to yield OF. The preferential loss of sulphur relative to oxygen in the case of polysulfone and polyethersulfone during plasma treatment is consistent with it being energetically more favourable for the formation of SF6 compared to OF. Preferential loss of oxygen over nitrogen from nylon could be attributed to the fact that oxygen is terminally bonded to the polymer structure whereas nitrogen is actually part of the polymer backbone, hence it should be easier to lose oxygen via the cleavage of one bond, rather than breaking two bonds to eliminate nitrogen as NFx. The slight loss of nitrogen during CF4 plasma treatment of nylon may be due to the attack of CF4 (x < 4) species at nitrogen-containing functionalities to form gaseous by-products containing CeN linkages. Although the surface chemical reactivity may be influenced by morphology, the structureebehaviour relationship derived in Fig. 5 indicates that the chemical structure of each polymer is the major contributing factor towards the attained level of plasmachemical surface fluorination. 5. Conclusions XeF2 glow discharges are highly effective at fluorinating polymer surfaces. The extent of fluorine incorporation is greater

for alkene and aromatic CeH bonds compared to their saturated analogues. This can be accounted for in terms of a structuree behaviour relationship based on extended Huckel molecular orbital calculations. A comparison between XeF2 and CF4 plasma treatment under similar experimental conditions has shown that the former feed-gas gives rise to more extensive surface fluorination.

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