J. Phys. Chem. 1996, 100, 6755-6759
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Plasma Fluorination versus Oxygenation of Polypropylene J. Hopkins, R. D. Boyd, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham UniVersity, Durham DH1 3LE, England, U.K. ReceiVed: December 15, 1995X
CF4 and O2 glow discharge treatment of biaxially oriented polypropylene film results in the surface incorporation of fluorine and oxygen atoms, respectively, together with an increase in surface roughness. The stability and extent of substrate modification have been investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The observed differences in physicochemical behavior can be accounted for in terms of extended Huckel molecular orbital theory.
Introduction Cold plasmas are of great chemical and technological interest since they provide a highly reactive medium at ambient temperatures. Modification of polymer surfaces by such nonequilibrium electrical discharges comprises reaction and degradation of the substrate by ions, free radicals, excited species, electrons, and electromagnetic radiation.1 The precise mechanistic details of such processes remain a matter of much debate. Depending upon the feed gas employed, plasma treatment of polymers can produce either an enhancement in the hydrophobicity of a polymer (typically using fluorine-containing molecules, e.g. CF4, SF6) or an improvement in surface hydrophilicity (generally by oxygen-containing gases, e.g. O2, H2O). Low-pressure glow discharge excitation of CF4 gives rise to high concentrations of fluorine atoms (both radicals and ions) and relatively low concentrations of CFx species.2,3 The constituent F atoms can lead to direct surface fluorination.4,5 Similarly, the chemically reactive component of O2 plasmas is atomic oxygen.6 Since fluorine and oxygen are adjacent to each other in the periodic table (differing in electronic configuration by only one valence electron), it is instructive to examine the relative reactivities of CF4 and O2 plasmas toward polymer surfaces. For instance, polypropylene has a wide range of applications where its surface properties can play a crucial role.7 In this article, we compare the CF4 and O2 glow discharge modification of biaxially oriented polypropylene film under a fixed set of experimental conditions. Experimental Section Small strips of additive-free, biaxially oriented blown polypropylene film (ICI) were ultrasonically washed in a hexane/ isopropyl alcohol mixture for 30 s and dried in air. High-purity oxygen (99.8%, BOC) and carbon tetrafluoride gas (99.7%, Air Products) were used for low-pressure plasma treatment. Electrical discharge experiments were carried out in an electrodeless cylindrical glass reactor (4.5 cm diamenter, 515 cm3 volume, base pressure of 1.5 × 10-3 mbar, and a leak rate less than 2.0 × 10-3 cm3 min-1) enclosed in a Faraday cage.8 This was fitted with a gas inlet, a Pirani pressure gauge, and a 27 L min-1 two-stage rotary pump attached to a liquid nitrogen cold trap. A matching network was used to inductively couple a copper coil (4 mm diameter, 13 turns, spanning 9-18 cm from the gas inlet) wound around the reactor to a 13.56 MHz radio frequency (rf) source. All joints were grease-free. Gas * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
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flow and leak rates were calculated by assuming ideal gas behavior.9 A typical experimental run comprised initially scrubbing the reactor with detergent, rinsing with isopropyl alcohol, and oven drying; this was followed by a 60 min highpower (50 W) air plasma cleaning treatment. Next, the reactor was opened up to atmosphere, a strip of polymer was inserted into the center of the rf coils, and then the system was evacuated back down to its original base pressure. Subsequently the appropriate feed gas was introduced into the reaction chamber at 2 × 10-1 mbar and a flow rate of 1.9 cm3 min-1 (i.e. at least 99.6% of the reactor contents). After allowing 5 min for purging, the glow discharge was ignited at 20 W for 5 min. Upon termination of treatment, the rf generator was switched off, and the reactor flushed with gas for a further 5 min prior to venting to atmospheric pressure. The experimental parameters were kept fixed for both types of glow discharge treatment to ensure a valid comparison could be made between the CF4 and O2 plasma modification of polypropylene. There are reports in the literature of the formation of low molecular weight species during low-pressure glow discharge modification of polymer surfaces.10 This was checked for by washing the plasma-treated substrates with a 50:50 mixture of cyclohexane (nonpolar)/isopropyl alcohol (polar) solvents. The chemical nature of the treated polymer surfaces was evaluated by X-ray photoelectron spectroscopy (XPS). A Kratos ES300 electron spectrometer equipped with a Mg KR X-ray source (1253.6 eV) and a hemispherical analyzer was used for XPS surface analysis. Photoemitted core level electrons were collected at a take-off angle of 30° from the substrate normal, with electron detection in the fixed retarding ratio (FRR, 22:1) mode. XPS spectra were accumulated on an interfaced PC computer and referenced to the C(1s) core level shift at 285.0 eV for -CxHy. C(1s) XPS envelopes were fitted with Gaussian peaks of equal full-width-at-half-maximum (fwhm),11 using a Marquardt minimization computer algorithm. Instrumentally determined sensitivity factors for unit stoichiometry were taken as being C(1s):O(1s):F(1s) equals 1.00:0.55:0.53. XPS was used to check cleanliness of the polymer substrate and for the absence of any surface-active inorganic additives. Gross and experimental errors were calculated for each plasma treatment. Atomic force microscopy (AFM) offers structural characterization of surfaces in the 10-4-10-10 m range without the prerequisite of special sample preparation (e.g. metalization). A Digital Instruments Nanoscope III atomic force microscope was used to examine the topographical nature of the polypropylene surface prior to and immediately after electrical discharge exposure. All of the AFM images were acquired in air using the Tapping mode12 and are presented as unfiltered data. This © 1996 American Chemical Society
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Hopkins et al.
TABLE 1: Summary of XPS Data for the CF4 Plasma Treatment of Polypropylene % C(1s) treatment
F/C
CxHy
mCF
>CF2
-CF3
clean CF4 plasma CF4 plasma/wash
0.0 1.3 ( 0.1 1.3 ( 0.1
100 10.5 ( 2.2 11.2 ( 0.5
0.0 12.8 ( 0.5 12.0 ( 2.9
0.0 62.5 ( 1.9 61.8 ( 5.3
0.0 14.3 ( 0.2 15.1 ( 1.9
an optical beam system. A large rms amplitude is used to overcome the capillary attraction of the surface layer, while the high oscillation frequency allows the cantilever to strike the surface many times before being displaced laterally by one tip diameter. These features offer the advantage of low contact forces and negligible shear forces. Root mean square surface roughness measurements were taken over a 2 µm × 2 µm area. Results
Figure 1. C(1s) XPS spectra of (a) clean; (b) CF4 plasma treated polypropylene; and (c) CF4 plasma treated polypropylene that was then washed in a 50:50 mixture of cyclohexane/isopropyl alcohol.
Figure 2. AFM micrograph of biaxially drawn polypropylene film (2 µm × 2 µm).
technique employs a stiff silicon cantilever oscillating at a large amplitude near its resonance frequency (several hundred kilohertz). The root mean square (rms) amplitude is detected by
Clean polypropylene contains only one type of C(1s) XPS environment at 285.0 eV, which is characteristic of -CxHy, Figure 1a and Table 1. No surface impurities were found. The AFM micrographs of the untreated substrate clearly show the biaxial orientation generated during manufacture of the polypropylene film, Figure 2. These fibrils13,14 have a width of 82 ( 11 nm and a height of 9 ( 3 nm. CF4 glow discharge treatment of polypropylene results in fluorination, Figure 1b and Table 1. There is a large change in the C(1s) envelope, which can be attributed to formation of fluorinated carbon centres:15 mC-CFn at 286.6 eV, mCF at 288.3 eV, >CFCFn at 289.5 eV, >CF2 at 291.2 eV, and CF3 at 293.6 eV. The predominant functionality is the mCF2 group. Atomic force microscopy shows that the original biaxial surface texture seen for the untreated polymer is lost during CF4 glow discharge treatment to be replaced by globular moieties with a concomitant increase in surface roughness, Figure 3a and Table 3. Washing of the CF4 plasma fluorinated surface in a 50:50 mixture of cyclohexane/isopropyl alcohol results in virtually no change in the level of surface fluorination, Figure 1c and Table 1; however, AFM shows that the low molecular weight species have been washed off to leave behind a fine globular texture exhibiting a much higher rms surface roughness, Figure 3b and Table 3. O2 plasma treatment of polypropylene gives rise to oxygen incoporation at the surface, Figure 4b and Table 2. The high binding energy shoulder of the C(1s) envelope can be fitted to oxygenated carbon functionalities:15 carbon adjacent to carboxylate (mC-CO2) at 285.7 eV, ether/alcohol/hydroperoxide linkage (mC-O-) at 286.6 eV, carbonyl/double ether linkage (>CdO/-O-C-O-) at 287.9 eV, carboxylate (-O-CdO) at 289.0 eV, and carbonate (-O-CO-O-) at 290.4 eV. Again, the original biaxial surface texture observed by AFM for clean
Figure 3. AFM micrograph (2 µm × 2 µm) of (a) CF4 plasma treated polypropylene and (b) CF4 plasma treated polypropylene that was then washed in a 50:50 mixture of cyclohexane/isopropyl alcohol.
Plasma Fluorination versus Oxygenation of Polypropylene
J. Phys. Chem., Vol. 100, No. 16, 1996 6757
TABLE 2: Summary of XPS Data for the O2 Plasma Treatment of Polypropylene % C(1s) treatment
O/C
CxHy/mC-CO2
mC-O-
>CdO/-O-C-O-
-CO2-
-O-CO2-
clean O2 plasma O2 plasma/wash
0.0 0.3 ( 0.1 0.1 ( 0.1
100 76.4 ( 0.7 88.9 ( 1.1
0.0 8.6 ( 0.4 6.3 ( 0.6
0.0 5.7 ( 0.4 2.5 ( 0.2
0.0 5.8 ( 0.5 2.3 ( 0.9
0.0 3.5 ( 0.1 0.0
Figure 4. C(1s) XPS spectra of (a) clean; (b) O2 plasma treated polypropylene; and (c) O2 plasma treated polypropylene that was then washed in a 50:50 mixture of cyclohexane/isopropyl alcohol.
TABLE 3: Summary of AFM Measurements (rms Roughness of Clean Polypropylene ) 12 nm) globular features treatment
rms roughness/nm
width/nm
height/nm
CF4 plasma CF4 plasma/wash O2 plasma O2 plasma/wash
24 41 21 24
270 ( 69 157 ( 37 188 ( 29 137 ( 28
40 ( 18 65 ( 35 46 ( 11 52 ( 15
polypropylene is totally lost during O2 plasma modification, to be replaced by globular features and an increase in surface roughness, Figure 5a and Table 3. Rinsing in a 50:50 mixture of cyclohexane/isopropyl alcohol causes a significant loss of oxidized carbon centers (especially the highly oxygenated centers), Figure 4c and Table 2; this is accompanied by a shrinkage in size of the large globular features formed during O2 plasma treatment, which leads to an increase in surface roughness, Figure 5b and Table 3. Discussion Production of biaxially oriented blown polypropylene film normally consists of drawing a molten polypropylene tube over
a cooling mandrel (the drag effect of melt flow brings about an increased molecular orientation on the tube surface); this is followed by passage through a reheating unit prior to biaxial bubble formation.16 The biaxial alignment of the clean polypropylene film surface shown in Figure 2 can be attributed to one of the following: (i) microfibrillation where fracture occurs at the surface on bubble blowing;16 or (ii) retraction of the last traces of molten polymer due to volume decrease during solidification and crystallization of material below the surface;17 or (iii) non-homogeneous deformation of surface spherulites resulting in localized necking and plastic deformation along the biaxial stretching directions.16 Surface orientation appears to be more prominent in one direction (the machine direction); this can be attributed to the shear drag effect of the cooling mandrel on the drawn polypropylene tube prior to blowing.16 Clearly there are some significant differences between the CF4 and O2 plasma treatment of polypropylene. In the former case, there appears to be a greater increase in surface roughness and a much higher level of chemical incorporation. From the C(1s) peak fits, it can be concluded that nearly 90% of the polypropylene carbon centers have been modified during CF4 glow discharge exposure compared to only 24% oxygenation for the corresponding O2 experiment. CF4 is a non-polymerizable gas, and pure CF4 plasma treatment is known to result in surface fluorination rather than substrate etching, or deposition of plasma polymer.18 The presence of an alternating rf electromagnetic field across a CF4 plasma causes electron acceleration, which in turn leads to bond cleavage and ionization of CF4 molecules. The predominant reactive component of a CF4 plasma is fluorine atoms together with a small concentration of CF, CF2, and CF3 radicals at vibrational and rotational energies approximately equal to room temperature.3,19-21 This description is supported by electron impact studies with CF4, which demonstrate the efficiency of F atom production,22 which is a consequence of the high F/C ratio in the CF4 molecule.24 This abundance of fluorine radicals can easily graft onto a polymer surface19 to yield mCF, >CF2, and CF3 functionalities. Hydrogen abstraction by fluorine from -CHx to form hydrogen fluoride (HF) is the initiation step for saturated polymers; this is thermodynamically favorable since C-H bond strengths are in the 3-4 eV range4,26,27 compared
Figure 5. AFM micrograph (2 µm × 2 µm) of (a) O2 plasma treated polypropylene and (b) O2 plasma treated polypropylene that was then washed in a 50:50 mixture of cyclohexane/isopropyl alcohol.
6758 J. Phys. Chem., Vol. 100, No. 16, 1996 SCHEME 1
SCHEME 2
to 5.9 eV for H-F and 5.0 eV for C-F. There may also be some C-H bond rupture and surface activation by the VUV radiation component of the glow discharge.25,28 The resultant free radical sites can be subsequently fluorinated to yield saturated -CHx-1F functionalities, as shown in Scheme 1. Repetition of the abstraction/fluorination mechanism at the -CHx-1F moiety will eventually lead to the formation of -CFx groups. Although a large number of chemical reactions are possible in an oxygen plasma, oxygen atoms are generally regarded as being the primary reactive species in conjunction with VUV surface activation.29,30 Extended Huckel molecular orbital calculations31 have shown that hydrogen abstraction by atomic oxygen (or C-H bond rupture by VUV radiation28) followed by oxygen incorporation is the predominant reaction pathway for saturated polymers, as illustrated in Scheme 2. Atomic force microscopy shows that low molecular weight species are produced at the surface during CF4 and O2 plasma treatment of polypropylene which can be washed off using a nonpolar/polar solvent mixture. Formation of such low molecular weight species can be attributed to there being cleavage of the main polymer backbone by ion bombardment,32 vacuum ultraviolet irradiation,33 and chemical attack by fluorine/oxygen atoms during electrical discharge treatment. Removal of the low molecular weight species by washing shows that there is a much stronger level of fluorination relative to oxygenation for the respective glow discharge treatments, since there is a significant loss of oxygenated centers on washing compared to virtually no change in the surface concentration of fluorinated functionalities. These differences in surface treatment can be accounted for in terms of the mode of reaction between the incident fluorine/oxygen atoms and the polymeric substrate. Extended Huckel molecular orbital calculations31 predict that
Hopkins et al. SCHEME 3
attachment of a fluorine atom to a free radical site on a saturated backbone following hydrogen abstraction does not cause significant weakening of the adjacent carbon-carbon σ bond, while oxygenation of the free radical center results in one less electron in the highest occupied molecular orbital (this comprises mainly carbon-carbon σ-bonding interactions), thereby weakening the hydrocarbon backbone and leading to chain rupture (homolytic scission). This can also be expressed in terms of valence bond theory by the valence bond structures shown in Scheme 3. Therefore, chain scission will be much more favorable in the case of O2 plasma treatment, whereas the converse will be true for hydrogen substitution along the polymer chain due to the higher electronegativities of fluorine atoms. These results are consistent with the fact that pure CF4 plasmas display poor polymer-etching behavior relative to O2 glow discharges.34 Clearly, the reactive fluorine/oxygen atoms are able to diffuse through the generated low molecular weight overlayer in order to react with the underlying bulk polymer. The spherical structure observed following removal of the low molecular weight material by washing in a 50:50 mixture of cyclohexane (nonpolar)/isopropyl alcohol (polar) solvents can be assigned to a number of possibilities: There are reports in the literature of a low molecular weight layer being exuded to the surface of polypropylene during biaxial stretching and hence masking a subsurface texture of deformed monoclinic R-form spherulites;21 these are typically on the order of micrometers and increase in size on moving into the bulk due to greater nucleation occurring at the melt-air interface;21 such a texture could be potentially unveiled during plasma etching. Alternatively, the observed features could be small undrawn remnants of spherulites which did not succumb to necking and plastic deformation during the drawing process.21 Another possibility could be that the VUV component of the electrical glow discharge is responsible for the globular topography, since it is well-known that VUV radiation can penetrate into the subsurface of polymers to cause chain scission and cross-linking.35 Conclusions Fluorine and oxygen atoms are the respective chemically reactive species in non-isothermal CF4 and O2 glow discharges. In both cases, plasma treatment of polypropylene results in the formation of low molecular weight species which can be washed off using a nonpolar/polar solvent mixture to leave behind a globular surface texture. CF4 glow discharge treatment yields a much higher level of chemical modification and greater surface roughness. Fluorinated carbon functionalities are retained during washing, while the corresponding oxidized groups are lost. Mechanistically, this difference can be accounted for in terms of hydrogen abstraction from the polymer chain by a fluorine/oxygen atom followed by fluorine/oxygen atom addition at the generated free radical center, which gives rise to a saturated bonding configuration in the case of fluorine, whereas oxygen atom attachment generates a free radical site which is susceptible to homolytic main chain scission of the polymer backbone, leading to the formation of oxidized low molecular weight species. Hence, oxygen plasma treatment of polyprop-
Plasma Fluorination versus Oxygenation of Polypropylene ylene results in greater fragmentation and loss of polymer chains from the surface. Acknowledgment. We are grateful to British Gas and ICI for the award of EPSRC CASE studentships to J.H. and R.D.B., respectively. References and Notes (1) Samson, J. A. R. Techniques of Vacuum UltaViolet Spectroscopy; Wiley: New York, 1967. (2) d’Agostino, R.; Cramossa, F.; DeBenedictus, S. Plasma Chem. Plasma Processes 1982, 2, 213. (3) Truesdale, E. A.; Smolinsky, G. J. Appl. Phys. 1979, 50, 659. (4) Strobel, M.; Corn, S.; Lyons, C. S.; Korba, G. A. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 1125. (5) McCaulley, J. A.; Goldberg, H. A. J. Appl. Polym. Sci. 1994, 53, 543. (6) McTaggart, F. K. In Plasma Chemistry in Electrical Discharges; Elvesier Publishing Co.: New York, 1967. (7) Burkstrand, J. M. J. Vac. Sci. Technol. 1978, 1, 223. (8) Shard, A. G.; Munro, H. S.; Badyal, J. P. S. Polym. Commun. 1991, 32, 152. (9) Ehrlich, C. D.; Basford, J. A. J. Vac. Sci. Technol. 1992, A10, 1. (10) Garbassi, F.; Morra, M.; Occhiello, E.; Barino, L.; Scordamaglia, R. Surf. Interface. Sci. 1989, 14, 585. (11) Evans, J. F.; Gibson, J. H.; Moulder, J. F.; Hammond, J. S.; Goretzki, H. Fresenius Z. Anal. Chem. 1984, 319, 841. (12) Zhong, Q.; Inniss, D.; Kjoller, I. K.; Elings, V. B. Surf. Sci. 1993, 290, L688. (13) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N. AdV. Mater. 1994, 6, 477. (14) Cramer, K.; Schneider, M.; Mulhaupt, R.; Cantow, H.-J.; Magonov, S. N. Polym. Bull. 1994, 32, 637. (15) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; Wiley: New York, 1992.
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