Langmuir 2007, 23, 4293-4295
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Mechanism of Ultraviolet Bonding of Perfluoropolyethers Revisited Xing-Cai Guo*,† and Robert J. Waltman‡ San Jose Research Center, Hitachi Global Storage Technologies, Inc., 3403 Yerba Buena Road, San Jose, California 95135, and Media DeVelopment, Hitachi Global Storage Technologies, Inc., 5600 Cottle Road, San Jose, California 95193 ReceiVed NoVember 2, 2006. In Final Form: January 19, 2007 The mechanism of ultraviolet (UV) bonding of perfluoropolyether (PFPE) boundary lubricants on magnetic disk surfaces is re-examined experimentally. It is found that UV-emitted photoelectrons may contribute a negligible part, and instead the UV bonding correlates with the direct photodissociation of PFPE molecules. The UV-induced photodissociation is demonstrated to occur almost randomly on the PFPE molecular chain. The contribution from photooxidation is eliminated under nitrogen purge.
Introduction
Experimental Section
Ultraviolet (UV) bonding of perfluoropolyether (PFPE) boundary lubricants on solid surfaces is now widely employed in the magnetic storage industry. The methodology was first published in Langmuir 16 years ago.1 Its mechanism was proposed a few years later2,3 and was recently cited in several publications.4-6 In these papers, UV bonding of PFPE films to the underlying substrate was assumed to originate from the photoelectrons emitted by the substrate as a consequence of UV irradiation. However, there are many possible mechanisms in surface photochemistry which may be classified into two major categories: (1) excitation of the adsorbate and (2) excitation of the substrate.7-9 Substrate excitation may be further divided into several processes:9 (a) a process driven by photoemitted electrons above the vacuum level, (b) a process driven by photoexcited electrons below the vacuum level, (c) a process associated with plasmon excitation, (d) a process associated with phonon excitation, and (e) a process associated with other kinds of excitation. There are many examples in the literature for each of the mechanisms.9 In this paper, we examine the previously proposed UV bonding mechanism of PFPE (category 2a above) and explore the possibility of direct excitation of the PFPE adsorbate by UV photons (category 1). For simplicity, we report on the UV irradiation of PFPEs having no functional end groups. The UV irradiation of PFPEs with functional groups will be reported separately. Resolving the UV bonding mechanism of PFPE lubricants is of general importance in the refinement of current lubricant processing and the development of future lubricants.
Experiments were conducted on commercially available perfluoropolyethers (Fomblin Z, Solvay-Solexis). PFPEs are random copolymers of CF2CF2O and CF2O monomer units truncated by the perfluoromethyl group:
* To whom correspondence should be addressed. Telephone: 408-7175839. E-mail:
[email protected]. † San Jose Research Center. ‡ Media Development. (1) Saperstein, D. D.; Lin, L. J. Langmuir 1990, 6, 1522. (2) Vurens, G. H.; Gudeman, C. S.; Lin, L. J.; Foster, J. S. Langmuir 1992, 8, 1165. (3) Vurens, G. H.; Gudeman, C. S.; Lin, L. J.; Foster, J. S. IEEE Trans. Magn. 1993, 29, 282. (4) Chiba, H.; Nakamura, N.; Takeda, M.; Watanabe, K. IEEE Trans. Magn. 2002, 38, 2108. (5) Chiba, H.; Nakamura, N.; Takeda, M.; Watanabe, K. Tribol. Int. 2003, 36, 367. (6) Zhang, H.; Mitsuya, Y.; Imamura, M.; Fukuoka, N.; Fukuzawa, K. Tribol. Lett. 2005, 20, 191. (7) Chuang, T. J. Surf. Sci. Rep. 1983, 2, 1. (8) Guo, X.-C. Ph.D. Dissertation, University of Pittsburgh, 1990. (9) Zhou, X.-L.; Zhu, X.-Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73.
F3CO-(CF2CF2O)m-(CF2O)n-CF3 The polydisperse PFPEs were fractionated via supercritical fluid extraction (Phasex Corp.) into narrowly dispersed molecular weights, designated below as Z followed by its number average molecular weight, for example, Z11080. The fractionated PFPEs were applied to the magnetic disk surfaces using a standard dip-coating method from a hydrofluoroether (HFE-7100, 3 M) solution. The applied film thickness was quantified via infrared spectroscopy (Nicolet 670 FT-IR) whereby a characteristic PFPE absorption band was previously calibrated to film thickness via electron spectroscopy for chemical analysis (ESCA). The “bonded” PFPE thickness after UV irradiation was quantified as the portion of the PFPE film that remained on the disk surface after rinsing in 2,3-dihydroperfluoropentane (Vertrel-XF, DuPont). In some of the experiments, as noted below, neat fractionated PFPEs were used. UV irradiation of PFPE was carried out using three different types of lamps at various wavelengths. A supersil encapsulated mercury lamp (UVOCS) provided UV light at both 185 and 254 nm with an intensity ratio of approximately 1 to 5. The power at 185 nm was 3.2 mW cm-2 for direct irradiation and 2.6 mW cm-2 for irradiation through a quartz window, as measured by a UV meter (Hamamatsu Photonics). A second mercury lamp (UVOCS) with a crystalline quartz envelope provided only 254 nm light at 16 mW cm-2, and the incident power was quantified to be identical for UV irradiation through the quartz window. Finally, a 172 nm xenon excimer lamp provided 7.0 and 2.7 mW cm-2 for direct irradiation and irradiation through a quartz window, respectively. Unless otherwise noted, all UV irradiations were performed at room temperature under nitrogen containing ∼50 ppm of residual oxygen. A temperature rise due to UV irradiation was minimal for all lamps, and possible thermal effects were eliminated by controlled blank experiments.
Results and Discussion The 185 nm UV irradiation (hν ) 6.7 eV) of a carbon substrate having a work function φ equal to ∼5.0 eV10 would produce the emission of photoelectrons with a maximum kinetic energy (Ek ) hν - φ) of 1.7 eV, whereas the 254 nm (4.9 eV) UV irradiation (10) Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Baca Raton, FL, 1990.
10.1021/la063211b CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007
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Figure 1. Transmission infrared spectra of (a) gas-phase carbonyl fluoride; (b) low molecular weight PFPE adsorbed onto IR windows after 7 min of 172 nm UV irradiation on 0.2 g of neat PFPE (Z11080); and (c) the parent Z11080 without UV irradiation. The inset figure shows a schematic drawing of the UV/IR gas cell used in these experiments.
Figure 2. Relative yield of carbonyl fluoride as obtained from the IR spectra for (a) 0.20 g of Z11080 versus oxygen concentration in the gas cell and (b) 0.10 g of PFPE at various molecular weights. All data represent 7 min exposures to 172 nm UV irradiation.
is expected to produce negligible photoelectron emission. The proposal of the photoelectron mechanism2,3 for UV bonding was therefore based upon the observation that 185 nm UV irradiation produced significant PFPE bonding, while 254 nm light produced very little bonding. In particular, photoelectrons emitted by UV photons whose energy was greater than the substrate work function interacted with the PFPE molecules leading to their dissociation and subsequent bonding to the substrate. The mechanism was rationalized by analogy to the observation that low-energy electron bombardment could also bond PFPE.2,3 First, we estimated whether there were enough photoelectrons to account for all of the bonded PFPE thicknesses based on previous data. It was reported2,3 that bombardment by 92.5 µC cm-2 of 10 eV electrons could bond 20 Å of PFPE, while the photocurrent detected from a clean substrate irradiated by 185
Guo and Waltman
nm UV light was 17.6 nC cm-2 s-1. To generate enough photoelectrons, it would take 88 min of UV irradiation, assuming a maximum of bonding efficiency to be the same as in the case of the 10 eV electrons. In fact, it took just 5 min of UV irradiation to bond 20 Å of PFPE.1 This indicates that the photoelectron mechanism contributes, at most, ∼6% to the overall UV bonding. Similar bonding efficiency was observed with electrons of 2-18 eV kinetic energy.2 Next, we considered the possibility of direct excitation of the PFPE adsorbate by UV photons. For this purpose, a UV/IR gas cell was constructed to allow the possible detection via transmission IR spectroscopy of reaction products produced in the gas phase after UV irradiation of neat PFPE liquid, as shown schematically in the inset of Figure 1. The experimental concept is based on previous observations of gas-phase products such as carbonyl fluoride (CF2O) upon main chain scission of neat PFPE liquids.11 A small quantity (0.1-0.2 g) of neat PFPE was placed on a quartz window at the bottom of the UV/IR gas cell. The cell was purged with nitrogen and then isolated by gas valves. Transmission IR spectra were taken through the NaCl windows before and after UV irradiation and after venting the cell to nitrogen. Analyses of the IR spectra using the appropriate background spectra provided absorption IR spectra of both the gas-phase products inside the cell and those adsorbed to the NaCl windows. Representative IR spectra are presented in Figure 1 for Z11080 exposure to 172 nm UV irradiation for 7 min. The top spectrum is unequivocally carbonyl fluoride, CF2O, based on an authentic reference spectrum.12 The adsorbed material on the NaCl window after venting the gas cell, shown in the middle spectrum, is comparable to that of the parent PFPE shown in the bottom spectrum. Because Z11080 has a large molecular weight (11080 amu) and the temperature rise during UV irradiation is less than 10 °C above room temperature, the PFPE adsorbed to the NaCl windows does not originate from the evaporation of the parent PFPE but rather from the evaporation of a more volatile, lower molecular weight PFPE fragment produced by UV irradiation. Controlled thermal experiments for Z11080 in the absence of UV irradiation did not yield any carbonyl fluoride or lower molecular weight PFPE. Therefore, the following reaction is proposed to have occurred during UV irradiation:
F3CO-(CF2CF2O)m-(CF2O)n-CF3(l) f F3CO-(CF2CF2O)i-(CF2O)j-(a) + CF2O(g) + F3CO-(CF2CF2O)p-(CF2O)q-(a) where l, a, and g are the liquid, adsorbed, and gas phases, respectively. To confirm the occurrence of the photon-induced dissociation (photodissociation) process, we need to eliminate the possibility of a photon-induced oxidation (photooxidation) process. Although UV bonding of PFPE is performed under nitrogen purge to avoid ozone production and light attenuation due to atmospheric oxygen, photooxidation is still possible because of the existence of residual oxygen in the ppm range. The oxygen concentration in our dry nitrogen was measured to be ∼50 ppm. By mixing the dry nitrogen with clean dry air in different portions, we could vary the oxygen concentration inside the UV/IR cell during UV irradiation while keeping all other experimental conditions identical. The relative yield of carbonyl fluoride was quantified by IR spectoscopy from 50 to 20 000 ppm of oxygen (Figure 2a), for 0.20 g of (11) Pacansky, J.; Waltman, R. J. J. Phys. Chem. 1991, 95, 1512. (12) Carbonic Difluoride. http://webbook.nist.gov/chemistry (accessed July 2006).
Mechanism of UV Bonding of Perfluoropolyethers
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line. However, Figure 2b shows a very weak dependence on molecular weight, indicating that photodissociation occurs almost randomly on the molecular chain. Consequently, the UV-induced scission of PFPE at nonterminal positions may lead to chain shortening and backbone bonding. Finally, we demonstrate a correlation between the carbonyl fluoride yield of photodissociated neat PFPE and the bonded thickness of PFPE on magnetic disks. Carbonyl fluoride yield was measured as a function of 172, 185, and 254 nm UV irradiation after 10 min for the identical 0.20 g of neat Z5990 (Figure 3a). The inset of Figure 3a shows the IR spectra of actual CF2O evolution. The bonded thickness from the UV irradiation of nominally 38 Å of Z5990 on the carbon film of finished disks was also measured under identical conditions (Figure 3b). A plot of bonded thickness (Figure 3b) versus CF2O evolution (Figure 3a) shown in Figure 3c is indicative that the two processes are directly correlated. These data provide strong evidence that the photodissociation of PFPE leads to its bonding on the carbon film of rigid disks. Contrary to the photoelectron mechanism, 254 nm UV irradiation does lead to PFPE bonding: 0.3 and 2.8 Å bonded after 10 and 30 min irradiation, respectively. The wavelength dependence is consistent with the UV absorption spectrum of PFPE.13 The disk surface was covered with diamondlike amorphous carbon with nitrogen doping, the chemical structure of which has been studied extensively.14
Conclusions
Figure 3. (a) Carbonyl fluoride yield from the UV irradiation of 0.20 g of Z5999 at 174, 185, and 254 nm. The inset figure shows the gas-phase IR spectra of CF2O at the three wavelengths. (b) Bonded thickness of 38 Å Z5990 on an amorphous carbon film after 10 min of UV irradiation at the three different wavelengths. (c) Correlation between the bonded thickness and the photodissociation yield of carbonyl fluoride.
Z11080 after 7 min of exposure to 172 nm light. The constant nonzero yield of carbonyl fluoride below ∼0.1% O2 indicates that the photodissociation process is dominant for UV irradiation under nitrogen purge. Photooxidation becomes significant when the oxygen concentration exceeds ∼1%. To determine whether there is any preference as to where photodissociation occurs on the PFPE molecular chain, that is, the terminal groups (-OCF3) versus the middle units (CF2CF2O and CF2O), we measured the carbonyl fluoride yield using the same amount (0.10 g) of PFPE at various molecular weights from 950 to 71 600 amu, all after 7 min of 172 nm UV irradiation, as shown in Figure 2b. If the photodissociation occurred only at the terminal groups, the carbonyl fluoride yield would be proportional to the number of terminal groups and hence inversely proportional to the molecular weight, as illustrated by the dashed
An analysis of previously published data on the UV bonding mechanism of perfluoropolyethers has shown that the contribution of photoelectrons emitted from the substrate may be insignificant to the UV bonding of PFPE. The experimental results presented here suggest that the direct photodissociation of PFPE may play a dominant role in UV bonding on the underlying substrate. Evidence for direct adsorbate (PFPE) photodissociation was provided by the evolution of carbonyl fluoride in the gas phase after 172, 185, and 254 nm UV irradiation. Photodissociation occurs nearly randomly on the molecule chain irrespective of the chain length. A direct correlation has been found between the bonded PFPE thickness and the photodissociation yield of carbonyl fluoride. The contribution of CF2O from photooxidation was eliminated by demonstrating identical CF2O yields for O2 concentrations < 1000 ppm. Acknowledgment. The authors thank Drs. C. Mathew Mate and Bruno Marchon for fruitful discussions and K. Munoz of Radiant Source Technology (San Jose) for the use of the 172 nm excimer UV source. LA063211B (13) Nakakawaji, T.; Amo, M. To be published. (14) Scharf, T. W.; Ott, R. D.; Yang, D.; Barnard, J. A. J. Appl. Phys. 1999, 85, 3142.
