Langmuir 1998, 14, 7527-7536
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Concerning the Interactions between Zdol Perfluoropolyether Lubricant and an Amorphous-Nitrogenated Carbon Surface G. W. Tyndall* IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120
R. J. Waltman and D. J. Pocker IBM Storage Systems Division, 5600 Cottle Road, San Jose, California 95193 Received March 9, 1998. In Final Form: October 1, 1998 The interactions that occur between the hydroxyl-terminated perfluoropolyethers Zdol 2000/4000 and an amorphous-nitrogenated carbon surface (CNx) were studied via surface energy measurements, kinetic measurements, and ab initio calculations. The results of these measurements are compared with those of previous studies on the Zdol/amorphous-hydrogenated carbon (CHx) system and the major differences identified. The thickness dependence of the dispersive surface energy for the Zdol/CNx system can be fit using a repulsive van der Waals potential. Effective Hamaker constants determined for both the Zdol/CNx and Zdol/CHx systems demonstrate that Zdol is less effective at covering CNx as compared to CHx due to less favorable interactions between the Zdol backbone and the CNx surface. The Zdol thickness dependence of the polar surface energy for the Zdol/CNx system indicates that very few strong polar interactions are present between the initially applied Zdol and the CNx surface. A substantial decrease in the polar surface energy of the first Zdol monolayer however occurs on a time scale of 1-5 weeks after lubricant application. The attractive well that develops in the free energy versus thickness curve reflects the formation of attractive interactions between the polar hydroxyl end groups of Zdol and the polar entities on the CNx surface. A kinetic analysis of the Zdol + CNx system reveals that the rate at which the adhesive interactions are formed is limited by diffusion of the polar end groups to the surface active sites. Ab initio calculations indicate that attractive hydrogen-bonding interactions between the hydroxyl end groups of Zdol and imine (basic) sites on the CNx surface may be responsible for the Zdol adhesion. These calculations further suggest that the appearance of the diffusion step in the bonding kinetics and the less efficient coverage of Zdol on CNx are manifestations of repulsive interactions that exist between the basic imine surface sites and the basic perfluorinated Zdol backbone.
Introduction The interface of molecularly thin fluid films in contact with solid surfaces is an active area of research. In addition to being of fundamental interest, molecularly thin films are of critical importance to numerous technologies that rely on thin-film adhesion and lubrication. One application in which thin polymeric films are employed is in the lubrication of computer disks within hard-disk drives. The magnetic recording industry universally employs hardcarbon overcoats of nominally 100 Å in conjunction with sub-monolayer films of perfluoropolyether (PFPE) lubricants to protect the magnetic film of the computer hard disk from mechanical damage induced via contact with the magnetic recording head. Despite the importance of the materials utilized at the head-disk interface (HDI) to the reliability of the storage device, little advancement in these materials has been made in the past few years. While it is true that the PFPE lubricants and the carbon overcoats have performed adequately in past applications, it is also true that an incomplete understanding of the fundamentals of sub-monolayer lubrication, and the behavior of complex polymeric fluids on surfaces in the sub-monolayer thickness regime, has impeded efforts to optimize the lubricant/overcoat combination. Given that the performance requirements of the HDI will become increasingly stringent, a more thorough understanding of the lubricants, the wear resistant overcoats, and the interactions that take place between these materials will
be required for the successful extension of this technology in future disk drives. In previous papers,1-3 we have reported on the nature of the interactions that occur between perfluoropolyether lubricants and the amorphous-hydrogenated carbon (CHx) that has been historically used as the protective overcoat on computer hard disks. In the case of PFPE lubricants terminated with polar end groups, strong adhesive interactions can occur between the PFPE end groups and polar active sites on the CHx surface.1-5 In the specific case of Zdol on CHx, the adhesive interactions between the hydroxyl end groups of the lubricant and the surface oxides, for example, carboxylic acid and/or carbonyl sites, dominate the polymer adsorption energetics.2,5 The adhesive interaction strength between Zdol and CHx is strongly temperature dependent, with increasing temperature leading to an increase in the amount of lubricant “bonded” to the underlying carbon substrate.2 An implication of this latter result is that, at the temperatures attained in many current disk drives, the mobile Zdol lubricant system initially applied to the computer disk is thermodynamically unstable. Since mobility is widely (1) Tyndall, G.; Leezenberg, P. B.; Waltman, R. J.; Castaneda, J. Tribol. Lett. 1998, 4, 103. (2) Waltman, R. J.; Pocker, D.; Tyndall, G. Tribol. Lett. 1998, 4, 267. (3) (a) Waltman, R. J.; Tyndall, G. W. J. Phys. Chem., submitted (b) Waltman, R. J.; Tyndall, G. W. J. Phys. Chem., submitted. (4) Dai, Q.; Vurens, G.; Luna, M.; Salmeron, M. Langmuir 1997, 13, 4401. (5) Ruehe, J.; Blackman, G.; Novotny, V.; Clarke, T.; Street, G. B.; Kuan, S. J. Appl. Polym. Sci. 1994, 53, 825.
10.1021/la9802825 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/04/1998
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believed to be directly linked to tribological performance, one might reasonably expect the durability of the headdisk interface within the drive environment to be altered as a result of lubricant bonding during operation of the storage device. Recently, a number of manufacturers of magnetic recording media have replaced the CHx overcoat with an amorphous-nitrogenated carbon (CNx) overcoat. Reports extolling the tribological benefits of CNx have appearedl6-9 Most have attributed the apparent improvement in the tribology of CNx overcoats relative to CHx overcoats to the increased hardness of the CNx film. However, altering the nature of the carbon surface could also strongly influence the PFPE lubricant via modifying the adhesive strength, bonding kinetics, lubricant orientation, and/or lubricant mobility. A change in any of these properties could easily manifest itself in substantially different tribological properties of the computer hard disk. In the current work, we investigate the interfacial interactions that develop between the polydisperse Zdol 2000/4000 lubricants and a CNx surface comprised of 11 atom % nitrogen. The results of surface energy measurements, kinetic measurements, and ab initio calculations are then compared with those of the previously studied Zdol + CHx system,1-3 and the major differences are discussed. Experimental Section The Zdol lubricants used in this work were obtained from Ausimont. The chemical formula for these materials is
HO-CH2CF2-O-(CF2-O-)x-(CF2CF2-O-)y-CF2CH2-OH where x/y ) 1.2. Two molecular weight distributions were employed for this work, Zdol 2000 and Zdol 4000. Zdol 2000 is polydisperse (Mw/Mn ) 1.5) with an average molecular weight of 2.0 kg‚mol-1. Zdol 4000 is also polydisperse (Mw/Mn ) 1.3) with an average molecular weight of 3.9 kg‚mol-1. Both Zdol molecular weight distributions were used as received. We note that the critical molecular weight for entanglement of PFPE’s is significantly higher than those utilized in this work.10 The carbon surfaces used in these experiments were those present on 95 mm diameter computer hard disks. Our choice for using computer disks for substrates was motivated both by the obvious relevance to the magnetic storage industry and by the fact that these computer disks can be produced in large numbers with a high degree of reproducibility. The hard disks were manufactured from supersmooth Al-Mg substrates with an rms roughness of nominally 10 Å. Onto these substrates are sputterdeposited an underlayer of chromium, a cobalt-based magnetic layer, and nominally 100 Å of amorphous-nitrogenated carbon (CNx). The carbon surfaces were characterized prior to use via XPS. These measurements were made using a Surface Science Instruments Model SSX-100 spectrometer, where monochromatic Al KR radiation at 1486.6 eV, a pass energy of 55 eV, and a 36° mean exit angle were employed. The typical carbon (1s), nitrogen (1s), and oxygen (1s) binding energies for the CNx overcoat used in this work are summarized in Table 1. The levels of oxygen and nitrogen apparent at the surface of the carbon overcoat are 4 and 11 atom %, respectively. We note that comparison of grazing exit angle and perpendicular exit angle measurements reveal that the oxide concentration in the bulk is approximately 2-3 times less than that at the surface, that is, on the order of j2 atom %. The nitrogen distribution is more uniform throughout the carbon (6) Li, D.; Cutiongco, E.; Chung, Y.-W.; Wong, M.-S.; Sproul, W. D. Surf. Coatings Technol. 1994, 68/69, 611. (7) Cutiongco, E. C.; Li, D.; Chung, Y.-W.; Bhatia, C. S. ASME J. Tribol. 1996, 118, 543. (8) Huang, L.; Hung, Y.; Chang, S. IEEE Trans. Magn. 1997, 33, 4551. (9) Yun, X.; Hsiao, R. C.; Bogy, D. B. IEEE Trans. Magn. 1997, 33, 938. (10) Novotny, V. J. J. Chem. Phys. 1990, 92, 3189 and references therein.
Tyndall et al. Table 1. XPS Data for CNx Disks Used in These Studiesa element carbon nitrogen oxygen
binding energy (eV)
atomic percent (no wash)
atomic percent (wash)
284.6 286.1 287.9 398.4 400.6 403 532.5
45 24 0, or from the formation of attractive (polar) cohesive interactions between adjacent polymer chains, ξjj > 0. In either case, the time dependence of the surface energy decrease is indicative of a restructuring, or self-assembly, of the lubricant on the disk surface, with the restructuring being thermodynamically driven by the resultant drop in the free energy ∆Fp. To determine whether the attractive well in the free energy is attributable to an increase in the adhesive (ξ01) or the cohesive interaction energy density (ξ11), we conducted a series of surface energy measurements on annealed, Zdol 4000-lubricated CNx samples. The choice of Zdol 4000 was made to minimize the complications associated with lubricant evaporation that can become
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Figure 4. Time dependence of the Zdol 4000/CNx polar surface energy at elevated temperature (90 °C). The Zdol film thickness for these measurements was 18 Å.
substantial with Zdol 2000 even at relatively modest temperatures.3 In the first experiment, we lubricated a series of CNx substrates with 18 Å of Zdol 4000 and heated them at 90 °C for various times ranging up to 60 min. The surface energies were then determined as a function of anneal time. The initial surface energies for the unannealed Zdol 4000 disks were measured to be nominally 16 ergs‚cm-2. As is shown in Figure 4, the effect of annealing is to decrease the polar surface energy with the magnitude of the decrease being dependent on anneal time. The decrease in ∆Fp indicates that annealing promotes the formation of attractive, polar interactions in the lubricant/ carbon system. Evidence that this polar interaction results from enhanced adhesive interactions (as opposed to enhanced cohesion) comes from the observation that increased anneal time increases the fraction of lubricant that is unextractable from the disk using the solvent rinse methodology described above. Furthermore, the absolute value of the polarity after 1 h at 90 °C (2 ergs‚cm-2) is substantially less than that of bulk Zdol (10 ergs‚cm-2). Since Zdol confined to the 2d surface will have fewer adjacent chains with which to interact cohesively, the magnitude of ξjj will be less on the 2d surface. Our results thus imply that the attractive free energy well results largely from attractive adhesive interactions. The Zdol thickness dependence of the polar surface energy for the annealed Zdol 4000 + CNx system was also determined. In these experiments, the anneal step consisted of heating the disks at 150 °C for 90 min. The results of these experiments are shown in Figure 5. The polar surface energy rapidly decreases with lubricant thickness, approaching 0.5 ergs‚cm-2 at approximately 15 Å. The surface energy remains at this low value until nominally 40 Å, above which the surface energy increases. No bonding to the surface was obtained with Fomblin Z03, structurally analogous to Zdol but terminated with nonreactive CF3 end groups at the temperatures studied here. These results
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Figure 6. Zdol titration of the CNx and CHx surfaces as a function of Zdol molecular weight. Solid symbols correspond to the maximum Zdol thickness that can be bonded to the carbon surfaces, and open symbols correspond to the minima obtained for the polar surface energy versus Zdol thickness.
Figure 5. Polar surface energy of the annealed Zdol/CNx system as a function of applied Zdol 4000 thickness. Annealing was performed at 150 °C for 90 min.
demonstrate that the adhesive interactions involve the polar hydroxyl end groups. Previously, we interpreted the surface energy difference between the annealed and unannealed Zdol + CHx systems in terms of two rather distinct states of the lubricant on the surface.1 In particular we proposed that the unannealed system was characterized by hydrogen bonding of the hydroxyl end groups to the oxidized surface sites and that the annealed system was characterized by a much stronger interaction such as might be expected from chemisorption. In light of the current results, we believe the two states most appropriate to describe the Zdol + carbon systems in the monolayer/sub-monolayer thickness regime are (a) a mobile state characterized exclusively by repulsive van der Waals interactions between Zdol (predominantly the backbone) and the carbon surface and (b) a bonded state characterized by attractive polar interactions between the Zdol end groups and the surface active sites. The polar free energy of the mobile or “free hydroxyl” state is in excess of nominally 17 ergs‚cm-2 for one monolayer of Zdol 2000 and on the order of 1.0 ( 0.5 ergs‚cm-2 for one monolayer of the bonded state. The maximum amount of Zdol that can be bonded to the CNx and CHx surfaces was determined via “titration” of the surface. In these experiments, an excess of Zdol 2000 and 4000 was applied to CNx surfaces and then annealed until limiting Zdol thicknesses were obtained. The results of these titration experiments are shown in Figure 6. In the case of the CNx surface, we find that, with Zdol 2000, 23 Å could be accommodated by the CNx surface, and in the case of Zdol 4000, 37 Å could be bonded to the surface. We also include in Figure 6 the thickness of the surface energy minimum obtained from the unannealed Zdol/CNx disks. As is apparent from Figure 6, a close correlation exists between the free energy minimum of the unannealed disks and the maximum thickness that can be bonded to the surface. This same correlation holds
for the titration results obtained for Zdol on CHx, which are also included in Figure 6. Thus, while the depth of the surface energy minimum increases as the bonded fraction increases, the thickness at which this attractive well appears is not dependent on the amount of lubricant bonded. The data of Figure 6 illustrate that the maximum amount of lubricant that can be bonded to the CNx and CHx overcoats increases linearly with increasing molecular weight (MW). In the case of Zdol on CNx, the maximum bonded thickness scales as [0.0094 × MW] Å, whereas the titrated thickness versus Zdol MW scales as [0.0062 × MW] Å for the CHx surface. The difference in the amount of Zdol strongly adsorbed on CNx versus CHx either could reflect a difference in the number of active adsorption sites on the two surfaces or could result from differences in the structure of the first Zdol monolayer on these surfaces. If the number of surface adsorption sites limits the amount of lubricant that can bond to the surface, then our results would imply that the number of active sites on CNx is approximately 50% greater than the corresponding number on CHx. This interpretation of the titration results would then lead to a linear dependence of the titrated thickness on molecular weight, since the carbon surface could only accommodate a fixed number of hydroxyl moieties. If Zdol adsorption is instead mitigated by the structure of the polymeric film on the surface, that is, a surface packing density, then the amount that can be bonded to the surface will be dictated by the ability of the end group to gain access to the active adsorption sites. If we consider the specific example of Zdol 2000, the titrated thickness corresponds to 20-23 Å on CNx and 13 ( 1 Å on CHx. As we recall from the previous discussion on Hamaker constants ASL ≈ 0 for Zdol on CHx. Since A* ≈ ALL in this case, the thickness of the first Zdol monolayer should be close to that given by 2Rg (where Rg is the radius of gyration of bulk Zdol). For Zdol 2000, an estimate of Rg ) 7 Å yields 2Rg ) 14 Å, in agreement with our observations. In the interaction between Zdol and CNx, the negative values of ASL indicate that the Zdol/surface interaction is repulsive. This repulsion serves to increase the average distance between the Zdol backbone and the CNx surface to values greater than 2Rg. The less efficient coverage of the surface by the backbone allows the end groups from a greater number of Zdol chains to interact attractively with the surface, as observed.
Zdol/Amorphous-Hydrogenated Carbon Interactions
In addition to the attractive well that develops in the free energy at a thickness corresponding to one Zdol monolayer, a maximum in the free energy develops at two monolayers (see Figure 2). In the first Zdol monolayer, the hydroxyl end groups are on average preferentially adsorbed at the CNx active sites (after the initial restructuring). The surface thus produced would not provide as many sites capable of interacting adhesively with a subsequently added monolayer. Since ξ12 , ξ01, the surface energy increases as the second monolayer is formed. The magnitude of the surface energy increases to levels significantly above that of the bulk Zdol by the time formation of the second monolayer is complete (nominally 40 Å). On the basis of the magnitude of the free energy, the second monolayer is characterized by a high density of non-interacting hydroxyl end groups at the lubricantair interface. At thicknesses greater than 35-40 Å the surface energy appears to start decreasing with increasing lubricant thickness. Since addition of lubricant in excess of 2 monolayers would lead to attractive, interlayer hydrogen-bonding interactions (ξ32 > 0) between the hydroxyl groups of the third monolayer and the “free” hydroxyls of the second monolayer, a surface energy maximum results. Eventually, the surface energy will necessarily approach the value 10 ergs‚cm-2 measured for the bulk lubricant (100 Å of Zdol 200 on CHx). Thus a layering of Zdol on the carbon surface is suggested by our results. It is interesting to note that the effects of the “short-range” polar interactions between the CNx surface and the first Zdol monolayer propagate through successive layers. Thus, the short-range polar forces of the surface impact the film structure at effective Zdol thicknesses (>40 Å) which are significantly larger than the thicknesses over which the “longer-range” van der Waals forces of the surface act (nominally 21 ( 2 Å). Summarizing the surface energy results, a number of analogies can be drawn between the Zdol/CNx system and the Zdol/CHx system. In both cases, we find that, in the monolayer/sub-monolayer thickness regime, two states of Zdol exist. Following the terminology used in previous publications, we refer to these as the “mobile” state and the “bonded” state. The mobile state is characterized by an absence of polar adhesive interactions with the surface. Within the plane of the monolayer, attractive cohesive interactions between neighboring chains are possible. The mobile state is then close in structure to the bulk liquid state modified by the relatively small perturbation that results from the VDW interactions with the surface. The bonded Zdol state is dominated by the strongly attractive polar interactions of the hydroxyl end groups with the polar active sites on the carbon surfaces. Transitions from the mobile state to the bonded state occur in both systems as a result of a self-assembly of the lubricant on the surface that is thermodynamically driven by the decrease in free energy associated with the formation of bonded Zdol. The primary differences between the Zdol/CNx and the Zdol/ CHx systems are that the chemical natures of the active adsorption sites are different and that incorporation of nitrogen into the carbon overcoat leads to a repulsion between the lubricant backbone and the surface. The effects of the latter on the Zdol/CNx system include (a) an effective increase of nominally 50% in the thickness required to obtain complete physical coverage of the carbon surface, (b) an increase in the time constant for Zdol restructuring/bonding, and (c) an increase in the thickness corresponding to the minimum in the free energy or, equivalently, an increase in the amount of lubricant that can be bonded to the surface.
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Figure 7. Comparison of the Zdol 4000 bonding kinetics on CNx (9) and CHx (b) overcoated computer disks, as a function of time at 64 °C. The initial thickness of Zdol on both disks was 10 ( 0.5 Å.
B. Zdol Bonding Kinetics on CNx versus CHx. In the previous section, we showed that the changes in the polar surface energy of lubricated Zdol on CNx disk carbon overcoats are time dependent. We correlated the surface energy decrease of the first Zdol monolayer with the formation of bonded lubricant and qualitatively determined that the time constants for bonding differed substantially for the CNx and CHx carbon surfaces. In the following discussion, we focus on quantifying the time dependence for Zdol bonding on these two surfaces. The evolution of the Zdol/CNx system in time was investigated by studying the kinetics of the bonding reaction at 64 °C. In Figure 7, the rate of bonding for Zdol 4000 on CNx is shown together with comparable data on CHx. We find that the initial levels of bonding are similar in both systems; however, the time dependence for the formation of bonded Zdol is strikingly different on CNx versus CHx. For Zdol on these carbon surfaces, the bonding reaction typically competes with evaporation of lubricant from the surface, with the branching between evaporation and bonding being strongly dependent upon both temperature and the molecular weight of the lubricant. Thus, for a complete kinetic treatment, both reaction channels are usually considered.3 However, at the temperature considered here of 64 °C, the amount of Zdol 4000 evaporation from CNx is only (2 ( 1)%. Since the perturbation on the bonding kinetics due to lubricant evaporation is small, a more facile description of the bonding kinetics is possible. We note that a full kinetic treatment of the competition between the bonding and evaporation reactions of Zdol on CHx as a function of molecular weight has been disclosed elsewhere,3 and that of Zdol on CNx is forthcoming. Using the assumption that Zdol evaporation is minimal, the rate of bonding dB/dt (where B represents the fraction of lubricant that bonds) on CHx and CNx can be fit phenomenologically (for time t > 0) using the differential rate equations
dB τ ) k′BA dt t
() dB τ ) k′ A( ) dt t 1/2
B
for CHx
(4a)
for CNx
(4b)
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where k′B is the first-order rate constant for bonding and A is the fraction of mobile lubricant on the carbon surface at time t. The time constant τ is related to lubricant restructuring on the carbon surface (see below). The integrated rate equations take the form
B(t) ) A0[1 - e-k′Bτ ln t] + B0 B(t) ) A0[1 - e-2k′Bτ(
xt-1)
] + B0
for CHx
(5a)
for CNx
(5b)
where A0 and B0 are the initial amounts of mobile and bonded lubricant, respectively. The solid lines accompanying the data points in Figure 7 are derived from the solutions of the integrated rate equations 5a and 5b. A conceptual framework useful for interpreting the bonding kinetics for Zdol on the CNx and CHx surfaces is provided by considering the following simple schematic mechanism:
A(ads) f B
Figure 8. HF/3-21G* net atomic charges computed from a Mulliken population analysis for (a) ZD and (b) 2,4,6-imino1,3,5-heptene. The total energies of the optimized geometries are -758.150 010 hartrees for ZD and -317.010 239 hartrees for 2,4,6-imino-1,3,5-heptene.
(6)
In eq 6, A(ads) is the concentration of Zdol molecules with the hydroxyl end groups in a position at the carbon active sites conducive to bonding. Once this occurs, bonding of the mobile lubricant occurs via simple first-order kinetics from A(ads); that is, dB/dt ) kBA(ads). Our results suggest that the fraction of mobile Zdol at these adsorption sites, expressed as A(ads)/A, is time dependent and given by (τ/t)m. The change in the concentration of Zdol at the CNx bonding sites A(ads), which varies as t-1/2, suggests that bonding is mitigated by diffusion of the hydroxyl end groups to the active bonding sites on the surface. We note that while the same diffusion process likely precedes the bonding of Zdol on CHx, the t-1/2 dependence does not appear in eq 4a, since diffusion is not rate limiting in this case. While still under investigation, we believe the observed Zdol bonding kinetics on CHx results from a time dependent change in the number of surface active sites available to facilitate interaction with the hydroxyl end groups.3 On the basis of the observation that the initial bonding rate on CHx is higher than that on CNx, we conclude that Zdol on CNx is initially in a less favorable position to bond compared to Zdol on CHx. As is discussed in more detail below, this could be a manifestation of the repulsive interactions that develop between the perfluorinated Zdol backbone and the electron-rich nitrogenated surface. C. Ab Initio Calculations. The surface energy measurements and the bonding kinetic experiments demonstrate that the interactions between Zdol and the disk surface are strongly dependent on the nature of the carbon overcoat. In the following, we address the possible origins for these differences using ab initio quantum chemical calculations to investigate potential active sites present on the carbon surface and how these sites may interact with the Zdol backbone and hydroxyl end groups. In the previous sections data were presented which suggested that nitrogenated species make up the active sites on the CNx surface (Table 1). Detailed studies on the chemical structure of CNx, while somewhat inconsistent, indicate the surface imine (>CdNs) as a possibly important nitrogenated functionality.11 To gain insight into the adhesion of Zdol to the CNx surface, we performed ab initio quantum chemical calculations between a model lubricant and model CNx surfaces. In an effort to keep the computational studies tractable, we employ the fluorinated hydroxyl-terminated ether CF3OCF2CH2OH (hereafter referred to as ZD) as the model for the Zdol lubricant. As a reference point, the optimized geometry
and partial atomic charges for the isolated ZD molecule are shown in Figure 8a. We find that each fluorine atom in ZD has an excess negative charge of -0.37 e. The hydroxyl and ether oxygens are also negatively charged by -0.66 e and -0.79 e, respectively. The net gains in negative charge by the fluorine and oxygen atoms are compensated by a substantial loss of charge by the adjacent carbon atoms and the hydroxyl hydrogen atom in the end group. The fluorinated backbone of ZD (and presumably Zdol) would thus be expected to exhibit basic properties (in the Lewis sense) while the hydroxyl end group would tend to act as an acid. As models for the heterogeneous CNx surface, a number of small molecules were investigated. 1,4-Hexene was chosen to be representative of the non-nitrogenated portions of the CNx surface. To model the imine functionality on the carbon surface, we employ the small molecules 2-imino-1,4-hexene and 2,4,6-imino-1,3,5-heptene. The optimized geometry and partial atomic charges of isolated 2,4,6-imino-1,3,5-heptene are presented in Figure 8b. We find that the imine nitrogen atoms have excess negative charges of -0.60 e. The negative imine centers, which are basic in the Lewis sense, should therefore be attractive toward the acidic O-H in ZD but strongly repulsive toward the negative ether oxygen and fluorine atoms. Initially, we investigate the interaction of ZD with 1,4hexene, shown in Figure 9a, which represents a model of the interactions that might be expected on a purely hydrocarbon-like surface. We find that four nonbonding close contacts develop between ZD and the hydrogen atoms on 1,4-hexene. On the ZD molecule, these are the hydroxyl oxygen O17, the perfluoromethylene fluorine atoms F25 and F26, and the perfluoromethyl fluorine atom F29. The O17-H5, F25-H11, F26-H11, and F29-H14 interatomic distances are 2.46, 2.68, 2.31, and 2.55 Å, respectively. The results of the ab initio calculation performed on the ZD and 2-imino-1,4-hexene complex are shown in Figure 9b. When one of the carbon atoms of 1,4-hexene is replaced with a nitrogen atom, the interactions of the ZD molecule are dramatically altered. A strong hydrogen bond develops between the acidic OH end group on ZD and the basic imine nitrogen atom. The hydrogen bond distance is computed to be 1.81 Å. Therefore, the imine functionality if present on the carbon surface would act as an active site capable of interacting attractively with the hydroxyl end group of Zdol. Since the remaining portion of the 2-imino1,4-hexene molecule is analogous to 1,4-hexene, the
Zdol/Amorphous-Hydrogenated Carbon Interactions
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change of -5.6 kcal/mol. The increase in the hydrogen bond distance is a direct result of the repulsive interactions that develop between the negatively charged fluorine and oxygen atoms of ZD and the remaining (non-neutralized) nitrogen atoms of 2,4,6-imino-1,3,5-heptene. The effects of this repulsion are also evidenced in the orientation of ZD on this surface. While the hydrogen atoms H7 and H8 on ZD (adjacent to the hydroxyl end group) are weakly acidic (Figure 8b) and can interact with the imine nitrogen (the H7-N19 nonbond distance is 2.6 Å), the perfluorinated ether portion of the ZD molecule has extended spatially away from the imine surface. The interaction of the hydroxyl end groups of Zdol with the CNx and CHx active sites appears to be comparable in the sense that strong hydrogen-bonding interactions can occur. However, since the interactions of the perfluorinated backbone with imine functionalities are dramatically different than the interactions with oxidized moieties,2 the initial distribution and subsequent relaxation of Zdol conformers on CNx can be expected to be quite different from those on CHx surfaces. The repulsive interactions inferred from the ab initio calculations are consistent with the negative Hamaker constants for the Zdol/CNx interaction and the increased thickness (compared to that of the Zdol/CHx system) at which the minimum i ∆Fp is observed. While transitions to the bonded state are thermodynamically favorable as a result of the decrease in the free energy, the majority of the Zdol initially applied to the CNx surface is not bonded. This suggests that a barrier to Zdol bonding exists. We believe that the barrier to Zdol bonding/restructuring on CNx is attributable to the Zdol lubricant having to minimize or overcome the repulsive backbone interactions that will allow the hydroxyl end group to interact favorably with the active sites on the carbon surface, that is, internal rotations to position the hydroxyl end groups near carbon surface active sites. Once this is accomplished, the interactions provided via hydrogen bonding of the Zdol end groups with the imine surface sites are of comparable strength to those provide via hydrogen bonding with the oxidized active sites on CHx. Conclusions Figure 9. HF/3-21G* optimized geometries for the interaction of ZD with (a) 1,4-hexene, (b) 2-imino-1,4-hexene, and (c) 2,4,6imino-1,3,5-heptene. The total energies are -989.854 619 hartrees for the ZD/1,4-hexene complex, -1005.759 821 hartrees for the ZD/2-imino-1,4-hexene complex, and -1075.188 815 hartrees for the ZD/2,4,6-imino-1,3,5-heptene complex.
orientation of the ZD backbone remains essentially unchanged. To investigate the effect of increasing the relative number density of imine functionalities on the CNx surface, we modeled the interaction of ZD with 2,4,6-imino1,3,5-heptene. When the number density of imine functionalities is greater than the number density of hydroxyl end groups, complete neutralization of the surface will not result and the interaction of the perfluorinated ZD backbone with the imine functionality can be investigated. The result of the calculations performed on the ZD and 2,4,6-imino-1,3,5-heptene interaction pair is shown in Figure 9c. The same hydrogen bond between the hydroxyl end group of ZD and an imine functionality serves to anchor the ZD molecule to the model surface. The hydrogen bond length increases from 1.81 Å found for the interaction of ZD with 2-imino-1,4-hexene to 1.92 Å. The N17-H2 hydrogen bond, however, remains strong with a computed binding energy of -16.2 kcal/mol and a Gibbs free energy
The Zdol + CNx system is characterized by two distinct states of the PFPE polymer on the surface. The first, which we have termed the “mobile” lubricant, interacts with the carbon surface via repulsive van der Waals interactions. The effective Hamaker constants obtained from the dispersive component of the surface free energy demonstrate that the repulsion between Zdol and the carbon surface is greater on CNx than it is on CHx. As a direct result, Zdol is less efficient at covering the CNx surface. Qualitatively, we estimate from the dispersive surface energy measurements that full physical coverage of the CNx surface occurs at an applied lubricant thickness (21 ( 2 Å) that is nominally 50% greater than that necessary to cover the CHx surface (14 ( 1 Å). The second identified state of Zdol on the CNx surface is the bonded state. While the nature of the bonding cannot be unequivocally ascertained from these experiments, the forces that lead to the enhanced adhesion stem from attractive polar interactions between the hydroxyl end groups of the PFPE lubricant and the polar active sites on the carbon surface. The ab initio calculations suggest that nitrogenated species on the CNx surface should be capable of strong hydrogen bonding with the hydroxyl end groups of Zdol. The Zdol + CNx system is similar to the Zdol + CHx system in that the initially applied
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lubricant is thermodynamically unstable. Immediately after lubricant deposition, approximately 10-20% of the Zdol present on these surfaces is bonded, and the remainder is mobile. As a result of the intramolecular motion within the Zdol polymer on the surface, the free hydroxyl end groups of the mobile lubricant pass through orientations favorable for bonding. Since transitions from the mobile state to the bonded state are thermodynamically driven by the decrease in the free energy associated with this transition, the fraction of mobile Zdol present on these surfaces decreases with increasing time. The rate of this Zdol restructuring process is slower at room temperature on the CNx surface compared with the CHx surface. We attribute the slower kinetics to the less favorable van der Waals interactions between the perfluoropolyether backbone and the CNx surface. The kinetic analysis suggests that the bonding of Zdol on CNx is mitigated by the random walk (diffusion) of the Zdol reactive end groups to the surface active sites. Once the end group approaches an active site, bonding occurs. A similar diffusion is not observed in the bonding kinetics of Zdol on CHx, since diffusion in this case is not rate limiting. Evidence for Zdol layering on the carbon surface is found in the polar surface energy. The strong adhesive interactions between the lubricant end groups and the underlying substrate result in the adsorption of the first Zdol
Tyndall et al.
monolayer with the hydroxyl end groups preferentially located at the carbon surface. The number of hydroxyl groups at the air-lubricant interface is therefore low. The low free-hydroxyl density of the first monolayer then affects the structure of the second monolayer. Since the number of “free” hydroxyl groups present on a surface coated with one Zdol monolayer is low, the number of “active” adsorption sites with which the second monolayer can interact is low. The density of free-hydroxyl end groups present in the second monolayer is thus higher than that of the first monolayer and also higher than that of the bulk Zdol lubricant. The surface produced via adsorption of two Zdol monolayers should therefore provide a large number of “active” hydroxyl sites with which subsequent lubricant can “bond”. Thus, the effects of the short-range polar interactions between the lubricant end groups and the surface, which produce a preferential orientation of the first monolayer, propagate to the second and presumably subsequent Zdol monolayers. Acknowledgments. We gratefully acknowledge J. Castenada for his assistance in lubing the CNx substrates, and R. Wagler and R. L. White for sputtering the CNx overcoats. LA9802825
