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Z-Tetraol composition and bonding to the underlying carbon surface
Journal of Colloid and Interface Science 333 (2009) 540–547
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Z-Tetraol composition and bonding to the underlying carbon surface R.J. Waltman Hitachi GST, 5600 Cottle Road, San Jose, CA 95193, USA
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 December 2008 Accepted 22 January 2009 Available online 4 March 2009
The bonding kinetics for 10 Å Z-Tetraol films on the amorphous nitrogenated carbon surface (CNx) is investigated as a function of the Z-Tetraol molecular polarity. Z-Tetraol nominally contains 2 hydroxyl groups per chain end. The molecular polarity of Z-Tetraol is varied by the addition of a “bis-adduct” component of Z-Tetraol. This material contains 3 instead of 2 hydroxyl groups per chain end. The bonding kinetics and changes in the surface energy as a function of time in thin film mixtures is investigated under ambient conditions. The initial bonded fraction scales with the bis-adduct concentration and can be increased by as much as 50%. The asymptotic bonded fraction exhibits a more modest increase of approximately 10%. © 2009 Elsevier Inc. All rights reserved.
Keywords: Z-Tetraol Bonding kinetics Surface energy Solvent power Model intermolecular interactions
1. Introduction The interfacial behavior of molecularly thin fluid films in contact with solid surfaces is an active area of research [1,2]. However, since molecularly thin films employed as boundary lubricants reduce the wear between contacting or intermittently contacting surfaces, they are also important technological materials. In the magnetic recording industry, tribological robustness in hard disk drives (HDDs) is particularly challenging because at the extremely small clearance between the read-write element of the slider and the disk surface, approximately 3–5 nm, lubricant motion and displacement on the rigid disk surface can occur due to the combined effects of the air shear stress; the air bearing pressure; and the van der Waals attractive forces operating between the head and the disk surfaces [3,4]. The resulting isolated and/or periodic lubricant moguls and ripples can cause instabilities in the flying characteristics of the slider. These instabilities can result in the mechanical degradation of the drive performance. The ability of boundary lubricant films to resist the formation of thickness non-uniformity under HDD conditions is dependent primarily upon the lubricant film thickness and its physical adsorbed state. The disjoining pressure, which derives from the interactions between the thin liquid film and the underlying solid surface, plays a critical role in defining the ability of the lubricant film to provide additional slider-disk clearance by resisting the dynamic film expansion in the head–disk interface [5–7]. Lubricant “bonding” to the underlying solid surface is another critical interfacial parameter that provides increased resistance to the unwanted migration of the lubricant film on the disk surface. Lubricant bonding
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is achieved by the intermolecular interactions between the polar functional groups of the lubricant and the underlying carbon surface [8]. Since bonding under ambient conditions can be timedependent, the process is often accelerated by annealing or by UV curing [9]. In this report, we investigate the bonding properties of the perfluoropolyether (PFPE) lubricant “Z-Tetraol” to the underlying carbon surface. We choose Z-Tetraol because while it is presently the most widely utilized disk lubricant in the magnetic recording industry, its bonding properties to the underlying carbon surface is fundamentally not well-understood. This is because Z-Tetraol can contain a variety of OH adducts (i.e., dol, Tetraol, bis-adducts, etc.) that impacts both the deposition and the bonding kinetics on the underlying carbon film. Consequently its bonding properties can vary significantly depending upon the relative concentration of the various OH adducts and whether or not fractionation (e.g., supercritical fluid extraction) is employed to control the distribution of the various OH adducts in Z-Tetraol. Therefore we report on the bonding kinetics of Z-Tetraol on CNx as a function of its composition. This study will reveal the role that the various OH adducts play in the initial deposition and the subsequent bonding kinetics of the Tetraol lubricant. 2. Experimental The substrates used in these studies were 95 mm diameter AlMg disks of nominally 2 Å RMS roughness as measured by a Dimension 5000 AFM with a standard AFM tip used in the tapping mode. The typical scan size for these measurements was 5 × 5 μm with a scan rate of 0.5 Hz and 256 lines of resolution. Atop the substrates were sputter-deposited a cobalt-based magnetic record-
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Fig. 1. Chemical structures for some Tetraol adducts. Table 1 NMR data on Z-Tetraols. Structure
Tetraol 2000S
Tetraol 2000S fractionated
Tetraol 2000GT
Tetraol 2000GT bis-adduct enriched
CF2 O CF2 CF2 O CF2 O/CF2 CF2 O CF2 O bounded by CF2 O (%) Dol adduct (%) Tetraol adduct (%) Bis-adduct (%) Mn (Daltons)
10.7 10.2 1.05 22.1 12 71 16 2240
10.6 9.6 1.10 22.6 1 97 2 2130
10.2 9.9 1.03 21.1 7 93 0 2150
9 .4 8 .9 1.06 21.3 3 52 45 2050
ing layer (CoPtCr) and 40 Å of an amorphous nitrogenated carbon film (called “CNx”) comprised of nominally 18 at% N. A number of the perfluoropolyethers (PFPEs) used in this work were commercially available from Solvay–Solexis under the tradenames Z-Tetraol 2000S and Z-Tetraol 2000GT, which were used directly as received. Z-Tetraol 2000S is polydisperse (M w / Mn ∼1.5) while Z-Tetraol 2000GT is more narrowly fractionated (M w / Mn ∼ 1.1–1.2). A sample of the polydisperse Z-Tetraol 2000S was also fractionated (15-cut fractionation by supercritical fluid extraction, Phasex Corp., MA, USA) and the fraction with the largest Tetraol/Zdol adduct ratio was isolated and used in these studies. This Z-Tetraol is referred to here as “fractionated Z-Tetraol 2000S.” The polydispersity of the fractionated Z-Tetraol 2000S was not determined. Finally a bis-adduct enriched Z-Tetraol 2000GT (45 mole % bis-adduct) was also investigated. The chemical structures for some possible Z-Tetraol OH adducts are shown in Fig. 1. The NMR data characterizing the bulk chemical composition of each Z-Tetraol is presented in Table 1. These data show that the main chain structures are nearly identical for all Tetraol samples and differ primarily in the percentage of the various OH adducts. Specular reflection FTIR (Nicolet Magna Model 560) was used to quantify the thickness of the applied perfluoropolyether films. A standard specular reflection adapter (Harrick, New York) was used to maintain the proper angle of incidence to the sample. The changes in each of the PFPE film thickness were quantified by calibrating the FTIR absorption band maximum (∼1280 cm−1 ) to the film thickness determined by XPS using a takeoff angle of 45◦ and an electron mean free path of 25 Å [10]. Throughout the following, we refer to the PFPE lubricant on the disk surface as being “bonded.” To clarify our definition of these
terms, we briefly describe the methodology by which these quantities are determined. The initial lubricant thickness is measured via specular reflectance FTIR. The disks are then rinsed with 2,3dihydro-perfluoropentane (Vertrel-XF, DuPont) for 1 min to remove any soluble lubricant, and the lubricant thickness re-measured. The lubricant retained by the disk is defined to be the amount “bonded.” The bonded fraction is obtained by normalizing to the initial lubricant thickness. 3. Computational methodology Ab initio calculations were performed using density functional theory (DFT) at the 6-311++G(d,p) basis set using the Gaussian 98 computer code [11]. The DFT calculations employed Becke’s 3-parameter functional together with Perdew and Wang’s gradientcorrected correlation functional [12]. The 6-311++G(d,p) basis set includes d functions for oxygen atoms and p functions for hydrogen atoms, both of which allow a more quantitative description of hydrogen bonding. All model structures were fully optimized. The requested convergence on the density matrices was 10−8 . Residual forces were 1 × 10−4 hartree bohr−1 on the cartesian components. Harmonic vibrational frequencies were calculated by differentiation of the energy gradient at the optimized geometry. No imaginary frequencies were computed for all structures. The figures reproducing the computational results were made using the GaussView graphical user interface [13] and the CambridgeSoft graphical software [14]. The strength of the bonding interaction between polar groups on the model PFPE lubricant and either the model carbon surface, the Vertel-XF solvent, or another PFPE molecule was quantified by dimer calculations on representative models. In order to keep the computations tractable, the PFPE OH was represented by HCF2 CH2 OH (Zdol end group) while the CNx carbon surface was represented by the imine CH3 N=CH2 . The Vertrel-XF structure was represented by the authentic CF3 CHFCHFCF2 CF3 formula. The computed binding energy is obtained as the difference in total energies between the dimer and the isolated reactants, i.e., E = E AB − ( E A + E B ), corrected for zero-point energy contributions. 4. Results The results of the ambient bonding kinetics conducted on 10 Å of polydisperse Tetraol 2000S, fractionated Tetraol 2000S,
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Table 3 Kinetic parameters used to fit Fig. 3.
Fig. 2. The bonding kinetics at 20 ◦ C and 50% RH for 10 Å films of polydisperse Z-Tetraol 2000S, narrowly fractionated Z-Tetraol 2000S, and Z-Tetraol 2000GT on CNx. Four sets of data points per time for each sample provide an indication of reproducibility. Table 2 Kinetic parameters used to fit Fig. 2. Lubricant
kB
h
2000S 2000S frac 2000GT
0.0055 0.0057 0.0052
0.50 0.50 0.50
and Tetraol 2000GT on amorphous nitrogenated carbon (CNx) are shown in Fig. 2. Upon application of the Tetraols to the CNx surfaces, some fraction bonds to the carbon surface at room temperature within the time required to allow for solvent evaporation and to conduct the initial thickness measurements (1 min). We observe that the polydisperse Tetraol 2000S has the largest initial bonded fraction of 0.5 while the fractionated 2000S and 2000GT Tetraols have the significantly lower initial bonded fraction of 0.25–0.30. All of the Tetraols exhibit increasing bonding to the CNx surface with increasing time reaching an asymptotic bonded fraction of approximately 0.8 for 2000S and 0.70–0.75 for the fractionated 2000S and 2000GT samples. The bonding rates shown in Fig. 2 do not follow classical kinetics but instead are time-dependent with a rate coefficient that decreases with time. The theoretical fit to the bonding data can be derived by assuming a rate coefficient that is time-dependent, that is, k(t ) ∝ k0 t −h , where k(t ) is the instantaneous rate coefficient at time t, k0 is the time-independent initial rate constant, and h is the coefficient describing the power dependence in time. The differential rate equation describing the conversion of mobile lubricant A to bonded lubricant B is [8]: dB dt
= k B t −h A ,
(1)
where k B is defined as the initial bonding rate constant. The results of the calculation based on this model are shown in Fig. 2 as the solid lines. The kinetic parameters used to generate the theoretical fits are summarized in Table 2. From these data we conclude that Tetraol bonding on CNx ubiquitously follows k(t ) ∝ k B t −1/2 kinetics. The bonding of perfluoropolyethers (PFPEs) to carbon surfaces characterized by this time dependence has been attributed to diffusion-limited one-dimensional kinetics [8]. The bonding kinetics data for each of the Tetraols can be interpreted on the basis of their bulk chemical composition. Refer-
Lubricant acronym
Weight % of bis-adduct in 2000GT
Mole % of bis-adduct
kB
h
bis0 bis9 bis18 bis32 bis100
0 9 18 32 100
0 4 8 14 45
0.0045 0.0042 0.0043 0.0043 0.0041
0.50 0.50 0.50 0.50 0.50
ring to the Tetraol NMR data (Table 1), the main chain chemical structure is virtually identical amongst the three samples, i.e., the ratio of the perfluoromethylene oxide to perfluoroethylene oxide monomer units is 1.06 ± 0.03, and the % of CF2 O monomer units adjacent to another CF2 O monomer unit is 21.8 ± 0.7%. Since all of the Tetraol bonding rates can be fit using k B = 0.0055 ± 0.00025 (5%) and h = 1/2 despite the very different initial bonded fractions, we conclude that the main chain is the significant determinant of the bonding kinetics. Previously, structural differences in the PFPE main chain (e.g., C/O ratio) have been shown to impact the bonding kinetics and the resultant tribological reliability of the finished disks [8,15]. According to Table 1, the chemical structural component that differs the most amongst the three Tetraol samples investigated in Fig. 2 is the concentration of the bis-adduct end group (Fig. 1). The NMR data shows that there is approximately 16 mol% bis-adduct in the 2000S sample compared to almost none for the fractionated 2000S and the 2000GT samples, 2 and 0 mol%, respectively. Based on this analysis, we propose that the bis-adduct end group is the significant determinant for the observed differences in the Tetraol bonding kinetics. We note that while the % of dol adduct is also different amongst the three Tetraol samples, they represent the less polar adduct that preferentially remains in solution and hence should not significantly impact the bonding kinetics [16]. In order to test this hypothesis, bis-adduct enriched Tetraol was incrementally added to Tetraol 2000GT and the thin film bonding kinetics of the resultant mixture samples were investigated as a function of time on the CNx surface. It is important to clarify here that the bis-adduct enriched Tetraol 2000GT has 45 mol% of the bis-adduct end group and about 50 mol% of the Tetraol adduct (Table 1). The mixture samples were produced by mixing the bisadduct enriched Tetraol 2000GT (Table 1, column 5) and fractionated Tetraol 2000GT (Table 1, column 4) in a common solvent (Vertrel-XF). The quantification of the mixtures is reported on the basis of the weight % of the bis-adduct enriched Tetraol 2000GT in Tetraol 2000GT. The compositions are referred to as, e.g., “bis9” or “bis100,” where bis9 represents 9% by weight and bis100 represents 100% by weight of the bis-adduct enriched Tetraol 2000GT in Tetraol 2000GT, respectively. The details of the Tetraol mixture compositions and the acronyms used to identify each of the mixture samples are summarized in Table 3 along with the kinetic parameters used to fit the bonding data. The bonding kinetics for Tetraol 2000GT as a function of bis-adduct content is shown in Fig. 3. Again, all of the experimental data up to the asymptotic bonded level could be fit by Eq. (1) using k B = 0.0043 ± 0.00015 (4%) and h = 1/2. Thus, we observe again that while the end group composition is apparently a significant determinant of the initial and asymptotic bonded fraction, the parameters used to fit the bonding kinetics (k B , h) are more dependent upon the main chain structure which are virtually identical here (Table 1). However, as shown in Fig. 5 right, the bonding rates (dB /dt) are very different. Since dB /dt is dependent on the concentration of the mobile lubricant “A” (Eq. (1)), dB /dt decreases with increasing bis concentration in Tetraol. Lastly we note that the k B values used to fit the two sets of data shown in Figs. 2 and 3 are not the same. This is attributed
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Fig. 3. (Left) Bonding kinetics at 20 ◦ C and 50% RH for 10 Å films of Z-Tetraol 2000GT on CNx as a function of bis-adduct enriched Z-Tetraol in the lubricant bath. Four sets of data points per time for each sample provide an indication of reproducibility. See Table 3 for mixture composition details. (Right) The corresponding bonding rate dB/dt as a function of time. The curves from top to bottom are bis0, bis9, bis18, bis32, and bis100, respectively. A reference line with a slope of −0.5 is shown.
Fig. 4. The average initial and asymptotic bonded fractions for Z-Tetraol 2000GT as a function of the weight % of bis-adduct enriched Z-Tetraol in the lubricant bath (solvent: Vertrel-XF). Data compiled from Fig. 3.
entirely to having conducted the two sets of experiments at separate times using slightly different CNx films. This difference does not impact against the conclusions derived from this study. Since each set of experiments is conducted simultaneously using identical carbon films, the results within each figure may be compared in the usual manner. The dependence of the initial and asymptotic bonded fractions as a function the bis-adduct concentration is conveniently summarized in Fig. 4. This plot provides clear evidence that both the initial and the asymptotic bonded fractions are correlated to the concentration of the bis-adduct component in Tetraol and thus provides a reasonable explanation for the apparently larger initial and asymptotic bonded fractions observed in polydisperse Tetraol 2000S (Fig. 2).
It is relevant at this time to discuss in more detail the nature of the bonded Z-Tetraol. We remind the reader that the bonded fraction, as shown in Figs. 2 and 3, is quantified after the disk is sequentially spray-rinsed with solvent (Vertrel-XF) for approximately 10 s, immersed in clean solvent (Vertrel-XF) for 1 min, then sprayrinsed for another 10 s, a methodology that was used to quantify the bonded fraction of the less polar Zdol disk lubricant [8]. During the course of these experiments, we observed that the initial bonded fraction for the polar Tetraols was dependent upon solvent soak time while the asymptotic bonded fraction was not. Referring first to Fig. 5 (top), the changes in the asymptotic bonded fraction as a function of solvent soak time shows that the asymptotically bonded lubricants are not easily displaced by the Vertrel-XF solvent even after ∼1000 min in solvent. Thus, we conclude that the polar intermolecular interactions that develop between the lubricant OH end groups and the underlying carbon surface are stronger than the lubricant–solvent interactions. In contrast, Fig. 5 (bottom) shows that the initial bonded fraction for the polar PFPE samples (Tetraols, bis-adduct) decrease significantly (∼40%) as a function of soak time, eventually converging towards an asymptotic initial bonded fraction after several thousand minutes. The timedependent dissolution observed for initially deposited Tetraols and the bis-adduct enriched Tetraol suggests that solvent power may be the rate-determining step for dissolution. This aspect will be further discussed below. The effect of the 1 and 1000 min solvent soak time on the bonding kinetics of Tetraol 2000S is compared in Fig. 6. This comparison is utilized to probe any significant differences that could limit the interpretation of the Tetraol bonding kinetics using the current methodology of a 1 min rinse time compared to the 1000 min soak time. At the 1000 min solvent soak, the experimental data up to the asymptotic bonded level can apparently still be fit by Eq. (1) using k B = 0.0052 and h = 1/2, similar to the 1 min soak time data (k B = 0.0055 and h = 1/2). Thus, the bonding kinetics (k B , h) is still governed by main chain effects. Furthermore, the asymptotic bonded fraction converges to the same level if only not at the same rate (Fig. 6, right). Thus the bonding kinetics as determined by the 1 min soak methodology appears to be adequate when used in side-by-side comparisons.
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Fig. 5. (Top) The changes in the asymptotic bonded fraction of 10 Å films of Tetraol 2000GT and bis100 on CNx as a function of soak time in Vertrel-XF solvent. (Bottom) The changes in the initial bonded fraction of 10 Å films of Tetraol 2000GT, Tetraol 2000S, bis100, and Zdol 2540 on CNx as a function of soak time in VertrelXF solvent.
5. Discussion The driving force for the bonding of thin PFPE lubricant films to the underlying carbon surface is the reduction of the surface energy [17]. Therefore, the change in the surface energy as a function of time, Fig. 7, left, can be correlated to the bonding kinetics of the Tetraols on the carbon surface, Fig. 7, right. The bonding is attributed to the attractive intermolecular interactions that develop between the Tetraol hydroxyl end groups (R–OH) and the polar functional groups on the carbon surface [8]. The kinetics for this process has already been discussed. The observed surface energy reduction for sub-monolayer PFPE films can be described by [17]
γsp = γcp + γl p − (εcl + εll ), p
(2) p
where γs is the polar component of the surface energy, γc is pop lar surface energy of the carbon film, γl is the surface energy of the non-interacting polar end groups in the sub-monolayer lubricant film, εcl is the adhesive interaction energy density between the lubricant film and the carbon surface, and εll is the cohesive interaction energy density between the lubricant molecules (via the polar OH groups). Since attractive interactions between polar groups result in a drop in energy, the signs for both εcl and εll as defined in Eq. (2) will be positive. p Referring again to Fig. 7, the γs at the asymptotic bonded level is a substantially smaller 6–10 mJ/m2 compared to the 36– p 42 mJ/m2 observed at the initial bonded level. Note that γs also scales with the fraction of bis-adduct in Tetraol and hence to the p overall molecular polarity. The decrease in γs is readily explained on the basis that with increasing time the fraction of Tetraol that becomes unextractable from the carbon surface increases due to the end group bonding (εcl ). This occurs over ∼104 min for the
systems under investigation here (Figs. 2 and 3). Conversely, the time-dependent dissolution of the initially bonded Tetraol (Fig. 5) suggests that, shortly after deposition, there are solvent-extractable Tetraol molecules. Since PFPE solubility is expected to decrease with increasing molecular polarity, solvent power may be a significant determinant of the initial bonded fraction, resulting in the observed time-dependence (Fig. 5). These aspects are now discussed in more detail below. We first consider the relative strength of the intermolecular interaction between a PFPE OH end group and a model CNx carbon surface imine (CH3 N=CH2 ), another PFPE OH end group, and the Vertrel-XF solvent. The goal of this theoretical study is to quantify the relative interaction strength between the PFPE OH end group and model surfaces and hence provide insight into the effect of solvent power on the bonding measurements. The optimized geometries for the interacting dimers are shown in Fig. 8. The computed binding energies and hydrogen bond distances are summarized in Table 4. As shown in Table 4, the PFPE OH dimer binding energy decreases in the order: imine > PFPE OH > Vertrel-XF. It is reasonable to conclude from the computational results that Vertrel-XF can more easily dissolve Tetraol molecules with “free” hydroxyls while the dissolution of a Tetraol hydroxyl bonded either to another Tetraol hydroxyl or to a polar group on the carbon surface becomes increasingly energetically prohibitive, respectively. The results of the computations appear to be fully consistent with the results presented in Fig. 5. According to the discussions above, it appears that the more polar PFPE molecules will have more limited solubility in the relatively non-polar solvents typically employed to deposit these materials, such as Novec fluids (3M), hydrofluorocarbons like the Vertrel-XF (DuPont) used here, or AE-3000 (Asahi Glass Co.). Therefore it is possible that the more polar PFPE lubricant components are more readily deposited from a “poorer” solvent and, once deposited on the disk, is not easily re-dissolved in the solvent. Solubility originates from the interaction energy between the solute and the solvent. The solubility parameter is defined as the square root of the cohesive energy density [18]:
δi =
E iv Vi
cal/cm3
1/2
,
(3)
where E iv is the energy of vaporization and V i is the molar volume of component i. The solubility parameter, δ , therefore describes the attractive strength between molecules and can be used as a criterion for the solvent power. Equation (3) is related to the heat of mixing per unit volume as [18]
H m / V = (δ1 − δ2 )1/2 ϕ1 ϕ2 ,
(4)
where ϕi is the volume fraction of component i. Since the free energy of mixing is related to the enthalpy of mixing by G m = H m − T S m , the difference in the polymer–solvent solubility parameter (δ1 –δ2 ) must be small for miscibility. The solubility parameter is conveniently calculated by expanding E v and V as [19]
E v = and V =
e j
v j ,
(5)
(6)
where e j and v j are the additive atomic and group contribution for the energy of vaporization and molar volume, respectively. The computed solubility parameter for some of the OH adducts found in Tetraol (at nominally 2200 molecular weight) and VetrelXF are shown in Table 5. These data are used here strictly for qualitative purposes. Table 5 clearly shows that the enthalpy of mixing does not favor the solution of the more polar adducts.
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Fig. 6. (Left) Bonding kinetics at 20 ◦ C and 50% RH for 10 Å films of Z-Tetraol 2000S on CNx after 1 and 1000 min solvent soak time to determine the bonded fraction. Four sets of data points per time for each sample provide an indication of reproducibility. (Right) The corresponding bonding rate, dB /dt, as a function of time.
Fig. 7. The changes in the polar surface energy,
γsp , as a function of (left) time and (right) the average bonded fraction under ambient conditions (20 ◦ C, 50% RH).
Fig. 8. The B3PW91/6-311++G(d,p) optimized geometries for some model interactions between a PFPE OH end group (CFH2 CH2 OH) and (left) Vetrel-XF; (middle) an imine carbon surface; and (right) another PFPE OH end group. The equilibrium intermolecular distance between the PFPE OH end group and the surface polar groups are given in angstrom.
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Table 4 B3PW91/6-311++G(d,p) optimized hydrogen bond distance and E corrected for zero-point energy contributions. Dimer HCF2 CH2 OH HCF2 CH2 OH HCF2 CH2 OH HCF2 CH2 OH
+ + + +
CH3 N=CH2 HCF2 CH2 OH Vertrel-XF H2 O
H-bond distance (Å)
E (kcal/mol)
1.851 1.886 2.206 1.864
−6.14 −4.60 −2.74 −3.68
Table 5 Computed solubility parameter for Tetraol adducts and Vertrel-XF solvent. Lubricant or solvent
Molecular weight
δ (cal/cm3 )1/2
Vertrel-XF Zdol Tetraol Bis-adduct
252 2200 2200 2200
6 .2 8 .4 9 .2 10.1
Fig. 10. The displacement of 12 Å 2000GT on a carbon surface by a 0.5 μl water droplet is quantified on an optical surface analyzer (Candela 5120). The thickness profile shown to the right of each surface image is taken at the dotted line.
Fig. 9. The thickness of lubricant deposited on the CNx carbon surface as a function of the lubricant solution concentration in Vertrel-XF. The disk withdrawal rate is a constant 1.5 mm/s.
This is readily verified by quantifying the deposition of Tetraol 2000GT and bis100. As shown in Fig. 9, the more polar bis100 deposits more readily from solution (Vertrel-XF) compared to Tetraol 2000GT at the same solute concentration. These data indicate that with increasing polarity, the more polar components of the PFPE, i.e., more OH end groups and/or lower molecular weight components, are likely to deposit onto the carbon surface. These polar adducts are also less likely to re-dissolve in the solvent, giving rise to an apparently larger initial bonded fraction. Once the lubricant film is deposited onto the carbon surface, diffusive self-stirring (time-dependent bonding kinetics) governs the spatial delivery of the PFPE OH end groups to the polar capture sites on the carbon surface, eventually leading to the asymptotic bonded level. At the asymptotic bonded state, the PFPE has reconstructed substantially to provide a significantly more stable bonded state to the underlying carbon film. In order to further probe the nature of the PFPE/CNx polar interactions, the stability of both the initial and asymptotic bonded fractions to water was next investigated. This experiment originates from the data presented in Fig. 7 with the accompanying suspicion that the observed temporal changes in the surface energy derives from the water droplet penetrating and/or displacing some fraction of the total lubricant film thickness. In these stud-
Fig. 11. The displacement of 10 Å bis100 on a carbon surface by a 0.5 μl water droplet is quantified on an optical surface analyzer (Candela 5120). The thickness profile shown to the right of each surface image is taken at the dotted line.
ies 0.5 μl water droplets were placed on the disk surfaces of 10 Å films of 2000GT and bis100 at the initial and asymptotic bonded fractions and were allowed to evaporate under ambient conditions (5–10 min). The disk surfaces were then optically analyzed “immediately” (10 min) for possible lubricant displacement by the water droplet. Figs. 10 and 11 reveal that at the initial bonded fraction, labeled “as dipped” in the optical images, water droplets do displace the lubricant as shown by the lighter colored region in the images. The quantity of lubricant displaced is shown in the corresponding right figures and is a substantial ∼3 Å (30%) for both 2000GT and bis100 (Table 6, columns 2, 4). The darker region that appears towards the left of the droplet area in both optical images is assumed to be some of the displaced lubricant and/or residual water. In contrast, the water droplets are observed to displace very little lubricant at the asymptotic bonded fraction for both 2000GT
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Table 6 The lubricant thickness displacement (Å) by 0.5 μl water droplets on lubricated disk surfaces. These data were quantified using an optical surface analyzer (Candela 5120). Representative data are shown in Figs. 9–11. Sample #
10.5 Å 2000GT at initial bonded fraction
10.5 Å 2000GT at asymptotic bonded fraction
10.2 Å bis100 at initial bonded fraction
10.2 Å bis100 at asymptotic bonded fraction
23 Å Z2300 (no bonding)
19 Å 2000GT (3 Å bonded)
1 2 3
2.8 2.2 2.3
0.6 0.5 0.5
3.1 2.5 2.0
0.3 0.4 0.3
22.0 18.7 19.0
4.6 4.6 4.5
Tetraol OH groups and/or cohesively interacting Tetraol OH groups that gradually reconstruct on the carbon surface to produce more bonded Tetraol on the carbon surface. The reconstruction follows k(t ) ∝ k0 t −h kinetics and produces a lubricant state at the asymptotic bonded level that is stable to solvent dissolution. The technological implications of the studies presented herein are that the resistance of the boundary lubricant to surface disturbances and disk rotation effects is enhanced by the lubricant bonding, either adhesive (εcl ) or cohesive (εll ). Furthermore, as increasingly stringent HDD requirements mandate decreased lubricant disturbance on the disk surface, this work reveals a chemical-based methodology to increase the bonded fraction and hence the ability of the boundary lubricant to resist unwanted migration. Acknowledgment The author thanks J. Burns of this laboratory for the NMR characterization of the Tetraol samples, and Solvay–Solexis for a generous sample of the bis-adduct enriched Tetraol 2000GT. Fig. 12. The displacement of 23 Å Z2300 (top) and 19 Å 2000GT on a carbon surface by a 0.5 μl water droplet is quantified on an optical surface analyzer (Candela 5120). The thickness profile shown to the right of each surface image is taken at the dotted line.
and bis100. The optical images show ∼0.5 Å (5%) and ∼0.3 Å (3%) of lubricant displacement for Tetraol-GT and bis100, respectively (Table 6, columns 3, 5). Thus, the solvent extraction studies (Vertrel-XF and water) on the initial bonded fraction suggests that free OH end groups exist in the initially deposited thin film that is not efficiently removed by the solvent used to deposit the Tetraols in the first place. Water displacement of thicker lubricant films were next investigated to elucidate the possible role of cohesively bonded lubricant end groups (εll ). We first consider the non-functionalized Z2300 which has no polar hydroxyl end groups. In this case, the water droplet is observed to displace almost all of the lubricant film as shown in Fig. 12 (top). In contrast, a water droplet on a 19 Å Tetraol-GT film (3 Å bonded to the carbon surface (εcl )) displaces approximately 5 Å of film, but does not displace approximately 10 Å, suggesting some degree of cohesion (εll ) between Tetraol end groups exists. 6. Conclusions In this study, we have demonstrated that polar adducts in ZTetraol are significant determinants of Tetraol bonding to the underlying carbon surface. An increase in the overall Tetraol molecular polarity leads to increased bonding onto the carbon surface. The initial bonded state is characterized by the presence of free
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S. Sills, R.M. Overney, Phys. Rev. Lett. 91 (2003), 095501-1–1095501-3. S. Sinha, M. Kawaguchi, T. Kato, J. Phys. D Appl. Phys. 37 (2004) 2247–2259. R. Pit, B. Marchon, S. Meeks, V. Velidandla, Tribol. Lett. 10 (2001) 133–142. Q. Dai, B.E. Knigge, R.J. Waltman, B. Marchon, IEEE Trans. Magn. 39 (2003) 2459–2461. A. Khurshudov, R.J. Waltman, Tribol. Lett. 11 (2001) 143–149. C.M. Mate, V.J. Novotny, J. Chem. Phys. 94 (1991) 8420–8427. R.R. Dagastine, L.R. White, P.M. Jones, Y.-T. Hsia, J. Appl. Phys. 98 (2005), 124906-1–124906-6. R.J. Waltman, G.W. Tyndall, J. Pacansky, R.J. Berry, Tribol. Lett. 7 (1999) 91–102. D.D. Saperstein, L.J. Lin, Langmuir 6 (1990) 1522–1524. M.F. Toney, C.M. Mate, D.J. Pocker, IEEE Trans. Magn. 34 (1998) 1774–1776. Gaussian 98, Revision A.7: M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian, Inc., Pittsburgh, PA, USA. (a) A.D. Becke, J. Chem. Phys. 104 (1996) 1040–1046; (b) J.P. Perdew, K. Burke, Y. Wang, Phys. Rev. B 54 (1996) 16533–16539. GaussView 2.1, Gaussian, Inc., Pittsburgh, PA. CambridgeSoft Chem 3D Ultra, Version 4.0, Cambridge, MA, USA. T. Karis, G.W. Tyndall, R.J. Waltman, Tribol. Trans. 44 (2001) 249–255. R.J. Waltman, G.W. Tyndall, G.J. Wang, H. Deng, Tribol. Lett. 16 (2004) 215–230. G.W. Tyndall, R.J. Waltman, Soc. Proc. Mater. Res. 517 (1998) 403–414. E.A. Grulke, in: J. Brandrup, E.H. Immergut (Eds.), Polymer Handbook, third ed., vol. 7, Wiley, New York, 1989, p. 519. R.F. Fedors, Polym. Eng. Sci. 14 (1974) 147–154.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Z-Tetraol composition and bonding to the underlying carbon surface R.J. Waltman Hitachi GST, 5600 Cottle Road, San Jose, CA 95193, USA
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 1 December 2008 Accepted 22 January 2009 Available online 4 March 2009
The bonding kinetics for 10 Å Z-Tetraol films on the amorphous nitrogenated carbon surface (CNx) is investigated as a function of the Z-Tetraol molecular polarity. Z-Tetraol nominally contains 2 hydroxyl groups per chain end. The molecular polarity of Z-Tetraol is varied by the addition of a “bis-adduct” component of Z-Tetraol. This material contains 3 instead of 2 hydroxyl groups per chain end. The bonding kinetics and changes in the surface energy as a function of time in thin film mixtures is investigated under ambient conditions. The initial bonded fraction scales with the bis-adduct concentration and can be increased by as much as 50%. The asymptotic bonded fraction exhibits a more modest increase of approximately 10%. © 2009 Elsevier Inc. All rights reserved.
Keywords: Z-Tetraol Bonding kinetics Surface energy Solvent power Model intermolecular interactions
1. Introduction The interfacial behavior of molecularly thin fluid films in contact with solid surfaces is an active area of research [1,2]. However, since molecularly thin films employed as boundary lubricants reduce the wear between contacting or intermittently contacting surfaces, they are also important technological materials. In the magnetic recording industry, tribological robustness in hard disk drives (HDDs) is particularly challenging because at the extremely small clearance between the read-write element of the slider and the disk surface, approximately 3–5 nm, lubricant motion and displacement on the rigid disk surface can occur due to the combined effects of the air shear stress; the air bearing pressure; and the van der Waals attractive forces operating between the head and the disk surfaces [3,4]. The resulting isolated and/or periodic lubricant moguls and ripples can cause instabilities in the flying characteristics of the slider. These instabilities can result in the mechanical degradation of the drive performance. The ability of boundary lubricant films to resist the formation of thickness non-uniformity under HDD conditions is dependent primarily upon the lubricant film thickness and its physical adsorbed state. The disjoining pressure, which derives from the interactions between the thin liquid film and the underlying solid surface, plays a critical role in defining the ability of the lubricant film to provide additional slider-disk clearance by resisting the dynamic film expansion in the head–disk interface [5–7]. Lubricant “bonding” to the underlying solid surface is another critical interfacial parameter that provides increased resistance to the unwanted migration of the lubricant film on the disk surface. Lubricant bonding
E-mail address: [email protected]. 0021-9797/$ – see front matter doi:10.1016/j.jcis.2009.01.045
© 2009
Elsevier Inc. All rights reserved.
is achieved by the intermolecular interactions between the polar functional groups of the lubricant and the underlying carbon surface [8]. Since bonding under ambient conditions can be timedependent, the process is often accelerated by annealing or by UV curing [9]. In this report, we investigate the bonding properties of the perfluoropolyether (PFPE) lubricant “Z-Tetraol” to the underlying carbon surface. We choose Z-Tetraol because while it is presently the most widely utilized disk lubricant in the magnetic recording industry, its bonding properties to the underlying carbon surface is fundamentally not well-understood. This is because Z-Tetraol can contain a variety of OH adducts (i.e., dol, Tetraol, bis-adducts, etc.) that impacts both the deposition and the bonding kinetics on the underlying carbon film. Consequently its bonding properties can vary significantly depending upon the relative concentration of the various OH adducts and whether or not fractionation (e.g., supercritical fluid extraction) is employed to control the distribution of the various OH adducts in Z-Tetraol. Therefore we report on the bonding kinetics of Z-Tetraol on CNx as a function of its composition. This study will reveal the role that the various OH adducts play in the initial deposition and the subsequent bonding kinetics of the Tetraol lubricant. 2. Experimental The substrates used in these studies were 95 mm diameter AlMg disks of nominally 2 Å RMS roughness as measured by a Dimension 5000 AFM with a standard AFM tip used in the tapping mode. The typical scan size for these measurements was 5 × 5 μm with a scan rate of 0.5 Hz and 256 lines of resolution. Atop the substrates were sputter-deposited a cobalt-based magnetic record-
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541
Fig. 1. Chemical structures for some Tetraol adducts. Table 1 NMR data on Z-Tetraols. Structure
Tetraol 2000S
Tetraol 2000S fractionated
Tetraol 2000GT
Tetraol 2000GT bis-adduct enriched
CF2 O CF2 CF2 O CF2 O/CF2 CF2 O CF2 O bounded by CF2 O (%) Dol adduct (%) Tetraol adduct (%) Bis-adduct (%) Mn (Daltons)
10.7 10.2 1.05 22.1 12 71 16 2240
10.6 9.6 1.10 22.6 1 97 2 2130
10.2 9.9 1.03 21.1 7 93 0 2150
9 .4 8 .9 1.06 21.3 3 52 45 2050
ing layer (CoPtCr) and 40 Å of an amorphous nitrogenated carbon film (called “CNx”) comprised of nominally 18 at% N. A number of the perfluoropolyethers (PFPEs) used in this work were commercially available from Solvay–Solexis under the tradenames Z-Tetraol 2000S and Z-Tetraol 2000GT, which were used directly as received. Z-Tetraol 2000S is polydisperse (M w / Mn ∼1.5) while Z-Tetraol 2000GT is more narrowly fractionated (M w / Mn ∼ 1.1–1.2). A sample of the polydisperse Z-Tetraol 2000S was also fractionated (15-cut fractionation by supercritical fluid extraction, Phasex Corp., MA, USA) and the fraction with the largest Tetraol/Zdol adduct ratio was isolated and used in these studies. This Z-Tetraol is referred to here as “fractionated Z-Tetraol 2000S.” The polydispersity of the fractionated Z-Tetraol 2000S was not determined. Finally a bis-adduct enriched Z-Tetraol 2000GT (45 mole % bis-adduct) was also investigated. The chemical structures for some possible Z-Tetraol OH adducts are shown in Fig. 1. The NMR data characterizing the bulk chemical composition of each Z-Tetraol is presented in Table 1. These data show that the main chain structures are nearly identical for all Tetraol samples and differ primarily in the percentage of the various OH adducts. Specular reflection FTIR (Nicolet Magna Model 560) was used to quantify the thickness of the applied perfluoropolyether films. A standard specular reflection adapter (Harrick, New York) was used to maintain the proper angle of incidence to the sample. The changes in each of the PFPE film thickness were quantified by calibrating the FTIR absorption band maximum (∼1280 cm−1 ) to the film thickness determined by XPS using a takeoff angle of 45◦ and an electron mean free path of 25 Å [10]. Throughout the following, we refer to the PFPE lubricant on the disk surface as being “bonded.” To clarify our definition of these
terms, we briefly describe the methodology by which these quantities are determined. The initial lubricant thickness is measured via specular reflectance FTIR. The disks are then rinsed with 2,3dihydro-perfluoropentane (Vertrel-XF, DuPont) for 1 min to remove any soluble lubricant, and the lubricant thickness re-measured. The lubricant retained by the disk is defined to be the amount “bonded.” The bonded fraction is obtained by normalizing to the initial lubricant thickness. 3. Computational methodology Ab initio calculations were performed using density functional theory (DFT) at the 6-311++G(d,p) basis set using the Gaussian 98 computer code [11]. The DFT calculations employed Becke’s 3-parameter functional together with Perdew and Wang’s gradientcorrected correlation functional [12]. The 6-311++G(d,p) basis set includes d functions for oxygen atoms and p functions for hydrogen atoms, both of which allow a more quantitative description of hydrogen bonding. All model structures were fully optimized. The requested convergence on the density matrices was 10−8 . Residual forces were 1 × 10−4 hartree bohr−1 on the cartesian components. Harmonic vibrational frequencies were calculated by differentiation of the energy gradient at the optimized geometry. No imaginary frequencies were computed for all structures. The figures reproducing the computational results were made using the GaussView graphical user interface [13] and the CambridgeSoft graphical software [14]. The strength of the bonding interaction between polar groups on the model PFPE lubricant and either the model carbon surface, the Vertel-XF solvent, or another PFPE molecule was quantified by dimer calculations on representative models. In order to keep the computations tractable, the PFPE OH was represented by HCF2 CH2 OH (Zdol end group) while the CNx carbon surface was represented by the imine CH3 N=CH2 . The Vertrel-XF structure was represented by the authentic CF3 CHFCHFCF2 CF3 formula. The computed binding energy is obtained as the difference in total energies between the dimer and the isolated reactants, i.e., E = E AB − ( E A + E B ), corrected for zero-point energy contributions. 4. Results The results of the ambient bonding kinetics conducted on 10 Å of polydisperse Tetraol 2000S, fractionated Tetraol 2000S,
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Table 3 Kinetic parameters used to fit Fig. 3.
Fig. 2. The bonding kinetics at 20 ◦ C and 50% RH for 10 Å films of polydisperse Z-Tetraol 2000S, narrowly fractionated Z-Tetraol 2000S, and Z-Tetraol 2000GT on CNx. Four sets of data points per time for each sample provide an indication of reproducibility. Table 2 Kinetic parameters used to fit Fig. 2. Lubricant
kB
h
2000S 2000S frac 2000GT
0.0055 0.0057 0.0052
0.50 0.50 0.50
and Tetraol 2000GT on amorphous nitrogenated carbon (CNx) are shown in Fig. 2. Upon application of the Tetraols to the CNx surfaces, some fraction bonds to the carbon surface at room temperature within the time required to allow for solvent evaporation and to conduct the initial thickness measurements (1 min). We observe that the polydisperse Tetraol 2000S has the largest initial bonded fraction of 0.5 while the fractionated 2000S and 2000GT Tetraols have the significantly lower initial bonded fraction of 0.25–0.30. All of the Tetraols exhibit increasing bonding to the CNx surface with increasing time reaching an asymptotic bonded fraction of approximately 0.8 for 2000S and 0.70–0.75 for the fractionated 2000S and 2000GT samples. The bonding rates shown in Fig. 2 do not follow classical kinetics but instead are time-dependent with a rate coefficient that decreases with time. The theoretical fit to the bonding data can be derived by assuming a rate coefficient that is time-dependent, that is, k(t ) ∝ k0 t −h , where k(t ) is the instantaneous rate coefficient at time t, k0 is the time-independent initial rate constant, and h is the coefficient describing the power dependence in time. The differential rate equation describing the conversion of mobile lubricant A to bonded lubricant B is [8]: dB dt
= k B t −h A ,
(1)
where k B is defined as the initial bonding rate constant. The results of the calculation based on this model are shown in Fig. 2 as the solid lines. The kinetic parameters used to generate the theoretical fits are summarized in Table 2. From these data we conclude that Tetraol bonding on CNx ubiquitously follows k(t ) ∝ k B t −1/2 kinetics. The bonding of perfluoropolyethers (PFPEs) to carbon surfaces characterized by this time dependence has been attributed to diffusion-limited one-dimensional kinetics [8]. The bonding kinetics data for each of the Tetraols can be interpreted on the basis of their bulk chemical composition. Refer-
Lubricant acronym
Weight % of bis-adduct in 2000GT
Mole % of bis-adduct
kB
h
bis0 bis9 bis18 bis32 bis100
0 9 18 32 100
0 4 8 14 45
0.0045 0.0042 0.0043 0.0043 0.0041
0.50 0.50 0.50 0.50 0.50
ring to the Tetraol NMR data (Table 1), the main chain chemical structure is virtually identical amongst the three samples, i.e., the ratio of the perfluoromethylene oxide to perfluoroethylene oxide monomer units is 1.06 ± 0.03, and the % of CF2 O monomer units adjacent to another CF2 O monomer unit is 21.8 ± 0.7%. Since all of the Tetraol bonding rates can be fit using k B = 0.0055 ± 0.00025 (5%) and h = 1/2 despite the very different initial bonded fractions, we conclude that the main chain is the significant determinant of the bonding kinetics. Previously, structural differences in the PFPE main chain (e.g., C/O ratio) have been shown to impact the bonding kinetics and the resultant tribological reliability of the finished disks [8,15]. According to Table 1, the chemical structural component that differs the most amongst the three Tetraol samples investigated in Fig. 2 is the concentration of the bis-adduct end group (Fig. 1). The NMR data shows that there is approximately 16 mol% bis-adduct in the 2000S sample compared to almost none for the fractionated 2000S and the 2000GT samples, 2 and 0 mol%, respectively. Based on this analysis, we propose that the bis-adduct end group is the significant determinant for the observed differences in the Tetraol bonding kinetics. We note that while the % of dol adduct is also different amongst the three Tetraol samples, they represent the less polar adduct that preferentially remains in solution and hence should not significantly impact the bonding kinetics [16]. In order to test this hypothesis, bis-adduct enriched Tetraol was incrementally added to Tetraol 2000GT and the thin film bonding kinetics of the resultant mixture samples were investigated as a function of time on the CNx surface. It is important to clarify here that the bis-adduct enriched Tetraol 2000GT has 45 mol% of the bis-adduct end group and about 50 mol% of the Tetraol adduct (Table 1). The mixture samples were produced by mixing the bisadduct enriched Tetraol 2000GT (Table 1, column 5) and fractionated Tetraol 2000GT (Table 1, column 4) in a common solvent (Vertrel-XF). The quantification of the mixtures is reported on the basis of the weight % of the bis-adduct enriched Tetraol 2000GT in Tetraol 2000GT. The compositions are referred to as, e.g., “bis9” or “bis100,” where bis9 represents 9% by weight and bis100 represents 100% by weight of the bis-adduct enriched Tetraol 2000GT in Tetraol 2000GT, respectively. The details of the Tetraol mixture compositions and the acronyms used to identify each of the mixture samples are summarized in Table 3 along with the kinetic parameters used to fit the bonding data. The bonding kinetics for Tetraol 2000GT as a function of bis-adduct content is shown in Fig. 3. Again, all of the experimental data up to the asymptotic bonded level could be fit by Eq. (1) using k B = 0.0043 ± 0.00015 (4%) and h = 1/2. Thus, we observe again that while the end group composition is apparently a significant determinant of the initial and asymptotic bonded fraction, the parameters used to fit the bonding kinetics (k B , h) are more dependent upon the main chain structure which are virtually identical here (Table 1). However, as shown in Fig. 5 right, the bonding rates (dB /dt) are very different. Since dB /dt is dependent on the concentration of the mobile lubricant “A” (Eq. (1)), dB /dt decreases with increasing bis concentration in Tetraol. Lastly we note that the k B values used to fit the two sets of data shown in Figs. 2 and 3 are not the same. This is attributed
R.J. Waltman / Journal of Colloid and Interface Science 333 (2009) 540–547
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Fig. 3. (Left) Bonding kinetics at 20 ◦ C and 50% RH for 10 Å films of Z-Tetraol 2000GT on CNx as a function of bis-adduct enriched Z-Tetraol in the lubricant bath. Four sets of data points per time for each sample provide an indication of reproducibility. See Table 3 for mixture composition details. (Right) The corresponding bonding rate dB/dt as a function of time. The curves from top to bottom are bis0, bis9, bis18, bis32, and bis100, respectively. A reference line with a slope of −0.5 is shown.
Fig. 4. The average initial and asymptotic bonded fractions for Z-Tetraol 2000GT as a function of the weight % of bis-adduct enriched Z-Tetraol in the lubricant bath (solvent: Vertrel-XF). Data compiled from Fig. 3.
entirely to having conducted the two sets of experiments at separate times using slightly different CNx films. This difference does not impact against the conclusions derived from this study. Since each set of experiments is conducted simultaneously using identical carbon films, the results within each figure may be compared in the usual manner. The dependence of the initial and asymptotic bonded fractions as a function the bis-adduct concentration is conveniently summarized in Fig. 4. This plot provides clear evidence that both the initial and the asymptotic bonded fractions are correlated to the concentration of the bis-adduct component in Tetraol and thus provides a reasonable explanation for the apparently larger initial and asymptotic bonded fractions observed in polydisperse Tetraol 2000S (Fig. 2).
It is relevant at this time to discuss in more detail the nature of the bonded Z-Tetraol. We remind the reader that the bonded fraction, as shown in Figs. 2 and 3, is quantified after the disk is sequentially spray-rinsed with solvent (Vertrel-XF) for approximately 10 s, immersed in clean solvent (Vertrel-XF) for 1 min, then sprayrinsed for another 10 s, a methodology that was used to quantify the bonded fraction of the less polar Zdol disk lubricant [8]. During the course of these experiments, we observed that the initial bonded fraction for the polar Tetraols was dependent upon solvent soak time while the asymptotic bonded fraction was not. Referring first to Fig. 5 (top), the changes in the asymptotic bonded fraction as a function of solvent soak time shows that the asymptotically bonded lubricants are not easily displaced by the Vertrel-XF solvent even after ∼1000 min in solvent. Thus, we conclude that the polar intermolecular interactions that develop between the lubricant OH end groups and the underlying carbon surface are stronger than the lubricant–solvent interactions. In contrast, Fig. 5 (bottom) shows that the initial bonded fraction for the polar PFPE samples (Tetraols, bis-adduct) decrease significantly (∼40%) as a function of soak time, eventually converging towards an asymptotic initial bonded fraction after several thousand minutes. The timedependent dissolution observed for initially deposited Tetraols and the bis-adduct enriched Tetraol suggests that solvent power may be the rate-determining step for dissolution. This aspect will be further discussed below. The effect of the 1 and 1000 min solvent soak time on the bonding kinetics of Tetraol 2000S is compared in Fig. 6. This comparison is utilized to probe any significant differences that could limit the interpretation of the Tetraol bonding kinetics using the current methodology of a 1 min rinse time compared to the 1000 min soak time. At the 1000 min solvent soak, the experimental data up to the asymptotic bonded level can apparently still be fit by Eq. (1) using k B = 0.0052 and h = 1/2, similar to the 1 min soak time data (k B = 0.0055 and h = 1/2). Thus, the bonding kinetics (k B , h) is still governed by main chain effects. Furthermore, the asymptotic bonded fraction converges to the same level if only not at the same rate (Fig. 6, right). Thus the bonding kinetics as determined by the 1 min soak methodology appears to be adequate when used in side-by-side comparisons.
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Fig. 5. (Top) The changes in the asymptotic bonded fraction of 10 Å films of Tetraol 2000GT and bis100 on CNx as a function of soak time in Vertrel-XF solvent. (Bottom) The changes in the initial bonded fraction of 10 Å films of Tetraol 2000GT, Tetraol 2000S, bis100, and Zdol 2540 on CNx as a function of soak time in VertrelXF solvent.
5. Discussion The driving force for the bonding of thin PFPE lubricant films to the underlying carbon surface is the reduction of the surface energy [17]. Therefore, the change in the surface energy as a function of time, Fig. 7, left, can be correlated to the bonding kinetics of the Tetraols on the carbon surface, Fig. 7, right. The bonding is attributed to the attractive intermolecular interactions that develop between the Tetraol hydroxyl end groups (R–OH) and the polar functional groups on the carbon surface [8]. The kinetics for this process has already been discussed. The observed surface energy reduction for sub-monolayer PFPE films can be described by [17]
γsp = γcp + γl p − (εcl + εll ), p
(2) p
where γs is the polar component of the surface energy, γc is pop lar surface energy of the carbon film, γl is the surface energy of the non-interacting polar end groups in the sub-monolayer lubricant film, εcl is the adhesive interaction energy density between the lubricant film and the carbon surface, and εll is the cohesive interaction energy density between the lubricant molecules (via the polar OH groups). Since attractive interactions between polar groups result in a drop in energy, the signs for both εcl and εll as defined in Eq. (2) will be positive. p Referring again to Fig. 7, the γs at the asymptotic bonded level is a substantially smaller 6–10 mJ/m2 compared to the 36– p 42 mJ/m2 observed at the initial bonded level. Note that γs also scales with the fraction of bis-adduct in Tetraol and hence to the p overall molecular polarity. The decrease in γs is readily explained on the basis that with increasing time the fraction of Tetraol that becomes unextractable from the carbon surface increases due to the end group bonding (εcl ). This occurs over ∼104 min for the
systems under investigation here (Figs. 2 and 3). Conversely, the time-dependent dissolution of the initially bonded Tetraol (Fig. 5) suggests that, shortly after deposition, there are solvent-extractable Tetraol molecules. Since PFPE solubility is expected to decrease with increasing molecular polarity, solvent power may be a significant determinant of the initial bonded fraction, resulting in the observed time-dependence (Fig. 5). These aspects are now discussed in more detail below. We first consider the relative strength of the intermolecular interaction between a PFPE OH end group and a model CNx carbon surface imine (CH3 N=CH2 ), another PFPE OH end group, and the Vertrel-XF solvent. The goal of this theoretical study is to quantify the relative interaction strength between the PFPE OH end group and model surfaces and hence provide insight into the effect of solvent power on the bonding measurements. The optimized geometries for the interacting dimers are shown in Fig. 8. The computed binding energies and hydrogen bond distances are summarized in Table 4. As shown in Table 4, the PFPE OH dimer binding energy decreases in the order: imine > PFPE OH > Vertrel-XF. It is reasonable to conclude from the computational results that Vertrel-XF can more easily dissolve Tetraol molecules with “free” hydroxyls while the dissolution of a Tetraol hydroxyl bonded either to another Tetraol hydroxyl or to a polar group on the carbon surface becomes increasingly energetically prohibitive, respectively. The results of the computations appear to be fully consistent with the results presented in Fig. 5. According to the discussions above, it appears that the more polar PFPE molecules will have more limited solubility in the relatively non-polar solvents typically employed to deposit these materials, such as Novec fluids (3M), hydrofluorocarbons like the Vertrel-XF (DuPont) used here, or AE-3000 (Asahi Glass Co.). Therefore it is possible that the more polar PFPE lubricant components are more readily deposited from a “poorer” solvent and, once deposited on the disk, is not easily re-dissolved in the solvent. Solubility originates from the interaction energy between the solute and the solvent. The solubility parameter is defined as the square root of the cohesive energy density [18]:
δi =
E iv Vi
cal/cm3
1/2
,
(3)
where E iv is the energy of vaporization and V i is the molar volume of component i. The solubility parameter, δ , therefore describes the attractive strength between molecules and can be used as a criterion for the solvent power. Equation (3) is related to the heat of mixing per unit volume as [18]
H m / V = (δ1 − δ2 )1/2 ϕ1 ϕ2 ,
(4)
where ϕi is the volume fraction of component i. Since the free energy of mixing is related to the enthalpy of mixing by G m = H m − T S m , the difference in the polymer–solvent solubility parameter (δ1 –δ2 ) must be small for miscibility. The solubility parameter is conveniently calculated by expanding E v and V as [19]
E v = and V =
e j
v j ,
(5)
(6)
where e j and v j are the additive atomic and group contribution for the energy of vaporization and molar volume, respectively. The computed solubility parameter for some of the OH adducts found in Tetraol (at nominally 2200 molecular weight) and VetrelXF are shown in Table 5. These data are used here strictly for qualitative purposes. Table 5 clearly shows that the enthalpy of mixing does not favor the solution of the more polar adducts.
R.J. Waltman / Journal of Colloid and Interface Science 333 (2009) 540–547
545
Fig. 6. (Left) Bonding kinetics at 20 ◦ C and 50% RH for 10 Å films of Z-Tetraol 2000S on CNx after 1 and 1000 min solvent soak time to determine the bonded fraction. Four sets of data points per time for each sample provide an indication of reproducibility. (Right) The corresponding bonding rate, dB /dt, as a function of time.
Fig. 7. The changes in the polar surface energy,
γsp , as a function of (left) time and (right) the average bonded fraction under ambient conditions (20 ◦ C, 50% RH).
Fig. 8. The B3PW91/6-311++G(d,p) optimized geometries for some model interactions between a PFPE OH end group (CFH2 CH2 OH) and (left) Vetrel-XF; (middle) an imine carbon surface; and (right) another PFPE OH end group. The equilibrium intermolecular distance between the PFPE OH end group and the surface polar groups are given in angstrom.
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R.J. Waltman / Journal of Colloid and Interface Science 333 (2009) 540–547
Table 4 B3PW91/6-311++G(d,p) optimized hydrogen bond distance and E corrected for zero-point energy contributions. Dimer HCF2 CH2 OH HCF2 CH2 OH HCF2 CH2 OH HCF2 CH2 OH
+ + + +
CH3 N=CH2 HCF2 CH2 OH Vertrel-XF H2 O
H-bond distance (Å)
E (kcal/mol)
1.851 1.886 2.206 1.864
−6.14 −4.60 −2.74 −3.68
Table 5 Computed solubility parameter for Tetraol adducts and Vertrel-XF solvent. Lubricant or solvent
Molecular weight
δ (cal/cm3 )1/2
Vertrel-XF Zdol Tetraol Bis-adduct
252 2200 2200 2200
6 .2 8 .4 9 .2 10.1
Fig. 10. The displacement of 12 Å 2000GT on a carbon surface by a 0.5 μl water droplet is quantified on an optical surface analyzer (Candela 5120). The thickness profile shown to the right of each surface image is taken at the dotted line.
Fig. 9. The thickness of lubricant deposited on the CNx carbon surface as a function of the lubricant solution concentration in Vertrel-XF. The disk withdrawal rate is a constant 1.5 mm/s.
This is readily verified by quantifying the deposition of Tetraol 2000GT and bis100. As shown in Fig. 9, the more polar bis100 deposits more readily from solution (Vertrel-XF) compared to Tetraol 2000GT at the same solute concentration. These data indicate that with increasing polarity, the more polar components of the PFPE, i.e., more OH end groups and/or lower molecular weight components, are likely to deposit onto the carbon surface. These polar adducts are also less likely to re-dissolve in the solvent, giving rise to an apparently larger initial bonded fraction. Once the lubricant film is deposited onto the carbon surface, diffusive self-stirring (time-dependent bonding kinetics) governs the spatial delivery of the PFPE OH end groups to the polar capture sites on the carbon surface, eventually leading to the asymptotic bonded level. At the asymptotic bonded state, the PFPE has reconstructed substantially to provide a significantly more stable bonded state to the underlying carbon film. In order to further probe the nature of the PFPE/CNx polar interactions, the stability of both the initial and asymptotic bonded fractions to water was next investigated. This experiment originates from the data presented in Fig. 7 with the accompanying suspicion that the observed temporal changes in the surface energy derives from the water droplet penetrating and/or displacing some fraction of the total lubricant film thickness. In these stud-
Fig. 11. The displacement of 10 Å bis100 on a carbon surface by a 0.5 μl water droplet is quantified on an optical surface analyzer (Candela 5120). The thickness profile shown to the right of each surface image is taken at the dotted line.
ies 0.5 μl water droplets were placed on the disk surfaces of 10 Å films of 2000GT and bis100 at the initial and asymptotic bonded fractions and were allowed to evaporate under ambient conditions (5–10 min). The disk surfaces were then optically analyzed “immediately” (10 min) for possible lubricant displacement by the water droplet. Figs. 10 and 11 reveal that at the initial bonded fraction, labeled “as dipped” in the optical images, water droplets do displace the lubricant as shown by the lighter colored region in the images. The quantity of lubricant displaced is shown in the corresponding right figures and is a substantial ∼3 Å (30%) for both 2000GT and bis100 (Table 6, columns 2, 4). The darker region that appears towards the left of the droplet area in both optical images is assumed to be some of the displaced lubricant and/or residual water. In contrast, the water droplets are observed to displace very little lubricant at the asymptotic bonded fraction for both 2000GT
R.J. Waltman / Journal of Colloid and Interface Science 333 (2009) 540–547
547
Table 6 The lubricant thickness displacement (Å) by 0.5 μl water droplets on lubricated disk surfaces. These data were quantified using an optical surface analyzer (Candela 5120). Representative data are shown in Figs. 9–11. Sample #
10.5 Å 2000GT at initial bonded fraction
10.5 Å 2000GT at asymptotic bonded fraction
10.2 Å bis100 at initial bonded fraction
10.2 Å bis100 at asymptotic bonded fraction
23 Å Z2300 (no bonding)
19 Å 2000GT (3 Å bonded)
1 2 3
2.8 2.2 2.3
0.6 0.5 0.5
3.1 2.5 2.0
0.3 0.4 0.3
22.0 18.7 19.0
4.6 4.6 4.5
Tetraol OH groups and/or cohesively interacting Tetraol OH groups that gradually reconstruct on the carbon surface to produce more bonded Tetraol on the carbon surface. The reconstruction follows k(t ) ∝ k0 t −h kinetics and produces a lubricant state at the asymptotic bonded level that is stable to solvent dissolution. The technological implications of the studies presented herein are that the resistance of the boundary lubricant to surface disturbances and disk rotation effects is enhanced by the lubricant bonding, either adhesive (εcl ) or cohesive (εll ). Furthermore, as increasingly stringent HDD requirements mandate decreased lubricant disturbance on the disk surface, this work reveals a chemical-based methodology to increase the bonded fraction and hence the ability of the boundary lubricant to resist unwanted migration. Acknowledgment The author thanks J. Burns of this laboratory for the NMR characterization of the Tetraol samples, and Solvay–Solexis for a generous sample of the bis-adduct enriched Tetraol 2000GT. Fig. 12. The displacement of 23 Å Z2300 (top) and 19 Å 2000GT on a carbon surface by a 0.5 μl water droplet is quantified on an optical surface analyzer (Candela 5120). The thickness profile shown to the right of each surface image is taken at the dotted line.
and bis100. The optical images show ∼0.5 Å (5%) and ∼0.3 Å (3%) of lubricant displacement for Tetraol-GT and bis100, respectively (Table 6, columns 3, 5). Thus, the solvent extraction studies (Vertrel-XF and water) on the initial bonded fraction suggests that free OH end groups exist in the initially deposited thin film that is not efficiently removed by the solvent used to deposit the Tetraols in the first place. Water displacement of thicker lubricant films were next investigated to elucidate the possible role of cohesively bonded lubricant end groups (εll ). We first consider the non-functionalized Z2300 which has no polar hydroxyl end groups. In this case, the water droplet is observed to displace almost all of the lubricant film as shown in Fig. 12 (top). In contrast, a water droplet on a 19 Å Tetraol-GT film (3 Å bonded to the carbon surface (εcl )) displaces approximately 5 Å of film, but does not displace approximately 10 Å, suggesting some degree of cohesion (εll ) between Tetraol end groups exists. 6. Conclusions In this study, we have demonstrated that polar adducts in ZTetraol are significant determinants of Tetraol bonding to the underlying carbon surface. An increase in the overall Tetraol molecular polarity leads to increased bonding onto the carbon surface. The initial bonded state is characterized by the presence of free
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