Self-Assembled Monolayers on Engineering Metals: Structure

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Self-Assembled Monolayers on Engineering Metals: Structure, Derivatization, and Utility John G. Van Alsten* Central Research & Development, E. I. duPont de Nemours & Co., Experimental Station, Wilmington, Delaware 19880-0356 Received December 8, 1998. In Final Form: July 6, 1999 A methodology for the formation and derivatization of self-assembled monolayers (SAMs) of alkyl phosphonic acids on common engineering metals such as steel, stainless steel, aluminum, copper, and brass is described. This methodology is shown to be a versatile route for surface modification of such substrates. R,ω-Metal bisphosphonate SAMs are shown to be receptive to complexation by organic acids and acid-containing polymers such as fluoropolymers and ethylene-co-methacrylic acid. This latter attribute is exploited in the construction of polymer/SAM/metal interfaces of surprising durability. The durability of the interface is a strong function of the SAM chain length.

This paper concerns the utility of self-assembled monolayers (SAMs) for tuning the properties of metal surfaces. Although studies of assembled monolayers date back to the 1940s,1 this area of study has blossomed since its resurgence ca. 15 years ago. Since this time, the marriage of modern synthesis and surface characterization techniques has revealed tremendous insights into such fundamental issues as the range of interfacial forces2, structural organization in monolayer films,3-5 and growth mechanisms of monolayers.6,7 Several excellent reviews of this field are available, and will not be reiterated here.8,9 The art of surface coating is thousands of years old, encompassing thousands of patents even in modern times. Given this large body of work, it is likely that there are compositions in use that are “self assembling”, regardless of whether the practitioners are cognizant of the scientific details. Some of the more familiar examples include the use of small-molecule organic additives to reduce corrosion rates,10,11 the in situ treatment of minerals with fatty acids during polymer compounding,12 and the use of metal salt/ phosphorous acid anticorrosion recipes.13 Nevertheless, a survey of the patent literature shows that specific references to self-assembly are not nearly as plentiful as scientific studies. We shall briefly review some of the patent art below. One area of great interest is in the formation of small dimension patterns on solid substrates. These include methods using the deposition of polymerizable SAMforming molecules14 and stamping to leave a desired * Current address: Chemical Process R & D, Pharmacia & Upjohn, 7000 Portage Rd., Kalamazoo, MI 49001. (1) Zisman, W. A. Adv. Chem. Ser. 1964, 43. (2) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897. (3) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey; C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (5) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (6) Woodward, J. T.; Ulman, A., Schwartz; D. K. Langmuir 1996, 12, 3626. (7) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (8) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (9) Xu, J.; Li, H. J. Colloid Interface Sci. 1995, 176, 138. (10) McCafferty, E.; Hackendorn, N. J. Electrochem. Soc. 1972, 146. (11) Elachouri, M.; Hajji, M. S.; Kertit, S.; Essassi, E. M.; Salem, M.; Coudert, R. Corres. Sci. 1995, 37, 381. (12) Rothon, R. N. In Particulate-Filled Polymer Composites; Rothon, R. N., Ed.; Longman Scientific & Technical Publishers: 1995. (13) Hwa, C. M. U. S. Patent 3,803,047 and U. S. Patent 3,803,048, April 9, 1974.

pattern on the substrate.15 Another very active area is targeted toward producing sensors for biological molecules. These methods typically involve capturing and binding of the molecule of interest by the SAM.16,17 Several patents utilize SAMs to improve the performance of small parts. These include the treatment of micromechanical devices to achieve a reduction in friction18 and modifications of the surface energies of the orifices of ink-jet pens.19 The latter treatment is claimed to reduce residual ink build-up and metal corrosion, which cause deterioration in pen performance. Metallic surfaces of membranes may also be treated to render them resistant to fouling by bacteria.20 One interesting patent claims a specific SAM as a protective barrier and adhesion promoter.21 In this work, 12-thiol dodecanoic acid is claimed to form an “organized molecular assembly” that is “...impervious to water, alkali, and other impurities and corrosive substances that typically attack metal surfaces.” The monolayer is said to improve the adhesion of poly(methyl methacrylate) protective layers to silver mirror surfaces when the mirror is exposed to weathering. Despite these interesting works, a number of obstacles exist for the large-scale utilization of SAMs. Although thiol compounds are very useful for binding to coinage or noble metals (gold, silver, copper, platinum), their affinity to engineering metals such as steel, stainless steel, and aluminum is very limited. If SAMs are to be used for more common metal surfaces, they must be capable of binding to crystal faces more like goethite or gibbsite than gold, and must be robust enough to tolerate high temperatures and chemically aggressive environments. Although alkane carboxylic or phosphonic acids are known to be capable of binding to surfaces such as aluminum,22-24 their (14) Schnur, J. M.; Schoen, P. E.; Peckerar, M. C.; Marrian, C. R. K.; Calvert, J. M.; Georger, J. H. U. S. Patent 5,077,085, Dec. 31, 1991. (15) Kumar, A.; Whitesides, G. M. U. S. Patent 5,512,131, April 30, 1996. (16) Tarlov, M. J. U. S. Patent 5,514,501, May 7, 1996. (17) Bamdad, C. C.; Sigal, G. B.; Strominger, J. L.; Whitesides, G. M. U. S. Patent 5,620,850, April 15, 1997. (18) Wallace, R. M.; Webb, D. A.; Gnade, B. E. U. S. Patent 5,523, 878, June 4, 1996. (19) Halko, D. J.; Halko, B. T. U. S. Patent 5,598,193, Jan 28, 1997. (20) Sawan, S. P.; Shalon, T.; Subramanyam, S.; Yurkovetskiy, A. U. S. Patent 5,490,938, Feb. 13, 1996. (21) King, D. E.; Czanderna, A. W.; Kennedy, C. E. U. S. Patent 5,487,792, Jan. 30, 1996.

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fortitude in the face of thermal and chemical challenges has not been presented. In these cases, where the binding to the surface is ionic, it is an open question as to how well such layers will withstand aggressive aqueous environments encountered in actual processing and service. There is at least one other limitation to the widespread utilization of SAMs that does not appear to have been appreciated in the literature. Although graduate student time is inexpensive and plentiful, material processing equipment is generally costly and dear, meaning that economic viability is often critically dependent on achieving rapid cycle times. In polymer processing, for example, time scales range from a maximum of a few hours for the cure of thermosets to a few milliseconds in fiber spinning. If SAMs are to be adapted to such applications, SAM formation must be rapid enough to impart the desired functionality within what is normally a very short processing window. This paper presents an exploration into the utility of SAMs for large-scale materials engineering, such as in the lamination or coating of metals with polymers. The use of alkylphosphonic acids to form SAMs on engineering metals such as steel and aluminum will be described first, along with strategies to effect useful derivatizations of the SAMs. Results will be presented showing that there is a strong correlation between the length of the SAM backbone and the performance of metal/SAM/polymer laminates. Finally, we will investigate the effect of processing time on the performance of laminates in which self-assembly proceeds directly from the binding polymer phase. Experimental Section SAM Precursors. Phosphonic acids (PAs) were synthesized from their brominated precursors, namely R,ω-dibromoethane, -butane, -hexane, -octane, -decane, and -dodecane. These were obtained from either Aldrich or Fluka, and recrystallized from ethanol/water solutions before use. Perdeuterated 1-bromohexadecane was obtained from CDN Isotopes and used as received. The brominated materials were converted to the ethyl phosphonates via the Arbuzov reaction.25 The bromoalkane was mixed with a 5% stoichiometric excess of triethyl phosphite and heated in the melt at 150 °C under nitrogen purge. Formation of the ethyl phosphonate was tracked by the disappearance of the methylene bromide and appearance of the ethyl phosphonate resonances in the 31P and 1H NMR spectra. When the reaction was complete, the reaction temperature was lowered to ca. 100 °C and air was allowed into the vessel to oxidize any remaining triethyl phosphite to water-soluble triethyl phosphate. The ethyl phosphonates were hydrolyzed to the corresponding PAs using either refluxing aqueous HCl or HCl/dioxane. The latter reaction system was especially useful given the very limited solubility of the decyl and dodecyl compounds in water. Hydrolysis was tracked by disappearance of the phosphonate resonances in the 31P and 1H NMR spectra. When complete, the solvent was stripped by rotary evaporation, and the crude solid dissolved in ethanol. Recrystallization was effected from ethanol/water. The crystalline solids were characterized by 31P, 1H, and 13C NMR, and elemental analysis. In subsequent references to these materials the notation of Mallouk and colleagues will be adopted, in which the alkane bisphosphonic acids (BPAs) shall be referred to by CnBPA, in which n denotes the carbon number of the spacer between terminal PA groups.26 Perfluorooctanoic acid (PFOA) was obtained from Aldrich, deuterated stearic acid (d-C18COOH) from CDN Isotopes, and (22) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1997, 13, 115. (23) Gao, W.; Reven, L. Langmuir 1995, 11, 1860. (24) Bram, C.; Jung, C.; Sratmann, M. Fresenius J. Anal. Chem. 1997, 358, 108. (25) Bhttacharya, A. K.; Thyagarajan, G. Chem. Rev. 1981, 81, 415.

Van Alsten Scheme 1: Formation and Derivatization of Bisphosphonate SAMsa

a Routes: 1a, epitaxial growth of bisphosphonate multilayer; 1b, capping the SAM with a monofunctional surfactant; 1c, grafting a polymer (in this case a carboxylic acid-terminated fluoropolymer) to the SAM.

ethyl-, octyl-, and octadecylphosphonic acid (C2PA, C8PA, and C18PA) from Alfa. All were used as received. Polymers. Copolymers of ethylene and methacrylic acid of various monomer ratios were obtained from Dupont. These are random copolymers produced by high-pressure free radical polymerization, and are sold under the trade name of Nucrel. The polymers will henceforth be described as X/Y E/MAA, where X and Y are the mole fractions of ethylene and methacrylic acid, respectively. Copolymers of tetrafluoroethylene and hexafluoropropylene of two monomer ratios were kindly provided by Dr. R. A. Morgan of Dupont Fluoroproducts. These materials contain carboxylic acid end groups, and will be designated X/Y FEP-COOH, where X and Y are the mole fractions of tetrafluoroethylene and hexafluoropropylene, respectively. Substrates. A wide variety of substrates were used in this investigation. Chromium and aluminum were sputter-deposited onto silicon wafers to a nominal thickness of 2000 Å, and the uncharacterized ambient oxides utilized. Type 304 stainless steel, cold rolled steel (CRS), aluminum, copper, and brass coupons were cut from sheets of material obtained at the site metal shop. For delamination tests, automotive test panels of CRS and aluminum were also utilized. The panels of aluminum were 4 in. × 12 in. × 0.025 in. and those of CRS 4 in. × 12 in. × 0.032 in., and were obtained from ACT Laboratories, Hillsdale, MI. Self-Assembly and Derivatization of the SAMs. Selfassembly was performed using techniques well-described in the literature,1-8 and outlined as Scheme 1. Typically, 1mM solutions of the PA were prepared by dissolution in either ethanol or water, the latter by neutralization with 1 mol equiv of NaOH. Chromium on silicon, aluminum on silicon, and stainless steel were cleaned before treatment with methylene chloride and a 1-min soak in Nochromix/H2SO4 cleaning solution, followed by a rinse with distilled water. CRS and aluminum sheet coupons were rinsed with methylene chloride immediately before treatment. SAMs of the bisphosphonic acids were produced by immersion of the coupons into the appropriate solutions for 15-30 min, whereupon they were withdrawn and rinsed with copious quantities of fresh solvent. In most cases, the terminal PA group was capped with a metal ion by immersion in an aqueous metal salt solution. For metal ions of oxidation state +2, the metal acetate was used; for ions of oxidation state +3, the metal nitrate. For Zr4+, ZrOCl2 was dissolved in water. The metal phosphonate intermediate so formed is particularly useful for further derivatization, as shown in Scheme 1a-c. In the first approach (1a), multiple layers of metal phosphonates can be epitaxially formed using procedures developed by the Mallouk group,26,27 dubbed molecular beaker (26) Yang, H. C.; Katsuori, A.; Hong, H.; Sackett, D. D.; Arendt, M. F.; Yau, S.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855.

SAMs on Engineering Metals

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epitaxy. The PA terminus of the monolayer is first treated with a solution of a metal salt to form the metal phosphonate, followed by another dip in the solution of BPA to complex another BPA layer. This sequence of BPA/metal deposition is repeated until the desired film thickness is achieved. In the remainder of the text, such multilayers will be designated a × Cn BPA/M, where a is the total number of BPA films in the multilayer, n denotes the carbon number of the BPA, and M is the metal ion used to complex the multiple BPA layers. In a second use (1b), the metal phosphonate terminus becomes a template for anchoring another layer of small molecules, provided that they too are receptive to binding to the ion. Judicious choice of the terminus of these molecules results in a surface that can be tuned to a wide range of wettability or adhesion. For example, treatment with a monofunctional carboxylic acid or PA would present a terminus with a low surface energy. Treatment with a polymerizable monomer such as vinyl PA would impart surface sites available for copolymerization with a contacting solution of monomer. In a third manifestation, (1c), the terminus can be used as a binding site for the anchoring of polymer layers, particularly those that contain acidic or polar functionality. In this manner, the SAM becomes a method of attaching a wide variety of polymers to the metallic substrate by direct covalent and ionic linkages. Characterization of SAMs. Characterization of SAMs was performed using infrared and contact angle measurements. Contact angles of water and hexadecane were measured with a goniometer from Rame-Hart. Polarized reflection-absorption infrared measurements (RAIRS) were taken at an incident angle of 86° using a Harrick Seagull accessory mounted within either a Nicolet 60SX or Nicolet 860 spectrometer equipped with an MCT-A detector. Typically, 250-1000 scans were coadded to obtain spectra, but for cases of particularly difficult experiments, up to 8000 scans were utilized. Ellipsometric measurements were also attempted, but were found to be too unreliable even in evaluation of the optical constants of the substrates. Measurements of Self-Assembly from a Polymer Melt. To determine the rate of self-assembly of a PA SAM, the selfassembly process was monitored in situ using infrared spectroscopy in the mode of attenuated total reflectance (ATR). One milligram of perdeuterated hexadecylphosphonic acid (d-C16PA) was compounded into 1 g of 0.946/0.054 E/MAA, to produce an additive concentration of ca. 2 mM at 100 °C. A disassembled liquid ATR cell was filled with this material by melting in an oven; then it was removed from the oven, allowed to cool, and assembled into an ATR experiment. The ATR element was a freshly cleaned and polished ZnS ATR crystal that had been coated by sputter-deposited gold on its opposite side. The angle of incidence was 60 degrees, which allowed five bounces of the infrared radiation off of the active surface. One hundred scans were coadded per spectrum. The maximum peak height measured for the C-D peaks was ca. 6 × 10-3 absorbance units per reflection. Experiments commenced by heating the cell to the desired temperature and monitoring the growth of the C-D peaks (2195, 2090 cm-1) of the d-C16 PA (Figure 1). These data may be used to calculate an apparent diffusion coefficient by fitting the integrated intensities to the expression derived by Langmuir and Schaefer,28

(Dtπ )

Γ(t) ) 2c

1/2

In this expression, Γ is the surface excess, c is the concentration of d-C16PA, t is the experimental time, and D the diffusion coefficient. The term “apparent” is used because it is not clear in these experiments that diffusion to the interface is rate-limiting in the formation of a monolayer. To bind to the surface, the d-C16PA will probably need to displace bound carboxylic acid segments of the polymer, which may be a much slower process than diffusion to the interface. (27) Keller, S. W.; Kim, H.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (28) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1937, 59, 2400.

Figure 1. Spectra illustrating the adsorption of d-C16PA to the surface of a ZnS ATR crystal from a melt of 0.946 E/0.054 MAA copolymer at 100 °C. The spectrum is dominated by the absorptions characteristic of the copolymer. The inset is an expansion of the C-D region from 2300 to 1950 cm-1, and shows the buildup of d-C16PA at the crystal surface with time. From bottom, the elapsed times are 0, 56, and 971 min. Investigating the Performance of Polymer/SAM/Metal Composite Structures. To test the utility of the SAMs, laminates of polymers with SAM-coated metal panels were produced and subjected to challenge by various aggressive aqueous environments. Test panels of aluminum or CRS were first treated in baths to form SAMs, then laminated with polymer films to form a three-layer (polymer/SAM/metal) composite. Treatment was effected by first immersing the cleaned metal panel in a bath of the BPA, then withdrawing it and rinsing with pure solvent. Where indicated, the panel was subsequently immersed in a metal salt solution to form the metal phosphonate terminus, then removed and rinsed again. The concentration of all the treatment baths used was 1 mM of reagent in ethanol, and the immersion time 30 min. Polymer films ca. 100 µm in thickness were laminated to the metal panels by heating the panels to the temperature at which the polymer begins to develop tack, ca. 80 °C. The polymer films were then placed onto the panel and rolled flat with a rubber roller. A control sample was also prepared by laminating a polymer film to a cleaned metal plate that did not have a SAM deposited onto it. Once all the panels had been so prepared, they were placed together into a 150 °C, nitrogen-purged vacuum oven for 15 min. All the panels were then stored together at room temperature and allowed to age for 4 weeks before subsequent testing. After aging, the panels were sheared into test plaques of dimension ca. 10 cm × 2.5 cm. The edges of these test strips were sealed with silicone caulk and a groove cut through the polymer film into the metal using a carbide-tipped tool, following the procedures of ASTM D 1654-92. The panels were then immersed in aqueous baths of various aggressive agents for various periods of time. Upon removal, the extent of delamination of the metal/ polymer interface was evaluated by imaging the panel with an optical scanner (UMAX model “PowerLook”) and using image analysis software (NIH Image, version 1.6, available from the National Institutes of Health, Bethesda, MD) to quantify the delaminated area. In some cases where contrast between the delaminated and laminated areas was poor, the area was quantified by outlining the delaminated area on the image with a pen, cutting out that area with scissors, and weighing it on an analytical balance. Self-Assembly from a Contacting Polymer Melt. For one experiment, self-assembly was tested using additives within the laminating polymer film. In this case, the SAM-forming materials were compounded directly into the test copolymer, which had a composition of 0.946/0.054 E/MAA. To make the compounded material, C12BPA and zinc acetate were first dissolved in separate containers of ethanol and poured over separate batches of polymer pellets. These were dried in a vacuum oven, then physically mixed together. The coated pellets were fed to a mixing extruder operating at 150 °C, with the resulting stream of

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Figure 2. Comparison of peak intensities of the CH2 νas stretch for bifunctional C12BPA with those of monofunctional C2PA, C8PA, and C18PA. The good quantitative agreement shows that the C12BPA binds to the surface using only one of its two acid groups. Aluminum-on-silicon wafer substrate. extrudate cooled in a water bath and chopped into pellets. These pellets were then melt-pressed into a film ca. 0.08 in. in thickness. This film, and a ”virgin" companion film made of copolymer without additives, were laminated to CRS panels at 80 °C, then sheared into test plaques. Additive-containing and virgin plaques were subsequently placed side by side in a vacuum oven at 150 °C for various lengths of time to allow the self-assembly process to proceed. Once a series of test plaques with various annealing times had been generated, they were scribed and subjected to a delamination test in a solution of 0.001 gm/gm NaCl/water as described above.

Results and Discussion Growth of Phosphonate SAMs on Metal Surfaces. Monomolecular Layers. Monolayers of PAs may be deposited by contacting dilute solutions of the acid with a metal surface, as has been demonstrated for monofunctional acids in the work of Bram et al.24 and Goetting et al.29 To determine whether R,ω-BPAs would also form monolayers we compared the adsorption behavior of C12BPA with that of monofunctional C2PA, C8PA, and C18PA. Films were formed by overnight immersion of aluminum-on-silicon wafer coupons in 1 mM solutions of these surfactants in ethanol. Figure 2 compares the peak height of the CH2 νas stretch for these monolayers as measured by RAIRS. By calibrating the intensity of the absorption for the C12BPA with those of the monofunctional surfactants, it is readily apparent that the C12BPA adsorbs as a single monolayer consisting of molecules with one acid group bound to the surface and the other acid group presented to the solution. If the C12BPA were to bind to the surface with both acid groups in a loop conformation, thereby occupying two surface sites per molecule, the measured intensity would have been approximately one-half of that observed. The RAIRS spectra of C18PA and C12BPA monolayers produce peak positions of the CH2 νas stretch at 2920 and 2926 cm-1, respectively, which are significantly higher than the value of 2918 cm-1 typically found for well-ordered alkane thiols on gold4 or 2914 cm-1 for carboxylic acids on silver.3 Hence, although complete from a mass coverage viewpoint, these monolayers are not as well ordered as many other metal/surfactant SAM combinations. Similar (29) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182.

Van Alsten

Figure 3. Correlation of peak intensities of the CH2 νas stretch for epitaxially grown C12BPA/Zn+2 multilayers (boxes) and monofunctional phosphonic acids (circles) with the total number of CH2 units in the structure. The last datum represents a construction of C12BPA/Zn+2/C12BPA/Zn+2/C12BPA/Zn+2/ C18PA. Aluminum-on-silicon wafer substrate.

conclusions from contact angle analysis were found by Folkers et al.30 Multilayers. Growing Multilayer Films on Engineering Metals. The epitaxial growth of metal bisphosphonates has been explored in numerous studies since the pioneering work of Mallouk and colleagues.26,27,31-33 In all the work of which we are aware, gold or silica are used as the substrates, which necessitates an initial “priming” step with either an ω-terminated PA thiol or silane. Because the PA group is itself capable of binding to the surface of many metal oxide (hydroxides), BPAs are self-priming on these materials. To demonstrate this, a multilayer structure consisting of three alternating C12BPA/Zn layers and a capping layer of C18PA was constructed by sequential dips of a substrate of evaporated aluminumon-silicon wafer in 1 mM solutions of C12BPA and zinc acetate in ethanol. RAIRS spectra were obtained after each dip and the peak intensity of the CH2 νas absorbance measured. Figure 3 illustrates that the intensity increase is linear with the number of methylene units in the film backbone, and so is strong evidence that the growth of the film does indeed proceed layer by layer. One very interesting question concerns whether the BPA is bound to the metallic substrate or the layer of complexed metal ions by one or both of its acid groups. Allara et al. provide ellipsometric and contact angle evidence that 1, 32-dotriacontanoic acid binds in a loop configuration with perhaps a single carboxylic acidic group bound to the surface.34 In contrast, no evidence of such loop formation has been reported in the large number of studies on layered surface phosphonatessindeed, it is unlikely that epitaxial growth could be achieved if such loops were formed. Our observation that there is good agreement in the progression of CH2 intensities for the monofunctional series C2PA, C8PA, and C18PA with the multilayer construction is consistent with the latter body of work. On the basis of (30) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (31) Xu, X.; Yang, H. C.; Mallouk, T. E.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 8386. (32) Frey, B. L.; Hanken, D. G.; Corn, R. M. Langmuir 1993, 9, 1815. (33) Byrd, H.; Whipps, S.; Pike, J. K.; Talham, D. R. Thin Solid Films 1994, 244, 768. (34) Allara, D. L.; Atre, S. V.; Elliger, C. A.; Snyder, R. G. J. Am. Chem. Soc. 1991, 113, 1852.


Figure 4. Comparison of integrated peak intensities of the phosphonate infrared P-O absorption for C12 and C2 metal phosphonate multilayers grown on 302 stainless steel. The difference in slope of over 3× indicates a considerable degree of disorder in the C2 layers.

this volume of evidence, it appears clear that the dominant configuration of BPA layers, at least those of 14 carbon atoms or less, is with one acid group anchored and the other presented to the free surface. Although this discrepancy remains one ripe for further study, our suspicion is that the material used in the study of Allara et al. is far more likely to become kinetically trapped in nonequilibrium states by entanglement of the very long hydrocarbon chain. Although we have found that the RAIRS measurements of the intensities of both C-H and P-O vibrations increase linearly with layer number, it is interesting to note that the measured absorptivity of the P-O vibrations varies depending on the chain length of the hydrocarbon backbone. For the C12BPA and C2BPA layers we measured absorptivities of 1.6 AU cm-1/layer and 0.49 AU cm-1/ layer, respectively, a difference of over three (Figure 4). This is very surprising, because it would be anticipated that the absorptivities should be virtually identical. This is most likely indicative of a higher degree of disorder in the C2BPA layers, which results in a significant orientation of the P-O transition moment away from the surface normal. A large number of metal ions may be used to build multilayers, in accordance with the literature on bulklayered metal phosphonates.35 We have successfully grown layers using Zn2+, Al3+, Cu2+, Mg2+, Zr4+, VO2+, Cr3+, Mn2+, Fe3+, Ni2+, Pb2+, and Ce3+ ions. It is also possible to produce autogenous growth of metal bisphosphonates on the surfaces of reasonably active metals such as copper or brass. In these instances, simply immersing a metal coupon in a solution containing the BPA results in the growth of films of metal bisphosphonates on the coupon surface, albeit probably not in an epitaxially layered structure. For example, for a copper surface contacted with a 1 mM solution of C12BPA in ethanol, the infrared peak intensities plateau at a level ca. 5× that of a typical monolayer (Figure 5). Although this phenomenon has not been studied in detail, it should be noted that the spontaneous formation of such multilayered films is not observed for more refractory metal [or metal oxide(hydroxide)] surfaces such as stainless (35) Thompson, M. E. Chem. Mater. 1994, 6, 1168.

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Figure 5. The autogenous growth of a C12BPA/Cu+2 film on exposure of a copper coupon to a 1 mM solution of C12BPA in ethanol. The infrared intensities plateau at values ca. 5× that of a monolayer. It is unclear whether this film is an epitaxial multilayer or a collection of randomly deposited crystallites, but the latter possibility appears more likely.

steel, chromium, tantalum, and aluminum. It also does not appear for CRS in solutions of BPAs in ethanol, but does for solutions of these acids in water. Often these multilayer films result in a surface haze visible to the naked eye. On the basis of these observations, it is likely that the mechanism by which these films grow is a corrosion/precipitation process in which metal ions are electrochemically etched from the metal substrate and then complex with the BPA to form insoluble precipitates on the substrate surface. This process appears to be very similar to that used in the phosphating of engineering metals.36 Thermal Stability of Bisphosphonate Multilayers. Many practical applications of SAMs would require a fair degree of thermal stability. To evaluate this, the thermal decomposition of a 3× C12BPA/Zn multilayer was measured by tracking the attenuation of the alkyl C-H intensity after a given thermal treatment time. At 250 °C in air, decomposition of the hydrocarbon backbone was first order with a rate constant of 0.10 min-1. Interestingly, no attenuation was noted in the phosphonate P-O intensity, indicating that this process leaves metal phosphates on the surface. Considerably more stability of the hydrocarbon layers may be expected in a less hostile atmosphere. Derivatization of the SAM. The metal ion terminus of the SAM would appear to be an ideal site on which to coordinate other chemical groups, and so tailor the surface of the material in a variety of ways. Xu et al., for example, has shown that aluminum ion-terminated bisphosphonate layers can be used to bind DNA molecules.31 Our own interests are to utilize the self-assembly strategy to tailor the surfaces of engineering metals, particularly with respect to their interactions with synthetic polymers. Tracking the coordination chemistry that occurs at these sites is difficult, however. For example, to our knowledge the infrared hydroxyl stretch of the acidic PA has not been observed in a SAM, even by photoelastic modulated spectroscopy.32 We have explored details of binding by derivatization with model small molecules, and although the behavior appears very complex, we have acquired a rudimentary understanding that will be described below. (36) Freeman, D. B. Phosphating and Metal Pretreatment: a Guide to Modern Processes and Practice, 1986.

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Van Alsten Scheme 2: Cartoon Representation of Interfacial Structure Formed by a Carboxylic Acid Bearing Polymer with a Metal Bisphosphonate SAM

Figure 6. Derivatization of a 3× C12PA/Zn+2 multilayer on 302 stainless steel by perfluorooctanoic acid [(CF3(CF2)6COOH)]. The bottom spectrum is pretreatment, and shows the presence of acetate at 1584 cm-1. The top spectrum is posttreatment, showing fluorocarboxylate absorption at 1687 cm-1 and other absorptions (1366, 1325, 1252, 1212, 1149, 1106, and 1024 cm1) characteristic of the fluorocarbon tail.

Producing Low-Energy Surfaces by Capping the SAM With Small Molecules. Treatment of the metal-terminated phosphonate with a monofunctional organic acid should in principle “cap” the layer with said acid (Scheme 1b). If the acid has a hydrocarbon or fluorocarbon tail of sufficiently long chain length, then the surface becomes one of low energy. For example, treatment of a 3× C12BPA/ Zn multilayer with a 1 mM solution of octadecylphosphonic acid in ethanol changes the advancing contact angle for water from 64° to 114°. From IR measurements of band intensities, this reaction appears to proceed quantitatively, which is not surprising given the ease with which multilayers are grown with similar chemistry. Metal phosphonate layers may also be readily capped with fluorinated acids. Figure 6 present the RAIRS spectra of a 3× C12BPA/Zn film before and after contact with a 1 mM solution of PFOA in perfluorotetrahydrofuran solvent. The postcontact spectrum shows no diminution of intensity in the CH2 modes, indicating that the hydrocarbon multilayer remains intact upon treatment. The binding of fluorocarbon is confirmed by the appearance of C-F vibrational bands at 1241 and 1210 cm-1, and the exchange of PFOA (1687 cm-1) for acetate anion (1584 cm-1). The metal-terminated layers are less readily capped with hydrogenated carboxylic acids. Performing this reaction appears to depend in a complicated way on temperature, the nature of the coordinating ion, and solvent quality. For example, treatment of a 3× C12BPA/ Zn or 3× C12BPA/Zr layer with a 1 mM solution of deuterated stearic acid in ethanol at room temperature shows no capping by the acid, as indicated by the absence of C-D bands in the RAIRS spectrum. Changing the solvent to hexadecane, however, results in a small amount of stearate complexation on the 3× C12BPA/Zn, but far less (ca. 10%) than that expected for complete monolayer coverage. We presume that the greater difficulty observed in exchanging the terminal acetate group with long-chain hydrogenated acids is due to the much weaker acidity of the hydrocarbon acids (pKa ∼ 5) relative to the fluorinated acids (pKa ∼ 0.2). Polymer/MetalInterfaceModificationUsingSAMs. Grafting Fluoropolymers to the SAM. Bisphosphonate SAMs may also be used to bind polymers to surfaces, particularly those that contain acid groups. A cartoon of

Figure 7. Grafting a fluoropolymer to a 3× C12PA/Zn+2 multilayer. The 0.81/0.19 FEP-COOH derivatized surface (middle) may be compared with the underivatized precursor (bottom) and the perfluorooctanoic acid-derivatized surface (top). Integration of the peak areas of the resulting fluorocarboxylate and residual acetate absorptions yields an estimated surface site substitution of ca. 50%.

the structures so formed is presented as Scheme 2. One example of this is the ionic grafting of 0.81/0.19 FEPCOOH to stainless steel via a 3× C12BPA/Zn multilayer SAM. This was accomplished by contacting the SAM with a 0.1% (weight) solution of the FEP polymer in perfluorophenanthrene solvent at 50 °C for 1 h. Grafting to the SAM is monitored by changes in the carboxylate region of the infrared spectrum, as illustrated in Figure 7. This figure shows an “as prepared” SAM (bottom), a model surface prepared by treating a SAM with PFOA (top), and a SAM after copolymer binding (middle). These spectra clearly show that the copolymer binds to the multilayer through displacement of acetate by the end groups of the FEP, as evidenced by the replacement of the acetate peak at 1584 cm-1 with the fluorocarboxylate at 1687 cm-1. Integration of the peaks in these spectra indicate that ca. one-half of the acetate-containing binding sites have been displaced by polymer end groups. The surface of this film was highly hydrophobic. Although solution processing provides a facile means to investigate the fundamentals of grafting, it is much more important to demonstrate that similar behavior can be obtained directly from the polymer melt. For this reason, a 0.89/0.11 FEP-COOH was melt pressed onto a 3× C12BPA/Zn layer at 270 °C, followed by a 5-min anneal at 150 °C. The fluoropolymer film was carefully stripped with a razor blade down to what appeared visually to be


Langmuir, Vol. 15, No. 22, 1999 7611

Figure 8. Effect of SAM chain length on interface fortitude. The chain lengths and terminating ions of the SAMs are indicated. These interfaces were attacked with aqueous HCl; it is readily apparent that longer SAMs provide more robust interfaces.

bare metal, although still very hydrophobic. Subsequent interrogation of the surface using RAIRS showed the presence of a fluorocarboxylate peak at 1683 cm-1, thereby confirming coupling of the FEP to the SAM. Grafting Copolymers of Ethylene and Methacrylic Acid. Of polymers used for lamination to metals, copolymers of ethylene and an acrylic acid are the most common, with worldwide production exceeding several billion kilograms annually. These produce tough, clear laminates that adhere strongly to metals such as aluminum and steel. Because of the ionic nature of the metal-carboxylate bond, however, the metal/polymer interface can be susceptible to attack by aggressive environments such as aqueous solutions of mineral or organic acids. Hence, the challenges encountered in such situations are not necessarily ones of adhesion per se, but of resistance of the interface to attack by adventitious agents. The utility of a SAM in such a situation would at least partially be governed by its ability to impart improved resistance to attack. To test the ability of SAMs as adherends, we produced structures of polymers laminated to SAM-coated aluminum. BPA SAMs of carbon number 2, 4, 6, 8, 10, and 12 were used, and the polymer was a 0.987/0.013 E/MAA. After annealing and aging, this construction was immersed in a solution of 5 mol % aqueous HCl at room temperature to determine how well these interfaces withstood attack by aqueous acid. A photograph of the panels after 2.6 h of such exposure is shown in Figure 8. A number of qualitative observations are readily apparent on inspection of this photograph. First, it is evident that the severity of the delamination is inversely related to the chain length of the SAM: longer chain-length SAMs produce more robust interfaces than their short brethren. Second, the shorter chain-length SAMs actually accelerate attack of the interface relative to the simple polymer/ metal system with no SAM. Enhanced resistance to delamination over the no-SAM control is only observed when the carbon chain length of the SAM exceeds eight; a numerical presentation of the extent of delamination versus SAM chain length is presented as Figure 9. Empirically, this figure shows that the strength increase of the interface is logarithmic with SAM chain length. Another significant finding from this experiment concerns the utility of the metal counterion used to terminate the BPA SAM. One clear example is presented as Figure 10, which shows the extent of delamination of a polymer of 0.987/0.013 E/MAA from CRS caused by immersion in

Figure 9. Dependence of failure on bisphosphonic acid (BPA) SAM chain length. The interfaces are aluminum/SAM/0.987/ 0.013 E/MAA copolymer, and the attacking medium is 5 mol % HCl/water. The fortitude of the interface is only improved over the metal/polymer interface (“0”) for SAMs with carbon backbones greater than eight. The ions listed are those used to cap the BPA SAMs.

a 1 × 10-4 gm/gm solution of NaCl/water. The fortitude of the interface is always improved substantially with inclusion of a multivalent counterion to “cap” the PA terminus. As in Figure 10, we have found the performance of uncapped C12BPA monolayer (i.e., those with PA termini) to be inferior to that of the no-SAM metal/polymer interface, that is, no improvement in properties is accrued over the “control” case. It is therefore evident that the metal ion terminus provides an important link between the BPA SAM and the contacting polymer phase.39 Taken together, these results show important relations between microscopic order and bonding in the SAM and a macroscopically observable criterion for system performance, namely delamination under chemical attack. The extent of ordering in SAMs has long been recognized to increase with hydrocarbon chain length, as inferred from (37) Jennings, G. K.; Munro, J. C.; Yong, T.; Laibinis, P. E. Langmuir 1998, 14, 6130. (38) Klein, J.; Briscoe, B. J. Proc. R. Soc. London 1979, 365, 53. (39) further examples of these effects are provided in a patent application: Van Alsten, John G. U. S. 09/102, 230.

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Van Alsten

Figure 11. Rate of self-assembly of d-C16PA on a ZnS ATR crystal from a ca. 3 mM solution in 0.946-E/0.054 MAA at 100 °C. In contrast with self-assembly from surface-inactive smallmolecule solvents, this process is remarkably sluggish.

Figure 10. Influence of metal counterion on the fortitude of a CRS/C12BPA/polymer interface. The interfaces are, from left, C12BPA, C12BPA/Zn(Ac)2, and virgin surface. The test environment was a 1 × 10-4 gm/gm solution of NaCl/water. Without the presenceof the complexing counterion, the fortitude of the SAM-containing interface is substantially less than that of the no-SAM control.

red-shifting of the CH2 νas stretch. Yang et al.26 have reported that this frequency decreases with increasing carbon number for metal phosphonate SAMs, reaching a lower limit at a chain length of 12. It is reasonable to infer that the extent of order in the monolayer is in large part responsible for the relative fortitude of the polymer/ SAM/ metal interfaces investigated here: longer chain-length SAMs produce more ordered structures, which leads to

improved performance. Similar conclusions have been reached by other investigators, most recently by Jennings et al.37 in their study of the effect of alkanethiol monolayers on copper corrosion. In that study, the rate of breakdown of the protecting SAM was observed to decrease ca. 50% for every five methylene units in the SAM backbone. It is also interesting to note that enhanced resistance is not obtained until the point at which a relatively high degree of order is achieved, in these cases at a chain length of ca. eight. Coating metallic surfaces with shorter chainlength SAMs actually accelerates interfacial delamination. With diminished order in the monolayer, such short chain lengths are likely to be more susceptible to intrusion of water than the highly hydrophobic copolymer chain by itself, and so serve to concentrate the aggressive agents in the medium at the interface. Furthermore, it is clear that the complexing metal ion is also critical to producing robust interfaces with these SAMs, as C12BPA monolayers alone do not yield robust

Figure 12. Effect of annealing on the in situ formation of a C12BPA/Zn+2 SAM. Samples labeled A are laminates of 0.946 E/0.054 MAA onto cold rolled steel (CRS); samples labeled B are laminates of the same polymer that have had SAM-forming additives added. The plaques were annealed at 150 °C for the times indicated, then subjected to a hostile solution of 1 × 10-3 gm/gm NaCl/water, causing the observed delamination/corrosion. The in situ self-assembly of the C12BPA/Zn+2 SAM in the system creates an increased fortitude of the interface.


Langmuir, Vol. 15, No. 22, 1999 7613

Table 1: Apparent Diffusion Coefficients for Deuterated Hexadecylphosphonic Acid in 0.946/0.054-E/MAA temperature (°C)

Dapp (cm2/s) × 1010

50 75 100

0.12 0.16 2.3

interfaces. The necessity of the metal ion can be rationalized in two ways. First, as we have observed with small molecule model studies, the ion provides a strong link between acid groups on the polymer backbone and the SAM, without which the only interactions between SAM and polymer are relatively weak hydrogen bonds. Second, because the metal ion is coordinated to three PA molecules in the SAM,35 it imparts cross-linking to the structure, which may impart additional stability to the monolayer. SAM Formation from Competitive Polymer Melts. It would be most beneficial if SAMs could be formed at the polymer/metal interface in situ, thereby avoiding lengthy and expensive pretreatments of the metal substrates by SAM-forming solutions. To do so, however, the SAMforming molecule must diffuse to the substrate surface quickly, as well as displace previously bound groups from the surface. Neither of these criteria is expected to be facile in the E/MAA copolymers in which we are primarily interested. To investigate this issue, we undertook a twopronged approach. First, we determine the rate at which a PA SAM forms on a model surface from a E/MAA polymer melt. Then, in a more practical vein, we infer the rate of self-assembly by measuring the impact of annealing time on the delamination of metal/polymer laminates that contained SAM-forming chemicals. Rate of SAM Formation from a Competitive Polymer Melt. There have been numerous studies that have included rate studies on SAM formation from smallmolecule solvents that do not compete with the surfactant for the surface. It is generally observed that the mass adsorbed to the surface grows very quickly in low-viscosity liquids of little surface activity, with surface saturation occurring in ca. 1 h.7,26 The building of SAMs from polymer melts, particularly polymers that themselves contain surface-active groups, would be expected to be more sluggish because of slower mass transport of the SAMforming molecules to the surface as well as the need to displace surface-bound segments. Our experiments show that this is indeed the case (Figure 11). For example, coverage is only ca. 50% complete after 1 h in a melt of 0.946/0.054 E/MAA at 100 °C, and is still increasing after 16 h. Using the Langmuir-Schaefer analysis, the apparent diffusion coefficients calculated for this self-assembly (Table 1) are ca. three orders of magnitude lower than coefficients measured for similar materials traversing polyethylene.38 Because a slowing of this magnitude due to transport through the bulk polymer is unlikely, it is plausible that the diminished rate instead reflects a very slow accumulation of PA on the substrate surface. Hence, the rate-limiting step for self-assembly is probably not transport of molecules to the surface per se, but the displacement of previously bound methacrylic acid segments on the substrate surface. Building Robust Interfaces in Situ. To investigate whether robust interfaces could be formed over technologically reasonable time scales, we performed a series of side-by-side comparisons of CRS/polymer (0.946/0.054 E/MAA) laminates in which the polymer film either contained no additives or ca. 3 mM each of self-assembling C12BPA and zinc acetate. Self-assembly was allowed to progress to varying degrees by annealing the panels for

Figure 13. Effect of diffusion on fortitude of the interface. Compositions: CRS/0.946 E/0.054 MAA copolymer (no additive) and CRS/0.0946 E/0.054 MAA copolymer + 1000 ppm C12BPA and 1000 ppm zinc acetate [C12BPA/Zn(Ac)2]. Test solution: 1 × 10-3 gm/gm NaCl/water. Annealing at 150 °C activates diffusion of the C12BPA and Zn+2 ions to the metal surface and allows self-assembly to proceed, producing the observed increase in integrity.

various lengths of time at 150 °C. They were then scribed and placed in a bath of 1 × 10-3 gm/gm NaCl in water. The performance results are presented as an image in Figure 12, where the annealing time is indicated and A designates pure polymer and B designates polymer containing the SAM additives. The numerical results are plotted versus annealing time in Figure 13. Several interesting points emerge from this experiment. First, it is noteworthy that annealing is required to obtain optimal performance of both the undoped and SAMcontaining structures. Because the polymer phase is always in intimate contact with the metal surface, this suggests that considerable rearrangement of the polymer/ metal interface to a more optimal configuration must occur. Second, there is clear differentiability between the performance of the virgin and SAM-containing specimens beginning after 10 min of annealing, and after 30 min of annealing there is virtually no delamination of the SAMcontaining film. It is therefore plausible that thermal activation allows the C12BPA molecules and zinc ions in the copolymer to assemble into the linking SAM structure necessary for a robust interface. Although the process is sluggish compared with self-assembly from low-viscosity solutions, it is evident that under certain circumstances in situ self-assembly can be an effective strategy to realize improvements in adhesion. Conclusions We have presented a study of the formation and utility of SAMs on common engineering metals. PAs selfassemble on common metals and metal oxide(hydroxides) to produce monolayers, albeit with less order than alkylthiols on coinage metals. The terminus of a SAM of R,ω-bisphosphonates has been shown to be amenable to complexation by other organic acids, including both smallmolecule surfactants and polymers. This approach allows metal surface properties to be tuned with appropriate treatments. Tests of laminates of polymers to SAM-coated metals shows that the ability of the interface to withstand attack depends critically on the chain length, and therefore the degree of order in the SAM. Chain lengths ca. eight and longer produce materials with a surprising improvement

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in the ability of the interface to withstand aggressive environments. Self-assembly has been demonstrated directly from competitive polymer melts. Although monolayer formation appears considerably more sluggish than that from dilute

Van Alsten

solutions of low-viscosity solvents, property improvements of additive-containing polymers can be observed after several minutes of annealing time. LA981694G

Self-Assembled Monolayers on Engineering Metals: Structure

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