Langmuir 1999, 15, 8783-8786
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Acid-Base Interaction in the Adhesion between Two Solid Surfaces Gun Young Choi, Jung F. Kang, Abraham Ulman,* and Walter Zurawsky Department of Chemical Engineering, Chemistry, and Materials Science, and the NSF MRSEC for Polymers at Engineered Interfaces, Polytechnic University, Six Metrotech Center, Brooklyn, New York 11201
Cathy Fleischer Imaging Support and Technology Division, Eastman Kodak Company, Rochester, New York 14650-2158 Received September 15, 1999. In Final Form: October 28, 1999 A study of acid-base interactions in the adhesion between solid surfaces is presented. Adhesion was studied between various OH-functionalized self-assembled monolayer (SAM) surfaces and poly(dimethylsiloxane) (PDMS) cross-linked networks using the contact deformation mechanics method. An increase in adhesion strength was observed in the order of increasing acidity of surface OH protons [Au/S(CH2)11OH < Au/S(C6H4)2OH < Au/S(CH2)15COOH < Au/S(CH2)11PO(OH)2]. Systematic control of surface OH concentration revealed that the strength of adhesion is proportional to the number of surface OH groups but that the relationship is highly nonlinear, probably due to the collective nature of H-bonding between the PDMS chain and surface OH groups. Intermolecular H-bonding at the surface of phosphonic acid (11-mercaptoundecyl phosphonic acid SAMs) results in decreased adhesion.
The ability to tailor adhesion properties of surfaces and interfaces for specific applications is a challenge. Often, efforts in understanding adhesion between solid surfaces have emphasized surface energy and fracture mechanics, and in the case of polymer surfaces, the latter has been related to phenomena such as interfacial chain interdiffusion, interdigitation, and entanglement. While interfacial chemical interactions have been used to enhance adhesion between polymers, the coupling between interfacial chemistry and chain dynamics makes it difficult to ascertain the contribution of chemical interactions to adhesion. Fundamental understanding of the relationship between surface chemistry and adhesion requires stable chemically engineered surfaces, and self-assembled monolayers (SAMs) of thiolates on gold surfaces are ideal model surfaces for such studies. For example, van der Waals and dipolar interactions can be investigated using the series X ) F, Cl, Br, and I. On the other hand, mixed SAMs of CH3 and OH groups can be used to control stronger, H-bonding interactions by tuning the surface OH concentration.1,2 Interfacial acid-base interactions have been invoked in the past to explain wetting and adhesion phenomena.3,4 However, to date, there have been no systematic studies of acid-base interactions in adhesion using SAMs.5 The equivalent interactions between cross-linked poly(dimethylsiloxane) (PDMS) networks and silanol (Si-OH) * To whom correspondence should be addressed: (718) 260-3119 (voice), (718) 260-3125 (fax),
[email protected]. (1) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (2) Kang, J. F.; Jordan, R.; Ulman, A. Langmuir 1998, 14, 3983. (3) (a) Jensen, W. B. The Acid-Base Concept; Wiley-Interscience: New York, 1980. (b) Jensen, W. B. In Surface and Colloid Science in Computer Technology; Mittal, K. L., Ed.; Plenum Press: New York, 1987. (4) Good, R. L.; Chaudhury, M. K.; van Oss, C. J. In Fundamentals of Adhesion; Lee, L.-H., Ed.; Plenum Press: New York, 1991; p 153.
groups on SiO2, created by the oxidation of PDMS or Si wafer surfaces, have been studied6,7 and have been mainly referred to as H-bonding interactions. The H-bonding, in a broad sense, is an acid-base interaction, since the protons in the OH can be considered as Lewis acids, while the oxygen nonbonded electrons in the PDMS Si-O-Si groups can be considered as Lewis bases.8 We have studied in detail the adhesion of PDMS crosslinked lenses to Si-OH groups on a SiO2/Si substrate.9 A significant increase of adhesion hysteresis with contact time at a maximum load was reported and attributed to network reorganization. In studies using gelatin spheres (25% weight gelatin in water), we found very strong adhesion to different OH surfaces, in some cases resulting in the rupture of the hemisphere upon separation.10 Clearly gelatin is too strong a base to distinguish between different surface acid groups. On the other hand, the SiO-Si group is a very weak base, and the (CH3)2Si groups provide steric hindrance that further decreases acid-base interactions. The strong adhesion between PDMS and SiOH groups gave us hope that PDMS cross-linked networks may be good systems for systematic studies of interfacial acid-base interactions. The underlying assumption is that Brønsted acidity (proton transfer in aqueous media) and H-bonding between two solid surfaces (in nonpolar media) are related. In other words, we expected that a stronger acidic group in aqueous media, which serves as a surface functionality, will result in stronger adhesion to PDMS (5) For review on SAMs of thiols on gold see: (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (6) Chaudhury M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013. (7) Silberzan P.; Perutz, S.; Kramer, E.; Chaudhury, M. K. Langmuir 1994, 10, 2466. (8) Pimental, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman & Company: San Francisco. (9) Kim, S.; Choi, G. Y., Ulman, A.; Fleischer, C. Langmuir 1997, 13, 6850. (10) Kim, S.; Ulman, A.; Fleischer, C. Unpublished results.
10.1021/la991222h CCC: $18.00 © 1999 American Chemical Society Published on Web 12/03/1999
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cross-linked networks. We base this assumption on Bolger and Michaels’ quantitative assessment of the energy of H-bonds.11 We have used a method developed by Johnson, Kendall, and Roberts (JKR)12 to study the adhesion between various OH-functionalized SAM surfaces and poly(dimethylsiloxane) (PDMS) cross-linked networks. This method is based on a continuum contact mechanics model, which considers the effect of surface energy on the properties of an elastic contact. It has been used in recent years for quantitative measurements of adhesion properties and surface energetics of various polymeric surfaces. Because we are mainly interested in obtaining a fundamental insight into the chemical foundation of adhesion, our approach has been to use this method to probe the roles of surface chemical functionalities. Many other investigators have reported work conducted to elucidate the mechanistic aspects of contact and separation processes.13 Our model surfaces for the adhesion study were prepared as self-assembled thiolate monolayers on Au(111) with different chemical functionalities.5 Since most macroscopic interfacial phenomena, including adhesion, wetting, and friction, are determined primarily by the properties of the outer ∼5-10 Å surface molecular groups,14 such SAMs are excellent model systems for polymer surfaces. SAMs and mixed SAMs of thiolates on Au(111) have been used extensively as model systems for the studies of wetting.15 In this paper we present a quantitative demonstration of acid-base interactions in adhesion between solid surfaces using SAM surfaces. To produce model surfaces with increasing acidity, we have used the following thiols: 12-hydroxyundecane-1thiol, HS(CH2)11OH, which gives alcohol surfaces, 4′hydroxy-4-mercaptobiphenyl, HS(C6H4)2OH, which results in phenol surfaces, 16-mercaptohexadecanoic acid, HS(CH2)15COOH, which forms carboxylic acid surfaces, and 11-mercaptoundecane-1-phosphonic acid, HS(CH2)11PO(OH)2, which provides phosphonic acid surfaces. All SAMs were formed by the spontaneous adsorption onto Au (111) substrates.15 Gold substrates were prepared by the evaporation of gold onto preheated (300 °C) glass slides under high vacuum (10-7 Torr). Prior to adsorption, the gold substrates were cleaned in an Ar plasma reactor (∼1 Torr) for 2 min. The substrates were immersed in 0.1-1 mM thiol solutions in ethanol for several hours, washed with fresh ethanol and distilled water, and dried with N2. Thickness values estimated by ellipsometry were 14 ( 1, 15 ( 1, 20 ( 1, and 15 ( 1 Å for the SAMs of HS(CH2)11OH, HS(C6H4)2OH, HS(CH2)15COOH, and HS(CH2)11PO(OH)2, respectively. Contact angles of distilled H2O were e20° in all cases, indicating almost complete wetting. Cross-linked PDMS elastomers have been used in various adhesion studies using the JKR method6,7,13a,d because their elastic modulus is in the range ideal for the (11) (a) Bolger, J. C.; Michaels, A. S. In Interface Conversion for Polymer Coatings; Weiss, P., Dale Cheever, G., Eds.; Elsevier: New York, 1968. (b) Bolger, J. C. In Adhesion Aspects of Polymeric Coatings; Mittal, L. K., Ed.; Plenum Press: New York, 1983. (12) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301. (13) (a) Maugis, D.; Barquins, M. J. Phys. D: Appl. Phys. 1978, 11, 1989. (b) Brown, H. R. Annu. Rev. Mater. Sci. 1991, 21, 463. (c) Tirrell, M. Langmuir 1996, 12, 4548. (d) Ahn, D.; Shull, K. R. Macromolecules 1996, 29, 4381. (e) Merill, W. W.; Pocius, A. V.; Thakker, B. V.; Tirrell, M. Langmuir 1991, 7, 1975. (14) Allara, D. L. Polymer Surfaces and Interfaces; Feast, W. J., Munro, H. S., Richards, R. W., Eds.; Wiley: Chichester, 1993; Vol. II, p 27. (15) (a) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (b) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E. Adv. Colloid Interface Sci. 1992, 39, 175. (c) Ulman, A. Chem. Rev. 1996, 96, 1533.
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JKR theory, and it is relatively easy to prepare samples with optically smooth surfaces and transparency. Using established synthetic methods to synthesize “model” PDMS networks,16 we have prepared cross-linked PDMS hemispheres with a radius of curvature of about 1 mm, following the procedures developed by previous investigators.6,7 Narrow molecular weight distribution (polydispersity ∼1.2) vinyl-terminated PDMS was mixed with an appropriate amount of tetrafunctional hydrosilylation cross-linker to yield as complete cross-linking as possible. These “model” networks have been assumed to contain a very small amount of defects and unconnected chain ends,16 thus allowing nearly ideal elastic behavior which is free from other mechanical effects that we want to eliminate in our study such as interpenetrating pendant chains, which could otherwise interfere and complicate the adhesion measurements. Extraction experiments using toluene consistently gave weight losses of e1%, in agreement with this assumption. The PDMS hemispheres used in this work were prepared from one batch of PDMS having a number average molecular weight of 48 000, and all have the same treatment history. The JKR apparatus in our laboratory is based on a Nikon microscope equipped with vertical illumination that allows using opaque substrates without the loss of image sharpness. The instrument is placed on an environmental chamber (Hotpack, A HEICO Co., model 317573) on an electronic vibration isolation table (EVIS, Newport Corp.). All measurements were carried out at 25 °C and 30% relative humidity. The PDMS lenses can be pressed on or released from the surface by the controlled movement of a micromanipulator that holds the lens and is connected to an accurate speed controller. The load at the interface is measured by a sensitive electrobalance with computerized automatic data acquisition. Loading and unloading rates were 0.17 and 0.15 µN/s, respectively. The optical microscope images of the contact circle formed at the interface between the elastic PDMS lens and the flat SAM surface were captured by a CCD camera and stored in a computer along with the corresponding load values measured at the interface. Image analysis is carried out using NIH software. We note that at 25 °C and 30% relative humidity there are certainly a number of water layers adsorbed on these polar surfaces. The exact nature of the adsorbed water and it role in adhesion are unknown, and it is not clear if water remains at the interfaces or is “squeezed” out by the formation of the joint. Nevertheless, we assumed that by keeping experimental conditions strictly constant the effect of adsorbed water could be considered similar in all cases. Figure 1 presents data of energy release rate (G) as a function of crack propagation rate (da/dt) for the adhesion of PDMS cross-linked 68 000 network lenses to surfaces of Au/S(CH2)11OH, Au/S(C6H4)2OH, Au/S(CH2)15COOH, and Au/S(CH2)11PO(OH)2. The order is consistent with the relative acidity of the protons. Thus, as proton acidity increases, the O-H:O interaction increases as well, but in the case of a weak base such as the Si-O-Si group, there is no proton transfer, and the interaction stays in the realm of H-bonding.17 Although pKa is defined for dissociation of acid molecules in water solutions, it may serve as an approximation for the relative acidities of the different SAM surfaces. The ionization constant, Ka, of (16) Patel, S. K.; Malone, C.; Cohen, J. R.; Gilmore, J. R.; Colby, R. H. Macromolecules 1992, 25, 2541. (17) Bolger, J. C.; Michaels, A. S. Interface Conversion for Polymer Coatings; Weiss, P., Cheever, G. D., Eds.; Elsevier: New York, 1968; p 3.
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Figure 1. Energy release rate (G) as a function of crack propagation rate (da/dt) for the OH surfaces studied.
Figure 2. A plot showing strain energy release rate plotted with respect to pKa for da/dt ) -0.02 mm/min. -18
the analogous acidic compounds is in the order 10 (alcohol) < 10-10 (phenol) < 10-5 (carboxylic acid) < 10-2 (phosphonic acid), and Figure 2 presents a plot of G with respect to these pKa values. Whitesides and co-workers observed that the dissociation constant of COOH groups shifts to higher pKa values at a surface.18 However, even if each data point in Figure 2 is shifted by 2-3 pKa units to higher values because of electrostatic repulsion of the negatively charge conjugate bases, it will not change the general trend. As evident from Figure 2, adhesion increases with the increasing interfacial H-bonding interaction, which, in turn, increases with the increasing Ka of the surface acid protons that are involved in H-bonding. To further investigate the effect of interfacial H-bonding on adhesion, we have studied SAM surfaces with different concentrations of phenol groups. This was accomplished by systematically changing the concentration of HS(C6H4)2OH in mixed SAMs of HS(C6H4)2OH and HS(C6H4)2CH3. (18) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870.
Figure 3. Energy release rate plotted with respect to dA/dt for different surface OH concentrations in experiments with continuous loading and unloading.
Figure 4. Plots of G values with respect to contact areas for adhesion to SAM with different surface OH concentration for experiments using the stepwise loading and unloading protocol: loading, open circles; unloading, filled circles.
Figure 3 presents G values, plotted with respect to da/dt for continuous loading and unloading experiments. Notice that surface OH concentration clearly affects the energy release rate, as can be expected for interfaces with H-bonding interactions. When experiments were repeated using the stepwise loading and unloading protocol (Figure 4), the G values were an order-of-magnitude larger, in agreement with our previous experiments with different dwell times at maximum loading.9 Figure 5 presents plots of G with respect to surface OH concentrations for different contact areas. The nonlinear nature of the plots is a signature of the collective nature of interfacial H-bonding interactions between PDMS and OH surfaces. In a 48 000 PDMS chain there are close to 700 oxygen atoms, and their ability to participate in H-bonding depends on the free energy needed to change chain conformation and bring them to the required distance from the surface as well as the correct orientation. However, one cannot assume that the strength of all H-bonds is the same, because the
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Figure 5. Plots of G with respect to surface OH concentrations for different contact areas.
entropy investment needed to form a contact point between a PDMS chain and the surface decreases with the increasing number of contact points. Chaudhury and co-workers19 have recently measured the interaction between a single PDMS chain and a SiOH group using atomic force microscopy. Preliminary results suggest that a strength of a single H-bond between silanol and siloxane groups is 1.5 kT or 3 × 10-3 kcal mol-1. They also used calorimetry to measure the heat of adsorption of PDMS on silica gel and found energies in the range of 130-150 mcal g-1 or 16 mJ m-2.19 Evidently, the magnitude of G measured cannot be explained simply by multiplying the number of interfacial H-bonds by the energy of one bond. This is because G represents predominantly dissipative processes such as the elastic energy associated with the stretching of polymer chains at the interface during crack propagation (unloading) UE, and the energy associated with the deformation of the PDMS network as a result of interfacial interactions, UD. Both UE and UD scale with the total energy of interfacial H-bond interactions, but since this energy does not increase linearly with the number of H-bonds, the relationship is highly nonlinear. In an attempt to gain further understanding of interfacial H-bonding and its effect on adhesion, we carried out experiments using SAMs of HS(CH2)11CH3 and HS(CH2)11PO3H2, as well as a 1:1 mixed SAM. Figure 6 presents plots of G values with respect to da/dt. Interestingly, G values for the mixed SAM are larger than those obtained for the pure phosphonic acid system. We believe that this results from extensive intermolecular H-bonding in the phosphonic acid SAM that makes OH groups less available for H-bonding with the PDMS chains. In the (19) Chaudhury, M. K. Private communication.
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Figure 6. Plots of G values with respect to da/dt for experiments using SAMs of HS(CH2)11CH3 and HS(CH2)11PO3H2, as well as a 1:1 mixed SAM.
mixed SAM, on the other hand, dilution of phosphonic acid groups makes them stronger acids and hence increases adhesion hysteresis. One could argue that the difference between the SAM and mixed SAM of the phosphonic acid might be the result of difference in adsorbed water. While we have no direct evidence to prove or disprove this mechanism, our studies of concentrationdriven surface transition in the wetting of mixed alkanethiol monolayers on gold1 suggest that differences in water adsorption may not be the dominating mechanism. In conclusion, we have presented a systematic investigation of acid-base interactions in the adhesion between solid surfaces. The adhesion studied between various OHfunctionalized SAM surfaces and PDMS cross-linked networks using the JKR technique revealed an increase in adhesion strength in the order of increasing acidity of surface OH protons. The studies revealed that the strength of adhesion is proportional to the number of surface OH groups but that the relationship is highly nonlinear, probably due to the collective nature of H-bonding between the PDMS chain and surface OH groups. Intermolecular H-bonding at the surface of phosphonic acid, 11-mercaptoundecyl phosphonic acid SAMs, results in decreased adhesion. Acknowledgment. This work was funded by the NSF through the MRSEC for Polymers at Engineered Interfaces. We thank Eastman Kodak Company for funding of the early experiments of this work and for the donation of the environmental chamber and the vibration-isolation system. The donation of 11-mercaptoundecyl phosphonic acid by Professor George Whitesides of Harvard University is gratefully acknowledged. LA991222H
