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Notes Nanostructured Multilayers and Their Influence on Tack of Cross-Linked Elastomers M. O. David,* T. Gerriet, M. Nardin, and J. Schultz Institut de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, B.P 2488, 68057 Mulhouse Cedex, France Received December 2, 1999. In Final Form: April 25, 2000
Introduction The adhesion developed between two materials in contact for very short times and under low pressure is called tack. Numerous practical applications take advantage of this property, observed generally for elastomers and molten polymers, and extensively used, for example, in adhesive tapes or industrial processes to produce multilayer assemblies. Tack is evaluated by the energy required to separate the materials.1-3 The surface properties of the materials (e.g., surface energy) and their mechanical and rheological behavior are fundamental parameters in determining the formation of the junction as well as the separation step.4,5 The contact time being very short, the development of adhesion can solely arise from wetting and, for two polymers in contact, limited interdiffusion of chain ends. However, the energy of adhesion can be several orders of magnitude higher than the level of the thermodynamic work of Dupre´ (reversible energy of adhesion), originating from the surface and interfacial energies of the materials in contact.6 Such a behavior is generally observed when at least one polymer is involved in the system being considered. This energy gap arises from dissipation mechanisms occurring in the polymer during the separation, such as viscoelastic losses6-9 or cavitation and fibrillation.10-12 Depending on the type of polymer, the dissipation can be limited to a region close to the interface (interfacial dissipation) or can extend to nearly all the material (bulk dissipation). This latter case was recently studied in several theoretical and experimental papers.10,12 When elastomers are used, as in the present study, interfacial dissipation has to be considered. The coupling between dissipation and interfacial interactions is not well understood. More specifically, an unanswered question remains: To what extent can the presence of a thin film at the surface of a rigid substrate contribute to the dissipation mechanisms involved during tack? The objective of this work is, therefore, to determine how the presence of nanostructured multilayers at the (1) Wetzel, F. H. ASTM Bull. 1957, 221, 64. (2) Hamed, G. R. Rubber Chem. Technol. 1981, 54, 403. (3) Hamed, G. R. Rubber Chem. Technol. 1981, 54, 576. (4) Russell, T. P.; Kim H. C. Science 1999, 285, 1219. (5) de Crevoisier, G.; Fabre, P.; Corpart, J. M.; Leibler, L. Science 1999, 285, 1246. (6) Gent, A. N.; Schultz, J. J. Adhes. 1972, 3, 281. (7) de Gennes, P. G. C. R. Acad. Sci. 1988, 307, 1949. (8) de Gennes, P. G. C. R. Acad. Sci. 1991, 311 II, 1415. (9) de Gennes, P. G. C. R. Acad. Sci. 1995, 320 IIb, 193. (10) Zosel, A. Int. J. Adhesion Adhes. 1998, 18, 265. (11) Creton, C.; Leibler, L. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 545. (12) Gay, C.; Leibler, L. Phys. Rev. Lett. 1999, 82, 936.
surface of a rigid substrate can modify the tack between this substrate and a cross-linked elastomer. For that purpose, multilayers with alternating layers of gold and alkanethiol molecules were grafted on a glass substrate. The number of layers was varied as well as the nature of the top surface layer (gold or alkanethiol) (Figure 1). Experimental Section Preparation of Multilayers. Microscope glass slides are used to support the grafted multilayers. To provide good adhesion between glass and the first gold layer, (3-mercaptopropyl)trimethoxysilane (MPS) is grafted on the activated glass plate surface. 1,9-nonanedithiol comprises the intermediate self-assembled molecular (SAM) layer between two successive gold layers. 1-hexadecanethiol is grafted on top of the last layer of gold. The surface of the glass plate is hydroxylated, after thorough ultrasonic cleaning in ethanol, with piranha solution (30/70 H2O2/ H2SO4) at 50 °C for 30 min (Caution: These solutions are highly oxidizing and should be handled with extreme caution!!) The glass plates are then rinsed with bidistilled water and dried under N2 gas jet. Alkanethiol (MPS and 1-hexadecanethiol) and alkanedithiol (1,9-nonanedithiol) assemblies are prepared by immersion of the substrates (glass plate with a fresh hydroxylated surface or with a fresh gold layer on top) in 1 mmol/L solutions of respective thiol in toluene at room temperature overnight. The grafted substrates are then rinsed with toluene in an ultrasonic bath to remove adsorbed aggregates and are then dried under N2 gas jet. Gold layers, about 5 nm thick, are prepared by evaporation under low pressure (2-4 × 10-4 Pa) at a low rate (0.1 nm/s). This specific thickness value was chosen to ensure transparency of the multilayer structure to visible light. This transparency is required for tack tests. The schematic structure of the prepared multilayers is shown in Figure 1. The surface morphology is characterized with atomic force microscopy (AFM) (Nanoscope III) in the tapping mode. Surface energy is determined by measuring static contact angles of two different liquids (diiodomethane and water) at room temperature.13 Elastomer. Synthetic polyisoprene (Natsyn 2200 from the Goodyear Tire and Rubber Company) is used. It is first mixed with 1 part per hundred (pph) dicumyl peroxide (DCP) and 1 pph antioxidant (N-isopropyl-N′-phenyl-p-phenylene diamine (IPPD)) by milling on a two-roll mill. Then, hemispheres of cross-linked elastomer are obtained by curing at 150 °C for 2 h in a semihemispherical mold of 40 mm diameter. The network is characterized by swelling measurements14,15 in toluene: molar mass between cross-links, Mc, and sol fraction, fs, equal to 30,600 and 0.06, respectively, are obtained. Tack Test. To evaluate tack between the multilayer and the elastomer, the geometry of Johnson, Kendall, Roberts16 test (JKR) is used: It considers a contact between a hemisphere and a plane substrate. This particular geometry was chosen because the mechanical analysis of the results is easier than in the case of a contact between two plane samples, and the contact area can be easily measured. With the apparatus described in Figure 2, the impact and separation rates (10 mm‚min-1) as well as the (13) Neumann, A. W.; Good, R. J. In Surface and Colloid Science; Good, R. J., Stronberg, R. Eds.; Plenum: NY, 1979. (14) Flory, P. J.; Rehner, J. J. Chem. Phys. 1943, 11, 521. (15) Flory, P. J. J. Chem. Phys. 1950, 18, 108. (16) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301.
10.1021/la9915670 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/12/2000
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Figure 1. Schematic representation of the nanostructured multilayers. The scale between molecules and gold layers is not in proportion. (a) The interface at the level of the molecules; and (b) at higher scale, deduced from AFM analysis (see Figure 3).
Figure 3. Morphology of the nanostructured multilayers as determined by AFM in tapping mode. (a) Nude glass; (b) glass I; (c) glass III; (d) glass I Au. Note the difference in height range between (a) and (b)-(d). (equation [1])17 gives what we call tack energy Gr (J‚m-2) with an accuracy of about 15%:
Gr )
∫σ(x)dx ) ∫ A
F(x) dx
(1)
0
where F is the measured force; A0 is the contact area before the beginning of debonding; x is the displacement and σ the nominal stress. Note that the JKR theory,16 which predicts the area of contact between a hemisphere and a plane as a function of the reversible work of adhesion and mechanical properties of the elastomer, cannot be used. This theory applies to the equilibrium state; in our case, the system is never at equilibrium.
Results
Figure 2. Experimental setup used for the tack measurements. contact time (0.1 s at maximum compression) can be controlled. A high-speed camera (250 images‚s-1) is used to follow the area of contact between the elastomer and the substrate. Thus, displacement, applied force, and contact area can be measured simultaneously. The total energy of adhesion or energy needed to break the bond Ga (J) is obtained by integration of positive values of the force as a function of the displacement. However, to compare the different samples, the energy of adhesion, Gr, has to be expressed by unit area. For that, the nominal stress σ is plotted as a function of displacement x and the integration of the positive values (17) Ondarc¸ uhu, T. J. Phys. II 1997, 7, 1893. (18) Zhang, L.; Cosandey, F.; Persaud, R.; Madey, T. E. Surf. Sci. 1999, 439, 73. (19) Petri, D. F. S.; Wenz, G.; Shunk, P.; Schimmel, T. Langmuir 1999, 15, 4520.
Different types of multilayers were prepared on the glass substrate by varying the number of layers and the nature of the terminal top layer. They are referenced as follows: A Roman numeral reference gives the number of SAMs of dithiol or thiol on top of the first gold layer. When the terminal top layer is made of gold, the term “Au” is added. For example, “glass I Au” means glass/MPS/gold / dithiol/ gold, and “glass II” means glass/MPS/gold / dithiol/gold/ thiol. Characterization of the Nanostructured Multilayers. Typical morphology of three different multilayers ending with gold or thiol, as well as the reference substrate (nude glass), is shown in Figure 3. A wormlike structure is obtained for every multilayer (Figure 3b-d). It does not vary with the number of layers or the nature of the top layer except some aggregates of thiol molecules (white spots in Figure 3b-d) remain after the grafting step. The manner of gold deposition (evaporation) and its thickness can explain this wormlike structure. As the average Au thickness increases, the morphology evolves from islands to wormlike (at about 5 nm) and finally to a continuous but rougher structure.18
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Notes
Table 1. Surface Energies of Substrates and Elastomer and Spreading Coefficient Between PI and the Substrate (Se) and Measured Tack Energy (Gr) sample glass glass Au glass I Au glass I glass II glass III glass IV PI (Natsyn 2200)
roughness (0.5 µm2) Rms (Å)
contact angle of water at equilibrium θ ( 2(°)
hystersis (water ) test liquid) ∆θ ( 4(°)
γd(mJ/m2) ( 10%
γp(mJ/m2) ( 3%
Se (mJ/m-2) ( 10%
Gr (J‚m-2) ( 15%
1.48 ( 0.05
45 67 65 103 101 99 101
38 44 48 17 15 20 23
42 49 50 40 35 28 27 27
20.1 6.2 7.0 ≈0 ≈0 0.7 0.6 3
23 21 23 6 2 -2 -3
7.6 16.3 26.9 5.4 1.5 1.6 2.2
a
7.7 ( 0.5 8.5 ( 0.5 10.6 ( 0.5
a γd, γp: Dispersive and polar component of the surface energy, respectively. The roughness of glass Au was not measured but its value should be less than or equivalent to the value of glass I, which differs only by the presence of the monoalkanethiol grafted at the surface of the gold layer. Such a grafting is known to lead to a very smooth variation of the roughness.27,28
The roughness was also determined by means of AFM measurements (Table 1): it increases with the presence and the number of layers on the glass but remains in all cases at very low values comparable to those reported by Petri et al.19 for gold deposited on amino-terminated monolayers. This very low roughness is in agreement with the mirror optical appearance of the samples. The values of the surface energy and wetting hysteresis of water obtained for the various structures are given in Table 1. The dispersive (γd) and polar (γp) components of the surface energy found for the glass plate show that the surface is not recovered by a layer of adsorbed water as expected for clean glass; it rather suggests the presence of a remaining organic contamination layer.20 Structures with a gold layer on top are characterized by a high-polarity component of the surface energy. Both values of dispersive and polar components are in accordance with values already published21 and remain approximately constant with the number of layers as well as the hysteresis of water.22 The variation of roughness between glass Au and I Au is too low to influence contact angle measurements.23 Surface properties of the top gold layer do not depend on the number of intermediate layers. This is not the case for the structures terminated by a SAM of alkyl chains. While the polar component stays approximately constant and close to zero,24 the dispersive component decreases from a very high value (40 mJ‚m-2) for glass I to a constant value (27 mJ‚m-2) for glass III and glass IV. The roughness (Table 1) cannot be responsible for such high variations because it remains at very low value. Differences in the grafting density and/ or organization of the top SAM is a better explanation. The γd values obtained for glass III and IV are in agreement with those of a disorganized alkanethiol SAM presenting both -CH3 and -CH2- groups at the surface. The higher γd values of glass II and I are close to the values obtained for gold-terminated glasses; a lower grafting density is expected in these cases. The organization of the intermediate dithiol-grafted layers has not been studied yet. Dithiols grafted at both (20) The glass plates used for tack and contact angle measurements were degreased only with acetone and alcohol: this is not sufficient. Subsequent treatment with a low concentration of sodium hydroxide in water leads to a surface completely covered with water (David, M. O.; Nipithakul, T.; Nardin, M.; Schultz, J.; Suchiva, K. J. Appl. Polym. Sci., in press). (21) Cognard, J. J. Chim. Phys. 1987, 84, 2. (22) The values are slightly higher but comparable to hysteresis obtained with glass. This variation does not necessarily reflect a variation of roughness because hysteresis is also sensitive to the chemical composition of the surface and especially its reorganizaton (orientation of groups interacting favorably with water) when it is in contact with the liquid. (23) Eick, J. D.; Good, R. J.; Neumann, A. W. J. Colloid Interface Sci. 1975, 53, 235. (24) The contact angle of water is about 100° in accordance with values obtained for alkanethiol SAM grafted on gold.27
Figure 4. Total energy of adhesion determined with the JKR geometry test. Evolution as a function of maximum compression. Contact occurs for 0.1 s between cross-linked PI and different nanostructured multilayers on top of glass plates.
ends to the supporting gold layer are certainly present. However, the thiol groups standing at the surface of dithiol layers are still sufficiently numerous to react with the new layer of gold and promote its adhesion because no delamination is observed even under the ultrasonic solicitations in solvent during the cleaning step. Further characterization is now under way, including characterization at intermediate steps (after grafting of dithiol), measurement of electrical conductivity of these systems, and X-rays analysis that provide a better image of the underground layers organization. Tack. Tack between the nanostructured multilayers and the cross-linked polyisoprene PI hemispheres was first evaluated as a function of maximum force of compression. The results in terms of tack energy (in J) are shown in Figure 4. As expected, tack energy increases with maximum compression because contact area increases. More noteworthy, however, is the variation of the tack energy as a function of the structure of the multilayers and nature of the top layer. Nanostructures ending with a gold layer always lead to the highest tack. Conversely, when SAMs of alkyl chains are grafted at the surface of gold, tack decreases drastically, leading to the lowest values. The unmodified glass plate shows intermediate values. Moreover, the results unambiguously show that the number of layers in the structure is a fundamental parameter which leads to significant variations of the tack
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energy. However, its influence varies with the nature of the upper surface: Increasing the number of layers for a gold-terminated nanostructure leads to an increase of tack energy. The behavior is more complex when SAMs of alkyl chains are present at the surface: As the number of layers increases, there is first a decrease followed by an increase of the tack energy. These variations are significant because they exceed the uncertainty (10%). All of these observations concerning the evolution of tack with the structure of the multilayer are confirmed when tack energy is expressed per unit surface area (Jm-2) (Table 1) except, perhaps, for glass II to glass IV where the variation approaches uncertainty. Discussion Only one reference elastomer was used in this study. Thus, the observed behaviors only depend on the structure of the multilayers prepared on top of the glass plates. The results will be analyzed in terms of wetting behavior and dissipation mechanisms. First, we will discuss the influence of the nature of the top layer on the tack energy. Tack increases from alkyl SAM to glass and gold. This evolution can be compared to the spreading coefficient between PI and the different substrates (Table 1). The surface energies of alkylterminated multilayers being the lowest, corresponding spreading coefficients are also the lowest. Poor wetting can then explain why the level of tack remains lower with these substrates. Conversely, the differences between tack properties of bare glass and gold-terminated multilayers cannot be explained by wetting differences, the spreading coefficients being roughly the same. Variation of relaxation behavior of the PI elastomer during contact with the different substrates cannot be invoked because the contact time is the same and short (time at maximum compression is equal to 0.01 s) for all samples. Energy dissipated through the fracture propagation appears to be responsible for the different tack values. Let us examine first the possible effects of an increased roughness from glass to gold-terminated glass. Depending on the contact conditions and materials, two opposite trends can be observed in connection with the true contact area. For a perfect contact, increasing the roughness increases the contact area and then, the tack energy.25 But short times of contact and light pressure can lead to incomplete contact between the two materials when the roughness is important and a decrease of tack energy should be observed.11 In the present study, the contact conditions correspond to this case but the behavior observed is the reverse. Because the roughness remains low even for gold-terminated glass, a good contact is possible, and an increase of the contact area could be responsible for the increase of tack energy from nude glass to gold glass. Recently, a new theoretical approach12 has shown that roughness can also be responsible for high dissipation related to air suction phenomena due to bubbles present at the interface between the materials. However, in our case, the geometry of the system does not involve plane contact. Furthermore, whatever the sample, we could clearly see that the fracture always propagated along the substrate from the outside to the interior of the contact zone and the presence of bubbles was never observed either during the contact or the separation step. Fibrillation of the elastomer was also never observed. Thus, the mechanism of air suction appears to be an improbable explanation of the different tack energy values with a nude glass substrate or a gold(25) Gent, A. N.; Lai, S. M. Rubber Chem. Technol. 1995, 68, 13.
terminated multilayer, even if it cannot be completely excluded. However, the presence of a gold-terminated multilayer at the glass surface seems to give rise to another source of dissipation. Indeed, tack energy increases with the number of layers for gold-terminated structures while the surface energy and roughness remain almost constant (Figure 4 and Table 1). The more complicated evolution of tack energy with the number of layers for an alkyl-terminated structure (Figure 4, Table 1) can be explained by a competition between wetting and dissipation phenomena. The highest tack energy is found for glass I, which has the lowest roughness. This parameter appears then to be nonessential. The decrease from plate I to plate II can be attributed to the important decrease of the spreading coefficient which is not compensated by an increase of the dissipation. Conversely, the low increase from plate II to plate IV could be due to a higher amount of energy dissipated while wetting parameters remain almost constant. In this context, note that the apparent different trends observed for Ga and Gr from glass II to glass IV (Figure 4, Table 1) cannot be ascertained because the variation of Gr approaches uncertainty. The location and the mechanisms of this high dissipation of energy observed for both gold- or alkyl-ended multilayers are yet not understood, but some assumptions can be invoked. We might expect that dissipation will mainly occur in the elastomer due to the very low thickness of the nanostructured layers (less than a fraction of a micrometer). Roughness of the structures is certainly partly responsible: it can affect both the mode and the rate of propagation of the fracture. However, this parameter remains at an almost low and constant value when the nanostructures are present, whereas dissipation of energy seems to increase with the number of layers. We suggest a modification of stress transfer at the interface between the substrate and the elastomer through the nanostructure which could affect the size of the dissipation zone in the elastomer and/or the rate of fracture propagation. The following hypothetical mechanism could then be proposed. The gold layer structure is wormlike. If it is considered as noncontinuous, very small displacements of the gold patches (grafted or not) which are at the surface of a nonrigid molecular layer could occur during contact or separation steps. These movements could be amplified by increasing the number of layers and could explain the increase of dissipation observed. Because the direct contribution of these movements to the global dissipation is certainly negligible, an effect on the size of the dissipation zone in the elastomer and/or rate of fracture is expected to explain the variation of dissipation. However, these are only preliminary assumptions which have to be verified. Studies are now underway by using different thicknesses of the gold layer (in order to reach a continuous), by varying the alkanethiol chain length as well as the tack measurement parameters (such as temperature and test geometry). Conclusion The results show unambiguously that the structure and the nature of a very thin layer of material present at the surface of a rigid substrate can influence the tack or (26) Kinloch, A. J. Adhesion and Adhesives Science and Technology; Chapman and Hall: New York, 1987; Chapter 3. (27) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (28) Wohlfart, P.; Weiss, J.; Ka¨shammer, J.; Winter, C.; Scheumann, V.; Fischer, R. A.; Mittler-Neher, S. Thin Solid Films 1999, 340, 274.
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adhesion behavior of an elastomer. Furthermore, the organization on a nanometric scale of these structures can induce macroscopic modifications. The contribution of surface energy appears to be prominent. However, most interesting is the influence of the number of layers present in the nanostructure on tack energy. Variation of roughness does not to be a sufficient explanation of the observed differences. Modification of the stress transfer at the interface induced by the nanostructure is invoked. Further investigations are currently underway.
Notes
Acknowledgment. The authors wish to thank Drs. H. Haidara and J. Ducret, ICSI, for helpful advice to prepare the samples and help in the development of the JKR geometry apparatus, respectively. They want also to thank Dr. C. Creton, ESPCI, France, for fruitful discussions. This work was supported by the CNRS within the framework of the research program: “Programme Mate´riaux”. LA9915670
