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Adhesion between Chemically Heterogeneous Switchable Polymeric Brushes and an Elastomeric Adhesive Haris Retsos,† Ganna Gorodyska,‡ Anton Kiriy,‡ Manfred Stamm,‡ and Costantino Creton*,† Physico-Chimie des Polyme` res et des Milieux Disperse´ s, UMR 7615, Ecole Supe´ rieure Physique et de Chimie Industrielles, 10, rue Vauquelin, 75231 Paris, France, and Leibniz-Institut fu¨ r Polymerforschung Dresden, Hohe Strasse 6, 01069 Dresden, Germany Received January 7, 2005. In Final Form: April 5, 2005 We investigated the adhesive properties of binary heterogeneous polymer brushes made from endfunctionalized polystyrene (PS) and poly(2-vinylpyridine) (P2VP) chains. The molecular organization of the mixed brush could be varied reversibly by exposure to selective solvents for PS (toluene) and for P2VP (acidic water). This exposure results in reversible switching of adhesive and wetting properties. The manner in which the adhesion switching occurs can be tuned by the composition of mixed brushes. However, the outer surface composition could be enriched more effectively in PS after the toluene treatment than in P2VP after the acidic water treatment. As a result, the mixed brush compositions that showed the largest difference in properties between an exposure to toluene and an exposure to water were the P2VP-rich compositions. Adhesive properties, tested against a soft hydrophobic pressure-sensitive adhesive (PSA) using a probe test, always showed smaller differences between solvent treatments than wetting properties with water, suggesting a much higher sensitivity of the hydrophobic/hydrophilic brushes to polar molecules than to nonpolar molecules.
Introduction Adhesion at polymer/polymer and polymer/solid interfaces is of great importance for numerous applications from microelectronics to the aircraft industry.1,2 It is essential for permanent, as well as for reversible, adhesion that the chemical composition and morphology of the materials at the interfaces are perfectly controlled. The purpose of the present work is to investigate the development of a new generation of adaptive surfaces. These surfaces, based on chemically heterogeneous switchable thin polymer films, are covalently bonded to solid substrates, therefore allowing us to modify their surface chemistry in a well-controlled and reproducible way.3-6 Such films were fabricated from two incompatible endfunctional homopolymers (mixed polymer brush)6 terminally tethered from one of their ends to properly modified solid substrates,6,7 using the technologically simple “grafting to” approach of polymer deposition. This method leads to polymer brushes of relatively low grafting densities which are, however, sufficiently dense to cause a significant altering of the surface properties. These mixed * Corresponding author. Tel: +33140794683. Fax: +33140794686. E-mail:
[email protected]. † Ecole Supe ´ rieure Physique et de Chimie Industrielles. ‡ Leibniz-Institut fu ¨ r Polymerforschung Dresden. (1) Creton, C.; Fabre, P. I.; Dillard, D. A.; Pocius, A. V. Adhesion Science and Engineering, The mechanics of adhesion; Elsevier: Amsterdam, 2002; p 535. (2) Creton, C. MRS Bull. 2003, 434. (3) Levicky, R.; Koneripalli, N.; Tirrel, M. Macromolecules 1998, 31, 2616. (4) Ionov, L.; Minko, S.; Stamm, M.; Gohy, J. F.; Jerome, R.; Scholl, A. J. Am. Chem. Soc. 2003, 125, 8302. (5) Minko, S.; Mu¨ller, M.; Motornov, M.; Nitscke, M.; Grundke, K.; Usov, D.; Scholl, A.; Luchnikov, V.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896. (6) Minko, S.; Mu¨ller, M.; Usov, D.; Scholl, A.; Froeck, C.; Stamm, M. Phys. Rev. Lett. 2002, 88 (3), 035502-1. (7) Sidorenko, A.; Minko, S.; Schenk-Meuser, K.; Duschner, H.; Stamm, M. Langmuir 1999, 15, 8349. Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K. J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289.
(binary) brushes undergo phase segregation, depending on the environment, resulting in a remarkable switching of both morphology and surface energetic state.4-8 With such a binary system, investigations so far have only demonstrated a switching of surface morphology, wetting, swelling, and adsorption properties.4-8 Here, we report on the switching of the adhesive properties of the binary brush responding to external stimuli. This investigation aims to study the influence of the switching phenomenon of mixed brushes on the strength of the interface that such a surface can form with a model soft adhesive. This class of adhesives is gaining increasing interest in industry and medicine for its low toxicity and ease of use. Knowing the main requirements for such adhesives provides the capability of generating reversible, fast, and easy-adhering bonds to different thin coatings (thin films) in a controlled environment. Experimental Section Materials. Silicon disks, 10 mm in diameter and polished to λ/4, with the (100) crystal planes parallel to the surface were used as substrates to build the binary polymer brushes using the “grafting to” method. Both end-functionalized PS-COOH (MW ) 48.4 kg/mol, PDI ) 1.05) and P2VP-COOH (MW ) 41.5 kg/ mol, PDI ) 1.06) were purchased from Polymer Source Inc. Details about the brush layer preparation have been presented extensively in other publications.6-8 For the present work we have used a series of six different grafted PS/P2VP layers, with various ratios, from pure PS to pure P2VP (Table 1). The hydrophobic pressure-sensitive-adhesive (PSA) layer, approximately 100-µm thick, is a blend from 40% of a symmetric (8) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33, 1043. Minko, S.; Luzinov, I.; Luchnikov, V.; Muller, M.; Patil, S.; Stamm, M. Macromolecules 2003, 36, 7268. Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677. Ruhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Grohn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Adv. Polym. Sci. 2004, 165, 79. Luzinov, I.; Minko, S.; Tsukruk, V. V. Prog. Polym. Sci. 2004, 29, 635. Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950.
10.1021/la050054s CCC: $30.25 © 2005 American Chemical Society Published on Web 07/23/2005
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Table 1. Spatial Characteristics of Polystyrene and Poly(2-vinylpyridine) Polymer Brushes Chemically Grafted on a Silicon Wafer PS-COOH thickness P2VP-COOH thickness sample
nm
%
nm
%
P2VP 100 PS 7/P2VP 93 PS 14/P2VP 86 PS 31/P2VP 69 PS 41/P2VP 59 PS 53/P2VP 47 PS 100
0 0.61 1.06 2.54 3.48 4.79 14.87
0 6.51 13.47 31.20 41.01 53.34 100
7.59 8.76 6.81 5.60 5.01 4.19 0
100 93.49 86.53 68.80 58.99 46.66 0
(polystyrene-polyisoprene-polystyrene) triblock copolymer (Vector 4100), MW ) 154 kg/mol (with 15.1 wt % styrene) and 60% of a C-5 hydrogenated resin, completely miscible with the isoprene phase, both provided by the ExxonMobil Chem. More details on the adhesive can be found in previous publications.9 Probe Tack Tests. We performed probe test experiments on our custom-designed apparatus, built on an MTS 810 hydraulic testing machine.10 A typical probe test can be divided into three stages. First, a cylindrical stainless steel probe with a silicon wafer coated with the binary brush film glued on its end approaches and comes in contact with the soft adhesive layer deposited on a glass microscope slide. When a maximum contact pressure of 1 MPa is reached, the probe then stops for a contact time of 1 s. Finally, the probe is removed with a constant debonding velocity of 10 µm/s, applying a traction force on the adhesive layer until a complete detachment and separation of the probe from the layer occurs. Events occurring at the adhesive/ brush interface were recorded with a CCD camera and synchronized with the stress/strain curve. Values of the maximum area of contact were determined by inspection of the images obtained during the compression stage. The interpretation of the debonding mechanisms of such a PSA have been reviewed11 recently, and we outline here only the main features. For such an elastic pressure-sensitive adhesive, the bonding of the adherent to the probe surface essentially depends on the applied pressure, elastic modulus, and surface roughness of the probe.12 If the surface of the probe or of the films has some degree of roughness, pockets of air can remain trapped during the compression stage and act as germs for cavities during the traction stage. The debonding begins with initiation of the failure process through the formation of cavities.13 This cavitation process, however, is not sensitive to the composition of the surface unless the adhesive interactions are extremely weak. In other words, the peak tensile stress of the probe test curve will be dependent only on the properties of the adhesive. The foam formed by cavities14 is then stretched in the tensile direction, and finally the separation of the two surfaces is achieved by the detachment of the feet of the cavity walls. During the stretching process, the cavity walls store elastic energy, which they subsequently release when the walls detach from the probe surface. Hence, the more extension the walls experience before detaching from the surface, the stronger the probe/adhesive interactions are.15 In our particular set of experiments, all the information about the differences in adhesive properties between the elastomer and the various binary brush coatings are visible at the end of the experimental curves, when the cavity walls (fibrils) start to detach from the brush layer until the complete failure. (9) Roos, A.; Creton, C. Macromol. Symp. 2004, 214, 147. Daoulas, K.; Theodorou, D. N.; Roos, A.; Creton, C. Macromolecules 2004, 37, 5093. (10) Lakrout, H.; Sergot, P.; Creton, C. J. Adhes. 1999, 69, 307. (11) Shull, K. R.; Creton, C. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 4023. (12) Dahlquist, C. A. Treatise on Adhesion and Adhesives; Patrick, R. L., Ed.; Dekker: New York, 1969; Vol. 2, p 219. Creton, C.; Leibler, L. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 545. (13) Chikina, I.; Gay, C. Phys. Rev. Lett. 2000, 85, 4546. Chiche, A.; Dollhofer, J.; Creton, C. Eur. Phys. J., in press. (14) Brown, K.; Hooker, J. C.; Creton, C. Macromol. Mater. Eng. 2002, 287, 163. (15) Roos, A. Sticky Block Copolymers: Structure Rheology and Adhesive Properties; Ph. D., University Paris VI, 2004.
Figure 1. Schematic description of the switching scenario from the hydrophilic to the hydrophobic state and vice versa, by using the appropriate selective solvents. Contact Angle Measurements. Over the years, contact angle measurements have proven to be a very powerful and versatile technique to examine the wetting of switchable polymer films.3-8,16-19 An extensive series of static advancing contact angle measurements were carried out before and after every probe test, to verify the surface composition of the samples and prove the reversibility, switchability, and stability of all the binary brushes. A Tracker model of I.T. Concept is the apparatus we have used for the wetting measurements, while the droplets deposited on the brush layers were pH 5.5 deionized water (Millipore, resistivity ) 18.2 MΩ cm). The droplets were monitored by a CCD camera and analyzed by drop-shape analysis methods. The complete profile of the sessile droplet has been fitted by the tangent method to a general conic section equation. The derivative of this equation at the baseline gives the slope at the three-phase contact point and thus the contact angle. In this way, the contact angles are determined both on the right and the left side, and therefore reproducibility is within 0.5°.
Results and Discussion Appropriate solvents have been used to modify the organization of the binary heterogeneous brush layers. Toluene as a selective solvent for PS gives a top layer much richer in PS than in P2VP. To make the surface of the binary brush layer more hydrophilic, we expose it to a selective solvent for P2VP, such as ethanol; to further improve the hydrophilicity, we also immerse the mixed brush in acidic water (pH 2). The P2VP after this treatment becomes ionized and can remain at this state for several hours after drying the sample, as systematic water contact angle measurements have shown. To return to the hydrophobic state requires the neutralization of the ionized state of P2VP by immersion in basic water (pH 10) before the toluene treatment. The switch from the hydrophobic to the hydrophilic state and vice versa is schematically described in Figure 1 and was proven by static contact angle measurements of water droplets on the sample surface, after drying the brush layers with a nitrogen flow. The contact angle measurements for a series of various brush compositions (Table 1) show clearly the switchable wetting behavior of such a binary system after selective solvent treatments (Figure 2). It is remarkable that after toluene treatment, even for very low PS concentrations (∼7%), the water droplets have a very high contact angle with water, very close to the contact angle found for pure PS. This result suggests a preferential segregation of polystyrene to the top layer. On the other hand, after ethanol exposure and ionization of the P2VP, the contact angles vary more or less linearly with the P2VP concentration of the binary thin film. (16) Ulman, A. ‘An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (17) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458. (18) Genzer, J.; Efimenco, K. Science 2000, 290, 2130. (19) Anastasiadis, S. H.; Retsos, H.; Pispas, S.; Hadjichristidis, N.; Neophytides, S. Macromolecules 2003, 36, 1994.
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Figure 2. Distilled water advancing contact angle measurements for a series of binary brushes with various compositions from pure PS to pure P2VP after toluene (O) and ethanol, acidic water (b) treatment.
Figure 3. Stress/strain curves from probe tack tests between a 100-µm thick SIS/C-5 elastomeric layer and pure PS and P2VP (in protonated state) (a), a PS 14/PVP 86 (b), and a PS 31/PVP 69 (c) brush thin film. Static water droplets on those model surfaces are included as insets.
The main purpose of this work was to investigate the differences in adhesion after the solvent treatments on this series of binary brushes with various surface ratios of PS and P2VP. To establish the experimental window for the two extreme cases in adhesion and wetting, we present in Figure 3a the stress/strain curves and the contact angles for a pure PS and for a protonated P2VP brush layer. The stress/strain curves obtained from probe tests are actually normalized force vs separation curves where the stress is the force divided by the initial area of contact and the strain is defined as ) (h - h0)/h0, where h0 is the initial thickness of the adhesive film and h is the actual thickness. For these materials,3,14,15,20 the plateau in stress after the first peak is due to the formation of bridging adhesive fibrils between the glass substrate and the probe.21 These fibrils start to detach when the plateau stress starts to decrease. Hence, the two important (20) Chiche, A. In Decollement d’un Adhesif Souple: Rupture et Cavitation; Ph. D., University Denis Diderot - Paris VII, 2003. (21) Zosel, A. J. Adhes. 1989, 30, 135.
Retsos et al.
parameters that characterize the degree of adhesion here, apart from the practical work of detachment (the normalized integral under the stress/strain curve), are the value of where detachment starts (deb start) and the value where ) . In terms of adhesion, the long it is completed (deb stop fibrillation plateau of the adhesive when it is debonded from the pure PS sample is a signature of the good adhesion with the hydrophobic elastomer, in contrast with the case of pure ionized P2VP. The average strain level at which the fibrils of the adhesive start to debond, deb start, from the P2VP solid brush layer is 1.90, in comparison with a much higher value of 2.85 for the PS. The same shift is also observed for the deb start strain of complete debonding, which for P2VP is 3 while for PS it reaches 6. The good water wetting of the protonated P2VP due to its polarity gives a 40° contact angle in contrast with the poor wetting on the hydrophobic PS, giving a high contact angle of 85°. Probe tests and contact angle measurements have been made for four different brushes with PS/P2VP ratios varying from ∼14 to ∼50% PS, after toluene or ethanol followed by acid water treatments. We present in Figure 3b,c the data from the PS14/P2VP86 and PS31/P2VP69 samples, respectively, to underline the switching effect in adhesion after appropriate solvent treatments. The exdeb perimental results of deb start, stop, and contact angles (C.A.) presented in Table 2 for all the samples are reproducible within experimental error ((0.1 for , (0.05 for σ, and (2° for C.A.) and show that adhesion properties can be reversibly switched by exposure of the sample to different solvents. Furthermore, the maximum of the adhesion switching can be systematically shifted from lower values of (switching of the deb stop between 2.85 and 4.15 for ∼14% of PS) to higher values of (switching of the deb stop between 4.55 and 5.45 for ∼41% of PS) by increasing the PS content of mixed brushes. The results also demonstrate both the insensitivity of the adapting process to the sample history and the remarkable stability of such a brush layer even after several probe tests where a hydrophobic adhesive is pressed in contact with 1 MPa of pressure. Our results are best presented by first directly comparing the adhesion tests and the contact angle measurements on the same surfaces. In Figure 4, the deb stop from all the binary brush mixtures are plotted. If we consider those surfaces after they have been treated by toluene, the contact angle is the most sensitive to the presence of PS. In contrast to the very low (∼7%) PS concentration in wetting experiments (Figure 2), the adhesive properties become identical to those of pure PS for an average PS composition of only ∼31%. If we now consider the surface after it has been conditioned with acidic water, the picture is remarkably different. In this case, the contact angle varies continuously when the average fraction of PS in the brush increases, and only for 7% PS is the effect of the PS completely invisible. Adhesion is once again less sensitive and becomes equivalent to that for pure P2VP with 14% PS, while the surface hydrophilicity that the P2VP can provide to the layer is screened when its fraction is no longer the majority at the thin film. To summarize the adhesion data, we present in Figure 4 the deb stop from all the samples to illustrate the switching adhesion behavior of the brushes after preferential solvent treatments as a function of the PS average concentration of the thin film. The first message from all these results is clear: although the changes are measurable and significant, the surface composition of the brushes is not changed all that
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Table 2. Experimental Results Extracted from Probe Tests and Contact Angle Measurements for a Series of PS/P2VP Brushes with Different Surface Ratiosa samples
deb Toluene [start ]
deb Toluene [stop ]
Toluene [C.A.]Water
deb Ethanol [start ]pH2, Water
deb Ethan ol [stop ]pH2, Water
Ethanol pH2 Water [C.A.]Water
P2VP 100 PS 14/P2VP 86 PS 31/P2VP 69 PS 41/P2VP 59 PS 53/P2VP 47 PS 100
1.90 3.10 3.05 3.50 2.85
4.15 5.50 5.45 6.10 6.00
60° 85° 85° 85° 85° 85°
1.90 1.45 2.95 2.75 3.50
3.00 2.85 4.70 4.55 6.00
40° 55° 67° 65° 70° 85°
a deb is the strain at which the fibrils start to debond from the brush layer, and deb is the strain of complete debonding. C.A. represents start stop the static contact angles of water droplets. The experimental errors are (0.1 and (2° for and C.A., respectively.
Figure 4. Experimental results of deb stop for a series of binary brushes with various compositions from pure PS to pure P2VP after toluene (O) and ethanol, acidic water (b) treatment.
much, or at least not enough to cause dramatic adhesion changes to a hydrophobic adhesive by the exposure to preferential solvents. In all cases, PS prefers to be on the surface, and an exposure to acidic water only reduces moderately the PS concentration on the surface. As a result, if the average composition of the brush is PS-rich, no switchability at all is observed in adhesion and only a moderate switchability is observed in C.A. Only the P2VP-rich brush with 14% content of PS displays a reasonable change in properties, both in contact angles and in adhesive properties. The second important message of these results is that the sensitivity to surface composition of adhesion and wetting experiments is different. In a general sense, this decoupling between wetting and adhesion has been reported previously but always when comparing surfaces with widely different surface mobilities.22,23 In our case, both polymers in the brush are glassy and should have a very limited mobility. Of course, one should take into account that the comparison, unfortunately, is made only between a polar interrogating liquid (water) for wetting and a hydrophobic elastomer for adhesion, something that involves a different kind of interaction with the treated binary films. This choice was originally made because hydrophobic adhesives are the most widely used pressure-sensitive adhesives in (22) Zhang Newby, B.-M.; Chaudhury, M. K.; Brown, H. R. Science 1995, 269, 1407. Zhang Newby, B. M.; Chaudhury, M. K. Langmuir 1997, 13, 1805. (23) Blum, F. D.; Gandhi, B. C.; Forciniti, D.; Dharani, L. R. Macromolecules 2005, 38, 481.
the industry, while the water contact angle measurement is the standard tool used to check the hydrophobic/ hydrophilic behavior of a surface. However, given these results, we also placed diiodomethane (a nonpolar but high-surface-tension liquid) droplets on our brushes, and indeed the two extreme cases showed much less difference in contact angles than water. For pure PS, the contact angle was 46°; for pure ionized P2VP the contact angle was 58°. This difference is not large and may explain why we observe only moderate differences in adhesion between a hydrophilic and a hydrophobic brush layer against a hydrophobic adhesive. Concluding Remarks Both the wetting with water and the adhesive behavior against a soft hydrophobic pressure-sensitive adhesive of a dual polymer brush made of grafted PS and P2VP chains were investigated. Exposure of the brush to selective solvents for PS and for P2VP modifies its organization and outer surface composition in a reproducible and reversible way that results in reversible switching of the adhesion properties. For both wetting and adhesion, the highest switchability in properties is observed for brushes which are P2VPrich, suggesting that the outer surface composition is PSenriched for all compositions. Adhesive properties seem to be more sensitive to different features of the brush composition than the water contact angle experiments. Clearly, more work is needed on model systems to better understand the subtle interactions taking place at the interface between a brush with mixed composition and an adhesive. In particular, we have used brushes which are fully glassy and well below their glass transition temperature. It may be interesting to investigate brushes with glassy and elastomeric components and also to use hydrophilic adhesives. Such investigations are currently under way. Acknowledgment. The authors are grateful for financial support from the “Centre National de la Recherche´ Scientifique” (CNRS) and the “Deutsche Forschungsgemeinschaft” (DFG) within the DFG/CNRS German-French bilateral program (STA 324/13). LA050054S
