Adhesion between Chemically Heterogeneous SwitchablePolymeric Brushes and an Elastomeric Adhesive

Haris Retsos,† Ganna Gorodyska,‡ Anton Kiriy,‡ Manfred Stamm,‡ andCostantino Creton*,†

Physico-Chimie des Polymeres et des Milieux Disperses, UMR 7615, Ecole Superieure Physiqueet de Chimie Industrielles, 10, rue Vauquelin, 75231 Paris, France, and Leibniz-Institut fur

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 end-functionalized polystyrene (PS) and poly(2-vinylpyridine) (P2VP) chains. The molecular organization ofthe 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 mannerin which the adhesion switching occurs can be tuned by the composition of mixed brushes. However, theouter surface composition could be enriched more effectively in PS after the toluene treatment than inP2VP after the acidic water treatment. As a result, the mixed brush compositions that showed the largestdifference in properties between an exposure to toluene and an exposure to water were the P2VP-richcompositions. Adhesive properties, tested against a soft hydrophobic pressure-sensitive adhesive (PSA)using a probe test, always showed smaller differences between solvent treatments than wetting propertieswith water, suggesting a much higher sensitivity of the hydrophobic/hydrophilic brushes to polar moleculesthan to nonpolar molecules.

Introduction

Adhesion at polymer/polymer and polymer/solid inter-faces is of great importance for numerous applicationsfrom microelectronics to the aircraft industry.1,2 It isessential for permanent, as well as for reversible, adhesionthat the chemical composition and morphology of thematerials at the interfaces are perfectly controlled.

The purpose of the present work is to investigate thedevelopment of a new generation of adaptive surfaces.These surfaces, based on chemically heterogeneous swit-chable thin polymer films, are covalently bonded to solidsubstrates, therefore allowing us to modify their surfacechemistry in a well-controlled and reproducible way.3-6

Such films were fabricated from two incompatible end-functional homopolymers (mixed polymer brush)6 termi-nally tethered from one of their ends to properly modifiedsolid substrates,6,7 using the technologically simple “graft-ing to” approach of polymer deposition. This method leadsto polymer brushes of relatively low grafting densitieswhich are, however, sufficiently dense to cause a signifi-cant altering of the surface properties. These mixed

(binary) brushes undergo phase segregation, dependingon the environment, resulting in a remarkable switchingof both morphology and surface energetic state.4-8

With such a binary system, investigations so far haveonly demonstrated a switching of surface morphology,wetting, swelling, and adsorption properties.4-8 Here, wereport on the switching of the adhesive properties of thebinary brush responding to external stimuli. This inves-tigation aims to study the influence of the switchingphenomenon of mixed brushes on the strength of theinterface that such a surface can form with a model softadhesive. This class of adhesives is gaining increasinginterest in industry and medicine for its low toxicity andease of use. Knowing the main requirements for suchadhesives 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 toλ/4, with the (100) crystal planes parallel to the surface wereused 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. Detailsabout the brush layer preparation have been presented exten-sively in other publications.6-8 For the present work we haveused a series of six different grafted PS/P2VP layers, with variousratios, 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

* Corresponding author. Tel: +33140794683. Fax:+33140794686. E-mail: Costantino.Creton@espci.fr.

† Ecole Superieure Physique et de Chimie Industrielles.‡ Leibniz-Institut fur Polymerforschung Dresden.(1) Creton, C.; Fabre, P. I.; Dillard, D. A.; Pocius, A. V. Adhesion

Science and Engineering, The mechanics of adhesion; Elsevier: Am-sterdam, 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.; Muller, 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.; Muller, 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.

(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.

7722 Langmuir 2005, 21, 7722-7725

10.1021/la050054s CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 07/23/2005

(polystyrene-polyisoprene-polystyrene) triblock copolymer (Vec-tor 4100), MW ) 154 kg/mol (with 15.1 wt % styrene) and 60%of a C-5 hydrogenated resin, completely miscible with the isoprenephase, both provided by the ExxonMobil Chem. More details onthe adhesive can be found in previous publications.9

Probe Tack Tests. We performed probe test experiments onour custom-designed apparatus, built on an MTS 810 hydraulictesting machine.10 A typical probe test can be divided into threestages. First, a cylindrical stainless steel probe with a siliconwafer coated with the binary brush film glued on its endapproaches and comes in contact with the soft adhesive layerdeposited on a glass microscope slide. When a maximum contactpressure of 1 MPa is reached, the probe then stops for a contacttime of 1 s. Finally, the probe is removed with a constantdebonding velocity of 10 µm/s, applying a traction force on theadhesive layer until a complete detachment and separation ofthe probe from the layer occurs. Events occurring at the adhesive/brush interface were recorded with a CCD camera and syn-chronized with the stress/strain curve. Values of the maximumarea of contact were determined by inspection of the imagesobtained during the compression stage. The interpretation ofthe 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 theapplied pressure, elastic modulus, and surface roughness of theprobe.12 If the surface of the probe or of the films has some degreeof roughness, pockets of air can remain trapped during thecompressionstageandactas germs for cavities during the tractionstage.

The debonding begins with initiation of the failure processthrough the formation of cavities.13 This cavitation process,however, is not sensitive to the composition of the surface unlessthe adhesive interactions are extremely weak. In other words,the peak tensile stress of the probe test curve will be dependentonly on the properties of the adhesive. The foam formed bycavities14 is then stretched in the tensile direction, and finallythe separation of the two surfaces is achieved by the detachmentof the feet of the cavity walls. During the stretching process, thecavity walls store elastic energy, which they subsequently releasewhen the walls detach from the probe surface. Hence, the moreextension 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 aboutthe differences in adhesive properties between the elastomerand the various binary brush coatings are visible at the end ofthe experimental curves, when the cavity walls (fibrils) start todetach from the brush layer until the complete failure.

ContactAngleMeasurements. Over the years, contact anglemeasurements have proven to be a very powerful and versatiletechnique to examine the wetting of switchable polymerfilms.3-8,16-19 An extensive series of static advancing contact anglemeasurements were carried out before and after every probetest, to verify the surface composition of the samples and provethe reversibility, switchability, and stability of all the binarybrushes. A Tracker model of I.T. Concept is the apparatus wehave used for the wetting measurements, while the dropletsdeposited on the brush layers were pH 5.5 deionized water(Millipore, resistivity ) 18.2 MΩ cm). The droplets weremonitored by a CCD camera and analyzed by drop-shape analysismethods. The complete profile of the sessile droplet has beenfitted by the tangent method to a general conic section equation.The derivative of this equation at the baseline gives the slope atthe three-phase contact point and thus the contact angle. In thisway, the contact angles are determined both on the right and theleft side, and therefore reproducibility is within 0.5°.

Results and Discussion

Appropriate solvents have been used to modify theorganization of the binary heterogeneous brush layers.Toluene as a selective solvent for PS gives a top layermuch richer in PS than in P2VP. To make the surface ofthe binary brush layer more hydrophilic, we expose it toa selective solvent for P2VP, such as ethanol; to furtherimprove the hydrophilicity, we also immerse the mixedbrush in acidic water (pH 2). The P2VP after this treatmentbecomes ionized and can remain at this state for severalhours after drying the sample, as systematic water contactangle measurements have shown. To return to thehydrophobic state requires theneutralizationof the ionizedstate of P2VP by immersion in basic water (pH 10) beforethe toluene treatment. The switch from the hydrophobicto the hydrophilic state and vice versa is schematicallydescribed in Figure 1 and was proven by static contactangle measurements of water droplets on the samplesurface, after drying the brush layers with a nitrogen flow.The contact angle measurements for a series of variousbrush compositions (Table 1) show clearly the switchablewetting behavior of such a binary system after selectivesolvent treatments (Figure 2). It is remarkable that aftertoluene treatment, even for very low PS concentrations(∼7%), the water droplets have a very high contact anglewith water, very close to the contact angle found for purePS. This result suggests a preferential segregation ofpolystyrene to the top layer. On the other hand, afterethanol exposure and ionization of the P2VP, the contactangles vary more or less linearly with the P2VP concen-tration of the binary thin film.

(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 andAdhesive Properties; Ph. D., University Paris VI, 2004.

(16) Ulman, A. ‘An Introduction to Ultrathin Organic Films fromLangmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.

(17) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science1997, 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.

Table 1. Spatial Characteristics of Polystyrene andPoly(2-vinylpyridine) Polymer Brushes Chemically

Grafted on a Silicon Wafer

PS-COOH thickness P2VP-COOH thickness

sample nm % nm %

P2VP 100 0 0 7.59 100PS 7/P2VP 93 0.61 6.51 8.76 93.49PS 14/P2VP 86 1.06 13.47 6.81 86.53PS 31/P2VP 69 2.54 31.20 5.60 68.80PS 41/P2VP 59 3.48 41.01 5.01 58.99PS 53/P2VP 47 4.79 53.34 4.19 46.66PS 100 14.87 100 0 0

Figure 1. Schematic description of the switching scenario fromthe hydrophilic to the hydrophobic state and vice versa, byusing the appropriate selective solvents.

Adhesive Behavior of a Dual Polymer Brush Langmuir, Vol. 21, No. 17, 2005 7723

The main purpose of this work was to investigate thedifferences in adhesion after the solvent treatments onthis series of binary brushes with various surface ratiosof PS and P2VP. To establish the experimental windowfor the two extreme cases in adhesion and wetting, wepresent in Figure 3a the stress/strain curves and thecontact angles for a pure PS and for a protonated P2VPbrush layer. The stress/strain curves obtained from probetests are actually normalized force vs separation curveswhere the stress is the force divided by the initial area ofcontact and the strain is defined as ε ) (h - h0)/h0, whereh0 is the initial thickness of the adhesive film and h is theactual thickness. For these materials,3,14,15,20 the plateauin stress after the first peak is due to the formation ofbridging adhesive fibrils between the glass substrate andthe probe.21 These fibrils start to detach when the plateaustress starts to decrease. Hence, the two important

parameters that characterize the degree of adhesion here,apart from the practical work of detachment (the normal-ized integral under the stress/strain curve), are the valueof ε where detachment starts (εstart

deb ) and the value whereit is completed (εstop

deb ). In terms of adhesion, the longfibrillation plateau of the adhesive when it is debondedfrom the pure PS sample is a signature of the good adhesionwith the hydrophobic elastomer, in contrast with the caseof pure ionized P2VP.

The average strain level at which the fibrils of theadhesive start to debond, εstart

deb , from the P2VP solid brushlayer is 1.90, in comparison with a much higher value of2.85 for the PS. The same shift is also observed for theεstart

deb strain of complete debonding, which for P2VP is 3while for PS it reaches 6. The good water wetting of theprotonated P2VP due to its polarity gives a 40° contactangle in contrast with the poor wetting on the hydrophobicPS, giving a high contact angle of 85°.

Probe tests and contact angle measurements have beenmade for four different brushes with PS/P2VP ratiosvarying from ∼14 to ∼50% PS, after toluene or ethanolfollowed by acid water treatments. We present in Figure3b,c the data from the PS14/P2VP86 and PS31/P2VP69samples, respectively, to underline the switching effect inadhesion after appropriate solvent treatments. The ex-perimental results of εstart

deb , εstopdeb , and contact angles (C.A.)

presented in Table 2 for all the samples are reproduciblewithin experimental error ((0.1 for ε, (0.05 for σ, and(2° for C.A.) and show that adhesion properties can bereversibly switched by exposure of the sample to differentsolvents. Furthermore, the maximum of the adhesionswitching can be systematically shifted from lower valuesof ε (switching of the εstop

deb between 2.85 and 4.15 for ∼14%of PS) to higher values of ε (switching of the εstop

deb between4.55 and 5.45 for ∼41% of PS) by increasing the PS contentof mixed brushes. The results also demonstrate both theinsensitivity of the adapting process to the sample historyand the remarkable stability of such a brush layer evenafter several probe tests where a hydrophobic adhesive ispressed in contact with 1 MPa of pressure.

Our results are best presented by first directly compar-ing the adhesion tests and the contact angle measurementson the same surfaces. In Figure 4, the εstop

deb from all thebinary brush mixtures are plotted. If we consider thosesurfaces after they have been treated by toluene, thecontact angle is the most sensitive to the presence of PS.In contrast to the very low (∼7%) PS concentration inwetting experiments (Figure 2), the adhesive propertiesbecome identical to those of pure PS for an average PScomposition of only ∼31%.

If we now consider the surface after it has beenconditioned with acidic water, the picture is remarkablydifferent. In this case, the contact angle varies continu-ously when the average fraction of PS in the brushincreases, and only for 7% PS is the effect of the PScompletely invisible. Adhesion is once again less sensitiveand becomes equivalent to that for pure P2VP with 14%PS, while the surface hydrophilicity that the P2VP canprovide to the layer is screened when its fraction is nolonger the majority at the thin film.

To summarize the adhesion data, we present in Figure4 the εstop

deb from all the samples to illustrate the switchingadhesion behavior of the brushes after preferential solventtreatments as a function of the PS average concentrationof the thin film.

The first message from all these results is clear:although the changes are measurable and significant, thesurface composition of the brushes is not changed all that

(20) Chiche, A. In Decollement d’un Adhesif Souple: Rupture etCavitation; Ph. D., University Denis Diderot - Paris VII, 2003.

(21) Zosel, A. J. Adhes. 1989, 30, 135.

Figure 2. Distilled water advancing contact angle measure-ments for a series of binary brushes with various compositionsfrom pure PS to pure P2VP after toluene (O) and ethanol, acidicwater (b) treatment.

Figure 3. Stress/strain curves from probe tack tests betweena 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/PVP69 (c) brush thin film. Static water droplets on those modelsurfaces are included as insets.

7724 Langmuir, Vol. 21, No. 17, 2005 Retsos et al.

much, or at least not enough to cause dramatic adhesionchanges to a hydrophobic adhesive by the exposure topreferential solvents. In all cases, PS prefers to be on thesurface, and an exposure to acidic water only reducesmoderately the PS concentration on the surface. As aresult, if the average composition of the brush is PS-rich,no switchability at all is observed in adhesion and onlya moderate switchability is observed in C.A. Only theP2VP-rich brush with 14% content of PS displays areasonable change in properties, both in contact anglesand in adhesive properties.

The second important message of these results is thatthe sensitivity to surface composition of adhesion andwetting experiments is different. In a general sense, thisdecoupling between wetting and adhesion has beenreported previously but always when comparing surfaceswith widely different surface mobilities.22,23 In our case,both polymers in the brush are glassy and should have avery limited mobility.

Of course, one should take into account that thecomparison, unfortunately, is made only between a polarinterrogating liquid (water) for wetting and a hydrophobicelastomer for adhesion, something that involves a differentkind of interaction with the treated binary films. Thischoice was originally made because hydrophobic adhesivesare the most widely used pressure-sensitive adhesives in

the industry, while the water contact angle measurementis the standard tool used to check the hydrophobic/hydrophilic behavior of a surface. However, given theseresults, we also placed diiodomethane (a nonpolar buthigh-surface-tension liquid) droplets on our brushes, andindeed the two extreme cases showed much less differencein contact angles than water. For pure PS, the contactangle was 46°; for pure ionized P2VP the contact anglewas 58°. This difference is not large and may explain whywe observe only moderate differences in adhesion betweena hydrophilic and a hydrophobic brush layer against ahydrophobic adhesive.

Concluding Remarks

Both the wetting with water and the adhesive behavioragainst a soft hydrophobic pressure-sensitive adhesive ofa dual polymer brush made of grafted PS and P2VP chainswere investigated. Exposure of the brush to selectivesolvents for PS and for P2VP modifies its organizationand outer surface composition in a reproducible andreversible way that results in reversible switching of theadhesion properties.

For both wetting and adhesion, the highest switchabilityin properties is observed for brushes which are P2VP-rich, suggesting that the outer surface composition is PS-enriched for all compositions.

Adhesive properties seem to be more sensitive todifferent features of the brush composition than the watercontact angle experiments. Clearly, more work is neededon model systems to better understand the subtle inter-actions taking place at the interface between a brush withmixed composition and an adhesive. In particular, we haveused brushes which are fully glassy and well below theirglass transition temperature. It may be interesting toinvestigate brushes with glassy and elastomeric compo-nents and also to use hydrophilic adhesives. Such inves-tigations are currently under way.

Acknowledgment. The authors are grateful forfinancial support from the “Centre National de la Re-cherche Scientifique” (CNRS) and the “Deutsche Fors-chungsgemeinschaft” (DFG) within the DFG/CNRS Ger-man-French bilateral program (STA 324/13).

LA050054S

(22) Zhang Newby, B.-M.; Chaudhury, M. K.; Brown, H. R. Science1995, 269, 1407. Zhang Newby, B. M.; Chaudhury, M. K. Langmuir1997, 13, 1805.

(23) Blum, F. D.; Gandhi, B. C.; Forciniti, D.; Dharani, L. R.Macromolecules 2005, 38, 481.

Table 2. Experimental Results Extracted from Probe Tests and Contact Angle Measurements for a Series of PS/P2VPBrushes with Different Surface Ratiosa

samples [εstartdeb ]Toluene [εstop

deb ]Toluene [C.A.]WaterToluene [εstart

deb ]pH2, WaterEthanol [εstop

deb ]pH2, WaterEthan ol [C.A.]Water

Ethanol pH2 Water

P2VP 100 60° 1.90 3.00 40°PS 14/P2VP 86 1.90 4.15 85° 1.45 2.85 55°PS 31/P2VP 69 3.10 5.50 85° 2.95 4.70 67°PS 41/P2VP 59 3.05 5.45 85° 2.75 4.55 65°PS 53/P2VP 47 3.50 6.10 85° 3.50 6.00 70°PS 100 2.85 6.00 85° 85°

a εstartdeb is the strain at which the fibrils start to debond from the brush layer, and εstop

deb is the strain of complete debonding. C.A. representsthe static contact angles of water droplets. The experimental errors are (0.1 and (2° for ε and C.A., respectively.

Figure 4. Experimental results of εstopdeb for a series of binary

brushes with various compositions from pure PS to pure P2VPafter toluene (O) and ethanol, acidic water (b) treatment.

Adhesive Behavior of a Dual Polymer Brush Langmuir, Vol. 21, No. 17, 2005 7725