Article pubs.acs.org/Langmuir
Intelligent Materials with Adaptive Adhesion Properties Based on Comb-like Polymer Brushes A. Synytska,*,†,‡ E. Svetushkina,†,‡ D. Martina,§ C. Bellmann,† F. Simon,† L. Ionov,† M. Stamm,†,‡ and C. Creton*,§ †
Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, D-01069 Dresden, Germany Physical Chemistry of Polymer Materials, Technische Universität Dresden, 01062 Dresden, Germany § Laboratoire de Physico-Chimie des Polymeres et Milieux Disperses, CNRS-UPMC-ESPCI Paristech, 10 rue Vauquelin, 75231 Paris, Cedex 05, France ‡
ABSTRACT: We investigated the adaptive adhesion properties of comb-like random copolymer brushes made of poly(ethylene glycol) (PEG)−poly(dimethylsiloxane) (PDMS) grafted on flat and rough substrates. The properties of the brush layers were investigated using ARXPS, contact angle, electrokinetics, null ellipsometry, and adhesion measurements. It was found that hydrophobic PDMS segments segregate at the brush topmost layer in the dry state. However, hydrophilic PEG chains segregate at the brush topmost layer in the wet state. The adhesion properties of fabricated materials were tested using the AFM colloid probe technique and probe tack tester. It was found that the adhesive properties depend strongly on the mechanical properties (stiff/soft) and chemical functionality (hydrophobicity/hydrophilicity) of the applied adhesion tester as well as on the chemical composition, surface roughness, and thickness of the brush. In particular, hydrophobic PDMS and hydrophilic PEG adhere more strongly to hydrophobically modified and hydrophilic native colloid probes, respectively. Thick brushes are more adhesive than thin ones, and brushes grafted to flat substrates are stickier than those grafted to rough substrates when measured with a hard AFM probe. Unlike the results of adhesion measurements performed using hard AFM probes, the PDMS surface probed by soft pressuresensitive adhesives (PSA) is almost nonadhesive. However, PEG is strongly adhesive, and the adhesion increases with the PEG fraction in the brush when probed by both hydrophilic and hydrophobic soft adhesives. The surfaces roughness also has a considerable effect on adhesion. Contrary to the adhesion measurements performed by hard AFM colloid probes, the adhesion of rough surfaces measured with a soft PAA or SIS tack tester is greater than that on the corresponding flat one.
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films and allows the design of surfaces with tunable properties. Polymer brushes were demonstrated to be very promising for controlling biomolecular transport,10−12 sensor design,11,13 microfluidic devices,14 logical devices,15 functional coatings,16 cell culturing,17 patterning or proteins,18 and wetting control and adhesion.16 Recent efforts were focused on the design of responsive and adaptable polymer brushes, which are either biocompatible or both biocompatible and biodegradable.19 Poly(ethylene glycol) (PEG) and poly(dimethylsiloxane) (PDMS) are synthetic biocompatible polymers that are widely used in medicine, the food industry, and cosmetics. Both polymers have a low glasstransition temperature, and the polymer chain segments are mobile at room temperature. Recently, Minko et al.20 have exploited this effect to design smart mixed polymer brushes, which immediately adapt to changing environmental conditions. Because of very quick and dynamic molecular reorganization, PDMS-PEG brushes have a low level of
INTRODUCTION The control of adhesion is very important for many industrial processes,1 health care applications, and everyday use. Much effort has been directed toward the fabrication of materials with either weak or strong adhesion depending on the field of application. However, design of “smart” surfaces with reversibly controllable adhesion is still a very challenging task. Such materials can be of great importance for numerous applications ranging from microelectronics to pharmaceutical and medical applications. One way to tune adhesion is to change the geometry, a strategy that is implemented in natural adhesives of gecko lizards 2 and has been mimicked in synthetic equivalents.3,4 Another approach is to fabricate surfaces with controlled chemical functionality. In this approach, smart surfaces are generated from chemically heterogeneous thin polymer films covalently bonded to solid substrates, therefore allowing a modification of their surface chemistry in a wellcontrolled and reproducible way.5−7 Functionalization with polymer brushes, which are formed by polymer chains attached by one end to a substrate,8,9 represents a particular method of obtaining robust responsive surfaces. Anchoring to a substrate provides chemical stability for such © XXXX American Chemical Society
Received: September 19, 2012 Revised: October 29, 2012
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dx.doi.org/10.1021/la303773b | Langmuir XXXX, XXX, XXX−XXX
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Figure 1. Schematic chemical structures of random copolymer PEG-PDMS polymer brushes grafted to (a) flat and (b) rough substrates and (c) representative AFM image of a rough surface made of core−shell particles. APS particles were placed in 50 mL of anhydrous dichloromethane. Then 250 μL of α-bromoisobutyryl bromide and 500 μL of ethylenediamine were added to the prepared suspension. Samples were stored in the prepared solution for 2 h. Finally, modified samples were washed several times with dichloromethane, distilled water, and ethanol when dried under vacuum at 60 °C and used to graft polymers. Synthesis of Polymer Brushes. Polymer brushes were synthesized on flat, rough substrates using AGEP ATRP as described in ref 10. The thickness of the polymer layer on the particles was determined from the results of thermogravimetric analysis. Null Ellipsometry. The thickness of the polymer layers on the smooth substrates in the dry state was measured at λ = 632.8 nm and an angle of incidence of 70° with a null ellipsometer (Multiscope, Optrel Berlin, Germany) in a polarizer−compensator−sample− analyzer configuration as described elsewhere.23,24 Contact Angle Measurements. Advancing and receding water contact angles were measured by the sessile drop method using a conventional drop-shape analysis technique (Krüss DSA 10, Hamburg, Germany). Deionized reagent-grade water was used for contact angle measurements. Liquid droplets (10 μL) were dropped carefully onto the sample surface, and the average of five measurements, made at different positions on the same sample, was adopted as the average value of the contact angles of the substrates. The error of the mean contact angle values, calculated as the standard deviation, did not exceed 2 and 3°. All contact angle measurements were carried out at 24 ± 0.5 °C and a relative humidity of 40 ± 3%, which were kept constant. Electrokinetic Measurements. Electrokinetic measurements were performed as streaming potential experiments with an electrokinetic analyzer (EKA) by Anton Paar GmbH (Graz, Austria). Pairs of samples were mounted in a special rectangular cell (developed and constructed at the Leibniz Institute of Polymer Research, Dresden, Germany, for small, flat pieces) where the samples formed a thin streaming channel. To carry out measurements as a function of pH, the pH value of the measuring solution (10−3 mol·L−1 KCI) was adjusted with 0.1 mol·L−1 HCl or 0.1 mol·L−1 KOH, respectively. The apparent electrokinetic potential (zeta potential, ζ) values were calculated from the measured streaming potential values according to Smoluchowski’s equation. Details of the measuring technique are reported elsewhere.25−27 X-ray Photoelectron Spectroscopy (XPS). All XPS spectra were recorded by means of an Axis Ultra photoelectron spectrometer (Kratos Analytical, Manchester, U.K.). The spectrometer was equipped with a monochromatic Al Kα (hν = 1486.6 eV) X-ray source of 300 W at 15 kV. The kinetic energy of the photoelectrons was determined with a hemispheric analyzer set to a pass energy of 160 eV for wide-scan spectra and 20 eV for high-resolution spectra. To improve the surface sensitivity of the XPS method, wide-scan and high-resolution spectra were recorded from the polymer brush samples applied to silicon wafers at different takeoff angles (angle-resolved Xray photoelectron spectroscopy, ARXPS, with takeoff angles of Θ = 0, 60, and 75°). Here, the takeoff angle is defined as the angle between the sample’s surface normal and the electron-optical axis of the XPS spectrometer. The corresponding maximum information depths of the XPS method are 10 nm for measurements at Θ = 0°, 5 nm for measurements at Θ = 60°, and 2.5 nm for measurements at Θ = 75°.
adhesion to most surfaces in both wet and dry states that is intrinsic to PEG and PDMS in water and in air, respectively.21 In this article, we investigate the adhesion properties of more complex responsive/adaptive brushes formed by comb-like random PEG-PDMS copolymers. Finally, the adhesion of the hard and soft tack tester on smooth and rough hard surfaces with different chemical compositions will be compared.
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EXPERIMENTAL SECTION
Materials. Highly polished single-crystal silicon wafers (Semiconductor Processing Co.) were used as a substrate. Monomethacryloxypropyl-terminated PDMS (asymmetric 6−9 cSt PDMS, Gelest) and oligo(ethylene glycol) methyl ether methacrylate (Mn = 475 g/ mol, OEGMA, Aldrich) were purified by passage through an Al2O3 column. 3-Aminopropyltriethoxysilane (APS, ABCR), anhydrous dichloromethane (Aldrich), 2-bromo-2-methylpropionyl bromide (BMPB, Aldrich), triethylamine (Fluka), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDTA, 99%, Aldrich), ethyl-2-bromoisobutyrate (EBiB), copper bromide (Aldrich), and tin(II) 2-ethylhexanoate (Aldrich) were used as received. Fabrication of Microstructured Rough Surfaces from Core− Shell Particles. Modification of the Supporting Substrates. The planar silicon wafers were cleaned in an ultrasonic bath for 30 min, placed in a hot ammonium hydroxyl/peroxide solution (3:1 concentrated ammonium hydroxyl/30% hydrogen peroxide; the mixture reacts violently with organic solvents and should be handled with care) for 1 h, and then rinsed several times with Milli-Q water. The thickness of silicon oxide was measured to be 1.3 nm after the cleaning procedure. Then, PGMA was spin-coated from the 1 wt % solution in chloroform. Spin-coating was performed in two steps as follows: (1) rpm = 100, time = 11 s, R/s2 = 1900; (2) rpm = 2000, time = 30 s, R/s2 = 1900. Afterward, PGMA was annealed at 180 °C for 30 min in a vacuum oven. The thickness of the PGMA layers was measured to be about 80 nm. Preparation of Core−Shell Particles: Modification of Spherical Silica Particles by 3-Aminopropyltriethoxysilane (SP-APS). Dry spherical silica particles (SP) prepared using the Stöber approach as described in ref 22 and having a diameter of 1 μm (0.5 g) were added to a 1.5 mL solution of 3-aminopropyltriethoxysilane (APS) in 50 mL of ethanol and stirred for 4 h with a magnetic stirrer. Afterward, particles were centrifuged and washed several times with ethanol to remove nonreacted APS. Immobilization of Spherical Particles onto PGMA-Coated Substrates. A dispersion of SP-APS-coated particles in ethanol (10 wt %) was used for deposition. For this, SP-APS was spin-coated onto supported PGMA-modified silicon wafers in two steps: (1) rpm = 100, time = 11 s, R/s2 = 95; (2) rpm = 500−700, time = 50 s, R/s2 = 95. Afterward, prepared rough surfaces were annealed for 2 h at 150 °C in a vacuum oven in order to graft SP-APS chemically onto PGMAmodified surfaces. Notably, prepared particle-covered substrates were robust as proven by AFM topography evaluation in contact mode (images not presented). The quality of the surface covering was checked by AFM (Figure 1) and SEM (images not presented) measurements. Immobilization of the Initiator on Spherical Particles (SP-Br). Robust rough substrates with chemically grafted 1000 nm large SPB
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contact between the probe and the film, the probe and films were aligned as described in refs 34 and 31. During the test, the cylindrical flat-ended probe comes in contact with a soft adhesive layer previously deposited on a glass slide. After a set contact time, the probe is withdrawn at a constant velocity. A mirror is installed behind the glass slide and allows the visualization of the debonding mechanism and the measurement of the real contact area for each test.35 We used standard parameters for the temperature (room temperature), approach velocity (Vapp = 10 μm/s), contact time (tc = 1 s), and contact force (Fc = 70 N). The debonding rate was varied from 1 to 10 to 100 μm/s. The force was measured by a load cell (250 N, resolution 0.2 N). The displacement of the probe was measured with an LVDT extensometer (range 5 mm, resolution 0.5 μm). Events occurring at the adhesive/brush interface are recorded with a CCD camera and synchronized with the stress/strain curve. Values of the maximum area of contact were determined by an inspection of the images obtained during the compression stage. The force and displacement for each curve were normalized by the contact area of the probe (as visualized with the camera) and by the initial thickness of the film to obtain a nominal stress versus nominal strain curve, which will be used to compare adhesive films. The adhesion energy Wadh was defined as the integral under the stress versus strain curve multiplied by the initial thickness of the film. In our study, we used hard hydrophilic and hydrophobic colloid probes for AFM force measurements and soft model pressure-sensitive adhesive (PSA) layers for the probe tests. The more hydrophilic PSA used was an emulsion (poly(n-butyl acrylate) containing 2 wt % acrylic acid comonomer synthesized by emulsion polymerization and with a molecular weight distribution typical of emulsion acrylic (PSA).36,37The more hydrophobic soft adhesive was based on a blend of styrene-isoprene-styrene block copolymer and a commercial C5 resin (Escorez 5380).38,39 The model adhesives had a tunable elastic modulus and viscoelastic character for probe tack investigations. PSA films were prepared by water evaporation for the waterborne emulsion and by solvent evaporation (toluene) for the SIS-based polymer. We used a method previously developed to study the adhesion of functionalized surfaces to soft adhesives.32,40
During all measurements, the electrostatic charging of the sample was overcompensated for by means of a low-energy electron source working in combination with a magnetic immersion lens. Later, all recorded peaks were shifted by the same amount, which was necessary to set the C 1s peak equal to 285.00 eV for saturated hydrocarbons. Quantitative elemental compositions were determined from peak areas using experimentally determined sensitivity factors and the spectrometer transmission function. The spectrum background was subtracted according to Shirley. The high-resolution spectra were deconvolved by means of a computer routine (Kratos Analytical, Manchester, U.K.). Free parameters of component peaks were their binding energy (BE), height, full width at half-maximum, and the Gaussian−Lorentzian ratio. Adhesion Measurements by the AFM Colloidal Probe Technique. The adhesive properties between the surfaces and silica were investigated by means of AFM force−distance experiments using a spherical particle/colloidal probe (CP), R = 2420 nm. The hydrophobic particles were prepared by the modification of hydrophilic particles using 1H,1H,2H,2H-perfluorodecyltrichlorosilane. Colloidal probes were prepared by gluing dry silica spheres (Bang Laboratories, USA, mean diameter 4.8 μm) onto tipless native oxide NSC12 AFM cantilevers (MikroMasch, Estonia) by a micromanipulator using a two-component epoxy resin (UHU plus endfest 300, UHU GmbH, Germany). All details of the determination of the cantilever sensitivity and the distance between the probe and the surface are given elsewhere.28 The spring constant of each cantilever was determined before the gluing of the spheres using the thermal noise method,29 which was on the order of 7 to 12.5 N/m. The results of the method showed an error of up to 20%. The effect of the glued sphere on the spring constant was within this error limit. The diameter of the spheres was determined after the measurement from the scanning electron microscope images (Phenom, FEI Co., USA) to an accuracy of ±0.05 μm.28,30 Interaction forces were measured in a MultiMode AFM with a NanoScope III Controller (Veeco Instruments, Inc., USA) equipped with a closed fluid cell (contact mode fluid cell) at room temperature in dry and aqueous media (distilled water). The samples were equilibrated in cells over 30 min before the measurement. In all measurements, AFM probes were first brought into contact with the substrate. When the AFM cantilever was withdrawn from the contact, the adhesion force was measured between the probe and the sample. The adhesion force, which is defined as the maximum force required to pull apart two surfaces after initial contact, was studied. The colloidal probe approached the brush at a speed of 300 nm/s until they came into a contact and a certain maximum force was reached. Then the probe was retracted at the same speed. Force− distance curves (256) were recorded in force−volume mode over a 10 × 10 μm2 area of the surface. The trigger threshold was varied between 5 and 45 nm, causing a change in the maximal load from 75 to 500 nN in air and from 210 to 440 nN in water. As expected, there is a linear dependence between these two parameters in air as well as in water. All force measuremenst were performed at 350 nN, which is the minimum load needed for equilibrium. At least 20 representative single curves were chosen for each sample and averaged to reduce the influence of inhomogeneities and statistical scattering. Force measurements yield the force F as a function of the distance D between two surfaces. For topography images, the Dimension V AFM (Veeco Instruments, Inc., USA) was used. The measurements were made in tapping mode in both dry and aqueous media (with a fluid tip folder) with a silicon tapping mode tip (resonance frequency 200−400 kHz) and a CSC11 silicon tip (resonance frequency 20−40 kHz) (MikroMasch, Estonia), respectively. Adhesion Measurements by Probe Tack Testing. The macroscopic adhesive properties of smooth and rough modified substrates were characterized with a laboratory probe test.31−33 To carry out the test, a smooth or rough 1-cm-diameter silicon wafer functionalized with a polymer brush is first glued to a stainless steel probe, and a soft adhesive layer (100−120 μm thick) is deposited from solution or from emulsion on a clean glass slide. In order to avoid poor
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RESULTS AND DISCUSSION Synthesis of Polymer Brushes. We synthesized random comb-like PEG-PDMS brushes (Figure 1a) on flat (silica wafer) and rough substrates (formed by a monolayer of colloid particles, Figure 1b,c). The rough substrates were prepared by the immobilization of APS-modified 1 μm large silica particles on an 80-nm-thick PGMA layer, which was preliminarily grafted to the silica wafer as described elsewhere41 (Figure 1c). The brushes were synthesized using ATRP, and then the surface topography was evaluated by AFM and SEM methods as described in our earlier publications.10,41,42 The comb-like brushes were prepared by simultaneously grafting methacrylatebased monomers with side PEG and PDMS chains: poly(oligo(ethylene glycol) methyl ether methacrylate) (Mw = 475 g/mol, eight ethylene glycol monomer units) and monomethacryloxypropyl-terminated PDMS (six to nine dimethylsiloxane monomer units). We prepared two sets of brushes with different thicknesses. The thicknesses of the brushes in the first and second sets were around 15 and 40 nm, respectively. The grafting density calculated on the basis of the results of GPC (typical molecular weight of brushes with thicknesses of H = 15 and 40 nm are Mn = 20 and 60 kg/mol, respectively; PDI = 1.7−2.5) and ellipsometry was ca. Γ = 0.4 chain/nm2, and the distance between grafting points was around D = 2 nm. The radius of gyration (Rg) of the used polymers is unknown but expected to be more than 5 nm. For example, Rg of polystyrene with a molecular weight of about 30 000 g/mol in a good solvent C
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Figure 2. Morphology of random copolymer PEG-PDMS brushes grown from flat substrates with different compositions in air and in water. The z scale for all AFM topography images corresponds to 6 nm.
Figure 3. (a) Results of elemental analysis using XPS measurements and C/O and Si/C ratios in random copolymer PEG-PDMS brushes as a function of the fraction of PEG in the polymerization mixture at the deepest XPS information depth (10 nm). (b) Correlation between the Si/C atomic ratios at different takeoff angles (0, 60, and 75°) corresponding to different information depths (10, 5, and 2.5 nm) obtained using ARXPS on the samples (thickness = 40 nm).
(toluene) is 10 nm.43 Because the distance between grafting points is smaller than the thickness of the polymer layers, the polymer-grafted layers can be considered to be brush-like in both the dry and swollen states.9 Brush Morphology. First, we investigated the surface topography/morphology of homopolymer and copolymer brushes in water and in air using AFM. The surface morphology of homopolymer PEG and PDMS brushes grown from flat substrates was, as expected, smooth and featureless independent of the environmental conditions and chemical composition (Figure 2). Because of the random distribution, incompatible, relatively short PEG and PDMS side chains (1 to 2 nm) most probably undergo segregation on a very small scale and are unable to form clusters of significant size that can be revealed by AFM. The surface root-meansquare roughness (rms) was very low for all samples (about 1 nm); therefore, it should not affect the XPS, contact angle, swelling, and electrokinetic measurements. Brush Chemical Composition. First, the chemical composition of the random copolymer layer was quantified by XPS measurements in high vacuum at a takeoff angle of Θ = 0°, which corresponds to a 10 nm information depth. A linear change in C/O and Si/C with the feed ratio of comonomers indicates that the composition of the resulting random copolymer brushes corresponds to feed ratio of comonomers (Figure 3a).
Afterward, the surface segregation in dry PEG-PDMS brushes and their chemical composition in the topmost surface region were quantitatively analyzed by ARXPS. For this, the samples were probed at three different angles of incidence (0, 60, and 75°) corresponding approximately to 10, 5, and 2.5 nm probing depths. Notably, the amount of silicon in the Si/C atomic ratio results only from the contributions of the PDMS sequences of the brush and not from the contributions of the silicon oxide to the Si 2p spectra. The latter were subtracted from the Si 2p spectra. It was found that the Si/C ratio decreased with increasing probing depth for all samples (Figure 3b), indicating a preferential segregation of hydrophobic PDMS chains in the topmost surface layer of the brushes and its depletion in the bulk. According to Schmidt and de Jong,44,45 this effect is due to intramolecular phase separation occurring in copolymer cylindrical brushes with two chemically different types of side chains statistically attached to a flexible backbone. Wetting Properties. Next, the wetting properties of the polymer brushes were investigated using contact angle measurements. PDMS and PEG are hydrophobic and hydrophilic polymers, respectively (Figure 4). Advancing (θADV) and receding (θREC) water contact angles on the PDMS surface are θADV = 112° and θREC = 85°, respectively. Advancing and receding water contact angles on the PEG surface are θADV = 42° and θREC = 22°, respectively. The results summarized in Figure 4a show that both advancing and receding water contact angles decrease nonlinearly with the increase in the PEG D
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chains to the top layer of brushes in air. This behavior corresponds to the fundamental thermodynamic principle that each material seeks to minimize its free energy. Because the surface free energy always makes a positive contribution to the total free energy of the material, hydrophobic parts of the flexible polymer chains will be directed to the air. Hence, in the case of PEG-PDMS copolymers, the more hydrophobic PDMS sequences were preferably used to form the outermost surface layer of the brush. Moreover, our results are found to be in good agreement with previous findings of Minko's group.21 Swelling and Electrokinetic Properties. To understand better the surface properties of prepared homo- and heterogeneous brushes in contact with an aqueous environment, swelling measurements were performed using null ellipsometry. It was found that surfaces with grafted monocomponent PDMS and those with a PEG-PDMS brush with the smallest PEG content of 20% almost do not swell in water (Figure 5a). Further increases in PEG content in the brush lead to stronger swelling of the film. The degree of swelling increased linearly with a PEG fraction of >20% (40, 60, and 80%) and reached a value of 4 for the monocomponent PEG layer (Figure 5a). The formation of surface charges during the contact of the brushes with aqueous solutions and the changes in these charges as a function of the pH of the aqueous solution were studied by streaming potential measurements (Figure 5b,c). It was found that the value of the apparent zeta potential changes linearly with pH as a result of charge formation by selective ion adsorption (Figure 5b). Because the investigated PEG-PDMS brushes do not have dissociating groups, the change in the absolute value of the zeta potential with increased pH is caused by the preferential adsorption of hydroxyl (OH−) ions at pH > IEP and hydronium (H3O+) at pH < IEP from electrolyte solutions, respectively. This is supported by the result that independent of the kind of grafted polymer brush the isoelectric point (IEP = pH|[ζ=0]) was about pH 4, which is typically observed for the charge-formation processes driven by ion adsorption. Moreover, investigated films showed a flattish curve for the pure PEG brush (Figure 5b, red dots). This behavior can be mainly attributed to the strong swelling of the polymer layer. Furthermore, concurrent adsorption between water molecules and ions from the surrounding aqueous electrolyte
Figure 4. (a) Advancing (θADV) and receding (θREC) water contact angles on PEG-PDMS random copolymer brushes (thickness = 40 nm) with different chemical compositions. (b) PEG surface fraction estimated by using Cassie's equation.
volume fraction. We applied the Cassie equation in order to estimate quantitatively the fraction of PEG in the topmost layer considering the advancing and receding contact angles cos θ = αPEG cos θPEG + (1 − αPEG)cos θPDMS
(1)
where αPEG is the PEG surface fraction and θPEG and θPDMS are the advancing/receding water contact angles on monocomponent PEG and PDMS brush surfaces, respectively. It was found that the PEG fraction on the surface is smaller than that in the volume. Moreover, the fraction of PEG on the surface obtained from advancing angles is smaller than that obtained from receding angles (Figure 4b). The difference between values of the PEG surface fraction obtained from an evaluation of the advancing and receding angles can be easily explained by considering the state of the polymer film. First, water droplets contact the surface that was exposed to air; the surface is dry during the measurements of the advancing contact angle, and the hydrophobic component (PDMS) is expected to dominate in the topmost brush layer.21 However, the polymer surface is wet during measurements of the receding angle, and the fraction of the hydrophilic component in the topmost brush layer is increased.21 The results of ARXPS and contact angle measurements are in reasonable agreement and show a preferential surface segregation of the hydrophobic PDMS
Figure 5. (a) Swelling properties of PEG-PDMS brushes, (b) apparent zeta potential vs pH (measured in 10−3 M KCI) for mono-PEG and monoPDMS and PEG-PDMS random copolymer brushes with different chemical compositions (PEG volume fraction in the brush). (c) Summary of zetapotential values extracted at pH 10 for all systems vs PEG volume fraction in the brush. E
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Figure 6. Effects of brush thickness (thickness = 15 and 40 nm), chemical composititon (PEG fraction), and surface roughness on the adhesion properties of PEG-PDMS brushes as measured by a hard AFM native silica colloid probe (blue columns, in water; empty columns, in air).
solution occurs on the brush layer during the measurements. In the case of hydrophilic PEG, a stronger adsorption of water is expected. PDMS is a nonhydrated hydrophobic polymer which does neither swell nor incorporate ions from aqueous solutions. Hence, brushes of PDMS showed higher apparent zetapotential values (Figure 5b, black dots). Moreover, it was found that the apparent zeta potential of random copolymer brushes with a different PEG fraction linearly depends on the ratio between comonomers and shows intermediate trends between monocomponent PEG and PDMS (Figure 5c). In fact, because PEG swells strongly in water, one can expect that values of the apparent zeta potential of the brushes must be close to that of the pure PEG layer. The observed linear change in the zeta potential (Figure 5c) with an increasing volume fraction of PEG in the brush could be explained by the resolution of the streaming potential method. In fact, a deeper penetration into the flexible brush layer “open” structure could be expected during the streaming of the electrolyte solution; therefore, the topmost layer is not necessarily measured. For instance, Duval et al.46 discussed so-called open structures of soft multilayered polyelectrolyte films illustrated by streaming current measurements. Furthermore, the overall tendencies in the changes of the contact angle, swelling ratio, and zeta-potential values with the brush composition are in good agreement. The lower value of the PDMS content in the system corresponds to the lower values of advancing/receding contact angles, lower values of the swelling property, and lower values of the apparent zeta potential (Figures 4a and 5a−c). Adhesion on Smooth and Rough Surfaces. Finally, we systematically investigated the adhesion properties of homopolymer and copolymer brush layers, with different chemical compositions, layer thicknesses, and substrate roughnesses
against surfaces with variable chemical functionality and mechanical properties under dry and wet conditions. For this, the adhesive properties of monocomponent and bicomponent random copolymer brushes were investigated on the microscopic scale by AFM with a hard colloidal probe (CP) (ca. 5 μm large particle) and on the macroscopic scale by probe tack measurements in contact with elastic model pressuresensitive adhesives. Notably, the colloidal probe and soft adhesive for the probe tack measurements were chosen from the point of view of their chemical composition (hydrophilicity/hydrophobicity) and mechanical properties (hardness/softness), respectively. Monocomponent PEG and PDMS brushes were found to be adherent to the hard hydrophilic colloidal probe in the dry state, with PEG being more sticky than PDMS (Figure 6). The difference between the adhesive properties of PEG and PDMS brushes with a colloidal probe becomes stronger when brushes are exposed to water. In particular, PDMS retains its sticky properties in water but PEG becomes completely nonadherent because of the strong hydration of polymer chains (Figure 6, blue columns). In fact, we observed that the PDMS layer is sticky in air, which contradicts previous findings of Minko's group.21 Because PDMS has a very low surface tension, it must be nonadhesive. However, adhesion is also related to the mechanical properties of materials that result in the stickiness of soft PDMS. It is most probable that the reason for the difference between our results and the results of Minko's group is the different thickness of the layer. The PDMS film used by Minko's group is very thin (
