Effect of Polymer Architecture on the Adsorption Properties of a

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Langmuir 2008, 24, 6676-6682

Effect of Polymer Architecture on the Adsorption Properties of a Nonionic Polymer Ali Naderi,*,† Joseph Iruthayaraj,† Torbjo¨rn Pettersson,†,‡ Ricˇardas Makusˇka,§ and Per M. Claesson†,| Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, Department of Polymer Chemistry, Vilnius UniVersity, Naugarduko 24, LT-03225 Vilnius, Lithuania, YKI, Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden, and ForceIT, MossVa¨gen 14, SE-153 37 Ja¨rna, Sweden ReceiVed January 10, 2008. ReVised Manuscript ReceiVed March 4, 2008 The adsorption of a linear- and bottle-brush poly(ethylene oxide (PEO))-based polymer, having comparable molecular weights, was studied by means of quartz crystal microbalance with dissipation monitoring ability (QCM-D) and AFM colloidal probe force measurements. The energy dissipation change monitored by QCM-D and the range of the steric forces obtained from force measurements demonstrated that linear PEO forms a more extended adsorption layer than the bottle-brush polymer, despite that the adsorbed mass is higher for the latter. Competitive adsorption studies revealed that linear PEO is readily displaced from the interface by the bottle-brush polymer. This was attributed to the higher surface affinity of the latter, which is governed by the number of contact points between the polymers and the interface, and the smaller loss of conformational entropy.

1. Introduction Polymers have for a long time found use as stabilizers in application areas such as pigment slurries and emulsions. To achieve good steric stabilization, the surface should be fully coated, and the solvent quality should be good. Further, the polymer-surface affinity should be sufficiently high to allow for adsorption and the formation of large loops and tails, but a too high polymer-surface affinity can be detrimental since it leads to a flat adsorbed layer. Even though thick layers with long tails are highly effective from a steric stabilization point-ofview, when the coverage is high, such layers also can induce destabilization at low coverages due to bridging effects by adsorption onto parts of other colloids that are not effectively protected.1 The layer structure and its ability to sterically stabilize colloids also may change with time2 due to relaxation/rearrangement of the polymer at the interface. An alternative to linear polymers is to make use of bottlebrush polymers, also known as comb polymers. These are polymers where side chains are grafted onto a linear main chain. The potential use of comb polymers is realized from the works of Balazs and Siemasko,3 van der Linden et al.,4 and Striolo et al.,5 in which the structures of adsorbed linear and comb (homoand co-) polymer chains at an adsorbing interface were compared. The general picture that emerged from the mentioned contributions was that comb homopolymers produce thinner layers and display a higher surface coverage with smaller numbers of adsorbed polymers chains as compared to linear equivalents. * To whom correspondence should be addressed. E-mail: ali.naderi@ surfchem.kth.se. † Royal Institute of Technology. ‡ ForceIT. § Vilnius University. | Institute for Surface Chemistry. (1) Pelssers, E. G. M.; Stuart, M. A. C.; Fleer, G. J. Colloids Surf. 1989, 38, 15. (2) Stuart, M. A. C.; Tamai, H. Langmuir 1988, 4, 1184. (3) Balazs, A. C.; Siemasko, C. P. J. Chem. Phys. 1991, 95, 3798. (4) van der Linden, C. C.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1996, 29, 1000. (5) Striolo, A.; Jayaraman, A.; Genzer, J.; Hall, C. K. J. Chem. Phys. 2005, 123, 64710/1.

From an adsorption point-of-view, the bottle-brush homopolymers have the ability to attach to the surface through both side chain and backbone segments. If adsorption occurs through the attachment of the side chains on the surface, then the polymer attains a flat conformation at the interface since the gain in adsorption energy increases with the number of contact points between the side chain segments and the surface. According to the simulation work of Balazs and Siemasko,3 side chain-mediated attachment is the preferred adsorption mechanism of comb (homo-) polymers. However, when adsorption is achieved through backbone segments (as concluded by van der Linden et al.4 but in contrast to the conclusions of Balazs and Siemasko3), the polymer gains favorable adsorption energy (through contact between backbone segments and adsorption sites at the interface) at the same time as much of the conformational entropy of the side chains is retained. Both adsorption mechanisms, backboneand side chain-mediated, predict the formation of thin layers due to spreading of the chains on the interface. A flat conformation in turn results in a large occupied area per chain. As a consequence, fewer bottle-brush chains are adsorbed as compared to the case when linear polymers are adsorbed. Experimental results6–8 indicate that the conformation of bottle-brush polymers in solution resembles that of a cylinder, with the side chains protruding from every side of the backbone chain. It is expected that much of this conformation is retained (especially for highly grafted bottle-brush polymers) when the polymer adsorbs onto a surface, as the confinement of all the side chains at one side of the chain is not possible due to strong steric and excluded volume effects and due to entropic reasons. It appears that “hairy” well-anchored polymer layers that may be formed through the adsorption of bottle-brush polymers will possess the potential to become effective (steric) stabilizers. To our knowledge, the work of (6) Zhang, M.; Mueller, A. H. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3461. (7) Bastardo, L. A.; Iruthayaraj, J.; Lundin, M.; Dedinaite, A.; Vareikis, A.; Makuska, R.; van der Wal, A.; Furo, I.; Garamus, V. M.; Claesson, P. M. J. Colloid Interface Sci. 2007, 312, 21. (8) Dedinaite, A.; Bastardo, L. A. ; Oliveira, C. L. P.; Pedersen, J. S.; Claesson, P. M.; Vareikis, A.; Makusˇka, R. Proceedings of the Baltic Polymer Symposium, Druskininkai, Lithuania, 2007, in press.

10.1021/la800089v CCC: $40.75  2008 American Chemical Society Published on Web 05/30/2008

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Figure 1. Molecular structure of poly(PEO45MEMA).

Kawaguchi and Takahashi,9 who conducted ellipsometric studies (at the theta temperature of the system) on the adsorption of comb and linear polystyrene on a metal surface, is the only attempt to experimentally test the findings in the simulation and theoretical studies3–5 described previously. The authors reported several interesting results, some of which are highlighted here. Kawaguchi and Takahashi9 observed (when investigating comb polymers with different side chain densities and side chain lengths, which were longer than the parent backbone) that the adsorbed mass of the comb polymers was higher than that obtained by linear equivalents, which they attributed to the higher segment density of the branched polymers. Furthermore, the authors showed that comb polymers form thinner layers than their linear equivalents, which is in line with results3–5 presented earlier. It is clear that the interesting results of Kawaguchi and Takahashi9 merit further investigations in this area. We characterized10 and studied bulk7,8 and adsorption properties10,11 of a series of poly(ethylene glycol) methyl ether methacrylate (PEO45MEMA)-based bottle-brush polymers (Figure 1). In this work, we compared the adsorption properties of the nonionic poly(PEO45MEMA) bottle-brush polymer to that of linear poly(ethylene oxide) (PEO), which has a comparable molecular weight to that of the comb polymer. The poly(PEO45MEMA) bottle-brush polymer has a very high side chain density, one 45 unit long PEO side chain is attached to each segment, and the backbone is longer than the side chains. Thus, the architecture is significantly different from that considered by Kawaguchi and Takahashi.9 The comparisons were made through the adsorption of the polymers on hydrophilic silica at pH 2. At this pH, the silica substrate is close to uncharged as the isoelectric point12 of silica lies in the vicinity of pH 2. The silanol groups of the interface, which at this pH are undissociated, serve as anchoring points for the polymers. The conformational entropy loss during (9) Kawaguchi, M.; Takahashi, A. J. Polym. Sci., Part B: Polym. Phys. 1980, 18, 943. (10) Naderi, A.; Iruthayaraj, J.; Vareikis, A.; Makusˇka, R.; Claesson, P. M. Langmuir 2007, 23, 12222. (11) Olanya, G.; Iruthayaraj, J.; Poptoshev, E.; Makusˇka, R.; Vareikis, A.; Claesson, P. M. Langmuir 2008, 24, 5341. (12) Yan, X. P.; Perry, S. S.; Spencer, N. D.; Pasche, S.; De Paul, S. M.; Textor, M.; Lim, M. S. Langmuir 2004, 20, 423.

Figure 2. QCM-D results for changes in sensed mass and dissipation (∆D) vs time of poly(PEO45MEMA) adsorbed onto silica, at pH 2, from a 10 ppm solution. (a) Sensed mass (squares) and dissipation (circles) for the third overtone. The sample was injected into the chamber at t ≈ 15 min and was rinsed after ca. t ≈ 200 min. (b) Sensed mass (triangles and stars) and dissipation (crosses and pluses) for the fifth and seventh overtone. QCM-D properties for the third overtone are denoted with the same symbols as in panel (a).

adsorption for the bottle-brush architecture is expected to be lower than for the linear architecture. On the other hand, it is conceivable that the energy change due to adsorption may be more favorable for linear PEO since flexibility of the chain can allow for favorable orientations relative to surface silanol groups. The competitive adsorption experiments will shed light onto the relative importance of these factors. Valuable insight into the extension of the layer was obtained by the parallel use of QCM-D (quartz crystal microbalance with dissipation monitoring ability) and AFM (atomic force microscopy). QCM-D also was used to monitor the competitive adsorption of the different systems. In these studies, the polymer layers were formed from a dilute solution (10 ppm): as by adsorbing from a dilute solution, less protective polymer layers are formed, which considerably reduces the exchange time13 of the polymers.

2. Materials and Methods 2.1. Materials. The brush polymer investigated in this work, poly(PEO45MEMA), was prepared by radical polymerization10 of PEO45MEMA (Figure 1). The molecular weight10 of the polymer, as obtained by light scattering, is 410 kg/mol. The PEO component has a molecular weight of 2000 g/mol (ca. 45 repeating units) and a polydispersity index of less than 1.1. The synthesized polymer is, however, quite polydisperse with a polydispersity index of 2-3 (as (13) Frantz, P.; Granick, S. Macromolecules 1995, 28, 6915.

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determined by GPC14 for other similarly prepared brush polymers). The linear PEO used in this study (referred to as PEO500) was purchased from Polymer Laboratories. It has a molecular weight (Mw) of 527 kg/mol and a polydispersity index of less than 1.1. Thermally oxidized silicon wafers (used in the ellipsometry measurements) were purchased from Wafer Net, and as colloidal probes in the AFM measurements, silica particles (SS06N) of approximately 7 µm in diameter from Bangs Laboratories Inc. were employed. The water used in the experiments was first pretreated using a Milli-RO Plus unit and then purified further using a Milli-Q Plus185 system. The water resistivity after the treatment was 18 MΩ cm, and the total organic carbon content of the water did not exceed 2 ppb. Hydrochloric acid (HCl) (from Sigma-Aldrich) was used for pH adjustment. 2.2. QCM-D. A commercially available QCM-D apparatus, D300 (Q-Sense AB), and silica-coated quartz sensors (crystals) with a fundamental frequency of 5 MHz (also supplied by Q-Sense AB) were used in our investigations. A detailed description of the principals of the instrument can be found elsewhere.15 There are different theories16–19 for calculation of the sensed mass, which take into account the change in frequency registered by the sensor. In this work, we make use of the Johannsmann et al.18 model (eq 1), which has been derived for viscoelastic layers

(

Fq(2πf)2d2 3 C∆f m* ) n

m * ) m0 1 + Jˆ(f)

)

(1) (2)

In eq 1, m* is the sensed mass obtained by using the Sauerbrey16 relation (eq 2). This quantity is also sometimes called the equivalent mass. The variable m0 denotes the true sensed mass as it corrects the frequency response for the viscoelastic properties of the layer. This quantity contains contributions from the adsorbing species and from the solvent that oscillates with the crystal, Jˆ(f) is the complex shear compliance, Fq is the specific density of quartz (2648 kg/m3), f is the resonance frequency of the crystal in contact with solution, and d is the film thickness. In the Sauerbrey relation (eq 2), C is a constant that for our crystals is equal to 0.177 mg/m2, ∆f is the change in frequency during the adsorption/desorption process, and n is the overtone (3, 5, and 7 in this study). The QCM-D chamber and all other tools were cleaned by immersion in a hot 2% (w/w) Deconex 11 Universal solution (a cleaning agent, Borer Chemie AG) for 1 h. They were thereafter rinsed with plentiful amounts of Milli-Q water and left overnight in Milli-Q water. The sensors (crystals) were immersed in a 2% (w/w) Deconex 11 Universal solution for 1 h and then rinsed excessively with Milli-Q water and left in the same water overnight. Next, the sensors were cleaned with ethanol and blow-dried with N2 before being mounted inside the measuring chamber. The set temperature for the measurements was 25 °C. The experiments were initiated by first establishing a stable baseline using the background solution (Milli-Q water, pH 2). When this was achieved, the polymer sample (10 ppm (w/w) in Milli-Q water at pH 2) was introduced in the measurement chamber (about 1 mL was flushed through the measuring cell that had a volume of about 80 µL). The adsorption process was then monitored until ∆f reached a near-to-plateau value, upon which the chamber was rinsed with 1 mL of the background solution. (14) Iruthayaraj, J.; Poptoshev, E.; Vareikis, A.; Makuska, R.; van der Wal, A.; Claesson, P. M. Macromolecules 2005, 38, 6152. (15) Rodahl, M.; Hook, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. ReV. Sci. Instrum. 1995, 66, 3924. (16) Sauerbrey, G. Zeit. Phys. 1959, 155, 206. (17) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scripta 1999, 59, 391. (18) Johannsmann, D.; Mathauer, K.; Wegner, G.; Knoll, W. Phys. ReV. B: Condens. Matter Mater. Phys. 1992, 46, 7808. (19) Voinova, M. V.; Jonson, M.; Kasemo, B. Biosens. Bioelectron. 2002, 17, 835.

Figure 3. QCM-D results for changes in sensed mass and dissipation (∆D) vs time of PEO500 adsorbed onto silica, at pH 2, from a 10 ppm solution. (a) Sensed mass (squares) and dissipation (circles) for the third overtone. The sample was injected into the chamber at t ≈ 10 min, and the chamber was rinsed after t ≈ 75 min. (b) Sensed mass (triangles and stars) and dissipation (crosses and pluses) for the fifth and seventh overtone. The data for the third overtone are denoted with the same symbols as in panel (a).

2.3. AFM. Force measurements were performed in a fused silica liquid cell (volume ≈ 0.1 mL), using a Nanoscope Multimode III Pico Force atomic force microscope (Veeco Instruments Inc.). Rectangular tipless cantilevers (MikroMasch) with the approximate dimensions of 250 µm length, 35 µm width, and normal spring constants in the range of 0.02-0.2 N/m were employed. The exact values of the spring constant were determined using AFM Tune IT v2.5 software (ForceIT), which employs a method based on thermal noise with hydrodynamic damping.20–22 The silica colloidal probes23 were attached to the cantilevers with the aid of a high-temperature melting epoxy (Epikote 1004, Shell Chemicals). Surface force curves determined upon approach and separation were recorded at a constant driving velocity of 1 µm/s. The force curves were then analyzed with AFM Force IT software (ForceIT). The deflection sensitivity obtained for the pure silica-silica system was in all cases used when defining the constant compliance region and generating force curves in the presence of adsorbed polymers. This method was employed since the adsorbed polymer layer may have a finite compressibility at the highest loads applied. Finally, it is noted that the displayed force (F/R ) 2πGf) in this study was normalized by the radius of the sphere to, according to the Derjaguin24 (20) Green, C. P.; Lioe, H.; Mulvaney, P.; Sader, J. E. ReV. Sci. Instrum. 2004, 75, 1988. (21) Sader, J. E.; Chon, J. W. M.; Mulvaney, P. ReV. Sci. Instrum. 1999, 70, 3967. (22) Pettersson, T.; Nordgren, N.; Rutland, M. W.; Feiler, A. ReV. Sci. Instrum. 2007, 78, 93702. (23) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature (London, U.K.) 1991, 353, 239.

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Figure 4. Evolution of dissipation change (∆D) with sensed mass (registered by the third overtone) of the polymer layers formed by PEO500 (triangles) and poly(PEO45MEMA) (squares).

Figure 6. QCM-D results for changes in sensed mass (squares) and dissipation (circles) vs time. (a) Graph illustrates the displacing effect of PEO500 on the preadsorbed layer of poly(PEO45MEMA). The layer was formed through injection of a 10 ppm solution (at pH 2) into the chamber at t ≈ 15 min and the subsequent rinse with background solution (pH 2) at t ≈ 150 min. A 10 ppm PEO500 solution (pH 2) was thereafter introduced into the chamber at t ≈ 200 min. The exchange process was then terminated by injection of background solution into the measuring chamber at t ≈ 320 min. (b) Displacing effect of poly(PEO45MEMA) on a preadsorbed PEO500 layer, monitored in a similar manner as in panel (a). Injection of PEO500 at t ≈ 10 min, rinse of the chamber at t ≈ 75 min, followed by injection of poly(PEO45MEMA) at t ≈ 100 min. The experiment was terminated after about 400 min by rinsing the chamber.

Figure 5. Interactions between two polymer-coated silica surfaces. (a) Compression curves of poly(PEO45MEMA) (triangles) and PEO500 (dashes). These curves also are displayed in a semilog plot in the inset. (b) Decompression curves of poly(PEO45MEMA) (circles) and PEO500 (crosses).

approximation, be proportional to the interaction free energy per unit area between equivalent flat surfaces (Gf). Before each experiment, the fused silica liquid cell and all other tools were cleaned by immersion in warm Deconex 11 Universal for at least 1 h. They were then rinsed excessively with Milli-Q water and ethanol and blow-dried with nitrogen gas. Just before the start of the experiments, a QCM-D silica crystal (cleaned in the same manner as in the QCM-D measurements) acting as the flat substrate, together with the cantilever holding the colloidal probe (24) Israelachvili, J. Intermolecular and Surface Forces; Academic Press: San Diego, 1991.

(rinsed with Milli-Q water and dried with nitrogen gas), was mounted into the liquid cell. The cell was thereafter filled with a background aqueous solution at pH 2 (filtered through a 0.2 µm Acrodisc PTFE filter, Pall Life Science). After ∼10 min of equilibration, force-distance curves were recorded to ensure the cleanness of the substrates. The probe was then retracted (ca. 60 µm) from the surface, and the cell was filled with 10 ppm of the investigated polymer, at pH 2, which also was filtered through a 0.2 µm Acrodisc PTFE filter. The polymer was then allowed to adsorb for 3 h when poly(PEO45MEMA) was investigated and for 1.5 h in the case of PEO500. As will be discussed later, this is the time needed for the investigated polymers to reach the plateau values in their sensed masses. Thereafter, the chamber was rinsed with 1 mL of the background solution, and the system was allowed to equilibrate for 10 min before force-distance curves were recorded (across Milli-Q water at pH 2).

3. Results and Discussion 3.1. Adsorption Properties of Linear and Brush Polymers. The build-up of adsorbed polymer layers of poly(PEO45MEMA) and PEO500 on silica, from 10 ppm solutions at pH 2, as monitored by QCM-D is shown in Figures 2 and 3. The sensed mass of poly(PEO45MEMA) increases rapidly in the initial stage

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of the adsorption process, as seen in Figure 2a,b. It is further noted that approximately 3 h of adsorption is needed for poly(PEO45MEMA) to reach a near-to-plateau value. The long adsorption time is suggested to be caused by the steric hindrance posed by the protruding side chains of already adsorbed polymers, through which the arriving polymer chains have to diffuse to reach the interface. The crowding at the interface also has consequences for the polymer layer structure. When the first chains reach the bare surface, they meet no resistance and can therefore attain a relatively flat conformation. The polymer layer structure thereafter becomes more extended since the polymers arriving on the surface at a later stage can attach with a lower number of segments due to hindrance imposed by the alreadyadsorbed chains. This change in polymer layer structure, which is a common observation during polymer adsorption,25 can be excellently demonstrated by monitoring the evolution of the dissipation parameter of the QCM-D measurements. As seen in Figure 2a, and even more clearly in Figure 4 where the change in dissipation as a function of sensed mass is displayed, the dissipation of the polymer layer increases more quickly with increasing sensed mass in the later stages of the adsorption process, in agreement with the mechanism described previously. We note that the sensed mass evaluated using different overtones consistently decreases with the overtone number. This is a signature of a viscoelastic layer and necessitates further analysis beyond the Sauerbrey relation. For this reason, the true sensed mass was evaluated with the Johannsmann et al.18 model (eqs 1 and 2), which confirmed that the assumption of a frequency independence shear compliance was an appropriate one. The analysis also returned a sensed mass value of 5.9 mg/m2. This high value is due to a large contribution from water hydrodynamically coupled to the adsorbed poly(PEO45MEMA) layer, as discussed further next. QCM-D results displayed in Figure 3 show that the sensed mass of the PEO500 layer is about half of that registered for poly(PEO45MEMA), but the magnitude of the dissipation change is approximately the same for both systems. The PEO500 layer is also viscoelastic (Figure 3b), and the true sensed mass calculated from the Johannsmann et al.18 model is 2.8 mg/m2. As displayed in Figure 4, the linear polymer is more energy dissipative per unit adsorbed mass than the brush polymer, as judged by the slope of the dissipation versus sensed mass curve. It is evident that both polymers form nonrigid layers. The nonrigidity of the PEO500 coating on silica combined with its high-energy dissipative nature leads to the conclusion that it has a more extended conformation than the polymer layer formed by poly(PEO45MEMA). In contrast, the viscoelasticity of the poly(PEO45MEMA) coating can be attributed to the protruding side chains of the polymer,11 which are at the same time heavily hydrated. A measure of the layer thickness of the polymer coating (δeff) can be obtained if both the sensed mass as registered by QCM-D and the adsorbed mass of the polymer are known. The method first proposed by Hoeoek et al.26 makes use of eqs 3 and 4

Feff ) Fpol(M ⁄ MQCM-D) + (1 - M ⁄ MQCM-D)Fsol

(3)

δeff ) MQCM-D ⁄ Feff

(4)

where Feff is the effective density of the layer, Fpol is the bulk density of the polymers (set10,27 to be 1.1 g/cm3), Fsol is the solvent density (equal to 1 g/cm3), M is the dry mass of the (25) Schneider, H. M.; Frantz, P.; Granick, S. Langmuir 1996, 12, 994. (26) Hoeoek, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796. (27) Raviv, U.; Klein, J.; Witten, T. A. Eur. Phys. J. E 2002, 9, 405.

adsorbed polymer, and MQCM-D is the true sensed mass obtained by QCM-D. The dry mass of the polymers can be obtained by, for example, reflectometry. Adsorption studies (conducted under similar conditions as in this study) by this instrument11 have shown that the adsorbed mass of poly(PEO45MEMA) on silica11 is 1.17 mg/m2; the adsorbed mass of PEO500 (measured under the same conditions as PEO45MEMA) was found to be 0.46 mg/m2. The thickness values that eqs 3 and 4 provide is based on the assumption that all mass located at distances smaller than δeff oscillates with the crystal and that no mass located further away from the surface contributes to the true sensed mass. This is obviously a simplification since some chain regions (e.g., polymer tails) have highly extended conformations. However, reasonable values should be attained for thin and homogeneous polymer layers. Assuming that poly(PEO45MEMA) forms such a layer, using eqs 3 and 4 returns a thickness value of 5.8 nm. This value is in close agreement to what has been found with SAXS measurements (in bulk solution), where twice the radius of gyration of the cross-section of poly(PEO45MEMA)8 was found to be 6.5 nm. The QCM-D layer thickness obtained for PEO500 is 2.8 nm. However, for the extended layers formed by this polymer, the value of the layer thickness returned by different methods will be different. Techniques sensitive to long tails, such as force measurements, will report a large layer thickness, whereas an evaluation based on eqs 3 and 4, which are insensitive to long tails, will report smaller thicknesses. It is finally noted that the combined data of the dry mass of PEO45MEMA and PEO500 and their true sensed mass (5.9 and 2.8 mg/m2, respectively) obtained through eqs 1 and 2 show that hydrodynamically coupled water constitutes about 80% of the true sensed mass. Similar water contents have been reported11,28 for bottle-brush polyelectrolytes (with side chains of the same molecular weight as this study) adsorbing with their backbone segments on oppositely charged surfaces. 3.2. Interactions between Polymer-Coated Surfaces. The interactions between silica surfaces coated with PEO500 and poly(PEO45MEMA) are displayed in Figure 5. The force curves are in both cases repulsive upon approach due to steric interactions (Figure 5a) but show adhesive forces of different magnitudes upon separation. The hysteretic feature of the force curves has also been reported by other groups29,30 (for linear PEO) and attributed to dynamic effects that become significant when the rate of the measurements exceeds the relaxation rates of the adsorbed polymer. The interaction force between layers of poly(PEO45MEMA) becomes significant (Figure 5a) from a distance of close to 10 nm, which coincidently (as the true distance between the surfaces cannot be determined by AFM) is close to the sum of the thickness of the two polymer layers, as calculated using eqs 3 and 4. In contrast, the surface interaction curve for silica coated with PEO500 (Figure 5a) starts from a significantly larger distance of approximately 40 nm. As a comparison, the radius of gyration obtained by static light scattering for PEO500 is 30 nm. Thus, the range of the steric force is significantly shorter than would have been the case if PEO500 had adsorbed as undisturbed coils. In this situation, the onset of the steric repulsion would have occurred at separations in order of 4 Rg (120 nm). The larger range of the surface interaction between PEO500 layers, when compared to that observed for poly(PEO45MEMA), is consistent with the presence of some long tails and explains (28) Heuberger, M.; Drobek, T.; Voeroes, J. Langmuir 2004, 20, 9445. (29) McLean, S. C.; Lioe, H.; Meagher, L.; Craig, V. S. J.; Gee, M. L. Langmuir 2005, 21, 2199. (30) Braithwaite, G. J. C.; Luckham, P. F. J. Chem. Soc., Faraday Trans. 1997, 93, 1409.

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Figure 7. Schematic presentation of the layer structures of poly(PEO45MEMA) (left) and PEO500 (right) as suggested by the data presented in this paper.

the high-energy dissipation per unit adsorbed mass observed with QCM-D. The different layer structures also affect the separation force curves. This can be seen in Figure 5b as a rather abrupt disengagement of the surfaces coated with poly(PEO45MEMA) upon separation, indicating a flat layer structure. These observations suggest that the poly(PEO45MEMA) polymer is firmly attached to the surface and that no polymers are stretched between the surface during separation. In contrast, for the PEO500 layer, significant re-expansion occurs before the adhesive minimum is reached, providing further evidence for an extended layer structure. In this case (Figure 5b), a long-range attractive force is observed upon separation, which we identify with bridging polymers that are stretched between the surfaces. However, the typical saw-tooth pattern in the retraction curve that is observed when polymers with a high binding energy to the surface are stretched due to bridge formation was not seen in the present case. This indicates that the polymers are pulled out from the surface without any pronounced stick slip feature, which is related to the relatively low affinity between PEO and silica. The distance within with which this force acts is consistent with the bridging ability of a long and flexible polymer chain. Similar conclusions were drawn by Braithwaite and Luckham,30 who studied interactions between PEO-coated glass surfaces. Finally, we note that the higher adhesion found between poly(PEO45MEMA) layers, as compared to between PEO500 layers, demonstrates that more bridges are formed in the former case. 3.3. Competitive Adsorption of PEO500 and Poly(PEO45MEMA). The outcome of surface exchange experiments between PEO500 and poly(PEO45MEMA) carried out with QCM-D is displayed in Figure 6. Starting with Figure 6a, it is noted that rinsing the preadsorbed poly(PEO45MEMA) layer results in minor changes in sensed mass and dissipation, indicating a very slow desorption into polymer-free solutions at pH 2. Exposing the preadsorbed poly(PEO45MEMA) layer to a PEO500 solution also has a limited effect on the layer. However, a small decrease in sensed mass and increase in dissipation is observed and suggests some exchange of loosely attached poly(PEO45MEMA) chains for PEO500 equivalents. Rinsing the preadsorbed PEO500 polymer layer with polymerfree solution at pH 2 (Figure 6b) resulted in some decrease in sensed mass and dissipation, and the effect is clearly larger than for poly(PEO45MEMA). Exposing the preadsorbed PEO500 layer to a poly(PEO45MEMA) solution resulted in significant changes. Both the dissipation and the sensed mass increased immediately, and the dissipation change indicated a strong expansion of the layer. Thereafter, the sensed mass continued to increase with time, whereas the dissipation passed through a maximum and then decreased. After a long time, both the sensed mass and the dissipation approached the values obtained for poly-

(PEO45MEMA) adsorbing onto bare silica. Thus, we conclude that poly(PEO45MEMA) is able to replace PEO500 on silica at pH 2 and that the initial stage of the exchange process involves the inclusion of some extended chains in the layer structure. These can be partly desorbed PEO500 and/or poly(PEO45MEMA) chains that have attached partly to the surface. Clearly, poly(PEO45MEMA) has a higher affinity for silica at pH 2 than PEO500, which in turn can be related to the ability of poly(PEO45MEMA) to attach with a larger number of segments to a smaller loss of conformational entropy (than PEO500); this point schematically is presented in Figure 7. It is noted, however, that the rate of the exchange is very slow and that full completion of the exchange process is not reached within the experimental time of this study (5 h after the start of the desorption process). The slow exchange rate is likely related to the low concentration of the displacer and the stronger anchoring of part of the PEO500 chain population to the surface. These are most probably the chains that reached the surface in the early stages of the adsorption process and can therefore spread (as described earlier) on the surface to a larger extent than those chains that arrived in the later stages of the adsorption process.

4. Conclusion Adsorption layers formed by a linear- and a bottle-brush PEObased polymer, having comparable molecular weights, were studied by means of QCM-D and AFM. From the QCM-D studies, it was shown that the adsorbed layer formed by the linear polymer was more energy dissipating per unit mass than that formed by the bottle-brush polymer. Force measurements with the AFM colloidal probe technique revealed a significantly longer ranged steric repulsion between silica coated with linear PEO than with bottle-brush PEO. From these and other findings, it was concluded, in accordance with the outcome of the studies of, for example, Balasz and Siemasko,3 van der Linden et al.,4 and Striolo et al.,5 that linear PEO forms a more extended layer than the bottlebrush polymer. For both polymers, the mass sensed by QCM-D is significantly larger than the adsorbed mass, and about 80% of the mass sensed by QCM-D is due to water hydrodynamically coupled to the adsorbed layer. Force curves determined upon separation were significantly different for the two types of PEOcoated surfaces. For surfaces coated with linear PEO, long-range attractive forces assigned to stretching of bridging chains were observed, whereas no similar feature was observed between layers of bottle-brush PEO. In this case, a stronger but more shortrange attraction was observed, indicating that bridging occurs but that the bridges formed by the side chains are not strong enough to allow the whole polymer to be stretched between the surfaces. Competitive adsorption studies revealed that a preadsorbed layer of linear PEO is displaced from the surface when the bottle-

6682 Langmuir, Vol. 24, No. 13, 2008

brush polymer is introduced into solution. The surface exchange is a slow process, and in the initial part of the surface exchange process, some extended chains were present in the adsorbed layer. The feasibility of the exchange process is attributed to the higher surface affinity and smaller loss in conformational entropy upon adsorption for the bottle-brush polymer. Consequently, linear PEO was not able to displace the preadsorbed PEO bottlebrush layer to any large extent.

Naderi et al.

Acknowledgment. A.N., J.I., T.P., and P.M.C. acknowledge financial support from the Swedish Research Council (VR). Part of the work was funded by the European Community’s Marie Curie Research Training Network “Self-Organization under Confinement (SOCON)”, Contract MRTN-CT-2004-512331. Geoffrey Olanya is thanked for his help with reflectometry measurements. LA800089V