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Adsorption of a Polyelectrolyte on Surfaces with Nanometer Sized Chemical Patchiness Yu-Wen Huang, Kyoung-Yong Chun, and Vinay K. Gupta* Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801 Received July 23, 2002. In Final Form: December 8, 2002 The adsorption of a cationic polymer, poly(2-vinylpyridine) (P2VP), from aqueous solutions was investigated on model chemically heterogeneous surfaces. The heterogeneous surfaces were prepared using self-assembly of ω-functionalized alkanethiols on gold substrates such that the surfaces consisted of a patchwise distribution of two types of sites, hydrophobic methyl (A) and hydrophilic carboxylic acid sites (B), that interact differently with P2VP. Microscopic imaging by scanning tunneling microscopy and macroscopic characterization by cyclic voltammetry was used to demonstrate that the domains of A on the surface were of nanometer dimensions and commensurate with the dimensions of the polymer chain. Surface plasmon resonance was used to measure adsorption kinetics of P2VP at the solid-liquid interface under two pH conditions such that the polymer chain was either protonated or uncharged. Under these conditions adsorption of P2VP occurred on a heterogeneous surface containing a distribution of patchy sites that are either more attractive or more repulsive than their surroundings. These experiments represent the first realization of a system modeled or simulated in recent theoretical studies, and the results corroborate predictions of the theory. It is found that the surface heterogeneity does not alter the kinetics during the early stages of adsorption and that the rate of adsorption is same as that measured on a homogeneous surface containing only one type of chemical site (A or B). Presence of the heterogeneity influences the number of polymer chains adsorbed on the surface, and the final adsorption amount on the heterogeneous surface lies between the adsorbed amounts found on the compositionally homogeneous surfaces of A and B.

Introduction Adsorption of flexible macromolecules at a solid-liquid interface has been the subject of intense investigation over the past decade due to the critical role of adsorbed polymer layers in adhesion,1 biocompatibility,2 colloidal stabilization,3 and chromatography.4 Despite the advances in characterization and theoretical understanding of nearequilibrium properties of adsorbed polymer layers, several aspects of the adsorption process remain elusive.5,6 Among these elusive aspects is the effect of compositional and topographical heterogeneity of surfaces on the process of adsorption and the structural properties of the adsorbed layer. In recent years, theory and simulation have explored the influence of both chemical7-12 and physical13,14 het* To whom correspondence should be addressed. Email: [email protected]. Fax: 217-333-5052. Tel: 217-244-2247. (1) Luzinov, I.; Voronov, A.; Minko, S. Adsorpt. Sci. Technol. 1996, 14, 259-266. (2) Radler, J.; Sackmann, E. Curr. Opin. Solid State Mater. Sci. 1997, 2, 330-336; Mobed, M.; Chang, T. M. S. Biomaterials 1998, 19, 11671177. (3) Incorvati, C. M.; Lee, D. H.; Reed, J. S.; Condrate, R. A., Sr. Am. Ceram. Soc. Bull. 1997, 76, 65-68. Berry, A. K.; Bogan, L. E.; Agostine, S. E. Ceram. Trans. 1996, 62, 125-132. (4) Gailliezdegremont, E.; Bacquet, M.; Dauphin, J. Y.; Morcellet, M. Colloids Surf., A 1996, 110, 169-180. (5) Stuart, M. A. C.; Fleer, G. J. Annu. Rev. Mater. Sci. 1996, 26, 463-500; Ramsden, J. J. Surf. Sci. Ser. 1998, 75, 321-361. (6) Fleer, G. J.; Stuart, M. A. C.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (7) Balazs, A. C.; Huang, K.; McElwain, P.; Brady, J. E. Macromolecules 1991, 24, 714-717. (8) Balazs, A. C.; Gempe, M. C.; Zhou, Z. Macromolecules 1991, 24, 4918-4925. Balazs, A. C.; Singh, C.; Zhulina, E. Macromolecules 1998, 31, 6369-6379. (9) Chakraborty, A. K.; Bratko, D. J. Chem. Phys. 1998, 108, 16761682. Muthukumar, M. J. Chem. Phys. 1995, 103, 4723-4731. (10) Sumithra, K.; Sebastian, K. L. J. Phys. Chem. 1994, 98, 93129317. Genzer, J. J. Chem. Phys. 2001, 115, 4873-4882.

erogeneity on the adsorption behavior of polymeric chains. However, corroboration by systematic and well-defined experiments has been nonexistent because of the difficulty in meticulous and reproducible preparation of well-defined heterogeneous surfaces on nanometer length scale. Recently we reported the first experimental measurements of adsorption of a neutral polymer (poly(ethylene oxide)) on solid surfaces that were chemically similar but different in their physical roughness on the angstrom level.15,16 Our experiments exploited gold-coated substrates that supported a self-assembled monolayer (SAM) formed from n-alkanethiols.17 We showed that it is possible to design gold substrates that possess different degrees of topographical heterogeneity and that a SAM ensures identical chemical interfaces. Using these model surfaces, we established the effects of the topographical heterogeneity of the surface on polymer adsorption. Our results showed that the maximum amount of adsorption increases with roughness and there is smaller distortion on the rough surface relative to the smooth surface, which verifies past theoretical predictions.14 The experiments also revealed that dynamic aspects such as flattening of the polymer chains on smooth surfaces are intimately connected with (11) Zajac, R.; Chakrabarti, A. J. Chem. Phys. 1997, 107, 86378653. (12) van der Linden, C. C.; van Lent, B.; Leermakers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 1915-1921. (13) Ji, H.; Hone, D. Macromolecules 1988, 21, 2600-2605. Hone, D.; Ji, H.; Pincus, P. A. Macromolecules 1987, 20, 2543-2549. Blunt, M.; Barford, W.; Ball, R. Macromolecules 1989, 22, 1458-1466. Striolo, A.; Prausnitz, J. M. J. Chem. Phys. 2001, 114, 8565-8572. (14) Douglas, J. F. Macromolecules 1989, 22, 3707-3716. Baumgaertner, A.; Muthukumar, M. J. Chem. Phys. 1991, 94, 4062-4070. Edwards, S. F.; Muthukumar, M. J. Chem. Phys. 1988, 89, 2435-2441. (15) Huang, Y.-W.; Gupta, V. K. Langmuir 2002, 18, 2280-2287. (16) Huang, Y.-W.; Gupta, V. K. Macromolecules 2001, 34, 37573764. (17) Ulman, A. Chem. Rev. 1996, 96, 1533-1554.

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the rate of transport of macromolecules to the surface as well as their molecular weight. A phenomenological model based on a two-state picture18 wherein a polymer adsorbs irreversibly in an initial “a” state and then spreads to a second state “b” with a different cross-sectional area could be used to interpret the experimental results. However, the experiments showed that there is a need for modification of existing theoretical models to capture kinetics effects such as the molecular weight dependence of the rate of adsorption from a fundamental perspective. In this paper, we report the first experimental results of the impact of chemical heterogeneity of a surface on polymer adsorption. We demonstrate that SAMs formed by coadsorption of two types of ω-functionalized alkanethiols provide a means to control the chemical patchiness of a surface on the nanometer length scale, which is commensurate with the typical dimensions of a polymer chain. In addition to elucidating the adsorption behavior of a polyelectrolyte on chemically patchy surfaces under two different conditions of ionization (fully charged and uncharged), the results presented here demonstrate that experimental realization of systems studied by previous theory on chemical heterogeneity is feasible for the first time. Experimental Section The cationic polymer, poly(2-vinylpyridine) (P2VP) with Mw ) 385 kDa (Mw/Mn ) 1.08; Polymer Source Inc., Quebec, Canada), was used in the adsorption experiments. P2VP was adsorbed from aqueous solutions onto gold surfaces covered by a monolayer formed from either decanethiol (H3C(CH2)9SH) (C10SH) or mercaptopropionic acid (HOOC(CH2)2SH) (MPA) or mixtures of the two compounds. Template stripped gold surfaces rather than gold evaporated on glass were used as substrates for the SAMs in order to facilitate imaging with STM. Preparation of the template stripped gold substrates has been described in detail elsewhere.16,19 For preparation of monolayers, substrates were rinsed with ethanol and then immersed in 1 mM ethanolic solutions of the alkanethiols for 1 h. Each surface used in adsorption experiments was characterized using three methods. To characterize the chemical patchiness over macroscopic sample areas, cyclic voltammetry was performed in a cell with three electrodes: platinum wire as the counter electrode, Hg|Hg2Cl2 saturated KCl as the reference electrode, and the gold substrates as the working electrode. The gold film was mounted using an O-ring with a working area of 1.5 cm2, and the potential-current characteristics were measured using a potentiostat (EG&G model 173) and universal programmer (model 175) from Princeton Applied Research (Oak Ridge, TN). The cyclic voltammetry (CV) was performed in 0.5 M KOH solution at a scan rate of 20 mV/s. Before each measurement, argon was bubbled through the aqueous solution for 30 min. To image the chemical patchiness of the surface used in adsorption experiments, scanning tunneling microscopy was performed using a Nanoscope IIIa Multimode AFM (Digital Instruments, CA) equipped with a low-current amplifier with 1010 gain and 4 kHz filter and mechanically cut Pt-Ir tips (Digital Instruments). Advancing (θa) and receding (θr) contact angle measurements were performed using a model 100-00 goniometer (Rame´-Hart, NJ) to characterize the hydrophobic chemical interface formed by C10SH. An average of 12 measurements showed θa = 45 ( 1° and θr = 42 ( 1°, which is consistent with an array of methyl groups at the interface. Polymer adsorption was monitored using a home-built surface plasmon resonance (SPR) instrument in the Kretschmann20 that (18) Van Tassel, P. R.; Talbot, J.; Tarjus, G.; Viot, P. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 53, 785798. (19) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 3946. (20) Kretschmann, E.; Raether, H. Z. Naturforsch., A 1968, 23, 21352136. Kooyman, R. P. H.; Lenferink, A. T. M.; Eenink, R. G.; Greve, J. Anal. Chem. 1991, 63, 83-85. Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569-638.

Huang et al. allowed acquisition of the reflectivity curve with high time resolution (∼400 ms). A detailed description of the apparatus, data acquisition, and processing as well as the experimental procedure has been described elsewhere.16 Like other techniques such as ellipsometry and reflectometry that are routinely used to study the adsorption of polymers on planar surfaces, SPR has high surface sensitivity and can noninvasively monitor the adsorption process in real time. The surface excess of the polymer was obtained from the fit of the reflectivity with an optical model based on Fresnel formulas.

Results and Discussion Design and Characterization of Chemically Heterogeneous Surfaces. Previous theoretical studies have considered several means to model composition heterogeneity of a surface and its effect on polymer adsorption. In these approaches, the heterogeneity has been such that surface sites are attractive, repulsive, or inactive toward a polymer chain and their distribution ranges from a periodic pattern to a random pattern on the surface. Furthermore, the surface heterogeneity can be “quenched” such as in a case wherein the sites represent irreversibly attached chemical species that do not diffuse on the surface or “annealed” such as in a case wherein the chemical sites on the surface are free to laterally diffuse on the surface. Design of surfaces using self-assembled monolayers provides a hitherto unexplored approach to experimentally understand the effects of chemical heterogeneity of surfaces on polymer adsorption. It is well-known that millimeter- or micrometer-sized patterns in self-assembled monolayers can be created using techniques such as UV photooxidation, microcontact printing, and micromachining.21 However, these patterns are large compared to dimensions of a polymer chain and not predicted by previous Monte Carlo studies7 to dramatically alter the adsorption behavior of polymer chains. Patterning on length scales smaller than 200 nm using nanolithography is possible but impractical for areas as large as 1 cm2 that are necessary for measurements using optical methods such as SPR. Previous studies have shown that the chemisorption behavior of the different alkanethiols onto gold depends22 on the chain lengths, solvent, and time. Therefore, mixed SAMs formed by coadsorption from a mixture of long and short chain (e.g., XCmSH and YCnSH; m , n) can produce quenched heterogeneous surfaces with a statistical distribution of domains of nanometer size when the relative proportion of the two species and the time of formation are carefully controlled.23,24 Figure 1a shows a low-current STM image of the typical mixed surface used in the polymer adsorption experiments. Bright domains correspond to areas dominated by the longer methyl-terminated alkanethiol (C10SH) while the dark regions are areas rich in the carboxylic acid terminated MPA. The C10SH domains are highlighted using image processing in Figure 1b, which shows that the bright domains occupy 35-40% of the total lateral area. Furthermore, the highlighted image shows that the surface consists of isolated C10SH areas that are as small as a few (21) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (22) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560-6561. Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (23) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1358-1366. (24) Hobara, D.; Ota, M.; Imabayashi, S.-i.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. Hobara, D.; Miyake, K.; Imabayashi, S.-i.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 35903596. Hobara, D.; Sasaki, T.; Imabayashi, S.-i.; Kakiuchi, T. Langmuir 1999, 15, 5073-5078.

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Figure 1. (a) Scanning tunneling microscopy image of mixed monolayer (bias voltage, 1.15 V; current setpoint, 8.0 pA; z-scale, 4.29 nm). (b) Image highlighting the C10SH domains in the STM image. (c) Cumulative area fraction of C10SH domains in the STM image. (d) Cyclic voltammograms of a homogeneous C10SH monolayer (dashed), a homogeneous MPA surface (solid), and the heterogeneous surface (dotted) shown in (a).

nanometers in dimension and also regions where the C10SH domains cluster into larger patches. Figure 1c shows the cumulative area fraction of the bright domains as a function of lateral area. It is evident that the area of the domains on the surface falls predominantly between 100 and 1200 nm2 or that the characteristic size of the domains is approximately between 5 and 20 nm. While imaging by STM gives a microscopic characterization of the nanometer scale morphology of the surfaces, recent electrochemical studies of binary self-assembled surfaces have demonstrated that reductive electrochemical desorption of the thiol using cyclic voltammetry (CV) allows a convenient macroscopic characterization of the nanometer scale surface structure.24,25 This method can be exploited to characterize areas as large as 1 cm2 on the surfaces of the type in Figure 1a. Cyclic voltammograms for homogeneous surfaces of single-component SAMs formed from C10SH and MPA show single peaks at -1.05 and -0.8 V, respectively (Figure 1d). In contrast, CV for the mixed SAM shows two distinct peaks in the reduction current indicating a heterogeneous surface where the COOH(CH2)2SH or CH3(CH2)9SH exist in domains rich in each type of species. The CV of the mixed surface shows that the area under the peak at -1.05 V due to the C10SH(25) Imabayashi, S.-i.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502-4504. Imabayashi, S.-i.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348-2351.

rich domains is approximately 30% of the total area under the two reduction peaks, which is consistent with the value estimated from the STM image. Analysis of the STM images shows that the heterogeneous surfaces possess a distribution of domains that are 5-20 nm in size. The radius of gyration of P2VP was measured to be 22 nm using light scattering and viscometry, which indicates that the polymer size is commensurate to the characteristic size of the heterogeneous sites on the surface. On the basis of results of previous MC simulations,7 the surfaces are well suited for investigation of the effects of chemical heterogeneity. The heterogeneous surfaces designed here also match the surfaces modeled by van der Linden and co-workers12 for self-consistent lattice calculations. In the lattice study, homopolymer adsorption onto heterogeneous surfaces containing attractive and repulsive sites was analyzed and the two types of sites were distributed in either a checkerboard, random, or patchwise manner (Figure 2). Comparison of the idealized surfaces with the monolayers used in experiments here indicates that the latter fits the model of a patchy surface. Adsorption of P2VP on Chemically Heterogeneous Surfaces. P2VP is a cationic polymer that is highly soluble in acidic aqueous solutions and becomes positively charged due to protonation of the pyridine ring.26 Increasing the (26) Marra, J.; Hair, M. L. J. Phys. Chem. 1988, 92, 6044-6051.

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Figure 2. Schematic representation of typical heterogeneous surfaces used in theoretical studies for comparison with the chemically heterogeneous surface used in experiments. Black and white areas indicate two different chemical sites, and the sites are distributed in (a) periodic checkerboard, (b) random, or (c) patchwise manner.

Figure 3. Kinetics of P2VP adsorption from aqueous solutions (concentration 1 ppm). Adsorption on a homogeneous MPA surface (squares), adsorption on a homogeneous C10SH surface (diamonds), and adsorption on a patchy heterogeneous surface (circles) are shown at (a) pH ) 4.5 and (b) pH ) 2.0. In (a), the vertical lines indicate the region where deviation from an initial linear rate of adsorption occurs for the homogeneous surfaces. The adsorbed amount calculated by linear superposition of the Γ(t) measured on the homogeneous surfaces is shown for a weighting of 30% (solid curve) and 55% (dotted curve). See text for discussion. Sparse markers are used with one symbol for every 15 data points.

pH of the aqueous solution reduces the linear charge density of the polymer chain. Between pH 4 and 5, the polymer chain becomes largely uncharged. In the experiments reported here, adsorption was performed from buffer solutions containing 1 ppm P2VP. Two different pH conditions were used such that the polymer chain either was in a fully charged state or was largely uncharged. To obtain a fully protonated P2VP, a pH of 2.0 was chosen and a phosphoric acid/sodium phosphate (monobasic) buffer was used. The ionic strength was kept low at 0.01 M using NaCl. Because the self-assembled surfaces used in the experiments contain carboxylic acid groups, variation of the solution pH can also affect the state of ionization of the acid groups on the surface. Self-assembled monolayers of MPA are known to exhibit a pKa greater than 5.27 Therefore, a pH of 4.5 was chosen to ensure that the

carboxylic acid groups on the surface are not ionized. As the surface remains largely uncharged, the adsorption energy has little contribution from electrostatic interaction of segments with the surface. A sodium acetate and acetic acid buffer was used to establish pH of 4.5, and the ionic strength was maintained at 0.01 M using NaCl. Figure 3 shows the typical kinetic curves for surface excess of P2VP (Γ) on the chemically heterogeneous surface containing both methyl (CH3) and carboxylic acid (COOH) sites. A comparison with Γ(t) on homogeneous surfaces of either MPA or C10SH is also shown. The kinetic curves show a characteristic shape where the initial adsorption shows a constant rate of adsorption. After the initial stage, Γ(t) starts to deviate from the linear part and slowly approaches a plateau. Because equilibration of adsorbed (27) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114-5119.

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Table 1. Initial Rate of Adsorption and Maximum Adsorption Amount for P2VP Adsorption on Homogeneous and Heterogeneous Surfaces of MPA and C10SH pH ) 4.5

homogeneous MPA surface homogeneous C10SH surface heterogeneous surface

pH ) 2.0

(dΓ/dt)0 (mg m-2 s-1)

Γmax (mg m-2)

(dΓ/dt)0 (mg m-2 s-1)

Γmax (mg m-2)

3 × 10-4 3 × 10-4 3 × 10-4

0.57 ( 0.01 1.0 ( 0.01 0.79 ( 0.01

3.1 × 10-4 3.1 × 10-4 3.1 × 10-4

0.32 ( 0.01 0.13 ( 0.01 0.22 ( 0.01

polymer layers occurs over long periods of time, the plateau value is generally assumed to be an adequate measure of the total adsorbed amount. Two characteristics can be noted in the adsorption on the homogeneous surfaces (Figure 3 and Table 1). First, for each type of surface (MPA or C10SH or heterogeneous surface) the value of Γmax is significantly larger at pH 4.5 (∼0.6-1 mg/m2) when compared to the lower pH of 2.0 (∼0.1-0.3 mg/m2). Previous studies6 of polyelectrolyte adsorption have shown that electrostatic repulsion between neighboring segments of a charged polymer chain generally leads to lower Γ as accumulation of charge within the surface layer is not favorable. P2VP becomes ionized at the low pH of 2, and mutual repulsion between segments makes the effective solvency parameter of the polymer more negative. The effective increase in solvency leads to lower Γ at pH ) 2 compared to pH ) 4.5, where the polymer is largely uncharged. Second, in the case of adsorption at pH ) 4.5 (Figure 3a and Table 1), the value of Γmax on the homogeneous methyl (C10SH) surface is almost twice as large (∼1 mg/ m2) as that on the homogeneous carboxylic acid (MPA) surface (∼0.57 mg/m2). In contrast, Figure 3b and Table 1 show that protonation of the polymer chain at pH ) 2.0 causes Γmax on the homogeneous MPA surface to be nearly double that of the Γmax on the C10SH surface. At low pH, the largely hydrophobic backbone is shielded by the hydrophilic (protonated) pyridine group, which decreases the adsorption affinity to the hydrophobic methyl C10SH surface and results in a lower Γmax. At pH 4.5, the pyridine ring is deprotonated and becomes more hydrophobic. Now the adsorption affinity for the C10SH surface becomes higher than that for the MPA surface and leads to a higher Γmax. On the basis of the adsorption behavior on the two homogeneous surfaces, the heterogeneous surface containing domains of C10SH sites can be interpreted to represent two types of situations. At pH 2.0, the minority phase (C10SH) in the smaller domains on the surface corresponds to sites that are less attractive relative to the surrounding regions of MPA on the surface. In contrast at pH 4.5, the C10SH sites are more attractive relative to the MPA sites. The measured Γ(t) for P2VP (Figure 3) on the heterogeneous surface at the two pH conditions shows several interesting results. Figure 3a shows that the initial rate of adsorption on the heterogeneous surface is the same as that observed on the homogeneous C10SH and the MPA surfaces (the similarity is less clear in Figure 3b due to the lower adsorption amounts). In both cases the initial rate of adsorption is close to ∼3 × 10-4 mg m-2 s-1 (Table 1). This result indicates that the presence of heterogeneity has little affect on the kinetics during early times. Figure 3a also shows that Γ(t) has an abrupt plateau after 2000 s in the case of adsorption of P2VP on a MPA, whereas the abrupt plateau occurs after 3400 s in the case of adsorption on a C10SH surface. Physically, this is consistent with a picture of stretched out configuration of the adsorbed chains on the MPA surface and coiled up chains on the hydrophobic C10SH surface, which leads to earlier occurrence of crowding on the MPA surface. In contrast,

on the heterogeneous surface Γ(t) does not show a sharp break following the initial linear rise but instead a more gradual approach toward a plateau region is observed. It is generally accepted that after an initial period of rapid adsorption, chains arriving later in time adsorb onto a few available empty sites and spread to maximize their adsorption energy. In the process, already adsorbed chains also undergo conformational changes. Since on a heterogeneous surface the polymer chains encounter two types of sites and different segment-surface interactions drive the chain toward two different conformations, the gradual approach to a plateau in Γ(t) is, plausibly, a manifestation of slower conformational change of polymer chains on the heterogeneous surface. At both pH values (Figure 3), the adsorbed amount Γmax lies between the value observed on the two types of homogeneous surfaces (Table 1). Thus, even though the initial kinetics are not altered dramatically by the presence of heterogeneity, the surface heterogeneity does impact later stages of adsorption. Physically, the intermediate value of Γmax indicates that the effective surface adsorption energy of the polymer chain represents some average of the interactions with the two different sites. The minority phase (C10SH) in the smaller domains on the surface are less attractive at pH 2.0 but more attractive at pH 4.5 when compared to the surrounding MPA regions. However, Figure 3 shows an intermediate Γmax at both pH values on the heterogeneous surface. The experiments suggest that in the case of a flexible polymer such as P2VP, the average interaction with the surface is more significant in determining Γ rather than the repulsive or attractive nature of the minority domains. Figure 3a also shows the adsorbed amount calculated using a linear superposition of the Γ(t) measured on homogeneous surfaces:

Γ(t) ) xΓC10SH(t) + (1 - x)ΓMPA(t)

(1)

Although no weighting is found to give an exact match for the experimentally measured curve on the heterogeneous surface, the measured Γ(t) lies between the two cases illustrated at x ) 0.3 and x ) 0.55. The comparison between the weighted average and the measured Γ(t) indicates that adsorption amount on the heterogeneous surface is dependent on the percent fraction of C10SH domains but cannot be predicted using a simple linear relation such as eq 1. Ongoing experiments focus on the interesting question of how variation in the size and fraction of heterogeneous sites influences the adsorption behavior, especially the maximum adsorbed amount as well as the intermediate region between the initial rate and the plateau in Γ. These first experimental results on compositionally heterogeneous surface verify theoretical predictions regarding polymer adsorption on chemically heterogeneous surfaces. Van der Linden and co-workers12 used a selfconsistent lattice model to study the influence of the degree of clustering of attractive and repulsive sites. On a surface with a patchwise distribution of chemical sites, which is similar to the experiments reported here, the lattice model

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predicted that the homopolymer adsorption was proportional to the fraction of adsorbing sites. Figure 3 demonstrates the adsorption behavior in a real system closely matches the lattice model. Zajac and Chakrabarti11 used a Monte Carlo simulation to explore the adsorption of a homopolymer on chemically heterogeneous surfaces containing 30-50% of sites that either were repulsive to the polymer or were simply inactive. They predicted that during the early stages the presence of surface impurities did not alter the growth kinetics of the adsorbed layer. However, as chains spread on the surface and competed for sites, there was a net decrease in the number of chains that were adsorbed and that the decrease was proportional to the fraction of inactive or repulsive surface sites. The kinetic curves of Γ(t) measured in Figure 3 corroborate the predictions from the simulations. At both pH 2.0 and 4.5, where a charged or uncharged polymer adsorbs on an uncharged heterogeneous surface, the initial rate of adsorption remains identical to the rate on homogeneous surfaces with only one kind of chemical site. The experiments reported here and the agreement with theoretical studies demonstrate that in addition to fundamental insights into adsorption on heterogeneous surfaces, we can now pursue experimental systems that exploit the interactions between lateral nanoscale mor-

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phology of a surface and an adsorbing polymer chain. For example, recent simulations9 have shown that a pattern of distributed charges on a surface can be recognized during the adsorption of an oppositely charged polyelectrolyte chain. Similarly others have predicted that the nanoscale morphology of a surface and the heterogeneous chemical interactions with a polymer chain can be used to direct segregation or patterning within adsorbed layers.7,8 The present strategy of using molecularly designed self-assembled surfaces for polymer adsorption enables exploration of physical phenomena such as recognition and segregation in polymer adsorption on heterogeneous surfaces and promises both scientific and technological advances. Acknowledgment. This material is based upon work supported by the U.S. Department of Energy, Division of Materials Sciences under Award No. DEFG02-91ER45439, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign. Support from the National Science Foundation (Career Award to V.K.G.) and the University of Illinois is also acknowledged. LA026284D