Desorption of Mono- and Diblock Copolymers on Surfaces

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Adsorption/Desorption of Mono- and Diblock Copolymers on Surfaces Using Specific Hydrogen Bonding Interactions Amitav Sanyal, Tyler B. Norsten, Oktay Uzun, and Vincent M. Rotello* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003 Received January 30, 2004. In Final Form: April 27, 2004 Diblock copolymers containing recognition units designed to participate in specific three-point hydrogen bonding were adsorbed onto modified gold surfaces. Self-assembled monolayers (SAMs) containing complementary recognition units were used to direct the adsorption process. The polymer-modified surfaces obtained were characterized using X-ray photoelectron spectroscopy, water contact angle, and ellipsometry. The role of individual block lengths on the adsorption process was followed by observing frequency changes of thymine-SAM-modified quartz crystal microbalance chips during adsorption of diamidopyridinefunctionalized polymers from a nonpolar solvent. The renewable nature of these recognition unit functionalized surfaces was demonstrated by reversible binding of polymers. Adsorption onto fresh surfaces, followed by desorption and subsequent readsorption of monoblock and diblock copolymers was also investigated.

Introduction In recent years, polymer modification of surfaces has attracted widespread attention for applications in coatings, adhesives, sensors, and biomedical devices.1 Physisorption,2 electrostatic self-assembly,3 and covalent attachment4 of polymers onto surfaces have been extensively explored; however, use of thermoreversible hydrogen bonding interactions has been mainly limited to smallmolecule surface binding using self-assembled monolayers (SAMs).5 Use of SAMs allows for easy chemical modification of latent/unreactive surfaces such as noble metals and provides a simple means to tailor surface properties at the molecular level.6 To this end, various polymers have * To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J. Science 1997, 275, 1458. (b) Black, F. E.; Hartshorne, M.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M.; Shakesheff, K. M. Langmuir 1999, 15, 3157. (2) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 2229. (3) (a) Decher, G. Science 1997, 277, 1232. (b) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (c) van Manen, H.-J.; Auletta, T.; Dordi, B.; Scho¨nherr, H.; Vancso, G. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Funct. Mater. 2002, 12, 811. (d) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 8518. (e) Maskus, M.; Abrun˜a, H. D. Langmuir 1996, 12, 4455. (4) (a) Hussemann, M.; Malmstro¨m, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (b) Zhao, B.; Brittain, W. J. J. Am. Chem. Soc. 1999, 121, 3557. (c) Crooks, R. M. ChemPhysChem 2001, 2, 644. (d) Bruening, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770. (5) (a) Zou, S.; Zhang, Z.; Fo¨rch, R.; Knoll, W.; Scho¨nherr, H.; Vancso, G. J. Langmuir 2003, 19, 8618. (b) Ginzburg, M.; Galloro, J.; Ja¨kle, F.; Power-Billard, N. K.; Yang, S.; Sokolov, I.; Lam, C. C. N.; Neumann, W. A.; Manners, I.; Ozin, G. A. Langmuir 2000, 16, 9609. (c) Godı´nez, L. A.; Lin, J.; Mun˜oz, M.; Coleman, A. W.; Rubin, S.; Parikh, A.; Zawodzinski, T. A., Jr.; Loveday, D.; Ferraris, J. P.; Kaifer, A. E. Langmuir 1998, 14, 137. (d) Garcia-Lopez, J. J.; Zapotoczny, S.; Timmerman, P.; van Veggel, F. C. J. M.; Vancso, G. J.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Commun. 2003, 352. (e) Credo, G. M.; Boal, A. K.; Das, K.; Galow, T. H.; Rotello, V. M.; Feldheim, D. L.; Gorman, C. B. J. Am. Chem. Soc. 2002, 124, 9036. (f) Park, J. S.; Lee, G. L.; Lee, Y.-J.; Park, Y. S.; Yoon, K. B. J. Am. Chem. Soc. 2002, 124, 13366. (g) Miura, Y.; Xu, G.-C.; Kimura, S.; Kobayashi, S.; Iwamoto, M.; Imanshi, Y.; Umenura, J. Thin Solid Films 2001, 393, 59. (h) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206. (i) Arias, F.; Godı´nez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086.

been “grafted to” or “grafted from” using SAM-modified surfaces.7 In recent studies, we have demonstrated that the thymine-diamidopyridine (Thy-DAP) hydrogen bonding motif is a simple and specific interaction for recognition and immobilization of molecular and macromolecular systems at the surface (Figure 1). In a recent study, we disclosed that similar multivalent noncovalent interactions between DAP-containing monoblock copolymer and the surface can provide a robust immobilization strategy.8 Highly selective adsorption of DAP-containing monoblock copolymers onto a thymine-decorated gold surface was achieved under carefully controlled deposition conditions. While the use of a monoblock copolymer provides access to surface coatings with a variety of properties, diblock copolymers provide a more flexible strategy for both immobilization and surface modification. In diblock copolymers, one of the blocks can be utilized to provide the required complementary interactions with the underlying substrate, and the other block can then be designed to present the desired functional attributes at the interfacial region (Figure 1). A single complementary three-point hydrogen bond dyad (such as Thy-DAP) provides a labile interaction; however, when several of these dyads are applied in concert by multiple functionalizations along a polymer backbone stable polymeric films can be achieved.8 The reversible nature of such hydrogen bonding interactions can be exploited to engineer thermally and chemically responsive/ recyclable surfaces. The most commonly used technique for surface modification involves electrostatic adsorption of polyanions and polycations onto complementary charged substrates. In an alternative approach, fabrication of (6) (a) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (7) (a) Weck, M.; Jackiw, J. J.; Rossi, R. R.; Weiss, P. S.; Grubbs, R. H. J. Am. Chem. Soc. 1999, 121, 4088. (b) Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. 1999, 38, 647. (c) Isaacs, L.; Chin, D. N.; Bowden, N.; Xia, Y.; Whitesides, G. M. In Supramolecular Materials and Technology; Reinhoudt, D. N., Ed.; Wiley: Weinheim, Germany, 1999; pp 14-24. (d) Ulman, A. Chem. Rev. 1996, 96, 1533. (e) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (8) Norsten, T. B.; Jeoung E.; Thibault, R. J.; Rotello, V. M. Langmuir 2003, 19, 7089.

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Figure 1. Schematic illustration of the binding of monoblock copolymer and diblock copolymer to thymine-(NH)/octanethiol(OT) functionalized SAM1 via three-point hydrogen bonding at the interface.

multilayer thin films using hydrogen bonding interactions has been demonstrated by several groups.9 In contrast to the ionic layer-by-layer process, fabrication of polymeric thin films using noncovalent interactions provides chargeneutral surfaces crucial for protein resistive and biocompatible materials.10 Furthermore, one can develop orthogonal fabrication strategies using mixtures of complementary recognition dyads thus expanding upon the electrolyte-based binary systems. In this report, we demonstrate surface modifications of gold surfaces covered with mixed SAMs comprised of octanethiol and thymine-thiol (SAM1) using monoblock and diblock copolymers functionalized with diamidopyridine-based recognition units (Figure 2). We also explore the effect of individual block length on surface adsorption and film characteristics. Finally the reversible nature of hydrogen bonding is utilized to regenerate the original surface by desorption of the polymer films. In these studies, we utilized the regenerated surface to adsorb a different polymer, as well as demonstrate the ability to cycle through this process. Experimental Section Preparation of Polymers and SAMs. The diamidopyridine-containing monoblock and diblock copolymers (1 and 2a-d) were obtained by postpolymerization functionalization of chloromethyl styrene based polymers obtained using nitroxide-mediated living free radical polymerization, as reported previously.11 The functionalized block in the diblock copolymers consisted of 17(9) (a) Caruso, F.; Quinn, J. F. Langmuir 2004, 20, 20. (b) Kharlampieva, E.; Sukhishvili, S. A. Macromolecules 2003, 36, 9950. (c) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (d) Bergbreiter, D. E.; Tao, G.; Franchina, J. G.; Sussman, L. Macromolecules 2001, 34, 3018. (e) Sukhishvili, S. A.; Granick, S. J. Am. Chem. Soc. 2000, 122, 9550. (f) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (10) (a) Ostuni, E.; Chapman, R. G.; Holmlin R. E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605. (b) Sandana, A. Chem. Rev. 1992, 92, 1799. (11) Frankamp, B.; Uzun, O.; Ilhan, F.; Boal, A.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892.

Figure 2. Structures of the monoblock and diblock copolymers and thymine-(NH)/octanethiol-(OT) functionalized SAM1 and N-methyl thymine-(NMe)/octanethiol-(OT) SAM2.

20% chloromethyl styrene units. Complete conversion of all the chloro substituents to the diamidopyridine units was achieved using SN2 displacement with 1.1 equiv DAPOH/1.5 equiv K2CO3 in DMF at 65 °C for 16 h. The DAPfunctionalized polymer 38,12 (Mn ) 4800, PDI ) 1.80, Figure 6) containing a tag for X-ray photoelectron spectroscopy (fluorine) and UV (porphyrin) was used for preliminary adsorption-desorption-readsorption studies of the monoblock.13 The diblock copolymer 4 was synthesized using the nitroxide-mediated living free radical polymerization procedure using 4-fluorostyrene as a comonomer in the synthesis of the first block.14 Full details of the Mn and PDI of individual polymers are tabulated in the Supporting Information. Preparation of Monolayers. Gold substrates (50 Å Ti/1000 Å Au, 15 mm × 15 mm × 1.5 mm) for ellipsometry, contact angle, and X-ray photoelectron spectroscopy (XPS) measurements were purchased from Evaporated Metal Films (Ithaca, NY). Quartz crystal microbalance (QCM) chips (fundamental resonance frequency f0 ) 10 MHz) (12) IIhan, F.; Gray, M.; Rotello, V. M. Macromolecules 2001, 34, 2597. (13) (a) Niemz, A.; Jeoung, E.; Boal, A. K.; Deans, R.; Rotello, V. M. Langmuir 2000, 16, 1460. (b) Su, Z.; Wu, D.; Hsu, S. L.; McCarthy, T. J. Macromolecules 1997, 30, 840. (14) Benoit, D.; Chaplinski, V.; Braslau, R.; Hawker, C. J. J. Am. Chem. Soc. 1999, 121, 3904.

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Figure 3. (a) Data from XPS multiplex scans of unmodified and polymer-modified SAM1 recorded with takeoff angles of 15°. (Top) C1s peaks. Curve A1: unmodified SAM1. Curves B1, C1, and D1: SAM1 modified with polymer 1, 2a, and 2c, respectively. (Bottom) Au4f peaks. Curve A2: unmodified SAM1. Curves B2, C2, and D2: SAM1 modified with polymer 1, 2a, and 2c, respectively. (b) Graph of atomic % of C and Au for SAM1 and SAM2 as a function of diblock copolymers 1, 2a, and 2c.

were purchased from International Crystal Manufacturing Co. Gold substrates and QCM chips were cleaned for 10 min in a piranha etch (H2SO4 concentrated + 30% H2O2, 4:1, 70 °C; caution: strong oxidant), rinsed with distilled/ deionized H2O and EtOH, respectively, and placed directly into the thiol solution. The thymine SAM1 was prepared by immersing thin films of evaporated gold on the appropriate substrates (glass or quartz) into 0.01 M ethanol solutions of thymine-thiol and octanethiol (1:3 v/v) for 16-20 h under an argon atmosphere.8 Physisorbed materials were then removed by repeated rinses with ethanol followed by chloroform. SAMs containing the N-methylthymine (NMe) functionality (SAM2) were prepared likewise to provide a control surface incapable of participating in a three-point hydrogen bonding. QCM Apparatus. The QCM studies were conducted using a 10.000 MHz lever oscillator purchased from International Crystal Manufacturing Co. (no. 35366) and a Metex MXC-1600 universal counter with power supply. The frequency counter was interfaced to a standard PC, and Benchview V1.0 software was used to receive and view the data. Polymer adsorption experiments were performed by immersing a clean crystal in a fresh solution of chloroform (9 mL) to obtain the initial baseline frequency under liquid oscillation conditions. A chloroform solution of the polymer (1 mL) was injected into the cell to yield the final polymer concentrations (10 µg/mL). Polymers were desorbed by removing the crystal from the adsorption solution and subsequently subjecting the crystal to the desorption protocols outlined in the discussion. The crystal was then placed into a fresh solution of CHCl3, and the corresponding frequency was recorded. X-ray Photoelectron Spectroscopy. XPS spectra were obtained on a Physical Electronics Quantum 2000 XPS spectrophotometer using a monochromatic Al KR source. Spectra were acquired at takeoff angles of 15° from the surface unless otherwise specified. Survey spectra were recorded at a pass energy of 117.40 eV, while multiplex regions were recorded with a pass energy of 35.75 eV with acquisition times ranging from 10 to 15 min. Results and Discussion XPS, Ellipsometry, and Wettability of Modified Surfaces. Polymer-modified gold surfaces were obtained

by dipping SAMs 1 and 2 into chloroform solutions of polymer 1, 2a, and 2c (10 µg/mL) for a period of 15 min. The SAMs were then thoroughly rinsed with fresh chloroform to remove any physisorbed polymer. As expected, upon XPS analysis of polymer-coated SAM1 the Au4f doublet (centered at 86 eV) decreases upon deposition of both the monoblock and diblock copolymers (Figure 3a, curves B2-D2). Further evidence of polymer deposition was provided by the increase in the peak corresponding to C1s (286 eV) (Figure 3a, curves B1-D1). Collectively, these data indicate that the copolymers are spread over the SAM surface. In contrast, N-methylthymine-containing SAM2 showed no polymer uptake under identical conditions (Figure 3b), as demonstrated by very little changes in both the Au4f and the C1s peak intensities. These results indicate that multivalent specific hydrogen bonding interactions at the polymer-SAM interface provide efficient immobilization of the diblock copolymers. Ellipsometry and dynamic water contact angle measurements were used to determine the thickness of the polymer layer and homogeneity of the modified surface. The thickness of deposited films was determined for surfaces 1 and 2 after exposure to the various polymers (Table 1). These measurements were found to be comparable with those for other ultrathin polymer films adsorbed from solution15 and consistent with complete monolayer coverage. Also, the films obtained using the diblock copolymers were slightly thicker. As expected, virtually no film was detected by ellipsometry for the N-Me thymine SAM2. Alteration of the physical properties at the interfacial boundary also provides a good indication of surface modification.16 The dynamic water contact angle measurements of SAMs 1 and 2 prior to polymer deposition were consistent with previously reported values.8 In all cases, polymer deposition on surface 1 results in an increase in the advancing contact angles, with the final contact angle values of the polymer-modified surfaces similar to those previously reported for polystyrene films.17 (15) Motschmann, H.; Stamm, M.; Toprakcioglu, C. Macromolecules 1991, 24, 3681. (16) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (17) Extrand, C. W. Langmuir 1993, 9, 475.

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Table 1. Dynamic Water Contact Angles and Ellipsometric Analysis for Surfaces 1 and 2 for Adsorption of Polymersa SAM 1

2

polymer (DAP block/PS block)

ellipsometric analysisb of film thickness (Å)

0 1a (12K) 2a (7K/27K) 2c (27K/27K) 0 1a (12K) 2a (7K/27K) 2c (27K/27K)

0 15.4 19.2 21.6 0d 0d 0d 0d

dynamic water contact anglesc advancing (θ, deg ) receding (θ, deg ) hysteresis (θ, deg ) 71.0 85.8 85.8 86.0 76.8 76.4 80.3 77.4

49.7 56.4 68.6 68.6 62.0 63.8 65.0 61.6

21.3 29.4 17.2 17.4 14.8 16.2 15.3 15.8

a All adsorption was conducted at 10 mg/mL polymer for 15 min. b Values for the film thicknesses were calculated using the refractive index of octanethiol and polystyrene, represent an average of five measurements at multiple positions on the surface, and are within (1-2 Å. c Values reported represent an average of five separate measurements and are within (1-2°. d No film was detected.

The contact angle hysteresis provides an indication of surface heterogeneity and overall surface roughness. The water contact angle results indicate that the film becomes more uniform upon deposition of diblock copolymers, as demonstrated by the noticeable decrease in the hysteresis as compared to that of the monoblock film (Table 1). From the hysteresis data, one can deduce that the monoblock copolymer may not cover the surface efficiently, presenting both a rough and chemically nonhomogeneous surface (due to unbound DAP units) to the receding edge of the water droplet resulting in the larger measured hysteresis. In all cases, the diblock copolymer covered surfaces exhibited lower hysteresis compared to the surface obtained by coverage with the monoblock copolymer. The surface heterogeneity is reduced since the unfunctionalized polystyrene block aids in creating a smoother, slightly thicker, and more chemically uniform film. As expected, the control surface SAM2 gave no indication of polymer uptake by water contact angle analysis under similar conditions, in agreement with the results obtained by ellipsometry. QCM Studies: Role of Individual Block Lengths. In the past, we have demonstrated that QCM can be a powerful tool to monitor the adsorption of functionalized polymers onto SAMs decorated with complementary hydrogen bonding recognition units.8 To determine if there is any compromise in the selectivity due to an additional unfunctionalized block and also analyze the polymer uptake in real time, we studied the deposition of two diblock copolymers, differing only in the length of the unfunctionalized polystyrene block. To monitor polymer deposition using QCM experiments, SAMs of 1 and 2 were prepared as described above. These chips were mounted onto the QCM oscillator and submerged into a solution containing fresh chloroform. When a steady baseline had been achieved, a chloroform solution containing the appropriate concentration of polymer was injected to give the corresponding final polymer concentrations. Polymer adsorption on the surface was then followed by monitoring the corresponding change in resonant frequency of the modified quartz chips.18 Adsorptions on control surfaces were run in parallel to ascertain the extent of physisorption under similar conditions. After the initial rapid uptake of polymers by surface 1, the rate of deposition between the two surfaces became very similar (Figure 4), indicating that a small amount of nonspecific adsorption occurs in both cases. (18) For papers describing QCM surface adsorption studies see: (a) Marx, K. A. Biomacromolecules 2003, 4, 1099. (b) Ha, T. H.; Kim, K. Langmuir 2001, 17, 1999. (c) Kim, D. H.; Noh, J.; Hara, M.; Lee, Bull. Korean Chem. Soc. 2001, 22, 276. (d) Sastry, M. Bull. Mater. Sci. 2000, 23, 159. (e) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645. (f) Roush, J. A.; Thacker, D. L.; Anderson, M. R. Langmuir 1994, 10, 1642.

Figure 4. QCM frequency responses to SAM1 and SAM2 in CHCl3 after addition of polymer 2b and 2d.

A proportional decrease in the frequency was observed with increase in the length of the second block. This observation is consistent with the Sauerbrey equation that the decrease in the observed resonant frequency of the quartz chip is proportional to the mass uptake.19 However, some of the observed decrease in the resonant frequency may be attributed to crystal damping as a result of longer polymer chains dangling in solution. The diblock copolymer 13K/13K decreases the frequency by 310 Hz. Addition of an unfunctionalized block that is twice as long (27K) further decreases the frequency by another 150 Hz to a net decrease of 460 Hz. The fact that increasing the overall mass of individual polymer chains results in a decrease in the frequency in a proportional manner suggests that the number of polymer chains adsorbing onto the surface is mainly dependent on the block length of the “adhesive” unit. Next we investigated a set of diblock copolymers in which the length of the DAP-functionalized block was varied while keeping the length of the unfunctionalized block constant. The polymer with a shorter adhesive block produced a larger shift in the resonant frequency of the quartz chip (Figure 5). This implies that a larger number of polymer chains become immobilized onto the given surface when the length of the recognition unit functionalized block is decreased, since a smaller DAP-containing block will leave more free binding sites for subsequent polymer uptake. Renewable Surfaces. Aside from the generation of charge-neutral surfaces using noncovalent bonding, these systems offer the possibility of regenerating the thyminefunctionalized surface, allowing refunctionalization with different polymers (Figure 6a). For the initial study, we chose the 5K fluoro-porphyrin-tagged DAP-functionalized monoblock copolymer 3, which contained a fluoro-por(19) Sauerbray, G. Z. Phys. 1959, 155, 206.

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Figure 5. QCM frequency responses for DAP-functionalized diblock copolymers 2a, 2b, and 2c adsorbing onto SAM1.

Figure 6. (a) Schematic illustration of regeneration of SAM1 employing thermal desorption. (b) Structure of porphyrintagged monoblock copolymer 3.

phyrin tag as an XPS, UV-vis, and fluorescence marker (Figure 6b).20 The polymer was adsorbed onto the gold surfaces, and excess physisorbed polymer was rinsed off using chloroform. The XPS of the surface showed the appearance of the F1s peak (689 eV), increase in the C1s peaks (285 eV), and a concomitant decrease in the Au peak intensity (Figure 7), implying successful immobilization of the thin polymer film. The modified surface was further characterized using ellipsometry and contact angle measurements.21 Then the same surface was dipped in an EtOH/ CHCl3 (1:3) mixture at 60 °C for 10 min. After the surface was washed with chloroform, the XPS measurement was conducted. Virtually no corresponding peak for F1s was (20) Control experiments demonstrated that presence of the porphyrin unit on the polymer does not have an effect on polymer adsorption. (21) Film thickness and contact angle measurements during each stage of the adsorption-desorption protocol are tabulated in the Supporting Information.

Figure 7. (a) XPS multiplex regions for SAM1 from adsorption/ desorption/readsorption of polymer 3. Curves A1, B1, C1, and D1 denote changes in F1s peak intensity for the native SAM1, SAM1 modified with polymer 3, regenerated SAM1 upon polymer desorption, and SAM1 upon readsorption of polymer 3, respectively. Curves A2, B2, C2, and D2 denote C1s peaks at each stage, and curves A3, B3, C3 and D3 denote Au4f peaks at each stage. (b) A plot of the atomic percent of F, Au, and C as deduced from XPS at each step of the process.

detected, while a decrease in the C1s and an increase in the Au4f peak intensities were observed (Figure 7). This confirmed that the polymer was desorbed from the surface successfully. Additional measurements using ellipsometry and contact angle corroborated the observations. Once again, fresh polymer was specifically adsorbed onto the newly regenerated surface as characterized by XPS, thickness, and contact angle measurements. In a separate experiment, the N-H thymine containing surface (SAM1) was subjected to the desorption conditions at 60 °C, and XPS measurements revealed that the monolayer remains largely intact under the desorption protocol.22

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Figure 8. (a) QCM frequency responses for SAM1 from adsorption/desorption/readsorption of polymer 3. Points A, B, and C denote the frequency response of the polymer-coated QCM chip after a CHCl3 rinse, an EtOH/CHCl3 rinse at room temperature, and an EtOH/CHCl3 rinse at 55 °C, respectively. (b) UV-vis and fluorescence of the EtOH/CHCl3 solutions containing the desorbed polymer 3.

A closer look at the adsorption profiles and intermediate desorption steps was undertaken using QCM. The random copolymer (3) containing a porphyrin tag was adsorbed onto a freshly prepared SAM1 surface. Thereafter the QCM chip was removed from the adsorption solution and rinsed off with CHCl3 at room temperature. After drying in a stream of nitrogen, it was mounted back on the microbalance and dipped into fresh CHCl3 solution. A small amount of polymer that was physisorbed was washed off the surface, as suggested by the small change in the QCM frequency (Figure 8a). This was additionally confirmed by the presence of very weak UV and fluorescence signals (from the porphyrin tag), after evaporation of the CHCl3 wash (Figure 8b). Slightly more polymer was desorbed at room temperature upon using a 1:3 mixture of EtOH/CHCl3, a more competitive solvent system. Upon heating the surfaces in the EtOH/CHCl3 mixture for 15 min, most of the adsorbed polymer came off, as suggested by the increase in resonant frequency of the chip and strong UV and fluorescence signals from the evaporated residues. Addition of fresh polymer to the regenerated surface showed an uptake profile similar to that of a new surface. The QCM experiment provided the expected adsorption profiles and corroborated the observations deduced from previous surface characterization studies. (22) See the Supporting Information. XPS spectra before and after subjecting SAM1 to desorption conditions remain similar.

Figure 9. (a) Schematic illustration of adsorption-desorptionreadsorption of diblock copolymers 2a and 4 onto SAM1. (b) Percent change in F, Au, and C content on the surface at each stage of surface modification monitored using XPS. (c) Change in the thickness of the polymer film on SAM1 and advancing contact angle at each stage of the surface modification study.

The unfunctionalized block exposed on the surface and covering the functionalized block could make desorption of the bound polymer difficult. To investigate the effect of the unfunctionalized block, adsorption-desorption-readsorption studies similar to the ones carried out with the monoblock copolymer were conducted (Figure 9a). Surface analysis using XPS, ellipsometry, and contact angle measurements showed that while the expected efficient adsorption was observed for 4-fluoro styrene containing diblock copolymer 4 (PS-DAP/PS-F), little or no diblock copolymer was present on the surface after the desorption step (Figure 9b,c). At this time, a different diblock copolymer comprising both polystyrene blocks, polymer 2a (PS-DAP/PS (7k/27k)), was chosen for adsorption to

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the regenerated surface. Although this did not have any distinct XPS tag, the expected decrease in the Au peak intensity, increase in the C peak intensity, and increase in thickness and contact angle indicated effective polymer adsorption. Subsequently this polymer could be efficiently removed and the initially adsorbed diblock copolymer 4 could be readsorbed on the surface. Conclusions In summary, we have demonstrated that the highly specific Thy-DAP hydrogen bonding motif provides rapid and selective adsorption of diblock copolymers on the surface. The choice of the adhesive block length can provide control over the number of polymer chains adsorbed onto the surface. The nonadhesive block can be chosen depending on the chemical or physical properties desirable on the surface. Although the multiple hydrogen bonding interactions allow adsorption of the polymer layer, the reversible nature of such noncovalent interactions can be utilized for regenerating the surface by desorption of the

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polymer from the surface induced by an appropriate choice of solvent system and temperature. This approach to noncovalent surface modification provides a versatile route to the formation of reusable surfaces. Studies extending these concepts to silicon and other metallic surfaces are underway and will be reported in due course. Acknowledgment. This research was supported by the National Science Foundation (DMR-9809365, MRSEC) and (CHE-0213554). T.B.N. acknowledges NSERC (Canada) for their support in the form of a postdoctoral fellowship. Supporting Information Available: XPS spectra of SAM1 before and after being subjected to desorption protocol, thickness, and contact angle measurements for adsorption/ desorption/readsorption of polymer 3; GPC traces and NMR spectra of all discussed polymers. This material is available free of charge via the Internet at http://pubs.acs.org. LA049737I