Motion of Integrated CdS Nanoparticles by Phase Separation of Block

Jul 17, 2007 - Kai Yu, Hanfu Wang, and Yanchun Han*. State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,...
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Langmuir 2007, 23, 8957-8964

8957

Motion of Integrated CdS Nanoparticles by Phase Separation of Block Copolymer Brushes Kai Yu, Hanfu Wang, and Yanchun Han* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China ReceiVed April 11, 2007. In Final Form: May 24, 2007 A new method of reversibly moving CdS nanoparticles in the perpendicular direction was developed on the basis of the phase separation of block copolymer brushes. Polystyrene-b-(poly(methyl methacrylate)-co-poly(cadmium dimethacrylate)) (PS-b-(PMMA-co-PCdMA)) brushes were grafted from the silicon wafer by surface-initiated atom transfer radical polymerization (ATRP). By exposing the polymer brushes to H2S gas, PS-b-(PMMA-co-PCdMA) brushes were converted to polystyrene-b-(poly(methyl methacrylate)-co-poly(methacrylic acid)(CdS)) (PS-b-(PMMAco-PMAA(CdS))) brushes, in which CdS nanoparticles were chemically bonded by the carboxylic groups of PMAA segment. Alternating treatment of the PS-b-(PMMA-co-PMAA(CdS)) brushes by selective solvents for the outer block (a mixed solvent of acetone and ethanol) and the inner PS block (toluene) induced perpendicular phase separation of polymer brushes, which resulted in the reversible lifting and lowering of CdS nanoparticles in the perpendicular direction. The extent of movement can be adjusted by the relative thickness of two blocks of the polymer brushes.

Introduction The extensive study of motion of nano-objects bridges the gap between the present generation of synthetic molecular systems, which by and large rely upon external stimuli to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movement of smaller parts to perform specific tasks. Constructing such synthetic systems and applying those to nanoscale devices will provide new insights into both the fundamental research of mechanics of motion and potential technical applications in nanoengines.1-4 One of the central challenges in this scientific research area is how to assemble the nano-objects into the addressable location, where the nano-objects can be synthesized, self-assembled in solution, or fabricated at the nanoscale. Atomic force microscopy and nanomotors based on protein complex5-9 have recently been applied for the positioning of nano-objects. Several groups have demonstrated that, by using SFM probe tips, it is possible to arrange a small number of clusters or atoms on a flat surface.2,5,6 Although this approach is very promising, it is limited for its low throughput, for only moving one molecule at a time. The latter method of nanomanipulation, which holds the promise of applicability in a highly parallel manner, is the in vitro use of biomolecular * State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences; Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China. Tel: 86-431-85262175, Fax: 86-431-85262126, E-mail: [email protected]. (1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72-191. (2) Sheehan, P.; Lieber, C. M. Science 1996, 272, 1158-1161. (3) Hess, H.; Clemmens, J.; Qin, D.; Howard, J.; Vogel, V. Nano Lett. 2001, 1, 235-239. (4) Santer, S.; Kopyshev, A.; Donges, J.; Yang, H. K.; Ru¨he, J. AdV. Mater. 2006, 18, 2359-2362. (5) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K.; Tang, H.; Joachim, C. Science 1996, 271, 181-184. (6) Hsieh, S.; Meltzer, S.; Wang, C. R. C.; Requicha, A. A. G.; Thompson, M. E.; Koel, B. E. J. Phys. Chem. B 2002, 106, 231-234. (7) Schliwa, M.; Woehlke, G. Nature (London) 2003, 422, 759-765. (8) Hess, H.; Matzke, C. M.; Doot, R. K.; Clemmens, J.; Bachand, G. D.; Bunker, B. C.; Vogel, V. Nano Lett. 2003, 3, 1651-1655. (9) Ionov, L.; Stamm, M.; Diez, S. Nano Lett. 2006, 6, 1982-1987.

motors based on protein complex, such as kinesin/microtubules and myosin/actin.7-9 Applications of these biological motors in nanoengineering are intriguing, as they possess a high efficiency, are small, and are available in large numbers. However, the protein complex only can function in buffered aqueous solution within a narrow temperature range. Recently, another approach of moving nano-objects by using homopolymer brushes10-13 or binary-component polymer brushes4,14,15 was presented. Polymer systems in their diversity may offer a range of alternatives, especially in the form of suitably designed thin films. In the case of homopolymer brushes, movement of nano-objects is based on the switching of polymer brushes from extended conformation to collapsed conformation over wetting-drying,10 alternating submersion in its good solvent and poor solvent11,12 or upon pH change of aqueous solution.13 The mechanism of moving nanoobjects using binary-component polymer brushes is quite different from the homopolymer brushes. In the case of binary-component polymer brushes, motion of nanoparticles is based on the phase separation of the two chemically different polymers which constitute the binary-component brushes. To the best of our knowledge, motion of nanoparticles by binary-component polymer brushes was only presented by Santer. 4,14,15 In Santer’s work using binary-component polymer brushes to move silica nanospheres, periodic switching of the underlying topography of the polymer brushes changed the dynamically competing surface force field, which induced the motion of silica nanospheres adsorbed on the polymer brushes in the horizontal direction. By using this method, they can irreversibly move the isolated nanospheres across the polymer brushes to form larger aggregates. Actually, the nanospheres cannot be transported in a predefined (10) Bhat, R. R.; Genzer, J.; Chaney, B. N.; Sugg, H. W.; Liebmann-Vinson, A. Nanotechnology 2003, 14, 1145-1152. (11) Ionov, L.; Sapra, S.; Synytska, A.; Rogach, A. L.; Stamm, M.; Diez, S. AdV. Mater. 2006, 18, 1453-1457. (12) Westenhoff, S.; Kotov, N. A. J. Am. Chem. Soc. 2002, 124, 2448-2449. (13) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950-15951. (14) Prokhorova, S. A.; Kopyshev, A.; Ramakrishnan, A.; Zhang, H.; Ru¨he, J. Nanotechnology 2003, 14, 1098-1108. (15) Santer, S. A.; Ru¨he, J. Polymer 2004, 45, 8279-8297.

10.1021/la701053d CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

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direction and require many switching cycles to implement motion. In some respects, this motion is similar to Brownian motion. Besides motion of nanoparticles in the horizontal direction, motion of nanoparticles in the perpendicular direction is also important for technical applications in nanoengines. Motion of semiconductor nanoparticles in the perpendicular direction can change the distance between the nanoparticles and the substrate. Consequently, the fluorescence intensity was alternately changed because of fluorescence interference contrast. This strategy has been successfully explored to design fluorescence environmental sensors.11 On the other hand, motion of nanoparticles in the perpendicular direction can change the thickness and properties of the diffusional barrier that lay between the environment and the nanoparticles. Consequently, the activity of nanoparticles was alternately changed. This strategy can be designed for “smart” switches for the activity of nanoparticles.16,17 In this paper, we have reported a new approach of driving motion of nanoparticles in the perpendicular direction by perpendicular phase separation of block copolymer brushes. Different from Santer’s work in which the nanospheres were moved on the x-y plane indefinitely, the nanoparticles were moved in a predefined perpendicular direction. Because the nanoparticles were chemically bonded by the polymer brushes, the vertical movement of nanoparticles was completely reversible and reproducible, and the motion can be implemented over only one switching cycle. The motion of nanoparticles in large numbers can also be realized by this approach. Experimentally, PS-b(PMMA-co-PCdMA) brushes were grafted from the Si wafer by ATRP. After exposure of the polymer brushes to H2S gas, PSb-(PMMA-co-PCdMA) brushes were converted to PS-b-(PMMAco-PMAA(CdS)) brushes, in which CdS nanoparticles were chemically bonded by the carboxylic groups of the PMAA segment. Alternating treatment of the PS-b-(PMMA-co-PMAA(CdS)) brushes by selective solvents for the outer block and the inner PS block induced perpendicular phase separation of polymer brushes, which resulted in the reversible lifting and lowering of CdS nanoparticles in the perpendicular direction. Experimental Section Materials. Styrene and methyl methacrylate were purchased from Sinopharm Chemical Reagent Co., Ltd, China. The monomers were first washed three times with 5 wt % sodium hydroxide solution and once with water. After drying with anhydrous magnesium sulfate, the monomers were obtained in the pure form by distillation under reduced pressure. All of these monomers were stored in a refrigerator immediately after distillation. Cadmium dimethacrylate was prepared by the procedure reported previously.18 The ATRP initiator used for surface-initiated polymerization is (11-(2-bromo-2-methyl)propionyloxy)undecyl trichlorosilane, Br(CH3)2CCOO(CH2)11SiCl3. The initiator was synthesized by using a similar procedure reported in the literature19 and our previous work of synthesis of stimuliresponsive polyampholyte brushes.20 N,N-Dimethylformamide (DMF) was dried with anhydrous magnesium sulfate and filtered, followed by distillation under reduced pressure. N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine (PMDETA) was purchased from Aldrich. n-Dodecyl mercaptan was purchased from Acros. The highly polished silicon wafers were purchased from Shanghai Wafer Work Corp., China. They were first cleaned in a freshly (16) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Chem. Mater. 2007, 19, 1062-1069. (17) Kidambi, S.; Bruening, M. L. Chem. Mater. 2005, 17, 301-307. (18) Cui, T.; Zhang, J.; Wang, J.; Cui, F.; Chen, W.; Xu, F.; Wang, Z.; Zhang, K.; Yang, B. AdV. Funct. Mater. 2005, 15, 481-486. (19) Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724. (20) Yu, K.; Wang, H.; Xue, L.; Han, Y. Langmuir 2007, 23, 1443-1452.

Yu et al. prepared “piranha” solution (70/30 v/v concentrated H2SO4/30% H2O2. Warning: Piranha solution is an extremely strong oxidant and should be handled Very carefully!) at 100 °C for at least 2 h to remove contaminants and generate a hydroxyl-functionalized surface. The silicon wafers were then removed from the solution and sequentially rinsed with copious amounts of distilled water and ultrasonic-washed, followed by drying with nitrogen gas. Polymerization. Synthesis of Si/SiO2/PS Brushes. CuBr2 (6 mg, 0.027 mmol), CuBr (38 mg, 0.27 mmol), and PMDETA (65 µL, 0.31 mmol) were added successively into a 20 mL glass tube followed by adding 10 mL styrene (0.087 mol). The heterogeneous reaction solution was degassed with three freeze-pump-thaw cycles. In a second 20 mL Schlenk flask, the initiator-modified substrates were added and degassed via evacuation and nitrogen gas backfilling for three cycles. The reaction solution was stirred at 90 °C until a homogeneous solution formed. It was then transferred to the Schlenk flask via syringe. (In the case of determination of grafting density, the same procedure was followed except for no addition of CuBr2 in the beginning. Free initiator, ethyl 2-bromoisobutyrate (39 µL, 0.27 mmol) was added to the reaction solution after formation of a homogeneous solution. The reaction solution was then transferred to the Schlenk flask which contained the wafer.) The surface-initiated polymerization was allowed to proceed at 90 °C for 10 h. After polymerization, the substrates were removed from the Schlenk flask. To remove the untethered substance, the substrates were washed with tetrahydrofuran (THF) and sonicated in THF for 30 min. The samples were then rinsed by THF thoroughly, followed by drying with nitrogen gas. Synthesis of Si/SiO2/PS-b-(PMMA-co-PCdMA) Brushes. Preparation of PS-b-(PMMA-co-PCdMA) brushes was conducted in a manner similar to that for PS brushes. The final recipe for synthesis of PS-b-(PMMA-co-PCdMA) brushes was as follows: CuBr2 (8.4 mg, 0.038 mmol), CuBr (17.2 mg, 0.12 mmol), PMDETA (60 µL, 0.29 mmol), CdMA (2.2 g, 7.8 mmol), MMA (3.1 mL, 0.029 mol), and DMF (9 mL). The surface-initiated polymerization was allowed to proceed at room temperature (24 °C) for a determined reaction time. The reaction was stopped by removing the substrates from the Schlenk flask. To remove the untethered substance, the modified substrates were washed with DMF and sonicated in DMF for 30 min. The samples were then rinsed by DMF thoroughly, followed by drying with nitrogen gas. Synthesis of Si/SiO2/PS-b-(PMMA-co-PMAA(CdS)) Brushes. The Si wafers modified by the PS-b-(PMMA-co-PCdMA) brushes were then put into an evacuated vial, and an excess of H2S gas was injected into it. The reaction was carried out at 100 °C for 1 h to enable the Cd ions to be converted to CdS nanoparticles. Solvent Treatment of the PS-b-(PMMA-co-PMAA(CdS)) Brushes. The PS-b-(PMMA-co-PMAA(CdS)) brush samples were first immersed in a mixed solvent of acetone and ethanol (volume ratio 1:1), which is a selective solvent for the outer block, at room temperature for 45 min. After testing the samples, they were then exposed to toluene, which is a selective solvent for the inner PS block, at 60 °C for 4 h. After each solvent exposure, the samples were blown dry by nitrogen gas, followed by testing via tensiometry and X-ray photoelectron spectra (XPS). Characterization. X-ray reflectivity (XRR) was applied to determine the thickness and roughness of the grafted polymer brushes. All measurements were performed with a diffractometer Brucker AXS D8 Discover (Brucker, Germany) operated in the reflectivity mode. The X-ray source is an X-ray tube with copper anode working at 40 kV and 40 mA. The wavelength λ of Cu KR is 0.154 nm. The experimental data were analyzed by simulation of the reflectivity curves using the integrated software REFSIM. The water contact angles were determined using a KRU ¨ SS DSA10MK2 contact angle measuring system (Kru¨ss, Germany) at ambient temperature. The static water contact angle was measured by using deionized water with drop size of 2 µL. The static contact angle was determined by using the Young/Laplace fitting method over three measurements. X-ray photoelectron spectra (XPS) were measured with Thermo ESCALAB 250 (Thermo Electron Corporation, U.K.) at room

Motion of CdS Nanoparticles By Polymer Brushes temperature by using an Al KR X-ray source (hν ) 1486.6 eV). The main chamber of the XPS instrument was maintained at 10-9 Torr. Pass energies of 50 and 20 eV were used to obtain the survey scan spectra and high-resolution spectra, respectively. Atomic force microscopy studies were performed on a commercial scanning probe microscope (SPA300HV with a SPI3800N Probe Station, Seiko Instruments Inc., Japan). The tapping mode was used to map the film morphology at ambient conditions. Silicon tips with a spring constant of 2 N/m and a frequency of 63-70 kHz were used. Root-mean-square roughness (rms) was evaluated using the integrated software. The roughness was defined as the root-meansquare of height deviations taken from the mean data plane. Transmission electron microscopy (TEM) images were obtained using a JEOL-1011 electron microscope (JEOL, Japan) operated at an accelerating voltage of 100 kV. UV-vis adsorption spectra of the polymer brushes grown on the quartz slide were measured by a Shimadzu UV-2450 spectrometer (Shimadzu, Japan). Photoluminescence spectra were recorded on a Perkin-Elmer LS50B spectrofluorometer (Perkin-Elmer). FTIR spectra were recorded using a Bruker IFS 66v/S spectrometer (Bruker, Germany) with a DTGS detector operated under vacuum (1 mbar). Spectra of samples were taken in transmission mode using a background of the same initiator-coated wafer that was used for growth of polymer brushes. Spectra were recorded at 4 cm-1 resolution, and 512 scans were collected. Molecular weight of the free polymer produced in solution was determined by gel permeation chromatography (GPC). The measurement was performed on a Waters 410 differential refractometer using THF as the eluent (35 °C, flow rate 1 mL/min). Linear polystyrene standards were used for calibration.

Langmuir, Vol. 23, No. 17, 2007 8959 Scheme 1. Synthesis Route of PS-b-(PMMA-co-PMAA(CdS)) Brushes

Results and Discussion Synthesis of the PS-b-(PMMA-co-PMAA(CdS)) Brushes. Scheme 1 outlined the procedure for preparation of the PS-b(PMMA-co-PMAA(CdS)) brushes, including immobilization of the ATRP initiator silane 1 onto the Si wafer (I), synthesis of Si/SiO2/PS brushes (II), synthesis of Si/SiO2/PS-b-(PMMA-coPCdMA) diblock copolymer brushes (III), and exposure to H2S gas (IV). The ATRP of styrene from the initiator-modified Si wafer was carried out using a procedure similar to Ru¨he’s work.21 CuBr2 was added to the reaction solution to ensure a sufficient concentration of deactivating Cu(II) species to control polymerization from the substrate. After polymerization for 10 h, the thickness and roughness of the PS brushes were 23.1 ( 3 and 0.56 nm, respectively, as measured by XRR. As a result of employing the “grafting from” approach, we were able to obtain higher grafting density. The grafting density (σ) for PS brushes was estimated by using the equation σ ) (hFNA)/Mn, where Mn is the molecular weight of free polymer in the solution, NA is Avogadro’s number, h is the block layer thickness, and F is the bulk density of the polymer. The thickness of the PS brushes and the molecular weight of free polymer in solution were 22.7 nm and 17 700 g/mol, respectively. The calculated grafting density for PS brushes was about 0.81 chains/nm2. Figure 1a showed a typical 3D AFM topographic scan of the PS brushes. The surface was relatively smooth and uniform with an rms roughness of 0.8 nm. FTIR analysis of the PS brushes (Figure 2a) revealed peaks at 3026, 3060, and 3080 cm-1, which were all attributed to the aromatic CsH stretching vibration.22 In addition to these peaks, the aromatic CdC stretching vibrations can also be seen at 1493 and 1453 cm-1.

PMMA-co-PCdMA block was grown from the PS brushes for the “living” characterization of ATRP. The thickness of the PMMA-co-PCdMA block can be controlled from 4.4 to 35 nm by adjusting the reaction time from 10 min to 2 h. Figure 3 showed the X-ray reflectivity curves of the PS-b-(PMMA-coPCdMA) brushes with different reaction times. The surface roughness was enhanced to 1.1 nm after polymerization for 30 min as measured by XRR. Figure 1b showed a typical 3D AFM topographic scan of the block copolymer brushes. The surface became rough with an rms roughness of 1.2 nm. Since there are two CdC bonds in each CdMA molecule, a cross-linked PMMAco-PCdMA network was inevitably produced during polymerization, which brought the enhanced surface roughness.23,24 FTIR spectra of the block copolymer brushes (Figure 2b) revealed peaks appearing at 1730 and 1548 cm-1, which were assigned

(21) Jeyaprakash, J. D.; Samuel, S.; Dhamodharan, R.; Ru¨he, J. Macromol. Rapid Commun. 2002, 23, 277-281. (22) Boyes, S. G.; Akgun, B.; Brittain, W. J.; Foster, M. D. Macromolecules 2003, 36, 9539-9548.

(23) Wang, J. Y.; Chen, W.; Liu, A. H.; Lu, G.; Zhang, G.; Zhang, J. H.; Yang, B. J. Am. Chem. Soc. 2002, 124, 13358-13359. (24) Huang, W.; Baker, G. L.; Bruening, M. L. Angew. Chem., Int. Ed. 2001, 40, 1510-1512.

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Yu et al.

Figure 1. 3D AFM images and height profiles of (a) PS brushes and (b) PS-b-(PMMA-co-PCdMA) brushes.

Figure 2. FTIR spectra of (a) PS brushes and PS-b-(PMMA-coPCdMA) brushes (b) before and (c) after reaction with H2S gas.

to the carbonyl stretching vibration of the ester groups in PMMA and the asymmetric vibration of the carboxylate groups in PCdMA. After reaction with H2S gas, the carboxylate groups were converted to protonated carboxylic groups, which was confirmed by the presence of a shoulder peak at 1700 cm-1 attributed to the carboxylic stretching vibration and a broad acidic OH stretch at 3250 cm-1 (Figure 2c). These results indicated that, after exposure to H2S gas, CdMA was converted from the ionized carboxylate form to the protonated carboxylic form, which suggested the formation of PS-b-(PMMA-co-PMAA(CdS)) brushes.22,25 By comparing the IR absorbance of carboxylate groups at 1548 cm-1 in the PS-b-(PMMA-co-PMAA(CdS)) brushes with that in the PS-b-(PMMA-co-PCdMA) brushes, we estimated that approximately 34% of carboxylate groups remained in the brushes after reaction with H2S gas. This suggested that (25) Cui, T.; Cui, F.; Zhang, J.; Wang, J.; Huang, J.; Lu, C.; Chen, Z.; Yang, B. J. Am. Chem. Soc. 2006, 128, 6298-6299.

Figure 3. X-ray reflectivity curves of (a) PS brushes with 22.4 nm thickness and PS-b-(PMMA-co-PCdMA) brushes with different thickness of PMMA-co-PCdMA block: (a) 4.4 nm, (b) 5.5 nm, (c) 7 nm, and (d) 35 nm. The respective reaction times are 10 min, 20 min, 30 min, and 2 h, respectively.

approximately 66% of CdMA molecules participated in the formation of CdS nanoparticles. Figure 4 showed the typical UV-vis absorption spectra of the PS-b-(PMMA-co-PCdMA) brushes with 14 nm PS and 8 nm PMMA-co-PCdMA on the quartz slide (b) before and (c) after reaction with H2S gas. After reaction with H2S, The absorption onset occurred at 310 nm, which revealed a 205 nm blue-shift relative to that of bulk CdS at 515 nm (2.42 eV) due to the quantum size confinement effect. The average size of CdS nanoparticles was 2.4 nm estimated from the adsorption onset inferred from the UV-vis spectra by using the Brus equation.26 Upon excitation with light at a wavelength of 380 nm, the PS-b-(PMMA-co-PMAA(CdS)) brushes exhibited photolumi(26) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183-188.

Motion of CdS Nanoparticles By Polymer Brushes

Figure 4. UV-vis absorption for (a) PS brushes with 14 nm thickness and PS-b-(PMMA-co-PCdMA) brushes with 14 nm PS and 8 nm PMMA-co-PCdMA on the quartz slide (b) before and (c) after reaction with H2S gas. Inset: Photoluminescence spectra for the PS-b(PMMA-co-PMAA(CdS)) brushes.

Figure 5. TEM image of CdS nanoparticles extracted from the PS-b-(PMMA-co-PMAA(CdS)) brushes (scale bar is 10 nm).

nescence with an intense emission centered at 560 nm (Figure 4, inset), which was not present on the corresponding spectra of PS-b-(PMMA-co-PCdMA) brushes. This broad photoluminescence spectrum was attributed to the recombination of trapped electrons/holes in some surface defect states of CdS nanoparticles. The most common defects present in these small CdS nanoparticles were sulfur vacancies, which gave rise to the characteristic luminescence.27 CdS nanoparticles in the PS-b-(PMMA-co-PMAA(CdS)) brushes can be extracted by adding the sample to a solution of n-octadecyl mercaptan (10 µL) in DMF (0.5 mL). The mercapto groups replaced the carboxylic groups and bound to the Cd2+ at the surface of the CdS nanoparticles. Then, the CdS nanoparticles were extracted from the sample into the solution. The TEM image of CdS nanoparticles was shown in Figure 5. We can clearly see that the CdS nanoparticles were uniform with a diameter of 2.0 nm, in agreement with the result obtained from the UV-vis spectra. Solvent Treatment of the PS-b-(PMMA-co-PMAA(CdS)) Brushes. Block copolymer brushes is a particularly interesting system. It has been predicted theoretically and recently shown experimentally that the block copolymer brushes will undergo (27) Zhang, Z. H.; Chin, W. S.; Vittal, J. J. J. Phys. Chem. B 2004, 108, 18569-18574.

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phase separation during solvent treatment.28,29 The phase separation occurring during solvent treatment will result in surface rearrangement, which brings the conformational change of polymer brushes. Depending on the quality of solvent that the polymer brushes was exposed to, two states of polymer brushes brought by phase separation can be distinguished: One state is that the outermost layer was occupied by the outer block, while the inner block buried in the deeper region of polymer brushes; the other state is that the outermost layer was occupied by the inner block, while the outer block migrated into the deeper region of polymer brushes. More important, the polymer brushes can be switched from one state to the other state reversibly. Because the water contact angle measurement is sensitive to composition change at a depth of approximately 0.5-1 nm,30 it can serve as a predictor of the degree of surface rearrangement. The closer the water contact angle is to the characteristic value for the inner or outer block after each solvent treatment, the more complete the surface rearrangement, and thus the greater the extent that the blocks migrate.31 First, we investigated the reversible wetting behavior of PSb-PCdMA block copolymer brushes (with 21.7 nm PS and 6.5 nm PCdMA). After treatment with acetone, a selective solvent for the outer PCdMA block, the water contact angle on the polymer brushes was 52.3° ( 6.4°, which was higher than the characteristic value for PCdMA brushes (37° ( 2.7°). After treatment with toluene at 60 °C for 2 h, the water contact angle increased to 68.7° ( 3.3°, which was nowhere near the characteristic value of PS brushes (91.3° ( 0.7°). These results indicated that surface rearrangement of the PS-b-PCdMA brushes was very limited. Previous reports have indicated that surface arrangement is strongly dependent upon the Flory-Huggins interaction parameter between two blocks.31 In this case, it appeared that the interaction parameter between PS block and PCdMA block was high enough to inhibit surface rearrangement of the diblock copolymer brushes. Besides the high value of the interaction parameter between the blocks, the restricted mobility of the polymer chains due to the formation of a cross-linked network also suppressed surface rearrangement. To get complete surface rearrangement, we copolymerized CdMA with MMA. The static water contact angle for the PS-b-(PMMA-co-PCdMA) brushes (with 22.4 nm PS and 5.5 nm PMMA-co-PCdMA) under the alternating treatment with acetone and toluene changed from 59.9° ( 0.9° (characteristic value for PMMA-co-PCdMA was 57.3° ( 0.5°) to 92.1° ( 1.8°. The switching of water contact angle between the characteristic values for the inner and outer blocks indicated that the degree of surface rearrangement was almost complete. The similar solubility parameters between PS (18.72 MPa1/2) and PMMA (18.58 MPa1/2)32 was helpful for complete surface rearrangement. Mobility of the outer block was also improved due to the reduced cross-link density of the polymer network after copolymerization with MMA. These two key factors contributed to complete surface rearrangement of the PS-b-(PMMA-co-PCdMA) brushes. Motion of Integrated CdS Nanoparticles by Perpendicular Phase Separation of Block Copolymer Brushes. After exposure to H2S gas, PS-b-(PMMA-co-PCdMA) brushes were converted (28) Zhulina, E. B.; Singh, C.; Balazs, A. C. Macromolecules 1996, 29, 63386348. (29) Zhao, B.; Brittain, W. J. Macromolecules 2000, 33, 8813-8820. (30) Allara, D. L.; Atre, S. V.; Parikh, A. N. In Polymer Surfaces and Interfaces II; Feast, W. J., Monro, H. S., Richards, R. W., Eds. John Wiley and Sons: Chichester, 1993; pp 27-40. (31) Akgun, B.; Baum, M.; Bickle, C.; Boyes, S. G.; Granville, A. M.; Mirous, B.; Zhao, B.; Brittain, W. J.; Foster, M. D. AdV. Polym. Sci. 2006, 198, 125-147. (32) Zhao, B.; Brittain, W. J.; Zhou, W.; Cheng, S. J. D. Macromolecules 2000, 33, 8821-8827.

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Yu et al. Scheme 2. Proposed Mechanism of Motion of Integrated CdS Nanoparticles by Perpendicular Phase Separation of Polymer Brushes

Figure 6. Reversible change of static water contact angle for the PS-b-(PMMA-co-PMAA(CdS)) brushes (with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS)) under the alternating treatment with toluene (T) and acetone/ethanol (A/E).

Table 1. Mole Contents of Carbon, Oxygen, Cadmium, and Sulfur (Obtained from the XPS Survey Scan Spectra Obtained at Emission Angles of 90° and 60°) of the PS-b-(PMMA-co-PMAA(CdS)) Brushes with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS) after Treatment by Different Solvents element XPS emission angle

solvent treatment condition

C%

O%

Cd%

S%

90°

acetone/ethanol toluene acetone/ethanol toluene

76.7 85 76.7 84.5

20.9 14.1 21 15.4

1.41 0.34 1.49 0.1

1 0.54 0.78 0

60°

Figure 7. XPS survey scan spectra (obtained at an emission angle of 90°) of the PS-b-(PMMA-co-PMAA(CdS)) brushes (with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS)) after treatment with (a) acetone/ethanol (1:1) and (b) toluene.

to PS-b-(PMMA-co-PMAA(CdS)) brushes, in which CdS nanoparticles were chemically bonded by the carboxylic groups of the PMAA segment.23,25 The thickness of the outer block showed a small decrease after exposure to H2S gas. For the outer PMMAco-PCdMA block with 7 and 14 nm, the thickness decreased to 5.2 and 12 nm, respectively. Formation of CdS nanoparticles condensed the polymer brushes and brought a decrease in thickness. Treatment of the PS-b-(PMMA-co-PMAA(CdS)) brushes (26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS)) with selective solvent induced reversible switching of water contact angle between the characteristic values for the inner and outer blocks (Figure 6). It indicated that the outmost layer was preferentially occupied by the outer block after exposure of the polymer brushes to a selective solvent for the outer block (a mixed solvent of acetone and ethanol). Meanwhile, the outmost layer was preferentially occupied by the inner PS block after treatment of the polymer brushes with toluene, a selective solvent for the PS block. Solvent treatment resulted in the phase separation of polymer brushes in the perpendicular direction,32 which brought the switching of water contact angle between the characteristic values for the inner and outer blocks. Because CdS nanoparticles were chemically bonded by the carboxylic groups of the outer block, the reversible change of water contact angle, which was correlated with the migration of the inner and outer blocks, suggested that CdS nanoparticles moved in the perpendicular

direction. To elucidate the exact extent of motion of CdS nanoparticles, X-ray photoelectron spectroscopy (XPS) was used to get a quantitative analysis. XPS is a surface-sensitive quasiquantitative analytic technique which can determine the compositional information as a function of depth.33,34 The sampling depth of XPS measurement is dependent on the emission angle, t ) 3λe sin θ, where t is the sampling depth, λe is the electron inelastic mean-free path, and θ is the emission angle relative to the surface. By changing the emission angle, the compositional information at different depths can be obtained. Therefore, the exact extent of motion of CdS nanoparticles can be deduced from the composition depth profiles. Figure 7 depicted the XPS survey spectra obtained at an emission angle of 90° for the PS-b-(PMMA-co-PMAA(CdS)) brushes with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS). The mole contents of each element (Table 1) were quantified from the survey scan spectra of the sample after treatment with mixed solvent of acetone and ethanol (Figure 7a) and toluene (Figure 7b). The mole contents of carbon, oxygen, caladium, and sulfur after treatment with acetone/ethanol were 76.7%, 20.9%, 1.41%, and 1%. After treatment with toluene, the mole contents of oxygen, cadmium, and sulfur decreased to 14.1%, 0.34%, and 0.54%, while the carbon content increased to 85%. Those results indicated that perpendicular phase separation occurred during solvent treatment, consistent with the result of water contact angle measurement. After treatment with acetone/ ethanol, a selective solvent for the outer block, the outer block occupied the outermost layer, while PS block segregated to the grafting surface. After treatment with toluene, a selective solvent for the inner PS block, PS block migrated to the outermost layer, while the outer block migrated to the deeper region of the brushes to increase the favorable PS-toluene interactions and reduce the unfavorablePMMA-co-PMAA(CdS)-tolueneinteractions(Scheme (33) Briggs, D.; Seah, M. P. Practical Surface Analysis, Vol. 1: Auger and X-ray Photoelectron Spectroscopy; John Wiley & SonS: Chichester, England, 1995. (34) Briggs, D. Surface Analysis of polymers by XPS and Static SIMS; Cambridge University Press: Cambridge, U.K., 1998.

Motion of CdS Nanoparticles By Polymer Brushes

Figure 8. XPS spectra of S2p (obtained at an emission angle of 90°) of the PS-b-(PMMA-co-PMAA(CdS)) brushes (with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS)) after treatment with (a) acetone/ ethanol (1:1) and (b) toluene.

2). Because CdS nanoparticles were chemically bonded by the carboxylic groups of the outer block, the reversible migration of inner block and outer block brought by perpendicular phase separation simultaneously brought the motion of CdS nanoparticles in the perpendicular direction. For the non-equimolar ratio of Cd to S (1.4/1), the XPS spectra of S2p (Figure 8) were used to evaluate the mole percentage of CdS nanoparticles that have moved. The intensity of the S2p peak decreased by about 32% after treatment with toluene as compared with that after treatment with acetone/ethanol. The electron inelastic mean free path for S2p is 2.1 nm in CdS as determined from ref 35. On the basis of the equation t ) 3λe sin θ, the sampling depth for S2p at an emission angle of 90° is approximately 6.3 nm. These results indicated that 32% of CdS nanoparticles have been lowered by approximately 6.3 nm after treatment with toluene. Meanwhile, CdS nanoparticles can be subsequently lifted by approximately 6.3 nm after treatment with a mixed solvent of acetone and ethanol for the reversible behavior of the PS-b-(PMMA-co-PMAA(CdS)) brushes as indicated by the water contact angle measurement. The XPS survey spectra of the PS-b-(PMMA-co-PMAA(CdS)) brushes obtained at an emission angle of 60° are shown in Supporting Information Figure S1. The change of mole contents of each element showed very similar trends (Table 1). After treatment with toluene, the mole content of sulfur decreased from 0.78% to nondetectable in the XPS spectra (Figure 9). The sampling depth at this emission angle is approximately 5.5 nm. These results indicated that 100 mol % of CdS nanoparticles have been lowered by approximately 5.5 nm after treatment with toluene. After treatment with acetone/ethanol, CdS nanoparticles can be reversibly lifted by approximately 5.5 nm. The motion of the nanoparticles was reversible, as confirmed by the reversible change of mole contents of each element obtained from the XPS survey scan for the repeated switching (Supporting Information Table S1). The extent of movement can be adjusted by the relative thicknesses of two blocks of the polymer brushes. As for the polymer brushes with fixed thickness of the PS block (26.1 nm) and varied thickness of the PMMA-co-PMAA(CdS) block, the dependence of block thickness on the extent of movement was (35) Jablonski, A.; Powell. C. J. NIST Electron Inelastic-Mean-Free-Path Database, v 1.1; National Institute of Standards and Technology: Gaithersburg, MD, 2000.

Langmuir, Vol. 23, No. 17, 2007 8963

Figure 9. XPS spectra of S2p (obtained at an emission angle of 60°) of the PS-b-(PMMA-co-PMAA(CdS)) brushes (with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS)) after treatment with (a) acetone/ ethanol (1:1) and (b) toluene. Table 2. Mole Contents of Carbon, Oxygen, Cadmium, and Sulfur (Obtained from the XPS Survey Scan Spectra Obtained at Emission Angles of 60° and 30°) of the PS-b-(PMMA-co-PMAA(CdS)) Brushes with 26.1 nm PS and 12 nm PMMA-co-PMAA(CdS) after Treatment by Different Solvents element XPS emission angle

solvent treatment condition

C%

O%

Cd%

S%

60°

acetone/ethanol toluene acetone/ethanol toluene

71.8 75.1 73.9 75.5

24.6 22.8 22.7 23.7

1.2 1.34 1.4 0.83

2.4 0.86 1.98 0

30°

investigated. Supporting Information Figures S2 and S3 depict the XPS survey spectra obtained at emission angles of 60° and 30° for the PS-b-(PMMA-co-PMAA(CdS)) brushes with 26.1 nm PS and 12 nm PMMA-co-PMAA(CdS). By comparing the mole contents of each element (Table 2) quantified from the survey scan spectra, it can be concluded that 100 mol % of CdS nanoparticles can travel a distance of approximately 3.1 nm after solvent treatment. Meanwhile, 80 mol % of CdS nanoparticles can travel a distance of approximately 5.5 nm, as evaluated by the high-resolution XPS spectra of S2p (Supporting Information Figure S4). Further elongation of the thickness of the outer block produced a similar result of movement. The decrease in the distance of movement was due to the limited surface rearrangement of polymer brushes, as revealed by the water contact angle. The water contact angles on these PS-b-(PMMA-co-PMAA(CdS)) brushes can only be reversibly switched from 56.6° ( 2° to 75.2° ( 2.8°, which showed a deviation from the characteristic value of PS brushes. As the thickness of the outer block increased, the mobility of the inner PS block migrating to the solvent-polymer interface and the mobility of the outer block penetrating into the brushes were both restricted. The restricted conformational change of the polymer brushes eventually resulted in limited extension of movement of CdS nanoparticles.

Conclusions In summary, we have developed a new method of reversibly moving CdS nanoparticles in the perpendicular direction based on the perpendicular phase separation of PS-b-(PMMA-co-

8964 Langmuir, Vol. 23, No. 17, 2007

PMAA(CdS)) brushes. The vertical lifting and lowering of CdS nanoparticles were stimulated by the reversible conformational change brought by the phase separation of block copolymer brushes. Copolymerization of CdMA with MMA was helpful for improving the degree of surface rearrangement due to the reduced cross-link density and reduced difference of solubility parameters between the two blocks. The extent of movement can be adjusted by the relative thicknesses of two blocks of the polymer brushes. A relatively thick outer block restricted the surface rearrangement of polymer brushes, which resulted in limited extent of movement of CdS nanoparticles. Because the outmost layer of the polymer brushes can be preferentially occupied by the inner block and the outer block, which encapsulated the functional nanoparticles, we anticipate that the polymer brushes depicted here can find application in the field where finely tuning the activity of functional nanoparticles is demanded.

Yu et al.

Acknowledgment. This work is subsidized by the National Natural Science Foundation of China (20334010, 20621401, 50573077). Supporting Information Available: XPS survey scan spectra (obtained at an emission angle of 60°) of the PS-b-(PMMA-co-PMAA(CdS)) brushes (26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS)) after solvent treatment. XPS survey scan spectra (obtained at emission angles of 60° and 30°) of the PS-b-(PMMA-co-PMAA(CdS)) brushes (26.1 nm PS and 12 nm PMMA-co-PMAA(CdS)) after solvent treatment. XPS spectra of S2p (obtained at an emission angle of 60°) of the PSb-(PMMA-co-PMAA(CdS)) brushes (26.1 nm PS and 12 nm PMMAco-PMAA(CdS)) after solvent treatment. The reversible change of mole contents of each element (obtained from the XPS survey scan spectra obtained at 60°) for the PS-b-(PMMA-co-PMAA(CdS)) brushes with 26.1 nm PS and 5.2 nm PMMA-co-PMAA(CdS) after repeated cycles of treatment by different solvents. This material is available free of charge via the Internet at http://pubs.acs.org. LA701053D