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Surface-Initiated Atom Transfer Radical Polymerization from Halogen-Terminated Si(111) (Si-X, X ) Cl, Br) Surfaces for the Preparation of Well-Defined Polymer-Si Hybrids F. J. Xu, Q. J. Cai, E. T. Kang,* and K. G. Neoh Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260 Received October 26, 2004. In Final Form: February 7, 2005 The halogen-terminated Si(111) (Si-X, X ) Cl, Br) surfaces contain effective initiators for surfaceinitiated atom transfer radical polymerization. Well-defined polymer-Si hybrids, consisting of covalently tethered (Si-C bonded) functional polymer brushes, can be prepared directly from the Si-X surfaces.
Introduction Tethering of polymer brushes on a solid substrate is an effective method for modifying its surface properties.1,2 Recently, considerable attention has been paid to the manipulation and control of the physicochemical properties of oriented single-crystal silicon surfaces because of their crucial importance to the microelectronics industry.3-9 It is possible to prepare a wide range of well-defined polymer brushes on various substrate surfaces by surfaceinitiated atom transfer radical polymerization (ATRP).10-19 A number of reports on the modification of oriented singlecrystal silicon surfaces by ATRP are available in the literature.8,15-19 In the preparation of polymer brushes on silicon surfaces, initiators were immobilized on the silicon surfaces, usually through coupling agents and functional intermediates in multistep processes. In this work, it is shown that the halogen-terminated single-crystal silicon (Si-X, X ) Cl, Br) surfaces, obtained * To whom all correspondence should be addressed. Tel: +656874-2189. Fax:+65-6779-1936. E-mail:
[email protected]. (1) Milner, S. T. Science 1991, 251, 905. (2) Halperin, A.; Tirrell, M.; Lodge T. P. Adv. Polym. Sci. 1992, 100, 31. (3) Lasseter, T. L.; Clare, B. H.; Abbott, N. L.; Hamers, R. J. J. Am. Chem. Soc. 2004, 126, 10220. (4) Husemann, M.; Morrison, M.; Benoit, D.; Frommer, J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122, 1844. (5) Caro, D. D.; Fraxedas, J.; Faulmann, C.; Malfant, I.; Milon, J.; Lame`re, J. F.; Collie`ru, V.; Valade, L. Adv. Mater. 2004, 16, 835. (6) Le´tant, S. E.; Hart, B. R.; Kane, S. R.; Hadi, M. Z.; Shields, S. J.; Reynolds,J. G. Adv. Mater. 2004, 16, 689. (7) Condoreli, G. G.; Motta, A.; Fragala`, I. L.; Giannazzo, F.; Raineri, V.; Cuneschi, A.; Gatteschi, D. Angew. Chem., Int. Ed. 2004, 43, 4081. (8) Yu, W. H.; Kang, E. T.; Neoh, K. G. J. Phys. Chem. B 2003, 107, 10198. (9) Hurley, P. T.; Ribbe, A. E.; Buriak, J. M. J. Am. Chem. Soc. 2003, 125, 11334. (10) Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921. (11) Pyun, T.; Kowalewski, T.; Matyjaszewski, K. Macromol. Rapid Commun. 2003, 24, 1043. (12) Jeyaprakash, J. D.; Samuel, S.; Dhamodharan, R.; Ru¨he, J. Macromol. Rapid Commun. 2002, 23, 277-281. (13) Shen, Y. Q.; Zhu, S. Q.; Zeng, F. Q.; Pelton, R. Macromolecules 2000, 33, 5399. (14) Huang, W. X.; Kim, J. B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35, 1175. (15) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14. (16) Wang, J. Y.; Chen, W.; Liu, A. H.; Lu, G.; Zhang, G.; Zhang, J. H.; Yang, B. J. Am. Chem. Soc. 2002, 124, 13358. (17) Edmondson, S.; Huck, W. T. S. J. Mater. Chem. 2004, 14, 730. (18) Zhao, B.; He, T. Macromolecules 2003, 36, 8599. (19) Xu, F. J.; Yuan, Z. L.; Kang, E. T.; Neoh, K. G. Langmuir 2004, 20, 8200.
via chlorination and bromination of the hydrogenterminated Si(111) surfaces (Si-H surfaces), are themselves effective initiators for the surface-initiated ATRP. Well-defined polymer brushes of 2-hydroxyethyl methacrylate (HEMA) and (2-dimethylamino) ethyl methacrylate (DMAEMA), tethered directly on silicon surfaces via robust Si-C bonds, were prepared via surface-initiated ATRP using Si-X as the initiator. The novel one-step process for the preparation of well-defined HEMA polymer (P(HEMA)) and DMAEMA polymer (P(DMAEMA))-Si hybrids is shown in Scheme 1. Experimental Section (111)-oriented single-crystal silicon was sliced into square chips of about 1.2 cm × 1.2 cm in size. The pristine (oxide-covered) silicon chips were immersed in 10 vol % HF in individual Teflon vials for 2 min to remove the oxide film and to leave behind a uniform hydrogen-terminated Si(111) surface (Si-H surface). The Si-H surface was treated with PCl5 in chlorobenzene or N-bromosuccinimide (NBS) in dried DMF for 30 min at 85 °C, in the presence of benzoyl peroxide as the radical initiator, to give rise to the respective Si-Cl or Si-Br surfaces.20,21 For the preparation of the Si-g-P(HEMA) and Si-g-P(DMAEMA) hybrids, the reaction conditions were as follows: [monomer (3 mL)]:[CuX]: [CuX2]:[Bpy (2,2′-bipyridine) (for HMEA) or HMTETA (1,1,4,7,10,10-hexamethyltriethylenetetramine) (for DMAEMA)] ) 100:1:0.2:2 in 3 mL of anhydrous DMSO at room temperature (for HEMA) or 60 °C (for DMAEMA) in a Pyrex tube for a predetermined period of time. After the reaction, the polymerSi hybrids were washed and extracted thoroughly with doubly distilled water (for the Si-g-P(HEMA) hybrids) and doubly distilled water and ethanol (for the Si-g-P(DMAEMA) hybrids). The hybrids were subsequently immersed in a large volume of their respective solvent system for about 48 h to ensure the complete removal of the adhered and physically adsorbed polymer, if any. The solvents were stirred continuously and changed every 8 h. One of the unique characteristics of the polymers synthesized by ATRP is the preservation of active end groups during the polymerization process. To confirm the existence of the “living” chain ends, surface-initiated ATRP was again used to synthesize the P(HEMA)-b-P(DMAEMA) (or P(DMAEMA)-b-P(HEMA)) diblock copolymer brushes from the silicon surface, using the grafted P(HEMA) (or P(DMAEMA)) brushes as the macroinitiators. The reaction conditions were as follows: [monomer]:[CuX]: [CuX2]:[Bpy (for HMEA) or HMTETA (for DMAEMA)] ) 100:1:0.2:2 in DMSO at 60 °C (for DMAEMA), or in deionized water at room temperature (for HEMA), for 5 h. (20) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S.J. Am. Chem. Soc. 1996, 118, 7225. (21) He, J.; Patitsas, S. N.; Preston, K. F.; Wolkow, R. A.; Wayner, D. D. M. Chem. Phys. Lett. 1998, 286, 508.
10.1021/la0473714 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/08/2005
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Scheme 1. Schematic Diagram Illustrating the Process of Surface-Initiated ATRP from the Halogen-Terminated Silicon Surface
The P(HEMA) thickness in the starting Si-g-P(HEMA) (X ) Cl) hybrid was 10 nm, and the P(DMAEMA) thickness in the starting Si-g-P(DMAEMA) (X ) Br) hybrid was 12 nm. The washing/ extraction procedures for the diblock copolymer brushes were similar to those described above. The chemical composition of the modified silicon surfaces was determined by X-ray photoelectron spectroscopy (XPS). The static water contact angles were measured by the sessile drop method, using 3 µL water droplets, in a telescopic goniometer. The thickness of the polymer brushes was determined by ellipsometry.
Results and Discussion The Si 2p core-level spectrum of the hydrogenterminated silicon surface (Figure S1(a), Supporting Information) can be curve-fitted with a spin-orbit split doublet of the Si 2p3/2 (at the binding energy (BE) of about 98.4 eV) and Si 2p1/2 (at about 99 eV) peak components in the expected 2:1 area ratio and 0.6 eV in energy separation.20,21,24 No oxidized silicon species in the BE region of 101-103 eV was discernible in the spectrum, indicating negligible oxidation of this silicon surface. Exposure of the hydrogen-terminated Si(111) surface (Si-H surface) to a chlorobenzene solution of PCl5,20 or a DMF solution of N-bromosuccinimide (NBS),21 in the presence of the benzoyl peroxide initiator at 85 °C for 30 min gave rise to the respective Si-Cl and Si-Br surfaces. Figure 1 shows the respective Si 2p and Cl 2p (BE ∼ 200 eV)22 core-level spectra of the Si-Cl surface (part a) and the Si 2p and Br 3d (BE ∼ 68.5 eV)22 core-level spectra of the Si-Br surface (part b). The Si 2p core-level spectra were curve-fitted with a major spin-orbit split doublet (Si 2p3/2 peak component at the BE of about 98.4 eV and Si 2p1/2 at about 99 eV) in the expected area ratio of 2:1 for the Si substrate and a minor spin-orbit split doublet with the BE for the Si 2p3/2 peak component at about 99.4 eV for the surface Si-X species.20,21 From the unit cell dimension (about 12.77 Å2) of the Si(111) surface23 and the surface coverage (about 66% on the average) of chlorinated or brominated silicon atoms,24 the surface Si-X species density was estimated to be about 5 units/nm2. Since the average cross-sectional area of methacrylate polymers25 prepared by living radical polymerization is about 2 nm2, the surface initiator efficiency (22) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In X-ray Photoelectron Spectroscopy; Chastain, J., Ed; Perkin-Elmer: Eden Prairie, MN, 1992, pp 41, 46, 51, 60, 92, and 99. (23) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho¨lter, E. J. R. Adv. Mater. 2000, 12, 1457. (24) Bansal, A.; Li, X.; Yi, S. L.; Weiberg, W. H.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 10266. (25) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597.
of the present system is estimated to be about 10%. The Si-X surfaces were only mildly moisture-sensitive and could be handled in air for several minutes without hydrolysis.20,21,24 After the surface-initiated ATRP, the unreacted Si-X moieties were eventually hydrolyzed to form the oxide and hydroxide species during the subsequent cleaning process under atmospheric conditions.20,21 The formation of oxides on the silicon surface was discernible only in the case in which the graft layer thickness was less than the probing depth of the XPS technique of about 7 nm for an organic layer.26 (see Figure S1(d), Supporting Information, for the Na salt of styrene sulfonate (NaStS) graft-polymerized Si-Br surface). For the Si-H and polymer-Si hybrid surfaces, the BE values for the Si-H and Si-C species cannot be differentiated from those of the Si-Si species.24,27 Two functional monomers, viz., HEMA and DMAEMA, were selected as the model monomers for the preparation of the polymer-Si hybrids via surface-initiated ATRP from the Si-X (X ) Cl, Br) surfaces. Other monomers used included Na salt of styrene sulfonate (NaStS) and poly(ethylene glycol) monomethacrylate (PEGMA) (see Supporting Information). The HEMA polymer (P(HEMA)) is well-known as a highly hydrophilic and biocompatible material, exhibiting low protein absorption and good chemical stability.14 The hydroxyl end groups of the grafted P(HEMA)) can be converted into various functional derivatives.14 The DMAEMA polymer (P(DMAEMA)) is both thermal- and pH-sensitive.8,13 Well-defined P(DMAEMA) brushes can be used to prepare “smart” surfaces on a nanometer scale.8 Prior to the surface-initiated ATRPs, the reaction of Si-X surfaces with HEMA and DMAEMA monomers was studied. After exposure to HEMA, the Si-X (X ) Cl, Br) moieties on the surface were gradually hydrolyzed by the -OH group of HEMA over a period of about 10 min, as indicated by the gradual appearance of the Si-O species at the Si 2p BE of about 103 eV in the XPS surface composition analysis at fixed time intervals. The composition of the Si-X (X ) Cl, Br) surfaces remained practically unchanged in the dry and degassed DMAEMA monomer solution over the same period of time. To suppress the side reactions, especially that involved the hydrolysis reaction of HEMA, the Si-X substrate was introduced into the reaction mixture at the very last stage after the mixture had been degassed with argon for 30 (26) Tan, K. L.; Woon, L. L.; Wong, H. K.; Kang E. T.; Neoh, K. G. Macromolecules 1993, 26, 2832. (27) Tufts, B. J.; Kumar, A.; Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1992, 96, 4581.
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Figure 1. XPS Si 2p spectra of the (a) Si-Cl and (b) Si-Br surfaces, C 1s core-level spectra of the (c) Si-g-P(HEMA) (X ) Cl) and (d) Si-g-P(HEMA) (X ) Br) surfaces, and wide scan spectra of the (e) Si-g-P(DMAEMA) (X ) Cl), (f) Si-g-P(DMAEMA) (X ) Br), (g) Si-g-P(HEMA)-b-P(DMAEM A) (X ) Cl), and (h) Si-g-P(DMAEMA)-b-P(HEMA) (X ) Br) surfaces.
min. Since only about 10% of the surface Si-X moieties were used in the subsequent surface-initiated ATRP, the effect of the initial side reaction involving hydrolysis by HEMA was probably limited. This conclusion was borne out by the fact that not only was HEMA graft-polymerized readily on the Si-X surface in the presence of the ATRP catalyst system but also the halogen at the living chain ends of the grafted P(HEMA) brushes served as a macroinitiator for the subsequent block copolymerization with a second monomer (see below). In this work, the method of addition of Cu(II) complex (CuX2) was chosen to control the concentration of the deactivating Cu(II) complex12 during the surface-initiated ATRP on the Si-X surface. The ratio of [monomer]:[CuX (catalyst)]:[CuX2 (deactivator)]:[Bpy or HMTETA (ligand)] was controlled at 100:1:0.2:2. The presence of the grafted P(HEMA) and P(DMAEMA) on the Si-X surfaces was confirmed by XPS analysis and ellipsometry measure-
ment. These surfaces are referred to as the Si-g-P(HEMA) (X ) Cl or Br) and Si-g-P(DMAEMA) (X ) Cl or Br) surfaces, respectively. The C 1s core-level spectra (spectra c and d of Figure 1) of the Si-g-P(HEMA) (X ) Cl, Br) surfaces can be curve-fitted with three peak components with BE values at about 284.6, 286.3, and 288.8 eV, attributable to the CsH, CsO and OdCsO species, respectively.22 The wide scan spectra of the Si-g-P(DMAEMA) (X ) Cl, Br) surfaces (spectra e and f of Figure 1) reveal the presence of C 1s, N 1s, and O 1s signals. The above results and the appearance of an N 1s signal at the BE of about 399 eV in spectra e and f of Figure 1, attributable to the amine species of P(DMAEMA),22 are consistent with the fact that HEMA and DMAEMA have been successfully graft-polymerized on the silicon surfaces via surface-initiated ATRP using the Si-X surfaces as the initiators. The thicknesses of the grafted P(HEMA) (for the Si-g-P(HEMA) (X ) Cl, Br)
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Table 1. Layer Thickness, Static Water Contact Angle, and Surface Polymer Density of the Polymer-Silicon Hybrids Prepared via Surface-Initiated ATRP from the Si-X (X ) Cl, Br) Surfaces sample
layer thickness (nm)
static contact angle ((3°)
surface densityd (monomer units/nm2)
Si-g-P(HEMA) (X ) Cl)a Si-g-P(HEMA) (X ) Br)a Si-g-P(DMAEMA) (X ) Cl)a Si-g-P(DMAEMA) (X ) Br)a Si-g-P(HEMA)-b-P(DMAEMA) (X ) Cl)a,b Si-g-P(DMAEMA)-b-P(HEMA) (X ) Br)a,c
10 8 16 12 10 + 8 12 + 21
37 38 58 59 56 38
46 37 61 46 46 + 31 46 + 97
a Reaction time was 5 h under the ATRP reaction conditions. b Surface-initiated ATRP of DMAEMA from the Si-g-P(HEMA) (X ) Cl) surface (P(HEMA) thickness ) 10 nm). c Surface-initiated ATRP of HEMA from the Si-g-P(HEMA) (X ) Br) surface (P(DMAEMA) thickness ) 12 nm). d Surface density ) surface polymer coverage (film thickness × bulk density of the grafted polymer)/molecular weight of the monomer.
Figure 2. Dependence of the grafted polymer film thickness on the surface-initiated ATRP time for the (a) Si-g-P(HEMA) (X ) Cl), (a′) Si-g-P(HEMA) (X ) Br), (b) Si-g-P(DMAEMA) (X ) Cl), and (b′) Si-g-P(DMAEMA) (X ) Br) surfaces.
surfaces) and P(DMAEMA) (for the Si-g-P(DMAEMA) (X ) Cl, Br) surfaces) brushes, obtained after 5 h of ATRP, are about 10, 8, 16, and 12 nm, respectively. Their corresponding average surface densities were estimated to be about 46, 37, 61, and 46 equivalent monomer units/ nm2 (Table 1). The static water contact angles of the pristine (oxidecovered) Si(111) and Si-H surfaces were about 20° and 73°, respectively. It is difficult to obtain reliable static water contact angles for the Si-X surfaces because of their susceptibility to hydrolysis.20,21 In the presence of the grafted P(HEMA) and P(DMAEMA) brushes, the silicon surfaces became more hydrophilic, and their contact angles were about 37° and 58°, respectively (Table 1). The kinetics of P(HEMA) and P(DMAEMA) growth from the Si-X surfaces via surface-initiated ATRP, using the Si-X species as the initiators, were investigated. An approximately linear increase in thickness of the grafted P(HEMA) and P(DMAEMA) brushes on the Si-X surfaces with polymerization time is observed, as shown by lines (a) to (b′), respectively, in Figure 2. The results suggest that the chain growth from the Si-X surfaces was consistent with a “controlled” or “living” process. The Si-Cl surface gives rise to a faster growth of polymer brushes, probably because of the lower reactivity of the chlorinated surface toward hydrolysis in air (prior to the initiation of polymerization) compared to that of the brominated surface.21 Control experiments on the oxidecovered Si(111) and Si-H surfaces were also carried out. No increase in organic layer thickness was discernible when the two substrates were subjected to the “surfaceinitiated” ATRP of the two monomers under similar reaction conditions, followed by vigorous washing/extrac-
tion. Other convincing evidence that the Si-X (X ) Cl, Br) surfaces contain effective initiators for the surfaceinitiated ATRP was provided by the solution ATRP using a small molecule analogue, chlorodimethylsilane, as the initiator. Initial results indicate that the chlorosilane can indeed serve as an ATRP initiator. Detailed results on the chlorosilane-initiated ATRP will be communicated separately. One of the unique characteristics of the polymers synthesized by ATRP is the preservation of active end groups during the polymerization process. To confirm the existence of the “living” chain ends, surface-initiated ATRP is again used to synthesize the P(HEMA)-b-P(DMAEMA) (or P(DMAEMA)-b-P(HEMA)) diblock copolymer brushes from the silicon surface, using the grafted P(HEMA) (or P(DMAEMA)) brushes as the macroinitiators. After an ATRP time of 5 h, an N 1s signal at the BE of about 399 eV, attributable to the amine species,22 has appeared in the wide scan spectrum of the Si-g-P(HEMA)-bP(DMAEMA) (X ) Cl) surface (Figure 1g). On the other hand, the N 1s signal at the BE of about 399 eV has disappeared completely in the wide scan spectrum of the Si-g-P(DMAEMA)-b-P(HEMA) (X ) Br) surface (Figure 1h). The thicknesses of the second P(DMAEMA) and P(HEMA) blocks are about 8 and 21 nm, respectively, corresponding to the surface graft densities of about 31 and 97 units/nm2 (Table 1). The accelerating effect of water on the surface-initiated ATRP14 gave rise to a thicker P(HMEA) block, compared to only about 8 nm for the P(HEMA) film grown from the Si-Br surface in anhydrous DMSO (Table 1). The thickness of the P(HEMA) layer is larger than the sampling depth of the XPS technique (about 7.5 nm in an organic matrix).26 As a result, the N signal from the underlying P(DMAEMA) layer is no longer discernible in the wide scan spectrum (Figure 1h). The above results confirmed that the halogen dormant sites at the end of the grafted P(HEMA) and P(DMAEMA) chains on the silicon surfaces allowed reactivation during the subsequent block copolymerization process, resulting in the formation of the diblock copolymer brushes. In summary, it was demonstrated that the halogenterminated silicon (Si-X, X ) Cl, Br) surfaces were themselves effective initiators for the surface-initiated ATRP. Well-defined polymer-Si hybrids, consisting of covalently tethered polymer brushes on Si(111) surfaces via robust Si-C bonds, were prepared directly via surfaceinitiated ATRP from the Si-X surfaces. Kinetic studies indicated that the chain growth from the silicon surface was consistent with a “controlled” or “living” process. The “living” chain ends of the polymer brushes allowed further molecular design of the hybrid surfaces via, for example, block copolymerization. The most important crystallographic faces of silicon to the microelectronics industry are the (111) and (100) faces. The halogen-terminated
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Si(100) (Si(100)-X, X ) Cl, Br) surfaces can also be shown to be effective initiators for the preparation of well-defined polymer-Si(100) hybrids via surface-initiated ATRP. Polymer-Si(100) hybrids, prepared via surface-initiated ATRP of pentafluorostyrene, sodium 4-styrenesulfonate, poly(ethylene glycol) monomethacrylate, and (2-dimethylamino)ethyl methacrylate from Si(100)-Br surfaces, are described in the Supporting Information.
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Supporting Information Available: The preparation and characteristics of well-defined polymer-Si(100) hybrids via surface-initiated atom transfer radical polymerization using bromine-terminated Si(100) surfaces as the initiator. This material is available free of charge via the Internet at http://pubs.acs.org. LA0473714
