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Layer-by-Layer Deposition of Polyelectrolyte Macroinitiators for Enhanced Initiator Density in Surface-Initiated ATRP Steve Edmondson,* Cong-Duan Vo,† and Steven P. Armes Dainton Building, Department of Chemistry, The UniVersity of Sheffield, Brook Hill, Sheffield, S3 7HF, U.K.
Gian-Franco Unali UnileVer Research and DeVelopment, Port Sunlight, Quarry Road East, Bebington, Wirral, Merseyside, L63 3JW, U.K.
Michael P. Weir Department of Physics and Astronomy, UniVersity of Sheffield, Hicks Building, Hounsfield Road, Sheffield, United Kingdom S3 7RH, U.K. ReceiVed December 21, 2007. ReVised Manuscript ReceiVed March 17, 2008 The layer-by-layer (L-b-L) deposition of oppositely charged polyelectrolytic macroinitiators has been demonstrated on planar silica substrates. The build-up of the macroinitiator multilayers was monitored by ellipsometry (up to 21 layers) and dual polarization interferometry (up to 17 layers) and good agreement was found between these techniques. The increase in L-b-L thickness was approximately linear, with an average thickness of 2.3 Å per layer of deposited macroinitiator. Surface-initiated ATRP of a model nonionic methacrylic monomer, 2-hydroxyethyl methacrylate (HEMA) in a 1:1 methanol/water mixture was conducted at ambient temperature. Increasing the number of macroinitiator layers led to a significant increase in PHEMA brush thickness up to 110 nm, which is attributed to the greater surface grafting density. PHEMA brush thicknesses obtained after 22 h showed a linear dependence on the number of layers of deposited macro-initiator, with all layers exhibiting near-identical growth kinetics. X-ray photoelectron spectroscopy was used to monitor L-b-L assembly and also to confirm PHEMA growth. This technique indicated the loss of small counterions from the multilayers during L-b-L deposition and confirmed an increase in the surface density of bromoester initiator groups as the number of deposited macroinitiator layers was increased. For 17 macroinitiator layers, the bromoester initiator density is estimated to be ∼4.9 ( 0.2 nm-2 from the DPI data. This is comparable to that calculated for ATRP initiator monolayers obtained by either thiol or silane chemistry. Ellipsometry suggested that the macroinitiator multilayers were weakly hydrated prior to the in situ HEMA polymerization. AFM studies indicated that the PHEMA brushes had appreciable surface roughness, but this roughness became negligible compared to the brush thickness with increasing macroinitiator layers.
Introduction Surface-initiated polymerization (SIP), which involves growing polymer chains directly from initiators immobilized on a surface, has attracted a great deal of interest in recent years.1–3 Atom transfer radical polymerization (ATRP)4–6 has been used in the vast majority of SIP studies to date, since it allows well-controlled film growth under mild conditions, the synthesis of tethered block copolymers and is applicable to a wide range of functional monomers. Surface tethering of polymer chains confers excellent solvent stability and hence allows stimulus-responsive behavior * To whom correspondence should be addressed. E-mail: s.edmondson@ sheffield.ac.uk. † Present address: Laboratory of Polymers and Biomaterials, School of Pharmacy and Pharmaceutical Sciences, The University of Manchester, Stopford Building, Oxford Road, Manchester, M13 9PT, U.K. (1) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14. (2) Radhakrishnan, B.; Ranjan, R.; Brittain, W. J. Soft Matter 2006, 2, 386. (3) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. AdV. Polym. Sci. 2006, 197, 1. (4) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. (5) Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614. (6) Matyjaszewski, K.; Xia, J. Chem. ReV. 2001, 101, 2921.
without loss of the grafted chains.7–12 This has led to suggested applications in sensing,13–15 nanoactuation,16 and the fabrication of selective membranes,17,18 in addition to the investigation of (7) Ru¨he, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gro¨hn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. AdV. Polym. Sci. 2004, 165, 79. (8) Ayres, N.; Boyes, S. G.; Brittain, W. J. Langmuir 2007, 23, 182. (9) Geoghegan, M.; Ruiz-Perez, L.; Dang, C. C.; Parnell, A. J.; Martin, S. J.; Howse, J. R.; Jones, R. A. L.; Golestanian, R.; Topham, P. D.; Crook, C. J.; Ryan, A. J.; Sivia, D. S.; Webster, J. R. P.; Menelle, A. Soft Matter 2006, 2, 1076. (10) LeMieux, M. C.; Peleshanko, S.; Anderson, K. D.; Tsukruk, V. V. Langmuir 2007, 23, 265. (11) Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Angew. Chem., Int. Ed. 2005, 44, 4578. (12) Sakai, K.; Smith, E. G.; Webber, G. B.; Baker, M.; Wanless, E. J.; Bu¨tu¨n, V.; Armes, S. P.; Biggs, S. Langmuir 2006, 22, 8435. (13) Bumbu, G. G.; Wolkenhauer, M.; Kircher, G.; Gutmann, J. S.; Berger, R. Langmuir 2007, 23, 2203. (14) Zhou, F.; Shu, W.; Welland, M. E.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 5326. (15) Tugulu, S.; Arnold, A.; Sielaff, I.; Johnsson, K.; Klok, H. A. Biomacromolecules 2005, 6, 1602. (16) Ryan, A. J.; Crook, C. J.; Howse, J. R.; Topham, P.; Jones, R. A. L.; Geoghegan, M.; Parnell, A. J.; Ruiz-Pe´rezL.; Martin, S. J.; Cadby, A.; Menelle, A.; Webster, J. R. P.; Gleeson, A. J.; Bras, W. Faraday Discuss. 2005, 128–55. (17) Sun, L.; Baker, G. L.; Bruening, M. L. Macromolecules 2005, 38, 2307. (18) Sun, L.; Dai, J.; Baker, G. L.; Bruening, M. L. Chem. Mater. 2006, 18, 4033.
10.1021/la7039898 CCC: $40.75 2008 American Chemical Society Published on Web 06/13/2008
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Scheme 1. L-b-L Electrostatic Adsorption of Oppositely Charged Polyelectrolyte Macroinitiators on Oxidized Silicon Wafers, Followed by Surface-Initiated ATRP of HEMA
fundamental properties of grafted chains. Antibacterial19 and protein-resistant/biocompatible20–23 surfaces are also promising applications for grafted chains, since highly hydrophilic polymers can be utilized in water without dissolution. Surface-grafting also improves the robustness of the polymer film: for example, Ishihara’s group has recently reported that surface-initiated lubricious poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) chains had significantly improved wear resistance over a spin-cast film of the same polymer.24 High grafting density is important for the fabrication of thick films by SIP. The grafting density must be high enough that the grafted polymer chains form a continuous layer, interacting sterically to produce so-called “polymer brushes”.25,26 Furthermore, the polymer brush thickness increases with grafting density σ (for a given grafted polymer molecular weight, the thickness of an uncharged brush is proportional to σ in the dry state27 and σ1/3 in a good solvent28). In addition to increasing the brush thickness, higher grafting densities greatly improve both the lubricity29 and protein resistance of the polymer-modified surface.21,30 (19) Ramstedt, M.; Cheng, N.; Azzaroni, O.; Mossialos, D.; Mathieu, H. J.; Huck, W. T. S. Langmuir 2007, 23, 3314. (20) Iwata, R.; Suk-In, P.; Hoven, V. P.; Takahara, A.; Akiyoshi, K.; Iwasaki, Y. Biomacromolecules 2004, 5, 2308. (21) Mei, Y.; Elliott, J. T.; Smith, J. R.; Langenbach, K. J.; Wu, T.; Xu, C.; Beers, K. L.; Amis, E. J.; Henderson, L. J. Biomed. Mater. Res. 2006, 79A, 974. (22) Brown, A. A.; Khan, N. S.; Steinbock, L.; Huck, W. T. S. Eur. Polym. J. 2005, 41, 1757. (23) Feng, W.; Zhu, S.; Ishihara, K.; Brash, J. L. Langmuir 2005, 21, 5980. (24) Kobayashi, M.; Terayama, Y.; Hosaka, N.; Kaido, M.; Suzuki, A.; Yamada, N.; Torikai, N.; Ishihara, K.; Takahara, A. Soft Matter 2007, 3, 740. (25) Milner, S. T. Science 1991, 251, 905. (26) Brittain, W. J.; Minko, S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3505. (27) Tomlinson, M. R.; Genzer, J. Macromolecules 2003, 36, 3449. (28) Biesalski, M.; Ruhe, J. Macromolecules 2002, 35, 499. (29) Klein, J. Annu. ReV. Mater. Sci. 1996, 26, 581. (30) Feng, W.; Brash, J. L.; Zhu, S. Biomaterials 2006, 27, 847.
Figure 1. Thickness of polyelectrolytic macroinitiator multilayers measured by (a) DPI and (b) ellipsometry. Macroinitiators were deposited in a L-b-L fashion using 15 min adsorption periods from 1.0 g L-1 aqueous solutions at 20 °C.
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Figure 2. XPS core-line spectra for polyelectrolytic macroinitiator multilayers. (a) C 1s, (b) N 1s, (c) Si 2s, and (d) O 1s spectra are shown with numerical labels indicating the number of macroinitiator layers in each case.
Perhaps the most attractive and technologically relevant substrates for SIP are metal oxides, including silica and alumina. Chlorosilane- or alkoxysilane-based small-molecule initiators have been typically used for ‘priming’ these surfaces prior to SIP.31 Such initiators are commonly synthesized by hydrosilylation using reagents that are highly toxic and flammable (e.g., H2PtCl6 and HSiCl3), making scale-up potentially hazardous. This has hitherto limited commercial applications of SIP, especially for high-surface-area substrates for which large amounts of initiator are required. Moreover, silane initiators are prone to hydrolysis, which makes functionalization of waterborne colloidal substrates somewhat problematic. Recently, Armes and co-workers have reported the use of polyelectrolytic macroinitiators for SIP by ATRP.32–35 These macroinitiators are readily synthesized from polymeric precursors in which the side-chains have been functionalized so as to introduce either cationic or anionic groups for electrostatic adsorption and bromoester groups for subsequent SIP via ATRP. Such macroinitiators overcome many of the disadvantages of conventional silane-based small-molecule initiators: they can be conveniently synthesized on a 20-30 g scale under relatively mild conditions, have excellent long-term chemical stability, and can be readily and rapidly adsorbed onto either silica or other metal oxide substrates from aqueous solution at ambient temperature. Both cationic32,33 and anionic34,35 macroinitiators (31) Husseman, M.; Malmstrom, 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. (32) Chen, X.; Armes, S. P. AdV. Mater. 2003, 15, 1558. (33) Chen, X. Y.; Armes, S. P.; Greaves, S. J.; Watts, J. F. Langmuir 2004, 20, 587. (34) Vo, C. D.; Schmid, A.; Armes, S. P.; Sakai, K.; Biggs, S. Langmuir 2007, 23, 408. (35) Edmondson, S.; Vo, C. D.; Armes, S. P.; Unali, G. F. Macromolecules 2007, 40, 5271.
have been developed and used to prime both colloidal32–34 and planar35 surfaces for SIP. However, polyelectrolytic macroinitiators generally produce polymer brushes with significantly lower grafting densities than those grown from silane-based initiators.36 This leads to significantly thinner layers and may compromise the surface properties that denser grafting could confer for applications. Polyelectrolyte macroinitiators contain side chains bearing either charged groups or initiator groups. In principle, the grafting density can be increased by incorporating more initiator groups at the expense of charged groups. However, this reduces the overall charge density on the macroinitiator and may therefore compromise its electrostatic adsorption onto a surface, leading eventually to a reduced initiator density. Additionally, the mechanical stability of the brush layer could be reduced by weaker grafting due to reduced electrostatics.37 Since both cationic and anionic macroinitiators are now available, it was decided to attempt to increase the chain grafting density using the well-known layer-by-layer (L-b-L) technique38,39 using a pair of oppositely charged macroinitiators. A cationic macroinitiator is first adsorbed onto anionic silica and the excess cationic charge is then neutralized by adsorption of an anionic macroinitiator. This second layer confers excess anionic charge, which allows the process to be repeated sequentially. Thus, each cycle produces a thicker layer of macroinitiator and (36) In unpublished work, we have recently shown that a wafer functionalized with trichlorosilane ATRP initiator produces polymer brushes around five times thicker than those functionalized with our previously reported anionic macroinitiator under identical growth conditions. (37) For example, we have observed macroscopic film blistering and delamination when using macroinitiators with a relatively high proportion of initiator groups (2:1 bromoester/N+ molar ratio). (38) Decher, G. Science 1997, 277, 1232. (39) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 430.
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Figure 4. Ellipsometric thicknesses of PHEMA brushes grown from polyelectrolyte macroinitiator multilayers comprising 1 (9), 3 (b), 5 (2), 11 (1), and 21 ([) layers. Polymerizations were conducted at 20 °C using a 1:1 v/v methanol/water solvent mixture. [HEMA] ) 4.12 M, and the HEMA/CuCl/CuBr2/bpy molar ratio was 60/1/0.3/2.8. The estimated error bars are smaller than the data points.
Figure 3. (a) XPS Br 3d core-line spectra recorded for selected polyelectrolytic macroinitiator multilayer films. Numerical labels indicate the number of macroinitiator layers in each case. (b) Linear correlation between Br 3d signal intensity and the number of deposited macroinitiator layers.
thus a higher surface initiator density. In principle, this approach should allow thicker polymer brushes to be grown using SIP. L-b-L deposition has been used for a wide range of applications, such as tunable drug release,40 the fabrication of stimulusresponsive surfaces,41 free-standing functional films,42 and distributed Bragg reflectors.43 Advincula and co-workers have demonstrated the inclusion of anionic polyelectrolyte macroinitiator layers into L-b-L assembled films for SIP, using a poly(acrylic acid)-based macroinitiator on a colloidal substrate.44 Recently, Bruening and co-workers45 demonstrated the inclusion of a low molecular weight (
