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Polymer Brushes on Planar TiO2 Substrates Jiming Yang, Liman Hou, Bin Xu, Ning Zhang,* Yongjiu Liang, Wenjing Tian, Dewen Dong* A facile and universal method is presented for the preparation of polymer brushes on amorphous TiO2 film. Homogeneous and stable poly(methyl methacrylate), polystyrene, poly(4vinylpyridine), and poly(N-vinyl imidazole) (PNVI) brushes up to 550 nm are directly created onto TiO2 via UV-induced photopolymerization of corresponding monomers. Kinetic studies reveal a linear increase in thickness with the polymerization time. Characterization of the resulting polymer brushes by FTIR spectroscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy (AFM) indicates an efficient UV-grafting reaction. Finally, we have demonstrated the possibility in converting the PNVI brushes to poly(vinyl imidazolium bromide), i.e., poly(ionic liquid) brushes by polymer–analogous reactions.
1. Introduction Since its commercial production in last century, titanium dioxide (TiO2) has been widely utilized as sunscreen, paints, and toothpaste.[1] In recent years, TiO2 has been extensively applied as the material in areas ranging from photocatalysis and photovoltaics to nanoscience and biomedical fields.[2,3] In spite of the versatility of the material, the question remains open concerning how to introduce chemical and biochemical functionalities to the TiO2 surface in a defined manner to meet the desire for divergent and specific surface functionalization.[4] To date, different strategies have been developed to tailor the surface functionality of TiO2, i.e., physical adsorption, chemically anchoring small molecule or polymer chains onto the surface of the material.[5–14] Among all the surface modification approaches, polymer brushes are probably the most efficient way in governing the interaction between the
J. Yang, L. Hou, Prof. N. Zhang, Y. Liang, Prof. D. Dong Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China E-mail: [email protected]; [email protected] Dr. B. Xu, Prof. W. Tian State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China
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substrate and the environment.[15–17] Takahara obtained dense polystyrene brush on TiO2 nanoparticles by the “grafting-from” method using nitroxide-mediated radical polymerization (NMRP).[5] Zhou prepared low surface energy poly(pentadeca-fluorooctyl-5-norbornene-2-carboxylate) brushes (PNCA-F15) from sticky biomimic-initiator-modified Ti/TiO2 surfaces via a surface-initiated ring-opening metathesis polymerization.[6] Woisel used well-defined dopamine end-functionalized polymers to functionalize titanium by “grafting-onto” method.[7] However, in most of the reported procedures, an intermediate layer, e.g., self-assembled monolayers (SAMs) on TiO2 is required so that the following surface-initiated polymerization (SIP) can be conducted to form polymer layers with controlled functionality, thickness, and density with precision.[5–14] Recently, it could be shown that well-defined, homogeneous and highly stable polymer brushes can be prepared directly on carbonaceous materials, e.g., diamond, glassy carbon, carbon-polar hexagonal SiC, and electron-beaminduced carbon deposits by the self-initiated photografting and photopolymerization (SIPGP) process with vinyl monomers.[18,19] The formation of defined reactive interlayers, e.g., SAMs or premodified layer is no longer necessary, and polymer brushes can be realized in an one-step reaction.[20,21] Here, we report an facile method that the UVinduced photopolymerization of different monomers leads to homogeneous polymer brushes on TiO2 surfaces.
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DOI: 10.1002/marc.201400068
Macromolecular Rapid Communications
Polymer Brushes on Planar TiO2 Substrates
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2. Experimental Section 2.1. Materials All substances were purchased from Sigma–Aldrich or Aladdin and used without further purification unless otherwise noted. Photochemical reactor (model RPR-100,) with a spectral distribution from 300 to 400 nm, λmax = 350 nm was purchased from The Southern New Scheme 1. Schematic illustration for the preparation of polymer brushes on TiO2 surfaces. England Ultraviolet Company (USA). TiO2 substrates were fabricated by Beijing Ying Zuo Nano Science and Technology to the C1s hydrocarbon peak at 284.6 eV. The atomic concentraCompany (China), which were rinsed thoroughly with deionized tions of the elements were calculated by their corresponding water and ethanol, and dried in a stream of nitrogen prior to use. peak areas. Methyl methacrylate (MMA), N-vinyl imidazole, styrene, and 4-vinylpyridine (4VP) were purified by refluxing over CaH2 and 2.3.4. Static Water Contact Angles were subsequently distilled under vacuum before use. All contact angle profiles were obtained at room temperature using a contact angle goniometer (DSA, Krüss, GmbH, Germany) by the sessile drop method with a 3 μL water droplet. The contact angle values were taken after 15 s from droplet deposition on the film. For each sample, at least three measurements taken from different surface locations were averaged.
2.2. Polymerizations Procedures 2.2.1. Self-Initiated Photografting and Photopolymerization All the polymer brushes were prepared according to a previous procedure.[21] Briefly, polymerizations were performed by immersing freshly cleaned substrates in 1 mL of freshly distilled and degassed bulk monomer and irradiated in the Photochemical reactor. After UV irradiation, the polymer-modified substrates were exhaustively cleaned by ultrasonication in toluene, ethyl acetate, and ethanol to ensure that only chemically grafted polymers remained on the substrates.
2.3. Characterization 2.3.1. Atomic Force Microscopy Scans were obtained with a Nanoscope IIIa scanning probe microscope from Veeco Instruments (USA). The microscope was operated in a tapping mode using Si cantilevers with a resonance frequency of 305 kHz, a driving amplitude of 1.23 V at a scan rate of 0.5 Hz. The average roughness (Ra) was calculated from a 1 × 1 μm2 area.
2.3.2. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectra were obtained from a spectrometer of VERTEX 70 Bruker, Germany with a resolution of 4 cm−1. The background spectra were recorded on corresponding TiO2 bare substrates.
2.3.3. X-ray Photoelectron Spectroscopy All the samples were examined by using VG Scientific ESCA MK II Thermo Avantage V 3.20 analyzer with Al/K (hν = 1486.6 eV) anode mono-X-ray source. The take-off angle for photoelectron analyzer was fixed at 90°. Surface spectra were collected over a range of 0–1200 eV. All binding energy values were referenced
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3. Results and Discussion In this study, an amorphous TiO2 thin film with a thickness of ≈14 nm prepared by atomic layer deposition (ALD) on a silicon wafer was used as the substrate. TiO2 films on flat substrates facilitate the spectroscopic and morphological characterization of the anchored polymer brushes in comparison to widely used mesoporous or nanoparticle TiO2. The preparation of polymer brushes on TiO2 substrate is schematically outlined in Scheme 1. In an initial investigation, poly(methyl methacrylate) (PMMA) brushes were prepared by SIPGP of MMA through irradiation with a UV light of a spectral distribution between 300 and 400 nm (λmax = 350 nm). After the polymerization, the substrate was rigorously cleaned by ultrasonication in several solvents with different polarities to ensure that only chemically grafted polymer remains on the substrate. The polymer-modified substrate was then visualized by atomic force microscopy (AFM). It was found that the surface roughness of the native TiO2 substrate (Ra 0.27 nm) was rendered by the polymer layer to a higher value of 1.4 nm (Ra). The successful modification of the TiO2 substrate by PMMA brushes was further confirmed by infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) (Figure 1). As shown in FTIR spectrum (Figure 1A), the strong bands at 1729 and 1146 cm−1 originated from the stretching mode of (C O) and (C O) were observed, indicating the successful formation of PMMA
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Figure 1. A) IR spectrum, B) XPS survey scan, C) detailed C1s, and D) O1s spectra with fitted curves of PMMA brush on TiO2. For XPS survey scan, Si2s and Si2p peaks are visible, which are attributed to the silicon substrate.
brushes on TiO2. Moreover, the XPS survey scan (Figure 1B) and high-resolution scan (Figure 1C,D) further corroborate the formation of PMMA brushes (Figure 1B). Ex situ kinetic studies of the SIPGP of MMA were conducted on individual TiO2 substrates at different UV irradiation times (1–18 h). In order to determine the thickness of the PMMA layer, the sample was scratched with a sharp metallic needle to remove the polymer layer(s) locally. The borders of the scratches were investigated by AFM to determine the height difference, i.e., the brush layer thickness. As indicated in Figure 2, the thickness of the polymer brush layer as measured by AFM under ambient conditions is plotted as a function of the irradiation time. For polymerization times below 10 h, an almost constant growth rate of 30 nm h−1 is observed. Nevertheless, the layer thickness growth rate decreases significantly when photopolymerization times exceed 10 h. Actually, the bulk monomer phase became highly viscous for longer irradiation time due to photopolymerization
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Figure 2. Kinetics study for SIPGP of MMA after different UV irradiation time. Polymer layer thickness as a function of A) polymerization time and B) AFM scans of PMMA brushes after indicated polymerization time.
of MMA in the bulk phase. The reduced layer thickness growth rate is very likely caused by the lowered monomer mobility as the viscosity increases. As reported previously, using different substrates, e.g., polyethylene,[19] aromatic SAMs on gold,[22] carbonaceous materials,[23,24] and carbon deposits on inorganic substrates induced by electron beams,[25,26] the grafting reaction and the formation of polymer brushes occur via the SIPGP mechanism that a vinyl monomer acts as a photosensitizer to activate a surface functional moiety
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reactions employing other vinyl monomers, i.e., styrene, 4VP, and N-vinyl imimdazole (NVI) were performed, resulting in homogeneous and stable poly(styrene) (PS), poly(4-vinylpyridine) (P4VP), and poly(N-vinyl imidazole) (PNVI) brushes. AFM-scratch experiments (Figure 3) as well as XPS spectroscopic analysis (Supporting Information) revealed that these polymer brushes were successfully created on the TiO2 surfaces. It should be noted that the increase of the thickness of the polymer layers with the irradiation time was specific for each monomer. For instance, after 30 h of irradiation, SIPGP of 4VP gave a polymer layer thickness of 58 nm (thickness growth rate of 2 nm h−1). However, for SIPGP of MMA, a layer thickness of 321 nm was reached in 10 h with a polymer layer thickness growth rate of 32 nm h−1. The distinct polymer layer thickness growth rate is caused by the different polymerization velocity of monomers through a free radical polymerization mechanism.[30] The influence of the polymer composition on the hydrophilic/hydrophobic character of the polymer layer was investigated by contact angle (CA) measurements (Figure 4). The bare TiO2 Figure 3. AFM scans for polymer brushes a) poly (methyl mechacrylate) (PMMA), b) film gave a static water CA of 75 ± 3°, poly(N-vinyl imidazole) (PNVI), c) poly(4-vinylpyridine) (P4VP), and d) polystyrene (PS) while the more hydrophobic PMMA, PS, on TiO2 surfaces. PNVI, and P4VP resulted in similar but greater CAs of 77 ± 3°, 87 ± 2°, 97 ± 4° and 98 ± 2°, respectively. by hydrogen abstraction to initiate a free radical SIP.[20] The only requirement for the grafting reaction is the presRecently, much attention has been paid to polymerized ence of abstractable hydrogen on the surfaces, which may ionic liquids or polymeric ionic liquids, which are usually be abstracted by the monomer radicals. It is known that obtained from polymerizing ionic liquid monomers or TiO2 surface is covered by large number of active hydroxyl analogous reactions of preformed polymers.[31] The potenfunctionalities,[3,27] which provide the possibility that tial applications of poly(ionic liquid) involve polymeric electrolytes,[32] catalytic membranes,[33] and ionic conducSIPGP of vinyl monomers may proceed. On the other hand, surface-bound OH· radicals are formed on TiO2 subtive materials etc.[34] Promoted by these intriguing applistrates when subjected to UV light.[28,29] The formed radications, the as-prepared PNVI brushes were then treated by bromoethane at 60 °C in MeOH in order to convert cals successively initiate a surface-initiated free radical polymerization. In any cases, surfacebound polymer chains are formed via a free radical polymerization mechanism. In order to demonstrate that the universal approach is applicable to other monomers with different chemical structure and functionalities, besides Figure 4. Surface wettabilities of the prepared polymer brushes as determined by static the polymerization of MMA, the SIPGP water contact angle.
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Figure 5. Reaction scheme for the conversion from PNVI brushes to poly(ionic liquid) brushes and static water contact angle measurement on corresponding surfaces (a), AFM scans and section analysis for PNVI and poly(ionic liquid) brushes (b), XPS survey scans of PNVI and poly(ionic liquid)s (c). For the PNVI grafts only N1s and O1s peaks are visible; the O1s is attributed to water adsorption. In addition to the N1s and C1s signals, the XPS spectrum of poly(ionic liquid) reveals the Br3s, Br3p, and Br3d peaks.
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them to poly(ionic liquid) brushes. After the reaction with bromoethane, the thickness of PNVI brushes significantly increased from 187 to 299 nm as shown in Figure 5, indicating the successful reaction of PNVI with bromoethane. The conversion to poly(ionic liquid) brushes was further confirmed by static water contact angle and XPS analysis (Figure 5). After SIPGP, the static water contact angle obtained from PVNI brushes drastically dropped from 97° to 26° for poly(vinyl imidazolium bromide) brushes. The successful post-functionalization of PNVI brushes also demonstrates the stability of the polymer brushes prepared via the established protocol.
substrate. The protocol is applicable to a broad range of monomers, e.g., methacrylates, styrene, 4VP, and N-vinyl imidazole. Through polymer-analogous reaction, poly(ionic liquid), i.e., poly(vinyl imidazolium bromide) brushes are available by reaction of PNVI with bromoethane. In view of the versatility of TiO2 in fields of heterogeneous catalysis, optoelectronics and nanoscience, the established method may provide researchers a preparative alternative to easily tailor the surface functionality of TiO2 in planar, nanoparticle, or mesoporous forms.
4. Conclusions
Supporting Information is available from the Wiley Online Library or from the author.
A straightforward and universal SIPGP of vinyl monomers is described, affording stable polymer brushes on TiO2
Acknowledgements: The financial support of this research by the National Natural Science Foundation of China (51373170
Supporting Information
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and 21304086), Department of Science and Technology of Jilin Province (20130101003JC), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201426) is greatly acknowledged. Received: February 1, 2014; Revised: March 7, 2014; Published online: April 9, 2014; DOI: 10.1002/marc.201400068 Keywords: polymer–analogous reactions; brushes; polymer brushes; TiO2; SIPGP
poly(ionic
liquid)
[1] M. M. Alam, S. R. Snigdha, M. J. Uddin, Titanium Dioxide, LAP Lambert Academic Publishing Saarbrücken, Germany 2011. [2] X. Chen, S. S. Mao, Chem. Rev. 2007, 107, 2891. [3] A. L. Linsebigler, G. Lu, J. T. Yates, Chem. Rev. 1995, 95, 735. [4] M. A. Stuart, W. T. Huck, J. Genzer, M. Müller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk, M. Urban, F. Winnik, S. Zauscher, I. Luzinov, S. Minko, Nat. Mater. 2010, 9, 101. [5] R. Matsuno, H. Otsuka, A. Takahara, Soft Matter 2006, 2, 415. [6] Q. Ye, X. Wang, S. Li, F. Zhou, Macromolecules 2010, 43, 5554. [7] C. Zobrist, J. Sobocinski, J. Lyskawa, D. Fournier, V. Miri, M. Traisnel, M. Jimenez, P. Woisel, Macromolecules 2011, 44, 5883. [8] K. J. Son, S. H. Ahn, J. H. Kim, W.-G. Koh, ACS Appl. Mater. Interfaces 2011, 3, 573. [9] W. Wang, H. Cao, G. Zhu, P. Wang, J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1782. [10] J. Wang, X. Ni, J. Appl. Polym. Sci. 2008, 108, 3552. [11] X. Wang, X. Song, M. Lin, H. Wang, Y. Zhao, W. Zhong, Q. Du, Polymer 2007, 48, 5834. [12] J. L. Dalsin, L. Lin, S. Tosatti, J. Vörös, M. Textor, P. B. Messersmith, Langmuir 2005, 21, 640. [13] E. A. Smirnov, M. A. Meledina, A. V. Garshev, V. I. Chelpanov, S. Frost, J. U. Wieneke, M. Ulbricht, Polym. Int. 2013, 62, 836.
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[14] A. Mesnage, M. Abdel Magied, P. Simon, N. Herlin-Boime, P. Jégou, G. Deniau, S. Palacin, J. Mater. Sci. 2011, 46, 6332. [15] Polymer Brushes, (Eds: R. C. Advincula , W. J. Brittain, K. C. Caster, J. Rühe), Wiley-VCH, Weinheim 2004. [16] R. Barbey, L. Lavanant, D. Paripovic, N. Schüwer, C. Sugnaux, S. Tugulu, H. A. Klok, Chem. Rev. 2009, 109, 5437. [17] B. Zhao, W. J. Brittain, Prog. Polym. Sci. 2000, 25, 677. [18] P. Yang, W. Yang, Chem. Rev. 2013, 113, 5547. [19] J. Deng, W. Yang, B. Rånby, Macromol. Rapid Commun. 2001, 22, 535. [20] T. B. Stachowiak, F. Svec, J. J. M. Fréchet, Chem. Mater. 2006, 18, 5950. [21] T. Chen, I. Amin, R. Jordan, Chem. Soc. Rev. 2012, 41, 3280. [22] M. Steenackers, A. Küller, S. Stoycheva, M. Grunze, R. Jordan, Langmuir 2009, 25, 2225. [23] N. Zhang, M. Steenackers, R. Luxenhofer, R. Jordan, Macromolecules 2009, 42, 5345. [24] M. Steenackers, A. M. Gigler, N. Zhang, F. Deubel, M. Seifert, L. H. Hess, C. Lim, K. P. Loh, J. A. Garrido, R. Jordan, M. Stutzmann, I. D. Sharp, J. Am. Chem. Soc. 2011, 133, 10490. [25] M. Steenackers, I. D. Sharp, K. Larsson, N. A. Hutter, M. Stutzmann, R. Jordan, Chem. Mater. 2010, 22, 272. [26] M. Steenackers, R. Jordan, A. Küller, M. Grunze, Adv. Mater. 2009, 21, 2921. [27] K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. 2005, 44, 8269. [28] H. Lin, J. Long, Q. Gu, W. Zhang, R. Ruan, Z. Li, X. Wang, Phys. Chem. Chem. Phys. 2012, 14, 9468. [29] Y. Nakabayashi, Y. Nosaka, J. Phys. Chem. C 2013, 117, 23832. [30] G. Odian, Principles of Polymerization, 4th ed., John Wiley & Sons, New Jersey, USA 2004. [31] J. Lu, F. Yan, J. Texter, Prog. Polym. Sci. 2009, 34, 431. [32] M. A. B. H. Susan, T. Kaneko, A. Noda, M. Watanabe, J. Am. Chem. Soc. 2005, 127, 4976. [33] R. T. Carlin, J. Fuller, Chem. Commun. 1997, 1345. [34] N. Matsumi, K. Sugai, M. Miyake, H. Ohno, Macromolecules 2006, 39, 6924.
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Communication
Polymer Brushes on Planar TiO2 Substrates Jiming Yang, Liman Hou, Bin Xu, Ning Zhang,* Yongjiu Liang, Wenjing Tian, Dewen Dong* A facile and universal method is presented for the preparation of polymer brushes on amorphous TiO2 film. Homogeneous and stable poly(methyl methacrylate), polystyrene, poly(4vinylpyridine), and poly(N-vinyl imidazole) (PNVI) brushes up to 550 nm are directly created onto TiO2 via UV-induced photopolymerization of corresponding monomers. Kinetic studies reveal a linear increase in thickness with the polymerization time. Characterization of the resulting polymer brushes by FTIR spectroscopy, X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy (AFM) indicates an efficient UV-grafting reaction. Finally, we have demonstrated the possibility in converting the PNVI brushes to poly(vinyl imidazolium bromide), i.e., poly(ionic liquid) brushes by polymer–analogous reactions.
1. Introduction Since its commercial production in last century, titanium dioxide (TiO2) has been widely utilized as sunscreen, paints, and toothpaste.[1] In recent years, TiO2 has been extensively applied as the material in areas ranging from photocatalysis and photovoltaics to nanoscience and biomedical fields.[2,3] In spite of the versatility of the material, the question remains open concerning how to introduce chemical and biochemical functionalities to the TiO2 surface in a defined manner to meet the desire for divergent and specific surface functionalization.[4] To date, different strategies have been developed to tailor the surface functionality of TiO2, i.e., physical adsorption, chemically anchoring small molecule or polymer chains onto the surface of the material.[5–14] Among all the surface modification approaches, polymer brushes are probably the most efficient way in governing the interaction between the
J. Yang, L. Hou, Prof. N. Zhang, Y. Liang, Prof. D. Dong Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China E-mail: [email protected]; [email protected] Dr. B. Xu, Prof. W. Tian State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China
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substrate and the environment.[15–17] Takahara obtained dense polystyrene brush on TiO2 nanoparticles by the “grafting-from” method using nitroxide-mediated radical polymerization (NMRP).[5] Zhou prepared low surface energy poly(pentadeca-fluorooctyl-5-norbornene-2-carboxylate) brushes (PNCA-F15) from sticky biomimic-initiator-modified Ti/TiO2 surfaces via a surface-initiated ring-opening metathesis polymerization.[6] Woisel used well-defined dopamine end-functionalized polymers to functionalize titanium by “grafting-onto” method.[7] However, in most of the reported procedures, an intermediate layer, e.g., self-assembled monolayers (SAMs) on TiO2 is required so that the following surface-initiated polymerization (SIP) can be conducted to form polymer layers with controlled functionality, thickness, and density with precision.[5–14] Recently, it could be shown that well-defined, homogeneous and highly stable polymer brushes can be prepared directly on carbonaceous materials, e.g., diamond, glassy carbon, carbon-polar hexagonal SiC, and electron-beaminduced carbon deposits by the self-initiated photografting and photopolymerization (SIPGP) process with vinyl monomers.[18,19] The formation of defined reactive interlayers, e.g., SAMs or premodified layer is no longer necessary, and polymer brushes can be realized in an one-step reaction.[20,21] Here, we report an facile method that the UVinduced photopolymerization of different monomers leads to homogeneous polymer brushes on TiO2 surfaces.
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DOI: 10.1002/marc.201400068
Macromolecular Rapid Communications
Polymer Brushes on Planar TiO2 Substrates
www.mrc-journal.de
2. Experimental Section 2.1. Materials All substances were purchased from Sigma–Aldrich or Aladdin and used without further purification unless otherwise noted. Photochemical reactor (model RPR-100,) with a spectral distribution from 300 to 400 nm, λmax = 350 nm was purchased from The Southern New Scheme 1. Schematic illustration for the preparation of polymer brushes on TiO2 surfaces. England Ultraviolet Company (USA). TiO2 substrates were fabricated by Beijing Ying Zuo Nano Science and Technology to the C1s hydrocarbon peak at 284.6 eV. The atomic concentraCompany (China), which were rinsed thoroughly with deionized tions of the elements were calculated by their corresponding water and ethanol, and dried in a stream of nitrogen prior to use. peak areas. Methyl methacrylate (MMA), N-vinyl imidazole, styrene, and 4-vinylpyridine (4VP) were purified by refluxing over CaH2 and 2.3.4. Static Water Contact Angles were subsequently distilled under vacuum before use. All contact angle profiles were obtained at room temperature using a contact angle goniometer (DSA, Krüss, GmbH, Germany) by the sessile drop method with a 3 μL water droplet. The contact angle values were taken after 15 s from droplet deposition on the film. For each sample, at least three measurements taken from different surface locations were averaged.
2.2. Polymerizations Procedures 2.2.1. Self-Initiated Photografting and Photopolymerization All the polymer brushes were prepared according to a previous procedure.[21] Briefly, polymerizations were performed by immersing freshly cleaned substrates in 1 mL of freshly distilled and degassed bulk monomer and irradiated in the Photochemical reactor. After UV irradiation, the polymer-modified substrates were exhaustively cleaned by ultrasonication in toluene, ethyl acetate, and ethanol to ensure that only chemically grafted polymers remained on the substrates.
2.3. Characterization 2.3.1. Atomic Force Microscopy Scans were obtained with a Nanoscope IIIa scanning probe microscope from Veeco Instruments (USA). The microscope was operated in a tapping mode using Si cantilevers with a resonance frequency of 305 kHz, a driving amplitude of 1.23 V at a scan rate of 0.5 Hz. The average roughness (Ra) was calculated from a 1 × 1 μm2 area.
2.3.2. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Spectra were obtained from a spectrometer of VERTEX 70 Bruker, Germany with a resolution of 4 cm−1. The background spectra were recorded on corresponding TiO2 bare substrates.
2.3.3. X-ray Photoelectron Spectroscopy All the samples were examined by using VG Scientific ESCA MK II Thermo Avantage V 3.20 analyzer with Al/K (hν = 1486.6 eV) anode mono-X-ray source. The take-off angle for photoelectron analyzer was fixed at 90°. Surface spectra were collected over a range of 0–1200 eV. All binding energy values were referenced
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3. Results and Discussion In this study, an amorphous TiO2 thin film with a thickness of ≈14 nm prepared by atomic layer deposition (ALD) on a silicon wafer was used as the substrate. TiO2 films on flat substrates facilitate the spectroscopic and morphological characterization of the anchored polymer brushes in comparison to widely used mesoporous or nanoparticle TiO2. The preparation of polymer brushes on TiO2 substrate is schematically outlined in Scheme 1. In an initial investigation, poly(methyl methacrylate) (PMMA) brushes were prepared by SIPGP of MMA through irradiation with a UV light of a spectral distribution between 300 and 400 nm (λmax = 350 nm). After the polymerization, the substrate was rigorously cleaned by ultrasonication in several solvents with different polarities to ensure that only chemically grafted polymer remains on the substrate. The polymer-modified substrate was then visualized by atomic force microscopy (AFM). It was found that the surface roughness of the native TiO2 substrate (Ra 0.27 nm) was rendered by the polymer layer to a higher value of 1.4 nm (Ra). The successful modification of the TiO2 substrate by PMMA brushes was further confirmed by infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) (Figure 1). As shown in FTIR spectrum (Figure 1A), the strong bands at 1729 and 1146 cm−1 originated from the stretching mode of (C O) and (C O) were observed, indicating the successful formation of PMMA
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Figure 1. A) IR spectrum, B) XPS survey scan, C) detailed C1s, and D) O1s spectra with fitted curves of PMMA brush on TiO2. For XPS survey scan, Si2s and Si2p peaks are visible, which are attributed to the silicon substrate.
brushes on TiO2. Moreover, the XPS survey scan (Figure 1B) and high-resolution scan (Figure 1C,D) further corroborate the formation of PMMA brushes (Figure 1B). Ex situ kinetic studies of the SIPGP of MMA were conducted on individual TiO2 substrates at different UV irradiation times (1–18 h). In order to determine the thickness of the PMMA layer, the sample was scratched with a sharp metallic needle to remove the polymer layer(s) locally. The borders of the scratches were investigated by AFM to determine the height difference, i.e., the brush layer thickness. As indicated in Figure 2, the thickness of the polymer brush layer as measured by AFM under ambient conditions is plotted as a function of the irradiation time. For polymerization times below 10 h, an almost constant growth rate of 30 nm h−1 is observed. Nevertheless, the layer thickness growth rate decreases significantly when photopolymerization times exceed 10 h. Actually, the bulk monomer phase became highly viscous for longer irradiation time due to photopolymerization
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Figure 2. Kinetics study for SIPGP of MMA after different UV irradiation time. Polymer layer thickness as a function of A) polymerization time and B) AFM scans of PMMA brushes after indicated polymerization time.
of MMA in the bulk phase. The reduced layer thickness growth rate is very likely caused by the lowered monomer mobility as the viscosity increases. As reported previously, using different substrates, e.g., polyethylene,[19] aromatic SAMs on gold,[22] carbonaceous materials,[23,24] and carbon deposits on inorganic substrates induced by electron beams,[25,26] the grafting reaction and the formation of polymer brushes occur via the SIPGP mechanism that a vinyl monomer acts as a photosensitizer to activate a surface functional moiety
Macromol. Rapid Commun. 2014, 35, 1224−1229 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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reactions employing other vinyl monomers, i.e., styrene, 4VP, and N-vinyl imimdazole (NVI) were performed, resulting in homogeneous and stable poly(styrene) (PS), poly(4-vinylpyridine) (P4VP), and poly(N-vinyl imidazole) (PNVI) brushes. AFM-scratch experiments (Figure 3) as well as XPS spectroscopic analysis (Supporting Information) revealed that these polymer brushes were successfully created on the TiO2 surfaces. It should be noted that the increase of the thickness of the polymer layers with the irradiation time was specific for each monomer. For instance, after 30 h of irradiation, SIPGP of 4VP gave a polymer layer thickness of 58 nm (thickness growth rate of 2 nm h−1). However, for SIPGP of MMA, a layer thickness of 321 nm was reached in 10 h with a polymer layer thickness growth rate of 32 nm h−1. The distinct polymer layer thickness growth rate is caused by the different polymerization velocity of monomers through a free radical polymerization mechanism.[30] The influence of the polymer composition on the hydrophilic/hydrophobic character of the polymer layer was investigated by contact angle (CA) measurements (Figure 4). The bare TiO2 Figure 3. AFM scans for polymer brushes a) poly (methyl mechacrylate) (PMMA), b) film gave a static water CA of 75 ± 3°, poly(N-vinyl imidazole) (PNVI), c) poly(4-vinylpyridine) (P4VP), and d) polystyrene (PS) while the more hydrophobic PMMA, PS, on TiO2 surfaces. PNVI, and P4VP resulted in similar but greater CAs of 77 ± 3°, 87 ± 2°, 97 ± 4° and 98 ± 2°, respectively. by hydrogen abstraction to initiate a free radical SIP.[20] The only requirement for the grafting reaction is the presRecently, much attention has been paid to polymerized ence of abstractable hydrogen on the surfaces, which may ionic liquids or polymeric ionic liquids, which are usually be abstracted by the monomer radicals. It is known that obtained from polymerizing ionic liquid monomers or TiO2 surface is covered by large number of active hydroxyl analogous reactions of preformed polymers.[31] The potenfunctionalities,[3,27] which provide the possibility that tial applications of poly(ionic liquid) involve polymeric electrolytes,[32] catalytic membranes,[33] and ionic conducSIPGP of vinyl monomers may proceed. On the other hand, surface-bound OH· radicals are formed on TiO2 subtive materials etc.[34] Promoted by these intriguing applistrates when subjected to UV light.[28,29] The formed radications, the as-prepared PNVI brushes were then treated by bromoethane at 60 °C in MeOH in order to convert cals successively initiate a surface-initiated free radical polymerization. In any cases, surfacebound polymer chains are formed via a free radical polymerization mechanism. In order to demonstrate that the universal approach is applicable to other monomers with different chemical structure and functionalities, besides Figure 4. Surface wettabilities of the prepared polymer brushes as determined by static the polymerization of MMA, the SIPGP water contact angle.
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Figure 5. Reaction scheme for the conversion from PNVI brushes to poly(ionic liquid) brushes and static water contact angle measurement on corresponding surfaces (a), AFM scans and section analysis for PNVI and poly(ionic liquid) brushes (b), XPS survey scans of PNVI and poly(ionic liquid)s (c). For the PNVI grafts only N1s and O1s peaks are visible; the O1s is attributed to water adsorption. In addition to the N1s and C1s signals, the XPS spectrum of poly(ionic liquid) reveals the Br3s, Br3p, and Br3d peaks.
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them to poly(ionic liquid) brushes. After the reaction with bromoethane, the thickness of PNVI brushes significantly increased from 187 to 299 nm as shown in Figure 5, indicating the successful reaction of PNVI with bromoethane. The conversion to poly(ionic liquid) brushes was further confirmed by static water contact angle and XPS analysis (Figure 5). After SIPGP, the static water contact angle obtained from PVNI brushes drastically dropped from 97° to 26° for poly(vinyl imidazolium bromide) brushes. The successful post-functionalization of PNVI brushes also demonstrates the stability of the polymer brushes prepared via the established protocol.
substrate. The protocol is applicable to a broad range of monomers, e.g., methacrylates, styrene, 4VP, and N-vinyl imidazole. Through polymer-analogous reaction, poly(ionic liquid), i.e., poly(vinyl imidazolium bromide) brushes are available by reaction of PNVI with bromoethane. In view of the versatility of TiO2 in fields of heterogeneous catalysis, optoelectronics and nanoscience, the established method may provide researchers a preparative alternative to easily tailor the surface functionality of TiO2 in planar, nanoparticle, or mesoporous forms.
4. Conclusions
Supporting Information is available from the Wiley Online Library or from the author.
A straightforward and universal SIPGP of vinyl monomers is described, affording stable polymer brushes on TiO2
Acknowledgements: The financial support of this research by the National Natural Science Foundation of China (51373170
Supporting Information
Macromol. Rapid Commun. 2014, 35, 1224−1229 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Macromolecular Rapid Communications
Polymer Brushes on Planar TiO2 Substrates
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and 21304086), Department of Science and Technology of Jilin Province (20130101003JC), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201426) is greatly acknowledged. Received: February 1, 2014; Revised: March 7, 2014; Published online: April 9, 2014; DOI: 10.1002/marc.201400068 Keywords: polymer–analogous reactions; brushes; polymer brushes; TiO2; SIPGP
poly(ionic
liquid)
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