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Preparation and characterization of optical-functional diblock copolymer brushes on hollow sphere surface via atom transfer radical polymerization
Materials Research Bulletin 45 (2010) 1314–1318
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and characterization of optical-functional diblock copolymer brushes on hollow sphere surface via atom transfer radical polymerization Li-Ping Wang a,*, Wen-Zhi Li a, Li-Min Zhao a, Chun-Juan Zhang b, Yan-Dong Wang a, Li-Li Kong a, Ling-Ling Li a a b
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China The Management Office of Weifang High-tech Zone Biomedical Science and Technology Industrial Park, Weifang 261061, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 October 2009 Received in revised form 27 January 2010 Accepted 28 April 2010 Available online 6 May 2010
The optical-functional poly(methyl methacrylate)-block-Tb complex diblock copolymer brushes grafted from hollow sphere surface via atom transfer radical polymerization were investigated in this work. A sufficient amount of azo initiator was introduced onto hollow sphere surface firstly. Then the monomer methyl methacrylate was polymerized via surface-initiated reverse atom transfer radical polymerization using azo group modified hollow sphere as initiator. Following, the poly(methyl methacrylate) modified hollow sphere was used as maroinitiator for surface-initiated atom transfer radical polymerization of Tb complex. The samples were characterized by Fourier transform infrared spectroscopy, hydrogen nuclear magnetic resonance, gel permeation chromatographer and transmission electron microscopy, respectively. The results indicated that the poly(methyl methacrylate) had grafted from hollow sphere surface and the average diameter of hollow core was about 1 mm. The optical properties of the poly(methyl methacrylate)-block-Tb copolymer modified hollow sphere were also reported. Crown Copyright ß 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Polymers A. Surfaces D. Optical properties
1. Introduction Since Matyjaszewski et al. first reported the atom transfer radical polymerization (ATRP) in 1995 [1], it has been intensively investigated because it retains the advantage of a wide range of monomer applicability and a high tolerance to impurities of conventional free-radical polymerization. In addition, it has the ability to generate polymers with well-defined molecular weight and block copolymers. However, transition-metal-catalyzed ATRP exhibits two major drawbacks in initiation system, including the toxicity of the halide species RX and the instablity of the catalyst such as CuCl in air and moisture. To overcome these drawbacks, the use of conventional radical initiators in the presence of complexes of transition metals in their higher oxidation state has been reported and referred to as reverse atom transfer radical polymerization (RATRP) by Matyjaszewski [2,3] and the other researchers [4–8]. Some authors [9–13] proposed the combination of the previously described methods with surface-initiated polymerization. For example, in surface-initiated RATRP (SIRATRP), conventional initiators are immobilized on the substrate by reacting the superficial hydroxyl groups with an RATRP initiator precursor (usually azo and peroxide compounds). Then the initiator-modified substrate is used to perform RATRP grafting.
As a result, the grafted chains with well-defined molecular weights on the substrate will be achieved. Hollow sphere is a kind of inorganic microsphere with abundant hydroxyl on its surface. It has large inner void, low density and high surface area which account for its actual and potential applications in imaging [14], wave absorber [15], acoustic [16], sensing devices [17], controlled drug-delivery carriers [18] and so on [19,20]. In this study, silica hollow sphere was used as inorganic substrate to prepare well-controlled polymer brushes from its surface. The controlled poly(methyl methacrylate) (PMMA) grafting from the silica hollow sphere surface via SI-RATRP was performed firstly. The polymers grafting from the hollow sphere surface were endfunctionalized by chlorine atom and they were used as macroinitiators to proceed the chain extension polymerization in the presence of CuCl/bipy catalyst system using Tb complex (including methyl methacrylate component) as monomer via a conventional ATRP process. The PMMA-block-Tb complex copolymer grafted hollow sphere (hollow sphere-g-PMMA-b-Tb) hybrid materials with designed structures were obtained subsequently. The reaction schemes were listed in Fig. 1. 2. Experimental 2.1. Reagents
* Corresponding author. Tel.: +86 635 8230919; fax: +86 635 8239863. E-mail address: [email protected] (L.-P. Wang).
Unless otherwise indicated, chemicals were obtained from commercial suppliers and used as received. The monomer methyl
0025-5408/$ – see front matter . Crown Copyright ß 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.04.023
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conducted on a Hitachi Model H-7650 electron microscope. The thermal decomposition behavior of the materials was examined by means of thermogravimetry (TG) with a heating rate of 10 8C min1 in the nitrogen atmosphere on a model STA 449C simultaneous DSC-TGA (Netzsch Instruments, Germany). The elemental analyses (EA) for C, H, and N were performed on a GmbH VarioEL elemental analyse system. Fluorescence excitation and emission spectra were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter from a 450 W stable xenon lamp. 2.3. Synthesis 2.3.1. Compound 1 Hollow sphere (1 g), (3-aminopropyl) triethoxysilane (KH-550) (1 mL) and toluene 20 mL were mixed and heated at 80 8C for 11 h under a nitrogen atmosphere. The mixture was filtrated and the filtrate was washed with toluene to remove excess compound KH550 to give KH-550 modified hollow sphere (hollow sphere-KH550) (compound 1), which was dried at 25 8C in vacuum. Elem. Anal. Calcd. (%): C, 8.0; H, 2.7; N, 2.4; KH-550, 1.1 mmol g1 (calculated according to the N content).
Fig. 1. Synthetic pathway for hollow sphere-g-PMMA-b-Tb optical-functional hybrid material via ATRP.
methacrylate (MMA) (AR, Shanghai Chemical Reagent Plant) was washed with 10% NaOH and ion-free water, stirred over CaH2 and distilled under reduced pressure prior to use. 4,40 -azobis (4cyanopentanoic acid) (ACPA) was purchased from Sigumas Co. The hollow sphere (HS) and the 4,40 -azobis (4-cyanopentanoic acid chloride) (ACPAC) were synthesized following a previously described procedure, respectively [21,22]. 2.2. Instrumentation The structure of samples was characterized by Nicolet-5700 Fourier transform infrared spectroscopy (FT-IR) from 400 to 4000 cm1 by the KBr pellet methods. The molecular weights and polydispersities (PDI) of the polymers were determined via an alliance GPCV 2000 (Waters, USA) gel permeation chromatographer (GPC) using THF as the eluent at a flow rate of 1.0 mL min1 and operated at 40 8C. Hydrogen nuclear magnetic resonance (1H NMR) spectra were recorded on Varian Mercury Plus 400 spectrometer using CDCl3 as a solvent and tetramethylsilane (TMS) as internal reference. Scanning electron microscopy analysis (SEM) was performed on a JEOL JSM6380LV scanning election microscope. Transmission electron microscopy (TEM) analysis was
2.3.2. Compound 2 Compound 1 (1 g) was dispersed in dichloromethane (10 mL) and 0.2 g ACPAC was added. The reaction mixture was stirred at room temperature for 6 h and stopped to remove the excess ACPAC by filtration. The white filtrate was washed thoroughly with mixed solvent of ethanol and water (1:1, v/v), ethanol and aether to afford azo groups immobilized hollow sphere (compound 2). Elem. Anal. Calcd. (%): C, 11.7; H, 3.2; N, 4.0; azo group, 0.57 mmol g1 (calculated according to the N content). 2.3.3. Tb complex Salicylic acid (2 mmol) and MMA (1 mmol) were dissolved in anhydrous ethanol; 1 mmol terbium nitrate was added under stirring and refluxing for 3 h resulting in the Tb complex (Fig. 2). 2.3.4. PMMA brushes on the hollow sphere surface A mixture of azo groups immobilized hollow sphere (compound 2) (0.20 g, azo group content: 0.11 mmol) and cyclohexanone (12 mL) in the flask was ultrasonically agitated for 30 min and then 2,20 -bipyridine (bipy) (0.11 g, 0.70 mmol), CuCl22H2O (0.04 g, 0.23 mmol) and degassed MMA (12 mL, 0.11 mol) were added. The flask was subjected to three freeze–pump–thaw cycles and heated at 70 8C for 10 h. After the reaction, the flask was cooled in an ice bath and the mixture was diluted with THF. The hollow sphere substrate with grafted PMMA (hollow sphere-g-PMMA) was separated by centrifugation. The crude product was extracted with THF and dried at 40 8C in vacuum to afford hollow sphere-g-PMMA. 2.3.5. Diblock copolymer brushes on the hollow sphere surface As shown in Fig. 1, the synthesis of diblock copolymer brushes was carried out on the hollow sphere-g-PMMA substrate. The
Fig. 2. The preparation scheme of Tb complex.
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Fig. 3. FT-IR spectra of (a) hollow sphere and (b) hollow sphere-g-PMMA.
Fig. 4. TG curves of (a) PMMA (residue weight = 2.1%), (b) hollow sphere-g-PMMA (residue weight = 66.9%) and (c) hollow sphere (residue weight = 60.5%).
procedures were similar to those used for synthesis of the hollow sphere-g-PMMA hybrid material, except using hollow sphere-gPMMA hybrid particles as macroinitiator and Tb complex as the second monomer via a conventional ATRP process. A general procedure is as follows: the mixture of hollow sphere-g-PMMA hybrid particles (0.10 g) and 12.5 mL ethanol in flask was ultrasonically agitated for 30 min and then the CuCl (0.01 g, 0.10 mmol), bipy (0.047 g, 0.30 mmol) and Tb complex (2.50 mmol) were added. The flask was subsequently evacuated and flushed with nitrogen. The polymerization was carried out under nitrogen in a 70 8C oil bath for 16 h, stopped by filtration and washing with excess THF to remove ungrafted Tb complex. Finally, the resulting white powder was dried at 40 8C in vacuum.
The not very clear peak at 2917 cm1 resulted from the C–H stretching vibrations of the small quantity of undissolved polystyrene. As for Fig. 3b, the absorption peaks at 3000–2800 cm1 came from C–H stretching vibration and the peak at 1733 cm1 represented the C5 5O stretching vibration of PMMA which indicated the successful grafting between hollow sphere and PMMA. TGA curves were used to trace the grafting polymerization. The typical TGA curves were depicted in Fig. 4. The weight loss stage below 200 8C was assigned to the evaporation of physically adsorbed water and residual solvent in samples. The weight loss of PMMA (Fig. 4a) exhibited a broad stage range from 200 to 420 8C. For silica hollow spheres (Fig. 4c), the weight loss at 300–420 8C and at 420– 600 8C was associated with the decomposition of undissolved polystyrene and the decompositions of silica-bonded groups such as –OH and/or unhydrolyzed –OR of hollow spheres, respectively [21]. So the grafted PMMA amount on hollow sphere surface can be determined approximately from the maximal difference in weight loss at 200–420 8C between the hollow sphere-g-PMMA hybrid material and silica hollow spheres, which is about 10%. In order to get more information about the microstructures of hollow spheres, SEM was performed. The hollow nature of the ‘‘bayberry’’ was observed in the contrast between the thin pale edge and dark center in SEM observations, as shown in Fig. 5. It is also evident that the surface of the hollow spheres before graft polymerization is relatively smoother than that of after graft polymerization and the average diameter of the hollow core is about 1 mm. TEM images (Fig. 6) clearly revealed typical hollow structure of the microspheres with hollow core diameter of about 1 mm. The
3. Results and discussion 3.1. Surface-initiated RATRP polymerization of MMA on the functionalized hollow sphere substrate The hollow sphere-g-PMMA hybrid materials were prepared using RATRP polymerization. The chemical state, graft ratio and topography of the hollow sphere-g-PMMA hybrid materials were probed by FT-IR, TGA, SEM and TEM, respectively. Fig. 3 shows the infrared spectrum of hollow sphere before (Fig. 3a) and after (Fig. 3b) graft polymerization. In Fig. 3a, the peak at 3440 cm1 was attributed to the O–H stretching vibrations of hydroxyl group in hollow sphere, the peak at 1091 cm1 was assigned to the Si–O–Si stretching vibration and the peak at 959 cm1 was a result of Si–OH vibration.
Fig. 5. SEM images of hollow sphere (a) and hollow sphere-g-PMMA (b).
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Fig. 6. TEM images of hollow sphere (a) and hollow sphere-g-PMMA (b).
hollow spheres before graft polymerization all kept good spherical shape and the walls consisted of numerous nanoparticles. After graft polymerization, the hollow microspheres were seriously anamorphic as shown in Fig. 6b, probably because the numerous nanoparticles on hollow spheres surface were influenced by shear force during grafting polymerization. In addition, the fact that a part of hydroxyl groups on hollow spheres surface replaced by PMMA would reduce the binding force among the silica nanoparticles, which is another possible reason for the anamorphic hollow spheres. The monomer conversion, the molecular weight and the polydispersity index (Mw/Mn) of the polymer formed in solution were examined by 1H NMR and GPC, respectively. Fig. 7 shows the 1 H NMR spectra of the reaction mixture for hollow sphere-gPMMA, which was performed directly with the polymerization system in deuterated chloroform. As shown in Fig. 7, monomer conversion of MMA was obtained using Convð%Þ ¼
block polymerization or further functionalization. The Tb complex was chosen for the block copolymerization as the Tb complex repeat unit contains MMA group, which can serve as a monomer in the ATRP polymerization. After graft copolymerization for 16 h on the hollow sphere-gPMMA substrate in an ethanol medium, the diblock polymer brushes grafted from hollow sphere surface (hollow sphere-g-
Aa 100% Aa þ Ab
where Aa and Ab were the areas of peaks a (the H of OCH3 in PMMA) and b (the H of OCH3 in MMA), respectively. The 1H NMR result shows that the MMA monomer conversion is 74%. At this conversion, the number average molecular weight (Mn) value reaches 27,600 gmol1 and the polydispersity index is as low as Mw/Mn = 1.15 according to the GPC result (Fig. 8). This is the characteristic of a controlled/‘‘living’’ polymerization. 3.2. Synthesis of the rare earth optical-functional hybrid material
Fig. 8. GPC curves of MMA ATRP polymerization formed in solution.
According to the mechanism of ATRP, the polymer prepared via the RATRP-mediated process has an end functionality of alkyl chlorine group. Thus, the graft chains prepared in this way on the hollow sphere substrate can serve as macroinitiator for subsequent
Fig. 7. 1H NMR spectra of the reaction mixture for hollow sphere-g-PMMA.
Fig. 9. The excitation spectra of hollow sphere-g-PMMA-Tb hybrid material.
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Fig. 10. The emission spectra of hollow sphere-g-PMMA-Tb hybrid material.
PMMA-b-Tb) came into being. The fluorescence properties of the sample were determined by spectrofluorimeter. Fig. 9 shows the excitation spectra of hollow sphere-g-PMMA-b-Tb complex. The excitation spectrum of the resulting hybrid materials was obtained by monitoring the emission of Tb3+ ions at 545 nm. The excitation spectrum is dominated by a broad band from 290 to 330 nm in narrow region and the maximum peak is at 330 nm, which can be attributed to the characteristic absorption of the lanthanide complexes arising from the efficient transition based on the conjugated double bonds of the aromatic cycle of salicylic acid ligand. These excitation spectra bands are the effective absorption for the luminescence of Tb3+. As a result, the strong green luminescence was observed (see Fig. 10), indicating that the effective energy transfer took place between the aromatic ligand and the chelated Tb3+ ions. In the emission spectrum there are four characteristic fluorescence emission bands associated with Tb3+. The band at 489 nm is assigned to the 5D4 ! 7F6 electron transition of Tb3+. The band at around 545 nm is associated with the 5 D4 ! 7F5 transition of Tb3+, and the bands at about 585 and 621 nm corresponding to the 5D4 ! 7F4 and 5D4 ! 7F3 electron transition of Tb3+, respectively. The fluorescence properties analysis results show that the Tb complex has grafted from the hollow sphere-g-PMMA substrate successfully. 4. Conclusion The preparation of optical-functional diblock copolymer brushes grafted from hollow sphere surface via surface reverse
atom transfer radical polymerization was wholly studied. After immobilization of the azo initiator, poly(methyl methacrylate) chains were successfully grafted from the surface of hollow sphere. Moreover, the poly(methyl methacrylate) grafted hollow sphere can be used as macroinitiator to initiate Tb complex polymerize, yielding poly(methyl methacrylate)-block-Tb grafted hollow sphere hybrid material. Gel permeation chromatographer analysis showed that the poly(methyl methacrylate) chains had a narrow molecular weight distribution. Transmission electron microscopy observations clearly revealed the hollow structure of the hollow spheres and TGA results indicated that the grafted polymer content is about 10% at 74% monomer conversion. Fluorescence spectra confirmed that the poly(methyl methacrylate)-block-Tb grafted hollow sphere hybrid materials exhibited strong fluorescence properties. This work presented a new method to synthesize optical-functional hybrid material with controlled molecular weights and ‘‘well-defined’’ structures, which may extend potential applications of hollow sphere and atom transfer radical polymerization.
Acknowledgements This research was supported by a project of Shandong Province Higher Education Science and Technology Program (J09LD56).
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J.S. Wang, K.J. Matyjaszewski, Am. Chem. Soc. 117 (1995) 5614. J. Xia, K. Matyjaszewski, Macromolecules 32 (1999) 5199. J. Qiu, S.G. Gaynor, K. Matyjaszewski, Macromolecules 32 (1999) 2872. X.P. Chen, B.A. Sufi, A.B. Padias, H.K. Hall, Macromolecules 35 (2002) 4277. A. Limer, D.M. Haddleton, Eur. Polym. J. 42 (2006) 61. Z.Q. Lei, S.X. Wen, Mater. Lett. 61 (2007) 4076. R.W. Simms, M.F. Cunningham, Macromolecules 41 (2008) 5148. H. Chen, Y. Liang, M. Wang, P. Lv, Y. Xuan, Chem. Eng. J. 147 (2009) 297. F.J. Xu, Z.L. Yuan, E.T. Kang, K.G. Neoh, Langmuir 20 (2004) 8200. B.F. Senkal, F. Bildik, E. Yavuz, A. Sarac, React. Funct. Polym. 67 (2007) 1471. K. Nagase, J. Kobayashi, A. Kikuchi, Y. Akiyama, H. Kanazawa, T. Okano, Langmuir 24 (2008) 511. L.P. Wang, Y.P. Wang, X.W. Pei, B. Peng, React. Funct. Polym. 68 (2008) 649. A. Friebe, M. Ulbricht, Macromolecules 42 (2009) 1838. Q. Peng, Y.J. Dong, Y.D. Li, Angew. Chem. Int. Ed. 42 (2003) 3027. M. Han, Y. Ou, L. Deng, J. Magn. Magn. Mater. 321 (2009) 1125. M. Bassan, A. DeWaard, G. Frossati, S. Giannı´, Y. Minenkov, L. Quintieri, R. Simonetti, Classical Quant. Grav. 27 (2010) 025014. Z. Wu, M. Zhang, K. Yu, S. Zhang, Y. Xie, Chem. Eur. J. 14 (2008) 5346. S.H. Im, U. Jeong, Y.N. Xia, Nat. Mater. 4 (2005) 671. X. He, W. Yang, L. Yuan, X. Pei, J. Gao, Mater. Lett. 63 (2009) 1138. C. Oh, Y.G. Lee, C.U. Jon, S.G. Oh, Colloids Surface A 337 (2009) 208. M. Chen, L. Wu, S. Zhou, B. You, Adv. Mater. 18 (2006) 801. G. Zhai, W.H. Yu, E.T. Kang, K.G. Neoh, C.C. Huang, D.J. Liaw, Ind. Eng. Chem. Res. 43 (2004) 1673.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and characterization of optical-functional diblock copolymer brushes on hollow sphere surface via atom transfer radical polymerization Li-Ping Wang a,*, Wen-Zhi Li a, Li-Min Zhao a, Chun-Juan Zhang b, Yan-Dong Wang a, Li-Li Kong a, Ling-Ling Li a a b
College of Materials Science and Engineering, Liaocheng University, Liaocheng 252059, China The Management Office of Weifang High-tech Zone Biomedical Science and Technology Industrial Park, Weifang 261061, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 October 2009 Received in revised form 27 January 2010 Accepted 28 April 2010 Available online 6 May 2010
The optical-functional poly(methyl methacrylate)-block-Tb complex diblock copolymer brushes grafted from hollow sphere surface via atom transfer radical polymerization were investigated in this work. A sufficient amount of azo initiator was introduced onto hollow sphere surface firstly. Then the monomer methyl methacrylate was polymerized via surface-initiated reverse atom transfer radical polymerization using azo group modified hollow sphere as initiator. Following, the poly(methyl methacrylate) modified hollow sphere was used as maroinitiator for surface-initiated atom transfer radical polymerization of Tb complex. The samples were characterized by Fourier transform infrared spectroscopy, hydrogen nuclear magnetic resonance, gel permeation chromatographer and transmission electron microscopy, respectively. The results indicated that the poly(methyl methacrylate) had grafted from hollow sphere surface and the average diameter of hollow core was about 1 mm. The optical properties of the poly(methyl methacrylate)-block-Tb copolymer modified hollow sphere were also reported. Crown Copyright ß 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Polymers A. Surfaces D. Optical properties
1. Introduction Since Matyjaszewski et al. first reported the atom transfer radical polymerization (ATRP) in 1995 [1], it has been intensively investigated because it retains the advantage of a wide range of monomer applicability and a high tolerance to impurities of conventional free-radical polymerization. In addition, it has the ability to generate polymers with well-defined molecular weight and block copolymers. However, transition-metal-catalyzed ATRP exhibits two major drawbacks in initiation system, including the toxicity of the halide species RX and the instablity of the catalyst such as CuCl in air and moisture. To overcome these drawbacks, the use of conventional radical initiators in the presence of complexes of transition metals in their higher oxidation state has been reported and referred to as reverse atom transfer radical polymerization (RATRP) by Matyjaszewski [2,3] and the other researchers [4–8]. Some authors [9–13] proposed the combination of the previously described methods with surface-initiated polymerization. For example, in surface-initiated RATRP (SIRATRP), conventional initiators are immobilized on the substrate by reacting the superficial hydroxyl groups with an RATRP initiator precursor (usually azo and peroxide compounds). Then the initiator-modified substrate is used to perform RATRP grafting.
As a result, the grafted chains with well-defined molecular weights on the substrate will be achieved. Hollow sphere is a kind of inorganic microsphere with abundant hydroxyl on its surface. It has large inner void, low density and high surface area which account for its actual and potential applications in imaging [14], wave absorber [15], acoustic [16], sensing devices [17], controlled drug-delivery carriers [18] and so on [19,20]. In this study, silica hollow sphere was used as inorganic substrate to prepare well-controlled polymer brushes from its surface. The controlled poly(methyl methacrylate) (PMMA) grafting from the silica hollow sphere surface via SI-RATRP was performed firstly. The polymers grafting from the hollow sphere surface were endfunctionalized by chlorine atom and they were used as macroinitiators to proceed the chain extension polymerization in the presence of CuCl/bipy catalyst system using Tb complex (including methyl methacrylate component) as monomer via a conventional ATRP process. The PMMA-block-Tb complex copolymer grafted hollow sphere (hollow sphere-g-PMMA-b-Tb) hybrid materials with designed structures were obtained subsequently. The reaction schemes were listed in Fig. 1. 2. Experimental 2.1. Reagents
* Corresponding author. Tel.: +86 635 8230919; fax: +86 635 8239863. E-mail address: [email protected] (L.-P. Wang).
Unless otherwise indicated, chemicals were obtained from commercial suppliers and used as received. The monomer methyl
0025-5408/$ – see front matter . Crown Copyright ß 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.04.023
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conducted on a Hitachi Model H-7650 electron microscope. The thermal decomposition behavior of the materials was examined by means of thermogravimetry (TG) with a heating rate of 10 8C min1 in the nitrogen atmosphere on a model STA 449C simultaneous DSC-TGA (Netzsch Instruments, Germany). The elemental analyses (EA) for C, H, and N were performed on a GmbH VarioEL elemental analyse system. Fluorescence excitation and emission spectra were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter from a 450 W stable xenon lamp. 2.3. Synthesis 2.3.1. Compound 1 Hollow sphere (1 g), (3-aminopropyl) triethoxysilane (KH-550) (1 mL) and toluene 20 mL were mixed and heated at 80 8C for 11 h under a nitrogen atmosphere. The mixture was filtrated and the filtrate was washed with toluene to remove excess compound KH550 to give KH-550 modified hollow sphere (hollow sphere-KH550) (compound 1), which was dried at 25 8C in vacuum. Elem. Anal. Calcd. (%): C, 8.0; H, 2.7; N, 2.4; KH-550, 1.1 mmol g1 (calculated according to the N content).
Fig. 1. Synthetic pathway for hollow sphere-g-PMMA-b-Tb optical-functional hybrid material via ATRP.
methacrylate (MMA) (AR, Shanghai Chemical Reagent Plant) was washed with 10% NaOH and ion-free water, stirred over CaH2 and distilled under reduced pressure prior to use. 4,40 -azobis (4cyanopentanoic acid) (ACPA) was purchased from Sigumas Co. The hollow sphere (HS) and the 4,40 -azobis (4-cyanopentanoic acid chloride) (ACPAC) were synthesized following a previously described procedure, respectively [21,22]. 2.2. Instrumentation The structure of samples was characterized by Nicolet-5700 Fourier transform infrared spectroscopy (FT-IR) from 400 to 4000 cm1 by the KBr pellet methods. The molecular weights and polydispersities (PDI) of the polymers were determined via an alliance GPCV 2000 (Waters, USA) gel permeation chromatographer (GPC) using THF as the eluent at a flow rate of 1.0 mL min1 and operated at 40 8C. Hydrogen nuclear magnetic resonance (1H NMR) spectra were recorded on Varian Mercury Plus 400 spectrometer using CDCl3 as a solvent and tetramethylsilane (TMS) as internal reference. Scanning electron microscopy analysis (SEM) was performed on a JEOL JSM6380LV scanning election microscope. Transmission electron microscopy (TEM) analysis was
2.3.2. Compound 2 Compound 1 (1 g) was dispersed in dichloromethane (10 mL) and 0.2 g ACPAC was added. The reaction mixture was stirred at room temperature for 6 h and stopped to remove the excess ACPAC by filtration. The white filtrate was washed thoroughly with mixed solvent of ethanol and water (1:1, v/v), ethanol and aether to afford azo groups immobilized hollow sphere (compound 2). Elem. Anal. Calcd. (%): C, 11.7; H, 3.2; N, 4.0; azo group, 0.57 mmol g1 (calculated according to the N content). 2.3.3. Tb complex Salicylic acid (2 mmol) and MMA (1 mmol) were dissolved in anhydrous ethanol; 1 mmol terbium nitrate was added under stirring and refluxing for 3 h resulting in the Tb complex (Fig. 2). 2.3.4. PMMA brushes on the hollow sphere surface A mixture of azo groups immobilized hollow sphere (compound 2) (0.20 g, azo group content: 0.11 mmol) and cyclohexanone (12 mL) in the flask was ultrasonically agitated for 30 min and then 2,20 -bipyridine (bipy) (0.11 g, 0.70 mmol), CuCl22H2O (0.04 g, 0.23 mmol) and degassed MMA (12 mL, 0.11 mol) were added. The flask was subjected to three freeze–pump–thaw cycles and heated at 70 8C for 10 h. After the reaction, the flask was cooled in an ice bath and the mixture was diluted with THF. The hollow sphere substrate with grafted PMMA (hollow sphere-g-PMMA) was separated by centrifugation. The crude product was extracted with THF and dried at 40 8C in vacuum to afford hollow sphere-g-PMMA. 2.3.5. Diblock copolymer brushes on the hollow sphere surface As shown in Fig. 1, the synthesis of diblock copolymer brushes was carried out on the hollow sphere-g-PMMA substrate. The
Fig. 2. The preparation scheme of Tb complex.
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Fig. 3. FT-IR spectra of (a) hollow sphere and (b) hollow sphere-g-PMMA.
Fig. 4. TG curves of (a) PMMA (residue weight = 2.1%), (b) hollow sphere-g-PMMA (residue weight = 66.9%) and (c) hollow sphere (residue weight = 60.5%).
procedures were similar to those used for synthesis of the hollow sphere-g-PMMA hybrid material, except using hollow sphere-gPMMA hybrid particles as macroinitiator and Tb complex as the second monomer via a conventional ATRP process. A general procedure is as follows: the mixture of hollow sphere-g-PMMA hybrid particles (0.10 g) and 12.5 mL ethanol in flask was ultrasonically agitated for 30 min and then the CuCl (0.01 g, 0.10 mmol), bipy (0.047 g, 0.30 mmol) and Tb complex (2.50 mmol) were added. The flask was subsequently evacuated and flushed with nitrogen. The polymerization was carried out under nitrogen in a 70 8C oil bath for 16 h, stopped by filtration and washing with excess THF to remove ungrafted Tb complex. Finally, the resulting white powder was dried at 40 8C in vacuum.
The not very clear peak at 2917 cm1 resulted from the C–H stretching vibrations of the small quantity of undissolved polystyrene. As for Fig. 3b, the absorption peaks at 3000–2800 cm1 came from C–H stretching vibration and the peak at 1733 cm1 represented the C5 5O stretching vibration of PMMA which indicated the successful grafting between hollow sphere and PMMA. TGA curves were used to trace the grafting polymerization. The typical TGA curves were depicted in Fig. 4. The weight loss stage below 200 8C was assigned to the evaporation of physically adsorbed water and residual solvent in samples. The weight loss of PMMA (Fig. 4a) exhibited a broad stage range from 200 to 420 8C. For silica hollow spheres (Fig. 4c), the weight loss at 300–420 8C and at 420– 600 8C was associated with the decomposition of undissolved polystyrene and the decompositions of silica-bonded groups such as –OH and/or unhydrolyzed –OR of hollow spheres, respectively [21]. So the grafted PMMA amount on hollow sphere surface can be determined approximately from the maximal difference in weight loss at 200–420 8C between the hollow sphere-g-PMMA hybrid material and silica hollow spheres, which is about 10%. In order to get more information about the microstructures of hollow spheres, SEM was performed. The hollow nature of the ‘‘bayberry’’ was observed in the contrast between the thin pale edge and dark center in SEM observations, as shown in Fig. 5. It is also evident that the surface of the hollow spheres before graft polymerization is relatively smoother than that of after graft polymerization and the average diameter of the hollow core is about 1 mm. TEM images (Fig. 6) clearly revealed typical hollow structure of the microspheres with hollow core diameter of about 1 mm. The
3. Results and discussion 3.1. Surface-initiated RATRP polymerization of MMA on the functionalized hollow sphere substrate The hollow sphere-g-PMMA hybrid materials were prepared using RATRP polymerization. The chemical state, graft ratio and topography of the hollow sphere-g-PMMA hybrid materials were probed by FT-IR, TGA, SEM and TEM, respectively. Fig. 3 shows the infrared spectrum of hollow sphere before (Fig. 3a) and after (Fig. 3b) graft polymerization. In Fig. 3a, the peak at 3440 cm1 was attributed to the O–H stretching vibrations of hydroxyl group in hollow sphere, the peak at 1091 cm1 was assigned to the Si–O–Si stretching vibration and the peak at 959 cm1 was a result of Si–OH vibration.
Fig. 5. SEM images of hollow sphere (a) and hollow sphere-g-PMMA (b).
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Fig. 6. TEM images of hollow sphere (a) and hollow sphere-g-PMMA (b).
hollow spheres before graft polymerization all kept good spherical shape and the walls consisted of numerous nanoparticles. After graft polymerization, the hollow microspheres were seriously anamorphic as shown in Fig. 6b, probably because the numerous nanoparticles on hollow spheres surface were influenced by shear force during grafting polymerization. In addition, the fact that a part of hydroxyl groups on hollow spheres surface replaced by PMMA would reduce the binding force among the silica nanoparticles, which is another possible reason for the anamorphic hollow spheres. The monomer conversion, the molecular weight and the polydispersity index (Mw/Mn) of the polymer formed in solution were examined by 1H NMR and GPC, respectively. Fig. 7 shows the 1 H NMR spectra of the reaction mixture for hollow sphere-gPMMA, which was performed directly with the polymerization system in deuterated chloroform. As shown in Fig. 7, monomer conversion of MMA was obtained using Convð%Þ ¼
block polymerization or further functionalization. The Tb complex was chosen for the block copolymerization as the Tb complex repeat unit contains MMA group, which can serve as a monomer in the ATRP polymerization. After graft copolymerization for 16 h on the hollow sphere-gPMMA substrate in an ethanol medium, the diblock polymer brushes grafted from hollow sphere surface (hollow sphere-g-
Aa 100% Aa þ Ab
where Aa and Ab were the areas of peaks a (the H of OCH3 in PMMA) and b (the H of OCH3 in MMA), respectively. The 1H NMR result shows that the MMA monomer conversion is 74%. At this conversion, the number average molecular weight (Mn) value reaches 27,600 gmol1 and the polydispersity index is as low as Mw/Mn = 1.15 according to the GPC result (Fig. 8). This is the characteristic of a controlled/‘‘living’’ polymerization. 3.2. Synthesis of the rare earth optical-functional hybrid material
Fig. 8. GPC curves of MMA ATRP polymerization formed in solution.
According to the mechanism of ATRP, the polymer prepared via the RATRP-mediated process has an end functionality of alkyl chlorine group. Thus, the graft chains prepared in this way on the hollow sphere substrate can serve as macroinitiator for subsequent
Fig. 7. 1H NMR spectra of the reaction mixture for hollow sphere-g-PMMA.
Fig. 9. The excitation spectra of hollow sphere-g-PMMA-Tb hybrid material.
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Fig. 10. The emission spectra of hollow sphere-g-PMMA-Tb hybrid material.
PMMA-b-Tb) came into being. The fluorescence properties of the sample were determined by spectrofluorimeter. Fig. 9 shows the excitation spectra of hollow sphere-g-PMMA-b-Tb complex. The excitation spectrum of the resulting hybrid materials was obtained by monitoring the emission of Tb3+ ions at 545 nm. The excitation spectrum is dominated by a broad band from 290 to 330 nm in narrow region and the maximum peak is at 330 nm, which can be attributed to the characteristic absorption of the lanthanide complexes arising from the efficient transition based on the conjugated double bonds of the aromatic cycle of salicylic acid ligand. These excitation spectra bands are the effective absorption for the luminescence of Tb3+. As a result, the strong green luminescence was observed (see Fig. 10), indicating that the effective energy transfer took place between the aromatic ligand and the chelated Tb3+ ions. In the emission spectrum there are four characteristic fluorescence emission bands associated with Tb3+. The band at 489 nm is assigned to the 5D4 ! 7F6 electron transition of Tb3+. The band at around 545 nm is associated with the 5 D4 ! 7F5 transition of Tb3+, and the bands at about 585 and 621 nm corresponding to the 5D4 ! 7F4 and 5D4 ! 7F3 electron transition of Tb3+, respectively. The fluorescence properties analysis results show that the Tb complex has grafted from the hollow sphere-g-PMMA substrate successfully. 4. Conclusion The preparation of optical-functional diblock copolymer brushes grafted from hollow sphere surface via surface reverse
atom transfer radical polymerization was wholly studied. After immobilization of the azo initiator, poly(methyl methacrylate) chains were successfully grafted from the surface of hollow sphere. Moreover, the poly(methyl methacrylate) grafted hollow sphere can be used as macroinitiator to initiate Tb complex polymerize, yielding poly(methyl methacrylate)-block-Tb grafted hollow sphere hybrid material. Gel permeation chromatographer analysis showed that the poly(methyl methacrylate) chains had a narrow molecular weight distribution. Transmission electron microscopy observations clearly revealed the hollow structure of the hollow spheres and TGA results indicated that the grafted polymer content is about 10% at 74% monomer conversion. Fluorescence spectra confirmed that the poly(methyl methacrylate)-block-Tb grafted hollow sphere hybrid materials exhibited strong fluorescence properties. This work presented a new method to synthesize optical-functional hybrid material with controlled molecular weights and ‘‘well-defined’’ structures, which may extend potential applications of hollow sphere and atom transfer radical polymerization.
Acknowledgements This research was supported by a project of Shandong Province Higher Education Science and Technology Program (J09LD56).
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