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PNIPAM hybrids by aqueous surface-initiated atom transfer radical polymerization
Materials Letters 61 (2007) 949 – 952 www.elsevier.com/locate/matlet
Synthesis of thermoresponsive silica nanoparticle/PNIPAM hybrids by aqueous surface-initiated atom transfer radical polymerization Kai Zhang a , Jia Ma b , Bao Zhang a , Shuang Zhao a , Yapeng Li a , Yaxin Xu a , Wenzhi Yu a , Jingyuan Wang a,⁎ a
College of Chemistry, Alan G. MacDiarmid Institute, Jilin University, Qianwei Road 10#, Changchun 130012, PR China b Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China Received 19 February 2006; accepted 10 June 2006 Available online 30 June 2006
Abstract This paper described the synthesis of a novel kind of thermoresponsive silica nanoparticle/PNIPAM hybrids by aqueous surface-initiated atom transfer radical polymerization at room temperature. These resulting hybrid particles were characterized by FT-IR, XPS, TEM, TGA and variable temperature DLS which indicated that they owned both core/shell structures and thermoresponsiveness. © 2006 Elsevier B.V. All rights reserved. Keywords: ATRP; Silica nanoparticles/PNIPAM hybrids; Core/shell structure; Ambient temperature; Thermoresponsiveness
1. Introduction During the past decade, nanoparticle/polymer (core/shell) hybrids have attracted strong interest [1,2] because of the combination of both the properties of the inorganic nanoparticles (optical, electronic, or mechanical) and those of the polymer (solubility, film formation, and chemical activity). Among the polymers employed in the formation of the core/shell hybrid structures, a stimuli-responsive polymer has gained much attention because the polymeric layer would endow the hybrid structure with an additional function/property on the top of the cores [3]. Thermoresponsive polymers are a class of the environmentally switchable materials and poly(N-isopropylacrylamide) (PNIPAM) has most intensively been studied [4]. The lower critical solution temperature (LCST) of PNIPAM is about 32 °C: the polymer is in the chain-extended, highly hydrated form below the LCST (hydrophilic state) and the polymer chains collapse above the LCST (hydrophobic state) in water [5]. Because of this unusual property, PNIPAM has been grafted from various substrates widely used in synthesizing thermoresponsive materials [6,7]. It was also reported that the LCST of PNIPAM grafted from the surface of solid substrates [8], cross-linked hydrogels [9], and block poly⁎ Corresponding author. E-mail address: [email protected] (J. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.021
mers [10] was nearly identical to the LCST of a homopolymer in aqueous solution. Covalent attachment of polymer chains to solid substrates is an effective method for tailoring surface properties [11]. As one of the most effective methods, surface-initiated atom transfer radical polymerization (ATRP) method has gained more and more attention recently [12] because it is a suitable method for the controllable preparation of polymer layers covalently bound to the surface of different surfaces such as SiO2 [13], Au [14], and Fe3O4 [15]. Although it is difficult to polymerize substituted acrylamides by ATRP in organic media, Brooks and coworkers have succeeded in polymerizing substituted acrylamides including N,N-dimethylacrylamide and NIPAM from polystyrene latex by aqueous surface-initiated ATRP [16,17]. Water can always act not only as solvent but also as accelerator in the ATRP process and enable the polymerization to be carried out under very moderate conditions [18]. In this letter, we report the preparation of a kind of thermoresponsive silica nanoparticle/PNIPAM hybrid particles by aqueous surface-initiated ATRP at room temperature. 2. Experimental N-isopropylacrylamide (NIPAM: Acros, 97%) was purified by crystallization from hexane and stored under argon at − 20 °C
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Fig. 1. Scheme of the synthesis of silica nanoparticle/PNIPAM hybrids. Conditions: (i) APTES, toluene; (ii) α-bromoisobutyryl bromide, TEA, toluene; (iii) NIPAM, CuCl, 2,2′-bipyridine, H2O, MeOH.
until used. CuCl purchased from Shanghai Chemical Co. (A.R., 97.0%) was purified according to the literature [19]. 2,2′bipyridine (bpy) (A.R., 97.0%) provided by Beijing Chemical Co. was recrystallized twice from acetone. 3-aminopropyl triethoxysilane (99.0%) (Aldrich) and α-bromoisobutyryl bromide (97.0%) (Fluka) were all used as received. Triethylamine (A.R., 99.0%) and toluene were all dried by CaH2 overnight, then distilled under reduced pressure before use. The water used throughout these experiments was either demineralised or purified using a Millipore Simplicity 185 system at 18.2 MΩ. Methanol (MeOH) (G.R.) purchased from Beijing Chemical Co. was used as supplied. Silica nanoparticles were prepared in ethanol according to the Stöber method [20,21]. Fourier transform infrared spectra (FT-IR) was carried out using a Nicolet FT/IR-410 instrument. The X-ray photoelectron spectroscopy (XPS) was recorded on a VG ESCA LAB MKII xprobe spectrometer with Mg Kα X-ray radiation to determine the surface composition of the substrates. Thermogravimetric analysis (TGA) was carried out using a Pyris Diamond TG-DTA (PerkinElmer) apparatus. The morphology of the initiator and polymer modified silica nanoparticles was observed by using
Fig. 2. FT-IR spectra of initiator (a) and PNIPAM (b) modified silica nanoparticles.
transmission electron microscope (TEM) (Hitachi H 8100). The hydrodynamic diameter of silica nanoparticle/PNIPAM hybrids was determined by variable temperature dynamic light scattering (DLS) (Wyatt Technology). 3. Results and discussion To prepare the polymer/silica hybrids from the surface of silica nanoparticles, a uniform and densely initiator-modified substrate is indispensable. Fig. 1 schematically outlines the procedure used for the preparation of initiator-modified silica nanoparticles and synthesizing the thermoresponsive hybrid particles. First, the α-bromoester initiator [22] on the silica nanoparticles was prepared by the self-assembly of aminopropyl triethoxysilane (APTES), followed by amidization with α-bromoisobutyryl bromide using triethylamine (TEA) as the catalyst. Then surface-initiated ATRP of NIPAM [23] in aqueous media at room temperature was carried out from the above initiator-modified particles as soon as the initiator-modified silica nanoparticles were obtained. Water was used as one of the solvents because it can act as a kind of
Fig. 3. C1s XPS spectra of PNIPAM modified silica nanoparticles.
K. Zhang et al. / Materials Letters 61 (2007) 949–952
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Fig. 4. TEM images of initiator-modified (a) and PNIPAM modified (b) silica nanoparticles.
accelerator and help the polymerization to be done at ambient temperature during the ATRP process [24]. Fig. 2 shows the changes in FT-IR spectra between the initiatormodified (a) and PNIPAM modified (b) silica nanoparticles. Compared with (a), two sharp absorption peaks appearing at 1645 cm− 1 (C_O stretching) and 1552 cm− 1 (N–H bending) are both the exact proofs that PNIPAM has grown from the initiator-modified particles successfully. The result from thermogravimetric analysis (TGA) shows that the proportion of PNIPAM in the hybrids is about 36%. The presence of the grafted PNIPAM on the surface of silica nanoparticles was also ascertained by XPS analysis. Fig. 3 is the PNIPAM's C1s core-level spectra and it can be curve-fitted with four peak components: hydrocarbon (CxHy: 285.0 eV), carbon adjacent to an amide group (C-C_O: 285.4 eV), carbon attached to nitrogen (C-N: 286.5 eV), and carbon attached to oxygen and nitrogen (N-C_O: 288.1 eV).Fig. 4(a) shows the TEM images of initiator-modified silica nanoparticles. These particles had a smooth, featureless surface morphology and nothing can be seen around them. The diameter was about 220 nm. Fig. 4(b) shows the PNIPAM modified silica nanoparticles. The diameter increased to about 260 nm and the clear core/shell structures were formed after the polymerization of NIPAM from the surface of the initiator-modified silica nanoparticles. The thickness of the PNIPAM shell was about 40 nm.
The thermoresponsiveness of silica nanoparticle/PNIPAM hybrids was studied by dynamic light scattering (DLS) at various temperatures increasing from 25 °C to 40 °C. Fig. 5 shows the relationship between the hydrodynamic diameter of the hybrids and temperature. The diameter did not change much with the temperature increasing from 25 °C to 32 °C. But from 32 °C to 33 °C, the diameter decreased from 366 nm to 284 nm, indicating that the temperature of phase transition was about 32 °C. We can also find that the diameter measured by DLS below 32 °C was bigger than that measured by TEM, while at 40 °C (262 nm) was almost the same as that measured by TEM (260 nm). This is because at the temperature of 40 °C, PNIPAM chains exhibited hydrophobic characteristic, they can hardly dissolve in the water and shrank around the silica nanoparticles leading to the same result measured by TEM.
4. Conclusion In conclusion, a novel kind of thermoresponsive silica nanoparticle/PNIPAM hybrid particles was synthesized by aqueous surface-initiated atom transfer radical polymerization at room temperature. They not only owned clear core/shell structures, but also showed good thermoresponsiveness. Furthermore, the LCST of PNIPAM is 32 °C and it approaches to the body temperature of human beings, so these thermoresponsive silica nanoparticle/PNIPAM (core/shell) hybrid particles must be widely used in the future. References
Fig. 5. The results of variable temperature DLS: diameter of the silica nanoparticle/PNIPAM hybrid particles vs. temperature.
[1] A.P. Alivisatos, Science 271 (1996) 933. [2] S. Förster, M. Antonietti, Adv. Mater. 10 (1998) 195. [3] V.P. Yissar, R. Gabai, A.N. Shipway, T. Bourenko, I. Willner, Adv. Mater. 13 (2001) 1320. [4] H.G. Schild, M. Muthukumar, D.A. Tirrell, Macromolecules 24 (1991) 948. [5] Y.H. Bae, T. Okano, S.W. Kim, J. Polym. Sci., Part A: Polym. Phys. 28 (1990) 923. [6] T.O. Collier, J.M. Anderson, A. Kikuchi, T. Okano, J. Biomed. Mater. Res. 59 (2002) 136. [7] P.S. Stayton, T. Shimoboji, C. Long, A. Chilkoti, G.H. Chen, J.M. Harris, A.S. Hoffman, Nature (London) 378 (1995) 472. [8] S. Balamurugan, S. Mendez, S.S. Balamurugan, M.J. O'Brien, G.P. Lopez, Langmuir 19 (2003) 2545. [9] N.C. Woodward, B.Z. Chowdhry, M.J. Snowden, S.A. Leharne, P.C. Griffiths, A.L. Winnington, Langmuir 19 (2003) 3202.
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K. Zhang et al. / Materials Letters 61 (2007) 949–952
[10] T.J.V. Prazeres, A.M. Santos, J.M.G. Martinho, A. Elaissari, C. Pichot, Langmuir 20 (2004) 6834. [11] S. Edmondson, V.L. Osborne, W.T.S. Huck, Chem. Soc. Rev. 33 (2004) 14. [12] J. Pyun, T. Kowalewski, K. Matyjaszewski, Macromol. Rapid Commun. 24 (2003) 1043. [13] T.Y. Cui, J.H. Zhang, J.Y. Wang, F. Cui, W. Chen, F.B. Xu, Z. Wang, K. Zhang, B. Yang, Adv. Funct. Mater. 15 (2005) 481. [14] K. Ohno, K.M. Koh, Y. Tsujii, T. Fukuda, Macromolecules 35 (2002) 8989. [15] L.J. An, Z.Q. Li, Z. Wang, J.H. Zhang, B. Yang, Chem. Lett. 34 (2005) 652. [16] J.N. Kizhakkedathu, D.E. Brooks, Macromolecules 36 (2003) 591.
[17] J.N. Kizhakkedathu, R.N. Jones, D.E. Brooks, Macromolecules 37 (2004) 734. [18] X.S. Wang, S.P. Armes, Macromolecules 33 (2000) 6640. [19] R.N. Keller, H.D. Wycoff, Inorg. Synth. 2 (1946) 1. [20] W. Stöber, A. Fink, J. Colloid Interface Sci. 26 (1968) 62. [21] A.P. Philipse, A.J. Vrij, J. Colloid Interface Sci. 128 (1989) 121. [22] K. Zhang, H.T. Li, H.W. Zhang, S. Zhao, D. Wang, J.Y. Wang, Mater. Chem. Phys. 96 (2006) 477. [23] T.L. Sun, G.J. Wang, L. Feng, B.Q. Liu, Y.M. Ma, L. Jiang, D.B. Zhu, Angew. Chem. Int. Ed. 43 (2004) 357. [24] W. Huang, J.B. Kim, M.L. Bruening, G.L. Baker, Macromolecules 35 (2002) 1175.
Synthesis of thermoresponsive silica nanoparticle/PNIPAM hybrids by aqueous surface-initiated atom transfer radical polymerization Kai Zhang a , Jia Ma b , Bao Zhang a , Shuang Zhao a , Yapeng Li a , Yaxin Xu a , Wenzhi Yu a , Jingyuan Wang a,⁎ a
College of Chemistry, Alan G. MacDiarmid Institute, Jilin University, Qianwei Road 10#, Changchun 130012, PR China b Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, PR China Received 19 February 2006; accepted 10 June 2006 Available online 30 June 2006
Abstract This paper described the synthesis of a novel kind of thermoresponsive silica nanoparticle/PNIPAM hybrids by aqueous surface-initiated atom transfer radical polymerization at room temperature. These resulting hybrid particles were characterized by FT-IR, XPS, TEM, TGA and variable temperature DLS which indicated that they owned both core/shell structures and thermoresponsiveness. © 2006 Elsevier B.V. All rights reserved. Keywords: ATRP; Silica nanoparticles/PNIPAM hybrids; Core/shell structure; Ambient temperature; Thermoresponsiveness
1. Introduction During the past decade, nanoparticle/polymer (core/shell) hybrids have attracted strong interest [1,2] because of the combination of both the properties of the inorganic nanoparticles (optical, electronic, or mechanical) and those of the polymer (solubility, film formation, and chemical activity). Among the polymers employed in the formation of the core/shell hybrid structures, a stimuli-responsive polymer has gained much attention because the polymeric layer would endow the hybrid structure with an additional function/property on the top of the cores [3]. Thermoresponsive polymers are a class of the environmentally switchable materials and poly(N-isopropylacrylamide) (PNIPAM) has most intensively been studied [4]. The lower critical solution temperature (LCST) of PNIPAM is about 32 °C: the polymer is in the chain-extended, highly hydrated form below the LCST (hydrophilic state) and the polymer chains collapse above the LCST (hydrophobic state) in water [5]. Because of this unusual property, PNIPAM has been grafted from various substrates widely used in synthesizing thermoresponsive materials [6,7]. It was also reported that the LCST of PNIPAM grafted from the surface of solid substrates [8], cross-linked hydrogels [9], and block poly⁎ Corresponding author. E-mail address: [email protected] (J. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.021
mers [10] was nearly identical to the LCST of a homopolymer in aqueous solution. Covalent attachment of polymer chains to solid substrates is an effective method for tailoring surface properties [11]. As one of the most effective methods, surface-initiated atom transfer radical polymerization (ATRP) method has gained more and more attention recently [12] because it is a suitable method for the controllable preparation of polymer layers covalently bound to the surface of different surfaces such as SiO2 [13], Au [14], and Fe3O4 [15]. Although it is difficult to polymerize substituted acrylamides by ATRP in organic media, Brooks and coworkers have succeeded in polymerizing substituted acrylamides including N,N-dimethylacrylamide and NIPAM from polystyrene latex by aqueous surface-initiated ATRP [16,17]. Water can always act not only as solvent but also as accelerator in the ATRP process and enable the polymerization to be carried out under very moderate conditions [18]. In this letter, we report the preparation of a kind of thermoresponsive silica nanoparticle/PNIPAM hybrid particles by aqueous surface-initiated ATRP at room temperature. 2. Experimental N-isopropylacrylamide (NIPAM: Acros, 97%) was purified by crystallization from hexane and stored under argon at − 20 °C
950
K. Zhang et al. / Materials Letters 61 (2007) 949–952
Fig. 1. Scheme of the synthesis of silica nanoparticle/PNIPAM hybrids. Conditions: (i) APTES, toluene; (ii) α-bromoisobutyryl bromide, TEA, toluene; (iii) NIPAM, CuCl, 2,2′-bipyridine, H2O, MeOH.
until used. CuCl purchased from Shanghai Chemical Co. (A.R., 97.0%) was purified according to the literature [19]. 2,2′bipyridine (bpy) (A.R., 97.0%) provided by Beijing Chemical Co. was recrystallized twice from acetone. 3-aminopropyl triethoxysilane (99.0%) (Aldrich) and α-bromoisobutyryl bromide (97.0%) (Fluka) were all used as received. Triethylamine (A.R., 99.0%) and toluene were all dried by CaH2 overnight, then distilled under reduced pressure before use. The water used throughout these experiments was either demineralised or purified using a Millipore Simplicity 185 system at 18.2 MΩ. Methanol (MeOH) (G.R.) purchased from Beijing Chemical Co. was used as supplied. Silica nanoparticles were prepared in ethanol according to the Stöber method [20,21]. Fourier transform infrared spectra (FT-IR) was carried out using a Nicolet FT/IR-410 instrument. The X-ray photoelectron spectroscopy (XPS) was recorded on a VG ESCA LAB MKII xprobe spectrometer with Mg Kα X-ray radiation to determine the surface composition of the substrates. Thermogravimetric analysis (TGA) was carried out using a Pyris Diamond TG-DTA (PerkinElmer) apparatus. The morphology of the initiator and polymer modified silica nanoparticles was observed by using
Fig. 2. FT-IR spectra of initiator (a) and PNIPAM (b) modified silica nanoparticles.
transmission electron microscope (TEM) (Hitachi H 8100). The hydrodynamic diameter of silica nanoparticle/PNIPAM hybrids was determined by variable temperature dynamic light scattering (DLS) (Wyatt Technology). 3. Results and discussion To prepare the polymer/silica hybrids from the surface of silica nanoparticles, a uniform and densely initiator-modified substrate is indispensable. Fig. 1 schematically outlines the procedure used for the preparation of initiator-modified silica nanoparticles and synthesizing the thermoresponsive hybrid particles. First, the α-bromoester initiator [22] on the silica nanoparticles was prepared by the self-assembly of aminopropyl triethoxysilane (APTES), followed by amidization with α-bromoisobutyryl bromide using triethylamine (TEA) as the catalyst. Then surface-initiated ATRP of NIPAM [23] in aqueous media at room temperature was carried out from the above initiator-modified particles as soon as the initiator-modified silica nanoparticles were obtained. Water was used as one of the solvents because it can act as a kind of
Fig. 3. C1s XPS spectra of PNIPAM modified silica nanoparticles.
K. Zhang et al. / Materials Letters 61 (2007) 949–952
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Fig. 4. TEM images of initiator-modified (a) and PNIPAM modified (b) silica nanoparticles.
accelerator and help the polymerization to be done at ambient temperature during the ATRP process [24]. Fig. 2 shows the changes in FT-IR spectra between the initiatormodified (a) and PNIPAM modified (b) silica nanoparticles. Compared with (a), two sharp absorption peaks appearing at 1645 cm− 1 (C_O stretching) and 1552 cm− 1 (N–H bending) are both the exact proofs that PNIPAM has grown from the initiator-modified particles successfully. The result from thermogravimetric analysis (TGA) shows that the proportion of PNIPAM in the hybrids is about 36%. The presence of the grafted PNIPAM on the surface of silica nanoparticles was also ascertained by XPS analysis. Fig. 3 is the PNIPAM's C1s core-level spectra and it can be curve-fitted with four peak components: hydrocarbon (CxHy: 285.0 eV), carbon adjacent to an amide group (C-C_O: 285.4 eV), carbon attached to nitrogen (C-N: 286.5 eV), and carbon attached to oxygen and nitrogen (N-C_O: 288.1 eV).Fig. 4(a) shows the TEM images of initiator-modified silica nanoparticles. These particles had a smooth, featureless surface morphology and nothing can be seen around them. The diameter was about 220 nm. Fig. 4(b) shows the PNIPAM modified silica nanoparticles. The diameter increased to about 260 nm and the clear core/shell structures were formed after the polymerization of NIPAM from the surface of the initiator-modified silica nanoparticles. The thickness of the PNIPAM shell was about 40 nm.
The thermoresponsiveness of silica nanoparticle/PNIPAM hybrids was studied by dynamic light scattering (DLS) at various temperatures increasing from 25 °C to 40 °C. Fig. 5 shows the relationship between the hydrodynamic diameter of the hybrids and temperature. The diameter did not change much with the temperature increasing from 25 °C to 32 °C. But from 32 °C to 33 °C, the diameter decreased from 366 nm to 284 nm, indicating that the temperature of phase transition was about 32 °C. We can also find that the diameter measured by DLS below 32 °C was bigger than that measured by TEM, while at 40 °C (262 nm) was almost the same as that measured by TEM (260 nm). This is because at the temperature of 40 °C, PNIPAM chains exhibited hydrophobic characteristic, they can hardly dissolve in the water and shrank around the silica nanoparticles leading to the same result measured by TEM.
4. Conclusion In conclusion, a novel kind of thermoresponsive silica nanoparticle/PNIPAM hybrid particles was synthesized by aqueous surface-initiated atom transfer radical polymerization at room temperature. They not only owned clear core/shell structures, but also showed good thermoresponsiveness. Furthermore, the LCST of PNIPAM is 32 °C and it approaches to the body temperature of human beings, so these thermoresponsive silica nanoparticle/PNIPAM (core/shell) hybrid particles must be widely used in the future. References
Fig. 5. The results of variable temperature DLS: diameter of the silica nanoparticle/PNIPAM hybrid particles vs. temperature.
[1] A.P. Alivisatos, Science 271 (1996) 933. [2] S. Förster, M. Antonietti, Adv. Mater. 10 (1998) 195. [3] V.P. Yissar, R. Gabai, A.N. Shipway, T. Bourenko, I. Willner, Adv. Mater. 13 (2001) 1320. [4] H.G. Schild, M. Muthukumar, D.A. Tirrell, Macromolecules 24 (1991) 948. [5] Y.H. Bae, T. Okano, S.W. Kim, J. Polym. Sci., Part A: Polym. Phys. 28 (1990) 923. [6] T.O. Collier, J.M. Anderson, A. Kikuchi, T. Okano, J. Biomed. Mater. Res. 59 (2002) 136. [7] P.S. Stayton, T. Shimoboji, C. Long, A. Chilkoti, G.H. Chen, J.M. Harris, A.S. Hoffman, Nature (London) 378 (1995) 472. [8] S. Balamurugan, S. Mendez, S.S. Balamurugan, M.J. O'Brien, G.P. Lopez, Langmuir 19 (2003) 2545. [9] N.C. Woodward, B.Z. Chowdhry, M.J. Snowden, S.A. Leharne, P.C. Griffiths, A.L. Winnington, Langmuir 19 (2003) 3202.
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K. Zhang et al. / Materials Letters 61 (2007) 949–952
[10] T.J.V. Prazeres, A.M. Santos, J.M.G. Martinho, A. Elaissari, C. Pichot, Langmuir 20 (2004) 6834. [11] S. Edmondson, V.L. Osborne, W.T.S. Huck, Chem. Soc. Rev. 33 (2004) 14. [12] J. Pyun, T. Kowalewski, K. Matyjaszewski, Macromol. Rapid Commun. 24 (2003) 1043. [13] T.Y. Cui, J.H. Zhang, J.Y. Wang, F. Cui, W. Chen, F.B. Xu, Z. Wang, K. Zhang, B. Yang, Adv. Funct. Mater. 15 (2005) 481. [14] K. Ohno, K.M. Koh, Y. Tsujii, T. Fukuda, Macromolecules 35 (2002) 8989. [15] L.J. An, Z.Q. Li, Z. Wang, J.H. Zhang, B. Yang, Chem. Lett. 34 (2005) 652. [16] J.N. Kizhakkedathu, D.E. Brooks, Macromolecules 36 (2003) 591.
[17] J.N. Kizhakkedathu, R.N. Jones, D.E. Brooks, Macromolecules 37 (2004) 734. [18] X.S. Wang, S.P. Armes, Macromolecules 33 (2000) 6640. [19] R.N. Keller, H.D. Wycoff, Inorg. Synth. 2 (1946) 1. [20] W. Stöber, A. Fink, J. Colloid Interface Sci. 26 (1968) 62. [21] A.P. Philipse, A.J. Vrij, J. Colloid Interface Sci. 128 (1989) 121. [22] K. Zhang, H.T. Li, H.W. Zhang, S. Zhao, D. Wang, J.Y. Wang, Mater. Chem. Phys. 96 (2006) 477. [23] T.L. Sun, G.J. Wang, L. Feng, B.Q. Liu, Y.M. Ma, L. Jiang, D.B. Zhu, Angew. Chem. Int. Ed. 43 (2004) 357. [24] W. Huang, J.B. Kim, M.L. Bruening, G.L. Baker, Macromolecules 35 (2002) 1175.