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Silica Nanorings on the Surfaces of Layered Silicate Qingjiao Duan,† Jian Zhang,† Jia Tian, and Hanying Zhao* Key Laboratory of Functional Polymer Materials, Ministry of Education, Department of Chemistry, Nankai University, Tianjin 300071, China ABSTRACT: A simple approach to the synthesis of claysilica nanocomposites is presented. Silica nanorings on the edges of clay sheets were synthesized by using a modified St€ober method. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and fluorescence spectroscopy were employed to characterize the prepared nanocomposites. TEM results show that the average size of the nanorings increases with the growth of silica. XRD results indicate that the layered structures of clay can be found in the nanocomposite and the growth of silica nanorings expands the d spacing of clay platelets. The mechanism of the formation of the nanorings is discussed. The preparation of polystyrene (PS) brushes on the surfaces of silica nanorings by atom-transfer radical polymerization is also reported. The polymer nanocomposite with negatively charged clay surfaces and hydrophobic polymer brushes on the silica nanorings can be used in Pickering emulsions, and PS colloidal particles with claysilica on the surfaces were prepared.
’ INTRODUCTION In the past decade, layered silicates have been widely used in the preparation of hybrid nanocomposites.1 With a relatively low loading of modified clay, a number of physical properties of polymer materials improve significantly.2 The salient feature that leads to such improvements is the dispersion of individual silicate layers in a polymer matrix with nanoscopic dimensions. Another type of interesting inorganic materials is silica particles. The nanometer-sized silica particles not only are scientifically important but also find practical applications in catalysis, drug delivery, and biosensors.3 Driven by potential applications as well as being promising building block candidates in nanoelectronics and nanooptical devices, research on silica nanowires has developed rapidly.4 Various techniques have been used to synthesize silica nanowires. For example, Paulose and co-workers produced thin amorphous silica nanowires with lengths in the 1001000 nm range by using gold as a catalyst on a silicon wafer.5 In their study, the solidliquidsolid (SLS) mechanism plays a key role in the growth of silica nanowires. In the SLS mechanism, the gold-rich catalyst particles remain on the surface of the silicon substrate during the growth of the nanowires. The use of a liquid drop to catalyze the growth of silica nanowires was also demonstrated,6 and this method was known as the vaporliquidsolid (VLS) growth mechanism. On the basis of the VLS mechanism, highly aligned and closely packed silica nanowire bunches were synthesized in high yield by using molten gallium as a catalyst.7 Efforts to prepare nanocomposites of the nanometer-sized silica and layered silicates have been reported.8 Microporous solpillared clay was prepared by the ion exchange of the interlayer sodium ions of montmorillonite with SiO2Fe2O3 mixed oxide sol particles. Upon pillaring with the particles, the basal spacing of the clay was expanded.9 Mesostructured intercalates, also known as porous clay heterostructures, were prepared through the surfactant-directed assembly of silica within the galleries of synthetic saponite clays. After calcination, porous clay r 2011 American Chemical Society
intercalates with expanded basal spacings were obtained.10 A different procedure was reported by Letaief and Ruiz-Hitzky.11 Alkylammonium-exchanged clays were prepared by ion exchange between montmorillonites and cetyl trimethylammonium bromide (CTAB). The slow addition of a mixture of water and alkoxysilane to the dispersion of modified clay gave rise to the spontaneous gelation of the system. After air drying and calcination, claysilica nanocomposites with elemental silicate layers dispersed in a silica matrix were produced. In another report, Qian and co-workers prepared a mesoporous silica network and silica nanoparticles covered or attached to the clay surfaces by a solgel approach.12 The St€ober synthesis is a widely used method in the preparation of silica particles.13 This one-step protocol involves the condensation of tetraethyl orthosilicate (TEOS) in ethanol/ water mixtures under alkaline conditions at room temperature. Good control of the size and shape of silica particles can be achieved by using this method. In this article, we report a simple approach to the synthesis of claysilica nanocomposites. In the presence of clay, silica nanorings on the edges of clay sheets were synthesized under moderate reaction conditions by using a modified St€ober method. The silica nanorings on the edges of clay sheets can be functionalized by chemical reactions or polymerizations, and in the meantime, the negatively charged clay surfaces remain, so this research provides a simple method for the preparation of novel claysilica hybrid nanomaterials.
’ EXPERIMENTAL SECTION Materials. Na+-montmorillonite (MMT) with an ion-exchange capacity of 80 mequiv/100 g was kindly provided by S€ud-chemie Received: April 4, 2011 Revised: September 8, 2011 Published: September 19, 2011 13212
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Figure 1. (a) TEM image of silica nanoparticles prepared after 30 min of reaction of tetraethyl orthosilicate (TEOS) in an ethanol/water mixture. (b) TEM image of silica nanorings on clay layers prepared after 0.5 h of TEOS reaction in the presence of clay. (ce) TEM images of silica nanorings on clay layers prepared after 1, 3, and 24 h of TEOS reaction in the presence of clay. Company under the commercial name of EX M 757. Tetraethyl orthosilicate (TEOS) purchased from Tian Jin Institute of Chemical Agents was distilled before use. Styrene (99%) purchased from Tian Jin Institute of Chemical Agents was purified by washing with an aqueous solution of NaOH, drying over MgSO4, and distilling under reduced pressure. N,N,N00 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA, 99%), 2-bromo-2-methyl propionyl bromide (98%), and trimethylamine
(99%) were purchased from Aldrich and were used as received. Methylene blue purchased from Alfa Aesar was used as received. All of the solvents used in this research were distilled before use. Preparation of Silica Nanorings on Clay Layers. The silica nanorings on the surfaces of clay layers were prepared on the basis of the modified St€ober method. To a 50 mL flask were added 20 mL of ethanol and 0.40 mL of TEOS, and then 0.30 mL of ammonia solution (25%) 13213
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Figure 2. Cartoon showing the formation of silica nanorings on the edges of clay layers. was injected into the solution. After being stirred at 30 °C for 30 min, a suspension of clay (20 mg) in 2.5 mL of doubly distilled water was added to the solution, and the mixture was stirred at 30 °C for 24 h. The clay silica nanocomposites were collected by centrifugation (10 000 rpm, 5 min), washed with ethanol and water, and dried at 110 °C under vacuum. A controlled experiment of the solgel reaction of TEOS without clay was also conducted. In the controlled experiment, the concentrations of reagents and the experimental process were the same as those in the preparation of silica nanorings.
Synthesis of Atom-Transfer Radical Polymerization (ATRP) Initiator-Anchored Silica Nanorings. In a round flask, 0.30 g of a claysilica nanocomposite was dispersed in dry toluene after 30 min of sonification, and 0.60 g of 3-(triethoxysilyl)-propylamin was added to the above solution dropwise. The solution was stirred at 95 °C for 18 h. After the reaction, the nanocomposite was filtered and washed with ethanol. To prepare ATRP-initiator anchored silica nanorings, 0.10 mL of 2-bromo-2-methyl propionyl bromide was added to 0.20 g of an amino-modified claysilica nanocomposite dispersed in 5.0 mL of dry THF with 0.20 mL of dry triethylamine. The solution was stirred at 0 °C for 2 h and at room temperature for 36 h. After the reaction, ATRP-initiator-anchored silica nanorings were filtered and washed with an ethanol/water (1:1 by volume) solution.
Preparation of Polystyrene (PS) Brushes on the Surfaces of Silica Nanorings. In a Schlenk flask, ATRP-initiator-anchored silica rings were dispersed in a mixture of dry toluene (0.25 mL) and styrene (0.25 mL). The flask was purged under vacuum and then flushed with nitrogen gas. Under a nitrogen atmosphere, 1.3 mg of CuBr and 2.0 μL of PMDETA were dissolved in 0.10 mL of DMF, and the solution was added to the dispersion with silica nanorings. The mixture was stirred at 110 °C for 24 h. After the polymerization, the claysilica nanocomposite with PS brushes was dispersed in toluene and centrifugated to remove Cu2+, ligand, excess monomer, and possibly untethered polymer. After being redispersed in toluene, the nanocomposite was precipitated in methanol. A white powder was obtained after filtration and drying under vacuum. Silica nanorings with PS brushes on the surfaces were etched with HF acid, and PS was obtained by pouring the polymer solution into 6-fold-excess methanol. In this article, the claysilica nanocomposite with PS brushes on the silica nanorings was assigned as claysilicaPS.
Preparation of PS Colloidal Particles Stabilized by Clay SilicaPS. To prepare PS colloidal particles, 15 mg of PS with an average molecular weight of 14K was dissolved in 1 mL of toluene, and the polymer solution was mixed with 10 mL of water in the presence of 10 mg of the claysilicaPS nanocomposite. After the addition of the Pickering emulsion into 5-fold-excess methanol, PS colloidal particles stabilized by claysilicaPS were obtained. Characterization. The 1H NMR spectrum was collected on a Varian Mercury Vx300 spectrometer. Transmission electron microscope (TEM) observations were carried out on a Tecnai G2 20 S-TWIN electron microscope equipped with a model 794 CCD camera (512 512).
X-ray diffraction (XRD) studies were carried out on a D/max-2500 diffractometer with Cu Kα radiation (λ = 1.5406 Å). The thermal properties of the nanocomposites were measured by thermogravimetric analysis (TGA). The samples were heated to 800 °C at a heating rate of 10 K/min under a nitrogen atmosphere on a Netzsch TG 209. Steadystate fluorescence spectra were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer. Ultravioletvisible (UVvis) absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer using a quartz cell of 1 cm path length. The scanning speed was set at 200 nm/min. The excitation and emission slits were both set at 5 nm. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra delay line detector (DLD) spectrometer employing a monochromated Al Kα X-ray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and DLD. All XPS spectra were recorded using an aperture slot of 300700 μm, survey spectra were recorded with pass energy of 160 eV, and highresolution spectra were recorded with pass energy of 40 eV. The apparent molecular weights (Mn) of the polymers were determined at 35 °C with a gel permeation chromatograph (GPC) equipped with a Waters 717 autosampler, a Waters 1525 HPLC pump, three Waters UltraStyragel columns with 5K600K (10 000 Å), 50030K (1000 Å), and 10010K (500 Å) molecular ranges, and a waters 2414 refractive index detector. THF was used as the eluent at a flow rate of 1.0 mL/min. Molecular weights were calibrated on PS standards.
’ RESULTS AND DISCUSSION Claysilica nanocomposites were prepared via a simple solgel method by using tetraethoxysilane (TEOS) as the Si source in the presence of clay layers. In brief, an ammonia solution (25%) was added to a mixture of ethanol and TEOS. After being stirred at 30 °C for 30 min, an aqueous suspension of clay was added to the solution, and the mixture was stirred at 30 °C for 24 h. Figure 1 shows TEM images of the nanocomposites prepared at different times. Before the addition of the clay dispersion, only silica particles with average size of 6 nm were observed (Figure 1a). After 0.5 h of reaction in the presence of clay, silica nanorings on clay layers were formed (Figure 1b). In the TEM image, the slight contrast between the clay surfaces surrounded by the silica nanorings and the background indicated that the nanorings were produced on the edges of clay layers. Figure 1c is a TEM image of silica nanorings on clay layers after 1 h of reaction in the presence of clay. As indicated by an arrow in the image, part of the clay edge is not covered with silica nanorings because of the short reaction time but the rest of the edge was covered with silica nanorings. The average width of the nanorings in Figure 1b is about 13 nm. The appearance of the broken lines with gray dots on the image suggested that the nanorings were composed of silica nanoparticles. A typical broken line was indicated by an arrow in Figure 1b. The average width of the 13214
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Figure 3. TEM image of silica nanoparticles prepared by the solgel method in the absence of clay.
nanorings represented the average diameter of silica nanoparticles on the nanorings. The average width of the nanorings increased with the growth of silica. The average width reached 18 and 22 nm after 3 and 24 h of reaction (Figure 1d,e), respectively. It was also noted that a small number of isolated silica nanoparticles were found in the nanocomposite (Figure 1d). Figure 2 is a cartoon showing a mechanism for the formation of silica nanorings on clay layers. Initially, silica nanoparticles were formed in solution via a condensation reaction. There are silanol groups on the edges of clay sheets.14 Upon addition of an aqueous dispersion of clay, silica nanoparticles are grafted to the clay layers by a condensation reaction with silanol groups on the edges of the clay layers. With the growth of silica, silica nanoparticles on the clay edges meet each other and nanorings are formed. The average size of the nanorings increases with the growth of silica. To prove the role of clay in the preparation of silica nanorings, a controlled experiment was conducted. In the experiment, the reaction conditions were kept the same only no clay was added. Narrow silica particles with an average diameter of 160 nm were prepared (Figure 3), and no silica nanorings were observed. The spherical silica nanoparticles were developed by means of the hydrolysis of alkyl silicates and the subsequent condensation reaction. Hence, clay layers played a key role in the fabrication of the silica nanorings. XRD patterns of original clay and the claysilica nanocomposite are shown in Figure 4a. The claysilica nanocomposite has a broad diffraction peak at around 2θ = 22.5° corresponding to the amorphous phase of silica, and no diffraction peak corresponding to a crystalline phase of silica can be observed, indicating that silica nanorings are amorphous. The d001 spacing of clay is 0.98 nm, and it increases to 1.27 nm after the formation of the silica nanorings.These results suggest that the layered structure of the clay is preserved in the preparation of the nanocomposite and the growth of the silica nanorings expands the d spacing of the platelets. Compared to the original clay, the d001 diffraction peak of the claysilica nanocomposite is broad and weak. It is noteworthy that some dark lines were observed on a TEM image of the nanocomposite (Figure 4b). A typical dark line was indicated by an arrow in the image. When the clay layers
Figure 4. (a) XRD patterns of clay and the claysilica nanocomposite. (b) TEM image of the claysilica nanocomposite. (c) Cartoon showing claysilica observed at different angles.
Figure 5. Si 2p XPS spectra of clay, silica nanoparticles, and the clay silica nanocomposite.
are perpendicular to the copper grid, dark lines are observed. A cartoon showing clay layers observed at different angles is shown in Figure 4c. The width of a dark line represents the thickness of a clay layer. The average width of the dark lines measured in the TEM image is about 3 nm, which corresponds to a two-layered structure. There is more evidence for the formation of silica nanorings from the XPS result. XPS is a powerful tool in the study of surface 13215
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Figure 6. Room-temperature photoluminescence spectra of clay, silica nanoparticles, and the claysilica nanocomposite.
properties of materials. An XPS spectrum provides information on the type and number of different species of a given atom in the molecules. Figure 5 shows the X-ray surveys of Si 2p of clay, silica nanoparticles, and the claysilica nanocomposite. The binding energies of Si 2p of clay, silica nanoparticles with an average diameter of 160 nm, and the nanocomposite are 100.9, 102.3 and 101.7 eV, respectively. The binding energy of Si 2p of the nanocomposite is higher than that of clay but lower than that of silica nanoparticles, which is consistent with the formation of silica nanorings on clay layers. Figure 6 shows the photoluminescence (PL) spectra of clay, silica nanoparticles, and the claysilica nanocomposite recorded at room temperature with 260 nm excitation. Clay has a broad, weak structureless emission in the range of 350390 nm, and silica nanoparticles with an average size of 160 nm also present weak structureless emission. However, the claysilica nanocomposite presents strong luminescence emission with peak positions at 377 nm (3.34 eV), 396 nm (3.18 eV), and 418 nm (3.01 eV). The PL properties of silica were reported to be strongly dependent on their structures.15 For example, Srivastava and coworkers reported that silica nanowires prepared by the thermal evaporation of silicon monoxide presented strong blue luminescence at 393 nm.16 Wu and co-workers synthesized amorphous silica nanowires by a carbothermal reduction reaction between silicon dioxide and active carbons and observed blue luminescence at 435 nm.17 Nishikawa and co-workers observed six luminescence bands in the range between 1.9 and 4.3 eV and suggested that the luminescence is related to preexisting defects or impurities introduced during sample preparation.18 Therefore, in this study the luminescence emission from the claysilica nanocomposites could be attributed to the structural defects produced during the growth of silica particles and the formation of nanorings. Clay particles are constructed of platelets with a thickness of ∼1 nm and a width of 100200 nm. The platelets with permanent negative charges on the surfaces are held together by charge-balancing cations such as Na+ and Ca2+. The clay platelets are able to undergo surface ion exchange with organic cationic molecules. To investigate the influence of the silica nanorings on the ion exchange capacity of clay platelets, methylene blue absorption by clay and claysilica was studied in this research. Methylene blue is a positively charged dye and can be
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Figure 7. Plot representing the absorption of methylene blue by clay and claysilica against dialysis time.
ion-exchanged onto clay layers from aqueous solutions.19 Clay or the claysilica nanocomposite was dispersed in water, and the dispersions were dialyzed against aqueous solutions of methylene blue. Because of the absorption by clay layers, the concentrations of methylene blue in aqueous solutions decreased. Figure 7 shows the amounts of methylene blue absorbed by clay or claysilica against the dialysis time, where the absorption is expressed in mequiv of methylene blue per 100 g of clay. For both clay and claysilica, the amounts of methylene blue absorbed by clay layers increased with time, which indicated that clay layers were still able to undergo ion exchange with positively charged molecules after the fabrication of silica nanorings on the edges. However, it is also noted that after 75 h of dialysis the absorption of methylene blue by clay and claysilica reached 46.5 and 41 mequiv/100 g, respectively. The lower absorption value of claysilica indicates that some negative charges on clay layers are protected by silica nanorings. The measurement of zeta potentials also supports this result. Because of negative charges on the surfaces of clay layers, the zeta potential of the original clay is about 48.6 mV at pH 6.75. However, after the preparation of silica nanorings on the clay layers, the zeta potential is about 30.5 mV. The decrease in the zeta potential is attributed to the loss of some negative charges on clay layers after the formation of the silica nanorings. The synthesis of silicapolymer nanocomposites has been studied widely.20 One of the possible applications of claysilica nanocomposites is the preparation of polymerinorganic nanocomposites. In this research, the preparation of polymer nanocomposites was investigated. To grow polymer chains on silica nanorings, the surfaces of the silica nanorings were modified by an ATRP initiator and PS brushes were prepared by in situ ATRP. The primary amino groups were introduced onto the surfaces of silica nanorings by the reaction of silanol groups with 3-(triethoxysilyl)-propylamine. ATRP initiator molecules were grafted to the nanorings via the reaction of aminated silica nanorings and 2-bromoisobutyryl bromide. PS brushes on the surfaces of silica nanorings were prepared by ATRP. The nanocomposites with primary amino groups and ATRP initiator on the surfaces of silica nanorings were assigned as claysilica NH2 and claysilicaATRP. TEM was used to confirm the preparation of PS brushes on the surfaces of silica nanorings. To increase the contrast, the specimen for TEM observation was 13216
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Langmuir stained with RuO4 vapor for 2 h. Figure 8 shows two TEM images of claysilica nanocomposites with PS brushes on the surfaces of silica nanorings. At low magnification (Figure 8a), nanoring structures on clay layers were observed. At high magnification (Figure 8b), PS brushes with a layer thickness of 11 nm were observed. The nanocomposite was dispersed in THF solution and etched with HF acid, and PS was obtained by pouring the solution into a 6-fold excess of methanol. On the basis of the GPC result, the number-average molecular weight of PS brushes is about 130 kg/mol. TGA results are shown in Figure 8c. ClaysilicaNH2 is found to have a 15 wt % weight loss in the range between 100 and 800 °C, and claysilica ATRP is found to have a 21 wt % weight loss. After PS brushes are grafted to the surfaces of silica nanorings, the weight loss is about 56 wt %, so the weight percentage of PS in the nanocomposite is about 35 wt %. As shown in the inset of Figure 8b, silica nanorings were produced on the edges of clay layers and PS polymer brushes were grown on the surfaces of silica via in situ polymerization. The negatively charged surfaces of clay layers remained in the nanocomposite. Because of the specific structure of claysilicaPS, hydrophilic clay layers, and hydrophobic PS brushes on silica nanorings, it is interesting to investigate the self-assembly of the nanocomposite in water. Figure 9 is a TEM image of clay silicaPS in water. The sample was prepared by dispersing the nanocomposite in THF and adding it to a 5-fold excess of water. In the image, the aggregation of the nanocomposite was
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observed. In an aggregated structure, there exist repulsive interactions among PS, water, and the water of hydration of cations on clay layers. To minimize the exposure of PS to water, PS brushes on silica nanorings collapse to form hydrophobic domains and the clay layers stay in the aqueous phase to stabilize the aggregated structures.
Figure 9. TEM image of claysilicaPS dispersed in water.
Figure 8. TEM images of claysilica nanocomposite with PS brushes on the surfaces of silica nanorings at (a) low and (b) high magnification and (c) thermogravimetric analysis thermograms of claysilicaNH2, claysilicaATRP, and claysilicaPS. 13217
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Figure 10. (a) TEM image of a dried droplet of an O/W emulsion with claysilicaPS in the aqueous phase. (b) TEM image of PS colloidal particles stabilized by the claysilicaPS nanocomposite.
In this article, the application of claysilicaPS in a Pickering emulsion was reported. An emulsion stabilized by particles instead of surfactant molecules is called a Pickering emulsion.21 After being dispersed in a mixture of oil and water, the solid particles have a strong tendency to locate at the interface between oil and water so that the total interfacial energy is reduced upon replacing part of the oilwater interface with an oilparticle or a waterparticle interface. In previous research, we studied the application of claypolymer nanocomposites in Pickering emulsions, and colloidal particles with clay layers on the surfaces were obtained.22 In claysilicaPS, there are negatively charged clay surfaces and hydrophobic PS brushes on silica nanorings, so the amphiphilic nanocomposite may be used as a stabilizer in the Pickering emulsion. To prepare the Pickering emulsion, clay silicaPS was dissolved in THF and the solution was added to a water/toluene mixture (10/1 v/v) drop by drop. Part a of Figure 10 shows a TEM image of the emulsion stabilized by the nanocomposite. The TEM specimen was prepared by dipping a carbon-coated copper grid into the emulsion and drying it in air. The TEM image indicates that the nanocomposite aggregates after the evaporation of solvents and no nanocomposites are observed outside of the droplet structure, which confirms the location of the nanocomposite at the liquidliquid interface.22,23 If claysilicaPS were just dispersed in an aqueous solution but on the surfaces of toluene droplets, then randomly distributed nanocomposites would be observed after drying in air. PS colloidal particles with claysilicaPS on the surfaces were prepared on the basis of the Pickering emulsion. To prepare PS colloidal particles, PS with an average molecular weight of 14K was dissolved in toluene, and the polymer solution was mixed with water in the presence of the claysilicaPS nanocomposite. After the addition of the emulsion to a 5-fold excess of methanol, PS colloidal particles were obtained. Part b of Figure 10 shows a TEM image of PS colloidal particles. The nanocomposite on the surfaces of particles can be observed. The PS brushes on the silica rings penetrate the PS collidal particles, and the colloidal particles are stabilized by the negatively charged clay layers on the surfaces.
’ CONCLUSIONS A simple synthesis route to the preparation of silica nanorings on the edges of clay layers has been presented in this article. This is the first report on the synthesis of claysilica nanocomposites with well-defined structure. In this research, silica nanorings on the edges of clay sheets were synthesized by using a solgel method in the presence of clay. The silica nanorings on the edges of clay sheets were chemically modified by ATRP initiators, and hydrophobic polymer brushes on the nanorings were prepared by in situ ATRP. The polymer nanocomposite with negatively charged clay surfaces and hydrophobic polymer brushes on the nanorings can be used in a Pickering emulsion. By introducing initiators onto the surfaces of silica nanorings, many different functional polymer brushes are able to be prepared. This procedure opens a new route to the fabrication of novel heterostructures, and new polymer materials can be prepared with this method. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions †
These authors contributed equally to this work.
’ ACKNOWLEDGMENT This project was supported by Science and Technology Committee of Tianjin under contract no. 10JCYBJC01900. ’ REFERENCES (1) (a) Okada, A.; Usuki, A. Macromol. Mater. Eng. 2006, 291, 1449–1476. (b) Pavlidou, S.; Papaspyrides, C. D. Prog. Polym. Sci. 2008, 33, 1119–1198. (c) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1–63. (2) (a) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Karauchi, T.; Kamigaito, O. J. Polym. Sci.; Part A: Polym. Chem. 1993, 31, 983–986. (b) Zhu, J.; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. A. Chem. Mater. 2001, 13, 3774–3780. 13218
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dx.doi.org/10.1021/la203180j |Langmuir 2011, 27, 13212–13219
