- Email: [email protected]
Synthesis and characterization of ZrO2 hollow spheres
Materials Letters 63 (2009) 1013–1015
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and characterization of ZrO2 hollow spheres Chenyi Guo a,b, Peng Hu a, Lingjie Yu a, Fangli Yuan a,⁎ a b
State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 11 November 2008 Accepted 9 December 2008 Available online 31 January 2009 Keywords: ZrO2 hollow spheres Sol–gel preparation Microstructure
a b s t r a c t ZrO2 hollow microspheres with the average diameter of about 500 nm and the shell thickness of about 50 nm were synthesized by a facile technique using carbon spheres as templates. The corresponding ZrO2 hollow microspheres were obtained by calcining the precursors of C–Zr(OH)4 core-shell heterostructures, which were synthesized with the precipitation of ZrCl4 solution with aqueous ammonia on the surface of colloid carbons. SEM, XRD, TGA and BET were used to characterize the composition, morphology, size and crystal structure of synthesized products. The effects of ultrasonic dispersion and separation process on the hollow spheres were studied, and the surfactant PEG-1000 was added to tune the shell structure of synthesized ZrO2 hollow spheres. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hollow inorganic micro- and nanostructures have recently attracted many interests because of their promising applications and functions, which may open new opportunities in catalysis, microelectronics, photonics, magnetics, ceramics, and pigments [1]. There are varieties of methods used to fabricate a wide range of hollow spheres with various compositions, such as hydrothermal technology [2]; emulsion droplets/micelles phase separation procedure [3]; kinetically controlled template-free synthesis method [4]; and sacrificial core technique [5]. Compared with other methods, the sacrificial core technique provides a facile, feasible and convenient method to prepare hollow spheres. ZrO2 is one of the most intensively studied materials owing to its technologically important applications in oxygen sensors, fuel cell electrolytes, catalysts and catalytic supports, metal oxide-semiconductor devices, superior thermal and chemical stability etc [6]. Since then, a great deal of efforts have been directed to the improvement of its structures and properties [7]. However, most of these approaches are focused on the synthesis of hollow spheres with a narrow size distribution and well defined pore size/shape, but limited to the fabrication of tunable shell structures. The development of a facile and economic method for the synthesis of ZrO2 hollow spheres with tunable structures would greatly promote their applications in catalytic and thermal isolated fields. In this paper, a more facile and exercisable method using colloid carbons as templates is developed to acquire the hollow ZrO2 microspheres with tunable shell structures. Ultrasonic dispersion and separation process were introduced to control the hollow
⁎ Corresponding author. Tel.: +86 10 82627058; fax: +86 10 62561822. E-mail address: fl[email protected] (F. Yuan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.12.053
structures, and surfactant was used to tune the shell structures of synthesized ZrO2 hollow spheres. 2. Experimental 2.1. Preparation of colloidal carbon spheres All reagents are analytical reagents and used as received without further purification. Colloidal carbon spheres were synthesized according to Sun's method [8]. In a typical synthesis process, 0.08 mol of glucose was dissolved in 80 ml of deionized water to form a homogeneous solution. The solution was then sealed in a
Fig. 1. Thermogravimetric curves of (a) colloidal carbon spheres and (b) C–Zr(OH)4 spheres.
1014
C. Guo et al. / Materials Letters 63 (2009) 1013–1015
creamy colloid of Zr(OH)4 was separated from the carbon spheres coated with Zr(OH)4 and was removed to get the C–Zr(OH)4 coreshell microparticles. Finally, the C–Zr(OH)4 core-shell microparticles were washed 3 times with deionized water and anhydrous ethanol respectively to remove impurities and dried at 60 °C for 5 h. ZrO2 hollow microspheres were obtained by calcination of the precursors at 600 °C for 1 h, and the final products were collected for further analysis. 2.3. Characterization
Fig. 2. XRD pattern of synthesized ZrO2 hollow spheres.
100 ml Teflon-lined autoclave and maintained at 170 °C for 10 h. The products were filtrated and washed by deionized water and anhydrous ethanol for three times, respectively. Subsequently, the colloidal carbon spheres were oven-dried at 60 °C for 5 h. 2.2. Preparation of hollow spheres The mixture of 0.2 g colloidal carbon spheres and 200 ml of 0.05 M zirconium chloride solution was sonicated for 10 min to get stable emulsion. In some experiments, 0.02 g surfactant PEG-1000 (polyethylene glycol-1000) was added into the emulsion during the ultrasonication procedure. After the ultrasonication, the mixture was transferred into a flask and stirred during the experiment followed by the dropwise of ammonia solution up to pH 7. Subsequently the obtained colloid solution was treated with ultrasonic dispersion for 0.5 h, and it was found that the creamy colloid of Zr(OH)4 was suspended on the upside and the carbon spheres coated with Zr(OH)4 subsided on the bottom of the flask. After it was centrifuged, the
The crystal structure and phase purity of the ZrO2 hollow microspheres were examined by means of X-ray Powder Diffraction (XRD, X-Pert, PAnalytic, Netherlands). The overview of the morphology was checked by a Field Emission Scanning Electron Microscope (FESEM, JSM-6700F, JEOL, Japan). Thermogravimetric analysis (DTATG, ZRY-2P, Shanghai) and surface area analyzer (Autosorb-1, USA) were used to characterize the ZrO2 hollow microspheres. 3. Results and discussion Fig. 1 shows the TGA measurement of the colloidal carbon spheres (curve a) and C–Zr(OH)4 microspheres (curve b). The curve a shows a weight loss process of carbon spheres by calcination. With the removal of absorbed water after 110 °C, carbon spheres experienced a calefaction process and began to lose weight at about 270 °C. Then the weight loss curve reveals a precipitous decline with the raising temperature. All the carbon spheres transformed into CO2 until the temperature went to about 490 °C. The C–Zr(OH)4 sample was determined until the weight loss ended. The curve b shown in the pattern reveals that the weight loss took place in two temperature ranges: the beginning to 200 °C and 300 °C to 550 °C. The first range occurred as soon as the sample was heated and the weight loss was about 9%. This phenomenon happens for the desorption of absorbed water on the surface of microspheres and crystal water. In the second weight loss procedure, the curve b is less precipitous compared with the curve a because colloidal carbon spheres transformed into CO2 by
Fig. 3. SEM images of (a) colloidal carbon spheres (b) C–Zr(OH)4 core-shell microparticles (c, d) synthesized ZrO2 hollow microspheres.
C. Guo et al. / Materials Letters 63 (2009) 1013–1015
Fig. 4. SEM images of synthesized ZrO2 hollow spheres with surfactant PEG-1000: (a) low magnification; (b) high magnification.
the calcination and at the same time the Zr(OH) 4 particles decomposed into ZrO2 for the dehydration. The weight loss process stopped until all the carbon spheres were removed and ZrO2 was formed. Then the curve b lasted for a straight line since the temperature was above 600 °C. The weight of ZrO2 shell occupied about 26% of total weight after the calcination process. According to the investigation, a suitable calcination temperature of 600 °C was confirmed to obtain final ZrO2 hollow spheres. The XRD pattern of the calcined products is shown in Fig. 2. All the diffraction peaks can be readily indexed to the tetragonal crystal system of ZrO2 (JCPDS 79-1771), and no peaks of other materials or phases are observed, which indicates the high purity of the products. Fig. 3a shows the typical size and morphology of the colloidal carbon spheres with a narrow size distribution of 800 nm. It should be noted that the diameter of colloidal carbon spheres can be controlled by adjusting the temperature, concentration of starting materials, and reaction time during the synthesis process [8]. Fig. 3b gives the image of obtained C–Zr(OH)4 core-shell microparticles. The fine particles deposited on the surface of colloidal carbon spheres confirm that the Zr(OH)4 nanoparticles continuously deposited on the surface of colloidal carbon spheres, which was due to residual hydroxyls and carboxylic groups on the surface of colloidal carbon spheres [9]. The diameter of the core-shell structures increased to about 1 µm because of the precipitation of Zr(OH)4. Fig. 3c shows the typical image of calcined products and it was found that the ZrO2 particles synthesized using carbon spheres (800 nm in diameter) were spherical and had
1015
uniform size of about 500 nm. Fig. 3d gives the magnified SEM image of the ZrO2 microsphere with a broken shell, which indicates that the sphere has a hollow structure with a shell thickness of about 50 nm. It is noted that there is a large shrinkage about 50% during calcinations, which is probably caused by further dehydration of the Zr(OH)4 shell and the loosely cross-linked structure of the carbon spheres [10]. The influences of experimental parameters on the structure of final products have been investigated, such as reaction temperature, the concentration of zirconium chloride and ultrasonic dispersion. In these parameters, ultrasonic dispersion has a greater effect on the morphology of synthesized particles than others. The ZrO2 products calcined from the precursor C–Zr(OH)4 without ultrasonic dispersion revealed lump sinter structures, while the hollow spheres were obtained only with ultrasonic dispersion. When ammonia solution was dropped into the emulsion, the fine Zr(OH)4 particles were hetero-nucleated on the surface of colloidal carbon spheres due to the low nucleation energy of hetero-nucleation, while a portion of Zr (OH)4 precipitated solely at the same time. And it is these precipitated Zr(OH)4 particles which were not coated on the carbon surface that caused the lump sinter. It is noted that the ultrasonic dispersion can make these Zr(OH)4 particles without coated on the carbon surface separated from those Zr(OH)4 particles coated on the carbon surface. Surfactant PEG-1000 can be easily adsorbed on the surface and interact with the groups of the carbon spheres [11], which can govern the absorbability of zirconium cations on the colloid carbon surface, and thus the deposited Zr(OH)4 can be tuned. As a result, ZrO2 hollow spheres with a porous shell were obtained in Fig. 4a. It was also found that the synthesized ZrO2 hollow spheres had a thinner shell with porous structure and the particles agglomerated on the shell were smaller than that without PEG-1000, as the image of one magnified particle shown in Fig. 4b. Specific surface area measurement reveals that the surface area of porous ZrO2 hollow spheres was 18.37 m2/g, and it was 10.94 m2/g for the ZrO2 hollow spheres synthesized without surfactant. It further confirmed the different shell structures of synthesized ZrO2 hollow spheres. 4. Conclusions ZrO2 hollow spheres were successfully prepared by coating Zr(OH)4 on the surface of colloidal carbon spheres, followed by the calcination of the carbon spheres and dehydration of Zr(OH)4 particles. Ultrasonic dispersion and separation process for C–Zr(OH)4 slurry are key factors for synthesized ZrO2 hollow spheres. The shell structure of hollow spheres can be well tuned by surfactant PEG-1000. The surface area of hollow spheres with compact shell synthesized without PEG-1000 was 10.94 m2/g, and that with the porous shell synthesized with PEG-1000 was 18.37 m2/g. The as-prepared ZrO2 hollow spheres have great potential in the development of nanoparticle-based functional materials. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
MacLachlan MJ, Manners I, Ozin GA. Adv Mater 2000;12:675–+. Wang X, Yuan FL, Hu P, Yu LJ, Bai LY. J Phys Chem C 2008;112:8773–8. Nakashima T, Kimizuka N. J Am Chem Soc 2003;125:6386–7. Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5. Li GC, Zhang ZK. Mater Lett 2004;58:2768–71. Liu RJ, Li Y, Zhao H, Zhao FY, Hu YQ. Mater Lett 2008;62:2593–5. Cao HQ, Qiu XQ, Luo B, Liang Y, Zhang YH, Tan RQ, et al. Adv Funct Mater 2004;14:243–6. Sun XM, Li YD. Angew Chem, Int Ed Engl 2004;43:597–601. Wang X, Hu P, Yuan FL, Yu LJ. J Phys Chem C 2007;111:6706–12. Sun XM, Li YD. Angew Chem, Int Ed Engl 2004;43:3827–31. Chen Y, Liu HR, Zhang ZC, Wang SJ. Eur Polym J 2007;43:2848–55.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and characterization of ZrO2 hollow spheres Chenyi Guo a,b, Peng Hu a, Lingjie Yu a, Fangli Yuan a,⁎ a b
State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 11 November 2008 Accepted 9 December 2008 Available online 31 January 2009 Keywords: ZrO2 hollow spheres Sol–gel preparation Microstructure
a b s t r a c t ZrO2 hollow microspheres with the average diameter of about 500 nm and the shell thickness of about 50 nm were synthesized by a facile technique using carbon spheres as templates. The corresponding ZrO2 hollow microspheres were obtained by calcining the precursors of C–Zr(OH)4 core-shell heterostructures, which were synthesized with the precipitation of ZrCl4 solution with aqueous ammonia on the surface of colloid carbons. SEM, XRD, TGA and BET were used to characterize the composition, morphology, size and crystal structure of synthesized products. The effects of ultrasonic dispersion and separation process on the hollow spheres were studied, and the surfactant PEG-1000 was added to tune the shell structure of synthesized ZrO2 hollow spheres. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hollow inorganic micro- and nanostructures have recently attracted many interests because of their promising applications and functions, which may open new opportunities in catalysis, microelectronics, photonics, magnetics, ceramics, and pigments [1]. There are varieties of methods used to fabricate a wide range of hollow spheres with various compositions, such as hydrothermal technology [2]; emulsion droplets/micelles phase separation procedure [3]; kinetically controlled template-free synthesis method [4]; and sacrificial core technique [5]. Compared with other methods, the sacrificial core technique provides a facile, feasible and convenient method to prepare hollow spheres. ZrO2 is one of the most intensively studied materials owing to its technologically important applications in oxygen sensors, fuel cell electrolytes, catalysts and catalytic supports, metal oxide-semiconductor devices, superior thermal and chemical stability etc [6]. Since then, a great deal of efforts have been directed to the improvement of its structures and properties [7]. However, most of these approaches are focused on the synthesis of hollow spheres with a narrow size distribution and well defined pore size/shape, but limited to the fabrication of tunable shell structures. The development of a facile and economic method for the synthesis of ZrO2 hollow spheres with tunable structures would greatly promote their applications in catalytic and thermal isolated fields. In this paper, a more facile and exercisable method using colloid carbons as templates is developed to acquire the hollow ZrO2 microspheres with tunable shell structures. Ultrasonic dispersion and separation process were introduced to control the hollow
⁎ Corresponding author. Tel.: +86 10 82627058; fax: +86 10 62561822. E-mail address: fl[email protected] (F. Yuan). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.12.053
structures, and surfactant was used to tune the shell structures of synthesized ZrO2 hollow spheres. 2. Experimental 2.1. Preparation of colloidal carbon spheres All reagents are analytical reagents and used as received without further purification. Colloidal carbon spheres were synthesized according to Sun's method [8]. In a typical synthesis process, 0.08 mol of glucose was dissolved in 80 ml of deionized water to form a homogeneous solution. The solution was then sealed in a
Fig. 1. Thermogravimetric curves of (a) colloidal carbon spheres and (b) C–Zr(OH)4 spheres.
1014
C. Guo et al. / Materials Letters 63 (2009) 1013–1015
creamy colloid of Zr(OH)4 was separated from the carbon spheres coated with Zr(OH)4 and was removed to get the C–Zr(OH)4 coreshell microparticles. Finally, the C–Zr(OH)4 core-shell microparticles were washed 3 times with deionized water and anhydrous ethanol respectively to remove impurities and dried at 60 °C for 5 h. ZrO2 hollow microspheres were obtained by calcination of the precursors at 600 °C for 1 h, and the final products were collected for further analysis. 2.3. Characterization
Fig. 2. XRD pattern of synthesized ZrO2 hollow spheres.
100 ml Teflon-lined autoclave and maintained at 170 °C for 10 h. The products were filtrated and washed by deionized water and anhydrous ethanol for three times, respectively. Subsequently, the colloidal carbon spheres were oven-dried at 60 °C for 5 h. 2.2. Preparation of hollow spheres The mixture of 0.2 g colloidal carbon spheres and 200 ml of 0.05 M zirconium chloride solution was sonicated for 10 min to get stable emulsion. In some experiments, 0.02 g surfactant PEG-1000 (polyethylene glycol-1000) was added into the emulsion during the ultrasonication procedure. After the ultrasonication, the mixture was transferred into a flask and stirred during the experiment followed by the dropwise of ammonia solution up to pH 7. Subsequently the obtained colloid solution was treated with ultrasonic dispersion for 0.5 h, and it was found that the creamy colloid of Zr(OH)4 was suspended on the upside and the carbon spheres coated with Zr(OH)4 subsided on the bottom of the flask. After it was centrifuged, the
The crystal structure and phase purity of the ZrO2 hollow microspheres were examined by means of X-ray Powder Diffraction (XRD, X-Pert, PAnalytic, Netherlands). The overview of the morphology was checked by a Field Emission Scanning Electron Microscope (FESEM, JSM-6700F, JEOL, Japan). Thermogravimetric analysis (DTATG, ZRY-2P, Shanghai) and surface area analyzer (Autosorb-1, USA) were used to characterize the ZrO2 hollow microspheres. 3. Results and discussion Fig. 1 shows the TGA measurement of the colloidal carbon spheres (curve a) and C–Zr(OH)4 microspheres (curve b). The curve a shows a weight loss process of carbon spheres by calcination. With the removal of absorbed water after 110 °C, carbon spheres experienced a calefaction process and began to lose weight at about 270 °C. Then the weight loss curve reveals a precipitous decline with the raising temperature. All the carbon spheres transformed into CO2 until the temperature went to about 490 °C. The C–Zr(OH)4 sample was determined until the weight loss ended. The curve b shown in the pattern reveals that the weight loss took place in two temperature ranges: the beginning to 200 °C and 300 °C to 550 °C. The first range occurred as soon as the sample was heated and the weight loss was about 9%. This phenomenon happens for the desorption of absorbed water on the surface of microspheres and crystal water. In the second weight loss procedure, the curve b is less precipitous compared with the curve a because colloidal carbon spheres transformed into CO2 by
Fig. 3. SEM images of (a) colloidal carbon spheres (b) C–Zr(OH)4 core-shell microparticles (c, d) synthesized ZrO2 hollow microspheres.
C. Guo et al. / Materials Letters 63 (2009) 1013–1015
Fig. 4. SEM images of synthesized ZrO2 hollow spheres with surfactant PEG-1000: (a) low magnification; (b) high magnification.
the calcination and at the same time the Zr(OH) 4 particles decomposed into ZrO2 for the dehydration. The weight loss process stopped until all the carbon spheres were removed and ZrO2 was formed. Then the curve b lasted for a straight line since the temperature was above 600 °C. The weight of ZrO2 shell occupied about 26% of total weight after the calcination process. According to the investigation, a suitable calcination temperature of 600 °C was confirmed to obtain final ZrO2 hollow spheres. The XRD pattern of the calcined products is shown in Fig. 2. All the diffraction peaks can be readily indexed to the tetragonal crystal system of ZrO2 (JCPDS 79-1771), and no peaks of other materials or phases are observed, which indicates the high purity of the products. Fig. 3a shows the typical size and morphology of the colloidal carbon spheres with a narrow size distribution of 800 nm. It should be noted that the diameter of colloidal carbon spheres can be controlled by adjusting the temperature, concentration of starting materials, and reaction time during the synthesis process [8]. Fig. 3b gives the image of obtained C–Zr(OH)4 core-shell microparticles. The fine particles deposited on the surface of colloidal carbon spheres confirm that the Zr(OH)4 nanoparticles continuously deposited on the surface of colloidal carbon spheres, which was due to residual hydroxyls and carboxylic groups on the surface of colloidal carbon spheres [9]. The diameter of the core-shell structures increased to about 1 µm because of the precipitation of Zr(OH)4. Fig. 3c shows the typical image of calcined products and it was found that the ZrO2 particles synthesized using carbon spheres (800 nm in diameter) were spherical and had
1015
uniform size of about 500 nm. Fig. 3d gives the magnified SEM image of the ZrO2 microsphere with a broken shell, which indicates that the sphere has a hollow structure with a shell thickness of about 50 nm. It is noted that there is a large shrinkage about 50% during calcinations, which is probably caused by further dehydration of the Zr(OH)4 shell and the loosely cross-linked structure of the carbon spheres [10]. The influences of experimental parameters on the structure of final products have been investigated, such as reaction temperature, the concentration of zirconium chloride and ultrasonic dispersion. In these parameters, ultrasonic dispersion has a greater effect on the morphology of synthesized particles than others. The ZrO2 products calcined from the precursor C–Zr(OH)4 without ultrasonic dispersion revealed lump sinter structures, while the hollow spheres were obtained only with ultrasonic dispersion. When ammonia solution was dropped into the emulsion, the fine Zr(OH)4 particles were hetero-nucleated on the surface of colloidal carbon spheres due to the low nucleation energy of hetero-nucleation, while a portion of Zr (OH)4 precipitated solely at the same time. And it is these precipitated Zr(OH)4 particles which were not coated on the carbon surface that caused the lump sinter. It is noted that the ultrasonic dispersion can make these Zr(OH)4 particles without coated on the carbon surface separated from those Zr(OH)4 particles coated on the carbon surface. Surfactant PEG-1000 can be easily adsorbed on the surface and interact with the groups of the carbon spheres [11], which can govern the absorbability of zirconium cations on the colloid carbon surface, and thus the deposited Zr(OH)4 can be tuned. As a result, ZrO2 hollow spheres with a porous shell were obtained in Fig. 4a. It was also found that the synthesized ZrO2 hollow spheres had a thinner shell with porous structure and the particles agglomerated on the shell were smaller than that without PEG-1000, as the image of one magnified particle shown in Fig. 4b. Specific surface area measurement reveals that the surface area of porous ZrO2 hollow spheres was 18.37 m2/g, and it was 10.94 m2/g for the ZrO2 hollow spheres synthesized without surfactant. It further confirmed the different shell structures of synthesized ZrO2 hollow spheres. 4. Conclusions ZrO2 hollow spheres were successfully prepared by coating Zr(OH)4 on the surface of colloidal carbon spheres, followed by the calcination of the carbon spheres and dehydration of Zr(OH)4 particles. Ultrasonic dispersion and separation process for C–Zr(OH)4 slurry are key factors for synthesized ZrO2 hollow spheres. The shell structure of hollow spheres can be well tuned by surfactant PEG-1000. The surface area of hollow spheres with compact shell synthesized without PEG-1000 was 10.94 m2/g, and that with the porous shell synthesized with PEG-1000 was 18.37 m2/g. The as-prepared ZrO2 hollow spheres have great potential in the development of nanoparticle-based functional materials. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
MacLachlan MJ, Manners I, Ozin GA. Adv Mater 2000;12:675–+. Wang X, Yuan FL, Hu P, Yu LJ, Bai LY. J Phys Chem C 2008;112:8773–8. Nakashima T, Kimizuka N. J Am Chem Soc 2003;125:6386–7. Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5. Li GC, Zhang ZK. Mater Lett 2004;58:2768–71. Liu RJ, Li Y, Zhao H, Zhao FY, Hu YQ. Mater Lett 2008;62:2593–5. Cao HQ, Qiu XQ, Luo B, Liang Y, Zhang YH, Tan RQ, et al. Adv Funct Mater 2004;14:243–6. Sun XM, Li YD. Angew Chem, Int Ed Engl 2004;43:597–601. Wang X, Hu P, Yuan FL, Yu LJ. J Phys Chem C 2007;111:6706–12. Sun XM, Li YD. Angew Chem, Int Ed Engl 2004;43:3827–31. Chen Y, Liu HR, Zhang ZC, Wang SJ. Eur Polym J 2007;43:2848–55.