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Synthesis and characterization of poly (o-phenylenediamine) hollow multi-angular microrods by interfacial method
Materials Letters 63 (2009) 334–336
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 poly (o-phenylenediamine) hollow multi-angular microrods by interfacial method Qingli Hao ⁎, Baoming Sun, Xujie Yang, Lude Lu, Xin Wang ⁎ Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 18 July 2008 Accepted 22 October 2008 Available online 30 October 2008 Keywords: Poly (o-phenylenediamine) Ferric chloride Interfacial polymerization Microstructure Characterization methods
a b s t r a c t Poly (o-phenylenediamine) (PoPD) hollow multi-angular microrods have been synthesized at 10 °C via an interfacial process using ferric chloride and o-phenylenediamine (oPD) as starting materials. The chemical structure of the one dimensional (1D) superstructure is proved to be phenazine-like, and contains the benzenoid and quinoid imine units doped partly with Cl. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations indicate that the resulting superstructure of PoPD is built from hollow nanofibers aligned parallel to the fiber axis by van der Waals' force. The possible formation mechanism of the structure has been proposed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In the past decades, poly(o-phenylenediamine) (PoPD) or oPD oligomer (OoPD), a highly aromatic polymer containing 2,3-diaminophenazine or quinoraline repeating unit [1], has received significant attention because it can be utilized in many fields, such as sensors [2], catalysts [3], rechargeable cells [4], anticorrosion coatings [5] and electrochromic devices[6]. Recently, many efforts have been devoted to fabricating one-dimensional (1D) nano- or micro-structures of the material obtained by the polymerization of o-phenylenediamine (oPD) mainly due to the possibility of enhanced performance in analytical application[7,8], or the easy self-assembly of π–π stacking in conducting polymers[9]. Some template-free chemical methods have been reported to shape the 1D micro- or nanostructure-based PoPD or OoPD. The reported micro- or nano-fibers or nanobelts are mainly obtained by solution polymerization through mixing [10,11], stirring [9,12,13], or reprecipitation [14] processes. The oxidants concern AgNO3, HAuCl4, FeCl3 (FC ) and CuSO4 [15]. In the case of AgNO3 [13], HAuCl4 [9,10] used as oxidants, the resulting precipitates are not “pure”, and there are some Au or Ag particle byproducts. Interfacial polymerization is a facile route to obtain the 1D conducting polymers [16]. Here, we have successfully obtained a 1D superstructure of PoPD hollow multi-angular microrods on a large scale by the interfacial route at 10 °C, using FC as an oxidant, without any surfactants. Such 1D superstructure self-assembled by this method is different from those obtained through other solution routes [9–14].
In a typical procedure, Solution A of 0.1 mol/L oPD (99.5%, SigmaAldrich) was prepared by dissolving oPD in chloroform; certain amount FC was dissolved in water respectively as Solution B with 0.05 and 0.4 mol/L FC. Solution B is transferred carefully upon to A in a breaker at 10 °C. A quick color change from light yellow to brown-red was observed within several seconds upon the addition of FC. Soon, red fine fibers of the products formed at the interface and diffused slowly into the aqueous solutions gradually. After 6 h, the aqueous phase was then collected and filtered, and the precipitate were rinsed thoroughly with water, and dried at 30 °C for further use. Fourier transform infrared (FT-IR) spectra were recorded with a Bruker Vector 22 FT-IR spectrometer in the region of 400–4000 cm− 1 using KBr pellets. Raman spectra were performed on a Renishaw Invia spectrometer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on a JEOL JSM6380LV and JEM-2100 electron microscopes, respectively. The X-ray diffraction (XRD) analysis of the samples was carried out on a Bruker D8 advanced X-ray diffractometer.
⁎ Corresponding authors. Tel./fax: +86 25 84315054. E-mail address: [email protected] (Q. Hao). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.041
3. Results and discussion The FT-IR spectrum of the product is given in Fig. 1(A). The single band at around 3301 cm− 1 is due to the N–H stretching vibrations of the –NH-group. The two peaks at 3380, 3153 cm− 1 are ascribed to the asymmetrical and symmetrical of N–H stretching vibrations of NH2 group, respectively. Two strong peaks at 1615 cm− 1 and 1533 cm− 1 are associated with the stretching vibrations of CfC and CfN group in phenazine ring. The peaks at 1367 cm− 1 and 1213 cm− 1 are associated with C–N–C stretching in the benzenoid and quinoid imine units. Furthermore, the bands at 750 cm− 1 and 610 cm− 1, which are the characteristic of C–H out-of-plane bending vibrations of benzene nuclei in the phenazine skeleton, are also observed.
Q. Hao et al. / Materials Letters 63 (2009) 334–336
335
Fig. 1. FT-IR spectrum (A) of PoPD and Raman spectra (B) of PoPD and oPD.
It is found that the Raman spectrum of PoPD microrods is different from that of the monomer oPD (Fig. 1B). There are three new peaks observed in PoPD Raman graph, 1570, 1369, and 1252 cm− 1, which corresponds to the stretching vibrations of CfN and C–N groups in phenazine ring along the polymer chain, and the C–N–C stretching in the benzenoid and quinoid imine units, respectively. The strong band at 1369 cm− 1 is assigned to the C–N•+ stretching modes of delocalized polaronic charge carriers [17] and which is further confirmed that the existence of the Cl doping in the PoPD backbone. The FT-IR and Raman results are similar to previous reports [10–14]. From the spectroscopic analysis, it can be concluded that the synthesized PoPD has a head-to-tail type arrangement with the benzenoid and quinoid structures in the phenazine-like backbone in which the –Nf group is doped partly by Cl atom. Fig. 2 shows the XRD patterns of oPD powder (A) and the PoPD microrods (B, C) obtained at different ratio of FC to oPD. In Fig. 2B and C, two PoPD products obtained at different ratio of FC to oPD present the same diffraction peaks, indicating the same chemical composition. And there is no broad reflection due to amorphous components, which indicates that the PoPD products obtained through interfacial polymerization have good crystallinity and long range ordering. Additionally, as shown in Fig. 2B and C, the better crystallinity is observed evidently in the PoPD microrods prepared with 0.4 mol/L FC and 0.1 mol/L oPD (FC to oPD, 4:1 ), indicating that the PoPD microrods could be well crystallized if the ratio of FC to oPD increases. Fig. 3 presents the SEM (A, B) and TEM (C, D) images of PoPD microrods. A lowmagnification image (Fig. 3A) reveals that the PoPD product consists of a large quantity of uniform 1D structure, straight multi-angular microrods, synthesized by interfacial method using FC. The long straight superstructure reveals the high rigidity of the synthesized PoPD. The high-magnification SEM image (left inset in Fig. 3A) demonstrates that the nature of the products is hollow multi-angular microrods with the length in the range from few tens micrometers up to several hundreds millimeters, and the width or the height of several micrometers. The number and the size of hollows per microrod are irregular. From the cutaway view of the microrods in Fig. 3B, one can see the hollow interior wall is not smooth and irregularly-shaped. For a better understanding of the growth mechanism of the hollow PoPD microrods, the lower reaction time was investigated. TEM images of the product obtained after 1 h (Fig. 3C, D) give a closer look at some fine PoPD microrods.
Remarkably, there is an apparent straight hollow with the inner diameter of about 20 nm in the microrod of Fig. 3C, by which a thinner hollow also can be observed. While Fig. 3D demonstrates a long microrod aggregation self-assembled with many long hollow nanofibers, and the thin nanofibers with an external diameter of ~ 100 nm and inner diameter of 15–50 nm are linearly aligned along the fiber axis. The hollows are found to pass totally or partly through the fibers. It is noted that a long hollow nanofiber partly separates from the microrod aggregation in Fig. 3D, and the marked bending part indicates the nanofiber is ductile to some degree. According to these results, we speculate that the hollow multi-angular microrods are in fact built from hollow nanofibers. These nanofibers are aligned parallel to the fiber axis by van der Waals' force. Subsequently, the aggregation and fusion processes of such hollow nanofibers will lead to the formation of microsized hollow multi-angular rods. The detailed growth mechanism of the hollow is still on the progress. An energy dispersive X-ray analysis (EDX) technique based on the SEM image was used to further determine the PoPD sample. The peaks of C, N, Cl are observed in the EDX spectrum, shown in upper-right insert of Fig. 3A. The other peaks are originated from the Pt coating for the measurement. Since there is no Fe detected, the strong Cl peak (atom content: ~ 4%) could only be ascribed to Cl doping of the PoPD backbone. In addition, the molar ratio of N/Cl obtained from the peak areas is about 4, indicating that these hollow microrods of PoPD are probably doped by one Cl atom per two units in the backbone. This analysis is in well agreement with the result of Raman spectra. The result reveals that the morphology of PoPD product prepared in our work is quite different from those reported previously, like solid nanobelt [12], microfiber [11], and microrods [18]. This implies that the interfacial route plays an important role in the formation of PoPD hollow microrods nanostructures, in addition to the reaction parameters. The driving force to fabricate such hollow 1D superstructure is proposed to be strong π–π stacking between the phenazine-like structure and hydrogen bonding due to a number of N, Cl and H in existence, as well as van der Waals' force.
Fig. 2. XRD patterns of oPD (A), and PoPD microrods obtained at different ratios of FC to oPD, 4:1(B), 1:2(C), respectively.
Fig. 3. SEM (A,B) and TEM (C,D) pictures of PoPD microrods. (Right inset in A: EDX spectrum of selected area; left inset in A: magnification of the selected part).
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Q. Hao et al. / Materials Letters 63 (2009) 334–336
4. Conclusions Hollow multi-angular PoPD microrods have been successfully prepared via an interfacial route using FC and oPD as raw materials without any templates at a low temperature (10 °C). The ratio of FC to oPD and the preparation route play important roles in the information and growth of the PoPD hollow superstructure. A possible mechanism for the formation of the hollow superstructure is proposed. Next, we will investigate the influence of other experimental aspects on the morphology and growth of the products through the same route. Acknowledgements This work was supported by the Natural Science Foundation of Jiangsu Province, China (BK2006201), Chinese Postdoctoral Science Foundation (20060400285), Jiangsu Planned Projects for Postdoctoral Research Funds (2006), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars from State Education Ministry and Ministry of Personnel (2006). We also thank Dr. Xu Chao for the assistance with SEM and the suggestions of the reviewers.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Sulimenko T, Stejskal J, Prokeš J. J Colloid Interface Sci 2001;236:328–34. Maiyalagan T. J Power Sources 2008;179:443–50. Cai LT, Chen HY. Sens Actuators, B 1999;55:14–8. Sivakkumar SR, Saraswathi R. J Appl Electrochem 2004;34:1147–52. Hermas AA. Prog Org Coat 2008;61:95–102. Losito L, Palmisano F, Zambonin P. Anal Chem 2003;75:4988–95. Curulli A, Valentini F, Orlanducci S, Terranova ML, Paoletti C, Palleschi G. Sens Actuators, B 2004;100:65–71. Valentini F, Salis A, Curulli A, Palleschi G. Anal Chem 2004;76:3244–8. Sun XP, Dong SJ, Wang E. Chem Commun 2004:1182–3. Han J, Song G, Guo R. Eur Polym J 2007;43:4229–35. Sun X, Hagner M. Langmuir 2007;23:10441–4. Guo S, Dong S, Wang E. Chem Mater 2007;19:4621–3. Sun X, Dong S, Wang E. Macromol Rapid Commun 2005;26:1504–8. Jiang HQ, Sun XP, Huang MH, Wang YL, Li D, Dong SJ. Langmuir 2006;22:3358–61. Wang L, Guo S, Dong S. Mater Lett 2008;62:3240–2. Huang J, Kaner RB. J Am Chem Soc 2004;126:851–5. Mallick K, Witcomb MJ, Erasmus R, Scurrell MS. Mater Sci Eng, B 2007;140:166–71. Lu X, Mao H, Chao D, Zhao X, Zhang W, Wei Y. Mater Lett 2000;61:1400–3.
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 poly (o-phenylenediamine) hollow multi-angular microrods by interfacial method Qingli Hao ⁎, Baoming Sun, Xujie Yang, Lude Lu, Xin Wang ⁎ Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 18 July 2008 Accepted 22 October 2008 Available online 30 October 2008 Keywords: Poly (o-phenylenediamine) Ferric chloride Interfacial polymerization Microstructure Characterization methods
a b s t r a c t Poly (o-phenylenediamine) (PoPD) hollow multi-angular microrods have been synthesized at 10 °C via an interfacial process using ferric chloride and o-phenylenediamine (oPD) as starting materials. The chemical structure of the one dimensional (1D) superstructure is proved to be phenazine-like, and contains the benzenoid and quinoid imine units doped partly with Cl. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations indicate that the resulting superstructure of PoPD is built from hollow nanofibers aligned parallel to the fiber axis by van der Waals' force. The possible formation mechanism of the structure has been proposed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In the past decades, poly(o-phenylenediamine) (PoPD) or oPD oligomer (OoPD), a highly aromatic polymer containing 2,3-diaminophenazine or quinoraline repeating unit [1], has received significant attention because it can be utilized in many fields, such as sensors [2], catalysts [3], rechargeable cells [4], anticorrosion coatings [5] and electrochromic devices[6]. Recently, many efforts have been devoted to fabricating one-dimensional (1D) nano- or micro-structures of the material obtained by the polymerization of o-phenylenediamine (oPD) mainly due to the possibility of enhanced performance in analytical application[7,8], or the easy self-assembly of π–π stacking in conducting polymers[9]. Some template-free chemical methods have been reported to shape the 1D micro- or nanostructure-based PoPD or OoPD. The reported micro- or nano-fibers or nanobelts are mainly obtained by solution polymerization through mixing [10,11], stirring [9,12,13], or reprecipitation [14] processes. The oxidants concern AgNO3, HAuCl4, FeCl3 (FC ) and CuSO4 [15]. In the case of AgNO3 [13], HAuCl4 [9,10] used as oxidants, the resulting precipitates are not “pure”, and there are some Au or Ag particle byproducts. Interfacial polymerization is a facile route to obtain the 1D conducting polymers [16]. Here, we have successfully obtained a 1D superstructure of PoPD hollow multi-angular microrods on a large scale by the interfacial route at 10 °C, using FC as an oxidant, without any surfactants. Such 1D superstructure self-assembled by this method is different from those obtained through other solution routes [9–14].
In a typical procedure, Solution A of 0.1 mol/L oPD (99.5%, SigmaAldrich) was prepared by dissolving oPD in chloroform; certain amount FC was dissolved in water respectively as Solution B with 0.05 and 0.4 mol/L FC. Solution B is transferred carefully upon to A in a breaker at 10 °C. A quick color change from light yellow to brown-red was observed within several seconds upon the addition of FC. Soon, red fine fibers of the products formed at the interface and diffused slowly into the aqueous solutions gradually. After 6 h, the aqueous phase was then collected and filtered, and the precipitate were rinsed thoroughly with water, and dried at 30 °C for further use. Fourier transform infrared (FT-IR) spectra were recorded with a Bruker Vector 22 FT-IR spectrometer in the region of 400–4000 cm− 1 using KBr pellets. Raman spectra were performed on a Renishaw Invia spectrometer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on a JEOL JSM6380LV and JEM-2100 electron microscopes, respectively. The X-ray diffraction (XRD) analysis of the samples was carried out on a Bruker D8 advanced X-ray diffractometer.
⁎ Corresponding authors. Tel./fax: +86 25 84315054. E-mail address: [email protected] (Q. Hao). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.041
3. Results and discussion The FT-IR spectrum of the product is given in Fig. 1(A). The single band at around 3301 cm− 1 is due to the N–H stretching vibrations of the –NH-group. The two peaks at 3380, 3153 cm− 1 are ascribed to the asymmetrical and symmetrical of N–H stretching vibrations of NH2 group, respectively. Two strong peaks at 1615 cm− 1 and 1533 cm− 1 are associated with the stretching vibrations of CfC and CfN group in phenazine ring. The peaks at 1367 cm− 1 and 1213 cm− 1 are associated with C–N–C stretching in the benzenoid and quinoid imine units. Furthermore, the bands at 750 cm− 1 and 610 cm− 1, which are the characteristic of C–H out-of-plane bending vibrations of benzene nuclei in the phenazine skeleton, are also observed.
Q. Hao et al. / Materials Letters 63 (2009) 334–336
335
Fig. 1. FT-IR spectrum (A) of PoPD and Raman spectra (B) of PoPD and oPD.
It is found that the Raman spectrum of PoPD microrods is different from that of the monomer oPD (Fig. 1B). There are three new peaks observed in PoPD Raman graph, 1570, 1369, and 1252 cm− 1, which corresponds to the stretching vibrations of CfN and C–N groups in phenazine ring along the polymer chain, and the C–N–C stretching in the benzenoid and quinoid imine units, respectively. The strong band at 1369 cm− 1 is assigned to the C–N•+ stretching modes of delocalized polaronic charge carriers [17] and which is further confirmed that the existence of the Cl doping in the PoPD backbone. The FT-IR and Raman results are similar to previous reports [10–14]. From the spectroscopic analysis, it can be concluded that the synthesized PoPD has a head-to-tail type arrangement with the benzenoid and quinoid structures in the phenazine-like backbone in which the –Nf group is doped partly by Cl atom. Fig. 2 shows the XRD patterns of oPD powder (A) and the PoPD microrods (B, C) obtained at different ratio of FC to oPD. In Fig. 2B and C, two PoPD products obtained at different ratio of FC to oPD present the same diffraction peaks, indicating the same chemical composition. And there is no broad reflection due to amorphous components, which indicates that the PoPD products obtained through interfacial polymerization have good crystallinity and long range ordering. Additionally, as shown in Fig. 2B and C, the better crystallinity is observed evidently in the PoPD microrods prepared with 0.4 mol/L FC and 0.1 mol/L oPD (FC to oPD, 4:1 ), indicating that the PoPD microrods could be well crystallized if the ratio of FC to oPD increases. Fig. 3 presents the SEM (A, B) and TEM (C, D) images of PoPD microrods. A lowmagnification image (Fig. 3A) reveals that the PoPD product consists of a large quantity of uniform 1D structure, straight multi-angular microrods, synthesized by interfacial method using FC. The long straight superstructure reveals the high rigidity of the synthesized PoPD. The high-magnification SEM image (left inset in Fig. 3A) demonstrates that the nature of the products is hollow multi-angular microrods with the length in the range from few tens micrometers up to several hundreds millimeters, and the width or the height of several micrometers. The number and the size of hollows per microrod are irregular. From the cutaway view of the microrods in Fig. 3B, one can see the hollow interior wall is not smooth and irregularly-shaped. For a better understanding of the growth mechanism of the hollow PoPD microrods, the lower reaction time was investigated. TEM images of the product obtained after 1 h (Fig. 3C, D) give a closer look at some fine PoPD microrods.
Remarkably, there is an apparent straight hollow with the inner diameter of about 20 nm in the microrod of Fig. 3C, by which a thinner hollow also can be observed. While Fig. 3D demonstrates a long microrod aggregation self-assembled with many long hollow nanofibers, and the thin nanofibers with an external diameter of ~ 100 nm and inner diameter of 15–50 nm are linearly aligned along the fiber axis. The hollows are found to pass totally or partly through the fibers. It is noted that a long hollow nanofiber partly separates from the microrod aggregation in Fig. 3D, and the marked bending part indicates the nanofiber is ductile to some degree. According to these results, we speculate that the hollow multi-angular microrods are in fact built from hollow nanofibers. These nanofibers are aligned parallel to the fiber axis by van der Waals' force. Subsequently, the aggregation and fusion processes of such hollow nanofibers will lead to the formation of microsized hollow multi-angular rods. The detailed growth mechanism of the hollow is still on the progress. An energy dispersive X-ray analysis (EDX) technique based on the SEM image was used to further determine the PoPD sample. The peaks of C, N, Cl are observed in the EDX spectrum, shown in upper-right insert of Fig. 3A. The other peaks are originated from the Pt coating for the measurement. Since there is no Fe detected, the strong Cl peak (atom content: ~ 4%) could only be ascribed to Cl doping of the PoPD backbone. In addition, the molar ratio of N/Cl obtained from the peak areas is about 4, indicating that these hollow microrods of PoPD are probably doped by one Cl atom per two units in the backbone. This analysis is in well agreement with the result of Raman spectra. The result reveals that the morphology of PoPD product prepared in our work is quite different from those reported previously, like solid nanobelt [12], microfiber [11], and microrods [18]. This implies that the interfacial route plays an important role in the formation of PoPD hollow microrods nanostructures, in addition to the reaction parameters. The driving force to fabricate such hollow 1D superstructure is proposed to be strong π–π stacking between the phenazine-like structure and hydrogen bonding due to a number of N, Cl and H in existence, as well as van der Waals' force.
Fig. 2. XRD patterns of oPD (A), and PoPD microrods obtained at different ratios of FC to oPD, 4:1(B), 1:2(C), respectively.
Fig. 3. SEM (A,B) and TEM (C,D) pictures of PoPD microrods. (Right inset in A: EDX spectrum of selected area; left inset in A: magnification of the selected part).
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Q. Hao et al. / Materials Letters 63 (2009) 334–336
4. Conclusions Hollow multi-angular PoPD microrods have been successfully prepared via an interfacial route using FC and oPD as raw materials without any templates at a low temperature (10 °C). The ratio of FC to oPD and the preparation route play important roles in the information and growth of the PoPD hollow superstructure. A possible mechanism for the formation of the hollow superstructure is proposed. Next, we will investigate the influence of other experimental aspects on the morphology and growth of the products through the same route. Acknowledgements This work was supported by the Natural Science Foundation of Jiangsu Province, China (BK2006201), Chinese Postdoctoral Science Foundation (20060400285), Jiangsu Planned Projects for Postdoctoral Research Funds (2006), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars from State Education Ministry and Ministry of Personnel (2006). We also thank Dr. Xu Chao for the assistance with SEM and the suggestions of the reviewers.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Sulimenko T, Stejskal J, Prokeš J. J Colloid Interface Sci 2001;236:328–34. Maiyalagan T. J Power Sources 2008;179:443–50. Cai LT, Chen HY. Sens Actuators, B 1999;55:14–8. Sivakkumar SR, Saraswathi R. J Appl Electrochem 2004;34:1147–52. Hermas AA. Prog Org Coat 2008;61:95–102. Losito L, Palmisano F, Zambonin P. Anal Chem 2003;75:4988–95. Curulli A, Valentini F, Orlanducci S, Terranova ML, Paoletti C, Palleschi G. Sens Actuators, B 2004;100:65–71. Valentini F, Salis A, Curulli A, Palleschi G. Anal Chem 2004;76:3244–8. Sun XP, Dong SJ, Wang E. Chem Commun 2004:1182–3. Han J, Song G, Guo R. Eur Polym J 2007;43:4229–35. Sun X, Hagner M. Langmuir 2007;23:10441–4. Guo S, Dong S, Wang E. Chem Mater 2007;19:4621–3. Sun X, Dong S, Wang E. Macromol Rapid Commun 2005;26:1504–8. Jiang HQ, Sun XP, Huang MH, Wang YL, Li D, Dong SJ. Langmuir 2006;22:3358–61. Wang L, Guo S, Dong S. Mater Lett 2008;62:3240–2. Huang J, Kaner RB. J Am Chem Soc 2004;126:851–5. Mallick K, Witcomb MJ, Erasmus R, Scurrell MS. Mater Sci Eng, B 2007;140:166–71. Lu X, Mao H, Chao D, Zhao X, Zhang W, Wei Y. Mater Lett 2000;61:1400–3.