Applied Surface Science 257 (2010) 1010–1014
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Green chemical functionalization of multiwalled carbon nanotubes with poly(-caprolactone) in ionic liquids Yingkui Yang a,∗ , Shengqiang Qiu a , Chengen He a , Wenjie He a , Linjuan Yu a , Xiaolin Xie b,∗ a b
Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430062, China School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 8 April 2010 Received in revised form 31 July 2010 Accepted 1 August 2010 Available online 6 August 2010 Keywords: Carbon nanotubes Room temperature ionic liquids Poly(-caprolactone) Green functionalization
a b s t r a c t Multiwalled carbon nanotubes (MWNTs) have been successfully functionalized by free radical addition of 4,4 -azobis(4-cyanopentanol) in aqueous media to generate the terminal-hydroxyl-modiﬁed MWNTs (MWNT–OH), followed by surface-initiated in situ ring-opening polymerization of -caprolactone in 1-butyl-3-methylimidazolium tetraﬂuoroborate (BmimBF4 ) to obtain poly(-caprolactone)-grafted MWNTs (MWNT-g-PCL). Spectroscopic methods in conjunction with electron microscopy clearly revealed that hairy PCL chains were chemically attached to the surface of MWNTs to form core–shell nanostructures with the latter as core and the former as shell. With increasing polymerization time from 2 to 8 h, the amount of the grafted-PCL synthesized in BmimBF4 varies from 30.6 to 62.7 wt%, which is clearly higher than that (41.5 wt%) obtained in 1,2-dichlorobenzene under comparable conditions (8 h). The proposed methodology here uses water and room temperature ionic liquids (RTILs) as the reaction media and promises a green chemical process for functionalizing nanotubes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have been proposed for many potential applications in biology , sensors , nanoelectronics and photonics , and high-performance polymer-based nanocomposites  due to their unprecedented physicochemical properties. However, nanosized CNTs are essentially insoluble and have a high tendency to form agglomerates in various solvents and polymer matrices . During the last decade, a variety of strategies have been devoted to improving the solubility and dispersibility of CNTs through covalent or noncovalent functionalization [6–8]. Up to now, functionalization of CNTs has been mainly performed in strong acids (e.g., nitric acid, sulfuric acid, or their mixture) or volatile organic solvents. The acidiﬁcation processing of CNTs is a currently dominant and powerful tool to create the carboxylic groups for subsequent functionalization by amidation or esteriﬁcation . Most of protocols involve the use of organic solvents as the reaction media in order to disperse CNTs and solubilize organic compounds . However, both strong acids and organic solvents often cause problems of environmental pollution, equipment corrosion and health hazard. Therefore, it is imperative to develop an environmentally friendly method to functionalize CNTs.
We report here a green method to functionalize CNTs as shown in Fig. 1. Initially, hydroxyl groups were covalently attached to the surface of multiwalled carbon nanotubes (MWNTs) by onestep free radical addition of 4,4 -azobis(4-cyanopentanol) (ACP) in water. Poly(-caprolactone)-grafted MWNTs (MWNT-g-PCL) was then synthesized by in situ ring-opening polymerization of caprolactone in the presence of MWNT–OH in room temperature ionic liquids (RTILs). The hydroxyl-functionalized CNTs are commonly obtained by three steps in turn [11–13], including oxidation in strong acids, acylation in thionyl chloride, and esteriﬁcation in organic solvents. Herein, only a one-step process for MWNT–OH is required to perform in water. More importantly, RTILs may provide environmentally benign “green” alternatives to volatile organic solvents for chemical synthesis due to their low volatility, non-ﬂammability and high thermal stability . In the present work, functionalization of CNTs using water and RTILs as the reaction media appears relatively facile and green in comparison with the one previously reported in strong acids and organic solvents [15,16].
2. Experimental 2.1. Materials
∗ Corresponding authors. Tel.: +86 27 8866 1729; fax: +86 27 8866 5610. E-mail addresses: [email protected]
(Y. Yang), [email protected]
(X. Xie). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.009
Pristine multiwalled carbon nanotubes (MWNTs) were purchased from Shenzhen Nanotech Port Co., Ltd., China (purity > 95%). 4,4 -Azobis(4-cyanopentanol) (ACP) was purchased from Langfang
Y. Yang et al. / Applied Surface Science 257 (2010) 1010–1014
Fig. 1. Schematic representation for covalent attachment of hydroxyl groups to MWNTs by free radical addition of 4,4 -azobis(4-cyanopentanol) in water and subsequent surface-initiated ring-opening polymerization of -caprolactone in room temperature ionic liquids (RTILs).
Triple Well Chemicals Co., Ltd., China. -Caprolactone (Alfa Asear, 99%) was puriﬁed by distillation after being dehydrated in CaH2 for 48 h before use. 1-Butyl-3-methylimidazolium tetraﬂuoroborate (BmimBF4 ) was purchased from Henan Lihua Pharmaceutical Co., Ltd. (China) and freshly treated for 6 h at 120 ◦ C under reduced pressure prior to use in order to eliminate any traces of water. 1,2-Dichlorobenzene (DCB) was dried over phosphorus pentoxide (P2 O5 ) and distilled before use. Stannous octoate [Sn(Oct)2 ] (Aldrich, 95%) was used without further puriﬁcation. Water was distilled and deionized before use. Other chemicals were purchased from Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai) and used as received unless otherwise stated. 2.2. Synthesis of MWNT-supported macroinitiators (MWNT–OH) in water As shown in Fig. 1, MWNT–OH was easily obtained by free radical addition of water-soluble 4,4 -azobis(4-cyanopentanol) in merely one reaction using water as the reaction medium . The feed ratio of 4,4 -azobis(4-cyanopentanol) to MWNTs was 20:1 in weight. TGA showed 12.2% weight loss at 600 ◦ C for MWNT–OH and hardly any weight loss for pristine MWNTs. The difference in weight loss corresponds to about 1.5 hydroxyl groups per 100 nanotube carbons. IR (cm−1 ): 3435 ( O–H), 2925–2847 ( C–H), 2225 cm−1 ( C≡N), 1580 ( C C), 1398 (ı C–H), and 1044 ( C–O). Raman (excitated at 632.8 nm, cm−1 ): 1326 (D-band), 1573 (G-band) and 1606 (D -band). 2.3. Synthesis of poly(-caprolactone)-grafted MWNTs in RTILs (MWNT-g-PCL) MWNT–OH (12 mg) was dispersed in a solution of caprolactone (2 mL, 18 mmol) and Sn(Oct)2 (0.12 mol% relative to monomer) by bath sonication for 10 min at room temperature. To this solution was added 6 mL of BmimBF4 followed by probesonicating for 5 min. The reaction mixture was continuously stirred for 15 min and then deoxygenated by ﬁve vacuum/nitrogen cycles. The resulting stable suspension was ﬁnally immersed in a preheated oil bath to 130 ◦ C under constant stirring. To determine the progression of the polymerization reaction with time, samples polymerized in BmimBF4 were carried out at 2 h intervals over a total period of 8 h (see Table 1). Upon completion, an eightfold excess of water was added into the ﬁnal reaction mixture in order to separate MWNT-g-PCL and free polymer from the aque-
ous solution of BmimBF4 . The precipitates of grafted MWNTs and free polymer were isolated by ﬁltration, and BmimBF4 could be recovered by distilling the ﬁltrate to remove water as described elsewhere . Finally, MWNT-g-PCL was collected by ﬁltering and washing with acetone for several cycles . The ﬁnal product was dried overnight at 50 ◦ C to afford a black solid. Moreover, free polymer was collected from the ﬁltrate by precipitation in mineral ether. In a control experiment (NT-PCL5), the same polymerization procedure as that of NT-PCL4 was carried out using DCB instead of BmimBF4 as the reaction medium. NT-PCL4 was selected as a representative sample for characterizing the chemical structure of MWNT-g-PCL, IR (cm−1 ): 2950–2865 ( C–H), 1726 ( C O), 1470 (ıas C–H), 1372 (ıs C–H), 1042 ( C–O), and 728 (in-plane rocking vibration of (CH2 )n ). 1 H NMR (600 Hz, CDCl3 ), ı (TMS, ppm): 4.06 (–CH2 O–), 2.31 (–OCCH2 –), 1.65 (–OCCH2 CH2 CH2 CH2 CH2 O–), and 1.38 (–OCCH2 CH2 CH2 CH2 CH2 O–). Raman (excitated at 632.8 nm, cm−1 ): 1327 (D-band), 1574 (G-band) and 1605 (D -band). 2.4. Characterization Fourier transform infrared (FTIR) spectra were recorded on a Bruker Equinox 55 spectrometer with a disc of KBr. Hydrogen nuclear magnetic resonance (1 H NMR) spectra were carried out on a Unity Inova 600 MHz spectrometer (Varian Co., USA) using TMS as an internal standard. Raman spectra were recorded on a Confocal Raman spectrometer (LabRAM HR, Jobin Yvon Co.) using the He–Ne laser excitation at 632.8 nm. Thermal gravimetric analysis (TGA) was conducted on a TGA-7 Perkin-Elmer calorimeter under argon ﬂow (20 mL/min) at a heating rate of 20 ◦ C/min. Transmission electron microscopy (TEM) analysis was performed on a Tecnai G220 electron microscope at 200 kV. Molecular weights and polydispersity index (PDI) were determined using gel permeation chromatography (GPC, Agilent 1100) with polystyrene (PS) as a standard and tetrahydrofuran (THF) as an eluent. 3. Results and discussion Fig. 1 shows a two-step procedure for synthesizing MWNT–OH and subsequent surface-initiated ring-opening polymerization of -caprolactone to yield MWNT-g-PCL. In addition to the derivation of the acid-oxidized CNTs, the CNT-supported macroinitiators have been obtained by either nitrene cycloaddition of organic azides  or radical coupling of aryl diazonium salts . Herein, the use of water as the reaction media to prepare MWNT–OH was carried out by one-step free radical addition of ACP that is commercially available and inexpensive. It has also been reported that CNT bundles can be exfoliated into smaller bundles and even individual nanotubes in RTILs [21,22]. The use of RTILs as the reaction media should herein conduce to the dispersion of CNTs during functionalization. Moreover, the present technique takes full advantage of the poor solubility of MWNT-g-PCL in water-soluble BmimBF4 to perform heterogeneous polymerization, which facilitates the separation of MWNT-g-PCL from BmimBF4 through ﬁltration and washing with water instead of organic solvents . Thereafter, BmimBF4 could be easily recovered from the aqueous ﬁltrate by distillation so as to reuse in new reaction cycles. The amount of the attached organic moieties in the functionalized MWNTs can be determined by thermogravimetric analysis (TGA). Fig. 2 shows the TGA curves of pristine MWNTs, MWNT–OH, and MWNT-g-PCL under nitrogen. MWNT–OH shows 12.2% weight loss at 600 ◦ C (Fig. 2b), corresponding to the decomposition of the attached radicals, while pristine MWNTs give hardly any weight loss under identical conditions (Fig. 2a). The coverage density in MWNT–OH is estimated to be 1.5 hydroxyl groups per 100 nanotube carbons by calculating the weight loss and molecu-
Y. Yang et al. / Applied Surface Science 257 (2010) 1010–1014
Table 1 Experimental conditions and selected results for MWNT-g-PCL. Sample
NT-PCL1 NT-PCL2 NT-PCL3 NT-PCL4 NT-PCL5d
2 4 6 8 8
33.8 45.1 67.4 86.4 52.5
30.6 43.9 57.1 62.7 41.5
2800 4570 7060 7260 5910
1.59 1.77 2.55 2.52 1.55
a The conversion of -caprolactone monomer calculated from the yield of product: (Wa + Wb − Wc )/Wd × 100%, where Wa is the weight of PCL-grafted MWNTs, Wb is the weight of free PCL collected from the reaction solution, and Wc and Wd is the weight of MWNT–OH and monomer, respectively. b The weight fraction of the grafted-PCL calculated from the TGA data at 600 ◦ C. c The number average molecular weight of free PCL collected from the reaction solution. d NT-PCL5 was prepared using DCB instead of BmimBF4 as the reaction medium.
lar weight of organic fragments. After covalent attachment of OH onto MWNTs, PCL chains were grown from the surface of MWNT–OH by ring-opening polymerization of monomer in the presence of Sn(Oct)2 as catalyst. TGA traces of MWNT-g-PCL give considerable weight loss between 200 and 600 ◦ C, corresponding to the decomposition of PCL covalently attached to MWNTs. As the polymerization time increases from 2 to 8 h, the conversion of monomer-to-polymer varies from 33.8 to 86.4%; accordingly, the amount of the grafted-PCL in MWNT-g-PCL increases from 30.6 to 62.7 wt% (Fig. 2 and Table 1) due to the grafted-PCL chains growth from the surface of MWNTs with increasing reaction time. Also, the number average molecular weight (Mn ) of free PCL collected from the reaction solution increases from 2800 to 7260 with increasing conversion ratio of monomer. Therefore, PCL-grafted MWNTs with different composition can be obtained by controlling the reaction time to some extent. In previous work, we have found that the amount of the grafted-polymer on the supported-surface can be controlled by adjusting the feed ratio of different monomers  or monomer to the supported-macroinitiator . In the control experiment, NT-PCL5 was synthesized by the same polymerization process as that of NT-PCL4 but using the organic solvent DCB instead of BmimBF4 . Some publications have demonstrated that the reactivity ratios of monomer are not independent of the reaction medium. The polarity and viscosity of solvent, interactions between solvent–monomer, solvent–initiator, and solvent–catalyst as well as heterogeneity of reaction system are all found to have some effect on the polymerization parameters [25–28]. It has been proposed that polymerization rate and molecular weight of polymers synthesized in ionic liquids are generally higher in comparison with those obtained in most organic solvents due to the high viscosity and polarity of the former [29–31]. In the
present work, the monomer conversion, the grafted-PCL amount and the Mn of free polymer in the case of NT-PCL4 are clearly higher than those obtained in the preparation of NT-PCL5 (see Table 1). This ﬁnding is possibly due to higher viscosity and polarity of BmimBF4 , stronger complexation between BmimBF4 and catalyst and better dispersibility of nanotubes when using BmimBF4 as solvent for polymerization compared to DCB [29–35]. It should be mentioned that some or even negative effects on the polymerization can be observed in the presence of ionic liquids as solvents . It herein seems that the use of BmimBF4 as green solvent is highly advantageous to functionalize MWNTs with polymers. FTIR spectroscopy was used in conjunction with NMR to characterize CNTs before and after covalent functionalization. Fig. 3 shows the FTIR spectra of pristine MWNTs, MWNT–OH and NT-PCL4 after KBr contribution subtraction. The peak at ∼1580 cm−1 for MWNTs (Fig. 3a) is assigned to the stretching mode of the benzenoid backbone. Covalent attachment of ACP to MWNTs can be conﬁrmed by emergence in the MWNT–OH spectrum (Fig. 3b) of several vibrational bands centered at 3435 cm−1 ( O–H), 2925–2847 cm−1 ( C–H), 1398 cm−1 (ı C–H), and 1044 cm−1 ( C–O). Moreover, a new weak peak at 2225 cm−1 corresponds to the stretching vibration of the remaining nitrile group (C≡N) in MWNT–OH. After grafting, the characteristic bands for PCL are clearly seen in the NT-PCL4 spectrum (Fig. 3c). The strong signal at 1726 cm−1 is contributed by the carbonyl group ( C O) of the ester moiety in the graftedPCL chains. The presence of ester bond can also be veriﬁed by the appearance of the C–O stretching at 1180 and 1042 cm−1 . It is noted that the infrared absorptions for the hydrocarbon chains of the grafted-PCL become more prominent when compared to those of MWNTs and MWNT–OH. Accordingly, the peaks at 2950 and 2865 cm−1 are attributed to the C–H antisymmetric and symmetric
Fig. 2. TGA curves of pristine MWNTs, MWNT–OH, and MWNT-g-PCL.
Fig. 3. FTIR spectra of pristine MWNTs (a), MWNT–OH (b) and NT-PCL4 (c).
Y. Yang et al. / Applied Surface Science 257 (2010) 1010–1014
H NMR spectrum of NT-PCL4 in CDCl3 .
stretch, 1470 and 1372 cm−1 to the C–H antisymmetric and symmetric bend, respectively. Moreover, the in-plane rocking vibration of the concerted (CH2 )n chain also appears at 728 cm−1 . The chemical structure of NT-PCL4 can be further conﬁrmed by its 1 H NMR spectrum in CDCl3 . As shown in Fig. 4, the proton signals occur at 4.06 (–CH2 O–), 2.31 (–OCCH2 –), 1.65 (–OCCH2 CH2 CH2 CH2 CH2 O–), and 1.38 ppm (–OCCH2 CH2 CH2 CH2 CH2 O–), corresponding to the labels of a, b, c, d in turn. These results prove that PCL chains are covalently grafted onto the surface of MWNTs. Raman technique has been proved to be an excellent tool to examine CNTs. Fig. 5 illustrates the Raman spectra of pristine MWNTs, MWNT–OH and NT-PCL4 with excitation at 632.8 nm using the He–Ne laser. The peaks centered at ∼1330 cm−1 (D-band) and ∼1610 cm−1 (D -band) are attributable to the functional defects of sp3 -hybridized carbons and disordered structures in the hexagonal framework, while the high frequency peak at ∼1570 cm−1 (G-band) corresponds to the sp2 -hybridized carbon atoms of CNTs (in-plane E2g stretching mode of graphite) . Therefore, the degree of functionalization for CNTs can be evaluated by the relative intensity ratio of ID /IG , which is calculated from the Lorentzian
Fig. 5. Raman spectra of pristine MWNTs (a), MWNT–OH (b), and NT-PCL4 (c).
ﬁtting areas of D- and G-band as described elsewhere . Accordingly, the ID /IG ratios for pristine MWNTs, MWNT–OH, and NT-PCL4 are 1.21, 1.56, and 1.55, respectively. It is clear that MWNT–OH has a higher ratio of ID /IG than that of pristine MWNTs. This ﬁnding is related to the attachment of functional groups to the nanotube lattice upon addition of azo initiators, leading to an increased number of the sp3 -hybridized carbons as a result of a higher disorder degree. The ID /IG ratio for NT-PCL4 is almost equal to that of MWNT–OH, which reveals that the graft polymerization is indeed initiated from the reactive sites of hydroxyl groups on the surface of MWNT–OH, thereby retaining the electron hybridization of the nanotube wall. The Raman signals of NT-PCL4 are relatively weakened with reference to those of MWNTs and MWNT–OH, as reported previously
Fig. 6. TEM images of pristine MWNTs (a), MWNT–OH (b), and NT-PCL4 (c and d).
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. However, all the spectra demonstrate an analogous proﬁle, indicating that the inherent graphite structure of MWNTs remains intact after free radical addition of ACP and subsequent grafting of PCL. As is well known, CNTs are promising building blocks for highperformance composites. An obstacle to facilitate their practical applications is how to better disperse and exfoliate them in solvents or polymer matrices . It is often difﬁcult to disperse pristine CNTs into solvents to obtain a stable suspension even after their mixture is subjected to ultrasonication. However, MWNT-g-PCL is well soluble in chloroform, dichloromethane, acetone and other organic solvents that are compatible with pure PCL. This observation proves that the hairy PCL chains break up the nanotube bundles to either individual or smaller aggregates, and accelerate the solubility and dispersity of MWNTs in solvents with the aid of the grafted-polymer chains [23,39]. Direct evidence for covalent functionalization with polymers can be clearly determined by TEM images. As shown in Fig. 6, the surface of MWNT–OH (Fig. 6b) seems to be smooth and clear similar to that of pristine MWNTs (Fig. 6a), due to the attachment of low-molecular-weight groups. A core–shell nanostructure is formed after grafting of PCL to the MWNT surface where MWNT acts as the core and PCL as the shell (Fig. 6c). The thickness of the shell layer is further estimated to be in the range of 6–8 nm according to the high resolution TEM image (Fig. 6d). It is widely accepted that incorporating CNTs into the polymer matrix leads to the great improvement of the thermal and mechanical properties of the latter. However, PCL is a class of aliphatic biodegradable polyester whose glass temperature, melting point and tensile strength are relatively low , and the PCL grafted onto MWNTs can be completely enzymatically degraded similar to the biodegradable properties of free PCL . Therefore, MWNT-g-PCL is expected to have potential applications in biomaterials, biomedicine, and artiﬁcial bones. 4. Conclusions Biodegradable poly(-caprolactone)-functionalized MWNTs has been successfully performed by a two-step process. In the ﬁrst step, the MWNT-supported macroinitiators (MWNT–OH) were synthesized in aqueous media by free radical addition of commercially available 4,4 -azobis(4-cyanopentanol) through a “grafting-to” approach. In the next step, poly(-caprolactone)grafted MWNTs (MWNT-g-PCL) was obtained by surface-initiated ring-opening polymerization of -caprolactone via a “graftingfrom” process in BmimBF4 . Spectroscopic methods (FTIR, NMR and Raman) were combined to identify the presence of covalent linkage between polymer chains and MWNTs. Core–shell nanostructures with nanotube as the hard core and PCL as the soft shell are clearly observed after grafting of PCL onto MWNTs. When increasing reaction time to 8 from 2 h, the relative amount of the grafted-PCL in BmimBF4 varies from 30.6 to 62.7 wt%, which is higher than that obtained in DCB under identical conditions. This ﬁnding implies that the chemical composition of polymergrafted nanotubes could be controlled by adjusting the reaction parameters (time and solvents). More importantly, the use of BmimBF4 as solvent cannot only improve the dispersibility and stability of nanotubes during polymerization, but also enhance the functionalization effectiveness as a result of higher polymerization rate and amount of the grafted-polymer compared to general organic solvents. In addition, previous strategies to functionalize nanotubes mainly use various acids and organic solvents, which
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