Hierarchical Hybrids of Carbon Nanotubes in Amphiphilic Poly

Feb 22, 2013 - Moreover, the BCPs were employed to noncovalently modify multiwalled carbon nanotubes (MWNTs) through either the direct or indirect ...
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Hierarchical Hybrids of Carbon Nanotubes in Amphiphilic Poly(ethylene oxide)-block-polyaniline through a Facile Method: From Smooth to Thorny Zhifang Yang,† Zhigang Xue,† Yonggui Liao,*,†,‡ Xingping Zhou,† Jinqiang Zhou,† Jintao Zhu,† and Xiaolin Xie†,‡ †

Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡ Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *

ABSTRACT: A facile approach was developed to synthesize conjugated block copolymer (BCP) poly(ethylene oxide)-b-polyaniline (PEO−PANI). Aldehyde group-terminated PEO was prepared by an esterification reaction of pformylbenzoic acid and PEO and then reacted with PANI from chemical oxidative polymerization. FT-IR, 1H NMR, and GPC results indicated that BCPs with different PEO block lengths were successfully synthesized. Moreover, the BCPs were employed to noncovalently modify multiwalled carbon nanotubes (MWNTs) through either the direct or indirect method. In the former method, transmission electron microscopy images showed that a core− shell MWNT@BCP hybrid with a shell thickness of gyration diameter of PEO block (2Rg,PEO) was obtained in 1-methyl-2-pyrrolidone (NMP). These hybrids can be well dispersed in many common solvents and poly(vinyl alcohol) matrix. With the increase of PEO block length, the stability of the MWNT dispersion would be highly improved. Interestingly, in the indirect method where deionized water was added to the NMP solution of BCP/ MWNT mixture, the surface of the hybrid micelles encapsulated with MWNTs changed from smooth into hierarchically thorny with the increase of BCP/MWNT weight ratio. In this case, the water contact angle had a minimum value of ∼70° at the ratio of 1/8, indicating that the hierarchical thorns followed a Cassie−Baxter regime rather than a Wenzel one. A possible formation mechanism of the unique structure was also proposed.

1. INTRODUCTION Polyaniline (PANI), one of the most important conjugated polymers, has widely been used ranging from sensors, transistors, displays, supercapacitors, and actuators to corrosion inhibitors, microwave absorbers, and electromagnetic interference shielding materials.1 Because of the poor processability of PANI, either oligo-anilines and their block copolymers (BCPs)2 or BCPs containing PANI3 have been attracted much attention during the past decades. However, some intrinsic properties of PANI for the former can be highly suppressed because of its much shorter conjugated structure. On the other hand, although some sulfonate or dodecyl side groups can improve PANI’s solubility, they will partially disrupt the conjugated structure of PANI as well.3a,b BCPs containing PANI without any side groups should be a good choice to overcome the above problem. Unfortunately, these BCPs have rarely been synthesized due to PANI’s ambiguous structure, variable oxidation state, and insolubility in most solvents, which result in a quite difficult postsynthesis processing and a copolymer topology diversity, i.e., diblock or comb-like copolymer.3c,d To the best of our knowledge, only the method of the chemical oxidative polymerization of aniline in the presence of an © 2013 American Chemical Society

aniline-terminated polymer has been presented to obtain the well-defined BCPs containing PANI without any side groups.4 The palladium-catalyzed Buchwald−Hartwig approach should be a possible route in which soluble tert-butoxycarbonyl (Boc)protected aniline is used as a monomer and Boc is removed by thermolysis or protonolysis after copolymerization.5 However, this approach is too complicated to obtain a BCP as yet. In addition, due to their extraordinary mechanical, thermal, and electrical properties, carbon nanotubes (CNTs) have made promising applications on electrochemical capacitors, sensors, actuators, solar cells, etc.6 However, the insolubility and infusibility of as-synthesized pristine CNTs arisen from their strong intertube van der Waals interactions have become an obstacle.7 Although the covalent method has been proved to be an efficient way to solve this problem, it is apt to disrupt the πconjugation structure of CNTs.8 On the contrary, the noncovalent functionalization method has been well recognized as a “nondestructive” way due to the aggregates from surfactant Received: October 29, 2012 Revised: February 20, 2013 Published: February 22, 2013 3757

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Scheme 1. Synthetic Routes of PEO-b-PANI Diblock Copolymer

modification, polymer wrapping, BCP absorption, and π−π interaction.8a BCP has been proved to be a good agent to disperse CNTs in a selective solvent. The solvophobic blocks are anchored to CNT surface, while the solvophilic blocks are dangling into solvent to form a steric hindrance for the aggregation of CNTs.9 The dispersibility of BCPs depends on both the attached density of solvophobic block on CNTs and the length of solvophilic block. The CNTs dispersed by BCPs are often with functions in anticancer drug delivery,10 temperature-responsive reversible dissolution and aggregation,11 flexible transparent conductivity,12 quantum effects,13 etc. In the past few years, a series of BCPs containing conjugated moieties including hexa-p-phenylene,14 naphthalene side group,15 large pyrene group,16 poly(3-hexylthiophene) (P3HT),17 PEDOT,18 etc., have been focused on enhancing the interaction between CNTs and solvophobic blocks. As a result, these conjugated copolymers become efficient dispersants for CNT in many solvent media and polymer matrices. Meanwhile, their microstructures and properties can be precisely tuned by both the interaction of CNTs/conjugated blocks and the microphase separation of BCPs. In this report, we have designed a facile approach to synthesize BCPs of PEO-b-PANI. It is expected that PANI block adsorbs on CNTs surface by π−π interaction and doping effect19 while PEO block is dangling outside and offers the steric hindrance to homogeneously disperse CNTs in water, organic solvents, and polymer matrix, respectively. Moreover, the microstructures and hydrophilicity of these hybrids have been investigated.

chemicals were used as received from Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China. PANI was synthesized by the oxidative polymerization of aniline in 1.0 mol/L aqueous hydrochloric acid with ammonium peroxydisulfate as oxidant followed by treatment with dilute ammonium hydroxide solution to obtain PANI emeraldine base (PANI-EB) according to the literature.20 2.2. Synthesis of PEO-b-PANI Diblock Copolymers. Scheme 1 illustrates the synthetic route of PEO-b-PANI. Formyl-capped PEO (PEO-CHO) was obtained by an esterification of PEO and pformylbenzoic acid. PEO-CHO was then reacted with the terminal primary amino group of the fully reduced PANI to form BCPs. Synthesis of PEO-CHO. Typically, dried PEO (22.05 g of PEO550, 40 mmol or 20.00 g of PEO5000, 4.0 mmol), p-formylbenzoic acid (15.01 g, 100 mmol), DCC (20.60 g, 100 mmol), and DMAP (0.63 g, 5 mmol) were dissolved in 250 mL of dried dichloromethane in a flask. The mixture was stirred under a nitrogen atmosphere at 40 °C for 48 h, and then 10 mL of acetic acid was added. After 10 min, the mixture was filtered and the precipitate was rinsed with dichloromethane. The combined filtrates were concentrated under reduced pressure and precipitated in 3-fold diethyl ether followed by filtering. The filtrate was concentrated and precipitated in excess diethyl ether more than 10-fold. Formyl-capped PEO (PEO550-CHO, 20.62 g, yield 73.6%; PEO5000-CHO, 18.77 g, yield 91.4%) was obtained by decanting the supernatant and drying in a vacuum at 30 °C for 24 h. Synthesis of BCP. First, PANI emeraldine base (PANI-EB) was purified by washing with ethanol and THF and reduced to leucoemeraldine base (PANI-LB) by excess hydrazine hydrate under a nitrogen atmosphere. 0.50 g of PANI-LB (0.022 mmol) was dried in vacuum at 50 °C, and 1.00 g of PEO-CHO, i.e., 1.47 mmol of PEO550-CHO or 0.19 mmol of PEO5000-CHO, was dissolved in 20 mL of degassed NMP in a flask. Then, 2 mL of trimethyl orthoformate (18.3 mmol) was added. The mixture was stirred at 40 °C for 48 h under a nitrogen atmosphere. Sodium cyanoborohydride (0.10 g, 1.6 mmol) was added and stirred for another 24 h. An equal amount of water was added, and the mixture was dialyzed against water with 8000−14 000 Da. After the solvent was removed by rotary evaporation, the blue sample was dried in vacuum at 50 °C. 2.3. Dispersion of MWNT in Solvents and PVA. MWNT was purified by refluxing in 6 mol/L hydrochloric acid for 12 h instead of concentrated nitric or sulfuric acid which can retain the conjugated structure of MWNT surface.21 Then, the purified MWNT and BCP were dispersed in solvents by the direct method and indirect one, respectively. In the direct method, the purified MWNT (10 mg) and BCP (10 mg) were dissolved in 10 mL of NMP. The mixture was ultrasonicated at 160 W, 43 kHz for 2 h and centrifuged at 4000 rpm for 20 min. The supernatant was filtered, and the precipitate was washed with excess NMP until the filtrate became colorless. The black powder was collected and referred as MWNT@BCP. The MWNT@ BCP dispersion in water and several organic solvents such as THF, DMF, and NMP was obtained by ultrasonic treatment for 10 min. In

2. EXPERIMENTAL SECTION 2.1. Materials. Pristine multiwalled carbon nanotubes (MWNTs, purity >95%, with diameter of 20−40 nm and length of 5−15 μm) were purchased from Shenzhen Nanotech Port Co., Ltd., China. Poly(ethylene oxide) monomethyl ethers (PEO, Aldrich Chemical Co., PEO with Mn = 550 and 5000 were denoted as PEO550 and PEO5000, respectively) were dried by azeotropic distillation with dry toluene before used. p-Formylbenzoic acid was generously supplied from Beijing Prina Chemical Industry Co., Ltd., China. Dichloromethane (99%, Acros) was dried over calcium hydride overnight and distilled. 1-Methyl-2-pyrrolidone (NMP) was degassed by bubbling with argon for 30 min before used. Sodium cyanoborohydride (95%) and trimethyl orthoformate (99%) were purchased from Acros and used as received. Poly(vinyl alcohol) (PVA with hydrolysis of 98%, Mw = 2400) was obtained from Kuraray (Japan) and used without further purification. N,N′-Dicyclohexylcarbodiimide (DCC), 4(dimethylamino)pyridine (DMAP), hydrazine hydrate, and other 3758

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the indirect method, all procedures for the purified MWNT and BCP with an appropriate weight ratio were the same as in the direct one except that 20 mL of deionized water was added dropwise and stirred for another 12 h between ultrasonication and centrifugation. The MWNT/PVA films with a MWNT content of 0.1 wt % were prepared by casting the aqueous solution of MWNT@BCP (0.05 mg/ mL) and PVA (5 wt %) on a glass slide and then evaporating at 50 °C. In a control experiment, MWNT@PANI was prepared in the similar way and dispersed in solvents and PVA film. 2.4. Characterization. FT-IR spectra were recorded on an Equinox 55 spectrometer (Bruker) using the KBr pellet technique. 1 H NMR spectra were obtained on a Bruker AV400 spectrometer using tetramethylsilane (TMS) as internal standard and deuterated dimethyl sulfoxide (d-DMSO) as solvent. UV−vis spectra measurements were carried out on a Shimadzu UV-2550 diode array spectrophotometer. Transmission electron microscopy (TEM) was performed on a Tecnai G220 electron microscope (PEI Co., Netherlands) at an accelerating voltage of 200 kV. The specimens for TEM examination were prepared by depositing a drop of aqueous solution on carbon-coated copper grids. The optical microscopy images were taken with a digital camera (Olympus C-4000 ZOOM). Gel-permeation chromatography (GPC) was taken out on an Agilent 1100 gel permeation chromatograph with refractive index detector, 0.35 mL/min flow rate, 60 °C column temperature, and polystyrene (PS) as the standard. 1-Methyl-2-pyrrolidinone (NMP) containing 0.5 wt % lithium chloride (NMP/LiCl) was used as the eluant. The polyaniline and BCPs (1 mg) were dissolved in NMP/LiCl (2 mL, containing 0.5 wt % LiCl) and then passed through a 0.45 μm filter prior to injection.21a Water contact angles (CA) were measured on a contact angle goniometer (DSA100, KRUSS) by the sessile drop technique. In each experiment, a drop of 3.0 μL water was placed at room temperature on the sample surface, and the measurements were carried out three times by the tangent method. The MWNT@BCP and MWNT@PANI were deposited on a glass plate from their aqueous dispersion and dried under vacuum at 50 °C for 24 h before examination.

Figure 1. FT-IR spectra of PEO550-CHO (a), PANI (b), BCP550 (c), and BCP5000 (d).

Figure 2. 1H NMR spectra of PEO550-CHO (a) and BCP550 (b).

PANI and the absorption bands of PEO, such as the C−H band at 3000 cm−1, the CO band at 1724 cm−1, and the C−O band at 1105 cm−1. Furthermore, these absorption bands increased with the increase of PEO block length. Although the 1 H NMR spectrum in Figure 2b appears to have some rough signals due to the solubility of BCP and structural complexity of PANI, the aromatic and aliphatic protons at 6.5−7.5 and 3.61 ppm can be observed clearly. The disappearance of the proton signal of −CHO at around 10 ppm and a new proton signal from the connection between PEO and PANI at 4.37 ppm suggest that PEO−CHO has reacted with PANI. Namely, the BCPs have been successfully synthesized. In the GPC experiments, NMP containing 0.5 wt % LiCl was used as the solvent and eluant since LiCl can block the hydrogen bond formation between interchains/intrachains of PANI EB molecules and improve EB solubility in NMP.21a Both molecular weights of BCP550 (Mn = 26 500, Mw/Mn = 2.29) and BCP5000 (Mn = 32 700, Mw/Mn = 1.91) were larger than that of PANI (Mn = 23 000, Mw/Mn = 2.08), which further confirmed the successful synthesis of BCP (see Figure 1S in the Supporting Information). A molecular weight increment larger than the molecular weight of single PEO chain may be originated from more than one primary amine groups on a PANI chain.21a Namely, taking the molecular weight of linker between PEO and PANI blocks into acount, about 5.25 and 1.90 PEO blocks have been connected to each PANI block for PEO550-PANI and PEO5000-PANI BCPs, respectively. 3.2. Dispersion of MWNT in Solvents and PVA Film with BCP Using a Direct Method. The dispersion stabilities of MWNT in water without and with PANI and/or PEO or BCPs were observed by visual observation after ultrasonication (Figure 3a−f). The pristine MWNT dispersion began to

3. RESULTS AND DISCUSSION 3.1. Synthesis of PEO-b-PANI Diblock Copolymers. Although there are some defective and branched structures,22 PANI synthesized by ammonium peroxydisulfate as an oxidant in strong acid is generally recognized as a linear para-coupled aniline chain with a N-phenylphenazine head22a and more than one primary amine group.21a Here, we developed a new method to prepare BCP containing PANI by an imidization reaction of primary amine, as displayed in Scheme 1. PEOCHO was synthesized by an esterification between pformylbenzoic acid and PEO in the presence of DCC and DMAP. PEO and PANI were then connected via an imidization reaction between capped formyl and primary amino groups. Notably, the terminated amine of PANI was necessary to be fully reduced to primary amine before the imidization. Finally, the imine was reduced to secondary amine. For PEO550-CHO, the FT-IR spectrum in Figure 1a obviously shows the C−H absorption peaks of the formyl group at 2745 and 2695 cm−1 and the CO absorption peak at 1724 cm−1. The 1H NMR spectrum in Figure 2a confirmed the proton signals (a and b) of −CH2CH2O− and −OCHH3 in PEO at 3.61 and 3.51 ppm as well as the proton signals (e, f, and g) of benzene ring and formyl group at 8.00 and 10.06 ppm. Interestingly, two proton signals (c and d) of methylene were also observed, which implied the covalent connection of p-formylbenzoide acid and PEO. In short, both spectra results proved the successful synthesis of PEO550-CHO. Compared to PANI (Figure 1b), the BCPs of BCP550 (Figure 1c) and BCP5000 (Figure 1d) presented both the characteristic band of 3759

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Figure 3. Optical images of 1 mg/mL aqueous dispersions of pristine MWNT (a), MWNT@PANI (b), MWNT/PEO5000 (c), MWNT/ PEO5000/PANI (d), MWNT@BCP550 (e), and MWNT@BCP5000 (f); 0.1 mg/mL MWNT@BCP5000 aqueous dispersions (g); and 1 mg/mL MWNT@BCP5000 dispersion in NMP (h), DMF (i), and THF (j). All photos were taken at standing time of 7 days after ultrasonication.

Figure 4. UV−vis absorption spectra of MWNT@BCP5000 aqueous dispersion with different concentrations. The arrow direction shows the decreasing direction of concentration. Inset: absorption intensity at 400 nm (squares) and 800 nm (circles) vs the concentration.

precipitate immediately once ultrasonic treatment was stopped, and it completely settled on the bottom of vial within several minutes as shown in Figure 3a. The MWNT@PANI dispersion became unstable within 2 h after ultrasonication, and MWNT@ PANI was precipitated out after 12 h (Figure 3b). Although many efforts have been done to disperse carbon nanotubes into the PEO matrix, most of them used some surfactants,23 including lithium/sodium dodecyl sulfate, suberic acid or Gum Arabic, dodecyltrimethylammonium bromide, etc. Abraham and co-workers24 prepared PEO/MWNT nanocomposites without any additives using a mechanical probe sonicator. Although the authors claimed that the nanotubes were well dispersed in the matrix at a MWNT loading of 0.4 wt %, some aggregates of MWNT could be observed from the SEM image of cryofractured sample of PEO/MWNT composite. The composite at a MWNT concentration of 2 wt % has many agglomerates. Namely, the loading of MWNT in a homogeneous MWNT/PEO composite without the aid of additives was very low. In this case, MWNT was precipitated out after 24 h in PEO5000 aqueous solution (Figure 3c) and in PEO5000/ PANI one (Figure 3d). In contrast, the ink-like dispersions were found for MWNT@BCP dispersions (Figure 3e,f). The MWNT@BCP5000 solution was still stable and without any sedimentation for several weeks, while there was some black sedimentation on the bottom of vial despite the supernatant retained black for MWNT@BCP550. The diluted MWNT@ BCP5000 dispersion with a concentration of 0.1 mg/mL was brown and transparent (Figure 3g), implying the homogeneous nature of the solution. This suggests that this type of BCP is an efficient dispersing agent for carbon nanotubes, wherein solvophobic PANI block might be adsorbed on the surface of nanotube, while solvophilic PEO block stretches into water to offer the steric hindrance for stabilizing the dispersion of CNTs. Furthermore, the longer the block length of PEO, the larger the repulsive energy is. For MWNT@BCP550, the PEO block length was too short to supply enough steric hindrance to stabilize the dispersion. MWNT@BCP5000 could be well dispersed in many organic solvents, such as NMP, DMF, and THF (Figure 3h−j). The stabilizing mechanism in DMF and THF was similar to that of water. Notably, although NMP was a good solvent for both PANI and PEO in this case, the interaction between MWNT and PANI may be stronger than that between PANI and NMP, inducing PANI to attach on the surface of MWNT and to disperse it. In addition, the UV−vis absorption spectrum was used to estimate the stability of MWNT dispersion (Figure 4). These curves were similar and featureless, as other functionalized MWNTs.25 The absorption intensity obeyed Beer’s law and was linearly dependent on the concentration of MWNT@BCP5000, which indicated that

there was no optical behavior typically caused by the aggregation of MWNT@BCP5000 species.26 TEM is a powerful tool to observe the morphology of materials at nanometer scale in real space. Compared with the smooth surface of pristine MWNT (Figure 5a), a thin polymer

Figure 5. TEM images and water contact angles (insets) of samples prepared by the direct method: pristine MWNT (a), MWNT@PANI (b), MWNT@BCP550 (c), and MWNT@BCP5000 (d).

layer with 1−2 nm was coated on MWNT for MWNT@PANI composites (Figure 5b), which confirmed that PANI was attached to the MWNT surface via the strong interaction between PANI and MWNT by specific π−π interaction and doping interaction.19 The coating layer thicknesses were 1−2 nm for MWNT@BCP550 (Figure 5c), which was similar to MWNT@PANI, and 3−4 nm for MWNT@BCP5000 (Figure 5d). Assuming that the plane of aniline units lies on the surface of MWNT through strong π−π interaction, the coating layer thickness outside MWNT should be mainly contributed from PEO blocks. Because the “grafting” density of PEO block is low enough, each PEO block can be considered as a “mushroom” on the MWNT surface. Its swollen thickness can be estimated 3760

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from the corresponding unperturbed radius of an equivalent freely jointed chain27 Rg = b

Mn 6M 0

(1)

where the statistical Kuhn’s segmental length b = 11 Ǻ and molar mass of Kuhn monomer M0 = 137 for PEO, and their number-average molecular weights Mn for PEO blocks were taken 550 and 5000, respectively. In addition, the thickness of a dried “mushroom” in the vacuum during the observation of TEM was usually about 40% of the diameter (2Rg) of swollen one.28 As a result, the estimated dried thicknesses were ∼7.2 and 21.7 Ǻ for PEO550 and PEO5000, respectively, which were in good agreement with those obtained from TEM observation. Namely, there was only about a monolayer of BCP on the MWNT surface. This assumption was further confirmed by water contact angle measurements shown in the insets of Figure 5. The contact angles for the hydrophobic pristine MWNTs with graphitic surface, MWNT@PANI, and MWNT@BCP were 149°, 142°, and ∼130°, respectively. These results demonstrated that the hydrophilic PEO block was on the surface of coated MWNTs. However, the coating thicknesses of MWNT@PANI and MWNT@BCP were too thin to behave the exact hydrophilicity as pure homopolymers of PANI (∼65°) and PEO (∼30°).29 The dispersion of CNTs plays a role key in improving the mechanical, thermal, optical, and electrical properties of CNTreinforced polymer nanocomposites.30 Figure 6 shows optical

Figure 6. Optical images of PVA films containing 0.1 wt % MWNT@ PANI (a), MWNT@BCP550 (b), and MWNT@BCP5000 (c).

microscopy images of PVA films containing 0.1 wt % MWNT@ PANI, MWNT@BCP550, and MWNT@BCP5000. MWNT aggregates can be observed as dark spots in the PANIdecorated system (Figure 6a). The diameter of the clusters was ca. 0.5−30 μm. On the other hand, homogeneous films can be achieved for MWNT@BCP/PVA composites (Figure 6b,c) with the same content of fillers. In the assistance of BCP550, the diameter of the dark particles was much smaller than that of PANI-decorated system (Figure 6b). It is noticeable that the MWNT@BCP5000/PVA film is homogeneous and transparent (Figure 6c). These results suggest that this type of BCP is an efficient dispersing agent for CNTs where PANI conjugated block attaches on CNTs and water-soluble PEO block provides a miscible interface between polymer and CNTs. This welldispersed composite film opens a green and environmentally friendly window for the fabrication of optical and electrical devices, sensors, etc. 3.3. Noncovalent Modification of BCP for MWNTs Using an Indirect Method. In the indirect method, the noncovalent modification of BCP for MWNTs was different from that in the direct method. Figure 7a−f shows TEM images for samples prepared by the indirect method at different weight

Figure 7. (a−f) TEM images for samples prepared by the indirect method at BCP5000/MWNT weight ratios of 1/32 (a), 1/16 (b), 1/8 (c), 1/4 (d), 1/2 (e), and 2/1 (f). (g) TEM image for MWNT@PANI (1/2, w/w) prepared by the indirect method. (h) Contact angle for samples prepared by the indirect method as a function of copolymer/ MWNT ratio. The square, circle, up-triangles, and down-triangles represent pristine MWNT, MWNT@PANI (1/2, w/w), MWNT@ BCP550, and MWNT@BCP5000, respectively.

ratio of BCP5000/MWNT. At the ratio of 1/32, the surface of MWNT was as clear and smooth as the pristine one (Figure 7a). In Figure 7b, an ultrathin coating layer with a thickness of about 1−2 nm could be observed at the ratio of 1/16. The thickness of the coating layer increased to 2−3 and 4−5 nm at the ratio 1/8 (Figure 7c) and 1/4 (Figure 7d), respectively. However, when the ratio increased further, the MWNT@BCP aggregate was thorny (Figure 7e,f), which was similar to the nanohybrid shish kebabs obtained from the crystalline homopolymers31 and BCPs containing crystalline blocks13a,31b 3761

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from water in order to stabilize the MWNT@BCP hybrid. Second, although the strong π−π stacking and doping effect19 between MWNT and PANI block can greatly compensate for the conformational entropy loss of PANI block, a part of PANI blocks cannot compact with MWNT surface as tight as ideal, even may coil or fold on the defects. And last but not least, the PEO-neighbored aniline repeat units tend to leave MWNT surface because of the strong interaction between the solvent and PEO block. When the amount of BCP is enough, the external layer of MWNT@BCP hybrid prepared by the indirect method is a layer of PEO. At this time, the dominant effect on the hydrophilicity of deposited hybrid is its microstructure. Compared with the classic Young state for a flat surface, Wenzel32 and Cassie−Baxter33 have successfully described the hydrophilicity of rough surface for a homogeneous solid/liquid interface and a heterogeneous one like the existence of some air bubbles, respectively. At the Wenzel state, the contact angle (θW) follows the equation32

on the surface of nanotubes, and even some isolated aggregates of the self-assembly of BCPs could be found at the higher ratios like 2/1 (Figure 7f). The size and morphology of these isolated aggregates were similar to the assemblies of neat BCP in solution (see Figure 2S). The thorny microstructure was different from the very smooth MWNT@PANI at the same ratio of 1/2 with a thickness of ∼15 nm (Figure 7g). The morphology evolution of MWNT@BCP550 was similar to that of MWNT@BCP5000 (see Figure 3S). This morphology evolution was further confirmed by the hydrophilicity from the contact angle measurement in Figure 7h. With the increasing of BCP/MWNT weight ratio, the contact angle showed a minimum value of 60° for MWNT@BCP550 and 69° for MWNT@BCP5000 at the ratio of 1/8, respectively, which was close to 70° for smooth MWNT@PANI (1/2, w/w). The hydrophilicity of surface is dependent not only on chemical compositions but also on microstructures. In this case, when the amount of BCPs is not enough to wrap MWNTs completely for regime I with low ratio of BCP/MWNT, the surface of hybrids is flat and contains both BCPs and part of MWNTs, so that the chemical composition is the dominant effect. On the other hand, for regime II with high ratio of BCP/ MWNT, all MWNTs are encapsulated into BCPs. At this time, the dominant effect is its rough microstructure. The critical transition from regime I to regime II can be estimated as follows. Assuming that a monolayer of BCP whose benzene ring and nitrogen atom in PANI block are coplanar closely aligns on the surface of MWNT through π−π stacking and doping effect,19 this critical transition ratio, (WBCP/WMWNT)crit, can be written as ⎛ WBCP ⎞ AMWNTM n,BCP/(NÃ NPANIAANI) ⎜ ⎟ = ρMWNT VMWNT ⎝ WMWNT ⎠crit

cos θ W = r cos θY

where θY is the contact angle at Young state and r the roughness ratio, i.e., the actual area of surface to its projected area. Obviously, due to r > 1, the Wenzel state can always amplify the hydrophilicity of solid substrate; i.e., θW is smaller than θY for the hydrophilic surface or larger for the hydrophobic one. In this work, due to the hydrophilicity of both PEO and PANI, the MWNT@BCP hybrid should be more hydrophilic if it followed the Wenzel state. At the Cassie− Baxter state, the contact angle (θCB) follows the equation33 cos θCB = φ(r cos θY + 1) − 1

Scheme 2. Schematic Illustration of the Formation of Hierarchical MWNT@BCP Thorn

(3)

where ϕMWNT is the average diameter of MWNT (∼30 nm). The average length and width of aniline repeat unit are 5.654 and 4.003 Ǻ , respectively; thus, AANI is 0.226 nm2 (see Figure 4S). Consequently, (WBCP/WMWNT)crit is 1/17.5 for BCP550 and 1/14.2 for BCP5000. Taking the following factors into account, the experimental result of 1/8 is rational. First of all, the occupied area fraction of PEO block coil (APEO) onto the ideal plane of PANI block (APANI) can be estimated as nPEOπR g,PEO2 APEO = APANI NPANIAANI

(6)

where φ is the area fraction of the projected wet area. In this work, only if φ < (1 + cos θY)/(1 + r cos θY), θCB is larger than θY, i.e., more hydrophobic, can even a superhydrophobic surface be achieved. Therefore, the hydrophobic hierarchical thorny hybrids should obey a Cassie−Baxter behavior rather than a Wenzel one. The formation mechanism of this hierarchical thorny morphology of MWNT@BCP can be illustrated by the conformation transition of PANI block and the microphase separation of BCP (Scheme 2). Initially, the benzene rings and

(2)

where AMWNT is the surface area of single MWNT which is close to lateral area; Mn,BCP, Ñ A, NPANI, AANI, ρMWNT, and VMWNT are number-average molecular weight of BCP, Avogadro’s number, degree of polymerization of PANI block, area of aniline repeat unit, theoretical density of MWNT (∼1.80 g/cm3),7 and volume of single MWNT, respectively. Then, this equation can be rewritten as ⎛ WBCP ⎞ 4M n,BCP ⎜ ⎟ = ̃ ρ W N ⎝ MWNT ⎠crit A MWNT φMWNTNPANIAANI

(5)

nitrogen atoms in PANI blocks are almost coplanar and tightly lie on the surface of MWNT due to their conformation entropy compensation from short-range strong π−π short-rang stacking. However, owing to weaker and weaker stacking interaction along the perpendicular direction from the surface of MWNT, this coplanar conformation for neighbored benzene rings will gradually change into an orthogonal one, thus leading to a coiled conformation of PANI blocks. Meanwhile, the microphase separation of BCP leads to the formation of aggregate with PANI block as a core and PEO block dangling into water as a shell. Consequently, the radius of PANI block coil along

(4)

where average block numbers of PEO in BCP, nPEO, can be obtained from the GPC results. The calculated ratios are 0.233 for BCP550 and 0.766 for BCP5000, which further confirms the hydrophobic behavior of MWNT@BCP hybrid prepared by the direct method in Figure 5c,d. In other words a little more PEO blocks are required to separate the naked PANI blocks 3762

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Response. Langmuir 2010, 26, 9386−9392. (b) Udeh, C. U.; Fey, N.; Faul, C. F. J. Functional Block-like Structures from Electroactive Tetra(aniline) Oligomers. J. Mater. Chem. 2011, 21, 18137−18153. (c) Guo, B.; Finne-Wistrand, A.; Albertsson, A. C. Enhanced Electrical Conductivity by Macromolecular Architecture: Hyperbranched Electroactive and Degradable Block Copolymers Based on Poly(εcaprolactone) and Aniline Pentamer. Macromolecules 2010, 43, 4472− 4480. (3) (a) Hua, F. J.; Ruckenstein, E. Synthesis of a Water-Soluble Diblock Copolymer of Polysulfonic Diphenyl Aniline and Poly(ethylene oxide). J. Polym. Sci., Polym. Chem. 2004, 42, 2179−2191. (b) Knaapila, M.; Garamus, V. M.; Almásy, L.; Pang, J. S.; Forster, M.; Gutacker, A.; Scherf, U.; Monkman, A. P. Fractal Aggregates of Polyfluorene-Polyaniline Triblock Copolymer in Solution State. J. Phys. Chem. B 2008, 112, 16415−16421. (c) Schemid, A. L.; Lira, L. M.; de Torresi, S. I. C. On the Electrochemical and Spectroscopic Properties of a Soluble Polyaniline Parent Copolymer. Electrochim. Acta 2002, 47, 2005−2011. (d) Motheo, A. J.; Pantoja, M. F.; Venancio, E. C. Effect of Monomer Ratio in the Electrochemical Synthesis of Poly(aniline-co-o-methoxyaniline). Solid State Ionics 2004, 171, 91−98. (4) (a) Yan, L.; Tao, W. Synthesis of Achiral PEG-PANI Rod-Coil Block Copolymers and their Helical Superstructures. J. Polym. Sci., Polym. Chem. 2008, 46, 12−20. (b) Kinlen, P. J.; Frushour, B. G.; Ding, Y.; Menon, V. Synthesis and Characterization of Organically Soluble PoIyaniline and Polyaniline Block Copolymers. Synth. Met. 1999, 101, 758−761. (5) (a) Eelkema, R.; Anderson, H. L. Synthesis of End-Functionalized Polyanilines. Macromolecules 2008, 41, 9930−9933. (b) Zhang, X. X.; Sadighi, J. P.; Mackewitz, T. W.; Buchwald, S. L. Efficient Synthesis of Well-Defined, High Molecular Weight, and Processible Polyanilines under Mild Conditions via Palladium-Catalyzed Amination. J. Am. Chem. Soc. 2000, 122, 7606−7607. (6) (a) D’Souza, F.; Ito, O. Photosensitized Electron Transfer Processes of Nanocarbons Applicable to Solar Cells. Chem. Soc. Rev. 2012, 41, 86−96. (b) Hu, L. B.; Hecht, D. S.; Grüner, G. Carbon Nanotube Thin Films: Fabrication, Properties, and Applications. Chem. Rev. 2010, 110, 5790−5844. (7) Xie, X. L.; Mai, Y. W.; Zhou, X. P. Dispersion and Alignment of Carbon Nanotubes in Polymer Matrix: A Review. Mater. Sci. Eng., R 2005, 49, 89−112. (8) (a) Kim, S. W.; Kim, T.; Kim, Y. S.; Choi, H. S.; Lim, H. J.; Yang, S. J.; Park, C. R. Surface Modifications for the Effective Dispersion of Carbon Nanotubes in Solvents and Polymers. Carbon 2012, 50, 3−33. (b) Lee, S. H.; Lee, D. H.; Lee, W. J.; Kim, S. O. Tailored Assembly of Carbon Nanotubes and Graphene. Adv. Funct. Mater. 2011, 21, 1338− 1354. (9) (a) Shvartzman-Cohen, R.; Nativ-Roth, E.; Baskaran, E.; LeviKalisman, Y.; Szleifer, I.; Yerushalmi-Rozen, R. Selective Dispersion of Single-Walled Carbon Nanotubes in the Presence of Polymers: The Role of Molecular and Colloidal Length Scales. J. Am. Chem. Soc. 2004, 126, 14850−14857. (b) Xin, X.; Xu, G. Y.; Zhao, T. T.; Zhu, Y. Y.; Shi, X. F.; Gong, H. J.; Zhang, Z. Q. Dispersing Carbon Nanotubes in Aqueous Solutions by a Starlike Block Copolymer. J. Phys. Chem. C 2008, 112, 16377−16384. (10) Adeli, M.; Hakimpoor, F.; Ashiri, M.; Kabiri, R.; Bavadi, M. Anticancer Drug Delivery Systems Based on Noncovalent Interactions between Carbon Nanotubes and Linear-Dendritic Copolymers. Soft Matter 2011, 7, 4062−4070. (11) Kitano, H.; Tachimoto, K.; Nakaji-Hirabayashi, T.; Shinohara, H. Wrapping of Single-Walled Carbon Nanotubes with A-B-A Block Telomers. Macromol. Chem. Phys. 2004, 205, 2064−2071. (12) Gonzalez-Dominguez, J. M.; Tesa-Serrate, M. A.; Anson-Casaos, A.; Diez-Pascual, A. M.; Gomez-Fatou, M. A.; Martinez, M. T. Wrapping of SWCNTs in Polyethylenoxide-Based Amphiphilic Diblock Copolymers: An Approach to Purification, Debundling, and Integration into the Epoxy Matrix. J. Phys. Chem. C 2012, 116, 7399− 7408.

the perpendicular direction to MWNT becomes smaller and smaller until it is completely random. Finally, the heat dissipation at the thorn front hinders its further growth,13a and the extra amount of BCP becomes micelles in water. In addition, the dry thickness of outside PEO layer is only ca. 40% compared to its swollen state, which leaves more free volume among the dried thorns observed from the TEM technique.

4. CONCLUSIONS The conjugated BCPs of PEO-b-PANI have been successfully synthesized by a facile approach, i.e., the esterification reaction of PEO and p-formylbenzoic acid followed by imination with PANI. These BCPs could noncovalently modify MWNTs through either the direct or indirect method. In the former method, TEM images showed that a core−shell MWNT@BCP hybrid with a shell thickness of gyration diameter of PEO block was obtained in NMP. With the increase of PEO block length, the stability of the MWNT dispersion in many common solvents and PVA matrix will be highly improved. Moreover, in the indirect method, with the increase of BCP/MWNT weight ratio, the morphology of hybrids changed from smooth to hierarchically thorny as well as the water contact angle had a minimum value at BCP/MWNT ratio of 1/8. Obviously, the hierarchical thorny hybrid was more like a Cassie−Baxter state instead of a Wenzel one. This noncovalent modification of nanotubes with water-soluble conjugated BCPs might find potential applications in microelectronics, electroactive pharmacies, chemical sensors, thermal conductive materials, etc.

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ASSOCIATED CONTENT

S Supporting Information *

GPC curves of PANI and BCPs, TEM images of the assemblies of PANI and neat BCPs in NMP, morphology evolution of BCP550@MWNT prepared by indirect method, and the molecular conformation of BCP calculated by Chem 3D. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-27-87558194; Fax +86-27-87543632; e-mail ygliao@ mail.hust.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the Outstanding Youth Fund of the National Natural Science Foundation of China (50825301), the National Natural Science Foundation of China (50903035), and Chinese Ministry of Education (NCET-11-0174). We thank the HUST Analytical and Testing Center for allowing us to use its facilities.

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REFERENCES

(1) (a) Long, Y. Z.; Li, M. M.; Gu, C.; Wan, M.; Duvail, J. L.; Liu, Z.; Fan, Z. Recent Advances in Synthesis, Physical Properties and Applications of Conducting Polymer Nanotubes and Nanofibers. Prog. Polym. Sci. 2011, 36, 1415−1442. (b) Lin, P.; Yan, F. Organic ThinFilm Transistors for Chemical and Biological Sensing. Adv. Mater. 2012, 24, 34−51. (2) (a) Yang, Z. F.; Wang, X. T.; Yang, Y. K.; Liao, Y. G.; Wei, Y.; Xie, X. L. Synthesis of Electroactive Tetraaniline-PEO-Tetraaniline Triblock Copolymer and Its Self-Assembled Vesicle with Acidity 3763

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Article

and Potentially Cost-Effective Evaporation Casting. Nanotechnology 2011, 22, 415703. (c) Dror, Y.; Salalha, W.; Khalfin, R. L.; Cohen, Y.; Yarin, A. L.; Zussman, E. Carbon Nanotubes Embedded in Oriented Polymer Nanofibers by Electrospinning. Langmuir 2003, 19, 7012− 7020. (d) Jung, Y. C.; Muramatsu, H.; Park, K. C.; Shimamoto, D.; Kim, J. H.; Hayashi, T.; Song, S. M.; Kim, Y. A.; Endo, M.; Dresselhaus, M. S. Transparent and Conductive Polyethylene Oxide Film by the Introduction of Individualized Single-Walled Carbon Nanotubes. Macromol. Rapid Commun. 2009, 30, 2084−2088. (e) Chatterjee, T.; Yurekli, K.; Hadjiev, V. G.; Krishnamoorti, R. Single-Walled Carbon Nanotube Dispersions in Poly(ethylene oxide). Adv. Funct. Mater. 2005, 15, 1832−1838. (24) Abraham, T. N.; Ratna, D.; Siengchin, S.; Karger-Kocsis, J. Rheological and Thermal Properties of Poly(ethylene oxide)/Multiwall Carbon Nanotube Composites. J. Appl. Polym. Sci. 2008, 110, 2094−2101. (25) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y. S.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Solution Properties of Single-Walled Carbon Nanotubes. Science 1998, 282, 95−98. (26) (a) Wang, Z.; Liu, Q.; Zhu, H.; Liu, H.; Chen, Y.; Yang, M. Dispersing Multi-Walled Carbon Nanotubes with Water-Soluble Block Copolymers and Their Use as Supports for Metal Nanoparticles. Carbon 2007, 45, 285−292. (b) Sinani, V. A.; Gheith, M. K.; Yaroslavov, A. A.; Rakhnyanskaya, A. A.; Sun, K.; Mamedov, A. A.; Wicksted, J. P.; Kotov, N. A. Aqueous Dispersions of Single-wall and Multiwall Carbon Nanotubes with Designed Amphiphilic Polycations. J. Am. Chem. Soc. 2005, 127, 3463−72. (c) Yang, Y. K.; Xie, X. L.; Wu, J. G.; Mai, Y. W. Synthesis and Self-Assembly of Polystyrene-Grafted Multiwalled Carbon Nanotubes with a Hairy-Rod Nanostructure. J. Polym. Sci., Polym. Chem. 2006, 44, 3869−3881. (27) Halperin, A.; Tirrell, M.; Lodge, T. Tethered Chains in Polymer Microstructures. Adv. Polym. Sci. 1992, 100, 31−71. (28) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukuda, T. Structure and Properties of High-Density Polymer Brushes Prepared by Surface-Initiated Living Radical Polymerization. Adv. Polym. Sci. 2006, 197, 1−45. (29) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. Formation of Self-Assembled Monolayers by Chemisorption of Derivatives of Oligo(ethylene glycol) of Structure HS(CH2)11(OCH2CH2)mOH on Gold. J. Am. Chem. Soc. 1991, 113, 12−20. (30) Ma, P. C.; Siddiqui, N. A.; Marom, G.; Kim, J. K. Dispersion and Functionalization of Carbon Nanotubes for Polymer-Based Nanocomposites: A Review. Composites, Part A 2010, 41, 1345−1367. (31) (a) Laird, E. D.; Wang, W. D.; Cheng, S.; Li, B.; Presser, V.; Dyatkin, B.; Gogotsi, Y.; Li, C. Y. Polymer Single Crystal-Decorated Superhydrophobic Buckypaper with Controlled Wetting and Conductivity. ACS Nano 2012, 6, 1204−1213. (b) Chen, X.; Dong, B.; Wang, B. B.; Shah, R.; Li, C. Y. Crystalline Block Copolymer Decorated, Hierarchically Ordered Polymer Nanofibers. Macromolecules 2010, 43, 9918−9927. (c) Li, C. Y.; Li, L. Y.; Cai, W. W.; Kodjie, S. L.; Tenneti, K. K. Nanohybrid Shish-Kebabs: Periodically Functionalized Carbon Nanotubes. Adv. Mater. 2005, 17, 1198−1202. (32) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (33) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551.

(13) (a) Li, B.; Li, L.; Wang, B.; Li, C. Y. Alternating Patterns on Single-Walled Carbon Nanotubes. Nat. Nanotechnol. 2009, 4, 358− 362. (b) Park, C.; Lee, S.; Lee, J. H.; Lim, J.; Lee, S. C.; Park, M.; Lee, S.-S.; Kim, J.; Park, C. R.; Kim, C. Controlled Assembly of Carbon Nanotubes Encapsulated with Amphiphilic Block Copolyme. Carbon 2007, 45, 2072−2078. (c) Mountrichas, G.; Pispas, S.; Tagmatarchis, N. Aqueous Carbon-Nanotube-Amphiphilic-Block-Copolymer Nanoensembles: Towards Realization of Charge-Transfer Processes with Semiconductor Quantum Dots. Small 2007, 3, 404−407. (14) Itzhak, R.; Raichman, D.; Shahar, Z.; Frey, G. L.; Frey, J.; Yerushalmi-Rozen, R. Tailoring Triblock Copolymers for Dispersion of Individual, Pristine, Single-Walled Carbon Nanotubes in Organic Solvents. J. Phys. Chem. C 2010, 114, 3748−3753. (15) Kim, K. T.; Jo, W. H. Noncovalent Functionalization of Multiwalled Carbon Nanotubes Using Graft Copolymer with Naphthalene and Its Application as a Reinforcing Filler for Poly(styrene-co-acrylonitrile). J. Polym. Sci., Polym. Chem. 2010, 48, 4184−4191. (16) (a) Etika, K. C.; Jochum, F. D.; Theato, P.; Grunlan, J. C. Temperature Controlled Dispersion of Carbon Nanotubes in Water with Pyrene-Functionalized Poly(N-cyclopropylacrylamide). J. Am. Chem. Soc. 2009, 131, 13598−13599. (b) Lou, X. D.; Daussin, R.; Cuenot, S.; Duwez, A. S.; Pagnoulle, C.; Detrembleur, C.; Bailly, C.; Jérôme, R. Synthesis of Pyrene-Containing Polymers and Noncovalent Sidewall Functionalization of Multiwalled Carbon Nanotubes. Chem. Mater. 2004, 16, 4005−4011. (c) Yan, Y.; Cui, J.; Pötschke, P.; Voit, B. Dispersion of Pristine Single-Walled Carbon Nanotubes Using PyreneCapped Polystyrene and Its Application for Preparation of Polystyrene Matrix Composites. Carbon 2010, 48, 2603−2612. (17) (a) Zou, J.; Liu, L.; Chen, H.; Khondaker, S. I.; McCullough, R. D.; Huo, Q.; Zhai, L. Dispersion of Pristine Carbon Nanotubes Using Conjugated Block Copolymers. Adv. Mater. 2008, 20, 2055−2060. (b) Mandal, A.; Nandi, A. K. Noncovalent Functionalization of Multiwalled Carbon Nanotube by a Polythiophene-Based Compatibilizer: Reinforcement and Conductivity Improvement in Poly(vinylidene fluoride) Films. J. Phys. Chem. C 2012, 116, 9360−9371. (18) Park, H. S.; Choi, B. G.; Hong, W. H.; Jang, S.-Y. Interfacial Interactions of Single-Walled Carbon Nanotube/Conjugated Block Copolymer Hybrids for Flexible Transparent Conductive Films. J. Phys. Chem. C 2012, 116, 7962−7967. (19) (a) Ali, S. R.; Ma, Y. F.; Parajuli, R. R.; Balogun, Y.; Lai, W. Y. C.; He, H. X. A Nonoxidative Sensor Based on a Self-Doped Polyaniline/Carbon Nanotube Composite for Sensitive and Selective Detection of the Neurotransmitter Dopamine. Anal. Chem. 2007, 79, 2583−2587. (b) Zengin, H.; Zhou, W. S.; Jin, J. Y.; Czerw, R.; Smith, D. W.; Echegoyen, L.; Carroll, D. L.; Foulger, S. H.; Ballato, J. Carbon Nanotube Doped Polyaniline. Adv. Mater. 2002, 14, 1480−1483. (20) Chen, S. A.; Lee, H. T. Polyaniline Plasticized with 1-Methyl-2pyrrolidone: Structure and Doping Behavior. Macromolecules 1993, 26, 3254−3261. (21) (a) Adams, P. N.; Apperley, D. C.; Monkman, A. P. A Comparison of the Molecular Weights of Polyaniline Samples Obtained from Gel Permeation Chromatography and Solid-State 15 N N.M.R. Spectroscopy. Polymer 1993, 34, 328−332. (b) Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical Oxidation of Multiwalled Carbon Nanotubes. Carbon 2008, 46, 833−840. (22) (a) Stejskal, J.; Sapurina, I.; Trchová, M.; Konyushenko, E. N. Oxidation of Aniline: Polyaniline Granules, Nanotubes, and Oligoaniline Microspheres. Macromolecules 2008, 41, 3530−3536. (b) Zujovic, Z. D.; Zhang, L.; Bowmaker, G. A.; Kilmartin, P. A.; Travas-Sejdic, J. Self-Assembled, Nanostructured Aniline Oxidation Products: A Structural Investigation. Macromolecules 2008, 41, 3125−3135. (23) (a) Ratna, D.; Abraham, T. N.; Siengchin, S.; Karger-Kocsis, J. Novel Method for Dispersion of Multiwall Carbon Nanotubes in Poly(ethylene oxide) Matrix Using Dicarboxylic Acid Salts. J. Polym. Sci., Polym. Phys. 2009, 47, 1156−1165. (b) Park, M.; Kim, H.; Youngblood, J. P.; Han, S. W.; Verplogen, E.; Hart, A. J. Excellent Dispersion of MWCNTs in PEO Polymer Achieved through a Simple 3764

dx.doi.org/10.1021/la304283n | Langmuir 2013, 29, 3757−3764