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RETRACTED: Polystyrene-b-poly(ethylene oxide) block copolymer thin films as templates for carbon nanotube dispersion
Thin Solid Films 536 (2013) 191–195
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Polystyrene-b-poly(ethylene oxide) block copolymer thin films as templates for carbon nanotube dispersion Jing Wang a, Fang Li a, Qifang Li b,⁎, Jianli Sun a, Guang-Xin Chen a, b,⁎⁎ a b
Key Laboratory on Preparation and Processing of Novel Polymer Materials of Beijing, Beijing University of Chemical Technology, Beijing 100029, PR China College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 29 March 2013 Accepted 5 April 2013 Available online 11 April 2013 Keywords: Carbon nanotubes Block copolymer Localization Thin films
a b s t r a c t A method for the selective self-assembly of polymer-functionalized carbon nanotubes (CNTs) in polystyreneblock-poly(ethylene oxide) (PS-b-PEO) copolymer was developed. Aqueous substrate was combined with solvent annealing. PEO-covered CNTs were prepared to form a PEO-covered CNT/water solution that was applied as a complex substrate for the PS-b-PEO template. The proposed method facilitated the selective assembly of the CNTs onto the PEO microphase. This selective assembly is a versatile approach that may open a route for the controlled assembly of anisotropic nanostructured materials with desirable patterns on soft substrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Diblock copolymers are powerful templates that can direct the self-assembly of inorganic particles (zero-dimensional nanomaterials) into nanometer-scale structures [1]. Significant progress has been experimentally and theoretically achieved on the control of spherical nanoparticle locations in diblock copolymer templates [2,3]. Compared with the organized assembly of spherical nanoparticles, the organization of one-dimensional nanomaterials (OdNMs) [4] is particularly significant. The collective properties of these anisotropic species are strongly affected by alignment and internal organization. The self-assembly of hybrid systems consisting of copolymers and OdNMs is more complex than that of spherical nanoparticles. Such complexity is due to the anisotropic shapes of the OdNMs [5], which tend to induce the formation of liquid crystalline phases [6]. Studies on these systems are usually complicated and challenging because of two main reasons: the interplay between the strong and directional van der Waals attraction among OdNMs and the entropy associated with their orientation. Changing the shape of nanoparticles from isotropic spheres to anisotropic OdNMs alters their properties and potential applications [7]. Carbon nanotubes (CNTs) are pseudo-one-dimensional carbon allotropes characterized by high aspect ratio, high surface area, and excellent material properties. CNTs have been studied to gain control
over their positional localization in block copolymers [8–11]. Park et al. [8] used a symmetric polystyrene (PS)-block-polyisoprene copolymer to achieve the two-dimensional alignment of PS-functionalized multi-walled CNTs (MWCNTs) in the lamellar PS microphase. In their experiment, MWCNTs were functionalized with PS via in situ emulsion polymerization, which generally renders PS layers with thicknesses of tens of nanometers on MWCNTs. To accommodate these large-diameter PS-functionalized MWCNTs in the PS microphase, an ultrahighmolecular-weight copolymer has to be specifically synthesized. Liu et al. [9] reported the assembly of PS-functionalized CNTs in the cylindrical PS microphase of an asymmetric styrene–butadiene–styrene triblock copolymer. Long PS-functionalized CNTs are not confined in the PS microphase but span both phases instead. In contrast, the short ones are confined in the PS microphase. Recently, Kenny et al. [10] and Mezzenga et al. [11] added CNTs with high aspect ratios to block copolymers. The localization of conducting CNTs inside the domains of the block copolymer matrix was investigated. In the present study, PS-block-poly(ethylene oxide) (PS-b-PEO) self-assembly was used to organize MWCNTs on a large scale by combining an aqueous substrate with solvent annealing. This floating template technique is a versatile approach that may open a route for controlled assembly with desirable patterns on a soft substrate. 2. Experimental details
⁎ Corresponding author. Tel.: +86 10 64445680; fax: +86 10 64421693. ⁎⁎ Correspondence to: G.-X. Chen, College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China. Tel.: +86 10 64445680; fax: +86 10 64421693. E-mail addresses: qfl[email protected] (Q. Li), [email protected] (G.-X. Chen). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.002
2.1. Materials PS-b-PEO with a polydispersity index of 1.03 was purchased from Polymer Source, Inc. The molecular weights of the PS and PEO blocks were 58,600 and 71,000 g/mol, respectively. The volume fraction of
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J. Wang et al. / Thin Solid Films 536 (2013) 191–195
H COOH
SoCl2 n
COCl
n
OCH2CH2
OH m
O
C
TEA
OCH2CH2
m
H n
Fig. 1. Synthesis scheme of MWCNT-g-PEO.
PEO blocks was calculated to be 51.49%, suggesting a symmetrical structure. Polyethylene glycol (Mn = 2000 g/mol) was purchased from Alfa Aesar, China. Multiwalled carbon nanotubes (MWCNTs) were provided by Aldrich, obtained by chemical vapor deposition and having a purity of > 95%. The outer diameter and the length of the MWCNTs were about 20 nm and 1–2 μm, respectively. Toluene and chloroform, which were bought from Beijing Chemical Reagents Corporation, were used as received. 2.2. Preparation of PEO-modified MWCNT PEO-modified MWCNT (MWCNT-g-PEO) was synthesized using a procedure similar to the previous literature [12]. A 250 ml roundbottomed flask was charged with 200 mg acid-treated MWCNT and 100 ml SOCl2. The flask was ultrasonicated in a water bath for 30 min. The solution was mixed at 80 °C for 30 h in a water condenser. Afterward, SOCl2 was removed by Heidolph rotary evaporators at 39 °C for 0.5 h. Then, 30 g polyethylene glycol was added to the solid mixture and stirred at 120 °C for 45 h in a water condenser. The resulting reaction medium was dissolved in excess THF and vacuum-filtered three times through a 0.22 μm PTFE membrane and vacuum-dried for 10 h to yield MWCNT-g-PEO. The reaction is schematized in Fig. 1. 2.3. Annealing of PS-b-PEO on the surface of water/MWCNT-g-PEO solution Approximately 80 mg of MWCNT-g-PEO was added to 80 ml of distilled water. The solution was placed in an ultrasonic bath for 30 min, and a homogeneous MWCNT-g-PEO aqueous solution was obtained. A clean culture dish filled with 60 ml of MWCNT-g-PEO aqueous solution was placed in a closed container with an open weighing bottle filled with toluene (Fig. 2). After 1 h, 15 μl of a PS-b-PEO/toluene solution (10 mg/ml), measured using a micropipettor, was dropped on the surface of the MWCNT-g-PEO/water solution. Toluene was allowed to evaporate to the confined space. The MWCNT-g-PEOs were absorbed by PS-b-PEO, and the PS-b-PEO/MWCNT nanohybrid film gradually formed under the toluene vapor. The annealing time was 3 h to 72 h.
Fig. 2. Schematic for the self-assembly of PS-b-PEO on the surface of water/MWNT-g-PEO solution under solvent vapor annealing.
2.4. Characterizations The annealed films were directly collected with carbon-coated copper grids and silicon wafer on the water surface. The films on carbon-coated copper grids were stained with RuO4 at room temperature for 20 min [13]. The morphology of the PS-b-PEO films was characterized using a transmission electron microscopy (TEM) system (Hitachi 800) at an accelerating voltage of 200 kV. The PS domains were preferentially stained and appeared dark. Raman spectroscopy (Renishaw inVia) was used to confirm the structure of MWCNTs and MWCNT-g-PEO/PS-b-PEO nanohybrid films (the films on silicon wafer) operating at 514 nm with a resolution of 1.5 cm−1. Thermogravimetric Analysis (TGA, HCT-2) was conducted in nitrogen atmosphere. MWCNT and MWCNT-g-PEO were heated with a heating rate of 10 °C/min from 50 to 600 °C to determine the grafting content of PEO. 3. Results and discussion The morphology of MWCNT-g-PEO after the complete washing with chloroform was observed by using TEM as exhibited in Fig. 3. As shown in Fig. 3, a core-shell structure with MWCNTs at the center can be clearly seen in MWCNT-g-PEO, indicating that MWCNTs were coated by PEO layer. Comparing the mass gain after the grafting reaction with the thickness of PEO layer observed in the TEM images of MWCNT-g-PEO samples, it can be clearly seen that the amount of the grafted PEO was larger. The amount of PEO grafted to MWCNT, defined as the ratio of the mass of the immobilized PEO to that of the MWCNT-g-PEO, was estimated from TGA (Fig. 4). As shown in Fig. 4 MWCNT-g-PEO exhibited major weight loss in the temperature range of 300–400 °C due to the degradation of PEO grafted on MWCNT. The residual fraction at the
Fig. 3. TEM image of MWCNT-g-PEO.
J. Wang et al. / Thin Solid Films 536 (2013) 191–195
Intensity
Fig. 4. TGA traces of MWCNT and MWCNT-g-PEO.
a b
500
1000
1500
2000
Wavenumber (cm-1) Fig. 5. Raman spectra of (a) pure MWCNTs, and (b) MWCNT-g-PEO/PS-b-PEO nanohybrid film.
temperatures above 600 °C corresponds mainly to the MWCNT. The amount of PEO bound to MWCNT is about 29 wt.%. Raman spectra of MWCNT and PS-b-PEO/MWCNT-g-PEO nanohybrid films are shown in Fig. 5. The peak around 1350 cm−1 is the disorder mode or SP3 mode, the peak around 1570 cm−1 is the graphitization mode. The intensity ratio of D peak and G peak reflects the extent of structure damage on the MWCNT surface. It's obvious that the intensity ratio of the D peak and G peak of MWCNT-g-PEOs is higher than that of pure MWCNTs. So the sidewall of MWCNTs is functionalized by covalent attachment of PEO. At the same time, the D peak of MWCNT-g-PEO in
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nanohybrid films moved to the right about 20 cm−1 compared to that of pure MWCNTs because MWCNT-g-PEOs disperse well in PS-b-PEO nanohybrid film owing to the enhanced interaction between carbon nanotubes and block polymers. PS-b-PEO/MWCNT-g-PEO nanohybrid films were prepared through a conventional method in our laboratory. The PEO-modified MWCNTs and PS-b-PEO solutions were either cast onto a silicon wafer (solid substrate) or dropped on the surface of pure water (flexible substrate). Fig. 6 shows the representative TEM images of the annealing time dependence of the structure evolution of the PS-b-PEO/MWCNT-g-PEO nanohybrid films prepared on silicon wafer. Clearly, the PS-b-PEO film did not exhibit any trace of microphase separation. CNT localization was not observed in the microphase of the PS-b-PEO copolymer. The TEM images shown in Fig. 7 exhibit the morphologies of the PS-b-PEO/MWCNT-g-PEO nanohybrid films prepared using pure water as substrate under toluene vapor annealing. The content of the MWCNTs was kept at 5 wt.%. Fig. 7a shows that the MWCNT-g-PEO particles are uniformly dispersed in the film without aggregation after 2 h of annealing with toluene vapor. After annealing under toluene vapor for 24 h, the PS-b-PEO films showed a multiple microstructure, in which the microphase separation of PS-b-PEO occurred, and the PS (black area) assembled into a segregated cylindrical structure. MWCNT-g-PEO was not confined to the preconceived PEO microphase (white area); they either aggregated with each other in the copolymer matrix (Fig. 7b) or spanned both microphases (Fig. 7c). The confinement of long MWCNT-g-PEO in the PEO microphase demands a huge translational entropy penalty. This demand could not be compensated by the enthalpy gain facilitated by the PEO ligand [9]. In our previous work, we described the formation of large lamellar structures in PS-b-PEO on pure water substrate [14] and responsive C60 assemblies [3] through annealing in different solvent vapors. In the present study, a floating method, which involved dropping pure PS-b-PEO/toluene solution on the surface of an MWCNT-g-PEO aqueous solution under solvent vapor, was described as a technique for processing nanohybrid films. MWCNT functionalization with PEO ligands provides MWCNT solubility in water and surface activity with the PEO microphase in the copolymer. Consequently, assembly by the flotation method is enabled. Russell et al. [4] reported the selective assembly of PEO-functionalized CdSe nanorods onto the defined regions of a patterned surface of a PS-b-PMMA film. The method involves floating the copolymer template on an aqueous solution of the nanorods. However, the copolymer template used for the floating technique is solid and cannot freely mediate the nanorods. Figs. 8a and 8b show the TEM images of microphase-separated PS-b-PEO films annealed under toluene vapor for 24 h on the MWCNT-g-PEO/water solution. Segregated PS cylindrical structures were found in the PS-b-PEO/MWCNT-g-PEO nanohybrid films. Most interestingly, the MWCNT-g-PEOs were found to be mainly located in
Fig. 6. TEM images of PS-b-PEO/MWCNT-g-PEO nanohybrid films on solid substrate under toluene vapor annealing for (a) 2 h and (b) 24 h: The MWCNT-g-PEO content was 5 wt.%; scale bar = 200 nm.
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J. Wang et al. / Thin Solid Films 536 (2013) 191–195
Fig. 7. TEM images of PS-b-PEO/MWCNT-g-PEO nanohybrid films on water surface under toluene vapor annealing for (a) 2 h and (b–c) 24 h: The MWCNT-g-PEO content was 5 wt.%; scale bar = 200 nm.
the PEO microphase. Many benzene ring structures found on the PS chains have π–π conjugations with CNTs [15]. The π–π conjugation between the PS and the MWCNTs induces the MWCNTs to penetrate the PS microphase [9]. However, PEO grafted onto the MWCNTs can cause the MWCNTs to remain in the PEO microphase of the diblock copolymer. The compatibility of MWCNT-g-PEOs with PEO is better than that with
PS in the copolymer because the solubility parameters of PEO in the diblock copolymer and those of PEO grafted onto the MWCNTs are very close. The two factors finally resulted in the MWCNT-g-PEOs being located in the PEO microphase (Fig. 8b). Annealing time was also increased to study the mutual influence of MWCNT-g-PEOs on the self-assembly of the diblock copolymer.
Fig. 8. TEM images of PS-b-PEO/MWCNT-g-PEO nanohybrid films prepared on the surface of water/MWCNT-g-PEO solution under toluene vapor annealing for: (a–b) 24 h and (c–d) 48 h with various magnifications: scale bar = 200 nm for a, and c; scale bar = 90 nm for b, and d.
J. Wang et al. / Thin Solid Films 536 (2013) 191–195
After 48 h of annealing under toluene vapor (Fig. 8c and d), more MWCNT-g-PEOs accumulated within the PEO microphase. No CNT orientation was observed in the PEO microphase. This result demonstrates that the surface affinity of PEO must aid the segregation of the MWCNT-g-PEOs to the water–air interface, where they are absorbed by the soft copolymer templates. Selective localization of the majority of CNTs in the copolymer microphase was achieved using our developed method. We present a versatile approach for the controlled assembly of anisotropic-nanostructured film materials with desirable patterns on soft substrate. 4. Conclusions A floating method for manipulating the localization of CNTs in a block copolymer was introduced. The method involved an aqueous substrate and organic solvent vapor annealing. The MWCNT-g-PEO/ water solution was prepared as the complex substrate for the PS-b-PEO copolymer template. The PS-b-PEO/MWCNT nanohybrid thin films were obtained after the solvent was vaporized and the PEO-functionalized MWCNTs were selectively distributed in the PEO microphase. The polymer ligand coverage on the carbon nanomaterials was found especially effective for achieving the desired assembly.
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Acknowledgments The authors gratefully acknowledge financial support of this work coming from the Natural Science Foundation of China (NSFC) (No. 21074009 and No. 51173009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
W. Yuan, Z. Zhao, S. Gu, T. Ren, J. Ren, Mater. Lett. 65 (4) (2011) 793. H.Y. Chen, E. Ruckenstein, Polymer 51 (24) (2010) 5869. J. Wang, G.-X. Chen, J. Sun, Q. Li, J. Phys. Chem. B 115 (12) (2011) 2824. Q.L. Zhang, S. Gupta, T. Emrick, T.P. Russell, J. Am. Chem. Soc. 128 (12) (2006) 3898. N.R. Jana, Angew. Chem. Int. Ed. 43 (12) (2004) 1536. A.J. Liu, G.H. Fredrickson, Macromolecules 29 (24) (1996) 8000. J. Hu, L.-s. Li, W. Yang, L. Manna, L.-w. Wang, A.P. Alivisatos, Science 292 (5524) (2001) 2060. I. Park, W. Lee, J. Kim, M. Park, H. Lee, Sensors and Actuators B 126 (1) (2007) 301. Y.T. Liu, Z.L. Zhang, W. Zhao, X.M. Xie, X.Y. Ye, Carbon 47 (7) (2009) 1883. L. Peponi, A. Tercjak, J. Gutierrez, M. Cardinali, I. Mondragon, L. Valentini, J.M. Kenny, Carbon 48 (9) (2010) 2590. C.X. Li, J.C. Hsu, K. Sugiyama, A. Hirao, W.C. Chen, R. Mezzenga, Macromolecules 42 (15) (2009) 5793. G.-X. Chen, H.S. Kim, B.H. Park, J.S. Yoon, J. Phys. Chem. B 109 (47) (2005) 22237. J.S. Trent, Macromolecules 17 (12) (1984) 2390. J.L. Sun, Dissertation, Beijing University of Chemical Technology, 2010. R.Q. Long, R.T. Yang, J. Am. Chem. Soc. 123 (9) (2001) 2058.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Polystyrene-b-poly(ethylene oxide) block copolymer thin films as templates for carbon nanotube dispersion Jing Wang a, Fang Li a, Qifang Li b,⁎, Jianli Sun a, Guang-Xin Chen a, b,⁎⁎ a b
Key Laboratory on Preparation and Processing of Novel Polymer Materials of Beijing, Beijing University of Chemical Technology, Beijing 100029, PR China College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China
a r t i c l e
i n f o
Article history: Received 13 February 2012 Received in revised form 29 March 2013 Accepted 5 April 2013 Available online 11 April 2013 Keywords: Carbon nanotubes Block copolymer Localization Thin films
a b s t r a c t A method for the selective self-assembly of polymer-functionalized carbon nanotubes (CNTs) in polystyreneblock-poly(ethylene oxide) (PS-b-PEO) copolymer was developed. Aqueous substrate was combined with solvent annealing. PEO-covered CNTs were prepared to form a PEO-covered CNT/water solution that was applied as a complex substrate for the PS-b-PEO template. The proposed method facilitated the selective assembly of the CNTs onto the PEO microphase. This selective assembly is a versatile approach that may open a route for the controlled assembly of anisotropic nanostructured materials with desirable patterns on soft substrate. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Diblock copolymers are powerful templates that can direct the self-assembly of inorganic particles (zero-dimensional nanomaterials) into nanometer-scale structures [1]. Significant progress has been experimentally and theoretically achieved on the control of spherical nanoparticle locations in diblock copolymer templates [2,3]. Compared with the organized assembly of spherical nanoparticles, the organization of one-dimensional nanomaterials (OdNMs) [4] is particularly significant. The collective properties of these anisotropic species are strongly affected by alignment and internal organization. The self-assembly of hybrid systems consisting of copolymers and OdNMs is more complex than that of spherical nanoparticles. Such complexity is due to the anisotropic shapes of the OdNMs [5], which tend to induce the formation of liquid crystalline phases [6]. Studies on these systems are usually complicated and challenging because of two main reasons: the interplay between the strong and directional van der Waals attraction among OdNMs and the entropy associated with their orientation. Changing the shape of nanoparticles from isotropic spheres to anisotropic OdNMs alters their properties and potential applications [7]. Carbon nanotubes (CNTs) are pseudo-one-dimensional carbon allotropes characterized by high aspect ratio, high surface area, and excellent material properties. CNTs have been studied to gain control
over their positional localization in block copolymers [8–11]. Park et al. [8] used a symmetric polystyrene (PS)-block-polyisoprene copolymer to achieve the two-dimensional alignment of PS-functionalized multi-walled CNTs (MWCNTs) in the lamellar PS microphase. In their experiment, MWCNTs were functionalized with PS via in situ emulsion polymerization, which generally renders PS layers with thicknesses of tens of nanometers on MWCNTs. To accommodate these large-diameter PS-functionalized MWCNTs in the PS microphase, an ultrahighmolecular-weight copolymer has to be specifically synthesized. Liu et al. [9] reported the assembly of PS-functionalized CNTs in the cylindrical PS microphase of an asymmetric styrene–butadiene–styrene triblock copolymer. Long PS-functionalized CNTs are not confined in the PS microphase but span both phases instead. In contrast, the short ones are confined in the PS microphase. Recently, Kenny et al. [10] and Mezzenga et al. [11] added CNTs with high aspect ratios to block copolymers. The localization of conducting CNTs inside the domains of the block copolymer matrix was investigated. In the present study, PS-block-poly(ethylene oxide) (PS-b-PEO) self-assembly was used to organize MWCNTs on a large scale by combining an aqueous substrate with solvent annealing. This floating template technique is a versatile approach that may open a route for controlled assembly with desirable patterns on a soft substrate. 2. Experimental details
⁎ Corresponding author. Tel.: +86 10 64445680; fax: +86 10 64421693. ⁎⁎ Correspondence to: G.-X. Chen, College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China. Tel.: +86 10 64445680; fax: +86 10 64421693. E-mail addresses: qfl[email protected] (Q. Li), [email protected] (G.-X. Chen). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.002
2.1. Materials PS-b-PEO with a polydispersity index of 1.03 was purchased from Polymer Source, Inc. The molecular weights of the PS and PEO blocks were 58,600 and 71,000 g/mol, respectively. The volume fraction of
192
J. Wang et al. / Thin Solid Films 536 (2013) 191–195
H COOH
SoCl2 n
COCl
n
OCH2CH2
OH m
O
C
TEA
OCH2CH2
m
H n
Fig. 1. Synthesis scheme of MWCNT-g-PEO.
PEO blocks was calculated to be 51.49%, suggesting a symmetrical structure. Polyethylene glycol (Mn = 2000 g/mol) was purchased from Alfa Aesar, China. Multiwalled carbon nanotubes (MWCNTs) were provided by Aldrich, obtained by chemical vapor deposition and having a purity of > 95%. The outer diameter and the length of the MWCNTs were about 20 nm and 1–2 μm, respectively. Toluene and chloroform, which were bought from Beijing Chemical Reagents Corporation, were used as received. 2.2. Preparation of PEO-modified MWCNT PEO-modified MWCNT (MWCNT-g-PEO) was synthesized using a procedure similar to the previous literature [12]. A 250 ml roundbottomed flask was charged with 200 mg acid-treated MWCNT and 100 ml SOCl2. The flask was ultrasonicated in a water bath for 30 min. The solution was mixed at 80 °C for 30 h in a water condenser. Afterward, SOCl2 was removed by Heidolph rotary evaporators at 39 °C for 0.5 h. Then, 30 g polyethylene glycol was added to the solid mixture and stirred at 120 °C for 45 h in a water condenser. The resulting reaction medium was dissolved in excess THF and vacuum-filtered three times through a 0.22 μm PTFE membrane and vacuum-dried for 10 h to yield MWCNT-g-PEO. The reaction is schematized in Fig. 1. 2.3. Annealing of PS-b-PEO on the surface of water/MWCNT-g-PEO solution Approximately 80 mg of MWCNT-g-PEO was added to 80 ml of distilled water. The solution was placed in an ultrasonic bath for 30 min, and a homogeneous MWCNT-g-PEO aqueous solution was obtained. A clean culture dish filled with 60 ml of MWCNT-g-PEO aqueous solution was placed in a closed container with an open weighing bottle filled with toluene (Fig. 2). After 1 h, 15 μl of a PS-b-PEO/toluene solution (10 mg/ml), measured using a micropipettor, was dropped on the surface of the MWCNT-g-PEO/water solution. Toluene was allowed to evaporate to the confined space. The MWCNT-g-PEOs were absorbed by PS-b-PEO, and the PS-b-PEO/MWCNT nanohybrid film gradually formed under the toluene vapor. The annealing time was 3 h to 72 h.
Fig. 2. Schematic for the self-assembly of PS-b-PEO on the surface of water/MWNT-g-PEO solution under solvent vapor annealing.
2.4. Characterizations The annealed films were directly collected with carbon-coated copper grids and silicon wafer on the water surface. The films on carbon-coated copper grids were stained with RuO4 at room temperature for 20 min [13]. The morphology of the PS-b-PEO films was characterized using a transmission electron microscopy (TEM) system (Hitachi 800) at an accelerating voltage of 200 kV. The PS domains were preferentially stained and appeared dark. Raman spectroscopy (Renishaw inVia) was used to confirm the structure of MWCNTs and MWCNT-g-PEO/PS-b-PEO nanohybrid films (the films on silicon wafer) operating at 514 nm with a resolution of 1.5 cm−1. Thermogravimetric Analysis (TGA, HCT-2) was conducted in nitrogen atmosphere. MWCNT and MWCNT-g-PEO were heated with a heating rate of 10 °C/min from 50 to 600 °C to determine the grafting content of PEO. 3. Results and discussion The morphology of MWCNT-g-PEO after the complete washing with chloroform was observed by using TEM as exhibited in Fig. 3. As shown in Fig. 3, a core-shell structure with MWCNTs at the center can be clearly seen in MWCNT-g-PEO, indicating that MWCNTs were coated by PEO layer. Comparing the mass gain after the grafting reaction with the thickness of PEO layer observed in the TEM images of MWCNT-g-PEO samples, it can be clearly seen that the amount of the grafted PEO was larger. The amount of PEO grafted to MWCNT, defined as the ratio of the mass of the immobilized PEO to that of the MWCNT-g-PEO, was estimated from TGA (Fig. 4). As shown in Fig. 4 MWCNT-g-PEO exhibited major weight loss in the temperature range of 300–400 °C due to the degradation of PEO grafted on MWCNT. The residual fraction at the
Fig. 3. TEM image of MWCNT-g-PEO.
J. Wang et al. / Thin Solid Films 536 (2013) 191–195
Intensity
Fig. 4. TGA traces of MWCNT and MWCNT-g-PEO.
a b
500
1000
1500
2000
Wavenumber (cm-1) Fig. 5. Raman spectra of (a) pure MWCNTs, and (b) MWCNT-g-PEO/PS-b-PEO nanohybrid film.
temperatures above 600 °C corresponds mainly to the MWCNT. The amount of PEO bound to MWCNT is about 29 wt.%. Raman spectra of MWCNT and PS-b-PEO/MWCNT-g-PEO nanohybrid films are shown in Fig. 5. The peak around 1350 cm−1 is the disorder mode or SP3 mode, the peak around 1570 cm−1 is the graphitization mode. The intensity ratio of D peak and G peak reflects the extent of structure damage on the MWCNT surface. It's obvious that the intensity ratio of the D peak and G peak of MWCNT-g-PEOs is higher than that of pure MWCNTs. So the sidewall of MWCNTs is functionalized by covalent attachment of PEO. At the same time, the D peak of MWCNT-g-PEO in
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nanohybrid films moved to the right about 20 cm−1 compared to that of pure MWCNTs because MWCNT-g-PEOs disperse well in PS-b-PEO nanohybrid film owing to the enhanced interaction between carbon nanotubes and block polymers. PS-b-PEO/MWCNT-g-PEO nanohybrid films were prepared through a conventional method in our laboratory. The PEO-modified MWCNTs and PS-b-PEO solutions were either cast onto a silicon wafer (solid substrate) or dropped on the surface of pure water (flexible substrate). Fig. 6 shows the representative TEM images of the annealing time dependence of the structure evolution of the PS-b-PEO/MWCNT-g-PEO nanohybrid films prepared on silicon wafer. Clearly, the PS-b-PEO film did not exhibit any trace of microphase separation. CNT localization was not observed in the microphase of the PS-b-PEO copolymer. The TEM images shown in Fig. 7 exhibit the morphologies of the PS-b-PEO/MWCNT-g-PEO nanohybrid films prepared using pure water as substrate under toluene vapor annealing. The content of the MWCNTs was kept at 5 wt.%. Fig. 7a shows that the MWCNT-g-PEO particles are uniformly dispersed in the film without aggregation after 2 h of annealing with toluene vapor. After annealing under toluene vapor for 24 h, the PS-b-PEO films showed a multiple microstructure, in which the microphase separation of PS-b-PEO occurred, and the PS (black area) assembled into a segregated cylindrical structure. MWCNT-g-PEO was not confined to the preconceived PEO microphase (white area); they either aggregated with each other in the copolymer matrix (Fig. 7b) or spanned both microphases (Fig. 7c). The confinement of long MWCNT-g-PEO in the PEO microphase demands a huge translational entropy penalty. This demand could not be compensated by the enthalpy gain facilitated by the PEO ligand [9]. In our previous work, we described the formation of large lamellar structures in PS-b-PEO on pure water substrate [14] and responsive C60 assemblies [3] through annealing in different solvent vapors. In the present study, a floating method, which involved dropping pure PS-b-PEO/toluene solution on the surface of an MWCNT-g-PEO aqueous solution under solvent vapor, was described as a technique for processing nanohybrid films. MWCNT functionalization with PEO ligands provides MWCNT solubility in water and surface activity with the PEO microphase in the copolymer. Consequently, assembly by the flotation method is enabled. Russell et al. [4] reported the selective assembly of PEO-functionalized CdSe nanorods onto the defined regions of a patterned surface of a PS-b-PMMA film. The method involves floating the copolymer template on an aqueous solution of the nanorods. However, the copolymer template used for the floating technique is solid and cannot freely mediate the nanorods. Figs. 8a and 8b show the TEM images of microphase-separated PS-b-PEO films annealed under toluene vapor for 24 h on the MWCNT-g-PEO/water solution. Segregated PS cylindrical structures were found in the PS-b-PEO/MWCNT-g-PEO nanohybrid films. Most interestingly, the MWCNT-g-PEOs were found to be mainly located in
Fig. 6. TEM images of PS-b-PEO/MWCNT-g-PEO nanohybrid films on solid substrate under toluene vapor annealing for (a) 2 h and (b) 24 h: The MWCNT-g-PEO content was 5 wt.%; scale bar = 200 nm.
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Fig. 7. TEM images of PS-b-PEO/MWCNT-g-PEO nanohybrid films on water surface under toluene vapor annealing for (a) 2 h and (b–c) 24 h: The MWCNT-g-PEO content was 5 wt.%; scale bar = 200 nm.
the PEO microphase. Many benzene ring structures found on the PS chains have π–π conjugations with CNTs [15]. The π–π conjugation between the PS and the MWCNTs induces the MWCNTs to penetrate the PS microphase [9]. However, PEO grafted onto the MWCNTs can cause the MWCNTs to remain in the PEO microphase of the diblock copolymer. The compatibility of MWCNT-g-PEOs with PEO is better than that with
PS in the copolymer because the solubility parameters of PEO in the diblock copolymer and those of PEO grafted onto the MWCNTs are very close. The two factors finally resulted in the MWCNT-g-PEOs being located in the PEO microphase (Fig. 8b). Annealing time was also increased to study the mutual influence of MWCNT-g-PEOs on the self-assembly of the diblock copolymer.
Fig. 8. TEM images of PS-b-PEO/MWCNT-g-PEO nanohybrid films prepared on the surface of water/MWCNT-g-PEO solution under toluene vapor annealing for: (a–b) 24 h and (c–d) 48 h with various magnifications: scale bar = 200 nm for a, and c; scale bar = 90 nm for b, and d.
J. Wang et al. / Thin Solid Films 536 (2013) 191–195
After 48 h of annealing under toluene vapor (Fig. 8c and d), more MWCNT-g-PEOs accumulated within the PEO microphase. No CNT orientation was observed in the PEO microphase. This result demonstrates that the surface affinity of PEO must aid the segregation of the MWCNT-g-PEOs to the water–air interface, where they are absorbed by the soft copolymer templates. Selective localization of the majority of CNTs in the copolymer microphase was achieved using our developed method. We present a versatile approach for the controlled assembly of anisotropic-nanostructured film materials with desirable patterns on soft substrate. 4. Conclusions A floating method for manipulating the localization of CNTs in a block copolymer was introduced. The method involved an aqueous substrate and organic solvent vapor annealing. The MWCNT-g-PEO/ water solution was prepared as the complex substrate for the PS-b-PEO copolymer template. The PS-b-PEO/MWCNT nanohybrid thin films were obtained after the solvent was vaporized and the PEO-functionalized MWCNTs were selectively distributed in the PEO microphase. The polymer ligand coverage on the carbon nanomaterials was found especially effective for achieving the desired assembly.
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Acknowledgments The authors gratefully acknowledge financial support of this work coming from the Natural Science Foundation of China (NSFC) (No. 21074009 and No. 51173009). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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