Crystallization induced block copolymer decoration on carbon nanotubes

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[10] Pandey D, Reifenberger R, Piner R. Scanning probe microscopy study of exfoliated oxidized graphene sheets. Surf Sci 2008;602:1607–13. [11] Nemes-Incze P, Osva´th Z, Kamara´s K, Biro´ L. Anomalies in thickness measurements of graphene and few layer graphene crystals by tapping mode atomic force microscopy. Carbon 2008;46:1435–42.

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Crystallization induced block copolymer decoration on carbon nanotubes Wei-ru Wang a, Xu-ming Xie a b

a,* ,

Xiong-ying Ye

b

Advanced Materials Laboratory, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Department of Precision Instrument and Mechanology, Tsinghua University, Beijing 100084, China

A R T I C L E I N F O

A B S T R A C T

Article history:

A unique noncovalent means to decorate block copolymers on carbon nanotubes (CNTs)

Received 17 August 2009

using a controlled polymer crystallization method is presented. Transmission electron

Accepted 31 December 2009

microscope observation and electron diffraction result demonstrated the surface function-

Available online 6 January 2010

alization of CNTs with a crystalline–noncrystalline triblock copolymer poly(vinylcyclohexane)-b-poly(ethylene)-b-poly(vinylcyclohexane) (PVCH-PE-PVCH), forming a novel nanohybrid epitaxial brush structure, which consists of a central CNT and disc-shaped foldedchain lamellae of PE blocks with random coils of amorphous PVCH blocks surrounding them. Ó 2010 Elsevier Ltd. All rights reserved.

Despite from extraordinary mechanical properties and thermal conductivity to unique electronic and optical properties [1] carbon nanotubes (CNTs) offer tremendous opportunities for the development of fundamentally new material systems, the intrinsic poor dispersibility and processability of CNTs have hindered their further use in the practical applications [2,3]. Accordingly, surface functionalization of CNTs becomes an essential step [4–6]. Using polymer chains to ‘wrap’ CNTs is a versatile and effective way for CNT functionalization. In particular, block copolymers (BCPs) may provide a series of attractive noncovalent wrapping and decoration methods for the functionalization of CNTs. These approaches can be driven by distinct interactions between CNTs and polymers including p-stacking, electrostatic interactions, and decoration of CNTs with micelles [7–9]. Compared with homopolymers, BCPs enhance the dispersibility and stability of CNTs in a wider range of organic solvents and host polymer matrices by means of the dual action: one block of the polymer forms a close interaction with CNTs, while the other block(s) provide the dispersibility and chemical compatibility

to the CNTs [10]. More attractively, the intriguing ability of BCPs to self-assemble into ordered nanostructures brings us an ideal nanoscale template for CNT alignment control, and can be utilized further for fabrication of functional hybrid materials and functional devices including flexible field emission display panels. Park et al. [11] reported a two-dimensional alignment of CNT in a lamellar polystyrene (PS) microdomain of PS-b-polyisoprene (PI) diblock copolymer. In our previous work [12], a selective quasi one-dimensional alignment of PS decorated CNTs in the cylindrical PS phase of a microphase-separated asymmetric styrene–butadienestyrene (SBS) triblock copolymer was successfully realized. The progresses give a new significance to the functionalization of CNTs with BCPs and dispersion of CNTs in BCP matrices. However, the challenge for the uniform dispersion of CNTs in selective microdomains of BCPs and self-assembly alignment in BCP ordered nanostructure is still huge. Recently, Li et al. [13,14] achieved a periodical surface decoration of CNTs using controlled crystallization of homopolymers such as poly(ethylene) (PE) and nylon-66. The

* Corresponding author: Fax: +86 10 62784550. E-mail address: [email protected] (X.-m. Xie). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.12.059

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heteroepitaxial growth of crystalline polymers on CNT surfaces provides a new way in noncovalent methods. Therefore, we explore herein a new attempt to decorate BCP on CNTs via a controlled polymer crystallization method, and investigation in the corresponding decoration morphology. Multi-walled carbon nanotubes (MWCNTs) were purified by washing in 2.4 M nitric acid before use. A crystalline–noncrystalline triblock copolymer, poly(vinylcyclohexane)-bpoly(ethylene)-b-poly(vinylcyclohexane) (PVCH-PE-PVCH) (Mw = 40,000; Mw (PE) = 22,000; polydispersity index (PDI) < 1.10) obtained from Dow Chemical Co. was used, and a polymer solution crystallization technique was employed in the experiment. MWCNTs (0.1 mg) were dispersed in 1 g of p-xylene by ultrasonication, followed by adding 4 g 0.018 wt.% PVCH-PE-PVCH/p-xylene solution. The mixture was then quenched to 103 °C, and isothermally crystallized for 0.5 h. The morphologies were investigated by using a JSM 7401F type scanning electron microscope (SEM), a Hitachi H-800 type transmission electron microscope (TEM) and high-solution TEM of JEM 2010. Before the solution crystallization, MWCNTs were effectively dispersed in the solvent by ultrasonication, separately dispersing in the suspension as shown in Fig. 1a. At the relatively high crystallization temperature of 103 °C, PE blocks in the PVCH-PE-PVCH preferred to heterogeneously nucleate on the surface of MWCNTs. As a result, after crystallization for 0.5 h, PE blocks formed periodic lamellar crystals along the MWCNT axis. Accordingly, as presented in Fig. 1b, BCP decorat-

Fig. 1 – (a) TEM micrograph of an individual MWCNT dispersed in p-xylene after ultrasonication. (b) TEM micrograph and (c) SEM micrograph of the PVCH-PE-PVCH/ MWCNT NHEB structure obtained after crystallization at 103 °C for 0.5 h.

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ing the MWCNT due to the heteroepitaxial growth of PE blocks on the surface, resulted in a test-tube brush-like nanostructure, which consists of a central MWCNT and surrounding ‘‘bristles’’ formed by PE crystals together with random coils of PVCH blocks. Therefore, we named this novel and interesting structure as so-called ‘‘nano-hybrid epitaxial brush’’ (NHEB). SEM image in Fig. 1c clearly exhibits the NHEB structure. In order to investigate the crystalline morphology of PE blocks decorated on the CNT surfaces, the sample was selectively etched by use of ruthenium tetraoxide. The PE crystals could be survived in this process. Fig. 2a and b show the TEM micrographs of the nanostructures of the NHEB before and after oxide etching, respectively. In comparison, the structure in etched sample clearly shows a central MWCNT periodically decorated with disc-shaped lamellae, similar with the reported nano-hybrid shish-kebab (NHSK) structure in morphology [13,14]. The PE single folded-chain lamella structure was further confirmed by electron diffraction (ED). Fig. 2c shows the corresponding ED pattern of the selected area (arrow in Fig. 2b). The symmetrical pattern indicates that PE crystallized chains inside the lamella align along the direction of the arrow in Fig. 2c, namely the MWCNT axis direction. The nanostructure is clearly illustrated in the schematic representation of Fig. 3b. Note that the PE lamellae in the NHSK structure shown in Fig. 2b (etched sample) possess an average lateral size of

Fig. 2 – TEM images of (a) the original PVCH-PE-PVCH/ MWCNT NHEB structure before oxide etching and (b) the NHSK structure obtained after oxide etching the original NHEB structure with ruthenium tetraoxide. (c) Corresponding ED pattern of the selected area (arrow in b), and the white arrow represents the orientation direction of PE chains in the lamella.

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Fig. 3 – Schematic representations of (a) the original PVCHPE-PVCH/MWCNT NHEB structure and (b) the NHSK structure obtained after oxide etching. The polymer chains of PE blocks and PVCH blocks in the figures are shown as real line (—) and dash line (ÆÆÆÆÆÆÆ), respectively.

460 nm, smaller than the average external lateral size of the original NHEB (560 nm). Moreover, after oxide etching, spaces are clearly visible between the adjacent disc-shaped PE lamellae. It implies that the amorphous chains of PVCH blocks mainly distribute in the surrounding areas of the PE lamellae, tethered by the covalently bonded PE blocks despite without direct interaction to MWCNT surfaces, as the dash line shown in the schematic representation of the NHEB structure (Fig. 3a). The controlled heteroepitaxial growth of PE blocks on MWCNT surfaces mainly induces the formation of the NHEB structure. In the NHEB structure, CNT provides a 1D nucleation surface to the crystalline PE blocks, while PVCH chains exist around the PE lamellae. Accordingly, the local phase separation of the BCPs causes the alternating decoration on MWCNTs. It was also observed that the decorated MWCNTs existed individually in the solution, revealing that the formation of the NHEBs enhances the dispersibility of MWCNTs. The polymer crystallization provides a simple and effective method for decorating crystalline–noncrystalline BCPs on CNTs. The surface functionalization method does not destroy the integrity of CNT structure, but leads to a strong interaction between CNT surface and BCP chains by means of crystallization. The interaction, together with the self-assembly microphase-separation of BCPs, could become an internal driving force for the entrance and existence of decorated CNTs in microdomains of BCPs, affording a promising approach to achieve uniform CNT arrays by controlling BCPs’ nanostructure. Furthermore, it is worthwhile to mention that by adding more PVCH-PE-PVCH/p-xylene solution at the higher concentration of 1.8 wt.% to the NHEB/p-xylene suspension and crystallizing for 3 h, BCP decorated the CNTs forming an epitaxial cylinder structure with an individual MWCNT core due to the further crystallization of PE blocks on the original NHEB struc-

Fig. 4 – TEM image of the epitaxial cylinder structure with an individual MWCNT core formed after adding more PVCH-PEPVCH into the NHEB/p-xylene suspension and further crystallizing at 103 °C. ture (Fig. 4). Interestingly, besides PE lamellae on the side, the central MWCNT is decorated with a half-spherulite like crystal on each end. More experimental works are ongoing. In conclusion, a crystalline–noncrystalline BCP has been successfully functionalized on MWCNT surfaces via a controlled polymer crystallization method. Individual MWCNTs were decorated with disc-shaped PE folded-chain lamellae surrounded by PVCH random coils, forming a novel NHEB structure. The method not only provides a noncovalent technique for BCP decoration on CNT surfaces, but also provides a promising approach to the uniform dispersion of CNTs in selective microdomains of BCPs and controlled alignment of CNTs in BCP ordered nanostructures.

Acknowledgements The authors sincerely thank Prof. Fei Wei for his providing us with purified MWCNTs. Financial support from the National Natural Science Foundation of China (Nos. 50573038, 20874056 and 50675118) , the 863 Program (No. 2009AA0 4Z308) and Specialized Research Fund for Doctoral Program of Higher Education (No. 200800030047) are highly appreciated.

R E F E R E N C E S

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[10] Szleifer I, Yerushalmi-Rozen R. Polymers and carbon nanotubes – dimensionality, interactions and nanotechnology. Polymer 2005;46:7803–18. [11] Park I, Lee W, Kim J, Park M, Lee H. Selective sequestering of multi-walled carbon nanotubes in selfassembled block copolymer. Sensor Actuat B-Chem 2007;126:301–5. [12] Liu YT, Zhang ZL, Zhao W, Xie XM, Ye XY. Selective selfassembly of surface-functionalized carbon nanotubes in block copolymer template. Carbon 2009;47:1883–5. [13] Li CY, Li LY, Cai WW, Kodjie SL, Tenneti KK. Nanohybrid Shish-kebabs: periodically functionalized carbon nanotubes. Adv Mater 2005;17:1198–202. [14] Li LY, Li CY, Ni CY. Polymer crystallization-driven, periodic patterning on carbon nanotubes. J Am Chem Soc 2006;128:1692–9.

Alkyl-functionalized graphene nanosheets with improved lipophilicity Yewen Cao, Jiachun Feng *, Peiyi Wu Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Graphene nanosheets grafted with long alkyl chains were produced by simply using an

Received 28 September 2009

amidation reaction. Compared with unmodified counterparts, the alkyl-functionalized

Accepted 27 December 2009

graphene nanosheets show largely enhanced lipophilicity, as illustrated by their remark-

Available online 6 January 2010

ably improved dispersion in the nonpolar solvents and polypropylene matrix. The polypropylene/alkylated nanosheets composites exhibit largely improved thermal stability, which suggests that this lipophilization method is a potential technique for developing high-performance composites. Ó 2010 Elsevier Ltd. All rights reserved.

Graphene nanosheets (GNSs) have attracted much attention in many potential applications, such as composites, transparent conductive films, field effect transistors and ultrasensitive sensors [1,2]. One practical route to harness the unique properties of GNSs would be to incorporate them into polymer materials [3]. Compared with exfoliation of graphite and epitaxial growth on SiC, the deoxygenation of graphite oxide (GO) nanosheets is widely considered as a more promising fabrication technique in that it enables mass production of GNSs at low cost [2–6]. Unfortunately, since the GO nanosheets are heavily oxygenated and the oxygen-containing groups are difficult to be completely removed during the deoxygenation process, the GNSs prepared by this strategy are decorated with some oxygen functionalities, such as carboxyl groups [4]. The presence

of these oxygen functional groups facilitates the uniform dispersion of GNSs in the polar polymers [5]. However, a huge challenge still lies in the generation of satisfactory compatibility between GNSs and nonpolar polymers, which hampers the development of such composites for high-performance applications. Considering that the nonpolar polymers are much more influential and versatile in industrial applications, it is of great significance to lipophilically modify the as-reduced GNSs so that they can be homogeneously dispersed in the nonpolar polymers. In this letter, by analogy with former works on GO nanosheets [7], we present an easy approach to covalently functionalize GNSs with long alkyl chains, which is demonstrated to be an effective way to enhance the compatibility between GNSs and nonpolar polymers. Synthesized and purified from

* Corresponding author: Fax: +86 21 6564 0293. E-mail address: [email protected] (J. Feng). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2009.12.061