Selective sequestering of multi-walled carbon nanotubes in self-assembled block copolymer

Abstract

Block copolymer (BCP) can self-assemble into various nanostructures thorough their unique microphase separation. The role of soft materials such as block copolymers in sequestering the functional materials (as nanoparticles and biomaterials) aims at templating the nanostructure to achieve its desirable function. Depending on functional elements incorporated in BCP, various applications have been proposed such as the mechanically reinforced materials or biosensors or wave-guiding photonic materials. While most of nano-elements are isotropic in shape (i.e. sphere), the carbon nanotubes (CNTs) exhibit the high aspect ratio. A typical CNT has the diameter of several nm and the length of a few μm. CNTs generally have a superior electrical conductivity and mechanical strength, thus its composite with soft materials can form a platform material for flexible display panel or e-paper. In this study, we demonstrated in situ functionalization of multi-walled carbon nanotube (MWNT) with polymer via emulsion polymerization. Polystyrene-functionalized MWNT was prepared in an aqueous solution containing styrene monomer, non-ionic surfactant and the cationic coupling agent ([2-(methacryloyloxy)ethyl]trimethylammonium chloride, MATMAC) and produced PS-functionalized MWNTs by dissolving them in toluene over 72 h. Consequently, it increased the solubility and the dispersion of nanotubes even with a low degree of functionalization. A lamellar forming polystyrene-b-isoprene (400k-360k, PS-b-PI) was used as the structure-guiding materials in ordering the PS-functionalized MWNTs anisotropically in PS domain. This method might contribute to the fabrication of CNT-embedded nanofunctional materials by controlling the orientation and the spatial distribution very effectively.

Introduction

Asymmetrically shaped nano-objects such as nanotubes, nanorod, nanosheet, etc., have been of great interest among researchers especially in manufacturing new-type of engineering materials and their use has been determined for anisotropic properties such as electric conductivity, thermal conductivities and so on. Among them, carbon nanotubes (CNTs) have been paid attention for various potential applications due to mechanical reinforcement, superior thermal and electrical conductivities and thermal resistance [1], [2].

The dispersion of fillers and the control of its alignment within the host polymer matrix have been expected to govern the overall properties in manufacturing the composite materials filled with CNTs. Rational ways to improve the degree of dispersion within host polymer matrix have been suggested briskly via chemical or physical surface modification with surfactants, polymers, acids and so on [3], [4], [5]. Recently the alignment of CNTs in matrix has received much attention. It has been mainly conducted when using well-aligned CNTs from the synthetic step or processing these CNT–polymer nanocomposites under the flow field, magnetic field or electrical field, in which some researchers have proposed that the increase of the alignment is a new efficient way to improve their desirable properties [5], [6], [7], [8], [9].

Block copolymer might be a suitable material to fulfill such demands. In the area of CNT-related researches, several researchers already have addressed the block copolymer as a good dispersant of CNTs. The stabilization of single-walled nanotubes (SWNTs) by block copolymer micelles adhered to the surface of CNTs enable SWNTs to disperse in either polar solvents or non-polar ones at the same time [10], [11]. The fabrication of amphiphilic layer-coated multi-walled nanotubes (MWNTs) was accomplished through in situ atom transfer radical polymerization (ATRP) resulting in MWNT grafted with various contents of polymer layers, which may contribute to the way to prepare CNT-based nanomaterials and molecular devices [12], [13]. These approaches have been based on the amphiphilic properties of block copolymer (BCP) and focus on the issue of dispersion within polymer matrix. Self-assembled BCP possesses various microstructures and have been applied as the superior structure-guiding materials in the application of nanocomposites [14], [15], [16], [17], [18], [19]. It seems to be able to open the door for promising, novel hybrid materials, ranging from the alignment control and the spatial distribution of the CNTs. The selective sequestering of nanoparticles such as gold particles or quantum dots has been investigated and sol–gel process within block copolymer microstructure demonstrated a way to build the inorganic–organic nanocomposite using block copolymer as the scafford materials. The sequestering of highly asymmetric nano-objects within block copolymer microstructures have not been explored experimentally, although dealt in theoretical approaches.

Here we report a fabrication of BCP-based nanocomposite with selectively located nanotubes in one microdomain. A lamellar forming polystyrene-b-polyisoprene (400k-360k, PS-b-PI) was used as a structure-guiding material for ordering MWNTs anisotropically in PS domain. To increase the interaction between nanotubes and one component of BCP preferentially, PS-grafted MWNTs were prepared by emulsion polymerization and placed within PS components of BCP. It was the first demonstration to the best of our knowledge and believed that the breakthrough associated with the formation of CNT-aligned BCP microstructures would enable us to continue advances on the novel fabrication of nanotube-based nanomaterials or nanodevices with desirable morphology and orientation.