Surface modified multi-walled carbon nanotubes in CNT/epoxy-composites

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Abstract

Multi-walled carbon nanotubes (MWCNTs), produced by arc-discharge method, were treated with oxidising inorganic acids. The surface modification of the oxidised nanotubes (o-MWCNTs) was achieved by refluxing the tubes with multi-functional amines. The functionalised nanotubes were embedded in the epoxy resin and the resulting composite was investigated by transmission-electron microscopy (TEM). The functionalisation led to a reduced agglomeration and evidences are given for improved interaction between the nanotubes and the epoxy resin.

Introduction

Since the first observation of carbon nanotubes in 1991 by Iijima [1] and the awareness of their novel mechanical and electrical properties [2], [3], [4], an extensive research in the field of CNT/polymer-nanocomposites has started. The introduction of carbon nanotubes as a structure element in nanocomposites should improve the material properties and therefore a large variety of applications, such as devices in nanoelectronics, field emitters or reinforcing materials are imaginable. However, due to their limited availability together with the high purchasing costs, only a few of these applications have been realised, yet. The progress in bulk synthesis and purification [5], [6], [7] make carbon nanotubes available in larger quantities at falling prices.

The progress in developing nanocomposites with carbon nanotubes approves their function as a reinforcing structural element. Studies using MWCNTs in polymer-composites reported an increased storage modulus [8], [9].

The interfacial interaction between the nanotubes and polymer is one of the issues of nanocomposites. First experimental work focusing on the interfacial interaction in MWCNT/epoxy-nanocomposites was performed by Cooper et al. [10]. They investigated the detachment of MWCNTs from an epoxy matrix. In a special pull-out test for individual nanotubes values for the interfacial shear strength were determined varying from 35 to 376 MPa. This large variation and the fact that no dependence on the size of interface and the shear strength could be observed requires further investigations.

Our research is focused on the evaluation of nanotube/epoxy-composites. The realisation of nanotube reinforced epoxies requires a homogeneous dispersion and strong interfacial interaction between the nanotubes and the polymer. We suppose that the use of MWCNTs as reinforcing elements in epoxies suffers from weak interfacial interactions.

An enhancement of the compatibility to the composite material can be achieved by a chemical modification of the MWCNTs, which could link nanotubes directly to the matrix. This linkage can be achieved by a reaction of functional groups with the epoxy, which enables a stress transfer between the polymer and the CNTs. The effect of a linkage between the nanotubes and the polymer, which should lead to improved interfacial interactions, is confirmed by calculations by Frankland et al. [11]. They predict that a functionalisation of less than 1% would improve the interaction between nanotubes and the polymer without decreasing their strength significantly.

In this Letter we present a procedure for a surface modification of carbon nanotubes which leads to a reduced agglomeration and furthermore, some evidences are given which indicate an improved interfacial interaction between the functionalised CNTs and the epoxy resin.

Section snippets

Experimental

The multi-walled carbon nanotubes used in this studies were arc-grown material produced at TUHH or obtained by MER Inc. The nanotubes were heated with an excess of a mixture of sulphuric and nitric acid in order to remove impurities of carbon black and graphite nanoparticles. These oxidised MWCNTs were separated by centrifugation and membrane micro-filtration (Millipore Durapore Membrane) and washed with distilled water and acetone.

In a second step the oxidised nanotubes were heated with an

Results and discussion

Our procedure to improve the interaction with the matrix is comparable to previous work by Chen et al. [12] using an ionic functionalisation to improve the stability of single wall carbon nanotube (SWCNT) suspensions. In order to remove amorphous carbon, we treated the nanotubes with oxidising inorganic acids. Besides the fast removal of these thermodynamically less stable impurities, a much slower oxidation of the nanotubes, starting at structural defects and at the caps of the tubes, occurs

Conclusion

A chemical functionalisation of MWCNTs can be a useful tool to improve the dispersion of the nanotubes in an epoxy system. The investigation of the CNT/epoxy-nanocomposites via TEM gives evidences for improved interfacial interaction between the functionalised nanotubes and the matrix. These interactions should be strong enough to potentially increase the mechanical properties of carbon nanotube reinforced epoxy-composites. The investigation of the effect of surface modification of the CNTs on

Acknowledgements

The support of the German Research Foundation (DFG Schu 926/8-1 and SFB 371-C9) is gratefully acknowledged.

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    Citation Excerpt :

    However, gamma (γ) ray irradiation, which was introduced as a control method to modify the physical and chemical properties of the carbon nanotubes (CNTs), has recently captured a lot of attention. Researches have shown that defects can form during the CNT growth and purification stages [1,2], during device or composite production [3,4], or under mechanical strains [5]. They have also indicated that even a small number of defects in the atomic network will cause some degradation [6,7] of their mechanical properties, resulting in scattering centres for phonon propagation along the tube axis and thus reducing intrinsic tube conductivity [8].

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