Enhanced thermal and mechanical properties of carbon nanotube composites through the use of functionalized CNT-reactive polymer linkages and three-roll milling

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Abstract

We report enhanced thermal and mechanical properties of carbon nanotube (CNT) composites achieved through the use of functionalized CNTs-reactive polymer linkages and three-roll milling. CNTs were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group resulting in a chemical reaction. To maximize CNT dispersion for practical usage, entangled CNTs are separated and then evenly dispersed within the polymer matrix using three horizontally positioned rotating rolls that apply a strong shear force to the composite. Consequently, accompanying with thermal stability, elastic modulus and storage modulus of such functionalized CNT/polymer composites were increased by 100% and 500% that of the untreated epoxy polymer.

Introduction

Polymer composites containing conducting fillers such as carbon black [1], carbon fiber, and metal fiber have been extensively investigated for various applications such as structural reinforcement, electromagnetic interference shielding, electronic packaging, radar absorption, heating element construction and high-charge storage capacitors [2], [3], [4], [5], [6], [7]. An enhancement in the mechanical properties has also been observed in the composites, facilitated through load transfer from a low elastic modulus (E) polymer matrix to a high E filler [8]. However, there is a limit to the impregnation of polymers with traditional filler material as the structural strength of these composites begin to breakdown (embrittlement) when the loading is increased significantly. Consequently, high aspect ratio fillers which favor reinforcement and electrical properties/percolation at lower volume fractions are desirable. Carbon nanotubes (CNTs) offer an attractive option in this regard, primarily due to their extremely large aspect ratio, which could be as high as 106, coupled with a large interfacial area >1300 m2/g [9]. Additionally, the CNT surfaces and interfaces can be functionalized and made to interact in a desirable manor with the polymer matrix groups [10]. However, there are still several challenges to the utilization of CNTs as filler. Aggregation and bundling can lead to non-uniform dispersion and poor reproducibility. Furthermore, designing a process that is amenable to large scale production is troublesome when high nanotube loading volume fractions are used [11].

The aim of this work is to present a method to enhance the thermal and mechanical properties of polymer composites containing homogeneously dispersed CNTs for practical usage. Enhanced nanotube polymer interface bonding is achieved, through mutual localized chemical reactions between functional groups on the CNTs and groups on the polymer. Homogeneous dispersion is further facilitated through an effective mixing process. Measurements are presented to show that the thermal and mechanical properties of such functionalized CNT/polymer composites, at identical CNT weight fractions, are superior to those using pristine nanotubes.

Section snippets

Experimental

For fabrication of the CNT composites, epoxy consisting of epoxy medium (Sigma 45345), DDSA (2-Dodecenylsuccinic anhydride), NMA (Nadic methyl anhydride) and DPM-30 (2,4,6-Tris dimethylaminomethyl phenol) was purchased from Sigma Aldrich and both pristine and carboxyl (single bondCOOH) functionalized multiwalled carbon nanotubes (MWCNTs) having 8–15 nm outer diameter and 20–50 μm length were purchased from Cheap-tube Inc. To ensure effective mixing and dispersion of the entangled CNTs within the polymer

Results and discussion

To maximize composite performance for a given CNT wt%, the nanotubes must be separated and then evenly dispersed within the polymer matrix. This was achieved using premixing and three-roll milling regardless of CNT type (COOH-CNT or pristine CNT), the results of which are shown Fig. 1(a)–(c). In both Fig. 1(b) and (c), a 5 wt% CNT loading is depicted. Through three-roll milling, shear force created by three horizontally positioned rolls rotating in opposite directions and at different speeds

Conclusions

It has been shown that the use of localized chemical reactions, between functional groups on CNTs and corresponding group in a polymer matrix, combined with three-roll milling, de-bundling and dispersing by applying a shear force, help enhance the thermal and mechanical properties of CNT composites. Such a method can be used to reliably produce uniform CNT composites. Consequently, the interfacial bonding and load transfer of composites can be enhanced enabling superior E and thermal properties

Acknowledgement

This research is the part of the result carried out by support of the naval institute for ocean research in Republic of South Korea.

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