Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology

https://doi.org/10.1016/j.compscitech.2009.08.006Get rights and content

Abstract

Advanced composites, such as those used in aerospace applications, employ a high volume fraction of aligned stiff fibers embedded in high-performance polymers. Unlike advanced composites, polymer nanocomposites (PNCs) employ low volume fraction filler-like concepts with randomly-oriented and poorly controlled morphologies due to difficult issues such as dispersion and alignment of the nanostructures. Here, novel fabrication techniques yield controlled-morphology aligned carbon nanotube (CNT) composites with measured non-isotropic properties and trends consistent with standard composites theories. Modulus and electrical conductivity are maximal along the CNT axis, and are the highest reported in the literature due to the continuous aligned-CNTs and use of an unmodified aerospace-grade structural epoxy. Rule-of-mixtures predictions are brought into agreement with the measured moduli when CNT waviness is incorporated. Waviness yields a large (∼10×) reduction in modulus, and therefore control of CNT collimation is seen as the primary limiting factor in CNT reinforcement of composites for stiffness. Anisotropic electron transport (conductivity and current-carrying capacity) follows expected trends, with enhanced conductivity and Joule heating observed at high current densities.

Introduction

Bulk nanostructured composites combining existing advanced fibers, structural polymers, and carbon nanotubes (CNTs) with tailorable and enhanced macroscopic engineering properties are being developed by many groups for aerospace and other applications [1]. Development of such materials requires scaling and establishing long-range order of the nanostructures, and also an understanding of the properties of the constituents and how they interact [2], [3], [4], [5], [6], [7]. Here, an aligned-CNT polymer nanocomposite (A-PNC) is fabricated using a high-performance (aerospace-grade structural epoxy) thermoset, and anisotropic multifunctional properties quantified and discussed relative to morphology of the samples. High volume fraction (Vf) continuous-CNT A-PNCs are fabricated via a novel mechanical densification [8] technique (see Fig. 1). This avoids the issue of dispersion, random orientation, and discontinuity (non-continuous CNTs) of the CNTs inherent in the extant work on PNCs which has focused almost solely on filler-like concepts [9], [10], [11]. Controlling morphology of the PNCs is critical for interpreting the multifunctional property results, and both SEM and X-ray scattering provide detailed quantification of PNC morphology herein. Randomly-oriented PNCs (R-PNCs) are also fabricated for comparison of unaligned vs. aligned reinforcement. Modulus and electrical conductivity are maximal along the CNT axis in A-PNCs, and are the highest reported to date due to the aligned CNT morphology and use of an unmodified aerospace-grade structural polymer [12], [13], e.g., the modulus is 1000× greater than recently reported for an elastomeric A-PNC [14], and the highest bulk conductivity (23 S/m) is reported for an epoxy PNC along the axis of the continuous CNTs. The measured modulus is in agreement with micromechanics predictions that consider waviness of the CNTs, and both modulus and electrical conductivity trends are consistent with standard physical models. Control of nanostructure morphology is essential for understanding and predicting properties of a broad array of new materials, including CNT-based fibers, 3D nano-engineered composites, and metamaterials [15], [16], [17], [18], [19].

Theoretical calculations and experimental measurements on individual CNTs show that these one dimensional materials have elastic moduli between 0.5 and 1 TPa and tensile strengths of perhaps 50–200 GPa [20], [21], [22] making them ideal reinforcement candidates for composites, and even more attractive given their low densities. From the existing literature, extensive efforts have focused on dispersing single or multiwalled CNTs in low modulus polymers for reinforcing thermoplastics [13], [23] for applications such as electrically conducting composites [24].

However, the poor properties of these polymers and the necessary processing conditions make these nanocomposites unsuitable for advanced structural composites. Dispersion and distribution challenges limit the reinforcement volume fraction typically to below 5% of randomly-oriented CNTs. Alternatively, some researchers have focused on synthesizing aligned CNTs [25], [26], [27], [28] and the combination of aligned and continuous CNTs with structural resins like epoxy could achieve maximal mechanical reinforcement and transport properties. In this work, both aligned and random PNCs are compared up to CNT volume fractions of 20% (A-PNCs only), close to practical and theoretical packing limits for 8–10 nm diameter CNTs.

Section snippets

Experimental

Fabrication of aligned-CNT and randomly-oriented CNTs polymer nanocomposites (A-PNCs and R-PNCs, respectively) are described before morphology characterization, mechanical, and electrical testing are discussed.

Results and discussion

Polymer nanocomposites were fabricated from vertically-aligned multi-walled CNT forests (sometimes called VANTA, vertically-aligned nanotube arrays) grown via a modified chemical vapor deposition (CVD) process [25]. The resulting forests have been characterized previously for alignment, distribution and spacing [32]. A-PNCs were fabricated for various volume fractions via mechanical densification of the VANTA followed by capillary-induced wetting with an aerospace-grade thermoset epoxy (see

Conclusions and recommendations

The inclusion of aligned CNTs in bulk materials such as existing advanced composites is an important step forward in engineering materials, and can provide several advantages including tailoring and manufacturability of complex architectures. Control of nanoscale morphology in polymer nanocomposite as demonstrated herein allows measured properties to be appropriately interpreted. The results and trends in this study demonstrate anisotropic behavior via morphology control makes clear the classic

Acknowledgements

This study was supported by Airbus S.A.S., Boeing, Embraer, Lockheed Martin, Saab AB, Spirit AeroSystems, Textron Inc., Composite Systems Technology, and TohoTenax Inc. through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium. Hülya Cebeci acknowledges support from Scientific and Technical Research Council of Turkey (TUBITAK) for a 2214-International Research Fellowship Programme. Roberto Guzman de Villoria is grateful for the support of the Ministry of Science and

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