Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites

Abstract

The novel properties of carbon nanotubes have generated scientific and technical interest in the development of nanotube-reinforced polymer composites. In order to utilize nanotubes in multi-functional material systems it is crucial to develop processing techniques that are amenable to scale-up for high volume, high rate production. In this research we investigate a scalable calendering approach for achieving dispersion of CVD-grown multi-walled carbon nanotubes through intense shear mixing. Electron microscopy was utilized to study the micro and nanoscale structure evolution during the manufacturing process and optimize the processing conditions for producing highly-dispersed nanocomposites. After processing protocols were established, nanotube/epoxy composites were processed with varying reinforcement fractions and the fracture toughness and electrical/thermal transport properties were evaluated. The as-processed nanocomposites exhibited significantly enhanced fracture toughness at low nanotube concentrations. The high aspect ratios of the carbon nanotubes in the as-processed composites enabled the formation of a conductive percolating network at concentrations below 0.1% by weight. The thermal conductivity increased linearly with nanotube concentration to a maximum increase of 60% at 5 wt.% carbon nanotubes.

Introduction

Developments in producing nanostructured materials with novel properties have generated significant excitement about potential for utilizing these materials in macroscopic engineering applications. Fiber-like carbon nanotubes are potentially ideal reinforcement for nanocomposites as well as for development of nanotechnologies in general [1]. At the nanoscale, this unique form of carbon shows extraordinary mechanical and physical properties, with predicted elastic moduli of about 1 TPa (1000 GPa), strengths in the range of 30 GPa, and exceptional resilience, showing large, nonlinear deformation before fracture [2]. Carbon nanotubes also possess exceptionally high axial thermal conductivities as well as electrical conductivity that can be metallic or semi-conducting depending on the atomic structure.

These nanocomposites represent a new frontier in materials science because the reinforcement scale has changed from micrometers with traditional carbon or glass fibers to nanometers. Conventional approaches to fabrication and characterization are not adequate and modeling often requires scaling down to atomistic levels. In composite materials there exists a strong interrelationship between the local structure at the micro or nanoscales and the bulk properties [3]. This internal structure of the composite is formed during the processing step. By tailoring properties at the nanoscale through controlling processing conditions the possibility exists to realize multi-functional nanocomposites with properties not attainable in traditional material systems. Toward development of nanostructured functional and structural materials and employment on a macroscopic scale, there are basic manufacturing issues that need to be addressed. It is crucial to develop laboratory-scale techniques amenable to scale-up for production of macroscopic structures. High volume, high rate and cost-effective means of manufacturing carbon nanotube-based composites have potential to dramatically enhance large-scale application of structural and functional nanocomposites.

As-processed nanotubes exist in a wide range of morphologies from single- and double-walled nanotubes to large diameter multi-walled nanotubes containing numerous concentric layers. Different morphologies present unique processing challenges. For effective reinforcement, nanotubes must be uniformly dispersed within the polymer matrix. Van der Waals interactions between small-diameter nanotubes result in aggregates of nanotube ropes. In addition to slipping of tubes not adhered to the matrix, aggregates of nanotube ropes effectively reduce the reinforcement aspect ratio. Also, agglomeration is significant in chemical vapor deposition (CVD) grown multi-walled nanotubes where there exists substantial nanoscale spaghetti-like entanglement of nanotubes. This mechanical interlacing of carbon nanotubes is a significant barrier toward achieving a homogeneous dispersion of nanotubes in a composite.

Characterization and structure/property nanomechanics modeling research has shown that enhancements in elastic properties of multi-walled nanotube composites are strongly diameter-dependent [4]. As a consequence of the intra-tube van der Waals interaction between the layers of the tube and the transfer of load to the nanotube via shear stresses at the nanotube/matrix interface, the effective stiffness of multi-walled nanotubes in a polymer matrix is reduced [5]. Completely exfoliated small-diameter single- or double-walled carbon nanotubes will be most effective for increasing elastic stiffness, but multi-walled carbon nanotubes offer significant potential as a possible multi-functional reinforcement. High purity multi-walled nanotubes are readily available from a wide-variety of sources at a fraction of the cost of single-walled carbon nanotubes. Their high aspect ratios and large interfacial surface area makes multi-walled carbon nanotubes an attractive candidate for potentially enhancing electrical and thermal conductivities as well as mechanical properties such as toughness [6], [7], [8], impact resistance [9], [10] and vibration damping [11], [12], [13].

A variety of processing techniques for nanotube-reinforced composites have been investigated, and these techniques have been highlighted in a recent review [1]. For processing nanotube composites using thermoset matrix materials, such as epoxies or vinyl esters, most approaches involve several processing steps that may include high speed mixing [14], [15], high-energy sonication and solution-evaporation processing [9], [16], [17], surfactant-assisted processing through formation of a colloidal intermediate, or functionalization of nanotubes with the polymer matrix [18], [19]. In our earlier research we developed solvent-assisted melt dispersion technique to produce highly dispersed and aligned nanotube/thermoplastic polymer composite films [20], [21]. We later adapted this technique to disperse multi-walled nanotubes in a thermosetting polymer to serve as a precursor to the fabrication of ceramic matrix composites [22]. A similar approach has been used recently by Moniruzzaman et al. [23] to disperse single-walled nanotubes in epoxy.

Most of these approaches are limited in scalability and not amenable to high volume, high rate production. To evaluate such key engineering properties as strength, toughness and electrical/thermal conductivity it is crucial to fabricate more macroscopic nanocomposite test specimens. Recent research by Gojny et al. [6], [24], [25], [26] introduced a calendering approach to achieve dispersion of amine-functionalized nanotubes. Calendering is used commercially for dispersing pigments in inks, paints and cosmetics. This approach utilizes adjacent cylinders rotating at different velocities to impart high shear stresses. Because the mixture must pass through the gap between the rotating cylinders the process also uniformly shears the entire volume of the material. This approach applied to nanotube composite processing represents a significant advance toward development of solvent-free, scalable manufacturing techniques. This type of technique is readily scalable from laboratory to manufacturing settings and can achieve high throughput for cost-effectiveness.

In the approach of Gojny et al. [6], [24], [25], [26] nanotubes were mixed into epoxy resin and then small quantities of the mixture added batch-wise to a three roll calendering mill with a gap setting of 5 μm between the rolls. The small gap between and mismatch in roll velocity results in enormous shear forces. A volume of nanotube/epoxy mixture was added to uniformly coat the rolls in the mill and was sheared for approximately two minutes before collecting. The shearing/collection process was subsequently repeated until the desired volume of dispersed nanotube/epoxy mixture was obtained.

In this research we utilize a similar calendering approach to disperse CVD-grown multi-walled carbon nanotubes in an epoxy matrix and study the evolution of nanocomposite structure during processing. Insight gained through studying the process-induced structural changes was used to develop optimized processing protocols for nanotube dispersion. After development of processing protocols for calendering several compositions of nanotube composites were processed and the influence of the nanoscale dispersion on the fracture toughness and electrical/thermal properties assessed.