Radial collapse of carbon nanotubes for conductivity optimized polymer composites

Abstract

The optimization of the electronic conduction of carbon nanotube polymer composites is studied by tuning the radial geometry of the carbon nanotubes in a compression cycle. We have investigated the structural evolution of multi-walled carbon nanotubes in a polyamide matrix as a function of applied high pressure. Combining high resolution electron microscopy and small angle neutron scattering experiments, we conclude that the nanotube radial cross-section is irreversibly deformed following applied pressures up to 5 GPa. Studying highly percolated composites we observe that the sample resistivity drastically decreases with pressure up to about 2 GPa with no further change up to the maximum 5 GPa applied pressure. An important hysteresis is observed upon decompression which leads to an enhanced electrical conductivity of the composite in all the studied compression cycles with maximum pressures ranging from 1 to 5 GPa. Modelling the radial collapse of single-walled carbon nanotubes shows that the modified radial geometry can considerably improve the electronic transport properties in contacted carbon nanotube junctions. Our results open opportunities for engineering nanotube composites by controlling the radial collapse.

Introduction

The exceptionally high elastic modulus and tensile strength of carbon nanotubes (CNT) is driving their development as reinforcing agents in next generation composite materials [1], [2], [3]. These properties, together with their electrical and thermal conductance, have been decisive for an emerging industrial interest in CNT in low weight reinforced composites, conductive polymers or advanced nanocomposites with multi-functional features. Nevertheless several challenges need to be tackled before CNT realize their full potential; such as poor dispersion or alignment [4], which together with higher production costs presently prevent CNT to fully compete with carbon fibers. Nevertheless, CNT composites remain attractive as they exhibit very low electric percolation thresholds [5]. Further developments of CNT-based composites, taking advantage of CNT exceptional physical properties, are in all cases needed from synthesis methods and scaling to system design.

Most of the physical properties of CNT rely on their particular geometry: an atomic carbon hollow cylinder with a very high aspect ratio. The circular cross-section constitutes an important characteristic feature, which is determinant for many CNT properties.

The modification of the CNT cross-section geometry constitutes a parameter which up to now has not attracted much attention in the design of CNT composite materials. It is nevertheless known that the radial cross-section adopted by CNT depends on their geometrical parameters (diameter and number of walls) and can be very far from circular [11]. The collapsed structure, also called dog-bone type has been observed in a number of large-diameter single-walled (SWCNT), double-walled (DWCNT) and few-walled (FWCNT) carbon nanotubes at ambient pressure [6], [12], [9], [7], [8], [13], [14]. Some of these observations are summarized in Fig. 1. For very large diameter FWCNT, the collapsed geometry has been proposed as an analogue for graphene nanoribbon structures [15]. The collapsed structure is stabilized through the additional van der Waals interactions provided by the inter-wall interaction which compensates for the involved elastic energies [16]. A number of precautions should be taken when considering published observations of collapsed tubes, such as channelling effects in TEM measurements that can contribute to the tube collapse [12], as well as substrate interactions [7], [17] that can modify the tube cross-section. The existence of a wide region of metastability of collapsed/non-collapsed structures has been predicted [10]. Atomistic calculations [18], [10] (also included in Fig. 1) seem to confirm the experimental trend indicating that at ambient pressure the collapsed tube geometry is governed by the number of walls and the radius of the tube.

Radial collapse of carbon nanotubes can be also obtained by applying pressure. In SWCNT the pressure induced radial collapse has been both theoretically predicted [19], [16], [20], [21] and experimentally observed [22], [23], [24]. There exists as well a number of predictions of pressure induced collapse in MWCNT [25], [26], [27].

There have been a few predictions on the electronic structure evolution of SWCNT subjected to radial deformations [28], [29], [30], [31], [32], [33], [34], [35]: they basically show that both metal-semiconductor and semiconductor-metal transitions are possible, depending on the tube chirality and on the degree of deformation. There have been some experimental confirmations of both a semiconductor to metal transition in SWCNT [36] and of a metal-semiconductor transition in DWCNT [37] upon radial collapse.

There have been many efforts on the optimization of polymer composites [38] and in particular the conductivity of polyamide-MWCNT nanocomposites [39], [5], [40]. In this work we show that the electrical conductivity of polyamide-MWCNT composites can be markedly improved through a high pressure treatment by surpassing the usual pressure limits of hot pressing or extrusion methods. We provide direct evidence of the pressure induced collapse of MWCNT in the compressive process and theoretical understanding of the relation between the geometrical changes of the nanotubes and the modification of the conductivity of the percolated network. Our work shows that the very high compression of MWCNT polymer composites can provide then an additional mechanism for electrical conductivity optimization, beyond the system consolidation.