Mechanical and electrical property improvement in CNT/Nylon composites through drawing and stretching

Abstract

The excellent mechanical properties of carbon nanotubes (CNTs) make them the ideal reinforcements for high performance composites. The misalignment and waviness of CNTs within composites are two major issues that limit the reinforcing efficiency. We report an effective method to increase the strength and stiffness of high volume fraction, aligned CNT composites by reducing CNT waviness using a drawing and stretching approach. Stretching the composites after fabrication improved the ultimate strength by 50%, 150%, and 190% corresponding to stretch ratios of 2%, 4% and 7%, respectively. Improvement of the electrical conductivities exhibited a similar trend. These results demonstrate the importance of straightening and aligning CNTs in improving the composite strength and electrical conductivity.

Introduction

Carbon nanotubes (CNTs) have highly desirable mechanical, thermal and electrical properties. They are promising candidates as reinforcement for the next generation of high performance composites. Significant effort has been focused on developing CNT composites over the last two decades. Methods for fabricating CNT composites include: dispersing short CNTs in polymer matrix [1], [2], [3], infiltrating CNT buckypaper with polymer solutions [4], [5], and reinforcing with CNT fiber assemblies [6], [7], [8].

Studies of CNT composites [9], [10], [11] so far have largely focused on improving the nanotube dispersion quality and the interface with the matrix. To achieve good quality CNT/polymer dispersion, short CNTs in low volume fractions are typically utilized. Although short CNT composites have some advantages in certain low volume fraction applications, such as thermally and electrically conducting materials, their mechanical properties fall far short of traditional high performance structural composites. It results largely from the short CNT length (usually <10 μm), which cannot efficiently transfer a mechanical load across the weakly bonded interface. Chemical modification may improve interfacial shear strength, at the expense of introducing defects in the CNT structures and thus degrading the properties [12]. Achieving high volume fractions of dispersed CNTs in polymer is difficult because the resulting high viscosity complicates further processing.

Another approach is to infiltrate CNT films (also known as buckypapers) with thermoplastic polymers or epoxy resin [4], [13]. The CNTs in the buckypaper sheets have no preferential orientation and each nanotube is curved and wavy. CNT fibers (yarns) can also be used to fabricate composites. They include plied or braided CNT fiber assemblies [6], [7] and long spun fibers infiltrated by polymer [8]. The most significant component in these composites is the CNT fiber. Techniques for making CNT fibers are classified into “liquid” methods [14], where CNTs are dispersed into a liquid and solution-spun into fibers, and “solid” methods [15], [16], where CNTs are directly spun into ropes or yarns. The last 10 years have seen rapid progress in the “solid” fiber spinning approach [17], [18], [19], [20], [21], [22], [23], [24]. While the mechanical properties of these fibers are promising, they both have limitations. The “liquid” method requires short CNTs for solution spinning, which limits the mechanical properties, while the “solid” method involves fiber twisting, which is a slow and expensive process. Scaling-up of these technologies presents a major obstacle for engineering applications.

In a recent work by Cheng et al. [25], a high volume fraction of highly aligned CNTs was homogeneously dispersed in an epoxy matrix. The CNT/epoxy composites were produced by drawing and stacking CNT sheets from aligned CNT arrays, and then infiltrating the stacked CNT sheets with epoxy. This method alleviates many limitations of other CNT processing methods. However, wavy nanotubes are still present, which reduces the mechanical properties of the composites.

Here we report a strategy, mechanical stretching of aligned CNT composites, to address the issue of CNT waviness. This approach involves drawing and winding thin CNT ribbons from free-standing CNT arrays, infusing a nylon 6,6 solution between layers of the CNT ribbon without disturbing the pre-existing alignment and stretching the composite while locally heating it to reduce the CNT waviness. The mechanical and electrical property results of this work identify a new mechanism of maximizing mechanical properties of CNT composites.