Alignment Induced Self-assemblage of Carbon Nanotubes for Structural Composite Applications

Abstract

Carbon nanotubes (CNTs) can be produced from floating catalyst synthesis to form random networks. However, the CNTs must be aligned and closely packed to eliminate molecular and microscale defects to reach levels of mechanical performance similar to carbon fiber systems. Previous research has shown that strain-induced alignment methods using bismaleimide (BMI) infiltration can effectively modify randomly oriented CNT networks. The BMI resin lubricates the network decreasing the friction between the CNT bundles and assists with load transfer between CNT bundles during the stretching process. This process yields unique CNT collapsing, self-assembling behaviors, and graphitic crystal packing at the nanoscale. In this study, the CNTs’ alignment degree was measured by Raman spectroscopy and X-ray scattering. Alignment degree analysis results showed a considerable increase at 40% stretch ratio with a plateau at 60% stretch ratio. An 80% stretch ratio achieved the highest alignment degree of 0.93 with noticeable graphitic crystal packing. Both scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of the CNT sheet surfaces and cross-sections show that as the stretch ratio increased, the CNTs started to align along the stretching axis and self-assembled into large bundles. Additionally, HRTEM analysis indicated that the CNTs exhibited collapsing and self-assemblage to form graphitic crystal packing. Tensile testing on the aligned CNT/BMI composite samples measured an increase in tensile properties. The ultimate tensile strength (UTS) and Young's modulus reached maximums of 1.58 GPa at 70% stretch ratio and 252 GPa at 80% stretch ratio, respectively. The high alignment degree and graphitic packing improved load transfer throughout the CNT network, and consequently, better mechanical performance in the CNT composites were achieved. Furthermore, this alignment process proved its scalability and potential to transcend into industrial-scale applications that utilize CNT material systems.