Atomic Resolution Imaging and Analysis of Microstructures and Interface of Aligned Carbon Nanotube Composites

Abstract

Higher concentrations of long carbon nanotubes (CNTs) that are aligned and functionalized can improve the mechanical properties of CNT composites. For this purpose, pristine/random CNT sheets were mechanically stretched at different strains of 40 %, 60 % and up to 80 %. The higher degree of CNT alignment was confirmed by scanning electron microscope (SEM) images and the degree of alignment was quantified by X-ray scattering and polarized Raman spectroscopy. The highest degree of alignment of 0.93 was achieved by using this method. Aligned CNT sheets were functionalized and CNT/bismaleimide (BMI) composites were fabricated. The mechanical properties of the composites were improved with a higher degree of alignment achieving a modulus of 252 GPa and tensile strength of over 1.4 GPa for an 80% stretched CNT/BMI composite. To understand the internal CNT packing and the CNT/resin interface, thin composite cross sections that are perpendicular to the alignment direction were prepared using focused ion beam (FIB) with a lift-out process. Using a high-resolution transmission electron microscopy (HRTEM), relatively large diameter CNTs (> 5 nm) cross sections and double-walled CNT bundles were observed. An increased number of densely packed and collapsed CNT structures were also observed with higher stretch ratio, which confirmed the CNT alignment and dense packing along the stretch direction. The CNT/resin interface was also analyzed by electron energy loss spectroscopy (EELS). Carbon K-edge peak was used to identify the functionalization of CNT, and a ratio of 1s-* to 1s-* transition was mapped by a scanning TEM. A decreased */* transition ratio was observed at the edge of CNT packing and could be related to the reduced sp2 bonding after functionalization. This leads efficient load transfer between CNT and resin, thus higher mechanical properties.