Mechanical properties of chemically-treated carbon nanotube nanocomposite microfibers: Experimental and modeling studies

We experimentally and theoretically investigated the mechanical properties of single-walled carbon nanotube (SWCNT)/epoxy nanocomposite microfibers, fabricated using the direct-write technique. The SWCNTs were functionalized by acidic oxidation during the SWCNT purification process. The nanocomposite suspensions were prepared using an efficient mixing method which involves the simultaneous heating and ultrasonicating of the nanocomposite solution diluted in acetone, followed by solvent evaporation and high shear mixing. Morphological characterization revealed that the SWCNTs and their aggregates with a size of up to ∼1.3 μm were fairly-well dispersed in the epoxy matrix. The mechanical characterization of the nanocomposite microfibers showed that, with only 0.5wt% loading of purified SWCNTs, the modulus and strength of the nanocomposite were improved by 33% and 27%, respectively. The overall effective stiffness of the nanocomposites was also estimated using micromechanical homogenization. Perfect bonding condition between carbon nanotubes and the epoxy matrix was assumed. The SWCNTs were assumed to be straight cylinders having an aspect ratio of 500. Since it is very difficult to control and experimentally determine SWCNTs orientation in the matrix, several orientation distributions were studied for comparison purposes: fully aligned, partially aligned and randomly oriented. The results were obtained for individually dispersed SWCNTs as well as dispersed SWCNT bundles featuring lower aspect ratios (25 to 100). Our experimental measured stiffness were found to be lower than the theoretical estimates for fully aligned SWCNTs or bundles of SWCNTs. However, the measured stiffness remains higher than randomly dispersed bundles of SWCNTS. These results suggest a partial alignment of the carbon nanotubes in the manufactured nanocomposites. The experimental stiffness was found to match with the theoretical results of different configurations of SWCNTs in the nanocomposite. All results are based on a perfect bonding assumption between straight SWCNTs and matrix. If imperfect bonding and/or SWCNT waviness are to be considered, the calculated alignment would be even better. This alignment achieved in the microfiber manufacturing process along with their chemical treatment and efficient mixing processes proposed help achieve full load transfer across SWCNT/epoxy interface, resulting in a considerable reinforcement of nanocomposite microfibers.