Structural evolution of OBC/carbon nanotube bundle nanocomposites under uniaxial deformation

Abstract

A regulated morphology of Multi-walled Carbon Nanotube Bundles (CNTBs) in an Olefin Block Copolymer (OBC) matrix is achieved via solution blending after short-time strong sonication which breaks the CNTBs into smaller bundles. Inside the CNTBs which consist of several to dozens of nanotubes, the nanotubes exhibit a well aligned structure. A hybrid shish-kebab (HSK) superstructure is observed where the nanotubes in the CNTBs act like a central stem and OBC crystals periodically locate perpendicular to the axis of the nanotubes. With 2 wt% incorporation of the CNTBs, dramatic mechanical reinforcement is achieved with both tripled tensile strength and Young's modulus. The reinforcement might result from an efficient load-transfer brought about by the HSK superstructure as well as the unique bundle morphology of the CNTBs with high robustness. In situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques were carried out to investigate the structural evolution of the OBC and nanocomposites upon uniaxial deformation. With a considerable proportion of OBC crystals attached onto the surface of the CNTBs, the scattered lamellaes in the nanocomposites are of lower density. With the contribution from the larger long period of the HSK superstructure and increased space between adjacent lamellaes, the long period of the nanocomposites is remarkably increased. Upon stretching, the decrease of the long period of the neat matrix is dominated by the density of the lamellaes, which increases upon the fragmentation of the lamellaes. Inversely, the increase in the long period of the nanocomposites is dominated by the stretching process, which leads to the increased separation of crystal lamellaes that are of lower density. The HSK superstructures in the nanocomposites act as much larger but fewer hybrid crystal junctions, thus the OBC chains in the nanocomposites are involved in even fewer physical junctions, indicating a less effective network structure than that of the neat matrix. Thus the Hermans' orientation factor of both the orthorhombic crystal and amorphous phases of the nanocomposites are lower than that of the neat matrix. With high incorporation of CNTBs and the prominent stereo hindrance brought about by the rigid CNTB network, the orientation behavior of the nanocomposites doesn't comply to the slip-link theory.