Hierarchical composites with high-volume fractions of carbon nanotubes: Influence of plasma surface treatment and thermoplastic nanophase-modified epoxy

Abstract

A scaled-up catalytic chemical vapor deposition synthesis approach has been used to grow carbon nanotubes (CNTs) onto quartz and alumina fibers to prepare novel hierarchically structured composites with high volume fractions of CNTs. The as-synthesized CNT-coated fibers were functionalized using atmospheric oxygen plasma, prior to infusing an epoxy polymer matrix, in order to improve wettability and bonding. The polymer matrix was further modified by adding a triblock copolymer to provide nanoscale toughening. Direct adhesion tests show that CNT and plasma treatments increase the shear strength of quartz or alumina/epoxy films by 30%. CNT–quartz/epoxy composites show an 80% increase in the in-plane shear strength while the CNT–alumina/epoxy composites show more than a threefold increase in shear strength. The failure mechanisms of the films and the fiber composites are dominated by fracture through the catalytic iron nanoparticles and the improvements are limited by the CNT to fiber adhesion. The composites also have very high in-plane electrical conductivity, over 3S/cm. The carbon nanotubes form a piezoresistive sensing network surrounding the fibers. Under flexural loading the change in electrical resistance can detect damage initiation and impending fracture, particularly for the alumina composites.