Modifying carbon nanotube fibers: A study relating apparent interfacial shear strength and failure mode

Abstract

Achieving high strength in fiber reinforced structural composites requires effective load transfer between the high-performance fiber reinforcement, e.g., carbon nanotube (CNT) yarn, and the matrix. Various processing approaches to enhance the interaction between CNT fiber and the matrix were investigated. The apparent interfacial shear strengths (IFSS) of pristine CNT yarns, post-treated CNT yarns (cross-linked, functionalized, and polymer incorporation by resistive heating), and pre-infiltrated polymer/CNT composite fibers measured using single fiber pull-out tests were used to screen the efficacy of the processing methods. Pristine CNT yarns had a low apparent IFSS (<5 MPa) due to shear failure within their dry cores. In post-treated CNT yarns which did not exhibit good IFSS, the failure surface consisted of a resin-infiltrated sheath near the surface of the yarn and a dry section within the yarn core; failure occurred in the dry core. This failure mode is unlike those observed in traditional carbon fiber reinforced composites which fail at the fiber/matrix interface. In contrast to the sword-in-sheath failure modes of post-treated CNT yarns, pre-infiltrated polymer/CNT composite fibers displayed high apparent IFSS (>20 MPa). Improved wet-out of the fiber eliminated the dry-core shear failure mode.