Nanosize Effects in Mechanical Properties of Nanotube Dispersed Composites

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In recent years an increasing number of studies have been published reporting mechanical properties of polymers added with carbon nanotubes (CNT). The addition of CNT does not necessarily improve the mechanical properties of polymers. The diversity of experimental results comes from the creation of defects due to aggregation of CNT and utilization of different specimen geometries, in addition to the difference in the CNT/polymer interaction depending on the kinds of polymers and the surface state of CNT. In the present study, time-resolved small-angle X-ray scattering measurements have been conducted using synchrotron radiation during deformation and fracture of processes of polymers added with CNT, carbon black (CB) and vapor grown carbon fiber. The formation and evolution of the precursory structure of fracture, i. e. crazes, have been investigated to clarify the essential effects of CNT exerted by their high strength, high aspect ratio and nanosize. As compared with CB, CNT showed larger effects to improve the fracture toughness of polymers. It was obtained that CB increases the number of sites where the crazes form whereas CNT increases the energy needed for craze growth due to their particular properties and geometries. The surface-notched films showed more significant effects of CNT addition as compared with the side-notched films. The essential work of fracture method and the finite element analysis were employed to rationalize these results. It was found that the surface-notched films have larger fraction of plastic deformation zone as compared with the side-notched films and the effect of the CNT addition to increase the plastic work of fracture is emphasized with the surface-notched films. In summary, the present study elucidated the essential effects of CNT addition exerted due to their particular properties and geometries, and suggested the material geometries with which the effects of CNT addition arise most effectively.