Synchrotron radiation small-angle X-ray scattering study on fracture process of carbon nanotube/poly(ethylene terephthalate) composite films

Time-resolved small-angle X-ray scattering (SAXS) measurements have been conducted during tensile deformation of carbon nanotube (CNT)/amorphous poly(ethylene terephthalate) (PET) composite films using synchrotron radiation in order to investigate the fracture process. The observed SAXS patterns consisted of the streaks parallel to the loading direction caused by the total reflection at craze/polymer interfaces, the streaks perpendicular to the loading direction caused by the fibril/void structure of crazes and the scattering from CNTs. The formation, widening and fracture processes of the crazes were investigated based on the changes of SAXS patterns during deformation and the fracture toughness of the composite films determined with essential work of fracture method. The influences of CNT addition on the mechanical properties of PET varied depending on the specimen geometries used for the mechanical tests and marked influences were obtained with surface-notched specimens. The CNT addition increased the energy needed to widen the crazes and retarded the growth and fracture of the crazes during deformation. This lead to the increases in the plastic work of fracture and the fracture toughness of PET. The CNT aggregates formed at the CNT fraction beyond 3 wt%, however, caused reduction of the fracture toughness.