In this work, the arising of stick-slip dissipation as well as the global mechanical response of carbon nanotube (CNT) nanocomposite films are tailored by exploiting a three-phase nanocomposite. The three phases are represented by the CNTs, a polymer coating localized on the CNTs surface and a hosting matrix. In particular, a polystyrene (PS) layer coats multi-walled carbon nanotubes (MWNTs) that are randomly dispersed in a polyimide (PI) matrix. The coating phase is strongly bonded to the CNTs outer sidewalls ensuring the effectiveness of the load transfer mechanism and reducing the material damping capacity. The coating phase can be thermally-activated to modify, and in particular, decrease the CNT-matrix interfacial shear strength (ISS) thus facilitating the stick-slip onset in the nanocomposite. The ISS decrease finds its roots in a partial degradation of the coating phase and, in particular, in the formation of voids. By weakening the CNT/polymer interfacial region, a significant enhancement in the material damping capacity is observed. An extensive experimental campaign consisting of monotonic and cyclic tensile tests proved the effectiveness of this novel multi-phase material design.
Due to their superior physical and electro-mechanical properties, Carbon Nanotubes (CNTs) are one of the most promising composite fillers to realize ultralight and flexible strain sensors that can be used, among others, to monitor strain concentrations within a structure when damage occurs. In this study, sensors are made of Multi-walled Carbon Nanotubes (MWNTs) embedded in a Polymide (PI) matrix. Nanocomposites are characterized under no-load conditions to study the electrical properties, and under tensile loading conditions, to evaluate the electromechanical and piezoresistive response. The results highlight a two orders of magnitude decrease in electrical resistivity if compared with previous studies, the capability to instantaneously respond to unpredictable deformations and to easily adapt to three-dimensional shapes. The beauty of the as conceived nanocomposite film, if compared with the commercially available strain gages, is its unprecedented potential expandability to monitor larger areas without the loss of ultra-low local (in scale) detection. Local detection is in fact allowed by nanoscale morphology changes that induce changes in local electrical conduction. The selected polyimide matrix allows the use of the proposed sensor to harsh and high temperature environments while keeping high flexibility and excellent mechanical properties, key parameters for the realization of reliable electromechanical films.
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