Revealing the mechanical strengthening mechanisms in twisting CNT ribbon with the effect of interface and boundary conditions

As an effective method to tune the strength and ductility of carbon nanotube (CNT) assembles, twist is very important for high performance CNT-based artificial muscles and actuators. During the twisting, the microstructural evolution and the accompanying strengthening effect would play key roles in determining the final functionalities of storage and release of mechanical energy. Toward the future development of twist-based CNT devices, especially by using CNT ribbons, it is of great necessity to understand the underlying twist-induced mechanical behavior. Here, based on an in-situ microscopic test, we report the origination of twist-enhanced ductility and the stiffened interfacial interconnection between CNTs caused by polymer infiltration. The distribution of stored mechanical energy in twisted ribbons is determined by the twist degree and strongly dependent on various boundary conditions. The fixed boundary results in a lower surface angle, higher interior stress and energy, and more packing density than those of the free boundary during the twisting process. The surface angle gradually decreases during the stretching process. The higher interface friction force inclines to increase the stability of the microstructure evolution. This study can cast light on developing high-efficiency CNT-based actuation devices.