Multiphysics characterization of multi-walled carbon nanotube thermoplastic polyurethane polymer nanocomposites during compression

Multiphysics properties of polymer nanocomposites (PNCs) are of interest for polymer-based microelectromechanical systems, tactile sensors, and flexible electronics. Coupling of mechanical (e.g., strain, stiffness, mechanical shock, etc.), electrical (e.g., resistance), and thermal (e.g., thermal expansion) effects has received little previous attention and is critical for performance and reliability. Compression experiments needed for sensitive touch sensors at low force and strain are rare with insufficient understanding of multiphysics mechanisms. This study investigates mechanical, electrical, and thermal properties of multi-walled carbon nanotube (MWCNT)/thermoplastic polyurethane PNCs during localized compression experiments. A novel correlation was established between increased electrical conduction through a spanning path(s) and higher stiffness giving insight into the mechanism of load transfer to MWCNTs. The correlation is attributed to MWCNT shell buckling-induced growth in the real area of contact between the metal contact electrodes and the PNC that occurs when a spanning path is compressed and begins to conduct electric current. Modulating electric current and power dissipation in a contact shows PNC thickness modulation, where higher power results in localized PNC elongation. The observed PNC thickness modulation is attributed to thermal expansion of the polymer.