On the mechanical and electrical relaxation in piezoresistive polymer composites

• Study of coupling between mechanical stress relaxation and electrical resistance behavior in flexible composites. • CNTs fillers reinforcing PVDF exhibit piezoresistive response without key mechanical impact. • Temperature critically affects the mechanical relaxation of PVDF, exhibiting slight impact on resistance behavior. • Under cyclic load-unloading force, the piezoresistive performance depends on temperature. Flexible piezoresistive sensors based on polymer composites are promising for real-time monitoring applications due to their mechanical flexibility, electrical sensitivity and capability of being processed by additive manufacturing. Despite its interest, the important issue of mechanical and electrical relaxation over time, influencing sensor performance, has not been properly addressed. In this study, poly(vinylidene fluoride) (PVDF) composites reinforced with 0.5 wt% carbon nanotubes (CNTs), with proven piezoresistive characteristics, have been evaluated under mechanical and electromechanical loading at various temperatures (room temperature to 100 °C). The study aimed to evaluate the coupling between mechanical stress relaxation and electrical resistance behavior under constant strain conditions. While mechanical stress exhibited significant viscoelastic decay, from ≈33 to ≈6 MPa at 5% strain and 100 °C during 1800 s, electrical resistance remained remarkably stable, fluctuating by less than 5% across most conditions. Cyclic deformation tests further confirmed this electrical stability under repeated loading. These results reveal a decoupled electromechanical response with respect to relaxation dynamics, suggesting that CNT networks near the percolation threshold remain largely unaffected by polymer matrix relaxation.