Characterizing the viscoelastic properties of layer-by-layer carbon nanotube-polyelectrolyte thin films

Many have sought to take advantage of carbon nanotubes for the fabrication of multifunctional thin films. However, before these carbon-nanotube-based films can be used for these intended functionalities, a complete understanding of the mechanical properties is crucial. While most studies have focused on characterizing just the thin film's stress-strain performance, their time-dependent mechanical properties still need to be investigated. In this study, free-standing layer-by-layer carbon-nanotube-based thin films have been assembled, and the objective is to investigate their viscoelastic stress relaxation and creep response. First, the results from stress relaxation tests are numerically fitted to two different models, namely the standard linear solid (Zener) model and the linear increasing viscosity-modified Zener (LIV-Zener) model. The results of the fitting suggest that the LIV-Zener model is better suited for describing thin film stress relaxation behavior, which also means that these nanocomposites exhibit time-dependent viscosity. Second, the experimental results from creep testing are also fitted to the LIV-Zener model. The results from both fittings are consistent and suggest that the LIV-Zener model is more capable of describing carbon-nanotube-based thin film viscoelasticity.