Viscoelasticity of carbon nanotube buckypaper: zipping-unzipping mechanism and entanglement effects

It has been reported that carbon nanotube (CNT) buckypaper demonstrates frequency-and temperature-invariant viscoelastic properties [Xu et al., Science, 2010, 330, 1364]. In an attempt to provide microscopic insight, we performed coarse-grained molecular dynamics simulations on the viscoelastic properties of model CNT buckypaper. First of all, the model is shown to exhibit the observed frequency-and temperature-invariant viscoelastic features. Analyzing snapshots of the buckypaper under cyclic shear deformation, we furthermore confirm that a zipping-unzipping mechanism plays an important role in the molecular origin of the particular viscoelastic properties of buckypaper. Quantitative inspection of the amount of inter-tube entanglements per CNT, < Z >, reveals that CNT buckypaper is faced with reversible entanglement loss during the shear loading process; the equilibrium < Zi > is fully recovered if shear loading is removed. The variation of < Z > during a loading-unloading-reloading cycle is found to be insensitive to both frequency and temperature. Our study highlights an energy dissipation mechanism for carbon nanomaterials, which can help us design new energy-adsorption coatings/damping devices under extreme conditions, and seems to rule out a recently proposed scenario of unstable detachments-attachments.