A New Aspect to Nano-Composite Rheology -- the Localized Memory Effect

We discovered that particle-reinforced elastomers after being sheared (or aged) in oscillation at a frequency $f_{a}$ at a small strain \textit{$\gamma $}$_{a}$ (e.g., $\sim $1{\%} strain) for time $t_{a}$ can often produce a spectrum hole or drop in their dissipation spectra. The location of the hole depends on the aging strain amplitude \textit{$\gamma $}$_{a}$. The depth of this hole is influenced by both the oscillatory aging frequency $f_{a}$ and the aging duration$ t_{a}$, and follows a simple power relationship of the product of $f_{a}$ and$ t_{a}$. Sequential shear at two strains reveals that when $\gamma _{a1}>$ $\gamma _{a2}$ the resulting dynamic spectra appear to be a combination of that aged at $\gamma _{a1}$and $\gamma _{a2}$; whereas for $\gamma _{a1}< \quad \gamma _{a2}$, the resulting dynamic spectra only reflect the characteristic hole burning of the second strain after holding at $\gamma _{a2}$. This new memory effect occurs at very small strains and involves material stiffening during the strain aging, and both of those features are quite different from the Mullins effect in filled elastomers.$_{ }$Also, this new memory is found to last for more than 10 days without noticeable sign of disappearing.