Opening and Closing of Nanocavities under Stress in Soft Nanocomposites: A Real Time Small Angle X-ray Scattering (SAXS) Observation

Cavitation occurring at the nanometer length scale has been recently demonstrated conclusively in rubbers$^{1}$. Real time SAXS with synchrotron radiation is employed to probe the structure changes in carbon black filled styrene-butadiene rubber (SBR) under uniaxial tension. The scattering invariant Q($\lambda )$, where $\lambda $ is the extension ratio, increases sharply, which we attribute to void formation, above a critical true stress ($\sim $25 MPa) that is roughly independent of both filler content and crosslinking density. During step-cycle tests Q decreases on unloading to Q$_{0}$, its value before any testing, and does not increase again until $\lambda $ exceeds the maximum previous $\lambda =\lambda _{max}$, showing that the voids close upon unloading and only reappear upon reloading when $\lambda \quad > \quad \lambda _{max}$ (Mullins effect). We attribute the increase of the scattering invariant once $\lambda $ exceeds $\lambda _{max}$ to the creation of new voids rather than to the reopening of old ones. The scattering of the voids in the region q $<$ 0.1 nm$^{-1}$ can be separated from that of the carbon black particles and provides information on average void size and shape.