Opening and Closing of Nanocavities under Cyclic Loading in a Soft Nanocomposite Probed by Real-Time Small-Angle X‑ray Scattering

The opening and closing of nanocavities in a model soft nanocomposite subjected to cyclic uniaxial tension were directly studied by real-time small-angle X-ray scattering (SAXS). The volume fraction and average shape of the nanocavities have been detected by a pronounced increase in the scattering invariant <i>Q</i>/<i>Q</i>₀ and a detailed analysis of the scattering patterns. Cavities appear upon loading past an intrinsic stress σ_int or intrinsic elongation λ_int. Upon unloading, nanocavities are progressively closed until the volume void fraction ϕ_void reaches 0 for a constant “closure stress” of about 3.5 MPa. As the sample is reloaded, no cavities are observed when the current elongation remains below the maximum elongation of previous cycles λ_max(<i>N </i>– 1) (<i>N</i> is the number of the cycles). Above this elongation, the void volume fraction ϕ_void of the sample increases again. In contrast with ϕ_void, the cumulative void volume fraction ϕ_cum<i>_</i>_void appearing in the sample to reach a given maximum historical elongation, λ_max, or equivalently maximum historical stress σ_max, was found to be independent of loading history. Both results point toward a process of creation of nanovoids in confined rubber domains that have not previously cavitated rather than to the reopening of the previously created cavities. All critical cavitation parameters display a strong memory effect, mostly captured in this uniaxial test by the maximum historical stress or elongation. The closure stress probably results from the Laplace pressure. A mechanism based on the rearrangement of filler agglomerates by strong shear stress after the emergence of nanocavities is proposed to account for the formation and release of the local geometric confinement and the non-reopening of the previously opened nanovoids upon reloading

Nanocavitation in Carbon Black Filled Styrene–Butadiene Rubber under Tension Detected by Real Time Small Angle X-ray Scattering

Nanocavitation was detected for the first time in carbon black filled styrene–butadiene rubber (CB-SBR) under uniaxial loading by real time small-angle X-ray scattering (SAXS) using synchrotron X-ray radiation. A three phase model was developed to calculate the void volume fraction from the scattering invariant <i>Q</i> determined from the observed SAXS patterns. The normalized scattering invariant <i>Q</i>/<i>Q</i>₀, where <i>Q</i>₀ is the invariant before deformation, greatly increased above a critical extension ratio λ_onset which we attribute to the formation of nanovoids. Analysis of the 2D scattering patterns show that voids formed are 20–40 nm in size and elongated along the tensile direction. Cavities formed beyond λ_onset are smaller as λ increases. Results from the scattering experiments are strongly supported by macroscopic volume change measurements on the samples under similar uniaxial strain. A nearly constant nanocavitation stress σ_onset (25 MPa) was observed when the filler volume fraction ϕ_CB was larger than 14%. This value is much higher than that predicted based on the elastic instability of small voids in an unfilled elastomer and shows only a weak dependence on the cross-linking density ν_C in heavily cross-linked samples. An energy based cavitation criterion stressing the importance of confined domains between particles or clusters of particles was adopted and found to be consistent with the observed results. The nanocavities are thought to alter the local stress state and promote local shear motion of filler particles