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

Abstract

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