Isostructural Softening of Vulcanized Nanocomposites

Following a previous work evidencing that short poly-propylene glycol (PPG) chains incorporated to crude SBR-silica nanocomposites act as filler-network softeners without changing their structure, we propose in the present letter to examine more operative cross-linked materials. We first evidence that the adsorption of PPG onto silica deactivates progressively the particle's catalytic effect on vulcanization, without perturbing however the cross-link density distribution that we investigate through multiple-quantum NMR. In addition, electron microscopy confirms that silica structure is conserved after vulcanization and that it does not depend on the PPG content either. Composites containing various amount of PPG can thus be seen as structurally identical, both from a matrix and filler point of view - which is confirmed by small and medium amplitude oscillation shear rheology showing strikingly identical viscoelastic properties. PPG signature only appears above 100% in tensile deformation where it is seen to soften dramatically the filler network. Our discovery makes it consequently possible to decorrelate the mechanical behavior of reinforced rubbers under normal conditions of use and urgent needs of energy dissipation.