Network evolutions in both pure and silica-filled natural rubbers during cyclic shear loading

Evolutions of chemical cross-linking and filler networks during sinusoidal small-strain (10%) shear loading (fatigue) process were studied in pure (unfilled) and silica-filled natural rubbers. The experimental results of dynamic mechanical analysis (DMA) and nuclear magnetic resonance (NMR) of pure natural rubber (PNR) indicated that the fatigued PNR has a more homogeneous cross-linking network than that of the virgin one, which can lead to a slight increase of the storage modulus; however, the change of cross-linking density and its effect on the viscoelastic properties of PNR are very limited. By analyzing the variation of storage and loss moduli and the transmission electron microscopy (TEM) images of silica-filled natural rubber (SFNR) during the cyclic loading process, we found that the loosely packed agglomerates were first disrupted, and then the closed ones could also be gradually broken down. Such a filler network evolution process also can be seen from our non-equilibrium molecular dynamics (NEMD) simulation results.