Direct Evidence of Nanoscale Interfacial Overlap in Elastomer Nanocomposites to Unravel the Origin of Mechanical Percolation

An interesting phenomenon has been observed in polymer nanocomposites under low filler loading (when the nanoparticles are still sparsely distributed before forming a filler network): when the filler concentration crosses a critical threshold, the mechanical strength of rubber nanocomposites undergoes a dramatic enhancement as if the sparsely dispersed nanoparticles suddenly initiate synergistic cooperation. This phenomenon is named the “mechanical percolation effect”. This study provides direct experimental evidence, through high-resolution atomic force microscopy (AFM) nanomechanical characterization of interfacial thickness, modulus, and content combined with interparticle distance measurements, that the “mechanical percolation effect” of carbon black/elastomer nanocomposites originates from the overlapping of interfacial layers at the microscopic level. Moreover, we found that the overlapping occurs in the outer loosely bound rubber (LBR) layers, while an increase in the tightly bound rubber (TBR) layer content is observed. This demonstrates that the overlapping effect of LBR layers elevates their moduli to TBR levels, thereby promoting the formation of additional TBR layers. Theoretical calculations using the Guth-Gold model confirm that the material exhibits a sharp increase in the thickness of interfacial regions (particularly TBR layers) during percolation. This study reveals that nanoscale interfacial overlap is the origin of mechanical percolation in rubber nanocomposites and elucidates the underlying mechanism of the percolation phenomenon.