The Payne Effect in Nanofilled Cross-Linked Polyisoprene

We present a systematic study of the Payne effect, i.e., the difference between the small- and large-amplitude oscillatory shear response of filled elastomers, which is relevant to the performance of tires under dynamic driving conditions. The elastomers consist of 14-nm-diameter spherical silica nanoparticles (NPs) grafted with polyisoprene chains, mixed with polyisoprene matrices, and subsequently cross-linked into the gel state. By changing the grafting density and the ratio of the graft-to-matrix chain length, we vary the NP assembly state and examine its response to large-amplitude oscillations. We find that the Payne effect is present at all filler contents but becomes more pronounced at higher NP loadings and for the connected sheet morphology, with the well-dispersed NP state showing the smallest effect. The key concept that apparently controls the strength of the Payne effect is that the core–core contacts are stronger but more fragile compared to interdigitated grafts. Therefore, to achieve increased modulus but reduced Payne effect, a compromise between enhanced NP core interactions and the interdigitation of grafts with other grafts and with the matrix chains is needed. This compromise is best achieved for a morphology intermediate between these two extremes, i.e., a connected network morphology.