Molecular mechanism of the fracture properties of slide-ring crosslinked elastomers

Whilst slide-ring (SR) crosslinked elastomers (SRCE) have attracted much attention due to their excellent fracture toughness, the molecular mechanism by which fracture occurs is unclear. Therefore, a coarse-grained model of SRCE with different crosslink densities and SR coverages is constructed in this work. By performing a triaxial deformation, the maximum fracture energy of the SRCE obtained at a moderate SR crosslink density is about 72 times higher than when no crosslink bonds are present. However, the fracture energy decreases by about 3.5 times when the SR coverage increases from 5% to 20%. To understand these findings, the asphericity factor of the chains, percentage of SR at the end region of the chains, and the average sliding distance of the SR are analyzed and found to rise at low strain and with increasing crosslink density or coverage of the SR. This indicates the presence of a high rate of sliding of the SR from the inner region to the end region of the chains. However, the strain at the maximum asphericity factor or SR percentage at the end region of the chains or the average sliding distance of the SR is reduced. This is because the number of broken bonds rises while the strain at the beginning of the bond-breaking process is reduced. Meanwhile, the percentage of broken backbone bonds is lower than that of broken crosslink bonds because one backbone bead bears less stress than a crosslink bead. Moreover, the position of bond breakage changes from the end region to the inner region of the chains with increasing crosslink density or coverage of the SR, which can be further proved by the ratio of the local stress induced by one end bead to that induced by one inner bead. Subsequently, the evolution process of the voids is quantified by calculating the number, volume fraction, and surface area of the voids. Voids are nucleated in the polymer region with a low local elastic modulus at low strain, while they appear in the positions of the broken bonds at large strain. The high crosslink density and low coverage of SR can reduce the growth and coalescence rate of the voids. In summary, this work presents deep and clear insights into the properties and molecular mechanism of the fracture of SRCE.