Mechanical behaviour of cis-1,4-polyisoprene under uniaxial tension with silica and POSS filling: a coarse-grained molecular dynamics simulation

In this paper, the iterative Boltzmann inversion (IBI) method has been employed to develop the coarse-grained (CG) force field of cis-1,4-polyisoprene (PI) rubber and its silica-filled and POSS-filled composites. Uniaxial tension simulations have been conducted for the pure PI model, the silica-filled PI model, and the POSS-filled PI model, as well as their particle grafting and matrix cross-linking models. By examining the evolution of molecular chains, filled particles, and the entire matrix in terms of mechanical behaviour, it was found that particle-induced molecular chain straightening is the primary factor contributing to the elevated macroscopic stress level observed in the particle-filled PI model. However, the ungrafted silica particles do not result in an increase in the stress level of the system due to the weaker interactions. The grafting and matrix cross-linking processes serve to enhance this induced process, with the former increasing particle roughness and the latter increasing molecular chain roughness. In terms of the entire condensed system, the macroscopic stress level of the system is strongly correlated with the inhomogeneity of the microscopic strain. The inhomogeneity of the strain distribution in the system demonstrates the existence of a region of strain concentration, which leads to large deformations, resulting in the loosening of the structure and a decrease in the load-carrying capacity. The nucleation of microvoids in the system cannot be attributed to microscopic stress concentrations, which are more likely to occur in the weak interaction region. In POSS-filled systems, the weak interaction region is located between the substrate and the substrate, which explains the formation of microvoids in this region. In contrast, in silica-filled systems, the weak interaction region is between the substrate and the filler, resulting in microvoids forming at the filler-substrate interface.