Molecular dynamics simulations of the structural, mechanical and visco-elastic properties of polymer nanocomposites filled with grafted nanoparticles

Through coarse-grained molecular dynamics simulations, we have studied the effects of grafting density (S) and grafted chain length (L-g) on the structural, mechanical and visco-elastic properties of end-grafted nanoparticles (NPs) filled polymer nanocomposites (PNCs). It is found that increasing the grafting density and grafted chain length both enhance the brush/matrix interface thickness and improve the dispersion of NPs, but there seems to exist an optimum grafting density, above which the end-grafted NPs tend to aggregate. The uniaxial stress-strain behavior of PNCs is also examined, showing that the tensile stress is more enhanced by increasing Lg compared to increasing S. The tensile modulus as a function of the strain is fitted following our previous work ( Soft Matter, 2014, 10, 5099), exhibiting a gradually reduced non-linearity with the increase of S and Lg. Meanwhile, by imposing a sinusoidal external shear strain, for the first time we probe the effects of S and Lg on the visco-elastic properties such as the storage modulus G ', loss modulus G '' and loss factor tan delta of end-grafted NPs filled PNCs. It is shown that the non-linear relation of G ' and G '' as a function of shear strain amplitude decreases with the increase of S and Lg, which is consistent with experimental observations. We infer that the increased mechanical and reduced non-linear visco-elastic properties are correlated with the enhanced brush/matrix interface and therefore better dispersion of NPs and stronger physical cross-linking. This work may provide some rational means to tune the mechanical and visco-elastic properties of end-grafted NPs filled polymer nanocomposites.