Molecular dynamics approach to locally resolve elastic constants in nanocomposites and thin films: Mechanical description of solid-soft matter interphases via Young's modulus, Poisson's ratio and shear modulus

摘要

A molecular dynamics approach based on a small-deformation mechanical response has been extended from the evaluation of locally resolved Poisson's ratios, ν_j, in nanocomposites to the calculation of local Young's moduli, E_j, (with j labelling a subvolume of the studied sample). On the basis of the ν_j and E_j, the local values of the shear modulus, Gj, can be derived as well. The capability of the developed method to derive locally resolved elastic constants of complex (nanocomposite) systems has been tested for an atomistic model of silica and atactic polystyrene. When measuring the interphase dimension of the composite in terms of local E_j, ν_j and G_j elements, a surface influence exceeding three times the polymer bulk radius of gyration (R_g ≈ 1nm in the studied 20mer composite) is predicted while for the majority of static quantities (e.g., polymer mass density, polymer orientation relative to the nanoparticle surface, radius of gyration, end-to-end distance) interphase dimensions only slightly larger than the polymer R_g are found. Calculated local values of mechanical descriptors can be adopted as input parameters in the micromechanical modelling of multicomponent nanocomposites.