Role of the interfacial area for structure and dynamics in polymer nanocomposites: molecular dynamics simulations of polystyrene with silica nanoparticles of different shapes

Polystyrene nanocomposites containing a fraction of silica nanoparticles of different geometries (sphere, cube and regular tetrahedron) have been investigated by coarse-grained molecular dynamics simulations. Structural and dynamic properties of the polymer chains in the presence of the nanoparticles have been analyzed as a function of the nanoparticle mass fraction and geometrical shape. It has been found that the dimension of the polymer chains in the interphase expands due to the polymer–nanoparticle interaction. Their global dimension (averaged over the whole sample), however, shrinks when increasing the total surface area of the nanoparticles. The conformational changes of polymer chains in the interphase are monitored by a chain orientation parameter. The profiles of the chain dimension and orientation as a function of their distance from the nanoparticle center of mass show that the interphase thickness is roughly equal to the radius of gyration of the polymer chains. Moreover, the dynamic behavior of the polymer chains in nanocomposites is analyzed by the center of mass diffusion coefficient, the relaxation time of the chain end-to-end vector and the characteristic escape time of the polymer chains from the interphase. Compared with neat polymers, both the global and local chain dynamics in nanocomposites are hindered with an increasing nanoparticle mass fraction and with an increasing surface area. The local chain dynamics in the interphase is stronger affected by the surface area of the nanoparticles than the global one. Specifically, the global diffusion coefficient of polymer chains is almost linearly reduced with the total surface area of the nanoparticles, whereas the global relaxation time of the chain end-to-end vector increases almost linearly with it. The interphase relaxation time of the polymer chains increases superlinearly with the surface area of an individual nanoparticle. Additionally, the characteristic escape time of polymer chains from the interphase is largely influenced by the geometrical shape of the nanoparticle. Due to their larger surface area, tetrahedral nanoparticles impede the global and local chain dynamics stronger than cubic nanoparticles, followed by spherical nanoparticles. Uniaxial tensile tests show that both the Young's modulus and yield strength of polymer nanocomposites increase monotonically with their total interphase area. Our simulations demonstrate that polymer structural and dynamic properties are both largely influenced by a common parameter, i.e. the interphase area which has a fundamental influence on the mechanical properties of polymer nanocomposites.