Confinement and shear effects for atactic polystyrene film structure and mechanics

We have performed the molecular-dynamics simulations for the atactic polystyrene (aPS) films supported by one substrate (SF, supported film) and films capped by two substrates (CF, capped film). The simulations of supported films have been carried out with the purpose to study the influence of confinement on the glass-transition temperature (Tg). We define the Tg by measuring the film density and thickness. We show that the Tg value of aPS films weakly depends on the film thickness and remains almost constant for films down to 2 nm, which is in agreement with recent experimental study. The simulations of capped films have been performed to study the statistical and mechanical properties of polymer chains under shear. The capped film has been loaded with different normal pressures (25–170 MPa) and sheared with different shear velocities (5 × 10-4–1 × 10-1 nm ps-1). In the absence of shear the internal structure of the aPS SF and CF films is different. We found that the internal structure, density and order parameter of the aPS CF films do not change with the small shear deformations