Surface Effect on Mechanical Properties and Atomic Mobility of Ultrathin Polystyrene Films using Molecular Dynamics Simulations

Thin polystyrene film has shown anomalous properties such as reduced glass transition temperature when the thickness is below 100 nm. However, few attentions have been paid to its mechanical properties, which are important in understanding the stability and reliability of polymer nano-structures. In this work, we aim at elucidating the size dependent mechanical properties of ultrathin polystyrene films using molecular dynamics (MD) simulations. Coarse grained MD samples of free-standing PS films with different thicknesses were generated using the augmented phantom chain growth method. Active deformations were applied by moving two repulsive walls to determine the size dependent mechanical properties of the films. The distribution of local atomic mobility was investigated through partitioning the films into equidistant bins along thickness and calculating the mean square displacement for each bin. The results indicate the existence of a softened surface layer with reduced modulus and enhanced local atom mobility compared to the bulk state. It shows the deformation has an enhancing effect on the local atom mobility, especially along the thickness direction. This work can provide insights into the size dependent mechanical properties of ultrathin polymer films.