Size Dependent Mechanical Behavior of Free-Standing Glassy Polymer Thin Films.

摘要

Mechanical properties of nanoscale free-standing polymer thin films exhibit size-dependence due to surface effects. However, it has remained a challenge to associate the length scales where such differences emerge with bulk polymer properties. Here we utilize molecular dynamics simulations to uncover the dependence of elastic modulus of freestanding films on film thickness and bulk properties. Comparison of glass transition temperature (Tg) and modulus (E) indicates that Tg converges to bulk value slightly faster as film thickness increases. The free surface effects that give rise to a depression in E and Tg are observed to be stronger for polymers with weaker intermolecular interactions. Our simulations suggest that the length scale of perturbation of free surface is only about several nanometers, but the effect is large enough that only films of 100 nm or larger exhibit negligible surface effects.