Effect of molecular weight and architecture on nanoscale viscoelastic heterogeneity at the surface of polymer films

Abstract

Dissipated energy mapping in amplitude-modulation atomic force microscopy was employed to characterize the nanoscale heterogeneous behavior of the viscoelastic response at the surface of polystyrene (PS) films of different molecular weight and architecture. By using an ultra-sharp probe with tip radius of ~1 nm, we were able to visualize the viscoelastic heterogeneities at the free surface of all PS samples. While the length scale of these heterogeneities at the surface of linear PS films was measured to be ~2.2 nm nearly independent of the molecular weight, it was significantly increased to ~3.1 nm at the surface of four-arm star-shaped PS films. We also demonstrated that similar nanoscale heterogeneous features were not observed when using a common probe with tip radius of ~10 nm, thereby confirming the intrinsic nature of the observed viscoelastic heterogeneities with sizes on the order of a few nanometers at the surface of PS films. This result provides important insights into the fundamental role of the molecular architecture in controlling the heterogeneous dynamics and many associated physical properties of amorphous polymer surfaces.