Elastic Mechanical Response of Thin Supported Star-Shaped Polymer Films

We show evidence of thickness-dependent elastic mechanical moduli that are associated largely with the effects of architecture (topology) and the overall shape of the macromolecule. Atomic force microscopy (AFM) based nanoindentation experiments were performed on linear chain polystyrene (LPS) and star-shaped polystyrene (SPS) macromolecules of varying functionalities (number of arms, J.) and molecular weights per arm M-w(arm). The out-of-plane elastic moduli E(h) increased with decreasing film thickness, h, for h less than a threshold film thickness, h(th). For SPS with f <= 64 and M-w(arm) > 9 kg/mol, the dependencies of E(h) on h were virtually identical for the linear chains. Notably, however, for SPS with f = 64 and M-w(arm) = 9 kg/mol (SPS-9k-64), the h(th) was over 50% larger than that of the other polymers. These observations are rationalized in terms of the structure of the polymer for high f and sufficiently small M-w(arm) and not in terms of the influence of interfacial interactions.