Comparison of Physical Aging and Local Glass Transition in Glassy-Rubbery Bilayer Films

Block copolymers consisting of glassy and rubbery polymers in direct contact with each other are common in many industrial applications, with the stability of the glassy domains critical to the material’s function. Recent fluorescence measurements by our group have shown large changes in local glass transition temperature T_g(z) across glassy-rubbery polystyrene (PS) / poly(n-butyl methacrylate) (PnBMA) multilayer films. Previously we have demonstrated a correlation between reduced physical aging rates in thin single layer PS films with local T_g reductions near the free surface. Here we explore if a similar correlation between physical aging and local T_g exists near glassy-rubbery polymer interfaces. Physical aging of PS films capped with rubbery PnBMA layers are measured using ellipsometry, avoiding the competing effects of the free surface and allowing us to isolate the impact of a single glassy-rubbery interface. Reductions in physical aging rate with decreasing PS layer thickness are observed, but the reduced layer thicknesses needed to make the ellipsometry measurements viable induce finite size effects altering the local T_g(z) profile relative to previous profiles on semi-infinite bilayers, necessitating direct comparisons on equivalent systems with both techniques.