(303a) Modulus, Confinement and Temperature Effects On Surface Capillary Wave Dynamics in Bilayer Polymer Films near the Glass Transition
For the first time, thin and ultrathin polymer layers [polystyrene] supported on substrates with modulus varying by 5 orders of magnitude [poly(isobutyl methacrylate), crosslinked polydimethylsiloxane, poly(4-vinyl pyridine), and silicon wafers] were characterized by synchrotron-based X-ray photon correlation spectroscopy (XPCS). At the polystyrene (PS) Tg + 9 K, the PS surface wave relaxation times characterized by XPCS were found to track with substrate modulus, with lower modulus leading to faster PS surface relaxations even when the PS top layers exceeded 100 nm in thickness. These results show that an immiscible polymer domain may significantly influence the dynamics of a second, neighboring immiscible polymer at a temperature near the Tg of the second polymer and over length scales that greatly exceed those of both cooperative segmental mobility near Tg (~1-4 nm) and the polymer radius of gyration. Results of this study also reveal the effect of confinement on surface wave relaxation time (t) in polymer films near Tg and at low qh (<1), where q is the scattering wavevector and h is the top layer thickness. In particular, at the PS Tg + 9 K, the values of t/h increased with decreasing PS layer thickness when measured on a given substrate. This is in direct contrast with prediction of simple capillary wave theory which does not consider confinement or substrate modulus effects. Instead, simple capillary wave theory indicates that values of t/h plotted as a function of qh should overlap for all film thicknesses tested. The fact that the data at Tg + 9 K differ dramatically from this prediction indicates the important role of confinement in altering the capillary wave response near Tg. In contrast, both substrate modulus and confinement effects are negligible when XPCS measurements are take at the PS Tg + 40 K. At this higher temperature, the XPCS response follows predictions of simple capillary wave theory. These results demonstrate the important role of temperature relative to Tg in defining the magnitude of confinement effects of polymer related dynamics, including surface capillary wave relaxations: confinement effects that may be substantial at temperatures within 10 K of Tg may be negligible at temperatures 40 K above Tg.