Structural relaxation of stacked ultrathin polystyrene films



The Tg depression and kinetic behavior of stacked polystyrene ultrathin films is investigated by differential scanning calorimetry (DSC) and compared with the behavior of bulk polystyrene. The fictive temperature (Tf) was measured as a function of cooling rate and as a function of aging time for aging temperatures below the nominal glass transition temperature (Tg). The stacked ultrathin films show enthalpy overshoots in DSC heating scans which are reduced in height but occur over a broader temperature range relative to the bulk response for a given change in fictive temperature. The cooling rate dependence of the limiting fictive temperature, Tf′, is also found to be higher for the stacked ultrathin film samples; the result is that the magnitude of the Tg depression between the ultrathin film sample and the bulk is inversely related to the cooling rate. We also find that the rate of physical aging of the stacked ultrathin films is comparable with the bulk when aging is performed at the same distance from Tg; however, when conducted at the same aging temperature, the ultrathin film samples show accelerated physical aging, that is, a shorter time is required to reach equilibrium for the thin films due to their depressed Tg values. The smaller distance from Tg also results in a reduced logarithmic aging rate for the thin films compared with the bulk, although this is not indicative of longer relaxation times. The DSC heating curves obtained as a function of cooling rate and aging history are modeled using the Tool-Narayanaswamy-Moynihan model of structural recovery; the stacked ultrathin film samples show lower β values than the bulk, consistent with a broader distribution of relaxation times. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2741–2753, 2008