The T g 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 (T f ) was measured as a function of cooling rate and as a function of aging time for aging temperatures below the nominal glass transition temperature (T g ). 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, T f 0 , is also found to be higher for the stacked ultrathin film samples; the result is that the magnitude of the T g 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 T g ; 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 T g values. The smaller distance from T g 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 b values than the bulk, consistent with a broader distribution of relaxation times.

The calorimetric glass transition (T g) is measured for single polystyrene ultrathin films using a commercial rapid-scanning chip calorimeter as a function of cooling rate and film thickness. Films have been prepared in two ways: spin-cast films placed on a layer of inert oil or grease and films directly spin-cast on the back of the calorimetric chip. For the films on oil or on grease, the 160 nm thick films show results consistent with those of a bulk sample measured by conventional DSC. On the other hand, the 47 nm thick film on oil and 71 nm thick films both on oil and on grease show a T g depression which decreases with increasing cooling rate; the magnitude of the T g depression is similar to results reported in the literature for the most mobile substrate-supported films. For films directly spin-cast onto the sensor, a T g depression is not observed for 47 and 71 nm thick films but is observed for a 16 nm thick film. These results are also within the range of the data on supported films in the literature but show a smaller depression than films on oil or grease. The effect of annealing is also investigated. For thick films and those directly spin-cast onto the sensor, annealing at 160 °C has no influence on heat flow curves; hence, T g values remain unchanged. For the 47 and 71 nm thick films on either oil or grease, the depressed T gs revert to the bulk values over the course of a day at 160 °C. Atomic force microscope (AFM) images show that annealing results in dewetting of the films with hole growth and thickening of the film to 200 nm, the latter of which is presumed to be the reason that T gs revert to bulk values.

A glass is not in thermodynamic equilibrium below its glass transition temperature (T g ), and consequently, its properties, such as enthalpy, volume, and mechanical properties, evolve toward equilibrium in a process known as structural recovery or physical aging. Several recent studies have suggested that the extrapolated liquid line is not reached even when properties have ceased to evolve. In this work, we present measurements of the enthalpy recovery of polystyrene at an aging temperature 15 °C below the nominal T g , for aging times up to 1 year. The results indicate that the equilibrium liquid enthalpy line can indeed be reached for aging 15 K below T g . The results are analyzed in the context of the TNM model of structural recovery.

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ABSTRACT:The absolute heat capacity and glass transition temperature (T g ) of unsupported ultrathin films were measured with differential scanning calorimetry with the step-scan method in an effort to further examine the thermodynamic behavior of glass-forming materials on the nanoscale. Films were stacked in layers with multiple preparation methods. The absolute heat capacity in both the glass and liquid states decreased with decreasing film thickness, and T g also decreased with decreasing film thickness. The magnitude of the T g depression was closer to that observed for films supported on rigid substrates than that observed for freely standing films. The stacked thin films regained bulk behavior after the application of pressure at a high temperature. The effects of various preparation methods were examined, including the use of polyisobutylene as an interleaving layer between the polystyrene films.