We have measured the temperature dependence of the physical aging rate β of thick (2430 nm) and thin (29 nm) polystyrene (PS) films supported on silicon using a new streamlined ellipsometry procedure that we have recently developed. The physical aging rates β(T) for the ∼30 nm thick films are found to be reduced at all temperatures, which is not consistent with a simple shift in β corresponding to the average reduced glass transition temperature (T g ) of these films. Instead, the β(T) results correspond well with there being a gradient in dynamics near the free surface. Our β(T) results can be well fit by both a two-layer model and a gradient model. The temperature-dependent length scale (of order 10 nm) that characterizes the depth to which the enhanced dynamics near the free surface propagate into the film is similar to that found previously by Forrest and Mattsson [Phys. Rev. E 2000, 61, R53-R56] for the molecular weight (MW)independent T g reductions of low-MW free-standing PS films, strongly suggesting that the same mechanism is responsible for both effects. This length scale grows with decreasing temperature, suggesting that the mechanism is cooperative in nature.

A eudiometer is commonly used to measure gas volumes over a liquid. It is well-known that the height of the liquid column is proportional to the difference between atmospheric pressure and the pressure of the trapped gas. It is therefore also well-known that changes in atmospheric pressure can be detected through changes in the height of the liquid column (1, 2). We demonstrate, perhaps for the first time, that absolute atmospheric pressure can be determined simply by moving the eudiometer up and down to vary the height of the liquid column. The analysis is simplest when the vapor pressure of the liquid is negligible. For this reason, we choose to use glycerol as our liquid.Suppose that a volume of air, V, is trapped in a eudiometer containing glycerol, as shown in Figure 1. The pressure of the trapped air, P, and the height of the glycerol column, h, are related to atmospheric pressure, P atm , according to