A Simple New Way to Account for Free Volume in Glassy Dynamics: Model-Free Estimation of the Close-Packed Volume from <i>PVT</i> Data

Pressure-dependent structural relaxation times for glass-forming systems, τ(T,V), depend on independent contributions from both temperature (T) and volume (V). Our recent work shows that free volume, V_free = V − V_hc, where V_hc is the system volume at close-packing, dictates the independent volume contribution. Specifically, isotherms of lnτ vs. 1/V_free are linear, with slopes that increase with decreasing T. We explained this behavior mechanistically through the cooperative free volume rate model (CFV). One way to calculate V_free is by applying our LCL equation of state to analyze pressure, volume, temperature (PVT) data to obtain V_hc from its molecular parameters. However, there is a very physically transparent alternative route, which is presented in this talk. V_hc values can be obtained by simple linear extrapolation of PVT isobars to T = 0. This new route for calculating V_free is another way to show the strong connection that exists between dynamics and PVT measurements. We will discuss results for a dozen polymer and small molecule glass forming systems.