The Importance of Density in Segmental Dynamics: Applications of the Cooperative Free Volume Rate Model and Connections with the Density Scaling Approach

A focus in our work is to make predictive connections with real experimental systems. For example, while segmental relaxation data is sometimes collected only at atmospheric pressure, deeper insight is only possible by accounting for pressure-dependent dynamics. This enables the analysis of the contributions due to independent changes in temperature (T) and volume (V), which gives a much deeper representation of the experimental system. It also leads to natural connections with that system's dynamics under confinement. In this talk we discuss our recent work in modeling and predicting alpha relaxation times, τ(T,V), using the cooperative free volume rate model (CFV), in which the system's (well-defined and thermodynamically quantified) free volume controls the molecular cooperativity, and thus the activation energy. In addition to presenting our analysis of experimental systems, we will also feature connections and comparisons with the widely applied density scaling approach. We will show how the key parameters of the two approaches are connected and contrast their predictive power. A strength of the CFV model is its more efficient use of the same thermodynamic information to characterize the form of the corresponding dynamics.