The Cooperative Free Volume Rate Model: Applications to Pressure Dependent Dynamics and Dynamics under Confinement

In this talk we discuss our recent work in modeling and predicting alpha relaxation times (τ) using the cooperative free volume rate model (CFV), an approach where the system's free volume determines the molecular cooperativity, and thus the activation energy. The term "free volume" carries some historical baggage, having been ill-defined and, at times, over-enthusiastically applied. However, our definition of free volume is grounded in analysis of experimental volumetric information (PVT data) using our locally correlated lattice (LCL) model equation of state. We have shown, for both polymers and small molecules, that the LCL predicted free volume is very effective in representing a system's volume-based contribution to dynamics. Another essential feature (ignored in historical models) is that the CFV model incorporates an independent temperature-based contribution. We have shown this contribution acts in a product form together with an inverse V_free dependence, a result that follows mechanistically from the rate model. This presentation will summarize some of our applications of the CFV τ(T,V_free) form to experimental systems, with a particular focus on pressure dependent dynamics and dynamics under confinement.