To Understand Film Dynamics Look to the Bulk.

We show that shifts in dynamics of confined systems relative to that of the bulk material originate in the properties of bulk alone, and exhibit the same form of behavior as when different bulk isobars are compared. For bulk material, pressure-dependent structural relaxation times follow tau(T, V) proportional to exp[f(T) x g(V)]. When two states (isobars) of the material, "1" and "2", are compared at the same temperature this leads to a form tau(2) proportional to tau(c)(1), where c = g[V-2(T)]/g[V-1(T)]. Using equation of state analysis and two models for P-dependent dynamics, we show that c is approximately T independent, and that it can be very simply expressed in terms of either the (free) volume above the close packed state (V-free) or the activation energy for cooperative motion. The effect of changing state through a shift in pressure (P-1 to P-2) is thus mechanistically traceable to cooperativity changing with density, through V-free. The connection with confined dynamics follows when 1 and 2 are taken as bulk and film at ambient P, differing in density only due to the film surface. The general form for tau(T, V) also illuminates why samples in different states (film vs bulk, high P vs low) trend toward the same relaxation behavior at high T.