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 $\tau (T,V)\propto \exp [f(T)\times g(V)]$. When two states (isobars) of the material, “1” and “2”, are compared at the same temperature this leads to a form ${\tau }_2\propto {\tau }_1^c$, 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 ($V_{\mathrm{free}}$) 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 $V_{\mathrm{free}}$. 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 $T$.

• Received 12 May 2020
• Revised 27 June 2020
• Accepted 8 July 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.058002