Relaxation of ultrathin polystyrene films hyperswollen in supercritical carbon dioxide

Relaxation of highly swollen (∼20–80 %) ultrathin (≤21 nm) polystyrene films exposed to supercritical carbon dioxide (at 35°C and 80 bar) is studied by monitoring the evolution of thickness as a function of time using X-ray reflectivity. Correlations between their swelling (that determines the departure from equilibrium), depressurization time, thickness relaxation rate and aging time are established. Swollen films that are depressurized very slowly (depressurization time ≈1 h) exhibit a small swelling and a very low relaxation rate at ambient condition, whereas fast depressurized films (depressurization time ≈2 min) show a large swelling and a high relaxation rate. The rate of depressurization not only impacts their out of equilibrium state but also their relaxation rate. The relaxation time is essentially governed by the temperature following an Arrhenius law below Tg and a strong temperature-dependent behavior above Tg and by the relative departure from equilibrium