CO₂-Enhanced Transport of Small Molecules in Thin Films:  A Fluorescence Study

Steady-state fluorescence measurements have been used to measure the rate of transport of a fluorescent probe (pyrene) out of ca. 250 nm thick films of polystyrene in contact with CO₂(sc) at pressures in the range 35−110 bar (estimated CO₂ content in the film up to 0.11 weight fraction) and several temperatures (35, 40, 50, and 60 °C). At constant temperature, the estimated pyrene diffusion coefficient increases by approximately 4 orders of magnitude from the lowest to the highest CO₂ content (e.g., from ca. 10^-14 cm²/s for ca. 0.04 CO₂ weight fraction to ca. 10^-10 cm²/s for ca. 0.11 CO₂ weight fraction at 60 °C). We estimate the glass transition pressure at a given temperature (P_g(T)) in two ways:  (1) from the inflection point of the log D vs P(CO₂) plot, (2) matching log D at a given temperature and P(CO₂) to the value obtained for an unswollen film at T_g (D is measured by the rate of loss of the probe into a vacuum).