Measurement of probe diffusion in CO2-Swollen polystyrene using in situ fluorescence nonradiative energy transfer

Tracer diffusion coefficients of decacyclene, perylene, and 9,10-bis(phenylethynyl)anthracene (BPEA) in CO2-swollen polystyrene (PS) films were measured in real time by high-pressure fluorescence nonradiative energy transfer (NRET) using pyrene-labeled polystyrene (PLPS) as the corresponding energy donor. The conditions studied, 55-75 degreesC and CO2 pressures between 55 and 104 bar, include regions near and well above the reported glass transition of PS in the presence of CO2. The diffusivity of the acceptor probes increased markedly upon modest increases in CO2 sorption. For example, decacyclene diffusivity increased from 6.6 x 10(-14) to 1.4 x 10(-12) cm(2)/s as CO2 sorption increased from 7 to 11 wt %. By comparison to the PS glass at ambient pressure and, equivalent temperatures, acceptor probe diffusivities increased by more. than 4 orders of magnitude. The concentration dependence of the probe diffusivity is in agreement with the Vrentas-Duda free volume model extended to ternary systems. The analysis indicates that probe diffusion in CO2-dilated films is strongly coupled to polymer segment mobility, but CO2 diffusion is not. The friction factor for the probes decreases with decreasing probe volume. The apparent activation energies for decacyclene diffusion in CO2-dilated films are considerably smaller than those in the PS melt at equivalent values of T-T-g.