CO₂-enhanced diffusion of azobenzene in glassy polystyrene near the glass transition

Forced Rayleigh scattering (FRS) has been used to measure the tracer diffusion coefficient of azobenzene in low molecular weight polystyrene (PS) as a function of temperature, and in high molecular weight PS plasticized by CO₂ at 35 °C and CO₂ pressures from 14 to 85 bar. In contrast to dye diffusion in pure PS and PS plasticized by tricresyl phosphate, where the effect of plasticization by temperature, chain ends, or added diluent can be accounted for by (T - T_g) scaling, dye diffusion in CO₂-plasticized PS is enhanced by 2-3 orders of magnitude over values predicted on the basis of T_g depression alone. This enhancement begins at surprisingly low CO₂ pressures (<15 bar at 35 °C) and is maintained across the CO₂-induced glass transition. A large difference in mobilities of the cis and trans isomers of azobenzene is also observed in the presence of CO₂, which is much greater than that seen in the experiments on pure PS. Two additional FRS relaxation modes unrelated to translational diffusion of the dye molecules have been identified in this study: a fast, local relaxation attributed to dye rotation, and a slow relaxation attributed to the dynamic response of the PS/CO₂ matrix to a chemical potential driving force associated with the azobenzene isomers.