A steady-state fluorescence study of low-molecular-weight polystyrene (MW = 1060 g/mol and 13,000 g/mol) plasticized in supercritical CO<sub>2</sub> is reported. In addition to excitation wavelength, molecular weight, and polystyrene concentration dependencies, CO<sub>2</sub> density also strongly affects the emission spectral contours. A major increase in the steady-state fluorescence intensity and a significant decrease in the polystyrene 320- to 365-nm fluorescence intensity ratio are observed when CO<sub>2</sub> density is increased. Concentration and conformational changes in the polystyrene molecules are used to explain the observations, and these results are proposed to arise from changes in the plasticization power of supercritical CO<sub>2</sub> over the density range studied. A theoretical model is proposed that is based on the assumption that, at low CO<sub>2</sub> densities and low polymer concentrations, polystyrene intermolecular interactions are negligible. The proposed model is able to fit our observed fluorescence data from a CO<sub>2</sub> reduced density of 0.3 to 1.4.

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