Dynamics of polymer thin films in supercritical carbon dioxide

In situ neutron reflectivity and dynamic secondary ion mass spectrometry measurements were used to investigate the dynamics of polystyrene thin films in supercritical carbon dioxide (sc$\chem{CO_2}$). The results show that the large degree of anomalous swelling observed is associated with a decrease in glass transition temperature that occurs only within a narrow region of the phase diagram associated with the density fluctuation ridge in sc$\chem{CO_2}$ region. Furthermore, the plasticization effect induced by the density fluctuations was found to enhance the polymer interdiffusion by 2 orders of magnitude as the ridge was approached. Within the ridge the diffusion coefficients scale as M2 and can be approximated by an effective William-Landel-Ferry (WLF) equation.