ABSTRACT We review the effects of solvent density inhomogeneties in CO2 on the swelling and dynamics of the polymer thin films. Using neutron reflectivity (NR) we found that CO2 could be sorbed to a large extent in polymer thin films even when the bulk miscibility of the polymers with CO2 was very poor. The swelling was reversible and reached a maximum at pressures and temperatures along the density fluctuation ridge which forms along the extension of the coexistence curve of gas and liquid in the pressure- temperature phase diagram for CO2. The functional form of the dilation amplitude followed that of the density fluctuations as a function of temperature and pressure. In addition, the NR measurements showed that uniform density films were formed with a maximum roughness less than 25 Å. Hence we concluded that the swelling did not seem to be associated with phase separation which ultimately leads to a void formation when CO2 is rapidly evaporated as previously reported for the bulk films. We have also studied the dynamics of polymer thin films in CO2 by means of NR and dynamic secondary ion mass spectrometry (DSIMS) measurements. 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 associated with the density fluctuation ridge. Within the ridge the diffusion coefficients scale as M-2 and can be approximated by an effective William- Landel-Ferry (WLF) equation.
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