Swelling of star polymer thin films exposed to supercritical carbon dioxide

Impact of polymer chain architecture on the swelling of supported single-layer polymer thin films with thicknesses below 80 nm in the presence of supercritical CO₂ (scCO₂) is investigated using in-situ neutron reflectivity (NR) and ex-situ x-ray reflectivity (XR). 4-arm star and 8-arm star deuterated polystyrene (dPS) having similar total molecular weights are used. NR results revealed that the magnitude of the swelling for star polymers increases proportionally with decreasing dry film thicknesses similar to linear polymers. For film thicknesses above 5R_g, architectural differences have no impact on the swelling behavior. However, below 5R_g as the branching increases from linear to 4-arm star and to 8-arm star, swelling decreases due to the presence of a relatively thicker adsorbed layer formed by more branched architectures. Ex-situ XR measurements verified that star architectures swell less than linear architecture and provided new information about the vitrification of the films through rapid quenching and its relation to the amount of CO₂ retention.