Gas transport characteristics of fluorinated polystyrene-b-polybutadiene-b-polystyrene (F-SBS)

Fluorinated polymers exhibit unique gas separation properties. However, the effect of the degree of fluorination in polymers on gas transport properties has not been elucidated. Herein, the alkenyl groups in the polybutadiene (PB) block of polystyrene-b-polybutadiene-b-polystyrene (SBS) were modified with gem-difluorocyclopropanation via difluorocarbene (∶CF₂). The F content in the fluorinated PB (FPB) phase ranges from 0 to 27 wt%, which was confirmed using ¹H NMR spectroscopy, and physical properties of the polymers were determined, including density, fractional free volume (FFV), and glass transition temperature (T_g). The Maxwell and additive models are used to estimate gas permeability and solubility in the FPB phase, respectively. Increasing the F content from 0 to 27 wt% in the FPB phase increases the CO₂ solubility from 0.58 to 1.1 cm³ (STP)/cm³ atm and CO₂/C₂H₆ solubility selectivity from 0.45 to 1.1 at 35 °C, suggesting that the F groups have induced favorable interaction with CO₂ and unfavorable interaction with C₂H₆ in these partially fluorinated polymers. However, a further increase of the F content in the perfluoropolymer (such as polytetrafluoroethylene) decreases CO₂ solubility and CO₂/gas solubility selectivity, indicating the unique CO₂/gas separation properties in partially fluorinated polymers. The introduction of the F groups to the polymer also increases T_g and decreases FFV and gas diffusivity, which can be satisfactorily described using a free volume model.