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

Abstract

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 (Tg). 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 Tg and decreases FFV and gas diffusivity, which can be satisfactorily described using a free volume model.