Effect of hexafluoro-2-propanol substituents in polymers on gas permeability and fractional free volume

A series of polymers were synthesized containing hexafluoro-2-propanol (HFP) pendant groups by facile reaction of lithiated polymers with hexafluoroacetone (HFA). The purpose of the study was to evaluate the effect of the HFP group in potential membrane gas separation materials. Dibromobisphenol A polysulfone (PSf-Br-2) tetramethylbisphenol A polysulfone (TMPSf), dibromohexafluorobisphenol A polysulfone (6FPSf-Br-2), tetramethylhexafluorobisphenol A polysulfone (TM6FPSf), and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were modified by using n-butyllithium to form lithiated intermediates and then treated with HFA to form the corresponding modified polymers PSf-HFP, TMPSf-HFP, 6FPSf-HFP, TM6FPSf-HFP, and PPO-HFP. Polymers with various degree of substitution per repeat unit of HFP were obtained, depending upon the extent of polymer lithiation and reactivity. PSf-HFP, TM6FPSf-HFP, and PPO-HFP were further derivatized at the hydroxyl group using iodomethane to obtain the respective methylated derivatives (HFM). The polymer structures were characterized by NMR spectroscopy, and thermal properties were investigated by differential scanning calorimetry and thermogravimetric analysis. The gas permeability coefficients of these polymers were measured for the gases He, O-2, CO2, and N-2. The gas transport properties of PSf-HFP, PSf-HFM, and TMPSf-HFP for He/N-2, He/CO2, O-2/N-2, and CO2/N-2 gas pairs were all improved in comparison with PSf and TMPSf. The permeabilities for all gases increased 2-3-fold, while the permselectivities of the four gas pairs increased as well. All the HFP derivatives showed improvements of this magnitude only for the He/CO2 gas pair.