On the sulfonation of fluorinated aromatic polymers: Synthesis, characterization and effect of fluorinated side groups on sulfonation degree

Series of partially sulfonated poly(F-STs-co-ST) copolymers and poly(TFMST-ter-ST-ter-FMST) terpolymer have been successfully synthesized via post-sulfonation method using acetyl sulfate as a sulfonating agent. The identical and mild sulfonation conditions with a controlled sulfonation degree (SD) in the range of 18-27 mol%, were designed for comprehensive evaluation of the effect of fluorine content on the efficiency of the electrophilic -SO3H substitution. The SD values of the sulfonated FAPs were determined using elemental analysis (EA). The identical, effective and versatile isolation method for all sulfonated polymers, regardless the amphiphilic dual nature of the resulting polymers (hydrophobic fluorine-containing, and also hydrophilic sulfonated unit) and also the presence of different fluorinated methyl moieties (-CH2F, -CF2H and -CF3) in polymeric chain, was developed. The resulting novel sulfonated FAPs were characterized by NMR and FTIR spectroscopies that evidenced a satisfactory incorporation of -SO3H functions into aromatic rings of polymer chain. Studies clearly revealed that the electron-deficient aromatic rings of F-ST units did not undergo sulfonation in comparison with electron-rich aromatic rings of ST units. The FTIR and H-1 NMR analysis showed that the sulfonation occurred only in aromatic rings of the ST units and -SO3H functions were introduced at the para position. Moreover, with an increase of the fluorine content in polymer structure, a decrease in the resulting SD values was noticed. The presence of fluorine reduced the electron density directly in fluorinated aromatic units (F-STs), and surprisingly gently affected neighboring ST units. The thermal properties of the resulting copolymers were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The observed higher T(g)s values of sulfonated polymers than those of unsubstituted counterparts indicated that the increase in the structural rigidity of the resulting materials as a function of the increase of the amount of sulfonic groups. Finally, the XPS analysis proved that under these conditions undesired sulfone crosslinking did not occur.