Synthesis and NMR studies of the polymer membranes based on poly(4-vinylbenzylboronic acid) and phosphoric acid

Proton conduction in novel anhydrous membranes based on host polymer, poly(4-vinylbenzylboronic acid), (P4VBBA) and phosphoric acid, (H₃PO₄) as proton solvent was studied. The materials were prepared by the insertion of the proton solvent into P4VBBA at different stoichiometric ratios to get P4VBBA·xH₃PO₄ composite electrolytes. Homopolymer and the composite materials were characterized by FT-IR, ¹¹B MAS NMR and ³¹P MAS NMR. ¹¹B MAS NMR results suggested that acid doping favors or leads to a four-coordinated boron arrangement. ³¹P MAS NMR results illustrated the immobilization of phosphoric acid to the polymer through condensation with boron functional groups (B-O-P and/or B-O-P-O-B). Thermogravimetric analysis (TGA) showed that the condensation of composite materials starts approximately at 140 °C. An exponential weight loss above this temperature was attributed to intermolecular condensation of acidic units forming cross-linked polymer. The insertion of phosphoric acid into the matrix softened the materials shifting T_g to lower temperatures. The temperature dependence of the proton conductivity was modeled with Arrhenius relation. P4VBBA·2H₃PO₄ has a maximum proton conductivity of 0.0013 S/cm at RT and 0.005 S/cm at 80 °C.