Synthesis And Properties Of Poly (Aryloxyphosphazenes) Functionalized With Pendant Perfluoroalkyl Sulfonic Acid For Proton Exchange Membrane

Polydichlorophosphazene(PDCP) is a kind of inorganic polymer, with a linear backbone of alternating phosphorus and nitrogen atoms, while the chlorine atoms in the side chain are unstable. Via substitution reaction with some small organic molecules with active hydrogen, it is easy to get poly-(organophosphazenes) of high thermal and chemical stability, which has the advantages of both inorganic and organic compounds. On the one hand, poly-(organophosphazenes) possesses high molecular weight, indicating it is an excellent film-forming material. On the other hand, the common synthetic route to poly-(organophosphazenes) via molecular substitution allows the properties to be tuned over a wide range of structure, in principle, it can meet the requirements of the proton exchange membrane. Hence, poly-(organophosphazenes) is a good candidate for proton exchange membrane.A new series of poly(aryloxyphosphazenes) functionalized with pendant perfluoroalkyl sulfonic acid(PTMPPx-r-PFSPPy) were successfully prepared. First, 4-trifluoromethylphenol and 4-methoxyphenol were introduced to polydichlorophosphazene; And then transformed some methoxy group to hydroxyl group by boron tribromide(BBr3); Finally, the functionalization of perfluoroalkyl sulfonic acid was finished via the lactone ring-opening reaction. After that, proton exchange membranes were made by solution blending method, at the same time, the crosslinking reaction was completed, for enhancing its methanol resistence.The new membranes get the comprehensive characteristics of poly(aryloxyphosphazenes) and perfluorosulfonic acid polymers: high proton conductivity, low permeability of methanol, good chemical and dimensional stability. Due to the presence of electron-withdrawing F atoms, the acid strength of the terminal sulfonic acid groups increases, the polarity between the polyphosphazene backbone architecture and the side chain enlarges, which facilitates phase separation between the hydrophobic and hydrophilic aggregates and forms interconnected nano-channels for efficient proton-transport. The skeleton structure of poly(aryloxyphosphazenes) is of good methanol resistance and chemical stability. In addition, the incorporation of perfluoroalkyl sulfonic acid group in the synthetic steps effectively controls the number of sulfonic acid group and avoids the swelling problems, therefore we get favourable dimensional stability.In the series of PTMPPx-r-PFSPPy proton exchange membranes, the PTMPP1-r-PFSPP9 membrane shows the best performance. Its proton conductivities are both close to the value of Nafion 117(0.1 S/cm, 0.19 S/cm) at 25 ℃ and 80 ℃, while its methanol permeability(5.95×10-7cm2/s) is much lower than that of Nafion 117(15.8×10-7cm2/s).Therefore, PTMPP1-r-PFSPP9 proton exchange membrane has broad prospects in the application of direct methanol fuel cell(DMFC).