Preparation And Properties Of Proton Exchange Membranes Based On Functional Poly-norbornene

Direct methanol fuel cell (DMFC) with a compact structure, light weight, abundant fuel source and easi to be carried and stored, is one of priority development of the energy technologies.The proton exchange membrane (PEM), is the key component of the DMFCs. Nafion(?), one of the perfluorosulfonic acid membranes, is the current state-of-the-art PEM material. However, several drawbacks of Nafion(?), such as high cost, high methanol permeability, low humidity and a major reduction in conductivity at high temperatures, have led researchers to investigate promising alternatives.Vinyl addition type polynorbornenes have both high rigidity main chain and strong polymer ring structure which limits the COC to move freely along the main chain, show a higher glass transition temperature. Because all this materials that don't have unsaturated double bond, triple bond or aromatic ring structure, usually have excellent heat resistance and anti-aging properties. Its thermal decomposition temperature is higher than 400℃. This material also has low water permeability. Based on its good film-forming, thermal stability, high chemical stability, and dielectric properties etc., the vinyl addition type polynorbornene is suitable for development as a direct methanol fuel cell proton exchange membrane material.The graft copolymer poly(butoxymethylenenorbornene-co-norbornenemethylene bromoisobutyrylate) grafted poly(hydroxy ethyl acrylate) [P(BN/NOH)Br-g-PHEMA] was synthesized by the atom transfer radical polymerization (ATRP) of hydroxy ethyl acrylate (HEMA) from the copolymer prepared by two functional norbornene monomers via vinyl addition mechanism. The graft copolymer [P(BN/NOH)Br-g-PHEMA] is further crosslinked with 4,5-imidazole dicarboxylic acid (IDA) and then doped with phosphoric acid (H3PO4) to form imidazole-H3PO4 complexes. The results showed that the polynorbornene backbone and crosslinked micromorphology produced low methanol permeability of the membranes (from 1.5×10-7 to 3.8×10-6 cm2/s) and endowed the membranes with good mechanical properties (from 692.7 to 159.7 MPa of elastic modulus, from 2.7 to 22.7% of elongation at break, from 14.4 to 5.5 MPa of tensile strength at break) and excellent thermal stability (up to 280℃). Furthermore, the proton conductivities of the membranes increased with increasing temperature and increasing contents of IDA/H3PO4 in the membranes.The vinyl addition type copolymer Poly(butoxymethylene norbornene-co-biphenyl oxymethoxy norbornene) (P(BN/BphN)) was synthesized by using bis-(β-ketonaphthylimino)nickel(Ⅱ)/B(C6F5)3 catalytic system. P(BN/BphN) was sulfonated with concentrated sulfuric acid (98%) as sulfonating agent in a component solvent. Degree of sulfonation (DS) was controlled by the reaction time and the SP(BN/BphN) membranes were obtained by solution casting method. The ion exchange capacity (IEC), DS, water uptake and methanol permeability of the SP(BN/BphN)s were increased with the sulfonated time. The sulfonated time was controlled to be 8h as the highest to obtain lower methanol permeability. The methanol permeability of the SP(BN/BphN) membranes were in the range of 1.8×10-7 to 7.5×10-7 cm2/s, which were lower than the value 1.3×10-6 cm2/s of Nafion(?)115. FTIR,1H NMR were involved to confirm the structure of achieved monomer, copolymer and sulfonated copolymers. The proton conductivity of SP(BN/BphN) membranes increased with the increase of IEC values, temperature and water uptake. Water uptake of the SP(BN/BphN) membranes were lower than that of Nafion(?) 115 and lead to low proton conduction. Microscopic phase separation occurred in SP(BN/BphN) membrane and domains containing sulfonic acid groups were investigated by TEM and SEM. SP(BN/BphN) membranes had good mechanical properties, high thermal stability and excellent oxidative stability.