Preparation And Properties Of Highly Branched Comb-shaped Sulfonated Poly (Arylene Ether)s As Proton Exchange Membranes

The proton exchange membrane(PEM) is a critical component in proton exchange membrane fuel cell(PEMFC). The PEM can transfer protons from the anode to the cathode and provides a barrier against electrons as well as fuel and oxidant pass through the electrodes. It is generally accepted that the perfluorosufonic acid polymers, such as Nafion, are typically used as the polymer electrolytes in PEMFCs because of their high proton conductivity, excellent chemical and oxidative stability. However, high cost is a major obstacle for their widespread application in PEMFCs. As an alternative PEM material, sulfonated poly(arylene ether)s has been developed over the past ten years. Sulfonated poly(arylene ether)s materials were exhibit several advantages, such as good thermal stability, appropriate mechanical strength and high proton conductivity. However, most of these materials are not suitable for PEM applicatons because of their short lifetimes resulting from hydrolysis and oxidative degradation. Cross-linking is an effective method to improve the oxidative stability of polymer membranes. However, cross-linked membranes are usually insoluble in common organic solvents and are difficult for processing, which hinders the development of a commercial process for PEM. Therefore, it is highly necessary to develop other methods to improve the durability of these membranes with excellent solubility. More recently, our group reported a highly branched PEM with 10% degree of branching(DB) value. the branched membrane can be soluble in common polar aprotic solvents, and its oxidative stability increases with increasing degree of branching. Unfortunately, the tensile strength of the membrane with 10% DB value is quite low(11.5 MPa) and cannot meet the requirements for PEMFC assembly application. This may be attributed to the decreased chain entanglement because of the formation of hard arms and short chains between the branching points with increasing branching agent. In this thesis, we synthesized a series of highly branched comb-shaped sulfonated polymers for PEMs by introducing flexible sidechains to improve the mechanical properties of membranes. The flexible sidechain increases the entanglement of polymer chains, which aids in forming good mechanical properties for the membrane. The specific contents and results are as follows:1) A series of highly branched comb-shaped sulfonated poly(arylene ether)s were successfully synthesized by introducing flexible sidechains, and the influences of the branched degree on the properties, especially, the mechanical properties of membrane material were investigated. The branched polymers are soluble in polar organic solvents and could be cast to form tough and smooth membranes. These membranes exhibit good overall properties for PEM aplications. With an increasing DB value, the membrane properties, such as oxidative stability, proton conductivity and swelling ratio, are significantly improved. Although the tensile strength decreases slowly with increasing DB value, all of the highly branched comb-shaped sulfonated polymer membranes show tensile strengths(30.15-20.51 MPa) comparable with that of Nafion 117(25.7 MPa). Compared with the highly branched sulfonated copolymer SPAES-main, the highly branched comb-shaped copolymers exhibit better oxidation stability and mechanical properties. The membrane with the highest DB value(8%) exhibits high proton conductivity(0.33 S cm-1 at 80 °C) and excellent oxidative stability(445 min) as well as acceptable mechanical properties(20.51 MPa), which indicate that this material is a good candidate for PEM in fuel cell applications.2) Two novel types of star-shaped block poly(arylene ether sulfone)s containing sulfoalkyl pendant groups with 6% branching agent were synthesized to improve the properties of the membranes. One of the novel block polymers has hydrophilic segments surrounded by hydrophobic segments(SPAES-6s), the other one has hydrophobic segments surrounded by hydrophilic segments(SPAES-6f). The effects of star-shape structure on the properties of the PEM membranes were investigated. Compared with the highly branched sulfonated copolymer SPAES-6-main, the highly branched comb-shaped copolymers exhibit better oxidation stability and mechanical properties. The block polymer SPAES-6f and SPAES-6s exhibited higher proton conductivity(0.42 S cm-1 and 0.30 S cm-1 at 80 °C) than that of the random polymer SPAES-6r and Nafion 117, whereas SPAES-6s exhibited excellent oxidative stability(435 min) and dimensional stability than SPAES-6f.