Synthesis And Properties Of Sulfonated Poly(Arylene Ether)s With Sulfonic Acid Groups On Pendant

The proton exchange membrane, which is a key component in proton exchange membrane fuel cell (PEMFC) acts as a separator to prevent mixing of reactant gas and provides ionic pathways for protons transport. The state-of-the-art proton exchange membranes are prefluorinated copolymers with pendant sulfonic acid group, such as Nafion membrane (Dupont) and Dow membrane (DOW). They are well known for their high conductivity and excellent chemical stability. However, high price and limited operating temperature of the perfluorinated sulfonated membranes have prevented their applications in large scale. In addition, they have the high methanol permeation when they use in DMFC. Many efforts have been spent on searching for alternative membranes, and sulfonated aromatic polymers are one of them due to their resistance to oxidation and stability under acidic conditions. A large number of research groups are involved into preparing and investing these new membranes.Directly sulfonated aromatic polysulfones and polyetherketones with the sulfonic acid groups on the main chain have been widely studied because of their high chemical, high thermal stability, and convenient sulfonation procedures. The directly sulfonated aromatic polymers are also promising materials for use in direct methanol fuel cells due to their low methanol permeability. Unfortunately, the materials like directly sulfonated polyetherketones typically were swollen extensively at temperatures above 80 ?C, and lost the dimensional stability as a consequence. Of course, this makes the use of these materials problematic at elevated temperatures. The situation is even more problematic in the case of directly sulfonated polysulfones, which have shown to be soluble into water at degrees of sulfonation corresponding to 0.8 sulfonic acid groups per repeating unit of the polymer. The reason for the excessive swelling may largely be due to the location of the sulfonic acid units, which are directly on the polymer main chain, resulting in a small spatial separation of the hydrophilic sulfonic acid units and the hydrophobic polymer main chains. This limits the cohesion of the hydrophobic parts as the membrane absorbs water. When the swelling pressure becomes too high, the morphology breaks up, resulting in disintegration of the membrane.Several attempts have been made to limit the swelling by cross-linking the membrane material. But unfortunately, covalent cross-linking increased the brittleness of the membranes. Another approach to limit the swelling is to control the morphology of the membrane. If the absorption of water can be restricted to a specific hydrophilic domain in the membrane, which is molecularly separated from the hydrophobic one, the latter domain may retain its cohesion to restrict the swelling. A successful way to separate the hydrophilic sulfonic acid units and the hydrophobic polymer main chain is to locate the sulfonic acid units at the chain ends of short side chains on the polymer main chain.In our work, we prepared some novel side-chain-style sulfonated poly(arylene ether) copolymer materials by molecular design, and investigated the relationship between structure and property in the view of the difference of side chain structure. Firstly, from the analysis of polymer molecular design, the introduction of short sulfonated side chain into aromatic polymer main chain via post sulfonation method was studied. This polymer exhibited good thermal stability and high proton conductivity. The conductivity value is higher than that of Nafion 117 at high temperature when higher IEC membrane was tested. Although sulfonic acid groups locate in side chain, the polymer did not exhibit excellent swelling ratio. We think that the length of side chain influences the performance of polymer. Thus, we prepared poly(arylene ether) with long sulfonated side chain (SC-SPAE) via direct copolymerization method. The novel SC-SPAE polymers exhibited good thermal stability and high proton conductivity. Compare with previous side-chain-style sulfonated polymer, the SC-SPAE copolymer exhibit excellent swelling ratio. The highest swelling ratio is below 20%. This value is lower Nafion 117 and most of reported sulfonated aromatic polymers.Compared with the structure of Nafion membrane, the sulfonated aromatic polymer has lower acidity. Increasing acidity for sulfonated aromatic polymer is the best approach for improving conductivity. From the analysis of polymer molecular design, we prepared high- acidity sulfonate poly(arylene ether) from high-acidity sulfonation monomer and functional polymer via graft method. These polymers exhibited excellent thermochemical stability and low swelling ratio. The conductivity value is higher than that of Nafion 117 at high IEC value.Generally, the flexibility of polymer chains is helpful to the motion of sulfonated groups and to increase the polymer conductivity. So, we prepared sulfonated poly(arylene ether)s with more flexible sulfoalkylated pendants from two kinds of sulfonated monomers via direct copolymerization method. These sulfonated polymers showed high thermal and anti-oxidative property and high proton conductivity. The most important is that the introduction of flexible segments between sulfonic acid groups and polymer mainchain greatly decreases the dependence of proton conductivity on temperature. Based on above results, the more hydrophobic mainchain and the more flexible sulfonic pendants favor the properties of sulfonated polymers.