Effect of Bridging Groups on Sulfonated Poly(Imide-Siloxane) for Application in Proton Exchange Membrane of Fuel Cells

A series of six-membered sulfonated poly(imide-siloxane)s were synthesized using 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), aminopropyl-terminated polydimethylsiloxane (PDMS) 2,2-benzidinedisulfonic acid (BDSA), as the sulfonation target diamine groups, and various non-sulfonated diamine monomers behaving as bridging groups. The structure-property relationship of SPI-SXx membranes is discussed in details according to the chemical structure of the nvarious non-sulfonated diamines of SPI-SXx membranes from the viewpoints of proton conductivity, ion exchange capacity (IEC) and membranes properties (water uptake, membrane swelling) at equal PDMS content SPI-SXx. They showed good solubility and high thermal stability up to 300 °C. The PDMS was introduced to enhance the proton conductivity and water uptake attributed from the highly flexibility of the siloxane segments. They showed a comparable or even higher proton conductivity than that of Nafion 117 in water at 60 °C. The conductivity and water uptake of angled, SPI-SXm and ODA-based SPI-SX membranes (SPI-SXO) are greater than those prepared from DDM-based SPI-SX membranes (SPI-SXD) at a given IEC. These differences resulted from the increased numbers of entanglements of the flexibility membrane. The SPI-SXD showed alomost isotropically dimensional changes with the increases of water uptake and the volume were slightly smaller than those estimated from the additivity rule. Microscopic analyses revealed that these smaller (<10 nm) and well-dispersed hydrophilic domains contribute to the better proton conducting properties. The new sulfonated poly(imide-siloxane)s have proved to be a possible candidate as the polymer electrolyte membrane for PEFCs and DMFCs.