Effects of Tetracarboxylic Dianhydrides on the Properties of Sulfonated Polyimides

A series of sulfonated polyimides (SPIs) were synthesized from a sulfonated diamine of 4, 4′-bis(4-aminophenoxy) biphenyl-3, 3′-disulfonic acid (BAPBDS), common nonsulfonated diamines, and various tetracarboxylic dianhydrides including 1, 4, 5, 8-naphthalene tetracarboxylic dianhydride (NTDA), 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTDA), 4, 4′-binaphthyl-1, 1′, 8, 8′-tetracarboxylic dianhydride (BTDA), 4, 4′-ketone dinaphthalene 1, 1', 8, 8'-tetracarboxylic dianhydride (KDNTDA), and isophthatic dinaphthalene 1, 1′, 8, 8′-tetracarboxylic dianhydride (IPNTDA). Their membrane properties were investigated to clarify the effects of the dianhydrides. They displayed reasonably high mechanical properties, thermal stability, and proton conductivity. The dianhydrides with flexible and non-coplanar structure (IPNTDA > KDNTDA > BTDA) led to the better solubility of the SPIs than those with rigid and coplanar one (NTDA, PTDA). The dianhydride with the smaller molecular weight led to the larger value of the number of sorbed water molecules per sulfonic acid group (k) in membrane, that is, NTDA (λ: 17) > PTDA(15) > BTDA (14) > KDNTDA (12) > IPNTDA (10), and as a result let to the larger proton conductivity in water. All of the BAPBDS-based SPIs showed the anisotropy in membrane swelling and in proton conductivity, of which the degree hardly depended on the dianhydride moieties. The water stability of SPI membranes against the aging in water at 130 °C for 192 h was in the order, PTDA = NTDA ≧ BTDA > KDNTDA > IPNTDA. The hydrolysis stability of polymer chain was similar between the BTDA- and KDNTDA-based SPIs. These results are discussed.