Structure-property relationship of polyimides derived from sulfonated diamine isomers

Four novel sulfonated diamine monomers which are isomers to each other, 4,4'-bis(4-aminophenoxy)biphenyl-3,3'-disulfonic acid (pBAPBDS), 4,4'-bis(3-aminophenoxy)biphenyl-3,3'-disulfonic acid (mBAPBDS), 2,2'-bis(4-aminophenoxy)biphenyl-5,5-disulfonic acid (oBAPBDS), and 4,4'-bis(4-amino-2-sulfophenoxy)biphenyl (iBAPBDS), were synthesized and the related sulfonated polyimides (SPIs) were prepared from these sulfonated diamines and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA). The resulting SPIs had the same chemical composition and ion exchange capacity (IEC) but quite different solubility behaviors and water stability. Nonlinear sulfonated diamine (m-BAPBDS and oBAPBDS)-based polyimides showed much better solubility in many organic solvents than the linear diamine (pBAPBDS and iBAPBDS)-based ones. It was found that the water stability of the SPI membranes was the total effects of solubility stability and hydrolysis stability. Solubility stability is mainly determined by the IEC level and the configuration of the sulfonated diamine moiety. High IEC and nonlinear configuration lead to poor solubility stability and vice versa. Hydrolysis stability is strongly dependent on the basicity of the sulfonated diamine moiety. Highly basic diamines give much better hydrolysis stability than weakly basic ones. Water stability is also affected by the chain flexibility of the SPIs, i.e., polyimides with flexible structure tend to have better water stability than the ones with rigid structure. As a result, NTDA-pBAPBDS, which has linear and highly basic diamine moiety and flexible main chain showed excellent water stability at high temperature. For example, it could maintain mechanical strength after being soaked in water at 80 degrees C for more than 1000 h and the pBAPBDS-based co-SPIs displayed much better water stability at 100 or 130 degrees C.