Proton conducting polyimides from novel sulfonated diamines - Polyimides and Other High Temperature Polymers - page 136

wide applications in many industrial fields [1, 2]. Recently sulfonated six-
membered ring polyimides have been identified to be promising proton conduct-
ing membrane materials for fuel cell application [3-8]. Mercier and coworkers
first synthesized a series of sulfonated copolyimides from naphthalene-1,4,5,8-
tetracaboxylic dianhydride (NTDA), 2,2'-bendizine sulfonic acid (BDSA, a
commercially available sulfonated diamine), and common non-sulfonated dia-
mine monomers [3-6]. These sulfonated polyimide membranes were tested in a
fuel cell system which showed fairly good performance. However, the proton
conductivity of these membranes was rather low (< 10 -2 S·cm -1 , in water) due to
the low ion exchange capacity (IEC) which is essential for maintaining hydrolysis
stability of the membranes. Here, IEC refers to the equivalent of exchangeable
ions (cations, in case of sulfonated polymers) of unit weight of polymer. The
fairly short lifetime (maximum: 3000 h) compared with the fuel cell with Nafion
membrane is another problem. Litt's group has also employed BDSA as the sul-
fonated diamine monomer for preparation of various random and sequenced co-
polyimides [7, 8]. They reported that some copolyimide membranes derived from
bulky or angled diamine comonomers displayed higher conductivities than Nafion
117. However, the water stability of their membranes is still a problem.
Proton conductivity and membrane stability are two important factors which
greatly affect the performance of a fuel cell system. The enhancement of proton
conductivity can be achieved by increasing the IEC. However, too high IEC gen-
erally leads to high degree of swelling or even dissolution in water of the mem-
branes. To maintain membrane stability, the IEC should be controlled at a rela-
tively low level. On the other hand, it is well known that the chemical structure of
polymers has considerable effect on the properties of the membranes. However,
little information on this aspect can be found in the literature. To develop polyim-
ide membranes with high proton conductivity and good water stability, it is essen-
tial to systematically study the “structure-property” relationships of the sulfonated
polyimides. Besides BDSA, the only other commercially available sulfonated
diamines are 2,4-diaminobenzenesulfonic acid (DABS) and 5,5'-
dimethylbenzidine-2,2'-disulfonic acid (DMBDS). DABS-based polyimides gen-
erally have poor solubility and poor mechanical properties, and, therefore, DABS
is seldom used. DMBDS-based polyimides are expected to have similar properties
to BDSA-based ones due to their similar structure. Therefore, the development of
novel sulfonated diamine monomers is strongly desired. In this paper, we report
on our recent progress on the development of novel sulfonated diamines and re-
lated polyimides and summarize the work in order to make a contribution to the
understanding of the relationships between polyimide structure and the proton
conductivity and water stability.