Modification of Sulfonated Polyimides

Watanabe and coworkers used different branching agents in order to synthesize branched/cross-linked sPIs. They introduced either melamine [83,111] in a ftiUy aromatic sPI (Fig. 3.3) or tris(aminoethyl)amine in an aromatic/aliphatic sPl backbone [14,89]. In both cases, the proportion of trifunctional branching agent (leading to 

Another triamine monomer (l,3.5-tris(4-aminophenoxy)benzene, TAPE) has been used as a cross-linker by Yin et al. [135,136] (Fig. 3.4). Typically, anhydride-terminated sulfonated ohgomers are prepared from BAPBDS and NTDA in m-cresol at 180°C for 20 h. After adding some triamine monomer, in the second step, the reaction medium is kept at moderate temperature (50 °C), resulting in a polyamic acid intermediate. By a thermal treatment at high temperature or in the presence of an acetic anhydride/pyridine mixture, complete imidization is performed during film formation. 

According to a similar approach. Park et al. [138] described the synthesis of cross-linked sPIs based on carboxylic acid-containing structures and the formation of ester bridges using various alkane diols. The low hydrolytic stability of ester linkages in 

9 Synthesis of sPI- and poly(ethylene glycol) diacrylate-based semi-IPN membranes. 

Organic-inorganic (sPI-SiOa) interpenetrating networks appear to be very promising materials as sohd electrolytes for fuel cell applications. Lee et al. [144] showed that the presence of sihca reduces the membrane water uptake and methanol permeability, while it increases the membrane selectivity (Fig. 3.10). In addition, the formation of an organic IPN improves the hydrolytic stability of the material. Hence, the mechanical properties (tensile strength and elongation at break) are increased by a factor of 100. 

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