216 HIGH PERFORMANCE POLYMERS
sodium salt and 4,4
-biphenol as hydroxy compounds,
the sulfonic acid group content can be varied by the
ratio of the hydroxy monomers [42].
In the polymer, the sulfonic acid group is in m-
position to the ether linkage. Therefore, it is believed
that sulfonic acid groups are being deactivated, giving
membranes with high proton conductivity due to
the increased acidity. In addition, the m-position is
expected to contribute to hydrolytic stability. The glass
transition temperature of the copolymers increases
with increasing sulfonic acid group content. It varies
from 233 to 336
◦
C in sodium form or the sulfonic
acid group and from 230 to 260
◦
C for acid form.
Instead of 2,8-dihydroxynaphthalene-6-sulfonate as
sulfonated monomer ...