As shown in Fig. 3A, the broad band near 3200 cm1
became evident after the first 5 min, when the NH2 groups
had already been converted to NH3
þ groups by protonation.
This absorption is assigned to the stretching vibration
of N+AH. This broad band is sensitive to SO4
2
ions, because of the strong interaction between the SO4
2
and NH3
þ ions. As the reaction time increased, more
SO4
2 ions interacted with the NH3
þ groups, so the band
at 3200 cm1 continued to broaden. Crosslinking of the
membrane occurs when a SO4
2 anion interacts with two
NH3
þ groups by ionic bonding, as depicted in Scheme 2.