It can be seen that the increase of sulfuric acid content in
the chitosan membrane was synchronous with protonation
and much quicker than ionic crosslinking. This suggests
that not all of the SO4
2 ions absorbed by the membrane
during protonation were crosslinking agents; only those
located between two NH3
þ groups on the chitosan chains
resulted in ionic crosslinking. As the reaction time
increased, more SO4
2 ions diffused to locations bridging
two NH3
þ groups, increasing the degree of ionic crosslinking.
Therefore, ionic crosslinking is dependent on the
mobility of SO4
2 ions in the membrane.