Comparative Study on Water Uptake and Ionic Transport Properties of Pre- and Post Sulfonated Chitosan/PVA polymer Exchange Membrane

Abstract

Chitosan/poly(vinyl alcohol) (PVA) blend composite was prepared through two strategies of chemical modification, namely pre- and post-sulfonation. The sulfonation was carried out by using 4-sulfopthalic acid (sPTA) as the sulfonating agent. The modified chitosan/PVA blend composite was prepared under a range of chitosan content (10:90, 25:75, 50:50, 75:25 and 90:10). Water uptake and ion exchange capacity (IEC) of modified membranes were evaluated by titration and gravimetry methods respectively. Accordingly, both pre- and post-sulfonated composite showed a decrease in water uptakes and IEC values with an increase in chitosan content from 10 to 50 vol.%, attributed to the greater number of hydrogen bond pairs between the two polymers. At 75 vol.%, the composite was predicted to be dominant by the hydrophilic nature of chitosan, in which IEC values and water uptakes were shown to increase. The composite with 90 wt.% chitosan was found to be excessively hydrophilic with tremendously high water uptake, hence not suitable for fuel cell application. Besides that, the post-sulfonated composite showed a trend of increase in the IEC values and water uptakes with a decrease in chitosan content from 10 to 50 vol.%, and bounced back at 75 vol.%. Despite both pre- and post-sulfonation methods demonstrated similar trends in the results, it was notable that post-sulfonation method emerged with higher water uptake and ionic conductivity was found more favorable, attributed to the possibility that sulfonation took place on both PVA and chitosan which has caused a significant increase in sulfonic groups that purportedly exhibited higher ion transport mobility.