Characterization of novel weak amphoteric charged membranes using zeta-potential measurements: Effect of dipolar ion structure

Weak amphoteric charged membranes were prepared by the radiation-induced gr aft copolymerization of the weakly charged poly(ethylene glycol) derivative s onto high-density polyethylene porous membranes. In this study, two kinds of amphoteric charged membranes, which are the amphoteric ion-pair side ch ain type and the mixed grafted chain type, were prepared and then character ized using the zeta -potential measurements. For the amphoteric ion-pair ty pe membrane, the zeta -potential/pH profile showed a plateau in the range o f pH 6-11 which was characteristic of the dipolar ion structure of an ampho teric ion-pair. On the other hand, the g-potential for the mixed grafted ch ain type membrane showed an intermediate pH dependence between the separate positively charged chain- and negatively charged chain-grafted membranes. The apparent surface charge density on these membranes was obtained from th e zeta -potential. For the amphoteric ion-pair side chain type membrane, th e experimental results could be explained by the theoretical model based on the dissociation of charge groups on the pore surface. For the mixed graft . chain type amphoteric charged membrane, however, the difference between t he experimental result and the calculated one appears in the pH range of 4- 9. This would be caused by the change in the surface states with pH, that i s, a conformational change in the grafted polyelectrolyte chains with the c hange in the charging property. Finally, the unique charging property of th e amphoteric ion-pair became apparent in this study.